Chiral phosphinamides: new catalysts for the asymmetric reduction of ketones by borane

We have identified a new class of catalysts for the asymmetric reduction of prochiral ketones by borane. Key to the architecture of effective catalysts is an N-P=O structural unit which may be part of a phosphinamide, phosphonamide or a related structure, Such catalysts are simple to prepare, are often crystalline solids and may be recovered from reduction reactions and reused. The catalysts act essentially as Lewis bases, serving to increase the reactivity of borane by electron donation. The incorporation of a hydroxy group into the catalyst provides an adjacent Lewis acid site upon reaction with borane and thus affords a superior catalyst capable of asymmetric inductions of up to 92% ee.