Modular Enantioselective Photocatalysts from Privileged Pybox Scaffolds

Modern organic synthesis relies upon the availability of chiral catalysts to control the stereochemistry of bond-forming reactions. As the field of enantioselective catalysis has matured over the past several decades, several families of chiral catalysts have become recognized as “privileged” structures because of their remarkable generality for diverse transformations with different reaction mechanisms. The ability to control the stereochemistry of photochemical reactions, however, has developed at a markedly slower pace. Examples of highly enantioselective photocatalyst structures remain scarce, and none have yet become widely adopted by the broader synthetic community. We have designed a new family of enantioselective photocatalysts by modifying the structures of privileged pyridine bis(oxazoline) (pybox) complexes with electron-donating carbazole units. The chiral ligands are accessible via a three-step synthetic sequence starting from commercially available chiral pool materials, and their charge-transfer photochemistry can be rationally tuned to optimize photocatalytic activity. We demonstrate the generality of these new chiral photocatalyst structures in a series of three model asymmetric reactions that include both photoredox and excited-state photoreactions.