Resolving Conflicting Mechanisms for Photoredox Allylic sp³-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-d4 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 kH/kD 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.