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 additive s, 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 wi th the catalysed reaction. When low substrate/catalyst ratios or addit ives are used, an acceleration of the epoxidation reaction is observed . As a result a larger difference in rate with the blank reaction occu rs, resulting in an increase in the enantioselectivity. (C) 1997 Elsev ier Science Ltd.