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

Abstract

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,β-unsaturated phosphonates is disclosed using a single enantiomer of the catalyst. This enables generation of the R- or S-configured β-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.