Silver-mediated epoxidations

Abstract

It has been shown that Ag₂O and AgNO₃, the latter in the presence of tertiary amines, e.g. pyridine, can achieve the oxidation of alkenes to epoxides with iodosylbenzene as the oxygen donor. The reactions have been studied with different alkenes and it has been found that the oxygen-transfer reaction takes place in a non-stereoselective manner, trans-epoxides being the major oxidized product when cis-alkenes are the substrates. The influence of different types of tertiary amines on the epoxidation with AgNO₃ as the ‘catalyst’ has been investigated and it has been found that the presence of electron-withdrawing groups in the pyridine ring leads to a better epoxidation system. Labelling studies with [¹⁸O]iodosylbenzene or [¹⁸O]water indicate that the epoxide oxygen is derived from iodosylbenzene or water, suggesting an oxosilver intermediate. The epoxidation of alkenes can also be achieved by Ag₂O₂, and the yield of epoxides and stereoselectivity are comparable to those with Ag₂O and iodosylbenzene, indicating an identical reaction path. The reactive high-valent oxosilver intermediate and the mechanism for the oxygen-transfer reaction are discussed in the light of the related transition metal–iodosylbenzene and the silver surface–molecular oxygen epoxidations.