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 landscape1. 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 challenge2, single, chiral photocatalysts that simultaneously regulate reactivity and selectivity are conspicuously scarce3. So far, precision donor-acceptor recognition motifs remain crucial in enantioselective photocatalyst design4. Here we show that chiral Al-salen complexes, which have well-defined photophysical properties, can be used for the efficient photochemical deracemization5 of cyclopropyl ketones (up to 98:2 enantiomeric ratio (e.r.)). Irradiation at λ = 400 nm (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.

Inverting External Asymmetric Induction via Selective Energy Transfer Catalysis: A Strategy to ß-Chiral Phosphonate Antipodes.

Enantiodivergent, catalytic reduction of activated alkenes relays stereochemical information encoded in the antipodal chiral catalysts to the pro-chiral substrate. Although powerful, the strategy remains vulnerable to costs and availability of sourcing both catalyst enantiomers. Herein, a stereodivergent hydrogenation of α,ß-unsaturated phosphonates is disclosed using a single enantiomer of the catalyst. This enables generation of the R- or S-configured ß-chiral phosphonate with equal and opposite selectivity. Enantiodivergence is regulated at the substrate level through the development of a facile E → Z isomerisation. This has been enabled for the first time by selective energy transfer catalysis using anthracene as an inexpensive organic photosensitiser. Synthetically valuable in its own right, this process enables subsequent RhI -mediated stereospecific hydrogenation to generate both enantiomers of the product using only the S-catalyst (up to 99:1 and 3:97 e.r.). This strategy out-competes the selectivities observed with the E-substrate and the R-catalyst.