B(C₆F₅)₃-Regulated Selectivity of Aryl Alkene Hydrosilylation Catalyzed by a [P,C]-Chelate Cobalt(I) Complex

In this article, two cobalt complexes bearing bidentate ligands, [Si,C]-chelate cobalt(I) complex [(Si,C)Co(PMe₃)₃] (1) and [P,C]-chelate cobalt(I) complex [(P,C)Co(PMe₃)₃] (2) were synthesized by activating Csp²-H of the corresponding 2-(diphenylsilylenoaminomethyl) pyridine (L1) PyN(Me)SiL (L = PhC(NBu)₂) or 2-(diphenylphosphinoaminomethyl) pyridine (L2) PyN(Me)PPh₂ with CoMe(PMe₃)₄. The catalytic performance of complexes 1 and 2 for alkene hydrosilylation was studied. Because of the different electronic properties of the phosphine and the silylene pincer ligand, the catalytic effect of phosphine complex 2 is superior to that of silylene complex 1 as indicated by faster conversion and higher selectivity for most of the selected substrates. Both aromatic alkenes and alkyl alkenes are mainly anti-Markovnikov converted to products. In the study of the catalytic mechanism, cobalt(III) hydride A' is proposed as the key intermediate. Unexpectedly, when B(C₆F₅)₃ is added to the system, the selectivity of the catalytic system for aromatic alkenes is reversed to afford mainly Markovnikov products. Through experimental exploration, it is proposed that the addition of B(C₆F₅)₃ induces a coordination vacancy at the Co center, which favors the initial coordination of the olefin and, in turn, changes the catalytic reaction mechanism, resulting in a reversal of regioselectivity.