Tetrabutylammonium bromide and K2CO3: an eco-benign catalyst for the synthesis of 5-arylidene-1,3-thiazolidine- 2,4-diones via Knoevenagel condensation
Phase-transfer catalyzed, energy-efficient and facile synthesis of 5-arylidene-1,3-thiazolidine-2,4-diones was developed. Three independent variables (temperature, bases and phase-transfer catalyst (PTC)) were screened through one-factor-at-a time (OFAT) study. The optimum reaction conditions suggested by the OFAT analysis were the use of tetrabutylammonium bromide (8 mol%) and potassium carbonate (1 mmol) for the reaction at 100°C. The nitrogen of PTC stabilizes carbonyl groups of thiazolidine-2,4-dione (TZD). The active methylene hydrogen of TZD forms potassium salt with potassium carbonate and generates 5-arylidene-1,3-thiazolidine-2,4-diones (1–16) through nucleophilic attack on the carbonyl carbon of arylaldehydes. The prominent advantages of this new process are economic viability, shorter reaction time (15 min), simple product isolation (non-chromatographic method), good to excellent yields (78–96%) and solvent-free conditions.