N-Heterocyclic Carbene-Photocatalyzed Tricomponent Regioselective 1,2-Diacylation of Alkenes Illuminates the Mechanistic Details of the Electron Donor–Acceptor Complex-Mediated Radical Relay Processes

Progress in the development of photocatalytic reactions requires a detailed understanding of the mechanisms underpinning the observed reactivity, yet mechanistic details of many photocatalytic systems, especially those that involve electron donor–acceptor complexes, have remained elusive. We report herein the development and a combined mechanistic and computational study of photocatalytic alkene 1,2-diacylation that enables a regioselective installation of two different acyl groups, establishing direct, tricomponent access to 1,4-diketones, key intermediates in heterocyclic and medicinal chemistry. The studies revealed the central role of the electron donor–acceptor complex formed from an N-heterocyclic carbene (NHC) catalyst-derived intermediate and an acyl transfer reagent, providing a detailed description of the structural and electronic factors determining the characteristics of the photoinduced charge-transfer process that mediates photocatalytic transformation. The in-depth investigation also illuminated the roles of other radical intermediates and electron donors relevant to the catalytic activities of N-heterocyclic carbenes in radical reactions.