Divergent Photocatalytic Reactions of α-Ketoesters Under Triplet Sensitization and Photoredox Conditions

The long-lived triplet excited states of transition metal photocatalysts can activate organic substrates via either energy- or electron-transfer pathways, and the rates of these processes can be influenced by rational tuning of the reaction conditions. The characteristic reactive intermediates that are generated by each of these activation modalities, however, are distinct and can exhibit very different reactivity patterns. Herein, we show that the photocatalytic reactions of benzoylformate esters with alkenes can be directed towards either Paternò–Büchi cycloadditions under conditions that favor energy transfer or allylic functionalization reactions under superficially similar conditions that favor electron transfer. These studies provide a framework for designing other divergent photocatalytic methods that produce different sets of reaction outcomes under photoredox and triplet sensitization conditions.