TFE assisted mechanochemical synthesis of new pyrazolones from Meldrum acid carbothioamides-Experimental and theoretical studies

Abstract

Facile synthesis of Meldrum acid carbothioamides 3 from condensation of Meldrum acid and aryl isothiocyanates under ultrasonication is reported. The structure of carbothioamides was investigated in solid and solution state from X-ray diffraction and spectroscopic studies, respectively. Studies on photo-tautomerism emanating from proton switching between O and S under UV irradiation have been reported. S-methyl derivatives 4 of carbothioamides 3 were also prepared and solid state structure of the representative compound was described. Novel synthetic mechanochemical route to synthesis of functionalized pyrazolones 5 from reaction of carbothioamides 3 or their S-methyl derivatives and hydrazine hydrate in presence of 2,2,2-trifluoroethanol has been developed. The structure and proton tautomerism of pyrazolones has been assigned on spectroscopic grounds and authenticated by single crystal X-ray diffraction (SCXRD) studies. The tautomeric equilibrium between different forms of pyrazolone 5a has been studied by ab initio calculations. Experimental studies are in congruous with theoretical results. Further, synthesis of new thiazole derivatives 6 and thiazolidin-5-one derivatives 7 linked with Meldrum's acid (MA) moiety was achieved from reaction of 3 with 3-chloro-2,4-pentanedione and chloroacetyl chloride, respectively. Spectroscopic studies confirmed the structure of new developed products.