Light-enabled deracemization of cyclopropanes by Al-salen photocatalysis

Privileged chiral catalysts-those that share common structural features and are enantioselective across a range of reactions-continue to transform the chemical-research landscape(1). In recent years, new reactivity modes have been achieved through excited-state catalysis, processes activated by light, but it is unclear if the selectivity of ground-state privileged catalysts can be matched. Although the interception of photogenerated intermediates by ground-state cycles has partially addressed this challenge(2), single, chiral photocatalysts that simultaneously regulate reactivity and selectivity are conspicuously scarce(3). So far, precision donor-acceptor recognition motifs remain crucial in enantioselective photocatalyst design(4). Here we show that chiral Al-salen complexes, which have well-defined photophysical properties, can be used for the efficient photochemical deracemization(5) of cyclopropyl ketones (up to 98:2 enantiomeric ratio (e.r.)). Irradiation at lambda=400nm (violet light) augments the reactivity of the commercial catalyst to enable reactivity and enantioselectivity to be regulated simultaneously. This circumvents the need for tailored catalyst-substrate recognition motifs. It is predicted that this study will stimulate a re-evaluation of many venerable (ground-state) chiral catalysts in excited-state processes, ultimately leading to the identification of candidates that may be considered 'privileged' in both reactivity models.