Coupling reactions and hydrogenations catalyzed by abundant transition metal complexes
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The field of homogeneous catalysis in the laboratory and on an industrial scale is dominated by transition metal catalysts. Most of these transformations require precious metal (PM) catalysts. However, due to their cost, toxicity, scarcity, and risk of depletion, research is underway to find chemistry that enables the replacement of PMs with abundant metal catalysts. Palladium (0) compounds have long been established as the catalyst of choice for most cross-coupling reactions. Lagging considerably behind the development of palladium-based systems, certain nickel-based compounds (particularly nickel compounds in (II) and (0) oxidation states) can also catalyze these reactions, potentially offering a lower cost and access to radical pathways. In this regard, the first half of the thesis describes the cross-coupling reactions by nickel-based catalysts. The study involves the comparison of Ni(0), Ni(I) and Ni(II) complexes containing triphenylphosphine (PPh3) ligands for their abilities to effect the various cross-coupling reactions. The Ni(0) complex was found to be catalytically active for representative Suzuki-Miyaura and Heck-Mizoroki reactions, and demonstrated comparable yields with many palladium-based catalysts. However, the Ni(I) complex seemed to convert to catalytically active Ni(0) species under Suzuki−Miyaura reaction conditions and it was found to be ineffective for Heck−Mizoroki cross-coupling. During this investigation, the paramagnetic Ni(I) complexes NiX(PPh3)3 (X = Cl, Br, I) were characterized for the first time by 1H NMR spectroscopy. The focus of the second half of the thesis is on the development of abundant-metal based homogeneous catalysts for the hydrogenation of amides. Catalysts were formed in–situ from abundant-metal salts and phosphine ligands, and these were screened for the deoxygenative hydrogenation of amides. It was found that Co(BF4)2·6H2O with triphos (1,1,1-tris-(diphenylphosphinomethyl)ethane) and Yb(OTf)3·H2O as co-catalyst were found to be efficient for the hydrogenation of various amides. In the final chapter, 1st and 2nd row transition abundant-metal complexes, generated in-situ, were tested as catalysts for the hydrogenation of carboxylic acids. MoCl3 and MoCl5 in combination with tetraphos ligand resulted in hydrogenation of 2-phenoxyacetic acid in higher yields (> 95 %). Cobalt precursors Co(acac)2 and Co(OAc)2, in combination with dcpe ligand, and Co(BF4)2·6H2O with triphos ligand hydrogenated 3-phenylpropionic in greater yields.