Distinct Catalytic Performance of Cobalt(I)-N-Heterocyclic Carbene Complexes in Promoting the Reaction of Alkene with Diphenylsilane: Selective 2,1-Hydrosilylation, 1,2-Hydrosilylation, and Hydrogenation of Alkene

Selectivity control on the reaction of alkene with hydrosilane is a challenging task in the development of non-precious-metal-based hydrosilylation catalysts. While the traditional way of selectivity control relies on the use of different ligand type and/or different metals, we report herein that cobalt(I) complexes bearing different N-heterocyclic carbene ligands (NHCs) exhibit distinct selectivity in catalyzing the reaction of alkene with Ph2SiH2. [(IAd)(PPh3)-CoCl] (IAd = 1,3-diadamantylimidazol-2-ylidene) is an efficient catalyst for anti-Markovnikov hydrosilylation of monosubstituted alkenes. [(IMes)(2)CoCl] (IMes = 1,3dimesitylimidazol-2-ylidene) shows Markovnikov-addition selectivity in promoting the hydrosilylation of aryl-substituted alkenes. [(IMe2Me2)(4)Co] [BPh4] (IMe2Me2 = 1,3-dimethyl-4,5-dimethylimidazol-2-ylidene) can catalyze hydrogenation of alkenes with Ph2SiH2 as the terminal hydrogen source. Mechanistic studies in combination with the knowledge on the steric nature of cobalt-NHC species suggest that (NHC)cobalt(I) silyl species and bis(NHC)cobalt(I) hydride species are the probable key intermediates for these hydrosilylation and hydrogenation reactions, respectively. The different steric nature of IAd versus IMes and the potential of IMes incurring pi center dot center dot center dot pi interaction with aryl-substituted alkenes are thought to be the causes of the observed 1,2- and 2,1-addition selectivity.