Some features of epoxidation of cyclohexene catalyzed by oxoperoxometallates under phase-transfer conditions

Abstract

The epoxidation of cyclohexene with hydrogen peroxide was investigated in the biphasic water-chlorobenzene system. It was observed that the autoxidation of cyclohexene to 2-cyclohexenyl-hydroperoxide is inhibited by the presence of the aqueous phase, but the inhibition caused by water is largely eliminated by the application of a phase-transfer catalyst (PTC). It was found that the yield of epoxide increased nearly linearly with increasing concentration of the molybdenum(VI) catalyst. A pH of 3–4 was found to be optimal for epoxide formation; the epoxide yield decreased with increase of the H + ion concentration mainly due to the H + ion-catalyzed hydrolysis of the epoxide formed. A PTC excess inhibits the epoxidation. In the case of molybdenum(VI), arsenate ion proved to be a more active promoter than phosphate ions by a factor of 1.6 when [Mo] ≥ [PTC]; however, the sequence of promoter activity was reversed when the PTC was applied in a 10-fold excess. Tungsten(VI) is about 3 times more active catalyst than molybdenum(VI). However, for tungsten(VI) the activity sequence was PO 4 3− > AsO 4 3−. The view is advanced that the enormous differences between the reactivities of the investigated oxoperoxometallate complexes and their heteropoly derivatives are connected with differences in the dissymmetry of the η 2 diperoxo-ligands in these complexes, which otherwise have similar structures.