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.