Highly facile homogeneous epoxidation of olefins using oxo-diperoxo tungstate(vi) complex as catalyst, bicarbonate as co-catalyst and hydrogen peroxide as a terminal oxidant

Abstract

Addition of a dilute acetic acid solution of 8-quinolinol to an H₂O₂ solution of freshly precipitated H₂WO₄·2H₂O furnishes a yellow adduct [WO(O₂)₂·2QOH] 1 which, on crystallization from a suitable solvent, affords orange-yellow complex [WO(O₂)(QO)₂] 2. When 2 reacts stoichiometrically with olefinic compounds in a 1∶1 molar ratio, the respective olefins are epoxidized and 2 is converted to the orange-red [WO₂(QO)₂] 3. When 1 is treated with an excess of H₂O₂ (greater than 6 equiv.) and PPh₄Cl, an anionic light yellow complex PPh₄[WO(O₂)₂(QO)] 4 is obtained. 4 reacts with cyclopentene (a representative olefin) in a 1∶1 molar ratio producing cyclopentene oxide and itself is converted to PPh₄[WO₂(O₂)(QO)] 5. If the above reaction is conducted at a 1∶2 molar ratio (instead of 1∶1) then 2 moles of the corresponding epoxide is formed and 4 is converted to PPh₄[WO₃(QO)] 6. All these peroxo complexes have remarkable catalytic efficiencies in the epoxidation of olefinic compounds when used in tandem with NaHCO₃ as co-catalyst and H₂O₂ as oxidant in a CH₃CN medium at room temperature, the method being green and economical. The catalyst 4under the above experimental conditions shows so far unmatched efficiency in epoxidizing a wide variety of olefinic substrates.