A spectroscopic study on the reaction-controlled phase transfer catalyst in the epoxidation of cyclohexene

The epoxidation of cyclohexene with hydrogen peroxide in a biphase medium (H2O/CHCl3) was carried out with the reaction-controlled phase transfer catalyst composed of quaternary ammonium heteropolyoxotungstates [pi-C5H5N(CH2)(15)CH3](3)[PW4O16]. A conversion of about 90% and a selectivity of over 90% were obtained for epoxidation of cyclohexene on the catalyst. The fresh catalyst, the catalyst under reaction conditions and the used catalysts were characterized by FT-IR, Raman and (PNMR)-P-31 spectroscopy. It appears that the insoluble catalyst could degrade into smaller species, [(PO4){WO(O-2)(2)}(4)](3-), [(PO4){WO(O-2)(2)}(2){WO(O-2)(2)(H2O)}](3-), and [(PO3(OH)){WO(O-2)(2)}(2)](2-) after the reaction with hydrogen peroxide and becomes soluble in the CHCl3 solvent. The active oxygen in the [W2O2(O-2)(4)] structure unit of these soluble species reacts with olefins to form the epoxides and consequently the corresponding W-Ob-W (comer-sharing) and W-Oc-W (edge-sharing) bonds are formed. The peroxo group [W2O2(O-2)(4)] can be regenerated when the W-Ob-W and W-Oc-W bonds react with hydrogen peroxide again. These soluble species lose active oxygen and then polymerize into larger compounds with the W-Ob-W and W-Oc-W bonds and then precipitate from the reaction solution after the hydrogen peroxide is consumed up. Part of the used catalyst seems to form more stable compounds with Keggin structure under the reaction conditions. (C) 2003 Elsevier B.V. All rights reserved.