(NHC)Ni(0)-Catalyzed Branched-Selective Alkene Hydrosilylation with Secondary and Tertiary Silanes

Hydrosilylation is a valuable approach for the construction of organosilanes, which are precursors to silicone materials that are widely incorporated in our everyday lives. The industry currently relies primarily on Karstedt’s catalyst, Pt2(dvtms)3 (dvtms = 1,3-divinyltetramethyldisiloxane), a precious metal catalyst that exhibits linear selectivity, with regioselectivity favoring the branched product remaining an outstanding challenge. The use of more Earth-abundant, base-metal catalysts has been a recent focus for hydrosilylation reactions, and most reports focus on the development of linear-selective catalysts and are commonly limited to primary and/or secondary silanes. We demonstrate that (NHC)Ni(0) (NHC = N-heterocyclic carbene) complexes are active in the branched-selective hydrosilylation of alkenes with secondary or tertiary silanes, including industrially relevant alkoxy- and chlorosilanes. The scope of alkenes and silanes has been expanded beyond what is currently known for Ni-catalyzed hydrosilylation reactions, including both steric and electronic profiles. In-depth mechanistic studies were also carried out, including stoichiometric and catalytic experiments investigating kinetic and thermodynamic reaction parameters. Radical trap experiments suggest against a one-electron pathway. The rate law of the reaction has a normal dependence on the Ni catalyst and silane and has an inverse dependence on the alkene. Deuterium-labeling studies reveal that hydrosilylation proceeds through a Chalk–Harrod-type mechanism, with the alkene reversibly inserting into a Ni–H bond. Hammett analyses show that the rate of reaction is faster with electron-rich alkenes and electron-poor silanes. Additional mechanistic evidence points to the resting state of the catalyst being a (NHC)Ni(alkene)2 complex, and the rate-determining step being migratory insertion and/or reductive elimination.