Carbon-Heteroatom Reductive Elimination and Catalysis Utilizing (POCOP)Rh and (POCOP)Co Complexes
Timpa, Samuel D
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Transition metal catalyzed cross-coupling reactions of aryl halides have revolutionized the synthesis of organic molecules. These reactions, which are commonly catalyzed by group 10 metals, have found applications including natural product synthesis, pharmaceuticals, and agrochemicals. Pd catalyzed cross-coupling reactions have undergone the greatest development due to their wide applicability, high efficiency, and selectivity. The success of Pd is attributed to its ability to readily traverse between Pd(0) and Pd(II) oxidation states, which is essential to the mechanistic steps oxidative addition and reductive elimination. The utility of transition metals outside of group 10 has largely been limited to Cu, but more recently several examples of Rh catalyzed cross-coupling reactions have been described. These examples propose a Rh(I)/Rh(III) cycle analogous to the Pd(0)/Pd(II) catalytic cycle involving aryl halide oxidative addition, transmetallation, and product forming reductive elimination; however, there has been little experimental evidence to support these claims. Examples of aryl halide oxidative addition to Rh(I) have been reported, but examples of reductive elimination from Rh(III) are less prevalent. Pincer ligands, tridentate ligands that typically coordinate in a meridional fashion, provide an excellent scaffold for the examination of both oxidative addition and reductive elimination at Rh due to their ability to access to three-coordinate Rh(I) and stable five-coordinate Rh(III) complexes. The ability of the (PNP)Rh center to undergo each of the stoichiometric reactions of catalytic C-C coupling reactions, including aryl halide oxidative addition and C-C reductive elimination, has been established. This dissertation describes the ability of the (POCOP)Rh system to catalytically form C-C as well as C-N and C-S bonds. Several proposed catalytic intermediates have been isolated and their reactivity examined to gain insight into the mechanism of these catalytic transformations. C-N and C-S reductive elimination from Rh(III) have been closely examined, with results providing insight to their respective steric and electronic properties. In addition, the potential for (POCOP)Rh systems to undergo C-F reductive elimination were also examined both theoretically and experimentally. Finally, early investigations into the synthesis of (POCOP)Co complexes will be described, with an emphasis on demonstrating the aptitude for this system to experience concerted reductive elimination. Numerous (POCOP)Co complexes were isolated and characterized, including Co(II) and stable Co(III) compounds.