Asymmetric Photocatalysis by Intramolecular Hydrogen‐Atom Transfer in Photoexcited Catalyst–Substrate Complex

Abstract

3‐(2‐Formylphenyl)‐1‐pyrazol‐1‐yl‐propenones undergo an asymmetric photorearrangement to benzo[ d ]cyclopropa[ b ]pyranones with up to >99 % ee , which is catalyzed by a bis‐cyclometalated rhodium catalyst in the presence of visible light. Mechanistic experiments and DFT calculations support a mechanism in which a photoexcited catalyst/substrate complex triggers an intramolecular hydrogen‐atom transfer followed by a highly stereocontrolled hetero‐Diels–Alder reaction. In this reaction scheme, the rhodium catalyst fulfills multiple functions by 1) enabling visible‐light π→π* excitation of the catalyst‐bound enone substrate, 2) facilitating the hydrogen‐atom transfer, and 3) providing the asymmetric induction for the hetero‐Diels–Alder reaction.