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Résumé :
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As disclosed in preceding studies [Munshi and Beckman Ind. Eng. Chem. Res. 2009, 48, 1059−1062 and Beckman and Munshi US Patent 7,271,286, 2007], aluminum chloride is superior among Lewis acids in making p-toluic acid from toluene and 6.9 MPa CO2 offering 80% stoichiometric yield, with respect to Al2Cl6, and 98% selectivity at 80 °C in 18 h. Attempts to improve the efficiency resulted in the finding that H+ exchange between HCl and Tol-COOAl2Cl5 is the key step in making the reaction catalytic. Introduction of a base complemented by a fluorinated solvent can only bring Al2Cl6 into catalytic cycle, provided pKa[Tol-COOH] > pKa[BH+] > pKa[HCl]. Efficiency of the reaction depends upon the nature of the fluorinated solvents and reaction parameters employed. Turnover number of 7.87 for p-toluic acid in 6 h is obtained employing 2.68 mmol quinoxaline, 15 mL 1,3,5-tris(trifluoromethyl)benzene, and 2.68 mmol of Al2Cl6 and CO2 at a pressure of 6.9 MPa CO2 and 100 °C. The base and fluorinated solvent could be recyclable and reused.
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