Precise construction of spiro stereocenters via enantioselective radical addition through modulating photocatalysis from redox to energy transfer

Abstract

Chiral hydrogen-bonding catalysis has been successfully applied in a wide range of asymmetric photocatalytic radical-based reactions. However, it faces intrinsic challenges in the reactions that rely on oxidative quenching to initiate transformations. A critical issue arises from the formation of anionic side intermediates, which preferentially interact with protons from chiral catalysts, undermining the essential enantiocontrol required for effective product formation. In this study, we demonstrate that creating energy transfer instead of single-electron transfer to trigger these transformations presents a promising solution. As a proof-of-concept, we report the first photocatalytic spirocyclization of olefinic sulfonyl oximes with vinyl azides, furnishing a diverse array of spiroaminals with high yields (up to 94%) and enantioselectivities (up to 99% ee). The success of this method hinges on employing a sulfonyl group as an N-protective group for oximes, which facilitates energy transfer as an alternative mechanism to initiate the transformation. This approach not only enhances reactivity and chemoselectivity but also creates an optimal environment for enantiocontrol. The synthetic significance of this work is underscored by the establishment of these products as a novel class of chiral ligands, with preliminary studies indicating their effectiveness in asymmetric alkynylation reactions.