Investigations into mechanism and origin of regioselectivity in the metallaphotoredox-catalyzed α-arylation of -alkylbenzamides.

A mechanistic study on the α-arylation of -alkylbenzamides catalyzed by a dual nickel/photoredox system using aryl bromides is reported herein. This study elucidates the origins of site-selectivity of the transformation, which is controlled by the generation of a hydrogen atom transfer (HAT) agent by a photocatalyst and bromide ions in solution. Tetrabutylammonium bromide was identified as a crucial additive and source of a potent HAT agent, which led to increases in yields and a lowering of the stoichiometries of the aryl bromide coupling partner. NMR titration experiments and Stern-Volmer quenching studies provide evidence for complexation to and oxidation of bromide by the photocatalyst, while elementary steps involving deprotonation of the -alkylbenzamide or 1,5-HAT were ruled out through mechanistic probes and kinetic isotope effect analysis. This study serves as a valuable tool to better understand the α-arylation of -alkylbenzamides, and has broader implications in halide-mediated C-H functionalization reactions.