Enantioselective enhancing selectivity, challenge

Terminal olefins represent another challenging substrate class. For olefins such as styrene, cis-trans partitioning leads to diminished catalyst enantioselec-tivity (60-70% ee). A viable solution was reported through an efficient low-temperature Mn(salen) epoxidation protocol employing N-methylmorpholine N-oxide and ra-CPBA [77]. Improved enantioselectivities were attainable for most substrates under low-temperature conditions, but the effect was especially pronounced in the case of terminal olefins. Epoxidation of styrene, for instance, occurred rapidly to afford the epoxide in 86% ee using catalyst 22g (Scheme 8). Deuterium-labelling experiments revealed that the improved enantioselectivity derived from enhancement of olefin facial selectivity in initial C-0 bond formation as well as suppression of deleterious cis-trans partitioning. 

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