DFT insight into asymmetric alkyl–alkyl bond formation via nickel-catalysed enantioconvergent reductive coupling of racemic electrophiles with olefins

Abstract

A DFT study has been conducted to understand the asymmetric alkyl–alkyl bond formation through nickel-catalysed reductive coupling of racemic alkyl bromide with olefin in the presence of hydrosilane and K₃PO₄. The key findings of the study include: (i) under the reductive experimental conditions, the Ni(II) precursor is easily activated/reduced to Ni(0) species which can serve as an active species to start a Ni(0)/Ni(II) catalytic cycle. (ii) Alternatively, the reaction may proceed via a Ni(I)/Ni(II)/Ni(III) catalytic cycle starting with a Ni(I) species such as Ni(I)–Br. The generation of a Ni(I) active species via comproportionation of Ni(II) and Ni(0) species is highly unlikely, because the necessary Ni(0) species is strongly stabilized by olefin. Alternatively, a cage effect enabled generation of a Ni(I) active catalyst from the Ni(II) species involved in the Ni(0)/Ni(II) cycle was proposed to be a viable mechanism. (iii) In both catalytic cycles, K₃PO₄ greatly facilitates the hydrosilane hydride transfer for reducing olefin to an alkyl coupling partner. The reduction proceeds by converting a Ni–Br bond to a Ni–H bond via hydrosilane hydride transfer to a Ni–alkyl bond via olefin insertion. On the basis of two catalytic cycles, the origins for enantioconvergence and enantioselectivity control were discussed.