Improvement of regioselectivity of alkene hydrosilylation catalyzed by [PNSiNP] pincer cobalt(iii) hydrides using sodium methoxide as an additive

Abstract

In this work, three silyl [PNSiNP] pincer cobalt(III) hydrides, Co^III(H)(Cl)(PMe₃)(R′Si(NCH₂PR₂)₂C₆H₄) (R′ = Me and R = Ph (1); R′ = Ph and R = Ph (2) and R′ = Me and R = ⁱPr (3)), were synthesized. Among the three complexes, complexes 2 and 3 are new and have been characterized and analyzed. The molecular and crystal structures of complexes 2 and 3 were determined by single crystal X-ray diffraction. The catalytic activity of cobalt hydrides 1–3 for alkene hydrosilylation was evaluated, revealing similar product selectivities, but the highest catalytic activity of the three catalysts for complex 1. The selectivity can be effectively regulated by using sodium methoxide as an additive. Under optimized catalytic reaction conditions a conversion of up to 98% with up to 99/1 (b/l) product selectivity was achieved. When aryl alkenes are used as substrates, the reactions mainly follow the Markovnikov rule. When using alkyl alkene substrates, the reactions tend to form anti-Markovnikov addition products. A plausible mechanism for this catalytic reaction was proposed and partly corroborated by experiments. A four-coordinated [PNSiNP] pincer cobalt(I) complex (1c) was considered as the active species for this catalytic system. NaOMe as an additive promotes the conversion of Co–Cl bonds to Co–OMe moieties, which, in the presence of silane, may facilitate the formation of a polyhydride species and accelerate the formation of the active species [PNSiNP] pincer cobalt(I) complex (1c) in the catalytic system. Compared with our reported [PSiP] pincer Co(III) hydride system, although the active intermediate in the catalytic cycle in both cases is the tetra-coordinated cobalt(I) complex, the selectivity of aromatic alkene hydrosilylation is reversed from anti-Markovnikov addition with [PSiP] pincer cobalt(III) hydride as the catalyst to Markovnikov addition with complex 1 as the catalyst. The catalytic selectivity of catalysts can be regulated by adjusting the properties of supporting ligands.