Photochemical Deracemization of Chiral Alkenes via Triplet Energy Transfer

A visible-light-mediated, enantioselective approach to axially chiral alkenes is described. Starting from a racemic mixture, a major alkene enantiomer is formed due to selective triplet energy transfer from a catalytically active chiral sensitizer. A catalyst loading of 2 mol % was sufficient to guarantee consistently high enantioselectivities and yields (16 examples, 51%-quant., 81–96% ee). NMR studies and DFT computations revealed that triplet energy transfer is more rapid within the substrate–catalyst complex of the minor alkene enantiomer. Since this enantiomer is continuously racemized, the major enantiomer is enriched in the photostationary state.