Tautomeric ratio in 4-methylthiazol-2-ylguanidine, a model guanidinoheterocycle

Abstract

The pK_a values of fixed model tautomers have been used to assess tautomeric ratio in the title compound. Although pK_T values of 3.62 and 5.32 between the major and the two minor tautomers result, reasons are given for believing that the method exaggerates both ratios. Nevertheless the arylimino tautomer is very dominant and evidence is adduced that this conclusion may be general for guanidinoheterocycles. If so, only this tautomer may be considered as the form that interacts with the histamine H₂-receptor.