Cooperative Catalysis-Enabled Asymmetric α-Arylation of Aldehydes Using 2-Indolylmethanols as Arylation Reagents

A catalytic asymmetric alpha-arylation of aldehydes using 2-indolylmethanols as arylation reagents has been established. This reaction was enabled by a cooperative catalytic system consisting of a gold complex, a Bronsted acid, and a chiral amine, which have a synergistic effect in the reaction process. By using this strategy, a series of alpha-arylation products of aldehydes were generated in overall acceptable yields and good enantioselectivities (up to 69%, 91:9 er). The control experiments demonstrated that the addition of PPh3AuCl as a gold complex was helpful to improve the yield, and trifluoroacetic acid as a Bronsted acid played a crucial role in the reaction by promoting the generation of carbocation and chiral enamine intermediates, which are two key intermediates of the asymmetric alpha-arylation reaction. In addition, the enantioselectivity of the reaction was mainly controlled by the chiral amine catalyst via forming a chiral enamine intermediate. This reaction has not only provided a useful protocol for catalytic asymmetric alpha-arylation of aldehydes but also enriched the research contents of 2-indolylmethanol-involved reactions and asymmetric cooperative catalysis.