Development of N‑Heterocyclic Carbene–Copper Complexes for 1,3-Halogen Migration

A series of NHC–copper complexes
was synthesized and their
potential to catalyze 1,3-halogen migration explored. Increasing the
steric bulk around the metal drastically improves the lifetime of
NHC–CuH species and promotes 1,3-halogen migration of both
2-bromo- and 2-chlorostyrenes through transfer of an aryl halogen
to a benzylic carbon with concomitant arene borylation. The NHC-based
system displays a broad substrate scope with notable advantages over
previously reported phosphine-based catalysts, including complete
selectivity for migration versus competing benzylic borylation, increased
steric tolerance, efficient aryl chloride migration, and facile formation
and characterization of organocopper catalytic intermediates. Experimental
evidence and DFT calculations support a mechanism proceeding through
dearomatization of a benzyl copper species, followed by a 1,4-halogen
shift and borylation of the resulting ArCu­(I) intermediate.