Carbocation/Polyol Systems as Efficient Organic Catalysts for the Preparation of Cyclic Carbonates

Carbocation/polyol systems are shown to be highly efficient catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide at 50 °C and 5 MPa CO₂ pressure. The best activity was shown by the combination of crystal violet and 1,1'-bi-2-naphthol (BINOL), which could be recycled five times with no loss of activity. The presence of specific interactions between the amino groups of the carbocation and the hydroxyl protons was confirmed by NMR experiments. The Job plots for the crystal violet iodide/BINOL and brilliant green iodide/BINOL systems showed that the catalytic systems consist of one molecule of the carbocation and one molecule of BINOL. Mechanistic studies using a deuterated epoxide indicate that there was some loss of epoxide stereochemistry during the reaction, but predominant retention of stereochemistry is observed. On this basis, a catalytic cycle is proposed.