Superacid counteranion as flexible-coordinating ligand for asymmetric organo-bismuth catalysis.

Abstract

Asymmetric binary catalysis, particularly combining chiral Brønsted acids with Lewis acids, is an emerging strategy in synthetic chemistry. Although a few catalyst combinations exist for stereoselective transformations, their scope is generally limited to pre-organized, activated substrates. Here, we report binary catalysis combining an organosuperacid with bismuth, where the counteranion of chiral N-triflyl phosphoramide acts as a flexible-coordinating ligand. This system demonstrates exceptional reactivity and enantioselectivity in the asymmetric allylation of α-keto thioesters, forming enantio-enriched α-hydroxy thioesters with a tetra-substituted stereogenic carbon center (up to > 99% yield and 97% ee). The success is attributed to bismuth's flexibility during activation, enhancing also interactions with the thioester-tethered substrate. Integrated experimental, analytical, and computational studies highlight the unique assembly enabled by the chiral Brønsted acid and bismuth salt system.