Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item Synthetic and mechanistic strategies to achieve unconventional site-selectivity in cross-couplings of dihalo-heteroarenes(Montana State University - Bozeman, College of Letters & Science, 2024) Norman, Jacob Patrick; Chairperson, Graduate Committee: Sharon Neufeldt; This is a manuscript style paper that includes co-authored chapters.Pd-catalyzed cross-couplings rank among the most powerful methods for constructing substituted biaryls, polyaryls, and heteroarenes. Frequently, di- or polyhalogenated (hetero)arenes are employed as starting materials in cross-couplings to access products with increased structural complexity via multiple cross-coupling or substitution steps. N-heteroarenes bearing multiple reactive handles--such as halides, are of particular interest as starting materials since their cross- coupled products can be medicinally relevant. Non-symmetrical dihalogenated N-heteroarenes typically exhibit a site-selectivity bias for C-X bonds which are adjacent to at least one heteroatom in Pd-catalyzed cross-couplings. However, some Pd catalysts--particularly those with hindered ligands, promote atypical selectivity at distal C-X bonds of 2,X-dichloropyridines and related heterocycles during the selectivity-determining oxidative addition step. This dissertation explores the mechanistic origins of these ligand trends and emphasizes the critical importance of Pd's ligation state--either mono (PdL) or bis (PdL 2), in controlling the site of oxidative addition. Ligation state is also relevant when selecting for the products of mono- vs difunctionalization in cross-couplings of dihalogenated substrates, since bisligated 14 e - Pd dissociates quickly from the monofunctionalized intermediate after an initial cross-coupling cycle, whereas monoligated 12 e - Pd is slow to dissociate and may "ring-walk" to the remaining reactive site(s). Additionally, this dissertation explores alternative methods to access minor regioisomers in cross-couplings of dichloro-azines. One approach involves ligand-free conditions where atypical site-selectivity at dichloropyridines and dichloropyrimidines arises from a change in Pd's speciation from mono- to multinuclearity. Another approach employs a thiolation/Liebeskind-Srogl arylation sequence to achieve site-selectivity which is orthogonal to that of Suzuki-Miyaura couplings.Item Multi-edge X-ray absorption near-edge spectroscopic analysis of palladium complexes in II, III and IV oxidation states(Montana State University - Bozeman, College of Letters & Science, 2013) Barton, Rhonda Lee Hoffert; Chairperson, Graduate Committee: Robert K. SzilagyiPalladium-based complexes have profoundly impact on the synthetic tools of organic chemists due to their importance as catalysts in a myriad of chemical transformations. Palladium in the 0, II, III and IV oxidation states have all been experimentally observed to have catalytic activity in carbon-carbon bond coupling reactions. A common organometallic research aim is to improve catalytic activity of these complexes by designing and optimizing new ligand systems to access more difficult transformations. In order to understand the electronic effects that ligand systems have on reactivity, X-ray absorption spectroscopy is used to characterize the electronic structure of the ligand and metal components of pre-catalysts and palladium model complexes. The multi-edge X-ray absorption near-edge absorption spectroscopic technique (XANES) is an element specific technique that excites core electrons of the 1s (K-edge) and 2p (L-edge) orbitals to frontier unoccupied molecular orbitals, providing a ground state picture of a complex's ligand and metal electronic structure. This thisis will describe a comparative analysis between homoleptic chloropalladium complexes and interesting heteroleptic palladium based complexes of II, III and IV oxidation states to understand the stabilizing effects of a unique ligand environment. Furthermore, it will emphasize the benefits of using multi-edge XANES technique in rationalized catalyst design.Item Mechanisms of platinum(II) and palladium(II) catalyzed organic transformations : hydration of alkynes and the cope rearrangement(Montana State University - Bozeman, College of Letters & Science, 1993) Hiscox, William Charles