L-proline-based DES in Knoevenagel synthesis of arylidene rhodanines, thiazolidine-2,4-diones, and barbituric derivatives

Deep eutectic solvents (DES) are environmentally friendly solvents that prevent the use of toxic organic solvents and have been extensively studied in recent years. However, volatile organic compounds (VOC) are often still used during workup and isolation of products. Here, a zero-VOC strategy for Knoevenagel reaction is reported. (Hetero)aromatic aldehydes are successfully condensed with rhodanine, thiazolidine-2,4-dione TZD, or barbituric acid under mild conditions in an L-proline-based DES and pure compounds are obtained after hydrolysis without any purification. For the less reactive TZD, activation by microwave allows diminution of reaction time from 24 h to just 1 h.