Regioselective Hydrosilylation of Olefins Catalyzed by Co-Iminobipyridine Complexes: The Role of Cyclohexyl Substituent on the Imino Nitrogen

Abstract

Abstract Regioselective hydrosilylation of aliphatic olefins catalyzed by Co-iminobipyridine complexes, Co〈R〉, were investigated, where R indicates a substituent on the imino nitrogen in an iminobipyridine ligand (iminobypyridine = [2,2′-bpy]-6-C(Me)=N-R). Syntheses of two complexes, Co〈Mes〉 (Mes = 2,4,6-trimethylphenyl) and Co〈Cy〉 (Cy = cyclohexyl), and comparison of their catalytic activity for hydrosilylation of 1-octene with diphenylsilane revealed that the reaction system using Co〈Mes〉 produced a mixture of anti-Markovnikov and Markovnikov hydrosilylation products, whereas that using Co〈Cy〉 produced the anti-Markovnikov product selectively. Crystal structures of these complexes implied that a catalytically active species derived from Co〈Cy〉 has narrower active space for hydrosilylation than that from Co〈Mes〉. According to the Chalk-Harrod mechanism, there are two types of hydrosilylated products (anti-Markovnikov and Markovnikov products), which stem from the way of terminal olefin insertion into an M-H bond (that is 1,2- or 2,1-insertion). One of the intermediates derived from Co〈Cy〉 in the Chalk-Harrod mechanism has hydride and silyl ligands. In the step of olefin insertion into the Co-H bond, 1,2-insertion is more likely to occur from the steric point of view, leading to selective formation of the anti-Markovnikov product.