Toward the Design of Halide‐ and Metal‐Free Ionic‐Liquid Catalysts for the Cycloaddition of CO 2 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 CO 2 to styrene oxide. At 100 °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 1–5 mol %, 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.