Ultrasound-assisted phase-transfer catalysis: Benzoylation of sodium 4-acetylphenoxide by dual-site phase-transfer catalyst in a tri-liquid system

A novel dual-site phase-transfer catalyst (PTC) was prepared and used to conduct the benzoylation of sodium 4-acetylphenoxide by ultrasound-assisted third-liquid phase-transfer catalysis. The catalyst 1,4-bis(tributylammoniomethyl)benzene dibromide (BTBAMBB) was synthesized from the reaction of p-xylylene dibromide and tributylamine in toluene at 70 °C. The dual-site PTC was employed to form the third-liquid phase by extra addition of 0.04–0.05 mol of NaCl into 10 cm3 of water. In the condition of 0.0425 mol of NaCl at 30 °C, the catalytic intermediate in the third-liquid phase reached a maximum value. Almost 80% of the catalyst was transferred from the aqueous phase into the third-liquid phase. The distributions of the catalytic intermediate and dual-site PTC between phases and the kinetics of benzoylation of sodium 4-acetylphenoxide catalyzed by BTBAMBB with ultrasound irradiation were performed. The pseudo-first-order kinetic equation was applied to describe the overall reaction. Under ultrasound irradiation (28 kHz/300 W) in a batch reactor, the yield of product 4-acetylphenyl benzoate in the organic phase was 98.1% in 2 min at 30 °C and 250 rpm with the apparent rate constant kapp to be 0.0075 s−1, which was 6 times faster than that without using ultrasound (yield = 14.4%, kapp = 0.0013 s−1). The present study provides a green method to synthesize esters by ultrasound-assisted third-liquid phase-transfer catalysis.