Visible Light-Induced Excited-State Transition-Metal Catalysis

Unlike many traditional transition metal (TM)-catalyzed reactions that require heat, in excited-state TM catalysis visible-light irradiation at room temperature is used. Compared with conventional metal/photoredox cooperative/dual catalysis that uses two different catalysts, the use of a single TM acting as both photo- and cross-coupling catalyst is economic and attractive. The inner-sphere mechanism via a substrate–TM interaction allows reactivity and selectivity that is not possible with conventional photosensitizers. In contrast to conventional photocatalysis, TM photocatalysis is less dependent on the often fine-tuned redox potentials of the substrate and catalyst. 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. 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. an energy level of a system (atom, ion, or molecule) in which an electron is at a higher energy level (high-energy orbital) than its ground state. a resonance process that increases the stability of the system through the interaction of the electrons in an σ-orbital with an adjacent empty or partially filled p- or π-orbital to provide an extended molecular orbital. an electron-transfer process in which the electrons are transferred between two redox partners via the formation of a direct bond or a strong electronic interaction between redox partners. an electron-transfer process in which the electrons are transferred between two redox partners without the formation of a direct bond or without any electronic interaction between redox partners. a substance capable of initiating a chemical reaction by absorbing light energy and transferring the energy to the desired reactants. the transfer of an electron from one atom or molecule to another. In redox chemistry, the donor atom or molecule loses electrons (oxidation) while the acceptor gains the electron (reduction). the cross-coupling reactions of an organohalides with terminal alkynes to give C–C cross-coupled products.