Photoinduced Enantioselective Triplet Radical Reaction on Metal: Copper‐Catalyzed Conjugate Addition of Acylsilanes to α,β‐Unsaturated Ketones and Aldehydes

A photoinduced copper‐catalyzed enantioselective conjugate addition of acylsilanes has been developed. The conjugate acylation of α,β‐unsaturated ketones and aldehydes was promoted by a copper(I)/chiral NHC catalyst under visible‐light irradiation for synthesizing various 2‐substituted 1,4‐dicarbonyl compounds in enantioenriched forms. Mechanistic studies combining experiments and quantum chemical calculations indicated a reaction mechanism involving copper‐to‐acyl charge transfer (i. e., metal‐to‐ligand charge transfer (MLCT)) excitation of an alkene‐bound acylcopper complex. The MLCT excitation is followed by an electronical and geometrical change to generate a triplet β‐radical‐ C ‐enolate‐Cu(II)‐acyl complex with an acyl radical character, which undergoes facile excited state C−C bond formation in the copper coordination sphere, affording the 1,4‐conjugate addition product.