Rapid and efficient Knoevenagel condensation catalyzed by a novel protic ionic liquid under ultrasonic irradiation

An efficient and reusable protic-ionic-liquid solvent-catalyst system, HMTA-AcOH-H2O, has been developed and used in the Knoevenagel condensation reaction of aromatic aldehydes with ethyl 2-cyanoacetate. Under ultrasonic irradiation, the Knoevenagel condensation promoted by the protic-ionic-liquid solvent-catalyst system proceeds smoothly and cleanly. Moreover, the HMTA-AcOH-H2O solvent-catalyst system could be recycled for at least 6 times and no significant loss of activity was observed. This protocol has notable advantages, such as being eco-friendly, the ease of the work-up and reuse of the ionic liquid conveniently, which could help reduce disposal costs and contribute to the development of new catalysts for use in green and continuous chemical processes.