An anion-binding approach to enantioselective photoredox catalysis.

Photoredox catalysis has emerged as a transformative strategy in synthetic chemistry, enabling a wide variety of valuable chemical reactions through generation of highly reactive radical ion intermediates. Pairing chiral counteranions with cation radical intermediates provides a potentially generalizable tool for controlling absolute stereochemistry in various reactivity contexts. However, ion-pairing effects on the efficiency of photoinduced processes and the reactivity of radical ion pairs impose severe limits on the chiral anions that can be engaged effectively. In this study, we report that association of neutral chiral small-molecule hydrogen-bond donors with the counteranions of cation radical intermediates can achieve enantioselectivity through ion-pairing and other noncovalent interactions. Applications to four different classes of cycloaddition reactions of electron-rich alkene substrates provide cyclic products with up to four new stereocenters in up to 99% enantiomeric excess. Editor's summary: Photoredox catalysis relies on light-absorbing compounds to activate reactants by charge transfer. Because the reactants are typically neutral, the intermediates become charged, and recent studies have introduced chiral counterions to steer their reactivity toward one of two mirror-image products. Vojáčková and Jacobsen now report an alternate, flexible method in which the chiral agent is neutral but can bind to a variety of counterions by hydrogen bonding. The authors demonstrated the selectivity of this approach in the preparation of a range of three-, four-, and five-membered ring products. —Jake S. Yeston [ABSTRACT FROM AUTHOR]