Substituent effect on the amino-imino tautomerism of aminothiazoles

The effect of type and substituent position on the conformational, structure and amino-imino tautomerism of aminothiazoles in the gas, water and carbon tetrachloride phases has been studied using the B3LYP/6-311++G(d,p) and MP2(full)/6-311++G(d,p) levels. They showed that the amino form is more stable than the imino in the three phases. This was in a good agreement with the experimental data. The electron donating group (-CH3); positioned at four and five, increased the amount of imino form. Despite the greater substituent stability of the imino, the amino tautomer was found to be the predominant species. The electron-withdrawing group attached to the exocyclic nitrogen increased the acidity of exocyclic -NH group, thus facilitating the hydrogen transfer to the endocyclic nitrogen atom. The total energy of the imino form was lower than that of the amino, exhibiting the coexistence of the two structures in the gas phase. The tautomerization became spontaneous in solutions at room temperature and the imino was the predominate.