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.