Asymmetric Visible-Light Photoredox Cross-Dehydrogenative Coupling of Aldehydes with Xanthenes

We report a methodology for the catalytic asymmetric cross-coupling of two C(sp3)–H bonds employing visible light as an economical and environmentally benign source of energy. In this protocol, photoredox catalysis is used for the oxidation of a xanthene to the corresponding cation, which is then trapped by an enamine intermediate generated in situ from an aldehyde reactant and a secondary amine organocatalyst. This mild method does not require preinstallation of functional groups and allows the formation of a C–C bond with simultaneous installation of one or two new stereocenters in a highly enantio- and diastereoselective manner. Mechanistic studies by experimental and computational methods aid to clarify the origin of the observed enantioselectivity.