Ultrasound-assisted third-liquid phase-transfer catalyzed esterification of potassium 4-methoxyphenylacetate by dual-site phase-transfer catalyst

The catalytic esterification of potassium 4-methoxyphenylacetate to synthesize 4-methoxyphenylacetic acid butyl ester (an ether–ester compound) by ultrasound-assisted third-liquid phase-transfer catalysis (U-TLPTC) using the prepared dual-site phase transfer catalyst 1,4-bis(tributylammoniomethyl)benzene dibromide (BTBAMBB) was investigated. The catalyst BTBAMBB was synthesized from p-xylylene dibromide and tributylamine, and the catalytic intermediate was also synthesized to verify the reaction mechanism. The third-liquid phase containing more than 50% of the added catalyst was formed by introducing BTBAMBB, potassium 4-methoxyphenylacetate and a critical amount of NaCl in the aqueous phase. Without using catalyst, the reaction system was liquid–liquid type and the product was not observed. With BTBAMBB in liquid–liquid system the product yield was only 39.3% at 70 °C in 4 h of reaction even under sonication (28 kHz/300 W), but was increased to 94.2% by forming the third-liquid phase with NaCl at the same reaction conditions. The green process to synthesize ether–ester by U-TLPTC was developed with good catalytic efficiency of dual-site phase-transfer catalyst.

► The third-liquid phase was formed from dual-site PTC BTBAMBB by using NaCl. ► Above 50% of catalyst was concentrated in a small volume of the third-liquid phase. ► The product yield 94.2% in 4-h reaction was achieved with the third-liquid phase. ► BTBAMBB in the third-liquid phase was more effective than in liquid–liquid phases. ► The green process to synthesize ether–ester by ultrasound-TLPTC was developed.