Construction of sulfur stereocenters by asymmetric geminate recasting

The reactions of radical pairs formed by light- or heat-induced homolysis are of fundamental importance to many biochemical processes, as well as production of pharmaceuticals, industrial chemicals, and polymers. For chiral compounds, cleavage of a bond to a stereocenter produces a radical pair whose recombination can reconstitute the stereocenter in either enantiomeric form. While racemization is typically observed, catalytic conversion of racemic mixtures to single enantiomers is a synthetically desirable direction. However, it poses the challenge of controlling the reactivity of rotationally and translationally free prochiral radicals that can engage in various deleterious processes independently of each other. Here, we report how such control can be achieved by means of asymmetric geminate recasting, which involves a sequence of homolysis and recombination of the radical pair in a solvent cage promoted by a chiral photocatalyst. This approach enabled construction of chiral sulfur stereocenters, addressing an important problem in asymmetric synthesis. A bespoke chiral indium photocatalyst promoted deracemization of sulfinamides that can serve as versatile precursors to valuable chiral sulfur functional groups. Asymmetric geminate recasting provides an approach to imparting stereocontrol in homolytic reactions that produce chemically unbound radical pairs. We anticipate that further exploration of this framework in broader structural settings can open new avenues for diversification of chiral motifs in medicinal chemistry, materials science, and catalysis.