Mild and expedient asymmetric reductions of α,β-unsaturated alkenyl and alkynyl ketones by TarB-NO2 and mechanistic investigations of ketone reduction.

A facile and mild reduction procedure is reported for the preparation of chiral allylic and propargyl alcohols in high enantiomeric purity. Under optimized conditions, alkynyl and alkenyl ketones were reduced by TarB-NO2 and NaBH4 at 25 °C in 1 h to produce chiral propargyl and allylic alcohols with enantiomeric excesses and yields up to 99%. In the case of α,β-unsaturated alkenyl ketones, α-substituted cycloalkenones were reduced with up to 99% ee, while more substituted and acyclic derivatives exhibited lower induction. For α,β-ynones, it was found that highly branched aliphatic ynones were reduced with optimal induction up to 90% ee, while reduction of aromatic and linear aliphatic derivatives resulted in more modest enantioselectivity. Using the (L)-TarB-NO2 reagent derived from (L)-tartaric acid, we routinely obtained highly enantioenriched chiral allylic and propargyl alcohols with (R) configuration. Since previous models and a reduction of a saturated analogue predicted propargyl products of (S) configuration, a series of new mechanistic studies were conducted to determine the likely orientation of aromatic, alkenyl, and alkynyl ketones in the transition state.