Enhancement of the enantioselectivity in the epoxidation of sterically hindered substrates by decreasing the contribution of the blank reaction

In the case of asymmetric epoxidation reactions catalysed by Jacobsen's catalyst, the substrate/catalyst ratios used are low. Often additives, such as N-oxides, are employed to enhance the enantioselectivity of the reaction. We have found that for reactions of sterically hindered, less reactive substrates, a non-selective blank reaction competes with the catalysed reaction. When low substrate/catalyst ratios or additives are used, an acceleration of the epoxidation reaction is observed. As a result a larger difference in rate with the blank reaction occurs, resulting in an increase in the enantioselectivity. (C) 1997 Elsevier Science Ltd.