Mechanistic Investigation of the Nickel-Catalyzed Transfer Hydrocyanation of Alkynes

The implementation of HCN-free transfer hydrocyanation reactions
on laboratory scales has recently been achieved by using HCN donor
reagents under nickel- and Lewis acid co-catalysis. More recently,
malononitrile-based HCN donor reagents were shown to undergo the C(sp3)–CN bond activation by the nickel catalyst in the
absence of Lewis acids. However, there is a lack of detailed mechanistic
understanding of the challenging C(sp3)–CN bond
cleavage step. In this work, in-depth kinetic and computational studies
using alkynes as substrates were used to elucidate the overall reaction
mechanism of this transfer hydrocyanation, with a particular focus
on the activation of the C(sp3)–CN bond to generate
the active H–Ni–CN transfer hydrocyanation catalyst.
Comparisons of experimentally and computationally derived 13C kinetic isotope effect data support a direct oxidative addition
mechanism of the nickel catalyst into the C(sp3)–CN
bond facilitated by the coordination of the second nitrile group to
the nickel catalyst.