Guanidinium-based phosphotungstates and ionic liquids as catalysts and solvents for the epoxidation of olefins with hydrogen peroxide

摘要

Several penta- and hexaalkylated guanidinium-based ionic liquids (GILs) were tested as solvents for the epoxidation of cyclooctene using the Venturello catalyst, [(C_8H_(17))_3N(CH_3)] _3[PO_4{WO(O_2)_2}_4], and hydrogen peroxide as the oxidant. Epoxide yields were obtained in a broad range between 13 and 79 % depending on both the anion and the substituents on the guanidinium moiety. Recycling experiments showed that the catalyst can be used at least three times. Furthermore, new guanidinium phosphotungstates with the PW_(12)O_(40)~(3-) anion were synthesized and characterized. Their catalytic performance was evaluated, and GILs as well as acetonitrile were employed as the solvent. Results were compared to those obtained with the comparable ammonium-based catalysts [NR_4] _3[PW_(12)O_(40)] (R = C_4H_9, C_6H_(13)) and the analogous imidazolium-based catalyst [BMIM]_3[PW_(12)O_(40)] containing the 1-butyl-3-methylimidazolium cation. Employing different GILs as solvents, similar results were obtained for the guanidinium and imidazolium catalysts but significantly lower epoxide yields were obtained using the ammonium catalysts. On using acetonitrile as the solvent, guanidinium-based catalysts exhibited a better performance than the imidazolium catalyst in the case of linear and branched olefins and vice versa in the case of cyclic olefins. Several guanidinium phosphotungstates and guanidinium-based ionic liquids have been synthesized and characterized. Their catalytic performance in epoxidation reactions with hydrogen peroxide was evaluated and compared to similar ammonium and imidazolium compounds. The reaction systems can be recycled and used in at least three consecutive reactions.