[CNC]-Pincer Cobalt Hydride Catalyzed Distinct Selective Hydrosilylation of Aryl Alkene and Alkyl Alkene

The reactions of unsymmetrical N-heterocyclic carbene (NHC) [CNC]-pincer preligands with CoMe(PMe3)(4) gave rise to NHC [CNC]-pincer cobalt(III) hydrides, [(CcarbeneNaminoCnaphthyl)Co(H)(PMe3)(2)] (3a) and (3b), via C-sp2-H activation and the unexpected trans-bischelate [C-carbene, N-amino] cobalt(II) complexes 4a and 4b via a disproportionation reaction, respectively. It was found that both 3a and 3b are efficient catalysts for hydrosilylation of alkenes. With aryl alkenes as substrates, 3a has high Markovnikov selectivity in excellent yields, while 3a is an efficient anti-Markovnikov catalyst in good yields with alkyl alkenes as substrates. The catalytic process could be promoted with pyridine N-oxide as an initiator. The catalytic mechanisms for the two different selectivities were proposed. Complexes 3a, 3b, 4a, and 4b were characterized by spectroscopic methods, and the molecular structures of 3b, 4a, and 4b were determined by single crystal X-ray diffraction.