Asymmetric synthesis using chirally modified borohydrides. Part 1. Enantioselective reduction of aromatic ketones with the reagent prepared from borane and (S)-valinol
The asymmetric reduction of aromatic ketones with the reagents prepared from borane and chiral aminoalcohols was studied under various conditions. The ratio of borane to (S)-valinol was found to be optimum at 2–3:1, when up to 65–73% selectivity was obtained in the reduction of n-propyl phenyl ketone. The amino-alkoxy-amine-boranes (2) and/or (3) were tentatively proposed as the reaction species responsible for asymmetric induction.