From Sacrificial to Catalytic Photogenerated Reductant Formation: A Sulfide-Mediated Platform in Photoredox Catalysis

Abstract In photoredox catalysis, reductive quenching defines a powerful manifold in which an excited photoredox catalyst is reduced by an electron donor to generate highly reducing photoredox catalyst-derived species. These photogenerated reductants can activate substrates beyond the direct redox window of the excited photocatalyst and have therefore become powerful intermediates in reductive photoredox synthesis. This account traces the development of this manifold, from classical sacrificial-amine-based systems to recent catalytic variants. After outlining the photochemical foundations of reductive quenching, we discuss how sacrificial amines established this manifold as a practical platform for generating radicals in organic synthesis, including dehalogenation and aryl radical chemistry. We then highlight newer sacrificial strategies that expand the reactivity through substrate pre-activation, proton-coupled electron transfer, hydrogen-atom transfer, and halogen-atom transfer. Finally, through several recent studies, our group developed a sulfide-mediated dual catalytic platform in which the electron donor is regenerated through sulfonium formation and substitution, converting a sacrificial process into a catalytic one. This design enables chlorotrifluoromethylation, modular heterocycle synthesis, and highly selective multicomponent radical coupling. Collectively, these studies show how mediator design can transform the generation of highly reducing species from a sacrificial redox strategy into a catalytic principle for reductive photoredox generation.