Visible Light-Induced Excited-State Transition-Metal Catalysis

In recent years, visible light-induced excited-state transition-metal (TM) (Mn, Co, Cu, and Pd) catalysis has attracted significant attention for the development of various chemical transformations. In contrast to metal/photoredox dual catalysis that uses conventional photosensitizers and TMs cooperatively, photoexcited-state TM catalysis uses a single TM complex as both the photocatalyst (PC) and the cross-coupling catalyst, resulting in more sustainable and efficient reactions. Unlike the outer-sphere mechanism active in conventional photocatalysis, these TM catalysts operate through a photoinduced inner-sphere mechanism in which the substrate–TM interaction is crucial for the bond-breaking or bond-forming steps, making this system an important advance in efficient carbon–carbon (C–C) bond formation reactions. Given the importance of these TM complexes as next-generation PCs with distinct mechanisms, in this review we highlight recent developments in photoexcited TM catalysis for C–C bond formation.