Scholarly Work - Chemistry & Biochemistry
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Item Solvent coordination to palladium can invert the selectivity of oxidative addition(Royal Society of Chemistry, 2022-01) Elias, Emily K; Rehbein, Steven M; Neufeldt, Sharon R.In the presence of the bulky monophosphine PtBu3, palladium usually prefers to react with Ar–Cl over Ar–OTf bonds. However, strongly coordinating solvents can bind to palladium, inducing a reversal of selectivity.Item Combined Experimental and Computational Mechanistic Investigation of the Palladium-Catalyzed Decarboxylative Cross-Coupling of Sodium Benzoates with Chloroarenes(American Chemical Society, 2021-08) Humke, Jenna N.; Daley, Ryan A.; Morrenzin, Aaron S.; Neufeldt, Sharon R.; Topczewski, Joseph J.Reported herein is a mechanistic investigation into the palladium catalyzed decarboxylative cross-coupling of sodium benzoates and chloroarenes. The reaction was found to be first order in Pd. A minimal substituent effect was observed with respect to the chloroarene and the reaction was zero order with respect to chloroarene. Palladium mediated decarboxylation was assigned as the turn-over limiting step based on an Eyring plot and DFT computations. Catalyst performance was found to vary based on the electrophile, which is best explained by catalyst decomposition at Pd(0). The COD ligand contained in the precatalyst CODPd(CH2TMS)2 (Pd1) was shown to be a beneficial additive. The bench stable Buchwald complex XPhos-PdG2 could be used with exogenous COD and XPhos instead of complex Pd1. Adding exogenous XPhos significantly increased the catalyst TON and enhanced reproducibility.Item Experimental and Computational Evaluation of Tantalocene Hydrides for C–H Activation of Arenes(American Chemical Society, 2021-08) Rehbein, Steven M.; Kania, Matthew J.; Neufeldt, Sharon R.Half a century ago, tantalocene hydrides (especially Cp2TaH3, where Cp = η5-C5H5) were reported to catalyze H/D exchange with arenes. However, there has been very little follow-up to the seminal reports, and numerous questions about this chemistry remain unanswered. In an effort to better evaluate the potential of tantalocene hydrides for processes involving C–H activation, we have conducted a series of experimental and computational studies on these complexes. Density functional theory (DFT) calculations support a mechanism for arene C–H activation involving oxidative addition at transient TaIII, rather than a σ-bond metathesis mechanism at TaV. Comparisons were made between thermal and photochemical conditions for the reaction of Cp2TaH3 with benzene-d6, and H/D exchange was found to be moderately faster under thermal conditions. In a reaction with toluene, Cp2TaH3 activates the aromatic C(sp2)–H bonds but not the benzylic bonds. DFT calculations suggest that benzylic C–H activation at TaIII has a barrier similar to aromatic C–H activation, but that formation of a π-complex with Cp2TaH directs preferential aromatic C–H activation. Analogous complexes containing the less labile permethylated ligand Cp* (Cp* = η5-C5Me5) were also evaluated for their ability to catalyze H/D exchange with benzene-d6, but these complexes are less active than Cp2TaH3. DFT calculations indicate that the methyl groups of Cp* disfavor π-coordination of an arene to the TaIII intermediate.Item Mechanistic Investigation into the Gold-Catalyzed Decarboxylative Cross-Coupling of Iodoarenes(American Chemical Society, 2021-07) Daley, Ryan A.; Morrenzin, Aaron S.; Neufeldt, Sharon R.; Topczewski, Joseph J.While many gold catalyzed reactions have been thoroughly developed, most are not thought to involve redox events at gold. In contrast, recent advances have demonstrated the feasibility of redox gold catalysis. This report describes a detailed mechanistic investigation of the gold catalyzed decarboxylative cross-coupling, which likely proceeds via a high valent Au(I/III) pathway. This investigation includes a kinetic analysis, stoichiometric experiments with Au(III) complexes, and DFT calculations. These data support a turnover limiting oxidative addition to form an Au(III) aryl complex, with an off cycle resting state. The dominant pathway appears to proceed through a silver mediated decarboxylation with a subsequent Ag(I) to Au(III) transmetalation. These data provide some rationale for the significant counterion effects between SbF6– and NTf2– and may explain why MeDalphos is not a superior ligand for the catalytic reaction.Item C–O-Selective Cross-Coupling of Chlorinated Phenol Derivatives(Georg Thieme Verlag KG, 2021-05) Neufeldt, Sharon R.; Russell, John E. A.Chemoselective cross-coupling of phenol derivatives is valuable for generating products that retain halides. Here we discuss recent developments in selective cross-couplings of chloroaryl phenol derivatives, with a particular focus on reactions of chloroaryl tosylates. The first example of a C–O-selective Ni-catalyzed Suzuki–Miyaura coupling of chloroaryl tosylates is discussed in detail.1 Introduction2 Density Functional Theory Studies on Oxidative Addition at Nickel(0)3 Stoichiometric Oxidative Addition Studies4 Development of a Tosylate-Selective Suzuki Coupling5 Conclusion and OutlookItem Solvent Effects on the Selectivity of Palladium‐Catalyzed Suzuki‐Miyaura Couplings(Wiley, 2020-03) Reeves, Emily K.; Bauman, Olivia R.; Mitchem, Gunner B.; Neufeldt, Sharon R.The use of polar solvents MeCN or dimethylformamide (DMF) was previously shown to induce a selectivity switch in the Pd/PtBu3-catalyzed Suzuki-Miyaura coupling of chloroaryl triflates. This phenomenon was attributed to the ability of polar solvents to stabilize anionic transition states for oxidative addition. However, we demonstrate that selectivity in this reaction does not trend with solvent dielectic constant. Unlike MeCN and DMF, water, alcohols, and several polar aprotic solvents such as MeNO2, acetone, and propylene carbonate provide the same selectivity as nonpolar solvents. These results indicate that the role of solvent on the selectivity of Suzuki-Miyaura couplings may be more complex than previously envisioned. Furthermore, this observation has the potential for synthetic value as it greatly broadens the scope of solvents that can be used for chloride-selective cross coupling of chloroaryl triflates.Item Chemodivergence between Electrophiles in Cross‐Coupling Reactions(Wiley, 2021-04) Reeves, Emily K.; Entz, Emily D.; Neufeldt, Sharon R.Chemodivergent cross-couplings are those in which either one of two (or more) potentially reactive functional groups can be made to react based on choice of conditions. In particular, this review focuses on cross-couplings involving two different (pseudo)halides that can compete for the role of the electrophilic coupling partner. The discussion is primarily organized by pairs of electrophiles including chloride vs. triflate, bromide vs. triflate, chloride vs. tosylate, and halide vs. halide. Some common themes emerge regarding the origin of selectivity control. These include catalyst ligation state and solvent polarity or coordinating ability. However, in many cases, further systematic studies will be necessary to deconvolute the influences of metal identity, ligand, solvent, additives, nucleophilic coupling partner, and other factors on chemoselectivity.Item Gold Catalyzed Decarboxylative Cross-Coupling of Iodoarenes(2020-07) Daley, Ryan A.; Morrenzin, Aaron S.; Neufeldt, Sharon R.; Topczewski, Joseph J.This report details a decarboxylative cross-coupling of (hetero)aryl carboxylates with iodoarenes in the presence of a gold catalyst (>25 examples, up to 96% yield). This reaction is site specific, which overcomes prior limitations associated with gold catalyzed oxidative coupling reactions. The reactivity of the (hetero)aryl carboxylate correlates qualitatively to the field effect parameter (Fortho). Reactions with isolated gold complexes and DFT calculations support a mechanism proceeding through oxidative addition at a gold(I) cation with decarboxylation being viable at either a gold(I) or a silver(I) species.Item Small Phosphine Ligands Enable Selective Oxidative Addition of Ar–O over Ar–Cl Bonds at Nickel(0)(2020-08) Entz, Emily D.; Russell, John E. A.; Hooker, Leidy V.; Neufeldt, Sharon R.Current methods for Suzuki-Miyaura couplings of nontriflate phenol derivatives are limited by their intolerance of halides including aryl chlorides. This is because Ni(0) and Pd(0) often undergo oxidative addition of organohalides at a similar or faster rate than most Ar–O bonds. DFT and stoichiometric oxidative addition studies demonstrate that small phosphines, in particular PMe3, are unique in promoting preferential reaction of Ni(0) with aryl tosylates and other C–O bonds in the presence of aryl chlorides. This selectivity was exploited in the first Ni-catalyzed C–O-selective Suzuki-Miyaura coupling of chlorinated phenol derivatives where the oxygen-containing leaving group is not a fluorinated sulfonate such as triflate. Computational studies suggest that the origin of divergent selectivity between PMe3 and other phosphines differs from prior examples of ligand-controlled chemodivergent cross-couplings. PMe3 effects selective reaction at tosylate due to both electronic and steric factors. A close interaction between nickel and a sulfonyl oxygen of tosylate during oxidative addition is critical to the observed selectivity.Item Ligation state of nickel during C-O bond activation with monodentate phosphines(2018-11) Hooker, Leidy V.; Neufeldt, Sharon R.The oxidative addition of phenolic electrophiles at Ni(0) in the presence of monodentate phosphine ligands was studied with both dispersion-free and dispersion-containing DFT methods. With the popular bulky ligand PCy3, consideration of dispersion has a striking effect on the predicted ligation state of nickel during oxidative addition of aryl sulfamates. Dispersion-containing methods such as M06L indicate a clear preference for a bis-phosphine ligated transition state (TS), while dispersion free methods like B3LYP strongly favor a mono-phosphine ligated TS. This discrepancy in predicted ligation state is also found with small phosphines (PMe3) in combination with some aryl electrophiles (carbamates, acetates, pivalates, chlorides), but a bis-PMe3-ligated TS is predicted regardless of dispersion for other electrophiles (sulfamates, mesylates, tosylates). DFT calculations that include dispersion also offer a possible explanation for the observed poor efficacy of PtBu3 as a ligand in Ni-catalyzed cross-coupling reactions.