Effect of Silylene Ligand on Catalytic Pathway of Alkene Hydrosilylation Catalyzed by Phosphine Cobalt(III) Hydrides

ABSTRACT In this paper, two cobalt(III) hydrides, [(PMe 3 ) 3 Co(H)(Cl)(SiH 2 Ph)] ( 3 ) and [(LSi:) 2 (PMe 3 )Co(H)(Cl)(SiH 2 Ph)] (LSi: = {PhC (N t Bu) 2 }SiCl) ( 4 ), were used as catalysts to compare their catalytic performance on the hydrosilylation of alkenes. The research results indicate that both cobalt(III) complexes 3 and 4 are good catalysts for the hydrosilylation of alkenes, affording Markovnikov products for aryl alkenes and anti‐Markovnikov products for alkyl alkenes. It was found that the catalytic activity of phosphine cobalt(III) hydride 3 is higher than that of silylene cobalt(III) hydride 4 for aryl alkenes. On the contrary, the catalytic activity of silylene cobalt hydride 4 is better than that of phosphine cobalt hydride 3 for alkyl alkenes. Different experiments were designed to study the catalytic mechanism for hydrosilylation of alkenes catalyzed by complexes 3 and 4 . We found that complex 3 first reacted with Ph 2 SiH 2 when it was used as a catalyst. From the stoichiometric reaction between complex 3 with Ph 2 SiH 2 we obtained the known complex 5 [(Ph 2 ClSi)Co(H) 2 (PMe 3 ) 3 ]. Further experiment indicates that hydrido cobalt(I) intermediate 5b , [HCo (PMe 3 ) 3 ], is an active intermediate in the hydrosilylation of alkenes catalyzed by complex 3 . When silylene cobalt(III) hydride 4 was used as a catalyst, complex 4 reacted at first with styrene, and four‐coordinated bis (silylene) cobalt(I) chloride Co (LSi:) 2 (PMe 3 )Cl was proposed as the active intermediate in the hydrosilylation of alkenes. Based on the experimental results, two different possible catalytic mechanisms catalyzed by complexes 3 and 4 were proposed, respectively.