SOME FEATURES OF EPOXIDATION OF CYCLOHEXENE CATALYZED BY OXOPEROXOMETALLATES UNDER PHASE-TRANSFER CONDITIONS

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 PO43- > AsO43-. 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 η2 diperoxo-ligands in these complexes, which otherwise have similar structures. © 1991.