A study of stereochemical and electronic factors in the nickel promoted dimerization of norbornadiene [pt.I] : electro-organometallic catalysts [pt.II] by Dale Grant Pillsbury A thesis submitted to the Graduate Faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Chemistry Montana State University © Copyright by Dale Grant Pillsbury (1973) Abstract: To gain a better understanding of metal promoted cycloaddition reactions, the nickel promoted dimerization of norbornadiene (NBD) was examined from three aspects; stereochemical and electronic factors in the substrate and the influence of ligands bound to the nickel which are not directly involved in the reaction. As an example of stereochemical perturbation, the 7-methyl derivative of NBD was dimerized using bis(tri-phenylphosphine)dicarbonylnickel, Ni(CO)2(P03)2 , as a promoter with both thermal and photochemical activation. It was found by an analysis of the nuclear magnetic resonance'spectra of the dimers that both 2+2 and 2+2+2 cyclodimerization products were produced. The major dimers had the same skeletal stereochemical configurations as the dimers produced from the corresponding thermal and photochemical reactions of NBD and to investigate substrate electronic factors, norbornene, pentacyclbtetradeca-5,11-diene (exo-trans-exo isomer) and [2,2,2]bicycloocta-2,5-diene were subjected, without success, to conditions found to produce dimers with NBD. A proposal describing the requisite electronic factors for active olefinic substrates was made. The influence of indirectly involved ligands was examined through a study of kinetics parameters in the thermal reaction of Ni(CO)2(P03)2 with NBD. The reaction was found to be very temperature sensitive, to be strongly inhibited by carbon monoxide and triphenylphosphine, and to exhibit an induction period followed by the major reaction period in which the rate of production of the two 2+2 dimers (both known) was constant. The kinetics results were viewed as the initial establishment of equilibrium between a number of competing reactions to produce very low steady state concentrations of active catalytic species. The substrate modification data and kinetics data were coordinated with the work of other investigators to give a unified picture of the nickel promoted cyclodimerization reaction. The use of electrochemical regeneration of active organometallic coupling reagents was also studied. It was found benzyl halides, bromo-benzene, benzoyl chloride, phenacyl bromide and n-hexyl bromide could be coupled with electro chemically produced metal species. The reactions were run in single compartment cells using N,N-dimethylformamide as solvent, a catalytic amount of a metal ion and stabilizing ligands such as triphenylphosphine to prevent zero valent metal from plating out. Carbonyl insertion and a unique formylation reaction were also studied. (Formula not captured by OCR) (Formula not captured by OCR)  PART I: A STUDY OF STEREOCHEMICAL AND ELECTRONIC FACTORS IN THE NICKEL PROMOTED DIMERIZATION OF NORBORNADIENE PART II: ELECTRO-ORGANOMETALLIC CATALYSTS ■ by DALE GRANT PILLSBURY A thesis submitted to the Graduate Faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in Chemistry Approved: Entropy Effects in Complexes Leading to the 2+2 xtn NBD Dimer .............................. 36. Entropy Effects in Complexes Leading to the 2+2 xtx NBD Dimer .................................... 37• Common Pathway to the 2+2 xtn NBD Dimer ................. 38. Non-carbonyl Containing Complexes Leading to the 2+2 xtn NBD Dimer ...................... ............. 39• Qualitative Picture of p Orbital Interactions in NBD .... 40. Proposed Intermediates.in the Formation of the 2+2 xtn NBD Dimer ................................... 41. Proposed Mechanistic Routes to the 2+2+2 xcn 7-MethyINBD Dimer ................................... 42. Alternative Pathways for the Non-concerted Production of the 2+2 xtn Dimer Skeleton ..................... . 43e Review of Proposed Routes to 2+2 and 2+2+2 NBD Dimers ... Figure Part II 1. Preparations of Allyl Nickel Reagents ...... . 2. Schematic for Electro-organometallic Catalysis . 72 74 83 83 84 85 8? 92 95 96 104 124 Page 134 ABSTRACT xiv To gain a better understanding of metal promoted cycloaddition re­ actions, the nickel promoted dimerization of norbornadiene (NBD) was ex­ amined from three aspects; stereochemical and electronic factors in the substrate and the influence of ligands bound to the nickel which are not directly involved in the reaction. As an example of stereochemical per­ turbation, the 7-methyl derivative of NBD was dimerized using bis(tri- phenylphosphine)dicarbonylnickel, Ni(CO)p(P0 ) , as a promoter with both thermal and photochemical activation. If was found by an analysis of the nuclear magnetic resonance'spectra of the dimers that both 2+2 and 2+2+2 cyclodimerization products were produced. The major dimers had the same skeletal stereochemical configurations as the dimers produced from the corresponding thermal and photochemical reactions of NBD and investigate substrate electronic factors, norbornene, 03»Sjpentacyclbtetradeca-5»11-diene.(exo-trans-exo isomer) and [2,2,2jbicycloocta-2,5-diene were subjected, without success, to conditions found to produce dimers with NBD. A proposal describing the requisite electronic factors for active olefinic substrates was made. The influence of indirectly involved ligands was examined through a study of kinetics parameters in the thermal reaction of NiCCO^CPiZWg with NBDe The reaction was found to be very temperature sensitive, to be strongly inhibited by carbon monoxide and tripheny!phosphine, and to exhibit an induction period followed by the major reaction period in which the rate of production of the two 2+2 dimers (both known) was constant. The kinetics results were viewed as the initial establishment of equilibrium between a number of competing reactions to produce very low steady state concentrations of active catalytic species. The substrate modification data and kinetics data were coordinated with the work of other inves­ tigators to give a unified picture of the nickel promoted cyclodimeriza­ tion reaction. The use of electrochemical regeneration of active organometallic coupling reagents was also studied. It was found benzyl halides, bromo- benzene, benzoyl chloride, phenacyl bromide and n-hexyl bromide could be coupled with electro chemically produced metal species. The reactions were run in single compartment cells using N,N-dimethylformamide as solvent, a catalytic amount of a metal ion and stabilizing ligands such as triphenylphosphine to prevent zero valent metal from plating out. - Carbonyl insertion and a unique formylation reaction were also studied. To 9. PAET I A STUDY OF STEREOCHEMICAL AND ELECTRONIC FACTORS IN THE NICKEL PROMOTED DIMERIZATION OF NORBORNADIENE INTRODUCTION Toward an Understanding of Metal Promoted Cycloaddition Among the various classes of organic reactions one may distinguish one group in which the construction of a ring occurs by the creation of two (or more) sides of that ring. The formation of these new sides occurs at the expense of unsaturation - frequently double or triple bonds - in the presursor addends. This class of organic reactions is known as cycloadditions. Thus, the formation of a cyclobutane ring from two molecules of ethylene would be the result of j 22 electrons + 2/n elec­ trons (two double bonds) going to two single bonds, or in short, a 2+2 cycloaddition. The formation of cyclohexane (three new single bonds) from three molecules of ethylene would be one example of a 2+2+2 cyclop- addition. The reaction of 1,3-butadiene ( W electrons) and ethylene (217 electrons) to form cyclohexene (two new single bonds and one new double bond) would be a 4+2 cycloaddition. In this case, the formation of two sides of a six member ring is attended by the creation of a new double bond as well. Cycloaddition" reactions may be divided into those which require only thermal or photochemical excitation and those in which a catalyst is used in conjunction with such excitation. The former group of cycload­ dition reactions encompasses a large body of "classical" organic reac­ tions with such notable examples as the Diels-Alder reaction, ozonization (a dipolar addition), the "ene" reaction, and the 3photochemical cyclobutanation of olefins. It appears these reactions •follow the conservation of orbital symmetry rules of Woodward and Hoffmann"*", if the reaction follows a concerted mechanism. In brief, the rules state that if there are an odd number of participating V electron pairs, the reaction will be thermally allowed, whereas, an even number of participating 77 electron pairs results in a photochemically allowed reaction. Catalyzed' cycloaddition reactions may be subdivided into general acid (or base) catalyzed cycloadditions, e.g. Lewis acid catalyzed Diels- • & Alder reactions, and metal catalyzed reactions. It is with a specific group of reactions in this latter class - cycloadditions catalyzed by transition metals in low oxidation states - that we shall be concerned here. These reactions hereafter shall be referred to as transition metal mediated ( or promoted) cycloaddition reactions. As implied above, in any cycloaddition reaction two alternative .mechanistic pathways are conceivable; one concerted and the other non- concerted. Specifically, with regard to transition metal promoted re­ actions one may visualize a concerted cycloaddition occurring on a metal template in which all new bonds to form the product ring are formed simultaneously with no intermediates being present from reactants to products. One could also visualize a pathway involving the formation of intermediates incorporating the metal. This non-concerted reaction would lead to the product by elimination of the metal with ring closure. 42 Examples of these two alternative mechanisms are given below for a metal promoted ethylene to cyclobutane reaction. r n IJ U M P A A Concerted Mechanism A Non-concerted Mechanism Figure I Alternative Cycloaddition Mechanisms Since the pioneering work of 0. Roelen in 1938 on the oxo process, through the increased interest in transition metal promoted reactions during the '50's and '60's, to current work, there has been a keen in­ terest in elucidating the mechanisms of such reactions. To be able to understand the role of a metal in the sequence, and hence predict new reactions, has become a formidable challenge. Some of the basic tenets of the two alternative mechanistic approaches (concerted and non-con certed) as related to transition metal promoted cycloadditions will be considered now. This shall be followed by a few specific examples of reactions which have been controversial 5The fundamental problem faced by advocates of a concerted pathway for metal assisted cycloadditions is the apparent violation of the afore­ mentioned Woodward-Hoffmaim rules in metal promoted cycloaddition re­ actions. Thus, concerted 2+2 cycloadditions are thermally forbidden (photochemically allowed) according to Woodward and Hoffmann, but in the presence of transition metals, reactions of this type occur with thermal I excitation. Without going into the details of the conservation of orbital -O symmetry, one may apply its principals. First, the participating re­ actant and product orbitals are placed within a system of symmetry ele- . ; jnents (planes of reflection, axes of rotation, etc.) and given symmetry designations, e.g. SA (Symmetrical with respect to one symmetry operation and Antisymmetric with respect to another). Second, these orbitals are ordered qualitatively according to energy. ' Third, the conservation of orbital symmetry restraint is applied, i.e„ ^reactant may go only to SS . . . Therefore, noting that the tie-lines in Figure 2 correlate product symmetry-allowed conversions; for two ethylenes going to a cyclobutane one has: 6V V I I- c — c-I I j<30C><2) olefin TT * orbitals empty metal orbital filled __ M --I SA AS 0 X 0 G><0 cyclobutane cr * orbitals empty metal orbital filled M olefin TT orbitals filled metal orbital empty cyclobutane a orbitals filled metal orbital empty Figure 3 Metal-Olefin Orbital Symmetry Correlations 8It has been suggested by van der Lugt that an alternative to the Mango theory exists involving a lowering of the degree of forbiddeness of thermal 2+2 cycloadditions by metal-substrate configurational inter- • 5 6action. Mango has answered this challenge and the Mango theory re­ mains the principal theoretical argument for advocates of a concerted thermal 2+2 cycloaddition mechanism. Proponents of the non-concerted mechanism circumvent any violation of the VJoodward-Hoffmann rules by the very fact that the rules only apply to concerted reactions. The theoretical basis for a non-concerted approach lies in an application of Occam's Razor which may be stated as: there is no necessity to formulate a new mechanism when a reasonable var­ iation of an established theory already exists. However, the very nature of a non-concerted reaction implies the presence of intermediates and the burden of proof for those intermediates lies with the advocates of a non- concerted mechanism. This is not always readily accomplished. ■ One case which has been examined from both the non-concerted and concerted points of view is the opening of quadricyclane to form norborna- diene. This reaction can be thought of as the reverse of a cyclobutan- \ ation reaction. 9n Q M = Rh(I), Pd(II), Pt(II)' or Ag(I) complexes Figure 4 Metal Assisted Conversion of Quadricyclane to Norbornadiene 4 9Mango 1 has asserted the cyclopropane rings fused to the cyclobutane ring ought to promote facile complexation with a metal allowing a concerted reaction to occur. However, L. Cassar and J . Halpern^^ report the iso­ lation of an acyl rhodium complex from the reaction of quadrieyelane and [ Rh(CO)^Gl]g, suggesting a non-concerted mechanism is a reasonable alterna­ tive. - --> NBD n Figure 5 Proposed Path for [Rh(CO)^GlJg Assisted Opening of Quadricyclane It is not clear in this case that the isolated rhodium complex is ef­ ficiently converted to norbornadiene, but it is noted that some quadri- cyclane is transformed to norbornadiene in the formation of the complex. Also, it is not obvious that intermediates are thereby indicated for other metals 10 Another case is the dimerization of butadiene to I,2-divinylcyclobutane with a Ni-Iigand catalyst."*""*" + Ni-L ---- Figure 6 Nickel Promoted Dimerization of Butadiene 12Initially, Mango suggested the plausibility of a concerted mechanism, but later he acknowledged a stepwise catalytic process based on isomer i % distributions. Intermediates of the type shown below were later iso- 14 lated by Wilke et al. Jl Jl /i-P(C6Hn )3 Isolated Intermediates in the Nickel Promoted Dimerization of Butadiene It is reported these ^-bonded intermediates are apparently in equil­ ibrium with a common TT allyI-a allyl bonded species shown by NMR to be: (C6RTI:3P ^ J A third system which has continued to provoke interest is the metal catalyzed conversion of the exo-tricyclooctene (I) to the 11 tetracyclooctene (Il). Two other isomers (III and IV) can also be produced.^3' ^ Figure 8 Products from the Rh(P0-,)_,Cl Promoted Opening of 2 k} ^ ^ Exo- I^ 1E 1I1O ’ J tricycloocta-6-ene It has been noted by Katz ^ that all the products are stable under the reaction conditions and further that it is difficult to relate products III or IV to I by a simple cycloaddition. He has presented arguments to support a common species, of unknown structure from which both II and III are formed. Mango, advocating the possibility of a concerted mech­ anism, accepts a common origin for the two products. He suggests this "parent" is a complex in which the metal is coordinated to I in a bi- dentate manner. He explains the lack of a clean transformation of this complex to II (the by-product formation of III and IV) as due to an energy barrier in­ volving non-reacting ligands. Thus it is proposed that when a metal acts as an electron reservoir-pump, electron density in the metal orbitals is 12 by necessity somewhat rearranged. If the metal has a strongly interact­ ing ligand field - exclusive of the organic substrate - that field could represent an impedance to the desired cycloaddition reaction. In systems where the ligands act principally as a donors the effect probably would be minimal, while ligands with strong backbonding capacity could cause a pronounced energy barrier. Since neither the non-concerted nor the concerted arguments can be shown to be faulty, this problem continues to be unresolved. One final molecule whose behavior in metal mediated cycloadditions remains a puzzle shall be examined. Norbornadiene (V) has been dimerized with a number of transition metal catalysts including derivatives of Cr, Fe, Co, Rh, Ir and Ni. Depending on the catalyst and the reaction con­ ditions a variety of products can be formed. However, the two most fre­ quently encountered types can be considered to be variants of VI (2+2 product) or VII (2+2+2 product) VII Figure 9 Dimer Types from the Metal Promoted Cyclodimerization of Norbornadiene 13 Both the concerted and non-concerted mechanisms have been applied to the norbornadiene (NBD) system. Advocates for the concerted approach have noted examples of "exo" as well as "endo" bonded transition metal com­ plexes. By selecting a suitable set of bonding modes, concerted path­ ways may be envisioned leading to the reported dimers. 2 On the other hand, the recent publication of the structure (IX) of an Ir complex derived as noted below is offered as proof of a non- concerted mechanism. acetone [lr(l,5-cyclooctadiene)Cl] 2 + NBD, ■> complex /empirical' VIII | formula / \ Ir(NBD) Cl Me _Pacetone CH-COCH-COCH. complex X Ir(NBD)-P(Me)-Cl Figure 10 Study of the[lr l,5(C0D)Cl]2 Promoted Dimerization of NBD It is also noted that there exist pronounced spectral similarities be­ tween IX, X and VIII, Nevertheless, even if one accepts this argument as proof for a non-concerted mechanism for an Ir catalyzed dimerization, the concerted alternative cannot be ruled out for other metal and ligand systems, Based on the four examples just given, one point should be clear: neither the concerted nor the non-concerted mechanism pathways for trans­ ition metal promoted cycloadditions can be discarded at this point. While the concerted mechanism is theoretically appealing it has had to be modified to accommodate such realities as the influences of nonre­ acting ligands» Furthermore, proponents of a concerted mechanism are placed in the difficult situation of either taking a defensive position in what is not clearly an impregnable fortress, or attempting to prove the non-existence of what their opponents claim are tangible species. Since the proof of intermediates lies with the advocates of the non-concerted mechanism they are faced with a serious epistemological problem. Do efforts to acquire intermediates alter the reaction such as to force the formation of intermediates? If this is the case, proof must be provided that intermediates exist in the unperturbed reaction. Since it would be impossible to observe the reaction without altering it to some extent the observation should be made to alter it as little as pos­ sible. Thus neither alternative is without its problems. It may be worthwhile at this point to summarize some of the impor­ tant parameters which must be considered in any metal mediated 14 15 cycloaddition, reaction. The first factor to be considered has just been noted above: when one is attempting to gain information about a reaction mechanism the system under study should be perturbed as little as pos­ sible. Frequently, this means only physical data such as spectra can be used. If intermediate species can be isolated, their spectra should be noted and compared to spectra obtainable from the system under actual reaction conditions. However, it is obvious the use of this approach is strongly dependent on the lifetime of the intermediates, the time scale for the method of measurement, and the degree to which a given species may be distinguished from other species in the reaction. A second factor is the nature of the metal used to catalyze a re­ action. It may be worth stressing that rarely, if ever, can the elec­ tronic or stereochemical characteristics of one metal-substrate system be duplicated by another. Even for second and third row transition metals in the same column significant differences exist.^ Very closely related to this is the nature of the ligands. While the nature of ligand-metal-substrate interactions is not completely understood some efforts have been made" to correlate experimental facts. One area which has received attention and is particularly relevant to metal mediated cycloadditions is the role of ligands in TT to a transfor­ mations. Two examples are given below. 16 substrate 77 bonded ■> substrate o bonded substrate 77 -allyI bonded V L substrate o -allyI bonded (with possible concurrent olefin 77 bonding) Figure 11 Two Examples of fT to a Transformations It may be noted that one of these examples has been encountered before in Figure I (page 4) to illustrate the non-concerted mechanism for cyclo­ addition. Species related to the 77-a allyl system have been studied 11 i4 with respect to the dimerization of butadiene. 1 A qualitative theore­ tical treatment has been given by Tsutsui1^ in which it is proposed that the "Addition of strong 77 ligands to the central metal is one method of inducing o-77 rearrangements. Only ligands...capable of 77 bonding are 20 capable of stabilizing the a state". However it has been suggested by Helmbach, Jolly and Wilke11 that 77-allyl to a-allyl conversions are more readily promoted by phosphines than by phosphites. They also note that phosphites are better back-bond acceptor ligands than are phosphines. 17 Their claim is based on a rather thorough study of the influence of lig­ ands ojj. the nickel catalyzed dimerization of butadiene and appears to be well supported. 21 This behavior has been explained by assuming the TT-allyI system contributes more electron density than the corresponding °-allyI system. When the other ligands are strong donors, the a form is favored. If one considered the formation of a a bonded complex an oxidative addition to the metal then one would expect ligands which increase electron density - strong ° donors, poor back donation acceptors - to enhance formation of 22 the o bonds. One final factor that bears mentioning is symmetry considerations. These considerations apply both to the symmetry constraints on orbital interaction for complexation and to the stereochemistry of the products. Thus, in order to secure a clear picture of the bonding in complexes (or intermediates) leading to a product it is necessary to interact substrate and metal orbitals of the same symmetry. For example, the structures of bis(77-allyl)nickel complexes have been found to prefer the geometry given below.^ Ni trans disposition A significant alteration of this bonding scheme must occur in the allyI- nickel intermediates leading from butadiene to cis,cis-cyclooctadiene. 18 To arrive at the cis,cis stereochemistry it is suggested that the phos­ phine ligand (P) allows either a o-allyl, 77-allyl nickel complex (XI) to form or it allows a cis disposition of two 77-allyl groups as in com­ plex XII. /~ x Figure 12 77 and a Bonded Allyl Nickel Intermediates in the Oligomerization of Butadiene Since it has been shown the addition of P(Et)^ to bis(*?-allyl)nickel results in a complex containing one 77-allyl and one c-allyl ligand (see XIV below), complex XI is most reasonable. It may also be noted that in the absence of the phosphine ligand, butadiene is converted to 19 principally the all trans 1,5,9-cyclododecatriene which could arise from complex XIII. XIV As noted before, cycloaddition reactions of norbornadiene have been treated from both a concerted and non-concerted mechanistic view­ point. However, it may be possible that proponents of neither approach have examined the problem unbiasedly. It is suggested here that the norbornadiene molecule may offer a unique model for examining the mech­ anism of metal promoted cycloadditions. This is proposed in light of: a) the formation of both 2+2 and 2+2+2 cycloaddition products, b) the number of 77-bonded complexes formed by norbornadiene, and c) the report of intermediates whose structures suggest partially formed dimers o-bonded to a metal. As an example of the variety one can encounter let us look at the 17aproducts obtained with just two nickel based "catalysts". It may be noted that these "catalysts" have been chosen because of their partic­ ular relevance to the work presented in the first part of this thesis and because of the wide spectrum of considerations they introduce. Nickel tetracarbonyl, Ni(CO)^, when used as a promoter with photo­ chemical excitation, produces two specific 2+2+2 dimers (XV and XVI 20 in Figure 13). Thermal excitation of Ni(CO)^ and NBD yields two specific 2+2 dimers (XVII and XVIII in Figure 13). The third 2+2 isomer with a trans disposition of norbomene moieties about the cyclobutane ring is a product (XIX in Figure 13) of both the thermally and photochemically induced dimerization of NBD using bis(tripheny!phosphine)dicarbonyl- nickel, Ni(Pj2f^ )2(CO)^1 as a promoter. Ni(CO)4 + h 2+2+2 exo-cis-endo 2+2 exo-trans-endo 2+2+2 exo-trans-endo XVIII 2+2 ' endo-trans-endo Figure 13 Dimers from the Cycloaddition Reaction of NBD Promoted by Ni(CO)4 and Ni(CO)2(P^5)2 21 It is suggested that the word promoter be used here rather than the word catalyst, since both Ni(GO)^ and NiCCCO^CP^)^ are apparently converted to other species which act as catalysts for the dimerizations. ^ The organic products are stable under the reaction conditions and neither thermal nor photochemical excitation alone (no promoter present) will re­ sult in dimerization. Therefore one is faced with accounting for quite an extensive range of products. One attempt to account mechanistically for the products would be to propose a number of steps leading to a complex which could undergo a 26 concerted cycloaddition to form dimers. As shown in Figure 14 , nickel carbonyl is converted to nickel tricarbonyl, Ni(CO)^, and to nickel di­ carbonyl, Ni(CO)^. Either Ni(CO)^ adds to the endo side of NBD in a bidentate manner, or Ni(CO)^ adds to the endo side in a monodentate manner followed by the loss of another CO. The net result is the for­ mation of endo,endo-NBD Ni(CO)-, XX. This molecule may photochemically lose another CO followed by the exo complexation of another NBD molecule to give complex XXI. Complex XXI could undergo a concerted 2+2+2 cyclo­ addition to give either dimer XF or dimer XFI» Complex XX could also be in thermal equilibrium with the coordinatively unsaturated complex endo-NBD Ni(CO)^, XXII. If complex XXII were to bond in an endo fashion to a second NBD molecule, complex XXIII would be formed and this could lead in a concerted fashion to the 2+2 dimer XFIII. On the other hand if complex XXII added another NBD in an exo manner the result would be Ni(CO) + CONi(CO) + Ni(CO) + Ni(CO) Ni(CO) Ni(CO)V-CO Ni(CO) + Ni(CO) XXIII XVIII XXII + Ni(CO) XVIIXXIV Ni(CO) (PO ) AT Ni(CO) + Ni(Pp ) Figure 14 Schematic of the Mechanism of NBD Dimer Formation Using Ni(CO)^ and Ni(CO)2(Pp^)2 Promoters 23 complex XXIV. A concerted cycloaddition of XXIV would give dimer XVII. In a similar fashion Ni(CO)^(P0^)^ could lead to complex XXII (and hence to dimers XVII and XVIII) thermally. However, since dimer XIX is pro­ duced thermally - and photochemically as well - another precursor com­ plex, e.g. XXV, would have to be formed. This non-carbonyl containing complex might be formed via an intermediate such as bis(triphenylphos- phine)nickel, NitP^)^, an^ could lead by a concerted cycloaddition to dimer XIX. There are also other mechanistic possibilities. A non-concerted 2 cycloaddition sequence based on the concepts developed by Osborn et al. , 27 vide ante, as well as Katz might be suggested. Thus 2+2+2 dimers could be explained as arising by the extrusion of a metal species from the incipient cyclopropane ring of the nortricyclane moiety. 24 Similarly, the 2+2 dimers would result from extrusion and closure to form the cyclobutane ring. Ni2w 3)2 + A severai ste- XVIII XXVIII (WyxW3)z XIX Another way in which the 2+2+2 dimers might arise by a non-concerted path could be by the closure of the five member ring. Pathways to these bis(a-alkyl)nickel species (XXVI-XXX) are many and varied. However, it may be worth-while looking at what appear to be some of the more reasonable possibilities. 25 (CO) Ni *  > 2+2 endo-trans-endo I 2+2+2 exo-cis-endo 2+2+2 exo-trans-endo I --NiCO ,4 closure '-Ni-6 closure and 2-6 cyclo­ propane180° rotation of endo bonded NBD closure -Ni-5closure corresponding dimer not foundClosure 2'-Ni- and 3-5 cyclo­ propane Figure 15 Modes of Closure of Precursor Complexes Leading to NBD Dimers 27 In each of the above cases the initial complex is the same as that which might lead to product in a concerted fashion. The point of con­ tention is simply the path of getting from these complexes to products. One path which has not previously been suggested as a route for metal mediated cycloadditions of NBD is that involving the so-called homo- allylic moiety: As can be seen in Figure 15, a complex of this type could allow for for­ mation of either 2+2+2 or 2+2 dimers depending on the mode of closure. The invocation of such an intermediate would add unity to the mechanism, and should be considered here. The formation of a "homoallylic" complex by the addition of meth- 28 oxide to the di-endo bonded complex (NBD)PdCl^ has been claimed and later this was expanded to the addition of other n u c l e o p h i l e s . ^ -- Ni XXXI homoallylic model XXXII o +77 model Figure 16 Reactions of PdCl21NBD Complex with Nucleophiles 28 There has been some controversy about the bonding in the product, XXXI pO or XXXII. The homoallylic model was proposed by Green and HancocIc to account for their interpretation of NI-IR spectra of the product. 29cStille and Morgan had originally suggested the product had a Pd-C CT bond and a Pd-C v bond. Chemical reactivity experiments"^ ^ were not very decisive since a dynamic equilibrium between two a-77 con­ tributors and a homoallylic species could readily be displaced by the gain or loss of a l i g a n d , ^ as shown below. XXXIV XXXIII XXXV Figure 17 Possible Reaction Pathways for a TT-allyI NBDePd Complex 2 8 30 32Further, proponents ^ of the n -homoallylic model in citing supposed similarities between it and known TT -allyI palladium compounds have made an analogy which is somewhat dubious. They have implied ex­ plicitly or implicitly that the formation of the cyclopropyl ring (forming a nortricyclane) is analogous to the formation of a double bond in Ti-allylie— > a -allylic transformations. They have apparently dismissed the similar process for norbornenyl systems. 29 Figure 18 Palladium 77 and o Allylic and 77 and o Homoallylic Complexes 34 A later NMR analysis by Haszeldine and workers refuted some of Green's assignments. They suggested the one 77, one a bonding picture is more appropriate and explained the propensity of palladium norbor- nenyl compounds to go to nortricyclane derivatives as being the result of steric approach and electronic factors. L = R^N, RjP, dienes Figure 19 Pd 77-allyI NBD to Pd a-nortricyclane Transformations 30 Another approach to the question of n homoallylic bonding to metals in norbornenyl systems may come from the bonding picture from the norbornenyl viewpoint. Semmelhack has noted that an allyI group bonded to a transition metal "shows chemical properties consistent with an 35electron density slightly greater than that of the allyI radical". If this is the case, would a delocalized radical be consistent with the behavior of a norbornenyl system? The general consensus appears to be that non-classical norbornenyl radicals such as XXXVI are not a very good representation. Rather there appears to be a preference for an equilibrium between the two classical radicals XXXVII and XXXVIII. XXXVII Figure 20 Structural Representations of the Norbornenyl Radical Based on this consideration, the cr and 77 model for the 2-methoxy- norbornenyl palladium chloride dimer would seem the more reasonable choice. It should be noted that while the nature of both the metal and any ligands on that metal may alter the situation there does not appear to be any compelling evidence in favor of a TT-homoallylic bonding mode. However, one is still faced with a decision between the concerted path to product or the non-concerted alternative. The problem of 31 resolving this question still presents a formidable challenge. It would be reasonable to ask if the solution of such a problem could have any significance other than establishing the mechanism for the cyclo­ dimerization of one molecule. While norbornadiene must be considered a rather unique molecule, an understanding of the mode by which it dimerizes in the presence of metals could have some far-reaching con­ sequences based on just that uniqueness. These consequences could reasonably include: 1) Contribution to the picture of the fundamental nature of bonding between metals and organic molecules - both TT and c bonded species being known. 2) Contributions to the relative significance of steric and ■ ■ electronic factors in metal promoted 2+2+2 as well as 2+2 reactions - thus allowing for the possible expansion of synthetic organic reactions. 5) Possible direct extrapolation to other hi- or polycyclic olefins including terpenoid molecules - perhaps providing an insight into the basis for stereoselectivity in natural products. Thus, while the solution of the problem may present a formidable challenge, the results of that solution would too. DISCUSSION The work reported in this thesis is the result - of a bifunctional approach to the problem of metal mediated cycloaddition reactions of ole­ fins. The first aspect of this approach is to explore the scope of the reaction and the second is to come to a better understanding of the mech­ anism or mechanisms operating in such reactions. Since norbornadiene (NBD) presents some particular advantages from both aspects, as noted in the Introduction to this part of the thesis, it was chosen as the mole­ cule to be studied initially. Although it would be somewhat artificial to isolate the two aspects of the problem from one. another, it is considered useful to differentiate them. To do so the following divisions have been made in the work to be reported: a) An examination of structural modification of the NBD mole­ cule, while still retaining the NBD skeleton. Emphasis is placed on steric considerations including substrate and metal-substrate factors. b) An examination of structural perturbations of the NBD mole­ cule in which the electronic nature of the NBD molecule has . been significantly altered. Emphasis is placed here on sub­ strate electronic effects as they influence complexation and subsequent cycloaddition. c) A kinetic study with emphasis placed on metal-ligand 33 interaction. It is realized that any structural modifications in the substrate would necessarily result in changes in electronic factors. Likewise, varia­ tions in ligands will result in new stereochemical demands. It is felt, however, that while in any given portion of the discussion emphasis may be placed on a particular point, this emphasis will not preclude the consideration of other factors. Structural Modifications - Steric Considerations Preliminary investigations. The first compound to be investigated from a structural viewpoint was 2,3-bis(trifluoromethyl)NBD, XXXIX. From an analysis of the anticipated 2+2 xtx dimer, vide infra, one might be able to gain an insight into the mode of bonding in intermediate com­ plexes and the role of stereochemistry in determining favorable bonding patterns. The following product variations are among those which could be considered. XXXIX CF3 34 Various concerted and non-concerted pathways involving directly or indirectly, metal mediated reactions could be suggested to account for these products and it was hoped an analysis of dimer product would be informative„ Furthermore, the ease with which the molecule could.be prepared from bis(trifluoromethyl)acetylene and cyclopentadiene was at­ tractive as compared to the difficulty anticipated with the correspond- * ing hydrocarbon, 2,3-dime thyIHBD„ .However, the extended photolysis . (l66 hours) of a benzene solution of this molecule using bis(triphenyl- phosphine) dicarbonylnickel, Ni(CO)2(P0^)2, as a promoter showed no ap­ parent dimer formation by gas chromatographic analysis. Bis(triphenyl- phosphine) dicarbonylnickel was chosen as a promoter in the reaction since, as noted earlier, it produces essentially only one NBD dimer (the 2+2 dimer, XIX) with photochemical activation; it is quite an active catalyst with NBD ( 20^ reaction in ?2 h o u r s ) a n d it is also a solid air stable compound providing experimental convenience. Feeling the negative result with 2,3-bis(trifluoromethyl)NBD might ,be a result of either too severe a steric demand or too drastic a change in the electronic configuration of the olefinic bonds, two ^-substituted derivatives (XL and XLI) were tried. The first of these was commercially available while the second could be fairly readily prepared. Jolly, * * A U photolysis reactions were carried out in pyrex tubes unless other­ wise noted. The amount of Ni(CO)2(P^)2 used was catalytic ( 1x10-4 1x10-° j which also applies to any thermal reactions run. 35 37Stone and MacKenzie reported XL could be thermally dimerized, albeit low yield, using the very active Promoter^a dicarbonyldinitrosyliron. However, irradiation of benzene solutions of XL (135 hours) and XLI (l40 hours) containing N i ( C O ) ^ showed no apparent dimer by gas XL R = t-butyl XLI R = H chromatographic analysis. This lack of success could be attributed to a very stable complex such as XLII being formed between the metal and the substrate. The formation of a stable chelate complex such as XLII XLII might prevent further reaction, since the bonded olefins may not be in a favorable disposition to interact (the bonded olefins are at right angles to each other). Other alternatives to explain the inactivity of this substrate might be suggested. For example, one could envision the blockage of one exo side of each norbornadiene simply by steric bulk. Thus if it were reasonable to assume an exo-exo diNBD specie (XLIII) 36 was necessary for dimerization (and involved in the rate expression), the rate would be one quarter of that in the unsubstituted case, based on statistical factors alone. In the case of 7-hydroxy-NBD (XLl), XLIII oxidation of the nickel with concomitant hydrogen gas formation is a reasonable explanation for lack of product formation. 7-Methylnorbornad;ene, Reactions and product structure assignments. In light of the difficulty encountered with substituents containing heteroatoms, the hydrocarbon derivative 7-methyINBD was synthesized and subjected to thermal and photochemical excitation in the presence of Ni(CO) .ptP^)^ • After photolyzing undiluted 7-me thy INBD for about 210 hours, a significant amount of the olefin had been converted to dimers. A thermal reaction was also run with this molecule. Starting at reflux (92°C) and heating for 40-70 hours (final temperature about 105°C) re­ sulted in nearly complete dimerization. Since gas chromatography showed that the major components of the photochemical reaction were also present in the thermal reaction and because the thermal reaction pro­ vided considerably more product to work with, the dimers from the thermal reaction were subjected to the following separation scheme. Mixture from Thermal Reaction . (6 components by GLG) XLIV, XLV, XLVI, XLVII, XLVIII, XLIX column chromatography AgNO^/silica gel (XLVII1 XLV-A)*---- Prep GLC 20' yy/o SE30 (XLVII)* very minor XLV-A MJOR XLIV1 XLV-B1 XLVI1 XLVIII1 XLIX Prep GLG XLIV - MAJOR (XLV-B1 XLVI1 XLVIII1 XLIX)* column chromatography AgClO^/silica gel (XLVI1 XLVIII)* XLV-B1 XLIX Prep GLC Prep GLC 23 XLVI MAJOR (XLVIlI)* trace XLV-B T-------(XLIX)* Minor Minor Note: Relative yields may be given as; XLV-A-"vyXLVT > XLIV» XLV-B > XLIX>>XLVII> > XLVIIT *Note: In each of the above subseparationsf.the dimer or dimers denoted by * were retained longer (had longer GLC retention times or required a greater quantity of,or more polar elutant for the column chromatography separations)« Figure 21 Although the initial gas chromatogram showed six components, column chromatography on silver nitrate impregnated silica gel followed by an­ other gas chromatographic analysis showed one of the "components" (XLV) to be two compounds (XLV-A and XLV-B)„ Only a limited separation could be achieved with silver nitrate impregnated silica gel column chromato­ graphy. However, by the use of preparative gas chromatography as well as column chromatography on silver ion impregnated silica gel (both the perchlorate and the nitrate), one could obtain complete separation. Attention is drawn to the fact that silver perchlorate which probably is a source of "rawer" silver ion'^5'^ could effect a separation which was impossible with silver nitrate. Once separated, the NMH spectra were taken for all the dimers except XLVIII since it was obtained in a very small quantity. Analysis of the NMR spectra (see Appendix I for the spectral data and its analysis) pro­ vided the structural assignments given in Figure 22. Note that the numbers following the thermal and photochemical designations refer to estimations■of the relative amounts of a particular dimer formed with . the specified mode of excitation. These estimates were based on peak • heights from gas chromatographic analysis of the product. The photochemically obtained products were assigned their struc­ tures by comparison with samples from the thermal reaction. Both silver ion impregnated silica gel thin layer plates and gas chromatography were used for these comparisons. t 38 39 Nomenclature for NBD dimers. An explanation of the nomenclature to be used here as well as in the remainder of this portion of the thesis is given here. I) Dimers are referred to either as 2+2 (two double bonds form a cyclobutane ring) or as 2+2+2 (three double bonds form cyclo­ pentane and cyclopropane rings). 2) 2+2 dimers are assigned three stereochemical designations. a) Two of these designations concern the sterochemistry of the carbon-carbon bonds to the two norbomene fragments, e.g. exo- -exo (abbreviated x-x). Note that in no case is there 2+2 2+2+2 4o any 1,2 (or 3,4) trans substituted cyclobutane formed, i.e. the cyclobutane ring is flat. b) The third designation concerns the disposition of 1,2 sub­ stituents (cyclopentene fragments, circled) relative to 3,4 substituents (also circled). Both trans and cis are con­ ceivable, but only the trans disposition is found in the 2+2 dimers reported in the literature. 3) 2+2+2 dimers are also assigned three stereochemical designations which are based on the following sequence. a) Norbomene fragment (always specified first) ; carbon-carbon bonds are either exo (x) or endo (n). b) The cyclopentane ring; 1,2 substituents (circled) are cis (c) or trans (t) to 3,5 substituents (marked by a square). Note that in all cases a substituent of carbon I is cis to one on carbon 2, and a substituent on carbon 3 is cis to one on carbon 5» c) Nortricyclane fragment; always bonded on the endo side in 2+2+2 dimers of NBD and always specified last. Thus, the total designation of a dimer should be noted since the follow­ ing dimers might otherwise be confused. 2+2 xtn 2+2+2 xtn Me Me Me 'I // XLV-A A 20 hv 2 2+2 exo-trans-endo (xtn) XLV-B 2+2+2 exo-cis-endo (xcn) A 7 hv I XLVI A 20 hv 2 XLIX 2+2+2 endo-cis-endo (ncn) A 5 I // ^ M e XLIV A 10 hv 7.5 2+2 exo-trans-exo (xtx) XLVII 2+2 endo-trans-endo (ntn) A 2 Figure 22 Structures of 7-MethyINBD Dimers 42 Structure and product distribution relationships for 7-methyINBD dimers. A cursory examination of these product assignments reveals some general relationships between structure and product distribution. a) The major "thermal" dimers are those having the 2+2 xtn and 2+2 xtx skeletons (xtn:xtx, 4:1). b) The major photochemical dimer has the xtx skeleton. These find­ ings are qualitatively consistent with the results from the reac­ tion of NBD with NiCCO^CPS^)^ run under similar conditions; photochemical xtx> 90%, thermal xtn:xtx, 2:1. c) The rate of dimerization is significantly slower than that of NBD itself. Therefore, side redactions in the dimerization of NBD could become more important with 7-methyINBD. However, it should be noted the 2+2 dimers do account for about 8l$ of the dimers produced. It is therefore suggested that a similar reaction mechanism is oper­ ating for both ?-methyINBD and NBD itself. The 7-methyINBD molecule is a perturbation on NBD and its dimerization reactions do not warrant a completely separate mechanistic picture. On further examination some other points may be made. d) There is no dimer formed in which a methyl group is syn to an 40 exo bonded cyclobutane. Brown’s work with 7-substituted NBD derivatives can be noted here. He found 7-methyl groups syn to the olefin would severely inhibit silver ion complexation and this inhibition can probably be generalized to other metals 43 Furthermore, there should be a strong steric interaction when a methyl group is syn to an incipient cyclobutane. Thus the pos­ sibility of a metal species bonding exo to one side of a NBD, thereby inducing (via a transannular interaction) a propensity for the other exo side to participate in dimer formation is un- e) In 2+2 dimers there appears to be a statistical distribution of methyl groups on norbornene fragments having the cyclobutane endo bonded, cf. the 2+2 xtn dimers XLV-A and XLVI and the 2+2 ntn dimer XLVII. Thus, formation of an endo bonded cyclobutane probably arises from a specie with the metal bonded endo to the incipient endo fragment. Based on statements d and e above, and assuming the mechanism for dimer formation involves the stereospecific complexation of two olefins 4 17a 25to the same metal, 1 1 followed by closure in either a concerted or a non-concerted manner, a statistical comparison of 7-methyINBD and NBD dimerizations can be made. Only one of the four possible modes of com­ plexation leading to dimer XLIV is possible with 7-methyINBD. Thus the effective concentration of 7-methyINBD is reduced by )4. Only two of likely. 44 four possible modes of complexation can lead to dimers XLV-A and XLVI. So the effective concentration of 7-methylNBD is reduced by # relative to these dimers. Since the 2+2 xtx dimer (XLIV) is reduced by # and 2+2 xtn dimers (XLV-A and XLVI) are reduced by 1Zz, dimer XLIV is reduced by # relative to dimers XLV-A and XLVI. Since the approximate ratio of XLV-A and XLVI to XLIV is experimentally found to be 4:1, a ratio of 0/2)4:1 or 2:1 would be suggested for the 2+2 xtn to 2+2 xtx ratio in the dimerization of unsubstituted NBD. This is the value reported. This provides further evidence the mechanism(s) operating in the dimerization is only a per­ turbation on that (those) for NBD. f) It may be noted that the 2+2+2 dimer XLV-B has the skeleton of the major 2+2+2 dimer xcn from Ni(CO)^. However, the 2+2+2 xtn dimer skeleton is not formed from 7-methyINBD and Ni(CO)2(P0^)2 although the corresponding NBD dimer is produced with Ni(CO)^. R 2+2+2 xcn skeleton Formed by Ni(CO)^ + NBD, R = H Formed by Ni(CO)2(PjZ^ )2 + 7-me thy INBD, R = methyl 2+2+2 xtn skeleton Formed by Ni(CO)^ + NBD, R = H Not formed by NiCCO^CPf#-^ + 7-methyINBDf R = methyl 2+2+2 non skeleton Not formed by Ni(CO)^ + NBDf R = H Formed by Ni(CO)2(P^)2 + 7-methyINBDf R = methyl Thus1 it is suggested that in the Ni(CO)2(P0^)2/7-methylNBD reac­ tion, at least, the formation of 2+2+2 xtn and 2+2+2 xcn type dimers are probably independent. The stereochemical combination of 2+2+2 dimer skeletons (xcn and ncn) formed in the 7-methylNBD^i(C0)2(P^^)2 system has been found in at least three other cases. The dimerization of NBD with 5/^ rhodium on carbon^ under thermal conditions provides these two, 2+2+2 xcn and 2+2+2 ncn (7:1), as nearly the exclusive dimeric product (2.8% 2+2 xtx is also formed). Dicobaltoctacarbonyl also produced these 25 two 2+2+2 dimers as well as the 2+2 xtx, xtn and ntn dimers. Using the promoter I,4-diphenylbutadienetricarbony!iron and NBD with photo­ chemical activation, the two 2+2+2 dimers and the 2+2 xtx dimer are formed 42 At this point one can examine the stereochemical dispositions of the various dimers,. both 2+2 and 2+2+2, formed in the thermal reaction of 7-methylNBD, and attempt to correlate their formation with mechanistic hypotheses. As in the application of deductive reasoning to any incom­ plete body of information, this can only result in the elimination of some possibilities as being unlikely. The application of inductive reasoning to the same body of information can only suggest likely pos­ sibilities. Thus, the application of both types of reasoning can possibly provide some working hypotheses to be tested. This is the in­ tended purpose in the following stereochemical-mechanistic correlation. To explain the absence of the 2+2+2 xtn dimer, a steric interference argument could be made involving the interaction between the bridge hydrogen and the bridgehead hydrogen (both circled) in structure XLV-Bf of Figure 23. However, it is not clear that in the concerted or the first non-concerted path shown, the alternative steric interactions forming the 2+2+2 xcn dimer, XLV-B, are any less of a problem. It would seem more reasonable to propose the second non-concerted path as the • predominant mode. Me * Note: Complex C could also be bonded endo to the 7-methyINBD in a monodentate fashion and the steric considerations would still be essentially the same. This is due to the restrictions placed on the spatial disposition of molecules in any intermediate which could reasonably lead to such a rigid structure as the 2+2+2 dimers. Figure 23 Stereochemical Interactions in Intermediates Leading to 2+2+2 Dimers ; 48 Sequences .are shown in Figure 24 below, where the two stereoisomers of complex C are distinguished as and C^. In complex C^, the ligand L, to relieve steric interaction with the bridge hydrogen, is not in the bridge carbon-nickel-bridgehead carbon plane; the plane of the paper in Figure 24» Thus the two norbornadiene molecules are juxtapositioned to favor formation of LI because carbons a and b are moved closer at the expense of the c-d distance, Furthermore, in the sequence -- & LI, the formation of the cyclopropane ring probably decreases steric interaction. ■ This is in contrast to the concerted path or the first non-concerted path where, as shown in Figure 23, the Ni-L/bridge hydrogen interactions prob­ ably increase as the reaction proceeds. The steric interference between the bridge hydrogen and the bridgehead hydrogen appears to be much more severe and much less readily relieved (by canting of the norbornadiene molecules) in the complex than are the steric interactions in C^. Thus, there ought to be a stereochemical preference in the formation of the initial complex (C^ or C^), with the formation of being favored. 49 Figure 24 Stereoisomers of Complex C Leading to 2+2+2 xcn and 2+2+2 xtn Further, in the case of the -- >L U sequence, steric interactions are also relieved but to a lesser extent than in the — > LI sequence. It is suggested complex leading to intermediate LI therefore is the favored process. As noted in Figure 2J, it is possible to propose that intermediate LI may also serve as one route to the 2+2 xtn dimers XLV-A and XLVI. Thus, in light of the electron donating character of the edge of a cyclo- 43.44propane ring one could expect the backside of the nickel to be available for attack as shown in Figure 25» Such an attack could lead to the intermediate LIII with no necessity to invoke a "homoallylic" bonding situation, (cf. page 2? ff.) although that possibility could not be ruled out 50 nortricycl fragment endo-bonded norbornene fragment 'ie   2+2 x tn LI LIII Figure 25 Stereochemical Considerations in Complex LI Influencing the Position of the Methyl Group in 2+2 xtn 7-methyINBD Dimer There is, however, one complication which must be considered here. If one proposes complex LI going to complex LIII1 then there ought to be a stereochemical preference of the position of the methyl group in the endo bonded norbornene fragment of the product. In the complex LI of Figure 25, both circled hydrogens would interact sterically about equally. These hydrogens constitute by far the major steric effect relative to the methyl group in the nortricyclane fragment. Thus there would be no particular preference for which side the methyl would be on in the nor­ tricyclane fragment. However, in complex LIII the endo bonded norbornene fragment - which was the nortricyclane moiety - presents two rather different steric demands to a bridge methyl group. The incipient cyclo­ butane side has two exo hydrogens (marked by squares in complex LIII) which ought to present significant steric bulk to a methyl group near 51 them. The olefinic side, however, presents no such interaction and, in fact, ought to provide steric interaction which is less than that found in the progenitorial complex LI. Preference for the methyl group being syn to the olefin is not found in the 2+2 dimers X L V - A and XLVI, and thus it is suggested the non-concerted path C -+LI— &-LIII— >2+2 xtn dimer is probably not reasonable. It is suggested, based on the same reasoning, that any sequence leading to 2+2 xtn dimer which involves the formation of complex LIII is unlikely, since it is inconsistent with the statistical disposition of methyl groups on the endo bonded norbornene fragment. It is therefore difficult to envision a non-concerted directly metal-mediated path to the 2+2 xtn dimers XLV-A,and XLVI. In a similar fashion one could visualize.complex LIV (Figure 26) leading to intermediate LV (and hence to the 2+2+2 non dimer) and pos­ sibly intermediate LVI, providing a path to the 2+2 ntn dimer. 45 52 2+2+2 endo-cis- endoL-Ni. 2+2 endo-trans-endo Figure 26 Stereochemical Considerations in the Formation of 2+2+2 non and 2+2 ntn 7-MethylNBD Dimers from Complex LV Why the methyl group should be syn to the olefin in this 2+2+2 product could thus be rationalized by two explanations. The first of these is 46.47 48 49based on the results of both proton ’ and carbon IJ 1 nuclear magnetic resonance spectroscopy from which it is clear the presence of a 7-methyl alters the electron density about the olefinic carbons (and presumably the olefinic bond itself). This might result in preferential bond making or breaking. Another explanation would be to look at complex LIV (Figure 26) as a bis(endo,endo-7-methylNBD)nickel species LIV (Figure 27). Thus, L in complex LIV is the remaining uncomplexed endo side of the 53 LIV L V Z / •X y rotate molecule ----- C----- > 1) l80° about y axis 2) l80° about z axis Me Me Figure 27 Stereochemical Considerations in the Formation of the 2+2+2 ncn 7-MethyINBD Dimer monodentate NBD and both NBD molecules become bidentate in complex L I V . As the intermediate L V is formed, the methyl could be either syn or anti to the olefin in the norbornene fragment. In this process of going from the77 -bonded intermediate LIV to the 0 -bonded intermediate L V , carbons 2 32 and 3 change from sp to sp hybrids. This results in the swinging up, to the exo side, of the hydrogens attached to these carbons. If the methyl group were on the same side as these "new" exo hydrogens the steric interference would be greater than if the methyl group were anti to them (syn to the olefin). Thus, the methyl is predicted to be syn to the olefin as is found to be the case. The position of the methyl on the nortricyclane fragment would be statistical since the steric require­ ment would be virtually the same on either side. An analysis of the path LV — > L V I ^ 2+2 ntn dimer (Figure 26) could be made in a fashion similar to that done for the LI— frLIII— >2+2 xtn dimer sequence (Figure 25). The prediction would be for a preferential formation of the 2+2 ntn dimer with both methyls syn to olefins. In fact, the statistically favored dimer with one methyl syn and one anti to the respective olefins is found. Thus here as well, it appears a common intermediate leading to both 2+2+2 and 2+2 dimers is unlikely. Further, by analogy with the arguments given for the 2+2 xtn dimers, page 50 , it is deemed unlikely that the 2+2 ntn • dimer (XLVII) is formed by a non- concerted process. The result of the dimerization of 7-methylNBD can therefore be summed up as follows: 1) The mechanism is probably quite similar to that of NBD itself for both the thermal and photochemical reaction. 2) Transannular energy transfer leading to dimer is unlikely. 3) 2+2+2 and 2+2 dimers are probably formed by different mechanisms. 4) It is suggested that the non-concerted paths C^ '— #*LI— >2+2+2 xcn (Figures 23 and 24) and LIV*— >LV'— >2+2+2 ncn (Figure 27) can 54 55 . best account for the formation of the observed 2+2+2 dimers. 5) Non-concerted mechanisms involving complexes LIII and LVI cannot explain the formation of the observed 2+2 xtn (XLV-A, XLVl) and 2+2 ntn (XLVII) dimers. Since' it is difficult to suggest non- concerted, direct metal-mediated paths which do not involve these complexes, it is suggested a concerted path may be more reasonable^ 6) Virtually nothing can be said about the formation of the 2+2 xtx dimer XLIV other than its formation may involve the initial • complexation of two NBD molecules on the exo side, anti to the methyl group. Structural Modifications - Electronic Factors As noted at the outset of this discussion, an examination of struc­ tural perturbations of the NBD molecule which result in a significant alteration of the electronic nature of that molecule were also studied. It is" very probable that the primary electronic interactions between NBD molecules and nickel species involve the olefinic bonds of NBD. Thus, modification of the olefinic bonds in NBD by alteration of its structure was .the method chosen to examine substrate electronic factors. Three molecules were chosen for study: norbornene, the 2+2 xtx NBD dimer and [2,2,2]bicycloocta-2,5-diene. Norbornene could be viewed as a model for one side of a NBD molecule and would provide a probe for the impor- 17atance of olefin-olefin interaction in the NBD molecule. It is known" that the 2+2 xtx NBD dimer will co-dimerize with NBD. Thus "it was felt a study of a norbornene-ole fin system, perturbed by an exo bonded cyclo­ butane, might provide a "graded" interaction scale from norbornene to NBD. Finally, the NBD homolog [2,2,2] bicycloocta-2,5-diene was to be studied for similar reasons„ The reaction of norbornene and NiCCO)^']?^)^ was run thermally as a melt of neat norbornene. The mixture was heated at about 90°C for three days. However, although the solution became yellowish, no dimeric product could be found by gas chromatographic analysis, A similar ex­ periment was run with the 2+2 exo-trans-exo dimer which may be viewed as a modified norbornene, except in this case just enough hexane was added to effect solution of the dimer. After refluxing six days, the solution * had become yellowish, but on work-up no tetramer could be detected. It is felt the results of these two experiments suggest either of two alternatives, a) no complex is formed, or b) a complex is formed but it does not lead to cycloaddition products, A thorough investigation of these systems was not attempted. However, the formation of yellow solutions is reminiscent of the behavior of norbornene and the 2+2 xtx dimer with Ni(CO)^ observed by Voecks. In that system, complex forma­ tion was readily observed, but dimerization could not be achieved. Thus 56 * Note: If one heats a neat mixture of NiCCO^CPJiC^ and the NBD 2+2 xtx dimer to 160S180°C, a solution is obtained and a gas, presumably carbon monoxide, is evolved. The infrared spectrum shows a significant de­ crease in the intensity of the two CO peaks (2000 cm”-'- and 1955 cm--'-). Prolonged heating under these conditions results in nickel being plated out. 57 the second alternative - "unfruitful" complex formation - may be a more suitable description of the observed behavior of norbornene and the 2+2 xtx dimer. The homblog of NBD, [2,2,2] bicycloocta-2,5-diene (dihydrobarrelene), shown in Figure 28, was synthesized and reacted as an isopentane solution with dinitrosyldicarbonyIiron(O), FeCCO^CNO^i using photochemical ac­ tivation. Three other dimerization reactions using thermal activation were also attemped with benzene solutions of dihydrobarrelene and the following promoters: FeCCO^CNO)^, Ni(CO)^1 and 5% rhodium on carbon. The choice of promoters used was made as follows. The NBD 2+2 xtx dimer is the most frequently encountered NBD dimer, considering all the pro­ moters which have been used for dimerization. Dinitrosyldicarbonyliron is the most active of the promoters reported for producing the 2+2 xtx NBD dimer, when used either photochemically or t h e r m a l l y . ^ The use of Ni(CO)^ was prompted by its ability to form the 2+2 xtn and ntn NBD 17adimers thermally. Rhodium on carbon is known to thermally dimerize 50NBD to give 2+2+2 xcn and non dimers. Thus, although a large number Figure 28 DihydrobarreIene 58 of promoters were investigated to provide a wide range of dimer skeletal types, none of the reactions produced dimer as shown by gas chromato­ graphic and/or NMR analysis; It was noted, however, that in the cases of Ni(CO)j^ and Fe(CO),,(NO)^, color changes (colorless to yellow and red to dark red, respectively) occurred, suggesting an1 "unfruitful" complex may have been formed here as suggested in the norbornene and 2+2 xtx dimer reactions with NitCO)^]?/^^ (vide supra). It was felt the reactivity of NBD in transition metal promoted cyclo dimerization reactions could be interpreted as a function of the inter­ action of its two double bonds» The lack of reactivity of the mono­ olefin, norbornene, would be in accord with this hypothesis« It was hy­ pothesized that interaction of a norbornene-type olefinic bond with the edge of an exo bonded cyclobutane might result in sufficient activation of the double bond to promote dimerization. The 2+2 xtx NBD dimer is an example of just such a system and its known co-dimerization with NBD 17ato form NBD trimer was encouraging. Part of the rationale for possibly expecting the cy­ clobutane to activate the olefin was based on an anal­ ogy with the cyclopropane analogs. It is known that 51 52norbornene derivatives such as LVII and LVTII will dimerize with 5% rhodium on carbon although in much ■ lower yields, considerably higher temperatures and with longer reaction times.than those used for NBD. 59 LVII LVIII IJO0C 48 hours 15/£ yield of dimer 185 0C 42 hours 5% yield of dimer The dimeric products are suggested to have the 2+2 xtx skeleton. It is also interesting to note that in the case of 5# rhodium on carbon, norbornene was found to be inactive alone or in crosscoupling ex- 52periments with NBD. As noted, it was found that, in fact, the 2+2 xtx dimer is inactive towards cycloaddition in the absence of NBD, and that dihydrobarrelene is also inactive. These results, in light of the above proposed hypoth­ eses, suggest a re-evaluation of the degree of olefin activation to be expected in these (and similar) cases is necessary. Equally important to consider is the influence of such activation on metal-promoted cyclo­ dimerization reactions. To aid in this re-evaluation, extensive use shall be made of the results from photoelectron spectroscopy, a technique which has been shown to yield valuable information about the nature of electronic interactions in molecules. Photoelectron spectroscopy shows there is an interaction between the exo cyclopropane and the olefin bond in LVII1 resulting in a lowering of the energy of the olefin by about .05 eV (interaction 60 parameter K 0.3 eV) relative to the olefin in norbornene. By comparison, the energy of the highest occupied olefin orbital in NBD is 0.25 eV higher than that in norbornene (interaction parameter U 0.4 eV), and .20 53 eV higher than the olefin in LVII. It is suggested that the cyclo­ butane moiety would be less effective in raising the energy of the high- * est filled n orbital than would a cyclopropane or another olefin. Thus an exo cyclobutane such as is present in the xtx dimer may provide suf­ ficient activation of a neighboring olefin to allow it to react with a complexed NBD (producing trimer), but may be insufficient to activate two dimers to form tetramer. It may be noted that photoelectron spectroscopy shows the highest occupied TT orbital of dihydrobarreIene to be 0.l8 eV below that of NBD 55or about equal to that of the exo cyclopropano-norbornene, LVTI. Note also that there appears to be a significantly * * Support for this argument comes from the photoelectron spectroscop­ ically obtained values for the energy of the highest occupied 77 orbit­ als in the following molecules.5^ + Energy of Highest Occupied Tl Orbital 61 more severe steric demand in the 2+2 dihydrobarrelene dimer with the xtx skeleton compared to the same NBD dimer skeleton. Since FeCCCO^CNO^ promotes the formation of the 2+2 xtx NBDddimer,"^a’ it might be expected, on steric grounds, to be less effective here than with NBD. Nickel tetracarbonyl favors the production of 2+2 endo bonded NBD dimers, but since it shows low activity as a promoter for NBD dimerization,it may not be ef­ fective with dihydrobarrelene. As noted earlier, rhodium on carbon will dimerize the less reactive exo cyclopropano-norbornenes, LVII and LVIII, but only under much harsher conditions than were used in the experiments run with dihydrobarrelene. As a result of the photoelectron spectral data given above for the various olefins the following sequence can be given. 62 Degree of Perturbation on the Olefinic Bond The influence of the activation of olefins by neighboring moieties in metal promoted cyclodimerizations might best be discussed in terms of the "fruitful" versus the "unfruitful" complex. It is Icnown that both NBD and norbomene are capable of bonding to metals on the exo side of the olefin. In cases where this behavior has been observed (Ag olefin, Cu olefin, TKOj-HjOl-InCCO^Colefin)), the complexing olefins (norbomene and NBD included) act principally as electron donors, with77-back dona­ tion being minimal.^^ The double bond in these cases is weakened only slightly as shown by the small shift in vc_c in the infrared spectra. Complexes of this sort do not generally lead to dimeric product (un­ fruitful complexation). In order to have an olefin complex lead to dimeric product, it is suggested that a significant weakening of the olefin bond must occur. Note: Photolysis of the AgNO^ complex of norbomene and NBD does not produce d i m e r s . T h e 77(C1-II'^ )IIn(CO)-NBD complex is formed by photoly­ sis of 77(C HcOMn(CO)^NBD with no dimer being formed.5° The photolysis of the Cul^complexes of norbomene, producing norbomene dimer, and NBD, producing quadricyclane,59 may well follow a path very much different from that operating with complexes which are known to promote NBD dimerization. The most probable way this could be achieved would be by 77-back donation of electron density into the anti-bonding orbital of the olefin. This can occur only if an approximate energy and symmetry match exists between filled metal d orbitals and olefin anti-bonding orbitals. Based on the photoelectron spectra and molecular orbital assignments for NBD1 the following correlation diagram can be drawn. 63 - 8- eV -9 - 10— n 8.69 eV SA S - - , - - ' " - 8.97 - -- 9.55 eVSS Figure 29 Correlation Diagram for Norbornene-Norbornadiene Thus the interaction of two "norbornene-type" olefins results in a raising in energy of the highest occupied 77 orbital, providing activa­ tion of the olefin. But, the important effect here is twofold and best illustrated in the diagram given by Hoffmann.^0 64 without after interaction interaction (norbornene-type orbitals) Figure JO Norbomadiene Orbital Interaction Diagram after Hoffmann60 Not only is the highest occupied 77 orbital raised, but the lowest anti­ bonding IT orbital is lowered by the interaction process. Lowering the energy of the anti-bonding TT orbital would most likely be conducive to a better energy match between it and a filled metal d orbital. The approximate atomic orbitals for nickel are given by T. L. Brown^ as Jd1-IO.5 eV; 4s,-9.8 eV; and 4p, -9.0 eV. The assignment ofn * orbitals based on ultra­ violet spectra of NBD has evoked some controversy in light of the apparent partial mixing of a and TT orbitals 62 in NBD. However, based on the analysis of VanCatledge and the ultraviolet spectra of NBD and norbornene,^ it appears that norborneneTT * orbitals are considerably more energetic than are the TT * orbitals associated with NBD. Backbonding should, therefore, be considerably more significant with NBD than with norbornene, and "fruitful" complexation, i.e. complexation leading to weakening of the olefinic bond, ought to be favored with NBD, Thus, a relationship between perturbations on an olefin by another moiety in the molecule and successful metal-mediated cyclodimerization of the perturbed olefins can be given. Perturbations on olefins which cause an increase in the energy of the highest occupied 77 orbitals at the expense of the energy of the lowest unoccupied 77* or­ bitals will favor transition metal cycloaddition reactions of the perturbed olefins. This proposal can be valid only if approximate metal-olefin energy and symmetry matches exist, and no dramatic new steric demands are intro­ duced by the perturbing moiety. The sequence given for the degree of perturbation of the olefinic bond, page 6l, provides a series of predicted reactivities in transition metal promoted cyclodimerization reactions. Thus, it is predicted that if dihydrobarreIene were subjected to the harsh conditions successful for dimerization of exo cyclopropano-norbornenes LVII and LVIII, it too would dimerize. It is further predicted that the molecule [2,2,2]bicyclo- octatriene ( b a r r e l e n e ) in which olefinic interaction raises the ■ highest occupied 77 orbital to -8.23 eV (cf. dihydrobarrelene highest -8.8? eV and NBD 77 , -8.69 eV), ought to dimerize. The dimer willhighest 65 66 probably not dimerize to tetrainer unless the fused cyclobutane activates the remaining two olefins considerably above those in dihydrobarrelene and 54this is not expected. It may also be noted the steric requirements imposed by dihydrobarrelene between the bridge hydrogens and the incipient Figure 31 Oligomerization of Barrelene cyclobutane (vide ante) are absent with barrelene, but are imposed by the dimer, a fact which also disfavors tetramer formation. Kinetic Considerations It has been implicitly and explicitly noted throughout the preceding discussion that electronic considerations are very important factors gov­ erning the behavior of substrate olefins. As noted at the onset of this discussion, this is also valid for coordinated ligands which are not directly involved in the cyclodimerization reaction. Thus, one is faced with a number of considerations pertinent to these indirectly involved ligands. Which ligands are lost and to what extent...temporarily or permanently? What relation do any remaining ligands have to the 67 electronic or stereochemical demands of the reaction? What role could ligands play in directing ir or cr bonding between the metal and substrate olefin molecules? It was with these questions in mind that an effort was initiated to probe some of these ligand related factors. A number of reactions were run with NBD, and NiCCO^CPS^,)^ as promoter. BisCtripheny!phosphine) dicarbony!nickel was chosen for a number of reasons. As mentioned before, it is an air stable solid thus allowing for ease of experimental manipulation.. Thermally, it promotes the formation of two major dimers, 2+2 xtx and 2+2 xtn, and one minor dimer, 2+2 ntn. Its reaction with 7-methyINBD was investigated. This work provided some insight into the course of the reaction as well as suggesting areas which required further investigationo With reference to coordination, NiCGCO^Cl^^ is either a disub- stituted nickel tetracarbonyl derivative or a disubstituted tetrakistri- pheny!phosphine nickel derivative„ The thermal reaction of nickel tetra- 17acarbonyl has been investigated by Voecks , and it has been shown to lead to 2+2 xtn (major) and 2+2 ntn (minor) dimers. The hypothesis could be made that if a dicarbonyl nickel specie such as NiCCCO^CNBD)^ was responsible for the formation of xtn (and ntn) dimer in the Ni(CO)^ case, then the same sort of specie could arise from NiCCCO^CPi^)^ by the loss of two ligands and the addition of two NBD molecules. To carry the hypothesis further, it could be suggested that a NiCPgf^^CNBD)^ specie formed by the loss of two carbon monoxides is responsible for 68 the formation of the xtx dimer and it would be predicted then, that Ni(P0^)^ would lead to only xtx dimer. Thus, two different active com­ plexes could be responsible for the products obtained from Ni(CO)„(P0_)_, d. 5 (L Exploratory experiments served to demonstrate some of the essential characteristics of the reaction of NBD with N i ( C O ) a) The reaction is very temperature sensitive; prolonged heat­ ing at 75°C produces virtually no dimer, while refluxing at an initial temperature of 85°C allows a facile reaction. , This is generally observed with metal carbonyls and has been interpreted as the minimum temperature for ligand ex­ pulsion. b) Using neat NBD, the reaction temperature increases from 85°G to ca. IOO0C during the reaction. The rate of reflux is decreased considerably as the pot temperature increases and the viscosity of the solution appears to increase. This behavior is attributed to the formation of dimer which be­ comes an increasingly important component in the refluxing medium. c) The reaction is very sensitive to refluxing. At a temper­ ature just below reflux, ca. 8l°G, very little dimerization occurs. However, if nitrogen is bubbled through the solu­ tion, reaction occurs quite readily. This suggests that "liberated" CO probably inhibits reaction by recombination. d) The substitution, of carbon monoxide for nitrogen as a sweeping gas inhibits the reaction virtually completely, e) Monitoring the formation of dimers as the reaction progresses shows that after an induction period, two major 2+2 dimers (xtx and xtn) are formed at a constant rate. A third 2+2 dimer, ntn, is also produced, but only in small amounts. f) The xtn:xtx ratio varies with time. Very early in the re­ action, during the induction period, this ratio is greater . than one. This value decreases to about .25 to .50, depend­ ing on the temperature at which the reaction is run, and remains constant after the induction period. One explanation of this result is the process by which xtn is produced is * perhaps less energetic than that for the production of xtx, As a result of these preliminary runs, a procedure was established for examining some of the kinetic parameters of the thermally induced cycloaddition reaction. The desired amount of N i C C O ^ C P ^ ^ dissolved in warm NBD was injected into a flask containing NBD which was already in equilibrium with a constant temperature bath. Prior to addition of the Ni(CO)2(P0^)2 solution, a stream of nitrogen was established in the reac­ tion flask by use of a glass tube fitted with a frit. The flow of * * Such an explanation does not distinguish between factors which influ­ ence the duration of the induction period and factors which govern dimer formation when the rate of dimer production becomes constant (cf. page 80 ). 69 70 nitrogen was monitored with a soap bubble flowmeter. The flask was fit­ ted with a long glass packed condenser to prevent loss of substrate by evaporation. Samples were withdrawn from the reaction and analysed by gas chrom- * atography using an internal standard. A plot of the concentration of dimer versus time was made which provided a graph with the characteristics given below, while data from the various experiments is given in Table I. dimer concentration linear portion induction period time Figure 32 Generalized Concentration versus Time Plot * For the derivation of the equation relating the standard and the dimer gas chromatographic data, see Appendix II. 71 Table I , Tabulation of Kinetic Data . Temperature 0C Promoter (initial concentration) Least squares slope of linear portion of graph • XlO5 Abscissa intercept xlO~5 xtx xtn xtx xtn 74.2« N i(C0)2(Pp )g .0112 M .5857 .2484 14.746 9.724 81.2« Ni(CO)2(Fgf3)2 .0112 M 12.165 2.896 5.266 5.354 85.1« Ni(CO)2(Pp3)2 .0112 M 16.827 4.969 2.634 2.135 81.2« Ni(CO)2 (P p ) 2 .0112 M + added Pp3 .00128 M 5.378 1.439 4.445 4.051 81.2« Ca. .0112 M 1.955 .5696 -6.926 - 2.953 a calculated from equation of least squares line It is estimated this data is accurate to about fifteen to twenty- five percent, depending on the particular data0 Thus, at best, the re­ sults should be treated as trends, not as absolute values. From the data for Ni(GO)^(P^^)g at the three temperatures, one can obtain an estimate 72 of the energy of activation (fr H*) for the formation of the 2+2 xtx and xtn dimers. The shape of the plots is given below. Figure 33 Plot of In Slope vs l/T A least squares analysis of the data yields values of 79*96 kcal/mole and A H*^tn~ 7^*24 kcal/mole. It may be noted that during the latter part of the reaction run at 85°C , it appeared that nickel was being plated out. This may imply the actual catalytic nickel specie may have been thermally decomposing, which would account for the non­ linearity of the DRY plot. Two important points concerning these reactions should be noted: a) The reaction has an unusually high energy of activation (vide infra). b) The implication that AH*^^ is greater than AH*x n^ is prob­ ably real. For example, if the data from the reaction at 83.2°G is neglected, the two remaining data points yield AH^xtxs 102.3 kcal/mole and AH*^^= 82.6 kcal/mole. 75 One can also examine the ratio of the amount of xtn to xtx produced versus temperature« Comparison can be made for reactions run at 8l.2°0 and 74e2°C, as well as for other preliminary experiments run under the same conditions ex­ cept at lower temperatures» This comparison shows that for mation of xtn increases in importance at lower temperatures implying its formation is a less energetic process. As noted, the experimentally determined DRO is unusually high. -How­ ever, extreme sensitivity to temperature is not unprecedented for re­ actions promoted by carbonyl containing nickel species. Meriwether and 65 co-workers studied the reaction of Mi(CO)^(P0^)^ with acetylenes lead­ ing to linear and aromatic oligomers. They reported: "These reactions are very sensitive to temperature and degree of agitation in the solution. No poly­ merization was observed in unstirred solutions of 1-heptyne and catalyst in cyclohexane or acetonitrile between 25 and 75°C. However, if the solution was brought to the boiling point (about 8l°) or if the solution at 75° was agitated, complete reaction oc­ curred after a short induction period." They also found that sweeping the solution with nitrogen or with carbon dioxide greatly facilitated reaction, whereas sweeping the solution with carbon monoxide had a strong inhibitory effect. They noted, "These results suggest that the reversible dissociation of carbon monoxide from Ni(CCO^CPS^),, is an essential step in the formation of the ’active' catalyst. Agitation simply promotes the release of carbon monoxide gas from the carbon monoxide supersaturated solutions." Their proposed 74 mechanism includes an initial equilibrium step whereby carbon monoxide is reversibly displaced by the acetylene. The acetylene coordinated complex is proposed as the active catalytic specie and thus the large inhibitory effect of carbon monoxide on the rate of reaction can be readily appreci­ ated. Ellgen also found the expulsion of carbon monoxide from a nickel carbonyl specie to be very temperature s e n s i t i v e T h e reaction in­ volved the substitution of carbon monoxide by alkynes in di-ju -carbonyl- bis(cyclopentadienyl)dinickel, (Ni^CCO^CC^Hr^) • + SCO Figure j4 Reaction of Alkynes with Ni2(CO)2(C^H^)2 Ellgen implies the loss of a carbon monoxide may be the first step in the reaction. It was further noted that the substitution reaction takes place at a convenient rate only at temperatures which are sufficiently high that the decomposition of Ni2(C0)2(C^ II^ )2 is difficult to avoid. A compromise temperature of SO0C was chosen. As will be seen later, these examples are used as models only to the extent they apparently provide instances where: 75 a) Difficult initial cleavage of the metal-carbonyl bonds occurs, and/or b) Great reluctance for the "liberated" carbon monoxide to be completely lost from the system is observed (recombination is very likely), Other information concerning the nature of the dimerization is provi­ ded by examining various slope ratios based on the data in Table I* 8l.2°C no added P/_ 8lo2°C' added P0 'slope xtn slope xtx slope xtn slope xtx ,238 »268 It is suggested either one of two explana­ tions may account for the observed in­ crease in xtn/xtx ratio» Either a) relatively more xtn is produced with added or b) relatively less xtx is produced with added However, for the same two .slope xtx (added P/-J , , slope xtx J ~ ’ reactions as given above slope xtn (added P0 )^ = .498 slope xtn ^ These two ratios provide evidence that the production of xtx is impeded more than is the production of xtn by the addition of P0^. An examination of the data (Table l) for Ni(P/^)^ and comparison with the data for IhUCCO^P/^)^ is also informative. Cf. slope xtn slope xtx = .292 Ni(CO)2(PZ5)2 slope xtn slope xtx = .238 Thus it would appear that carbon monoxide ligands are not necessary for the production of either 2+2 xtx or xtn dimers« It is also informative to note the dependence of the rate on the concentration of P0^o It is not unreasonable to expect that the promoter Ni(CO)g(P^^)g loses either one or more ligands to form a true catalytic specie which is responsible for the dimerization reaction. Thus a sequence similar to the following may pertain, x+y = 2 I) Ni(CO)3( ^ ) 2 __^L-^Ni(P0^)o JC0)o „ + xPpL, + yCO 76 -I 3 2-x 2-y ' k, 2) Ni(BP2>3_x(C0)3_y + 2NBD S=-S -2 Ni(B0_)_ _ (C 0 ), )2 (x+y=2) (x+y=2) The assumption of the simultaneous loss of two ligands in step I (page 80), followed by the addition of two NBD molecules in step 2, serves as a simplification for developing the rate equation. Unfortunately, an understanding of the degree of complexity of the equilibria leading to productive complexes Ni(CO)2 (P^13:BME LNKZ:2, x+y=2, is thereby made less obvious. The important point is that the number of productive (dimer producing) pathways relative to the number of probable reactions occurring is small. The induction period of the reaction involves-the establishment of equilibrium among all the reaction steps, and any process which would lead to the relatively easy production of coordinatively unsaturated species ought to shorten this period (vide post). The linear portion of the dimer concentration versus time plots corresponds to a constant rate of production of dimer once.the equilibria have been 82 established, d |imer]/dt = ky §i(C0)2_y(P^^)2 ^(NBD)^]= a constant,. Since a number of equilibria do exist involving the complex Ni(CO), (PGL), (NBD)2 , x+y=2, which lead to dimer, the slopes of the [d] vs t plots contain a large number of concentration and rate constant terms, In turn, the use of the slopes of [d]vs t plots for the determination of AH^ values necessarily implies that a AH^ value so obtained represents the total AH+ for a rather large number of reactions, although it is referred to as the enthalpy of activation for the dimerization of NBD to give a specific dimer. Thus, values of = 70,24 kcal/mole and A = 79,96 kcal/mole should each be treated as composite AH^ values for the various equilibria•involving intermediates which lead to each of the two dimers. The observed influence of the ligands (P0^ and CO) on reaction rates may now be correlated to the mechanistic picture of the dimeriza­ tion reaction. The production of xtx and xtn are both inhibited by the presence of which vies for vacant coordination sites on nickel, As noted in the introduction as well as earlier in the discussion, the following scheme can be proposed. To form the 2+2 xtn dimer, one starts with a bidentate endo,endo NBD nickel specie which then goes to a mono- dentate endo NBD nickel specie and finally to an exo NBD, endo NBD . nickel specie, leading to xtn dimer, • • [' d ] vs t = dimer concentration versus time 83 Figure 35 Entropy Effects in Complexes Leading to the 2+2 xtn NBD Dimer Further, in the case of the 2+2 xtx dimer, the following intermediate (LX) has been proposed. 2+2 xtx Figure 36 Entropy Effects in Complexes Leading to the 2+2 xtx NBD Dimer 84 Relative to entropy considerations, it is expected that if the mono- endo bonded NBD nickel specie temporarily complexes with a molecule, the nickel atom will be likely to recomplex with the endo side of the same NBD, since it is still bonded to the remaining endo side (Figure 35) However, if an exo,exo-bis(NBD)nickel specie temporarily forms a bonded complex, it is much less likely to reform an intermediate capable of leading to dimer (Figure 36). Thus, it would be expected that in­ creases in the [P^] would result in more significant changes in the rate * of xtx production than in the rate of xtn production. The possibility of two mechanisms for the production of the 2+2 xtn dimer should also be considered. One mechanism would involve the for­ mation of a dicarbonyl endo,endo-NBD nickel catalyst which could be an- 17aalogous to the proposed nickel tetracarbonyl derived specie. Ni(CO)4 NBD 2+2 > xtn Figure 37 Common Pathway to the 2+2 xtn NBD Dimer * * While it is possible a P0-, could substitute for a NBD (either exo or endo bonded) in the endo-NBD, exo-NBD nickel complex (LIX), it is more likely substitution of the exo-NBD by the remaining uncoordinated endo side of the mono-endo coordinated NBD will occur. In the absence of carbonyl ligands (and to a lesser extent even in their presence), a less efficient path to dimer might be operative involving endo,endo-NBD nickel catalysts. 85 NBD 2+2 ------> xtn Figure 38 Non-carbonyl Containing Complexes Leading to the 2+2 xtn NBD Dimer It is not completely clear why a dicarbonyl-endo,endo-NBD-nickel specie should be particularly efficient for the production of the 2+2 xtn dimer. However, a reasonable explanation involves the nature of the interaction between the endo side of NBD and the nickel dicarbonyl moiety. 70 It has been noted that valence bond theory predicts that in tetra­ hedral nickel(O) complexes only two orbitals will be available for 77- back-bonding. Thus, in nickel tetracarbonyl, two of the four carbonyls are readily removed, while strong 77-back-bonding to the remaining two carbonyls prevents their easy displacement. Accordingly, a relatively 86 stable but labile complex could be envisioned for a NBD-dicarbonylnickel system. The 77-back-bonding capability of nickel could be readily satis­ fied with two ' carbonyls, leaving two sites available for ligands having 71principally donor properties. Schrauzer has advocated, based on spec­ tral data for endo, endo-NBD duroquinone nickel, that the endo side of WBD acts principally as an electron donor. This is supported by the lability of NBD duroquinone nickel towards trimethyl phosphite 72 Based on photoelectron spectra it is clear the double bonds in NBD interact in a through-space manner. Thus, in a, qualitative way, one could develop the following picture for the orbitals in NBD. In addition to the "normal" interaction between the four p atomic orbitals - produc­ ing two 77 bonding and two 77 antibonding molecular orbitals - account should be made for interaction between the endo lobes of transannular p orbitals. This interaction may be thought of as a form of partial sigma bond formation. Thus, the situation can be pictured as given be­ low, where 77 and 77* notation refers to molecular orbitals in which p-77 bonding is stressed, while yS and a* notation refers to molecular orbitals in which p-°' bonding is stressed. 87 TT I Symmetry designations are relative to reflections through the m^ and m^ planes Figure 39 Qualitative Picture of p Orbital Interactions in NBD 88 In light of the much greater strain energy for quadricyclane (78<>7 kcal/mole) versus two norbornene molecules (2x17«6 = 35«2 kcal/mole) (NBD strain energy = 25«6 kcal/mole)^^, it is possible that energy dif­ ferences will preclude extensive mixing between the 17 and 0 states. Ufb bb The and 71 states, however, probably do mix, the degree once again depending on the relative energy levels. It is suggested that one result of such mixing would be to increase the electron density on the endo side of NBD6 The lowest antibonding orbital, would also be altered by mixing. Thus, to the extent mixing obtains, the region of maximum antibonding orbital probability would lie on the exo side of NBD, at the expense of a decrease on the endo side. This picture, if correct, would explain the preference of a di­ carbonyl nickel moiety to be bonded to the endo side since the two car­ bonyls.would act as excellent 77-back-bond acceptors, while the endo side of NBD would serve principally as a donor. The proposed model could also be invoked to explain why a NiCPjZf^ )^ could preferentially bond to the exo side of two NBD molecules and thus lead to the 2+2 xtx dimer. Triphenylphosphine is generally conceded to be a rather poor 77-back-bond 7k •acceptor, while nickel has been suggested to be perhaps the most basic 75metal for 77 -back donation to olefin 77 * orbitals. Thus, in the absence of carbonyl, ligands to act as 77-back donation acceptors, olefins (e.g. the exo sides of two NBD molecules) could serve as acceptor ligands. Note that in the above discussion any Ni(L)^ entity in which L is 89 - principally a cr donor (poor 77-back-bond acceptor), e.g. a dimer molecule, would be predicted to lead to the 2+2 xtx dimer. One final point should be made concerning the data from the kinetics experiments: duration of the induction period. From the abscissa inter­ cepts in Table I, page 71» it can be seen that the trend is as expected; the higher the temperature the shorter the induction period, correspond­ ing to an increased production of unsaturated species (cf. page 80 ff.). In general, the induction period for the 2+2 xtn dimer is shorter than that for the 2+2 xtx dimer. This is consistent with a less energetic process for xtn than for xtx formation, H-l* , K effecting the establishment of the equilibrium among the various ligands and complexes mirror the factors effecting the maintainance of that same equilibrium. Since it is known that tetrakis(triphenylphosphine)nickel dissoci­ ates in solution at room temperature to tris(triphenylphdsphine)nickel 74and tripheny!phosphine, a coordinativeIy unsaturated specie can be readily formed and thus, it is not surprising to see the induction period virtually disappear. Further, the absence of the strongly bound carbon monoxide ligand in the promoter, cf. NiCCCO^CPj^)^ , should also contribute to a shortening of the time required to form an intermediate capable of leading to dimer. v. Conclusions At this point it is felt a review of the proposed mechanism for the P , , provided the factors 90 dimerization of NBD by nickel promoters is appropriate. This review will be presented in light of the combined results from both the structural modifications and kinetic data reported in this portion of the thesis. In conjunction with these results the work of other investigators will be included (or referred to) as support for the proposed theories» The object of this section will therefore be to describe the author’s conception of the mechanistic picture« First, the formation of both the 2+2 xtx and 2+2 xtn dimers from NBD and NiCCCO^CP^)^ involves the coordination of two NBD molecules in a stereo-specific mannero Second, these coordinated NBD molecules join to form dimer„ The proposed mechanistic routes for the formation of each of the two 2+2 dimers will now be presented» This will be followed by some comments concerning the production of 2+2+2 dimers. 2+2 xtx: Two molecules of NBD are coordinated to nickel on their exo sides« This exo,exo-bis(NBD)nickel di-ligand complex would be favored by ligands which were not good Tl -back-bonders, e.g. P0^, dimers^ and 1,5-cyclooctadiene However, the presence of an excess of strong ySdonors will re­ sult in the effective inhibition of xtx formation, since once an exo,exo-bis(NBD)nickel-di-ligand specie is converted to a mono-exo-NBD tri-ligand moiety (e.g. LX) by replacement of one NBD by a ligand, there are no favorable entropy considerations 91 present to aid in the reformation of the exo,exo-bis(IfBD)nickeI di-ligand specie (cf. Figure 56). From the data presented in this thesis it is not possible to distinguish between a concerted or a non-concerted path for the production of the 2+2 xtx dimer from an exo,exo-bis(NBD)nickel di-ligand specie. However, it has been suggested the presence 21 22of good 0 donors ought to favor r to o' transformations. ’ But as noted above, the presence of excess good 0 donors al 0 ought to inhibit the formation of the exo,exo-bis(NBD)nickel di-ligand precursor to the di-o-bonded complex. Thus, one is faced with a dilemma and both factors must be considered in determining the likelihood of any proposed mechanism. 2+2 xtn: Initially a complex involving an endo,endo-NBD di-ligand com­ plex must be formed. This is favored by ligands capable of functioning as strong 77- back-bond acceptors such as carbonyls or activated olefins which could satisfy the propensity of nickel for 77-back-bonding. The endo side of NBD is proposed to be principally a a donor L: L 92 and a poor ^-back-bond acceptor. The presence of added strong than exo,exo-bis(IfBD)nickel di-ligand complexes because of the favorable entropy factor to reform the complex (cf. Figure 35)» Once the endo,endo-NBD-nickel di-ligand complex is formed it may lead by the dissociation of one endo bond and recomplexation with the exo side of a second NBD molecule to the immediate precursor to the 2+2 xtn dimer - an exo-NBD, endo-NBD-nickel di-ligand complex (LIX). Proposed Intermediates in the Formation of the 2+2 xtn NBD Dimer Based on the data from the dimerization of 7-methyINBD, it is proposed that to account for the formation of the statistical distribution of methyl groups in the endo-bonded fragment - in both the NBD 2+2 xtn and ntn dimers - a concerted path is most probable. Thus, it is proposed that the xtn dimer and the ntn dimer (by analogy) are produced by the concerted pathway o donors, e.g. ought to affect a complex of this type less Figure 40 93 particularly in cases where carbonyl ligands (Ni(CO),^ and .NiCCO)^]?^)^) are available. In the case of Ni(P07)^ , the formation of the 2+2 xtn dimer is accounted for by suggesting a specie such as complex LIX,, where L is an exo-bonded NBD *molecule, or less likely, F(3 , dimers, or similar molecules. It is suggested in these cases, the formation of 2+2 xtn dimer via complex LIX is in strong competition with the loss of the endo bonded NBD molecule. For tetrahedral nickel complexes, the presence of any less than two strong 77-back donation acceptors 78will promote the loss of cr donors. In the case of the Ni(P^)^ promoted dimerization, the best F -back-bond acceptor ligands available are the exo sides on NBD molecules. Since these exo sides are only moderately good IT - back-bond acceptors, the loss of ligands including the endo bonded NBD becomes more likely. 2+2+2: , -It has been proposed in the stereochemical modification portion of this discussion that the formation of 2+2+2 7-methylNBD dimers (using NiCCO)^]?!^^) follows a non-concerted pathway. It is.further proposed that if, for stereochemical • (or possibly * * As was noted earlier, Ni(Il) has been electrochemically reduced in the presence of NBD to yield the 2+2 xtn and 2+2 xtx dimers. These reac­ tions were run in pyridine and acetonitrile, solvents which may serve as ■ donor ligands. 94 electronic) reasons, the complex LXII, Figure 42, cannot readily result in 2+2 xtn dimer, it is possible a carbon monoxide may be lost to yield complex LXIII which would yield the five co­ ordinate di-o-bonded complex ^LXVII„ This would reductively eliminate the dimer» Other routes to complex LXIII could be suggested and may be even more probable<, Initial complexation of NBD on the exo side could give complex LXIV. The loss of carbon monoxide, prompted by the acceptor properties of the exo side of NBD, followed by complexation of a NBD molecule, this time on the endo side, would give LXVIo Loss of PJ2L from LXVL would yield j LXIIIo The complex LXIV could lose instead, giving LXV. This would be promoted by steric considerations of the more hindered y-methyINBD molecule, The coordinatively unsaturated complex LXV could bind with the endo side of a second 7-methyl- NBD molecule to give LXII and as noted, loss of a carbonyl from LXII would yield IXIIIo 27Note that whereas Katz proposed that a rhodium complex (LXVIII) is responsible for the 2+2+2 exo-cis-endo NBD dimer, it is proposed here, based on stereochemical considerations, that a nickel complex (LXIX) is responsible in the case of 7-methyINBD and Ni(CO)2(P^5)2 Proposed Mechanistic Routes to the 2+2+2 xcn V-MethylITBQ Dimer 'if LXIIICO CO 7-HeNBD LXII (CO or P0,) -Ni(CO)(Pp) 7-MeNBD LXIV LXVI Figure 4l 96 L LXVIII Figure 42 Alternative Pathways for the Non-concerted Production of the 2+2 xtn Dimer Skeleton It is also worth-while to note that in the case of nickel tetra- carbonyl, the two 2+2+2 dimers (XV and XVI) are formed, and their formation might well be explained by a concerted mech­ anism noting the presence of carbonyl ligands does not favor an oxidative addition (non-concerted) mechanism. XV XVI 97 A concerted 'pathway for the dimerization promoted by Ni(CO)^ would be favored from a stereochemical point of view too, since carbon monoxide is sterically quite small. Note that it would be predicted the non-concerted (oxidative addition) mechanism would be favored by the presence of PjZf (vide supra). The work reported and discussed in this portion of the thesis has been concerned with the cycloaddition reaction of NBD promoted by NiCOO^CPjZf^)^ and to a lesser extent Ni(CO)^. The metal is the sane for both promotors and the number of ligand types initially in solution are reasonable small (PjZf^, CO and NBD). However, as has been noted, the number of factors to be discussed is already quite large. It can well be imagined that the number of factors which must be considered is very large when: the substrate is changed from NBD; the promoter metal is changed from nickel; the formal oxidation state of the metal is no longer zero and the number of ligand types is increased to include trihalophos- phines, phosphites, arsines, stibines, nitrosyls and halide ions. The problem can quickly become virtually insolvable. Thus, despite the limited area of examination which is.reported here, it is felt the ev­ idence and proposals presented should aid in suggesting experiments for more detailed studies of metal promoted cycloadditions as a whole. SUMMARY The work reported in this thesis is the result of a bifunctional approach to the problem of metal mediated cycloaddition reactions of olefins. The first aspect of this approach was to explore the scope of the reaction and the second was to come to a better understanding of the mechanism(s) operating in such reactions. Norbornadiene (NBD) presents some particular advantages from both aspects. Thus this molecule and its derivatives were used as the principal substrates in the work presented here. The first general area of study was structural modifications. This area was subdivided into two sections. In,the first, emphasis was placed on substrate stereochemical considerations, while the second section stressed substrate electronic considerations. The second general area involved a kinetic study, with emphasis being placed on consideration of metal-ligand related phenomena. A number of substituted NBD derivatives were tried .as substrates ■ using bis(triphenylphosphine)dicarbonylnickel as a promoter (promoter being used to describe the initial metal complex which leads to the ac­ tive catalytic specie during the reaction). The compound 7-methylNBD was found to dimerize using NitCO)^?^)^ and activation by light or.heat. The thermally obtained product mixture was subjected to an extensive separation scheme using silver nitrate and silver perchlorate impregnated silica gel column chromatography in conjunction with preparative gas 99 chromatography. It was shown there are seven products, six of which were identified. The photochemically obtained product consisted of four major dimers. These were related to thermally produced products by using Ag+ ion impregnated silica gel thin layer plates and gas chromatography. The product structures are given below. Relative amounts are only internally consistent since amounts of thermally derived products were not compared to amounts of photochemically derived products. 2+2 xtn both isomers in equal amounts 2+2 xtx 2+2+2 xcn 2+2+2 ncn 2+2 ntn // AT hv 40 4 10 7.5 7 I 5 2 Me relative yields 100 A study of substrate electronic considerations was also made. It was assumed that substrate olefin-metal interactions constitute the major substrate electronic factors in metal promoted cycloaddition re­ actions of olefins, Thus, three olefins having "perturbed" olefin bonds (relative to NBD) were chosen. Norbornene - only one double bond The 2+2 xtx NBD dimer - one double bond interacting with a trans- annular exo bonded cyclobutane. (2,2,2| bicycloocta-2,5-diene (dihydrobarrelene) - a methylene homo­ log of [ 2,2,l] bicyclohepta-2,5-diene (NBD). • However, norbornene, the 2+2 NBD xtx dimer, and [ 2,2,2] bicycloocta-2,5~ diene were all found to be inactive toward dimerization. An explanation was offered, basedon reported and projected energy levels, for the lack of reactivity in these systems as compared to NBD. The fundamental tenet of the proposal is that both a relatively high-energy filled 77 olefin orbital and a relatively low-energy unfilled77 * orbital are necessary in the substrate. In NBD this situation is provided by the interaction.of the two olefin bonds,. It is proposed other similar mole­ cules where such interaction can occur will also be active towards dimer­ ization. Thus, based on photoelectron spectra, it is suggested that [2,2,2] bicycloocta-2,5,7-triene (barrelene) will dimerize, but the dimer being similar to dihydrobarrelene, may be inactive. 101 The kinetics study, as noted, was directed towards an understanding of metal-ligand interactions. The thermal reaction of NBD with Ni(CO) (P^)^ and Ni(P0^)^ was studied and the following information gathered. a) The reaction proceeds in two phases - an induction period, fol­ lowed by the principal reaction where dimers are produced at a constant rate. b) The reaction is extremely dependent upon three factors. i) Temperature ii) The rate at which carbon monoxide can escape from the re­ action iii) The concentration of tripheny!phosphine in solution c) From the data it was found: i) A values were obtained for the 2+2 xtx and xtn dimers. A = 79*96 kcal/mole A = 70.24 kcal/mole ii) Sweeping the reaction solution with N^ greatly facilitated reaction, while sweeping the reaction with CO caused nearly total inhibition. iii) strongly inhibits the reaction, the production of xtx being inhibited somewhat more than the production of xtn. 102 d) Using Ni(P0^)^ instead of NiCCO^CPf^^ as promoter: i.) Virtually eliminates the induction period. ii) Decreases the rates of xtx production (1/6) and xtn pro­ duction (1/5) for reactions run under the same conditons. Several inferences were made based on the kinetics data. These were coordinated with the results of the structural modification data as well as with the work of Voecks"*"^ 8-’^ and other investigators on similar sys­ tems . It is proposed the following picture for the Ni(O) promoted cyclo­ addition reaction of NBD emerges. A) Ligands (both CO and P0^) are lost to produce coordinatively un­ saturated nickel species, some of which may be described as "active catalysts". These ligands, as well as NBD and NBD dimers are in constant competition for sites on nickel, thus reducing the concentration of "active catalysts" to very low values. The establishment of an equilibrium between the promoter, the .ligands and the various modified nickel species, including "active catalysts" constitutes a mechanistic description of the induction period observed in these metal promoted cyclodimeri­ zations . B) Once this equilibrium is established, the rate of dimer produc­ tion is constant. Because of the very low concentrations of "active catalysts" caused by the efficient competition of inhib­ itory ligands (e.g. CO, P0^, and NBD dimers) for coordination sites on nickel, the enthalpy of activation for dimer formation 10) may appear to be very large. What, in fact, is being measured is the composite of enthalpy values for the many equilibrium processes involving the "active catalysts". C) NBD molecules stereospecifically disposed on nickel act as pre­ cursors to dimer. Which specific precursors are formed in any particular promoter substrate system depends on the nature of the ligands initially on the metal as well as any ligand in solution. This situation also holds for the fate of these precursor mole­ cules with respect to the routes they follow in forming dimer (see Figure 43). However, some generalizations can be made based on the available data and the proposal that the endo side of NBD is principally a a donor, while the exo side of NBD is a good 77-back-bond acceptor, i) Formation of complex LXXX is favored by and being good cr donors, e.g. 'P0^« ii) A non-concerted path from complex LXXX to the 2+2 xtx dimer is favored by good a donors in solution, but these .ligands would also promote the deactivation of complex LXXX by vying with NBD for coordination sites on nickel, iii) The formation of complex LXXXI would be favored by L^ and L^ being strong 77-back-bond acceptors, e.g. carbonyls. iv) The formation of 2+2 xtn and 2+2 ntn dimer proceeds via a concerted pathway // 2+2 xtx LXXX 2+2 xtn 2+2+2 2+2+2 LXXXII LXXXI 2+2+2 LXXXIII Figure kj> 2+2 ntn Review of Proposed Routes to 2+2 and 2+2+2 NBD Dimers 105 v) The formation of 2+2+2 dimer can occur either by a non- concerted route (2+2+2 xcn and 2+2+2 ncn) or by a con­ certed route (2+2+2 xcn and 2+2+2 xtn)„ The non-concerted path is favored by steric inhibition to 2+2 xtn formation and the presence of P0^. The concerted path is favored by the absence of a donors and the presence o f TT -back-bond acceptors. It is expected to be operative in the very ener­ getic process of photochemical activation of complex LXXXI (forming complex LXXXII) when L^ and L^ are- CO. ■ D) Trimer formation is expected to be favored when dimers can effec­ tively serve as ligands. i) Then their concentration is relatively large. ii) When there are the minimal number of other effective a donors. For example, the use of I,5-bis(cyclooctadiene)nickel pro- 7.7-motes extensive trimer formation. ' To further explore the above proposals, some experiments can be suggested. 1) To study the dissociation of carbon monoxide from Ni(CO) (P(ZL) , d. P d the experiment can be run in reverse. That is, measure the en­ thalpy change for the reaction : Ni(Pgf5)2(L) + 2C0 N i(PgL)2 (CO)2 + L1 where L is H2C=CH2 or :N=N: . 2) By using the substituted NBD, LXXXIV, it may well be possible to obtain the endo,endo-bis(7,7-polymethyleneNBD)nickel dicarbonyl 106 complex, since with all modes of exo bonding blocked, its rate of dimerization ought to be very slow. Infrared, NMR and UV ^ W nY n = 0 or 2 \ ZWS -3*OS•H y f LXXXIV or Ni(Pgfj)2(CO)2 \ IZM CO CO spectra of such a complex could aid in detecting the unsubsti­ tuted counterpart. 3) By running the reaction of NBD with a specie such as Ni(P0^)2N2 at low temperature under vacuum, one may be able to obtain evi­ dence for NBD-NiCPj^)^ species. 4) To explore attack by nickel on the exo side of NBD, 7-disubsti- 79 tuted methylenephosphinatoNBD could be used as substrates. It has not been possible to exchange norbomene for ethylene in the complex (F0 )0Ni(CH CU ).&0 However, with the dinitrogen complex (P^)2NiN2, NBD exchange may prove possible. 10? 5) Finally, to investigate the possibility of non-concerted inter­ mediates in the formation of 2+2 xtn and 2+2+2 xcn, as well as the possible interconversion of such intermediates, the follow­ ing sequences are suggested. . X = halogen In these cases, Ni(L)if could be Ni(CO)if1 Ni(Pj^)if1 Ni(CO)2(Pgf^ )2 , bis(I,5-cyclooctadiene)nickel or electrochemically obtained low valent Ni species. In light of the success obtained in the use of the dipyridyl ligand for stabilizing low valent bis-c -alkyl nickel complexes, it may prove to be a suitable ligand for X (L)X 3 these reactions EXPERIMENTAL Instruments and Equipment Gas chromatographic analyses were run on a Varian Model 1740 Gas Chromatograph using l/8 inch six or twenty foot columns of 5°/o SE 30 on 100-120 mesh Gas Chrom Z . Preparative gas chromatography for 7-methyl- norbornadiene dimers were run on an Aerograph Model A-700 Autoprep Gas Chromatograph, using a 3/8 inch twenty foot column of 30% SE 30 on 80- 100 mesh Chromosorb W at a column temperature of 225°C and helium flow of 200 ml/min. Nuclear Magnetic Resonance spectra were obtained on a Varian A-60 Proton NMR The light source for photochemical reactions was a Hanovia Type A 450 watt high pressure mercury arc, A pyrex filter was used for the NiCCO^CP/j,^ Promoted reactions, while a uranyl glass filter was used for the reaction of dinitrosyldicarbonyliron and 2,5-[2,2,2]-bicycloocta- diene. Reagents Norbornadiene was purchased from Frinton Laboratories and was purified by distillation under nitrogen on a Todd Model A Precise Frac­ tionation Assembly complete with automatic reflux ratio and fractionation still head. The column was four feet in length and packed with glass he­ lices. The first one-third or more was discarded due to the amount of 109 impurities still present and approximately one-half of the remaining solution was pure by gas chromatographic analysis on a six, foot column of 5% SE 30 on 100-120 mesh Gas Ghrom Z . Seven-t-butoxy-norbornadiene was purchased from Frinton Laboratories and was purified by column chromatography using Alfa Inorganics Alumina CAMAG NEUTRAL $07-0-1 as an absorbant and n-hexane as an elutant« Triphenylphosphine (Alpha Inorganics) was sublimed prior to use. 'Bis(triphenylphosphine)dicarbonylnickeI was prepared by the method 82reported in Organometallic Synthesis edited by Eisch and King. Dicarbonyldinitrosyliron(0) was a gift of G. Voecks. The 2+2 norbornadiene exo-trans-exo dimer was doubly sublimed from a dimer mixture and was free of impurities by gas chromatographic analysis. Dodecane, used as a standard for the gas chromatographic analysis of dimers in kinetics experiments was purchased from the Eastman Kodak Co. It showed one peak under the conditions of analysis: 20 foot column (vide supra), linear temparature program of 6-10°G per minute, starting at 50°C. Purification of di-n-amyl ether used in the preparation of 7-methyl norbornadiene was accomplished by running the ether through an activated alumina column followed by simple distillation under nitrogen. Seven-hydroxy-norbornadiene was prepared by an established liter- 83ature procedure via a benzoyl norbornadiene intermediate. H O Gases Hexafluoro-2-butyne was purchased from Peninsular ChemResearch and was used without further purification for the preparation of 2,3-bis(trifluoromethyl)norbornadiene. Nickel carbonyl (Matheson) was used without purification for the preparation of NiCCO^CPS^)^. Carbon monoxide (Matheson C.P»), used to sweep the NBD-NiCCO^CP^^ reaction, was deoxygenated by the use of BSTS catalyst (BASF Colors and Chemicals, Inc,). Nitrogen (Airco, Dry-High Purity) was used for distillations, re- fluxings, sweeping, and in all other cases was deoxygenated by use of BSTS catalyst. ' Preparations 2,3-bis(trifluoromethyl)norbornadiene. Eight ml (.137 moles) of freshly distilled 1,3-cyclopentadiene was placed at O0C in a bomb re­ actor. The bomb was sealed and cooled to -78°C. Hexafluoro-2-butyne (22.2 g, .137 moles) was introduced via an inlet valve. Gradually, the mixture was allowed to come to room temperature. The product was pur­ ified by distillation at 120 mm, bp 68.5-69»0°C. Its nuclear magnetic resonance as a neat sample, with tetramethyl silane added as a standard, showed: Triplet 2H 6.83 olefin Multiplet 2H 3.85 bridgeheads Multiplet 2H 2.125 bridge Ill Note the bridge and bridgehead protons appear to show extensive splitting 19presumably due to F coupling» gh 7-Methyl-norbornadiene. The preparation of Po Po Story was mod­ ified in the following manner, Methyl iodide (92 g, 065 moles) in 200 ml of di(n-amyl)ether was added dropwise to 158 g («65 moles) of magnesium turnings, maintaining the temperature at 35-^5°C » Thus a di(n-amyl)ether solution of methyl Grignard was prepared. Fifty grams («305 moles) of 7- t-butoxy-norbornadiene was added and the solution heated (refluxing en­ sues) to l40°C for 15 minutes 0 It was then cooled to 120°C and the 7“- me thy1-norbornadiene was removed by vacuum distillation through a 30 cm glass-packed column at 100 mm. The product was collected in a -87°C (ethyl acetate slush) trap. The distillation was continued until the di- (n-amyl)ether began to distill and provided a 25«17 S (77»8% yield) of nearly pure 7-methyl-norbornadiene, The bp is consistent with Story's value and the NME spectrum identical. This method of preparation is not only an improvement over Story's method based on yield, 77*8# vs 57»6#, but also based on convenience - Story's .procedure requires several days while the procedure reported here takes sixteen hours, [2,2,2lcycloocta-2,5-diene. The most facile route to this diene involved a modification of the electrochemical reduction of [2,2,2.]bicyclo 85oct-2-ene-5,6-dicarboxylic acid. The acid was prepared by the base hydrolysis of the corresponding anhydride., followed by careful acidifica- . Care must be exercised in acidification since the diacid readilytion 112 reconverts to the anhydrideThe anhydride was synthesized from eyeIo- hexa-1,3-diene and maleic anhydride•^ The reaction was run as a 10# water-90# pyridine solution with a small amount of tri-ethylamine added to aid in carboxylate formation= Because of the low conductivity of the reaction medium, a considerable amount of heat is evolved. Thus, in 8?light of the extreme volatility of the product, some provision had to be made to effectively disperse this heat from the reaction mixture. Using a thin-walled steel reaction vessel, a mechanical stirrer, a cold bath and two concentric platinum electrodes, the reaction could be con­ veniently carried out on a reasonably large scale with the minimal super­ vision. Work-up consists of adding water to the reaction mixture, ex­ tracting with isopentane and careful distillation to remove the solvent. Thus, one can convert 5-88 g (.03 moles) of [2,2,2]bicyclooct-2-ene-3»6- dicarboxylic acid to 1.11 g-1.2? g of product (35~^0# yield). The yields are compatible with reported results for reactions run on one-sixth the scale. Tetrakistriphenylphosphinenickel(O). All procedures were carried out under a protective atmosphere of nitrogen. A solution of sodium benzo- phenone ketyl was prepared by combining benzophenone (I.83 g, .01 moles) and sodium (.23 g, .01 moles) in 10 ml of anhydrous ether. The ketyl solution was added dropwise to a vigorously stirred solution of bis (pentan-2,4-dione)nickel(II) (1.0 g, .OO389 moles) and tripheny!phosphine (4.2 g, .016 moles) in 20 ml of anhydrous ether until the dark blue color of the ketyl just persisted. The mixture was centrifuged and the 113 red-brown precipitate washed with anhydrous ether, and the mixture centri fuged again= This operation was repeated two more times« The product was finally dried under a slow stream of nitrogen to yield 3»^5 g (8C$) NiCPp^ )^ . Dimerization Reactions These reactions can be put into two general classes, thermal and photochemical. The general procedure for each is as follows. Thermal» The reactions were run with or without solvent under nitrogen at a specific temperature or at reflux. Gas chromatographic an­ alysis of an aliquot sample was carried out using either a six foot or twenty foot 1/8 inch 5$ SE 30 column. Nuclear magnetic resonance and ir spectra were also used to confirm gas chromatographic data. Photochemical. These reactions were normally run in evacuated pyrex tubes. The appropriate, amount of substrate was either added to the tube or transferred into the tube by a trap to trap distillation on a vacuum line. The promoter was either directly added, NKCCO^CP^)^? or transferred into the tube on a vacuum line. The charged tubes were sub­ jected to several freeze-thaw cycles on a vacuum line, then sealed with a torch. The tubes were placed in a merry-go-round for irradiation with a high pressure mercury arc. Samples from the reaction mixtures were an­ alyzed as in the thermal reactions. In Table 2 below, a compilation of several specific experiments is given Table 2 Compilation of Attempted Dimerization Reactions with Norbomene, 2+2 xtx NBD dimer, and [2,2,2]bicycloocta-2,5-diene Substrate Promoter Activation Mode photochemical, hv thermal, (temp.) Activation time Solvent (if any) Method(s) of Analysis GC (gas chromatography) NMR (nulear magnetic resonance spectra) Result of Analysis and/or any other comments concern­ ing experiment 2,3-bis(tri- flubromethyI)- norboraadiene Ni(CO)2(P^3)2 hv, 166 hours benzene GC no dimer 7-t-butoxy- norbornadiene Ni(CO)2(Pp3)2 hv, 155 hours benzene GC no dimer 7-hydroxy- norbomadiene Ni(CO)2(Pp3)2 hv, l40 hours benzene GC no dimer norbomene Ni(CO)2(Pp3)2 A(90°C), 3 days neat GC no dimer, solution became yellowish 2+2 xtx norboraadiene dimer Ni(CO)2(Pp3)2 reflux, 6 days hexane GC no tetramer, solution became yellowish3- [2,2,2]bicyclo- octa-2,5-diene Fe(CO)2(NO)2 A, 3 hours isopentane GC no dimer, solution turns from red to dark red Fe(CO)2(NO)2 hv, 5 hours benzene GC no dimer, solution turns from red to 5°/o Rh/C reflux, 27 hours benzene  dark red NMR no dimer • Ni(CO)^ A, 71°C(reflux) 7 hours isopentane-benzene GC NMR no dimer, solution turns from colorless to yellow ■ a heating xtx + NiCCO^CP^),, results in apparent complex formation. See page 56. 115 7-Methyl-norbornadiene. This molecule was dimerized in the presence of NiCCO^CPj^^ by thermal and photochemical activation. Thermal. Nine ml of 7-methyl-norbornadiene was distilled on a vacuum line and transferred on the line into a flask containing ,20 g (,31 moles) of NiCCO^CP^)^. The reaction was run under nitrogen at reflux for 36 hours. The pot temperature rose from an initial 92°C to a final temperature of 105°C 0- After cooling, the unreacted 7-methyl-nor- bornadiene was removed under aspirator vacuum at 0-5°C. The resultant dimer mixture was subjected to the separation scheme diagrammed on page 36, Preparative column chromatography was performed on a 3/8 inch 20 foot 30# SB 30 column. Silver nitrate impregnated silica gel was prepared by adding a concentrated aqueous solution containing 10 g of silver nitrate to 90 g of silica gel in a 500 ml round bottom flask. The water was slowly removed under aspirator vacuum in the dark, . In a similar fashion, a twenty weight percent silver perchlorate impregnated silica gel was prepared, . Two dimers (XLV-A -and XLVII) were retained by silver nitrate impreg­ nated silica gel. The column was eluted with hexane, hexane-benzene, and finally benzene alone. Silver perchlorate impregnated silica gel retained five dimers. Hexane, hexane-benzene, benzene was the elution sequence, although a hexane, hexane-ethanol sequence could also be used. The five dimers in increasing order of retention tenacity were: . XLIX ~ XLV-B < XLVIII < XLVI < XLIV. See Appendix I for structure assignments of the isolated dimers Il6 based on their NMR spectra. • Photochemical. Five ml of 7-methyl-norbomadiene were trans- h ferred on a vacuum line into a pyrex tube containing .064 g (l x IO- moles) of Ni(CO)^CP ^ ^ - This was irradiated in a merry-go-round with a high pressure mercury arc using a pyrex filter for 8 days,' 18 hours and 30 minutes. The excess 7-methyI-norbornadiene was removed under aspir­ ator vacuum at 0-5°C. Comparison of dimeric products was made by gas chromatographic analysis using a 1/8 inch 20 foot 5°/o SE 30 column and by use of silver nitrate and silver perchlorate impregnated silica gel thin layer plates. These plates were prepared by dipping pre-cut Baker-flex Silica Gel IB thin layer plates into a saturated methanol solution of the appropriate silver salt. The degree of impregnation is determined by the concentration of the solution and the duration of submersion of the plates. The eluted (hexane or hexane-ethanol) plates were developed by spraying with a potassium permanganate solution. Kinetics Experiments The reaction vessel was a 50 ml 14/20 S three neck flask. The mid­ dle neck was fitted with a water cooled condenser which was packed (45 cm), with short lengths of 3 mm glass tubing to prevent losses by evaporation. One of the exterior necks was stopped with a septum cap while the other was fitted by means of tygon tubing with a glass tube having a medium porosity frit. This system allowed sweeping of the reaction by nitrogen -or carbon monoxide gas. Gas flow was monitored by a soap bubble flowmeter. The flow rate was kept at 13o^-l4»5 ml/minute during all reactions. The temperature was maintained to ± 0o3°C by a large constant temperature bath. All reactions were run at a promoter concentration of 1.12 x IO- M. The promoter (.2148 g NiCCCO^Cl^)^ or .3 g Ni(F0^)^) was dissolved in 7 ml of norbornadiene which had been warmed to about 60°C. The solution was quickly injected through the septum cap into the reaction flask, which contained 23 ml of norbornadiene (and any added ligands) already in thermal equilibrium with the constant-temperature bath. Prior to the addition of the NiCCO^CP^)^ solution, a stream of the sweeping gas had been established in the reaction flask. Samples (ca. .2 ml each) were withdrawn from the reaction and quickly cooled. The samples were pre­ pared for analysis by adding the appropriate amount of a benzene solution of dodecane (standard). The standard solution and diluted reaction sam­ ple solutions were prepared so as to produce gas chromatographic peaks of similar intensity for the standard and principal dimer at a given atten­ uation setting on the gas chromatograph. Each prepared sample was analyzed at least three times. The analyses were carried out using a 1/8 inch 20 foot 5% SE 30 column with a linear temperature program of S0-IO0CZminute starting at 30°C. The data was obtained as integrated areas under peaks and was treated as described in Appendix XI to give concentrations of dimer. Dimer concen­ tration vs time values were subjected to least squares analyses. . The., slopes and abscissa intercepts of the least squares lines are recorded in Table I on page 71« 117 PAET II ELECTRO-OEGANOMBTALLIC CATALYSTS INTRODUCTION Transition Metal Organometallic Reagents and the.Application of Electrochemistry to Their in situ Generation A large proportion of the organic reactions used to form carbon- carbon bonds can be thought of as involving organometallic reagents. This includes such well-known cases as the Reformatsky and Grignard re­ actions as well as the less obvious base-catalysed condensations involving alkali metal salts of stabilized carbanions such as enolates. There also exists within this large group, a body of classical coupling reactions in which the reagent is usually generated in situ. Under this heading the - following reactions may be noted. 1) Wurtz reaction: RX + R X --- > R-R a) Primary halides- work best. ' b) Metallic sodium is used except for ring closure of a , to polymethylene dihalides, where Zn is favored. . c) Cross coupling reactions generally give poor yields. d) Only ethers, of the common functional groups can survive re­ action conditions. 2) Wurtz-Fittig reaction: RX + ArX--- > R-Ar a) This reaction works well for bromides with sodium as a coupling agent. b) Most functional groups interfere with the reaction. 3) Ullman reaction: ArX + Ar1X - -- ^Ar-Ar1 a) Supplants Fittig reaction by substituting copper for sodium. 120 b) High temperature is required (150FCBthes:i c) Functional groups with hydrogens attached to oxygen or ni­ trogen inhibit reaction. d) Coupling of like substrates generally gives reasonable yields. e) Cross-coupling yields rarely exceed $0% and are usually lower. The general impression one gains upon examination of these reactions is the prevalence of harsh conditions. As a result of the demand in ' modern synthesis for the formation of carbon-carbon bonds under mild con­ ditions, efforts have been made to avoid these classical reactions, and a search made for new approaches. The majority of success in the area of reactions applicable to laboratory synthesis has come from two areas: the chemistry of organo-copper reagents and organo-nickel reagents. These re­ agents and their reactions shall now be discussed from the viewpoint of their scope and limitations. Other pertinent reactions shall also be - mentioned. Organo-copper Reagents I I 2Two reviews ’ have recently appeared covering organo-copper reagents ■ and these will be heavily relied upon in this portion of the introduction. The chemistry of organo-copper reagents could be divided into three areas based on reagent types: RCu, RCu-Iigand and LiGuR^0 However, in practice, it is only the more readily available and versatile lithium disubstituted cuprates (LiCuR^), which have found general use among chemists. Thus, this is the only reagent type to be discussed here. 121 These reagents are most often prepared in tetrahydrofuran from an organolithium and a cuprous halide in situ. In some cases it was found advantageous to add a ligand such as bipyridyl or triphenylphosphine during the preparation of the reagent. It is useful to classify reac­ tions using LiCuR^ as coupling and addition reactions. Representative examples of reactions in each of these categories are given below to pro vide an insight into the scope of these reagents. Coupling reactions: I LiCuR oxidizing agent , . R? + Cu(Il) eg. 0 , Cu(II) or 0UO2 R = Alkyl, vinyl, aryl II RX + LiCuR^---->R-R' . a) RX = I0 alkyl chloride, bromide, iodide or tosylate = 2° alkyl bromide (if R 1 = aryl) = a -bromoketone or a-hromocarboxylic ester. R 1 = alkyl or aryl An oxidizing agent, eg, or 0NO2>is necessary to effect coupling. b) RX = aryl or vinyl bromides or iodides R 1 = alkyl, aryl, vinyl c) RX = allyI or propargyl bromides R 1 = alkyl, aryl d) RX = A r L d or um yI R 1 = alkyl, aryl or vinyl 122 Addition reactions: I Ilx or l|l + xR R LiCuR' --- > /R' /C or C o o R = carbonyl group, C-Rm, C-O-R" R 1 = alkyl, aryl, vinyl IJ + LiCuR ------> C xCH, ^ | 2 OAc R = alkyl, aryl III a) alkyl-<^-CH2 + LiCuMe2 -> alkyl-C-CH -Me OH ^ + LiCu(Me). OH In the majority of the above cases it has been found a stoichiometry of greater than one mole of organo-copper reagent to one mole of substrate is optimumo Although it is not stated in the literature for each specific case, it appears that generally one obtains RCu molecules as relatively unreactive by-products in the above reactions. These two points can mean a considerable amount of organo-cuprate is not effectively used. If the organic moiety in the cuprate is difficult to obtain, this can be a severe limitation. If there is excess organo-lithium (or Grignard) re­ agent in a reaction to regenerate the disubstituted cuprate anion, only a catalytic amount of cuprous salt is necessary. However, this "catalytic" route, due to side reactions, is often less efficient for the production of the desired molecule 12^ One further limitation on the use of organo-copper reagents concerns the protection of functional groups. The reaction of LiCuE2 with car- ' bonyI groups is slow and thus carbonyls in the substrate do not need to be protected when such a reagent is used. However, one must protect car­ bonyl groups in the substrate if the above, simpler "catalytic" procedure is used since a very active organometallic is always present. Nor can one incorporate unprotected carbonyl functions in the cuprate since its preparation involves the use of active organometallics. This is probably also true for other carbonyl-type functional groups, eg. nitriles and im- ines. The presence of other functional groups also alters organo-copper reagents; amines tend to act as ligands, thereby altering the stereo­ chemical course of reactions, while active hydrogen groups, eg. -OH and -NH, oxidize the reagent. Thus, while organo-copper reagents are very useful synthetic tools, there are several restrictions on their applica­ tion. Organo-ni'ckel Reagents The most highly developed area of nickel reagent chemistry is that involving 77-allylie nickel compounds, either as discrete reagents or as 3 4transient intermediates. This area has also recently been reviewed ’ in detail and the following discussion relies extensively on these reviews, although reference will be made to other reports concerning related areas when appropriate.. 124 Two types of TT-allylie nickel reagents exist: bis(TT-allylic)nickel species, I, and ff-allylic nickel halides, II. < O ' II Both types of reagents are air sensitive and involve inconvenient prepar­ ations, as shown below. <5^ X / MEBr CIL + NiBr, polar solve: CODa + Ni++ + Al(Et)5 Ni 0. I Ni(CO)if very toxic + or Ni(COD)2 COD = 1,5-cyclooctadiene Figure I Preparations of Allyl Nickel Reagents 125 The reaction of an allylie Grignard with nickel bromide to give the bis (77-allylic)nickel complex would appear to be the most facile approach, but it is not well documented. Semmelhack states the Grignard route to bis(7r-allylic)nickel complexes is less convenient than the route via allyI nickel halides. However, this latter path involves the use of either the very toxic nickel tetracarbonyl or bis(l,5"cyclooctadiene) nickel, Wi(COD)^, which itself must be synthesized. The bis(ISCallylie) nickel complexes are generally thermally unstable, very volatile (the parent bis(77-allyl)nickel co-distills with ether) often making purification difficult. In general, synthetically, bis(77-allylic)nickel species offer no particular advantages over TT-allylie nickel halide complexes. The one significant exception is in the case of the oligomerization reactions of butadiene to give various products (eg. I,2-divinylcyclobutane, 4- vinyIcyclohexene, 1,5-cyclooctadIene or 1,5? 9-cyclododecatriene) depending on the conditions used. In this case, as well as in the crossed oligomer­ ization of 1,3-butadiene and ethylenes, bis(77-allylie)nickel species appear as1 transient intermediates. Whether generated as discrete reagents or as transient intermediates, the chemistry of TT-allylic nickel halides can be divided into three re­ action types; carbonyl insertion, coupling and addition. Carbonyl insertion reactions are most often carried out by the in situ generation of active nickel species from nickel tetracarbonyl. The reaction can be diagrammed as follows: 126 RX + Ni(CO)if * R-Ni(CO)xX R-C-OR1 <- % R-C-X R 1OH R-H-Ni(CO) ,X RX = -f5^ s< Jr , Ar(lor Br), alkyl-I. Often, a strong C base, eg. NaOMe or Ca(OH)^, is required for these substrates. At least with allylie halides, it is not easy to predict whether carbonyl insertion or coupling will occur for a given structure. However, compli­ mentary carbonylation reactions are available using sodium tetracarbonyl- ferrate(ll)^ or the olefin/palladium chloride/CO system.^ Coupling reactions with TT-allylie nickel lialides or the in situ generation of such species from allylie halides and nickel, offers a general route for the addition of an allyl group. Thus, in the sequence + RX -> R + NiX, R can be allyl, benzyl, alkyl, vinyl or aryl. X is usually Br or I, although in the allyl case the following relative reaction rate sequences have been established: I >Br >C1 and TsO >.0Ac. In general, bromides or iodides (2° alkyl, aryl) are used and the reaction is run in polar sol­ vents (eg. dimethylformamide, dimethylsulfoxide, N-methylpyrrolidone or hexamethylphosphoramide). Yields are usually quite good, 70-90^. 127 However, this reaction is stoichiometric in the nickel reagent. Other organic halogen compounds can be coupled by the action of Ni(CO)if or Ni(COD)2 . a)8 4 - 0 1 - J g - tc0 i^ , 4 1 ( 0 0 ) - THF Ni(CO)l (Xe I c0-C”CI 0—C—0 0—C —0 0-C=C-0 -> 0-C-OR b) 0-6-CH2-Br + Ni(CO)if-- dTora hCOCyRBCy RBCcChh 0, DMF c)10 0-CH2-Br + Ni(CO)4 Y EtOH --  \ O — > 2 2 I_™^—0Et 2 2 33^ 2 --- 2 67* d)11 2ArX + Ni(COD)2 ---— F -> Ar-Ar + NiX + COD X = I, Br, Cl TsO is inactive 128 * + Ni(COD)2 X = I, Br, Cl f)13 0-CH2-X + Ni(COD)2 > + NiX2 > 0-CH2-CH2-0 + NiX g) 13 + Ni(COD)2 Y X = Br or Cl Y and Z are often carbonyl functionalities, although fair to good yields can be obtained when they are H, alley I or aryl groups. A number of addition reactions involving a Ni(O) complex, butadiene and an organic carbonyl compound have been developed by Wilke.^ Cassar ^ has found Ni(CO)^1 an allylic compound, acetylene and carbon monoxide can be added to give 2,5-dienoic acids. While these reactions are useful, they have not found widespread use among chemists and wil'J not be dis­ cussed here. However, it has been found that in polar solvents at 50-60°C, 77- ally lie nickel halides can add across ketone carbonyls to give p-y unsaturated alcohols. Similarly, epoxides can be opened. It is inter­ esting that acrolein undergoes 1,2 addition of an allyI group to the carbonyl function while methyl acrylate affords the 1,4 addition product. In general, these additions may find special synthetic applications, but they do not offer any particular advantage over magnesium or lithium reagents 129 Although organo-cuprates and organo-nickel species are the most versatile transition metal organometallics, there exists a considerable body of data on reactions promoted by other transition metal organo- metallic reagents. Sodium tetracarbonyl ferrate and the olefin-PdCl^- CO system have already been noted. The fixation of nitrogen by low valent titanium compounds has also been realized. a)15 CC5H5)* TiCl2 + 0Li 0NIL b)lb (C5H5)2 TiCl2 + CHCH-^-CHCH 5 Z 2 5 N„ (I atm) Mg (CH5CH2)2CHNH2 + [(CH5CH2)2CHj2NH 16 (Ct-H-). TiCl + 0-&C1 + Mg ____ Np (l atm) c )— '-5 - 5 '2 ^^‘2 aC5H5 = cyclopentadienyl 0CN Using bis(cyclooctatetiaene)iron, (COT)2Fe, it has been possible to effect Diels-Alder type reactions. a)17 Fe(COT)2 : Fe(COT). ------ > 130 Often, the active catalytic species are not isolated or even identified For example, Kochi has reported?0 the following useful reactions which 21have been distilled from a large body of data. a) b) c) RMgX + R-X RMgX + Br RMgX + R 1Br soluble Ag catalyst (prepared from Rl-IgX + AgNO ) FeCl., -> R-R' + R-R + MgX2 + MgXBr MgXBr The last reaction can be recognized as probably involving an organo- cuprate reagent. In fact, reactions of this type have served as, and will continue to be, a source of inspiration and motivation for detailed investigations of transition metal based catalytic systems. One underlying similarity between the various organometallic reagents which have been discussed here is the preponderance of stoichiometric re­ actions. Thus, in most cases, unless the reactions are run in the pres­ ence of a strong reducing reagent (eg., an organo-lithium or Grignard re­ agent) to regenerate the catalyst, a relatively large amount of "catalyst" is needed. As noted in this discussion, the catalysts often are not 131 readily available for large scale use, and the presence of strong re­ ducing agents can limit synthetic scope and certainly encumber, it. Thus, it would be desirable to have a reduction method available that allows the production of "catalytic" species, but which does not cause unwanted side reactions. In light of the reduction potential range of transition metal cations and their compounds (relatively low) and the polar nature of the solvents in which many of the above discussed, "catalytic" reactions are run, electrochemistry suggests itself as a reduc­ tion mode. This method of producing active metal species shall now be discussed to provide background as well as an understanding of the prin­ ciples upon which this portion of this thesis is based. NJ The electrochemical generation of organometallic compounds can be divided into two general areas, non-transition metal organometallics and transition metal organometallics. The former area has received the greatest attention and organic derivatives of the following elements have been prepared electrochemically: Zn, Cd, Hg, B, Al, Ga, In, Tl, Si, Ge, Sn,■ Pb, As, Sb, and Bi. In most cases one of two techniques was used.^’^ a I. Reduction of an organic halide at a cathode of '.the appropriate element, or ■ 2. Reaction of an alkyl metal compound, AlR^, RMgX or their alkali metal derivatives, eg. NaAlR^, with an anodically generated cation of the appropriate element. Undoubtedly, the interest shown in electrochemically produced non­ transition organometallics is motivated by possible industrial applications. 132 In fact, a large portion o f the tetraalkyl lead compounds produced are prepared electrochemically. The electrochemical generation of transition metal compounds has been much more limited in scope» Exemplary of the work which has been done is that of Lehmkuhl and workers, who have prepared Ni, Co and Ti * containing organometallics by the electrochemical reduction of NiCacac)^ CoCacac)^ and TiCl^ in the presence of organic ligands. In this manner 2 h they have prepared, for example, bis(l,5-cyclooctadiene)nickel(0) , TT-cyclooctenyl-1,5-cyclooctadienecobalt(I)^ and cyclooctatetraene 26titaniumchloride pyridinate. The area of intentional in situ generation of very reactive metallic or organometallic reagents has been even more limited. Some represen­ tative examples are given below. I. The generation of a metal in an oxidation state suitable for reaction as a catalyst or reagent. 27a. Reduction of aromatics with Li0 from LiCI. b. Oxidation of aromatics with Mn, Co or Ag in high oxidation 23bstates to give aldehydes. a. Hydroxylation of aromatics with molecular oxygen and I 23belectrochemically generated Cu . d. Formation of "univalent magnesium" (benzophenone to benzo- pinacol) and "univalent aluminum" (nitrosobenzene to * acac refers to the mono anion of acety!acetone (pentan-2,4-dione). 133 sym-diphenyl hydrazine) e o Electrochemical generation of Ni2^ and Co2^ catalysts for conversion of butadiene to 4-vinyl cyclohexene and 1,5- cyclooctadiene. 2o Preparation of amalgams for hydrodimerizations of activated oIefins:,2^ 3 eg. (R, N) (Hg) -----— ---- N O CN % . 100# 3- Generation of reactive radical anions suitable as reagents2"'’0, ^ P b — 6 ■ > 0^Pb" ---1^— > 05Pb' (Cpd)Pe(CO)pX ^ X + ^Pb-Fe(Cpd)(CO)p Cpd = eye lopentadienyl Although these examples are not exhaustive they do show the trend of in situ generation of reactive metal containing species and imply the potential for further investigation» With a background of the sort of systems to which the electro­ chemical technique has been applied, it is now appropriate to discuss the basis for application of the technique to coupling and insertion reactions of organic substrates. As noted in the first portion of this introduction, frequently one encounters reactions in which the coupling of two organic substrate molecules is promoted by the presence of a transition metal in a low oxidation state. In addition to the coupled organic product, one obtains the oxidized transition metal. If the oxidized transition metal is electrochemically reduced, it is possible to provide the system with an electrochemically regenerated catalyst". The situation lor a divalent metal can be diagrammed as follows, where IiX refers to the substrate to be coupled. 134 reduction R-C-X reductive elimination reductive elimination oxidative addition R-C-K(L) acyl complex transmigratory insertion disproportionation Figure 2 Schematic for Electro-organometallic Catalysis Note, that included in the sequence is the alternative route in­ volving carbonyl insertion. The initial step is reduction of the transition metal in the presence of a number of possible ligands to yield the active transition metal complex. This reduction should be carried out at a potential low enough 155 to avoid reduction of the common functional groups. This, in fact, is possible for the majority of common functional groups, although closer regulation of potentials is required when transition metals on the left- hand side of the periodic table are used than when metals from the VII-B * and VIII groups are used. The oxidative addition step is strongly in­ fluenced by the nature of the metal and the coordinated ligands, L. 31 32 10Coilman, in two reviews, 5 has pointed out that d complexes undergo 8 53oxidative addition more easily than do d complexes. He has also noted that oxidative additions are promoted by the presence of strong electron donors, eg. R^P, and the absence of strong TT-back-bond acceptors, eg. CO. The complex, RMX(L)^, which is formed in the oxidative addition reaction, may be either Tf- or o -bonded. As noted in part I of this thesis, page 15, the presence of stronger-donor ligands favor Tl —» cr reactions. This complex may follow either of two routes depending on the nature of the ligands, L. If at least some of the ligands are carbonyls, then trans- migratory insertion can occur to give an acyl complex. The nature of the R group, the metal and any remaining coordinated ligands can determine the probability of this route. If excess CO is present, the resultant acyl complex can reductively eliminate to give a carbonylated organic product and the active catalyst. Thus, this cycle can be catalytic. * * To aid in the determination of the appropriate reduction potentials and the feasibility of any given reaction, reference can be made to the extensive lists of polarographic data available.^ This, of course, can be supplemented by small-scale control potential electrolyses. M(CO)x(L)y ■+ EX EMX(CO)x(L)y---> R-CO-MX(CO)x ^ (L ) +CO» R-CO-X + M(CO)x (L) Two molecules of the EMX(L)^ complex can also undergo disproportionation ■ to give a R^M(L)n complex and MX^. Thus, one M is returned to be reduced again while the other reductively eliminates to give product and a M° species. 33If oxidative addition is promoted by cr -donors and if ySCdonors also stabilize resultant, a-bonded organome tallies (alkyl, aryl or acyl complexes), it is probable that replacement of a cr -bonded ligand by a strong 77-back-- bond acceptor will promote the reverse process of reductive elimination. One is therefore faced with a dilemma, and the best compromise may be to use ligands such as triaryl phosphines, which can act as acceptors to at least some extent„ The net result of such a cycle is the use of catalytic amounts of metal and associated ligands , and the flow of two faradays of charge for each mole of coupled product produced. Finally, one may consider some of the electrochemical variables which influence these "catalytic" coupling reactions. Although there are a very large number of variables associated with any electrochemical re­ action only three of these shall be distinguished as being clearly impor­ tant here. These are electrode potential, electrode material and solvent. As will be seen, the first two of these variables are perhaps best treated together. It may be argued that the careful control of the electrode potential is critical to any electrochemical reaction and the importance of such control cannot be deniede The manner in which the potential is controlled, however, can vary widely=, In the case of the proposed "catalytic" coupling reactions there are two types of reduction potentials to be considered; the metal reduction potential and the sub­ strate reduction potential. Since most common functional groups have more negative reduction potentials than do transition metal ions, it is generally possible to reduce the metal without reducing the substrate. Furthermore, in light of the cyclic nature of the coupling reactions (M+n + ne ---» M0 — > M+n + E-R), the concentration of metal ions reaches an equilibrium value. The following points may be made: a) the metal ions are present in concentrations which are only a fraction of the substrate concentration(although they are, speak­ ing from a classical electrochemical point of view, quite high), b) the coupling reactions occur quite readily and hence c) the concentration of metallic "catalyst" species is probably quite low. Thus, the concentration of the metal ions doesn' t change very much until * the substrate is nearly completely reacted. Thus, monitoring the applied potential provides a reasonable check on the cathode potential. 137 * Note that the relation between emf and concentration is given by the Nernst equation as E = E0 + 0,0591 log [gx] . Thus to change the — ~ T i U value of E by 0.1 volt for a two electron reduction, one must change the ratio [px]/QredJ by 2400 ,which is a very large change for this system, * In order for an electrochemical reduction to occur, there must be an electrochemical oxidation as well® There are several ways to avoid difficulties with concurrent oxidation processes. The conceptually easiest route, and experimentally most inconvenient route is to use a divided cell. An alternative which is experimentally advantageous is to allow an innocuous oxidation to occur in an undivided cell. One of the most facile approaches is to use a sacrificial anode. This route was chosen in the electrochemical preparation of Ni(COD)^ by Lemkuhl^ who used two aluminum electrodes in a pyridine electrolyte. The two half reactions can be written: Ni++ + 2e ---> Ni0 — 2C0D— ^ Ni(GOD)^ Al0--- > 3e~ + Al+++ The anodically generated aluminum ions were probably quite readily complexed by pyridine. It can be noted also that a cell composed of these two half reactions would result in a spontaneous reaction (act as a battery) if it were not for the internal resistance of the cell. In fact, an applied voltage of about l8 volts is required for significant reaction. One could also use an anode made of the same metal as is used for the "catalyst". The concentration of metal ions in solution would thus stead­ ily increase as the reaction proceeded. But, since the half cell reactions are identical, there is only one solution, and barring concentration gra­ dients, only one concentration of metal ions, the cell potential would remain zero. The only significant change would be in the ionic strength of the solution, which initially is quite high in light of supporting electrolyte concentration and concentration of metal ion originally in 138 139 the cell. As noted in the first part of this introduction, many of the coupling reactions have been found to work best in highly polar solvents 13and it has been suggested by Semmelhack that this may be due to their role as very labile ligands, In particular, the aprotic, water soluble, high dielectric constant solvent N,N-dimethylformamide has been found to be very useful, although, other similar solvents such as dimethylsulfoxide and hexamethy!phosphoric triamide have found some usec The presence of of other good o'-donor ligands would allow the use of acetonitrile as wello All of the above solvents dissolve organics, inorganics and organometallies readily. Dimethylformamide offersthe additional advan­ tage of low reactivity towards anion radicals. In light of the introduction presented here it ought to be evident that the application of the electrochemical redubtion technique to transition metal organometallic promoted reactions ought" to be quite fruitful DISCUSSION Coupling Reactions Several coupling reactions of organic halogen compounds have been run in our laboratory. The reactions were conducted under a nitrogen atmosphere in glass, single compartment cells. Two general electrode systems were used: two aluminum electrodes or a copper cathode and nickel anode. Nickel ion was introduced as NiCacac)^, while tetraethyl ammonium bromide was the supporting electrolyte. The solvent was N,N- dimethylformamide in all cases. Unless otherwise noted, the products were indentified by infrared spectra (ir) comparisons and mixed melting point determinations with known samples. Chronologically the first, as well as the most thoroughly investi­ gated reaction, was the coupling of benzyl halide with low valent nickel species. The applied potential was such that in the absence of Ni++ no appreciable current would' flow, ie. no reaction would occur, while in the presence of Ni+"1", appreciable reaction would result*. The results can be tabulated as follows, where COD = cyclooctadiene. Table I Benzyl Halide Coupling Reactions Substrate Added Ligand Electrodes Isolated (cathode/anode) Yields 0CH2C1 Al/Al 69# pfCH^Cl COD Cu/Ni 56# 0CH2C135 (ZfCH2Br35 Cu/Ni Cu/Ni 87 85# (ZfCH2I35 Cu/Ni — I k l These results can be examined from the viewpoint of four closely interrelated variables: electrode material, added ligands, the halogen (X), and the appearance of side reactions. Using Al electrodes, P0^ as an added ligand and gfCH^Cl as the substrate, a 69% isolated yield of bibenzyl ,could be realized. During the course of the work-up, it was noticed there appeared to be a very fragrant by-product. Thin layer chromatography (TLC) of the crude pro­ duct showed the presence of a more polar component, while an infrared spectrum showed a carbonyl peak at ca. 1705 cm \ Based on this ir spectrum and a comparison.of TLC values with those of a known sample, this compound was identified as pheny!acetaldehyde. By changing to a nickel anode and a copper cathode and using the same substrate and added ligand (P0^) ,■ the elimination of this side product could be achieved. The yield of bibenzyl rose to 87%. The yield of coupled product would be predicted to decrease, of. page 154 of the Introduction, with the substitution of (a good a-donor) by cycloocta-1,5-diene (a poorer 0-donor) and this is observed (56# vs 87% yields). When the substitution of benzyl bromide for benzyl chloride is made keeping the concentration of substrate, P0^, NiOacac)^ and supporting electrolyte constant, and using a Cu/Ni electrode system, very little difference is observed in the course of the reaction. Thus the applied potential for a current of 50 mA remains constant during reaction and is the same value for both 0CIL,C1 and 0CEL,Br. The yields of coupled 142 product are also similar (gfCH^Cl = 87$, f i C E^Br = 85% ) „ That the potentials are nearly- identical suggests the same specie is being reduced in both cases. While it is conceivable this could be an organic substrate, it is very unlikely based on published^ reduc­ tion potentials of the various possible species. 0OH Br -1.22 vs SCE in DMF -1.324 vs Ag/AgCl in methanol + - 0 G E ^ P 0 ^ C 1 -I.826 vs Ag/AgCl in methanol Ni++ -0.835 vs SCE in DMF -0.917' vs Ag/AgCl in methanol Since the reduction potential of ECl > RBr, it can be assumed from the above data, that Ni++ would reduce at a less negative potential than ^CH^Cl. Furthermore, as noted earlier, in the absence of Ni++ very little current (< 5 mA) flows at a potential sufficient to cause a 50 mA current in the presence of Ni++. When (ZfCH^ I was used as a substrate, the liberation of iodine appeared to occur at the anode and the reaction did not run cleanly. To explore the scope of the application of this electrochemical technique to coupling reactions, bromobenzene was chosen as an example of an aryl halide. It could be coupled using NiCacac)^ and either COD or as an added ligand. The goal of the preliminary investigation using an Al/Al electrode system and COD as an added ligand wras simply to establish the feasibility of the reaction. When the reaction was ' run using P0^ , a nickel anode and a copper cathode, a 65% yield of biphenyl was isolated, No effort was made to optimize the yield of this reaction. Exploratory coupling reactions were also run with benzoyl chloride and Ni(acac)g, With a Cu cathode and a nickel anode, and COD as an added ligand, the major product was the 1,2-diketone, benzil. Using P0^ as an added ligand and two aluminum electrodes, the benzoate ester of the cor­ responding ketol, benzoin, was formed. This product was identified by comparison of ir spectra, melting point and a mixed melting point with a sample of benzoin benzoate prepared by the reaction of benzoyl chloride and benzoin , It is reasonable to propose that the dibenzoate of ben­ zoin (esterification of both the ketone and the alcohol) is actually produced in this latter reaction. This would be in accord with the re- g ported result of coupling acyl halides using Ni(CO)1^, However, in the work-up procedure employed by Kiely, acidic water (ca, 2M HCl) is used to facilitate- the break-up of emulsions, . These conditions would probably result in hydrolysis of the enol ester function, giving .the observed d-ketobenzoate, . From the coupling of a.-bromoacetophenone with N i C a q a c P j ^ , and using a copper cathode and a nickel anode, crystals of I,2-dibenzoylethane could be readily isolated. The identification of this compound was made * * This reaction was run by Mr, Dennis Miller, .Montana State University, ** This electrolysis and the subsequent product identification were carried out by Mr, John Kiely, Montana State University. ■2 z' by comparison with the reported-5 ir spectrum and melting point. As noted earlier, when benzyl chloride is coupled in an Al/Al electrode system, a by-product suggested to be pheny!acetaldehyde is formed. Al­ though Al+ 1"+ and Cl ions are formed in this reaction, dilution of the electrolyte with water results in an essentially neutral solution. VThen a nickel anode and copper cathode are used instead, no pheny!acetaldehyde is produced. This situation was enigmatic until it was found that a DMF solution of AlCl^, when diluted with water, also produced an essentially neutral solution. It has been found that DMF forms a strong complex with AlCl^ and evidence has been presented for complexation occurring "57through the carbonyl oxygen. f It is suggested such complexation would result in an activation of the carbonyl carbon. O-AlCl +n . H/VxN-Me Me Hz" O-AlCl 3 It would be expected such an activated carbonyl could react with an organometallic specie, R-Ni(L)n(X), such as would be produced by the ox-? idative' addition of RX to Ni(L) .n . ' f"^1C13 + ENi(L) X II-C-N(Me)2 n ^ A1C13 + Ni(L) X+ H-C-N(Me)2 n Ni(L) X+ + :N(Me)- + AlCl- + H-C-R n R p 0 * Ni(L) X+ + H-N(Me)0 + AlCl- + H-fi-R + OH n 2 3 H2° 3>5 Thus, either with or without the addition of water, one could visualize the formation of an aldehyde and an essentially neutral solution. To gain the maximum synthetic utility from such a reaction, several features would be helpful. a) Coupling of RX molecules by the metal promoter M(L) should .be minimized; a non-transition metal might be more suitable. b) The reactivity of the complex RMX(L)^, resulting from ox­ idative addition, would increase if the R-M bond were made more ionic; a non-transition metal complex would be expected to have a more ionic R-M bond than would a transition metal complex. c) However, to achieve substitution of the halide in RX by a formyl group, R-E-H, without subsequent reaction of the ■ aldehyde, the complex RMX(L)^ should not be too reactive. Moderated activity would also help to obviate the problem of the protection of other reactive functional groups which might be present in a substrate. d) The electrochemical generation of the low valent metal specie M(L) and its ease of subsequent reaction with RX to u give the RMX(L)^ complex should also be compatible with the above points. Zinc was chosen as the most suitable metal to meet these requirements. Its reduction potential and reactivity toward oxidative addition is a good compromise in light of the moderately ionic character of the R-Zn bond. However, the reactivity of organo-zinc compounds toward common 38functional groups is quite low. Further, organo-zinc compounds do not couple readily and dimethylformamide has been recommended as a solvent for their preparation from zinc dust and alkyl halides. 5 It was found that electrolysis of a DMF solution of n-hexy!bromide and ZnI^ resulted in the formation of a kQP/o yield of heptaldehyde, isolated as the 2,4dinitrophenyIhydrazone derivative. It was felt this yield could be improved. One factor limiting the yield could be the stoichiometry of the reaction. 3Zn++ + 6e"---->3Zn° .3RX > 3RZhX 2A1°--> 6e" + 2A1X^ - ^ -MF > 2 [DMF1AH^] but, 3RZnX + 2 [DMF-AlX^I--> 2K-0-H + HMe5N + H A H 5 + 2ZnX+. + RZnX It can, therefore, be seen that for the flow of six faradays of charge, three moles of RZnX are formed and two moles of DMFeAlX^1 activated DMF complex, are formed. If the reaction of one mole of RZnX with'one mole of DMF'AIX., results in the effective neutralization of the mole of 5 AlXj then only two-thirds of the RZnX would form product (RCHO). One way to approach this stoicheometry problem would be to add the "deficient" amount of AlCI^ to the DMF prior to electrolysis. However, it was found that under these Conditions (a high concentration of DMF- ■ AlCIj complex), the reaction did not run cleanly and little aldehyde was formed 14? This problem could also be approached by changing from an aluminum anode to an anode made of a metal which preferentially oxidizes to a divalent cation showing reasonable Lewis acidity. It is now felt that this would be the most successful solution to the stoichiometry problem. ' The approach which was, in fact, used involved the substitution of zinc by a trivalent metal, providing a balanced redox system. Thus Fe(acac)^ was used with a copper cathode, an aluminum anode and n-hexyl- bromide as a substrate= A gas chromatographic analysis of the product showed that the formation of heptaldehyde had not occurred to any major extent = It was established by sample admixture that the major product was n-dodecane, As noted in the Introduction, coupling methods are often limited by the reactivity of precursor organometallies. This has been especially true for the coupling of alkyl halides and thus the discovery of a new mild coupling method would be advantageous. The reaction has not been extensively studied but it does show promise = Carbonyl Insertion Some success was also achieved with carbonyl insertion using the electro-organometallic technique, A DMF solution of (ZfCH^ Cl and Nifacac)^ was reduced between two Al electrodes in the presence of a carbon monox­ ide atmosphere= Thin layer chromatography of the crude product showed the presence of unreacted 0 G E ^ 1 , 0 C H ^ G H ^ 0 and two other more polar 148 components, one of which appeared to be ^CH^CHO. The crude product was chromatographed on silica gel and the remaining unidentified component isolated. This compound's infrared spectrum was identical with that of dibenzylketone, while its NMR spectrum was also consistent with this assignment. It is suggested,the formation and reaction of Ni(CO)^ may have occurred in the system, thus providing a route to the insertion product. Since the insertion product was obtained in low yield by the initial technique of exposure to a carbon monoxide atmosphere, some reactions were carried out in which a stream of CO was swept through the system at a pressure of about 1.2 atm. However, this did not improve the yield of insertion product. It did result in removal of nickel from the reaction vessel, presumably as Ni(CO)^, since nickel could be readily detected as nickelbis(dimethylglyoximate) in a trap placed in the efferent CO stream. It is suggested a closed pressure electrolysis cell would be required to exploit this reaction. Suggestions for further Work, in this Area At this point a number of suggestions will be made for further ex­ ploration of the use of the electro-organometallic technique in organic chemistry. It is probable that not all the suggested reactions will behave as anticipated. However, it is hoped all will provide stimulation for further development and exploitation of this technique. Organic Coupling Reactions. While crosscoupling experiments were not investigated, the ability to perform this sort of reaction would increase 'the scope of the reaction. Since the reduction potential of alkyl tos- 30ylates is well above that of most transition metals and since they appear to have relatively low reactivity towards oxidative addition, it is suggested that crosscoupling may best be carried out using the EMX + ElOTs — > E-E' system. A likely possibility for a metal (M) would be copper. By using nickel as a metal, bipyridyl as a stabilizing ligand and a i+O small amount of tetracyanoethylene to promote coupling, it may be possible to improve the coupling of two alkyl groups. Formylation by Bimetallic Activation. (The Halide/Formyl Sibstitntian Reaction)-. It is suggested the stoichiometry problem (AIaZAl1 ' 1 and Zn1""1*/Zn0) may best be met using a magnesium anode since- MgX_ would be formed, and it is a good Lewis acid allowing activation of the DMF. It is also- suggested that other amides may prove to be reactive in this system enlarging the scope of the reaction. Insertion, Reactions. As noted, the use of a closed pressure elec­ trolysis cell would probably increase the production of carbonyl insertion producto It is also possible other metals* especially iron and cobalt, could be used to insert carbon monoxide into carbon-carbon or carbon- halogen bonds. Other groups such as SO^ and NO might also- be incorpor­ ated. In particular the in situ generation of a tri- or tetranitrosyl substituted alkyl chromium species from Cr(acac)^, EX and NO; or NOGl could be- suggested. ■ SUMMARY It has been found that using eIectrochemically generated low valent nickel species, the following substrates may be coupled to give the specified products. 0CHgX 0Br 0COC1 0COCH2Br » 0CH2CH20 > 0-0 0 O' O H 0 0-0-C-0 or 0-6-G-O-C-0 0 -> 0-0-CH2CH2-G-0 In these reactions it was found P0^ was a suitable added ligand. It probably acts a) by stabilizing transient low valent nickel species, pre­ venting the plating out of nickel on the cathode, and b) as a good o donor The use of an aluminum anode, generating Al+++ ions which can complex . with the dimethylformamide solvent, promotes a halide-formyl substitution reaction. 0 RMX + AlX-, oDMI1 -------3> R-5-H Zinc is suggested to be the best choice for a metal in RMX. However, because zinc favors a +2 oxidation state while aluminum favors a +3 state, problems with the stoichiometry of the reaction occur. It was also found that n-hexy!bromide would couple in the presence of an Fe/P0^ "catalyst" to give dodecane, Carbonyl insertion was also observed, using the electro-organo- metallic technique. Thus, benzyl chloride could be converted to 151 dibenzyl ketone in low yield0 It is felt the success of this preliminary work in the area of electro-organometallic catalysis provides a reasonable basis for expecting the technique to have wide application to organic synthesis« Further, based on preliminary experiments, some of which are referred to in part I of this thesis, page 77i it is expected at least two more areas of application exist» The first of these involves the application of the electrochemical technique to the synthesis of new organometallic compounds» The chemistry of transition metal organometallies not having carbonyl or cyclopenta- dienyl ligands is very limited, but potentially very useful. Thus, the ability to synthesize such compounds would be advantageous and'it is suggested the selectivity of the electrochemical reduction technique would be particularly helpful. It may also be noted the electrochemical technique may prove valuable in the synthesis of rare earth and actinide organometallies. Another area would be the selective generation of ligand stabilized metals in high or unusual oxidation states. In addition to - the intrinsic value of synthesizing exotic compounds of this sort, they may prove to be useful as synthetic reagents themselves. For example,' the availability of an organic solvent soluble form of the cer­ ic ion would provide organic chemists with a potentially very useful oxidizing agent. The second area involves the integration of electro-organometallic synthesis and electro-organometallic catalysis, ie. a study of the 152 mechanisms of coupling or insertion reactions. Unlike many other methods of generating highly reactive organometallic catalytic species, eg. the thermal or photochemical activation of relatively stable complexes, the electrochemical technique, because of its selectivity should allow the low energy generation of reactive organometallic species. The approach would be to build up the reactive species from reactive pre­ cursors under mild conditions, rather than tearing stabilizing ligands off a stable complex to produce this same reactive species. It may also be noted that specific ligand combinations which were otherwise difficult to produce might become available for investigation by the electrochemical technique. EXPERIMENTAL Instruments and equipment Gas chromatographic analyses were run on a Varian Model 1740 Gas Chromatograph using 1/8 inch, six or twenty foot columns' of 5/° SE 30 on 100-120 mesh Gas Chrom Z0 Nuclear Magnetic Resonance spectra were obtained on a Varian A-60 Proton NMR0 Electrolyses were carried- out with regulated voltage power supplies equipped with current limiters. Several power supplies were used, the only requirement being the ability to deliver from 0 to ca. 20 volts at currents at least up to ca. 250 mA. Reagents All of the organic substrates and the ligand 1,5-cyclooctadiene, were distilled under nitrogen prior to use. Triphenylphosphine was sublimed under vacuum. Gases Carbon monoxide (Matheson, C.P.) used for carbonyl insertion re­ actions, was deoxygenated by the use of BSTS catalyst (BASF Colors and Chemicals, Inc.). Nitrogen (Airco, Dry High Purity) used for distillation and as a protective atmosphere for electrochemical reactions, was deoxygenated by the use of BSTS catalyst. 154 Solvent The solvent for all electrolyses was N ,N-dimethylformamide, It was dried over Linde 4-A molecular sieves and distilled under a atmosphere, Preparations Ni(acac)^ . This was prepared by a variation of the procedure of Watson. An ammoniacal solution of NiCl^ was prepared by the addition of just enough water to effect the solution of a mixture of NiCl^ (32.4 g, 0.25 moles) and 150 ml of concentrated ammonia solution. This solution was, added with vigorous stirring to an ammoniacal solution prepared by the addition of just enough water to effect the solution of a mixture of acetylacetone (55 ml, .535 moles) and concentrated ammonia (4) ml, „60 moles)0 The resulting thick blue mixture was vigorously stirred for one-half hour, filtered, and air dried to give the di-ammine derivative of Ni(acac)0. To obtain ammonia free Ki(acac)0, the di-ammine derivative was boiled in toluene, or more quickly in xylene, to give a dark green solution of NiOacac)^ . The solution was concentrated and hexane added to precipitate the light green Nitacac)^ . After washing the precipitate several times with hexane, it was filtered, vacuum dried and stored in a desiccator. Fe(acac)^ . This was prepared from a stoichiometric amount of FeCl^ and sodium acetylacetonate (1:3) in an aqueous solution. The dark red product was extracted with chloroform, washed with water and the chloro­ form was removed. The red Fe(acac)^ was recrystallized from ethanol- water and air dried 155 Tetraethylammonium Bromide. This was prepared from equal molar quantities of triethylamine and ethyl bromide by refluxing overnight in a toluene solution0 The precipitate was washed with toluene and filtered. It was recrystallizaed from a benzene-ethanol mixture, dried under vacuum at -So0C and stored in a desiccator. Electrochemical Cells and Electrode Assembly The cells were pyrex cylinders of various diameters closed at one end. The electrode assemblies were of four types. A) Two parallel rectangular aluminum plates. B) Two parallel wires, one made of copper (cathode) and the other (anode) made of either aluminum or a heavily nickel plated copper wire. This assembly was used principally for small scale (.005 moles) reactions or about one-tenth of the scale of the reactions with other assemblies and cells. C) Two concentric cylindrical.electrodes,.the outer cathode made of copper screen and the inner anode made of nickel foil Wrapped around a long glass tube which had been closed off. D.) Two concentric cylindrical electrodes, the outer cathode made of copper screen and the inner anode a short cylinder of aluminum attached by means of septum caps to a long glass tube which was fitted with a frit. This electrode assembly was used for carbonyl insertion reactions so the electrolyte could be swept with carbon monoxide 156 Stirring could be effected by a teflon covered magnetic stir bar. Anode of Electrode Assembly D electrical connection to anode septum caps glass tube Al anode Procedures for Electrolysis Reactions General Considerations. The cell was charged with the appropriate metal salt (ca, 0.1 M NiCacac)^, Fe(acac)^ or ZnI^), the supporting electrolyte (ca. 0.1M) and the added ligand (P0^ or COD). The cell was flushed with nitrogen, and under a protective atmosphere of gas, the substrate and solvent added. The appropriate electrode assembly was then inserted in the electrolyte. The system was maintained under during the electrolyses, except in the carbonyl insertion reactions where either" a carbon monoxide atmosphere or sweeping of the system by carbon monoxide was usedo The electrolysis was usually carried out at constant current (ca« 50 mA, fixed by the current limiter) and both potential and current were monitored frequently during the reaction» In cases where an aluminum anode was used, a short period of electrode activation was observed. During this period, usually about an hour, the voltage dropped when the current was held constant. This period of activation is not observed when a nickel anode is used. It is probably attributable to the oxide coating on aluminum and can probably be eliminated by pre-electroIysis prior to the addition of the substrate. To obviate the activation period, it has been found advantageous to increase the voltage to a high value (ca. 10-15 V, corresponding to a current value of ca. I amp) for a very short (30 seconds) time, and returning it to the normal (ca. 1.0 V) range to maintain a current of about 50 mA. The electrolyses were run for about % more time than that required by Faraday's laws. Work-up con- sisted of addition of the electrolyte to water followed by repeated ex­ traction of the organics by ether. The combined ether extracts were washed with water several times, washed with a saturated salt solution and dried over magnesium sulphate. In the,following portion of the experimental, the treatment of the * 157 * In some cases the acidification of the water was found to be helpful in eliminating emulsions. dried ether extracts from the various reactions will be described,, The reactions were frequently carried out more than once in different cells with different electrode assemblies. Further, some of the reactions were not carried out by the author, although he supervised their running. Thus, in the following section of the experimental, only the electrode assembly types (designated as A,B,C or D), the approximate ratio (1:1:5, 1:2:6, etc.) of metal (ca. 0.1 M) to added ligand, to substrate, the nature- of the metal and ligand, the temperature of the reaction (if not room temperature), and the product isolation procedure shall be given. As noted, the approximate concentration of metal salt and supporting electrolyte was 0.1 M. Since the applied potential for any electrolysis is dependent on electrode assembly construction and many electrode assemblies were used, only guideline values for applied potentials will be given. For the electrode assemblies A,C and D an applied voltage in the range of ca. .8-1.5 volts was sufficient to maintain•a current of 50 mAo For electrode assembly B, an applied potential of 5-4 volts was required to maintain a current of 50 mA. Since it was felt this potential would be apt to produce side reactions, an applied voltage of 1.5 to 2.2 volts, corresponding to a current of between 30 and 40 mA was 158 used Table 3 Tabulation of Experimental Data from Electrochemical Reactions Substrate■ Electrode Assembly (Reaction Temperature) type (cathode/anode) Metal /Ligand/Substrate,s Work-upC 0CH2C1 (RT)* hyRByI Lld: A (Al/Al) C (Cu/^i) Ni (I)/Ppf5(A)/S(Il) Ni(l)/C0D(1.4)/8(7.45) solvent removed from ether extract,0 re­ crystallized from 95$ ethanol 0CHC1 (RT) C (Cu/Ni)a NiZRp5ZS0 solvent removed from ether extract under PfCH2Br (RT) C (Cu/Ni)d NiZPp5ZS0 aspirator at O0C, residue sublimed PfCH2I (RT) B (Cu/Ni) NiZRp5ZS0 ether extract sub­ jected to gas chrom- . atographic analysis pfBr (50-60°C) C (Cu/Ni) NiZRp5Zs*' solvent removed from ether extract, re- pfBr (50-60*C) A (Al/Al) Ni(I)ZPp (1)/S(8.2) ■ crystallized from 95% ethanol ■ pfC-Gl (RT) O C (Cu/Ni) Ni(I)/C0D(.7)/S(7.4) solvent removed from ether extract, pfO-Cl (RT) A (Al/Al) Ni/Pp^/Sg ' residue sublimed pG-CH Br (RT) B (Cu/Ni) Ni(I)/Rp3(I)Zs(8.2) solvent removed.from ether extract, re­ crystallized from methanol Work-upSubstrate Electrode Assembly Metal/Ligand/Substrate,s (Reaction Temperature) type (cathode/anode) CH5(CH2)4CH2Br (950C) (Br/formyl substitution) A (Al/Al) Zn( ca.. 18M),/S ( ca.. SM) ether extract was added dropwise to hot acetic acid, kept on steam bath /2 hour, solvent re­ moved. Recrystallized from ethanol-water. n-hexyl Br (33°C) (coupling) B (Cu/Al) Fe(l)/P/5(1.35)/S(2.5) ether extract sub­ jected to gas chroma­ tographic analysis /CH2Cl (RT) (carbonyl insertion) A (Al/Al) Ni(I)/Co atm./S(12) ether extract evapor­ ated and residue chrom­ atographed on silica gel. Use hexane to elute non-polar com­ ponents and use 15 vol% /H/85 vol$ hexane to get /CHpCOCHp/ product. /CH2Cl (RT) D (Cu/Al) stream m ( i ) / ^ % / n ( i 2 ) z pressure / ether extract concen­ trated and analyzed by thin layer chromato­ graphy. a RT = room temperature b concentration of metal equals ca. .1 molar unless otherwise noted c for origin of ether extract see general work-up procedure in General Procedure (previous) section d the same electrode assembly and cell were used for these two reactions e these reactions were run by Mr. Gary Hass f This reaction was run by Mr. Dennis Miller g This reaction was run by Mr. John Kiely APPENDICES APPENDIX I Nuclear Magnetic Spectral Data of Dimers Obtained from Ni(CO) (Pgf) and 7-MethyINBD with Thermal Excitation and Their Structural Assignments Prior to assigning structural features responsible for resonances found in the spectra some assumptions and their bases should be noted. When R group anti to olefin, olefinic protons are a "clean" triplet. When R group syn to olefin, olefinic protons are a perturbed triplet, usually a septet. Basis: I) NliR assignments of the two compounds shown; structures are based on oxymercuration studies. °° olefin protons clean triplet olefin protons perturbed triplet 2) Integration of these assignments and those for a number of 7-substituted norbornenes and norbomadienes. ° Assumption B: Bridge protons syn to an "exo" bonded cyclobutane ring resonate at relatively low field compared to protons syn to an endo" bonded cyclobutane. 16) Basis: I) Comparison with assignments for dimers from NBD itself and the corresponding saturated dimers. ?6iii 2) Assignments for resonances of protons over (low field) and edgewise (high field) to cyclobutane r i n g s . 9 Based on these assumptions, and the assignments (vide infra) from refer­ ence ?6iii for norbornadiene and norbornene dimers, preliminary assign­ ments may be made. Chemical Shifts (6) in Norbornene Derivatives 1.32 1.06 1.23 1.69 H ,H 1.32 // U H 5.97 H 2.62 exo trains exo xtx 1.02 1.55 bridge protons not assigned exo trains endo xtn endo trans endo ntn HCT\ -P- ,a H H 1.59 H (1.25- (L.50 1.8 a = mean value for both fragments 165 Preliminary Structural Assignments of Selected 7-MethyINBD Dimers Sample in CDCl Olefin(s) MethylCs) Cyclobutane(s) Bridgeheads Bridges XLlV 5.84 dSept ■ .84 D 1.45 M 2.38 M 2.35 Q integration 4 6 4 6 XLV-A 6.22 T .69 D 1.21 M 2.51 M, 2.33 M . 5»75 dSept .87 D 2.12 M, 2 ,01 M integration 2 5 2 8 2 5 XLVI 6.02 dSept .85 D 1.17 M, 2.54 M, 2,50 M, 2.08 ..M, 5.66 dSept 1.84 M integration 2 6 2 3 2 2 2 L XLVII 6.54 T .53 1) 2.41 M 1.94 M, 1.67 M 6.19 dSept .80 D integration 2 3 ■ 4 6 2 3 dSept = dirty septet D = doublet M = multiplet Q = quartet T = triplet Assignment for Dimer XLIV: a) One kind of olefin at 5=846 dSept, therefore methyl group syn to olefin. ■ • b) One kind of methyl at .846 D c) One kind of cyclobutane at 1.456 M Therefore, highly symmetric dimer either xtx or ntn skeleton; the chemical shift for the olefin supports an assignment of xtx. NBD dimer xtx 5«97» ntn 6.55« 166 H Me Note the similarity between chemical shift of methyl group here (.84) and in 7-methyl NBD (.87). Assignment for Dimer XLV-A: a) Two kinds of olefins i) Triplet 6.22 6 ii) Septet 5.73 6 b) Two kinds of methyl i) Doublet .69 6 ii) Doublet .87 6 c) Two kinds of cyclobutane d) Unsymmetrical; suggests xtn skeleton, of. NBD xtn olefins - exo fragment 5*82 6 endo fragment 6.20 6 e) Assign 6.22 to endo fragment, and 5»73 to exo fragment •F \ ri 4-vi4 4- ^f) Since 6.22 is a triplet, and since 5»73 is a septet, g) Thus assignment is: 16? Methyl groups are assigned as shown based on*. i) exo methyl would be closest to methyl in 7-methyl (.87) ii) the methyl on the endo portion ought to feel influence of the edge of the cyclobutane ring, therefore it ought to be shifted upfield and it is (.69). Assignment for Dimer XLVI: a) Two kinds of olefins b) i) Dirty septet 6.02 ii) Dirty septet 5.66 so both fragments have methyl syn to the olefin b) Although in DCCl^ only one methyl appears to be present, in Dg benzene two kinds of methyl are present(.97 D, and .92 D). c) Two types of cyclobutanes d) Probably xtn skeleton; cf. NBD xtn olefins - exo 6.20, endo 5.82 Since neither methyl feels direct influence of cyclobutane ring, both would be similar (.85) to 7-methyI NBD (.87) Assignment for Dimer XLVII: a) Two kinds of olefins i) Triplet 6.54 6 ii) Septet 6.19 & 168 b) Two kinds of methyl i) .53 6 ii) .80 6 c) Cyclobutane protons uncertain but no resonances at higher field than 1.5 ? ; also note assignments for XLIV, XLV-A and XLVI where it appears the cyclobutane protons at an exo norbornene cyclobutane fusion resonate at a higher field than 1.56. This is also true in NBD dimers. Likewise in NBD dimers, protons at the fusion of an endo norbornene-eyelobutane are at lower field than 1.50 6. d) Thus in light of these considerations and the chemical shifts for the olefins, an endo-trans-endo skeleton is suggested. e) Comparing the olefin chemical shifts: 169 Me XLV-A 5.73 H II 6 = 22 5.82 H H 6.20 Me € 6.02 XLVI 5.66 H H 6.35 XLVIi 6.19, 6.34 The most reasonable possibility is an ntn skeleton with one methyl syn and one anti to the olefins. One methyl is quite close to the 7-methyI NBD value (.87 ) while the other is shifted upfield considerably (edge effect of cyclobutane). The olefins may also be assigned since the 6.34 resonance is a triplet (a methyl is anti to it) and likewise the 6.19 resonance should be a septet. Notice that when a methyl is anti to an olefin its resonance is about that of the NBD dimer, while the presence of a syn methyl group shifts the resonance to a higher field. Thus, 80 Mo 6.19 H H 6.34 170 Dimer NBD (olefin) 7-methyINBD (olefin) xtx 5.97 syn,syn dimethyl xtx 5«84 xtn exo 5.82 endo 6.20 syn,anti dimethyl xtn exo 5«73 (syn) endo 6.22 (anti) syn,syn dimethyl xtn exo 5«66 (syn) endo 6.02 (syn) ntn 6.55 syn,anti dimethyl ntn 6,19 (syn) 6,54 (anti) * Methyl syn and anti relative to olefin bond of that fragment ' This can also be seen in the parent 7-methyINBD vs NBD, 7-methylNBD 6.715 triplet (methyl anti) 6.47 septet (methyl syn) NBD 6.7 septet The structures for two dimers remain to be assigned. The NMB data is given below. XLV-B 5.98 dSept .85 D i80 D 3.30 Q 2.38 M a series of res­ onances at 2.25- 0.75 integration 2 5 3 I 2 9 XLIX 5.74 dSept .84 D .75 D 2.5 M a series ances at of reson- 2.3-0.7 M .39 D integration 2 5 3 4 6 2 Based on the relative integration values for methyls and olefins (6:2), it appears there is only one olefin bond in these dimers, suggesting 2+2+2 dimers. This tends to be confirmed by the presence of high field (less than 1.0) resonances expected from the cyclopropane moiety which is found in 2+2+2 dimers 171 The following assignments have been made f o r 2 + 2 + 2 dimers of NBD Assignment for Dimer XLV-B: a) Olefin 5*98 septet, methyl syn to olefin b) A quartet (one proton) appears at low field (5*306). The coupling constant for this quartet matches the coupling constants for the methyl groups. c) In only one dimer, A, does the norbornene hydrogen, anti to the olefin, appear at an unusually low field. Since the published spectra of the NBD 2+2+2 dimers show no resonances which could 172 reasonably correspond in multiplicity to the quartet at 3.30 6 in XLV-B1 this quartet must be the result o f a methyl group splitting a bridge hydrogen. Thus the structure is assigned as: Me ■Me Assignment for Dimer XLlX: a) Olefin septet at 3*74 &, so methyl syn to olefin. b) Two cyclopropyl hydrogens at unusually high field (0.39'6). c) Only one 2+2+2 NBD dimer (D) has such high field cyclopropyl protons. Furthermore, D shows a relatively high field olefin resonance (5*886) which would be shifted to higher field by the presence of a syn methyl group. Thus the structure is assigned as: Me 173 It may also be noted these assignments are consistent with behavior of the dimers on silver ion impregnated silica gel plates: XLV-A and XLVII retained with silver nitrate; XLIV, XLV-B1 XLVI and XLIX not retain­ ed with silver nitrate. Decreasing order of retention with silver perchlorate: XLIV »XLVI »XLV-B K XLIX It is most likely that dimers with the exo side of the olefin free from the shielding of a methyl group ought to be bonded the strongest; XLV-A and XLVII are retained on silver nitrate while the others are not. It is also probable that any complexing that the remaining dimers show is due to endo side bonding of the dimer to the rawer silver ion of silver perchlorate. Further, to account for the given order of retention it is suggested an exo bonded cyclobutane either decreases the steric hinderance to endo bonding or the edge of the cyclobutane actually pro­ vides some electron density to aid complexation. Thus dimer XLIV has two sites for this type of bonding to occur while dimer XLVI has only one (an endo bonded cyclobutane only provides steric hinderance). 1?4 Dimers XLV-B and XLIX do not have any available double bonds to complex with and are not retained even with silver perchlorate. APPENDIX II Derivation of Equation Relating Gas Chromatographic Data All experimental data was reduced to ratios, relating the dimer concentrations of all samples to the concentration of the 2+2 xtx dimer in one particular experimental sample (base sample) containing both the 2+2 xtx and 2+2 xtn dimers = The 2+2 xtn values were related first to the 2+2 xtn value in the base sample. They were then related to the xtx value in the base sample by the ratio of xtn to xtx in the base sample. It was assumed xtx and xtn would analyze similarly using a flame ionization detector. This assumption is supported by noting the heat of combustion of the two isomers differs only by 2.4%.^^ It is also supported by the fact that a comparison of the ratios from a gas chromatographic analysis agrees with integration data of the olefin NMR peak spectra for the two dimers. The molar concentration of the 2+2 xtx dimer in the base sample was determined by use of a calibration curve prepared using known amounts of pure 2+2 xtx dimer. The following symbols will be used in the derivation of the equation. [A° = concentration of 2+2 xtx dimer in base sample = concentration of 2+2 xtx dimer in base sample after it is diluted with a benzene solution of standard (dodecane) Thus A0 refers to data for base sample before dilution and A^ refers to data for base sample after dilution. jl3°j = concentration of 2+2 xtx dimer in any other experimental sample 176 [B"J = concentration of 2+2 xtx dimer in this sample after it , . ">v, is diluted with a benzene solution of standard Wodecane) Thus B° refers to data for any other particular sample prior to dilution, whereas refers to data for that sample after dilution. S refers to standard (dodecane) related data X refers to 2+2 xtx related data CNTS (counts) refers to the integrated area (in arbitrary units) under a particular gas chromatographic peak. OBS refers to an observed variation ATT refers to an attenuation setting on the gas chromatograph INJ = injected sample OBS CNTS of S in A1 U OBS CNTS of S in B1 a OBS CNTS of X in A1 CG OBS CNTS of X in B1a npa£] (ml of A1 INJ) (ATT. A1 |Sgl] '(ml of B1 INJ)(ATT. B1 [XAf] (ml of A1 INJ) (ATT. A1 Jigl] (ml of B1 INJ)(ATT. B1 for S) for S) for X) for X) OBS CNTS of S in B1 = S i (ml of B1 INJ)(ATT. B1 for'S) OBS CNTS of S in A1 E (ml of A1 INJ)(ATT. A1 for S) ml of B1 INJ (OBS CNTS of S in B1) Iv (ATT, A1 for S) ml of A1 INJ (OBS CNTS of S in A1) CsB b (ATT. B1 for S) (I) Thus the sizes of injected samples can be related to the size of the in­ jected sample of diluted base sample. Similarly: 177 XeI xA1 (OBS CNTS of X in B1)(ATT0 A1 for X) I (DBS CNTS of X in A1)(ATT. B1 for X)/ml of B1 INJ Iml of A1 INJ (2) Substitution of equation I into equation 2, and assuming: ATT. A1 for S = ATT. A1 for X I IATT. B for S = ATT. B x for X Gives: _ OBS CNTS of X in B1/ OBS CNTS of S in B1 yE! OBS CNTS of X in A1/ OBS CNTS of S in A1 fq ■- (3) Thus the Xgl/X^l ratio is independent of ATT. values so long as they are consistent for any given run (ie. ATT. S^ = ATT. X^). We want jx^ Tj ? not Jx^ lJ , that is, the actual concentration of X in the undiluted experimental sample. In making up the diluted samples: jx^ lj = (ml of X^0 added) X ^ ________________________ ml of X^0 added + ml of standard solution added to X^0 [Xgl] = (ml of X30 added) [x^________________________________ ml of Xg0 added + ml of standard solution added to Xg0 = ml of X^0 added mLgA = ml of standard solution added to X^0 ralXB0 “ ml 01 xBo a^dea ml „ = ml of standard solution added to let - mlKA= 178 r q 3 = '(mLXAo) r q g f v3 _ (mlXBo) feg] mlXA0 + mlSA mlXB0 + iplSB From these two relationships one can write; i + - SB XB0/ ml. I + ■ SA l3ICAc (4) Substituting equation 3 into equation 4: , /oBS CNTS of X in BlN i + alXBc k<, a= J . Chatt and B= L= Shaw, J= Chem= Soc = , 705 (1959) ° bo G= E= Coates, M= L= H= Green, K= Wade, "Organometallic Compounds", Vol III, Methuen, London, I960= A brief statement of the theory is given by: Co G= W= Parshall and J « J= Mrowca in " o'-Alkyl and.o— Aryl Deriva­ tives of Transition Metals", Advances in Organometallic Chemistry, Po Ge A= Stone and E= West, ed=., Academic Press, New York, Vol 7, 1968, p. 176. 55« G= Hass, "Organometallic Electrochemistry", 55rd Annual Meeting of Montana Academy of Sciences, Dillon, Montana, April 27, 1975, Physical Sciences Section, Paper 16. 56» Spectrum 25265, Catalog of Infrared Spectrograms, Samuel P= Sadtler, Philadelphia, Pa.= 57« For 1:1 DMF/A1C1 complex (K = 8=15), see: 5 eH C= V= Pham and V= M. Samoilenki, Ukr Khlm = Zh., 57, 642 (1971), Chem Abs=, 75, l4424n (1971)« 58. Go Ei= Coates, M= L= H= Green and K= Wade, "Organometallic Compounds", Vol= I, The Main Group Elements, Methuen, London (1967)« 59« a= R= Joly and R= Bucourt, U= S= patent 5,040,079*, Chem Abs =, 58, 10258 (1965). bo R= Joly and R= Bucourt, Compt= Rend=, 254, 1655 (1962)= 40= T= Yamamoto, A= Yamamoto and S= Ikeda, J= Am. Chem= Soc., 91, 3350, 3360 (1970). 1977 • ' ' / >op.2 Pillsbury, Dale G A study of stereochemical and electronic factors .. N A M K A N D