Most biological functions are regulated by chiral molecules1 that contain at least one tertiary stereogenic carbon, that is, a carbon with one C(sp3)–H bond. Hydrogen atom transfer (HAT)2 is a straightforward strategy that can be used to either edit3 or introduce tertiary stereocentres in multiple synthetically useful transformations4, especially when coupled with photoredox catalysis5,6. However, traditional de novo design of chiral HAT catalysts that provide sufficient enantiocontrol over short-lived open-shell intermediates7 has represented a major hurdle in the development of enantioselective HAT reactions. Here we describe a distinct approach in which chiral HAT catalysts are obtained in situ by non-covalent self-assembly of privileged chiral phosphoric acids and commercial 2-mercaptopyridines. The phosphoric acid serves as a modular interchangeable chiral element that renders the achiral thiol effectively chiral, thereby allowing access to a previously inaccessible combinatorial space of chiral HAT catalysts. This platform enabled the photochemical deracemization of 2-aryl pyrrolidines, which are prevalent scaffolds in active pharmaceutical ingredients. Optical enrichment occurs by means of enantioselective hydrogen atom relay, in which a single chiral assembly orchestrates hydrogen atom abstraction and delivery. This conceptual approach of relaying chiral information through non-covalent assembly paves the way for discovery of numerous asymmetric radical transformations.
