Toward the Design of Halide- and Metal- Free Ionic- Liquid Catalysts for the Cycloaddition of CO2 to Epoxides

Carbonate and carboxylate methyltrioctylphosphonium ionic liquids [P-8881][A], made by a totally halide- and metal-free procedure, have been used as new organocatalysts for the cycloaddition of CO2 to styrene oxide. At 100 degrees C and ambient pressure, the performance of such compounds, particularly the acetate salts, was quite satisfactory. Even in the presence of catalyst amounts as low as 1-5mol%, reaction conversions of 35-75% were achieved with 100% selectivity towards the expected cyclic carbonate. An analysis on the effect of the catalyst structure and loading, as well as operative parameters including temperature and pressure, demonstrated that anions played a major role on the activity and the stability of the ionic liquids. A parallel transformation of the phosphonium cation into its phosphine oxide was also identified as a consequence of the combined action of epoxide reagents and weakly nucleophilic carbonate and carboxylate species. Accordingly, a reaction mechanism was formulated.