Highly Enantioselective Oxazaborolidine-Catalyzed Reduction of 1,3-Dicarbonyl Compounds: Role of the Additive Diethylaniline

The oxazaborolidine-catalyzed reduction of 2,2-disubstituted cycloalkan-1,3-diones or hindered 2,2-disubstituted cyclic ketones using catecholborane as reductant proceeds with greater enantioselectivity when N,N-diethylaniline is added. It has now been shown that the effect of this additive is to catalyze the conversion of a harmful minor impurity in catalyst preparations to the active catalyst.