Quantum Chemical Study of Tautomerism in 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol

Abstract

The relative stabilities of five tautomers of 2-[(4-phenylthiazol-2-yl)hydrazonomethyl]phenol were calculated at the B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) levels of theory. The possible tautomeric transformations were also analyzed at the same level taking into account the solvent effect with the integral equation formalism polarizable continuum model (IEF-PCM) using three different solvents. The enolhydrazo- thiazole (T1) form has been found to be the predominant tautomer, and the tautomers follow the stability pattern: enol-hydrazo-thiazole (T1) > enol-imine-thiazoline (T2) > keto-hydrazo-thiazoline (T4) > keto-amine-thiazole (T5) > enol-azo-thiazole (T3). The tautomeric barrier heights for T1 ⇌ T2, T1 ⇌ T3 and T4 ⇌ T5 reactions are very high in both the forward and reverse directions. In the case of T1 ⇌ T5 and T2 ⇌ T4 tautomerizations, the forward proton transfer is not possible because of large barrier height. According to the B3LYP results of the reverse reactions, only the proton transfer in the gas phase and in chloroform is allowed for T1 ⇌ T5 while it is found to be barrierless for T2 ⇌ T4. However, the MP2 computations show that the reverse T1 ⇌ T5 reaction impossible for all cases while the reverse T2 ⇌ T4 reaction needs very low energy. These results are also corroborated by the thermodynamic parameters obtained at the B3LYP/6-311+G(d,p) level. The barrier energy height increases with the increasing polarity of the solvent in general, however, the trend is not observed for all cases. © 2016 Elsevier B.V.