Epoxidation of Alkenes by Hydrogen Peroxide over 12-Heteropolyacids of Molybdenum and Tungsten (H 3 PMo 3 W 9 O 40 ) Combined with Cetylpyridinium Bromide

Abstract

In the epoxidation of 4-vinylcyclohex-1-ene with H₂O₂ in monophasic acetonitrile solution catalysed by Keggin-type 12-heteropolyacids, i.e., H₃PMo_12-nW _n O₄₀ (n = 0–12), which are precursors of active peroxo complexes, and phase transfer catalysts Q⁺Br^–, the catalyst H₃PMo₃W₉O₄₀ showed the highest activity, giving a conversion of 98% and a selectivity of 88%. By this method, a variety of water-insoluble unactivated alkenes, internal or terminal, open chain or cyclic and isolated, were epoxidised under mild conditions and after relatively short reaction times. The state of the H₃PMo₃W₉O₄₀/CPB/H₂O₂/CH₃CN system was studied using UV, IR, and ³¹P NMR spectroscopies with the [H₂O₂]: [HPA] ratio = 50. Several peroxo species were observed by ³¹P NMR spectroscopy at a lower field than the original heteropolyacids. Their composition varied regularly with that of the starting catalyst. The P-containing peroxo species formed were deduced as [(PO₄){Mo_4–xW _x O₂₀}]^3- (x = 0-4), which are the true catalytically active species under the reaction conditions.