Heteroleptic Nickel Complexes for the Markovnikov-Selective Hydroboration of Styrenes

First-row
transition metal catalysis offers a cheaper, more environmentally
sustainable alternative to second- and third-row transition metal
catalysts. Nickel has shown great promise as a tool for the borylation
of unsaturated compounds to yield boronic esters, but Markovnikov-selective
hydroborations of simple styrenes have not been well-explored. Herein,
we report the synthesis of benzyl boronic esters via nickel-catalyzed
hydroboration of styrenes using a heteroleptic N-heterocyclic carbene
(NHC)–phosphine nickel complex, IMes­(Cy3P)­NiCl2. The IMes­(Cy3P)­NiCl2 complex displays
a broad substrate scope and maintains the integrity of yield and regioselectivity
when challenged with substrates bearing increased steric hindrance.
The heteroleptic complexes also tolerate both electron-withdrawing
and -donating groups, in contrast to traditional bis-phosphine and
Ni(0) complexes.