Solvent-Free Hydrosilylation of Alkenes Catalyzed by Well-Defined Low-Valent Cobalt Catalysts

A solvent-free cobalt-catalyzed highly selective hydrosilylation of alkenes has been developed. It was found that both Co­(PMe3)4 and CoCl­(PMe3)3 are highly active catalysts for hydrosilylation of alkenes. The former promoted Markovnikov-type hydrosilylation of the aryl alkenes, while the latter catalyzed anti-Markovnikov-type hydrosilylation of the alkyl alkenes. These two catalytic systems tolerate a variety of functional groups and provide high selectivity and medium to high yield. In the exploration of the reaction mechanism, a dinuclear silyl cobalt­(I) complex [(PMe3)2Co­(μ–η2-HSiPh2)2Co­(PMe3)2] (4) from the Co­(PMe3)4 system and a silyl cobalt dihydride [(PMe3)3Co­(H)2SiClPh2] (5) from the CoCl­(PMe3)3 system were obtained. It is proposed that the silyl cobalt­(I) intermediate, [Co­(PMe3)3(SiHPh2)], is the real catalyst for the Co­(PMe3)4 system, while the hydrido cobalt­(I) intermediate, [HCo­(PMe3)3], is the real catalyst for the CoCl­(PMe3)3 system. Complexes 4 and 5 were characterized by spectroscopic methods and single-crystal X-ray diffraction.