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. Photoredox catalysis is used for the oxidation of a diarylmethane to the corresponding cation, which is then trapped by an enamine intermediate generated in situ from an aldehyde reactant and a secondary amine organocatalyst. Notably, this mild method is an ideal synthetic approach from the green chemistry point of view. It does not require preinstallation of functional groups, thereby constituting an atom economical, efficient transformation, and it 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.本研究报道了一种以可见光作为经济且环境友好的能源，实现两个C(sp³)-H键催化不对称交叉偶联的方法。该方法采用光氧化还原催化将二芳基甲烷氧化为相应阳离子，该阳离子可被由醛反应物与二级胺有机催化剂原位生成的烯胺中间体捕获。值得注意的是，从绿色化学的视角来看，该温和反应方法是一种理想的合成策略。其无需预先安装官能团，因此具备原子经济性且转化效率优异，可在形成碳-碳键的同时，以极高的对映选择性与非对映选择性构建1个或2个全新的立体中心。本研究通过实验与计算手段开展机理研究，阐明了所观测到的对映选择性的来源。