Visible-Light-Driven Catalytic Dehalogenation of Trichloroacetic Acid and α-Halocarbonyl Compounds: Multiple Roles of Copper.

Herein, we report the reaction development and mechanistic studies of visible-light-driven Cu-catalyzed dechlorination of trichloroacetic acid for the highly selective formation of monochloroacetic acid. Visible-light-driven transition metal catalysis via an inner-sphere pathway features the dual roles of transition metal species in photoexcitation and substrate activation steps, and a detailed mechanistic understanding of their roles is crucial for the further development of light-driven catalysis. This catalytic method, which features environmentally desired ascorbic acid as the hydrogen atom source and water/ethanol as the solvent, can be further applied to the dehalogenation of a variety of halocarboxylic acids and amides. Spectroscopic, X-ray crystallographic, and kinetic studies have revealed the detailed mechanism of the roles of copper in photoexcitation, thermal activation of the first C-Cl bond, and excited-state activation of the second C-Cl bond via excited-state chlorine atom transfer.