Development of hydrogen-mediated carbon-carbon bond formations
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Elemental hydrogen is the cleanest and most cost effective reductant available to mankind. Although catalytic hydrogenation has been practiced industrially on an enormous scale, use of hydrogen as a terminal reductant in catalytic C-C bond formation is restricted to only processes involving migratory insertion of carbon monoxide, e.g. alkene hydroformylation and the Fischer-Tropsch reaction. Given the profound socioeconomic impact of these processes, the development of general methods for hydrogen-mediated C-C bond formation under CO-free conditions represents a paramount scientific challenge. Hence, the interception of organometallic intermediates in the course of catalytic hydrogenation and subsequent C-C bond formations have become of great interest. Herein, related to hydrogenative C-C bond formation, the historical overview of transition metal catalyzed reductive cyclizations in the presence of reducing agents such as silane, borane, alane, stannane, and hydrogen is presented. Following chapters describe the research aimed at the development of hydrogenative C-C bond formation: 1) vii hydrogen-mediated intramolecular reductive cyclizations of 1,6-diynes and 1,6-enynes, 2) hydrogen-mediated intermolecular reductive couplings of π-unsaturated precursors (dienes, enynes, and diynes) with α-keto aldehydes (glyoxals), and 3) hydrogen-mediated intra- and intermolecular reductive aldol couplings. These results clearly demonstrate that hydrogen can be utilized for reductive C-C bond formation in good yield with high selectivity. In addition, mechanistic studies of reductive cyclizations of 1,6-enynes via crossover experiments and ESI-MS studies verify the mechanism of this transformation. Although several limitations of this methodology need to be resolved, the mechanistic insight achieved from these new transformations provide valuable information for future development.