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
generated, however, are distinct and can exhibit very different reactivity
patterns. This mechanistic diversity available to photocatalytic reactions
might thus offer an opportunity to engineer divergent reactions that
give markedly different chemical outcomes under superficially similar
conditions. Herein, we show that the photocatalytic reactions of benzoylformate
esters with alkenes can be directed toward either Paternò–Büchi
cycloadditions or allylic functionalization reactions under conditions
favoring energy transfer or electron transfer, respectively. These
studies provide a framework for designing other divergent photocatalytic
methods that produce different sets of reaction outcomes under photoredox
and triplet sensitization conditions.