Resolving Conflicting Mechanisms for Photoredox Allylic sp 3 -CH Arylation Using Deuterium-Labeling and Isotope Effects.

Two conflicting mechanisms have emerged for the direct arylation of allylic C-H bonds enabled by the combined use of thiol and photoredox catalysis. In the original report ( Nature, 2015 , 519 , 74-77), a radical coupling step-between a radical anion of an arene and an allylic radical-is proposed to be the key C-C bond-forming step. A recent mechanistic study ( J. Org. Chem . 2022 , 87 , 223-230) has suggested that the C-C bond formation occurs via radical anion capture by the olefin followed by an H atom transfer (HAT) event to deliver the allylic C-H arylation product. Utilizing cyclohexene-4,4,5,5- d 4 as a mechanistic probe to distinguish between otherwise indistinguishable regioisomeric allylic C-H arylation products in the reaction of cyclohexene and dicyanobenzene, we establish that the radical anion capture-HAT mechanism is not operative. Furthermore, experimental k H / k D studies and DFT calculations lend strong support to the radical coupling mechanism proceeding via irreversible HAT to form the allylic radical of cyclohexene, followed by regioselectivity-determining radical coupling (for unsymmetrical olefins) and facile decyanation.