Insights into the Mechanism of an Allylic Arylation Reaction via Photoredox Coupled Hydrogen Atom Transfer

Despite widespread use as a synthetic method, as well as rapid expansion of substrate scope, the precise mechanism and kinetics of photoredox coupled hydrogen atom transfer (HAT) reactions remain poorly understood. This results from a lack of detailed kinetic information, as well as the identification of side reactions and products. In this report, a mechanistic study of a prototypical tandem photoredox/HAT reaction coupling cyclohexene and 1,4-Dicyanobenzene (DCB) using an Ir(ppy)3 photocatalyst and thiol HAT catalyst is reported. Through a combination of electrochemical, photochemical, and spectroscopic measurements, key unproductive pathways and side products are identified and rate constants for main chemical steps are extracted. The reaction quantum yield was found to decline rapidly over the course of the 20-hour reaction. A previously unreported cyanohydrin side product was identified and thought to play a key role as proton acceptor in the reaction. Transient absorption spectroscopy (TAS) suggested a reaction mechanism that involves trapping of the DCB radical anion by cyclohexene with HAT occurring as the final step via a cooperative HAT step. Kinetic modeling of the reaction, using rate constants derived from TAS, demonstrates that the efficiency of the reaction is limited by parasitic absorption and unproductive quenching between excited Ir(ppy)3 and the cyanohydrin photoproduct.