Bifunctional catalysts for catalytic asymmetric sulfur ylide epoxidation of carbonyl compounds

Abstract

Carbonyl epoxidation using diazo compounds mediated by catalytic quantities of sulfide and metal catalyst has been investigated using sulfides linked to the metal catalyst. In this study a range of bis-oxazolines with pendant sulfides were tested in the asymmetric epoxidation process using Cu(I)Br (this was the optimal metal catalyst). Although the enantioselectivity was poor (< 24% ee), it was possible to use a much lower catalyst loading (5 mol%) than we had previously achieved (20 mol% was the lower limit) without compromising the yield. This is believed to be because ylide formation is now an intramolecular process and therefore fast compared to the reaction of the metal carbenoid with the diazo compound (resulting in stilbene formation) which no longer competes significantly.