17O, 95Mo and 1H NMR study of the mechanism of epoxidation of alkenes with hydrogen peroxide in the presence of molybdenum complexes

Abstract

Using 17O, 95Mo and 1H NMR spectroscopy, the composition and reactivity of molybdenum (VI) complexes formed in the course of the catalytic epoxidation of cyclohexene and allylic alcohol with hydrogen peroxide in aprotic solvents were studied. Dioxobis(acetylacetonato)molybdenum(VI) and molybdenum(VI) oxide were used as starting materials for the catalysts. Five molybdenum(VI) complexes ( I–V) were identified and characterized with NMR spectroscopy. The following compositions for I–V are suggested: I is the dimeric complex [MoO 2(acac)] 2O; II the hexamolybdate anion Mo 6O 2− 19, III the complex MoO(O 2)(acac) 2; IV and V are the diperoxo complexes MoO(O 2) 2L and MoO(O 2) 2L·H 2O, respectively (where L is H 2O or H 2O 2). Complexes III and V are inert towards alkenes. IV reacts with cyclohexene yielding cyclohexene oxide and trans-cyclohexane-1,2-diol. In the course of this reaction IV converts mainly into II in acetonitrile and into 1,2-diolo molybdenum (VI) complexes in dioxane. Further portions of H 2O 2 convert II and the 1,2-diolo-molybdenum(VI) complex back to IV and to V. To elucidate the role of the preliminary coordination of the double bond of alkene to molybdenum in the epoxidation with diperoxo molybdenum complexes, reactions Of MoO(O 2) 2·HMPA (HMPA = hexamethylphosphoric triamide) with cyclohexene and allylic alcohol were also investigated. Our 95Mo, 17O NMR data, when taken together with kinetic data available in the literature, suggest that such coordination is not important. These reactions are more likely to proceed via the direct interaction of alkenes with one of the peroxygens.