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 alpha,beta-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 beta-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.