La/Ag Heterobimetallic Cooperative Catalysis: A Catalytic Asymmetric Conia-Ene Reaction

摘要

Cooperative catalysis continues to attract growing interest as a particularly effective strategy to promote specific bond formation under mild reaction conditions. This intriguing approach allows for the activation of not only a single functionality through multiple coordination, but also chemically distinct multiple functionalities in a simultaneous manner. The latter option, under an asymmetric environment, is particularly powerful to assemble the multiple functionalities with a high degree of stereocontrol. A carbonyl group and an alkyne group represent hard and soft Lewis basic functionalities, respectively, and their simultaneous activation by hard and soft Lewis acid cooperative catalysis can be best manifested by a Conia-ene reaction. Pioneering work on the catalytic asymmetric Conia-ene reaction of β-keto esters and 1,3-diketones with a hard Lewis acid (Yb(OTf)3)/chiral soft Lewis acid (DTBM-Segphos/Pd~(II)) bimetallic catalytic system was reported by Corkey and Toste. Subsequently, Dixon and co-workers reported a distinct approach with chiral amine base/soft Lewis acid (CuOTf) cooperative catalysis. Meanwhile, our research group developed a La(NO3)3/amide-based ligand 1a/amine ternary catalyst that promotes the catalytic asymmetric animation of malonamates to azodicarboxylates. In this reaction a La/1a/amine ternary complex formed under equilibrium acts as a chiral Lewis acid (Scheme 1).