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.