Enantioselective Borane Reduction of Acetophenone Catalysed by Oxazaborolidines Derived from Chiral Diethanolamines

Article abstract of DOI:10.1016/0957-4166(95)00134-B

The synthesis of oxazaborolidines of types 1 and 2, derived from chiral diethanolamines and their behaviour, as catalysts for the asymmetric reduction of acetophenone by borane has been investigated.Bicyclic oxazaborolidines of type 2 afforded the expected alcohol in good yield but with modest ee's (ee<40percent).Monocyclic oxazaborolidines of type 1 gave better results.For instance, catalyst 16 allowed the isolation of (S) α-methyl benzyl alcohol with 72percent ee while oxazaborolidine 26, structurally close to 16, produced a remarkable reversal of enantiofacial selectivity.The origin of the enantioselectivity of these catalysts is discussed on the basis of transition state models.