Environmentally friendly epoxidation of olefins under phase-transfer catalysis conditions with hydrogen peroxide

Several catalysis systems, WO3⋅H2O/H2O2–H2O–H3O+/Q+A−/H3PO4/H2SO4/solvent (Q+A− = Arquad 2HT®, [CH3(n-C8H17)3N]+Cl−; [CH3(n-C8H17)3N]+HSO4−, [CH3(n-C8H17)3N]+H2PO4−; solvent: CHCl3 or toluene) were used to selectively and efficiently convert olefins to their corresponding epoxides at room temperature. With cyclooctene and using Arquad 2HT® as the phase-transfer agent, there is a synergy when both phosphate and sulfate anions are present in the reaction medium compared with systems using either one or the other. The importance of the tungsten(VI) source is, as found previously, underlined by the strong activity increase when WO3⋅H2O is used instead of Na2WO4⋅2H2O, even at room temperature. The influence of the phase-transfer agent Q+A− has been evaluated for the system WO3⋅H2O/H2O2–H2O–H3O+/Q+A−/toluene. With Q+A− (Q+ = [CH3(n-C8H17)3N]+ and A− = Cl−, HSO4−, and H2PO4−), the best results for the conversion of cyclooctene at room temperature are obtained with [CH3(n-C8H17)3N]+H2PO4−. 31P NMR experiments show the transfer in the organic phase of the [PO4{W2O2(μ-O2)2(O2)2}2]3− and [HPO4{W2O2(μ-O2)2(O2)2}]2− complexes with H3PO4 and H2PO4−, whereas only [PO4{W2O2(μ-O2)2(O2)2}2]3− can be identified with the addition of H3PO4/HSO4− or H2PO4−/H2SO4. Moreover, acid-sensitive epoxides can be prepared using buffers generated by the addition of sodium hydrogenocarbonate or, preferably, disodium hydrogenophosphate, leading to high selectivities toward the corresponding epoxides. The data show that disodium hydrogenophosphate gives the best results even if the reaction time has to be increased to obtain high conversions. The WO3⋅H2O/H2O2–H2O/[CH3(n-C8H17)3N]+H2PO4−/toluene catalysis system can be reused in 5 consecutive runs with no loss in activity.