It is thiazolidine-2,4-dione’ and not thiohydantoins as the reaction product of monosubstituted thioureas and chloroacetylchloride The reaction products of monosubstituted phenylthioureas with chloroacetylchlorlde in a polyethylene glycol (PEG-400) medium and K2 C03 as base/catalyst at an elevated temperature are exclusively thiazolidine-2,4-dlones and not thlohydantolns as has been reported. The core unit thiazolidine-2,4-dlone is essentially derived from chloroacetylchloride and the thioamidlc part of the phenylthiourea.
The scope of this unprecedented transformation has been evaluated with electron-rich and electron- poor phenylthioureas. O o s K2C03 (5 mol PEG-400, 80 oc x NH2 K2C03 (5 mol H20 NH2 Keywords: monosubstituted thiourea; chloroacetylchloride; PEG-400; thiazolidine-2,4- dione Introduction The mechanism of thiazole-2-imine formation has not been well understood leading to the proposal of incorrect structures for the products obtained by the reaction of thiourea analogs with ahaloketones by two independent research groups (1). Based on the principle of hard-soft reactivity, i. . hard nucleophile attacks on hard electrophile and the soft nucleophile on soft electrophile, we have suggested an alternative reaction mechanism for the formation of thiazole-2-imine. Our mechanism is supported by isolating the reaction intermediate, confirming its structure and that of the products by X-ray crystallography (2). Having understood the reactivity of thioureas toward a-haloketones, recently, we have suggested an lternative structure for the product obtained by the reaction of thiourea with chloroacetylchloride (3).
The structure of the product is found to have the 2-imino-4-thiazolidinones moiety (3) and not the thiohydantoin as has been Downloaded by [Florida Atlantic University] at 10:01 04 November 2011 Results and discussion Ar NH Scheme 1 . PEG/ K2C03 Our expected product Actual product obtained Reported structure, our proposed structure and the actual structure. H Mechanism 1: Mechanism 2: Scheme 2. Mechanism for the formation of iminothiazolidin-4-one (1 a). Scheme 3. s OH2 Mechanism for the formation of thiazolidine-2,4-dione (1 b). Conclusion In conclusion, we have unambiguously proved that the reaction product of monosubstituted thiourea with chloroacetylchloride in the presence of K2 C03 using PEG-400 solvent is thiazolidine-2,4-dione and not the reported thiohydantoins or the expected isomeric iminothiazolidine-4-one. Furthermore, the scope of this transformation was evaluated with different monothioureas under identical conditions.