Visible-Light-Induced Energy-Transfer-Mediated Hydrofunctionalization and Difunctionalization of Unsaturated Compounds via Sigma-Bond Homolysis of Energy-Transfer Acceptors

Over the past decade, visible-light-mediated energy-transfer (EnT) catalysis, particularly triplet-triplet energy-transfer (TTEnT) catalysis, has emerged as a mild and environmentally friendly approach for diverse organic synthetic transformations. In contrast to photoredox catalysis, which typically requires sacrificial electron donors or acceptors to complete the catalytic cycle, EnT photocatalysis generally proceeds with high atom economy while minimizing the generation of wasteful byproducts. Furthermore, while successful photoredox catalysis is contingent upon the precise control of redox potentials of both photocatalysts and organic substrates, EnT strategies are primarily influenced by the triplet energy compatibility between these entities. Considering the growing importance of EnT photocatalysis, as well as hydrofunctionalization and difunctionalization reactions in organic synthesis, this review systematically summarizes significant advancements in EnT-enabled hydrofunctionalization and difunctionalization of unsaturated compounds via sigma-bond homolysis over the past decade. Special emphasis is placed on elucidating substrate scopes and mechanistic scenarios. Additionally, this review discusses versatile synthetic applications of these methodologies and addresses current challenges and opportunities within this evolving research field. This review is structured into six main categories based on different types of energy-transfer acceptors and the sigma-bonds undergoing homolysis. These categories include EnT-enabled hydrofunctionalization and difunctionalization transformations mediated by 1) N-O bond homolysis of oxime esters and other N,O-radical precursors; 2) N-S bond homolysis of N-sulfonyl imines and other N,S-radical precursors; 3) chalcogen-chalcogen bond homolysis of disulfides and oxy/thio/selenosulfonates; 4) C-S bond homolysis of tri/difluoromethylated sulfinates, acetylenic triflones, and arylsulfonium salts; 5) C-X (X = halogen) bond homolysis of organic halides; and 6) sigma-bond homolysis of other energy-transfer acceptors. Through providing theoretical backgrounds of EnT catalysis, along with a comprehensive overview of currently employed energy-transfer acceptors, photosensitizers, and contemporary strategies for EnT-induced hydrofunctionalization and difunctionalization of unsaturated compounds, this review aims to serve as an invaluable resource for future innovations in this rapidly evolving field.