Efficient Access to Chiral Benzhydrols via Asymmetric Transfer Hydrogenation of Unsymmetrical Benzophenones with Bifunctional Oxo-Tethered Ruthenium Catalysts

A concise asymmetric
transfer hydrogenation of diaryl ketones,
promoted by bifunctional Ru complexes with an etherial linkage between
1,2-diphenylethylenediamine (DPEN) and η6-arene ligands,
was successfully developed. Because of the effective discrimination
of substituents at the ortho position on the aryl group, unsymmetrical
benzophenones were smoothly reduced in a 5:2 mixture of formic acid
and triethylamine with an unprecedented level of excellent enantioselectivity.
For the non-ortho-substituted benzophenones, the oxo-tethered catalyst
electronically discerned biased substrates, resulting in attractive
performance yielding chiral diarylmethanols with >99% ee.