Enantioselective Photocatalysis

This chapter discusses more recent work toward catalytic enantioselective photoreactions with particular emphasis on processes that are of potential use in synthetic applications. One of the most attractive approaches to asymmetric photocatalysis involves the application of circularly polarized light (CPL). Catalyst decomposition is thought to occur through hydrogen-atom abstraction by the triplet excited state of the photocatalyst; restricting the availability of labile C—H bonds would therefore be expected to limit the degradation. The chapter provides a summary of the most important pioneering contributions to enantioselective photocatalysis and discusses the more recent developments in the field. It then describes the three most successful approaches, namely, large-molecule chiral hosts, small-molecule chiral photosensitizers, and activation by chiral Lewis acids. The use of a substoichiometric amount of chiral Lewis acid clearly enables the effective suppression of the background reaction, such that high enantioselectivity is observed in the photoreactions.