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

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.