Nonadiabatic Curve-Crossing Model for the Visible-Light Photoredox Catalytic Generation of Radical Intermediate via a Concerted Mechanism

Abstract

Photoredox catalysis relies on the excited-state single-electron transfer (SET) processes to drive a series of unique bond-forming reactions. In this work accurate electronic structure calculations at the CASPT2//CASSCF/PCM level of theory together with the kinetic assessment of SETs and intersystem crossing are employed to provide new insights into the SET initiation, activation, and deactivation by calculating the SET paths for a paradigm example of photoredox α-vinylation reaction mediated by iridium­(III) catalysts. The concerted photocatalysis mechanism described by the nonadiabatic curve-crossing model, in essence of Marcus electron transfer theory, is first applied for the mechanistic description of the SET events in visible-light photoredox catalysis. The C–C bond functionalization has been revealed to take place in a concerted manner along an energy-saving pathway, in which the generated α-amino radical is unlikely independent existence but strongly depends on the mutual interaction with different substrates. These mechanistic insights offer a plausible picture for the excited-state SET-mediated chemical transformations that should be applicable to further studies of photoredox catalysis in organic chemistry