1,2,4-Triazoles. IV. Tautomerism of 3,5-disubstituted 1,2,4-triazoles.

Abstract

Tautomerism of ten 3, 5-disubstituted 1, 2, 4-triazoles, which can exist in three tautomeric forms, has been studied. The nuclear magnetic resonance (NMR) and ultraviolet (UV) spectra of these 1, 2, 4-triazoles were compared with those of the N-methyl derivatives, which are the model compounds of the three kinds of tautomeric forms. Results indicate that a tautomeric hydrogen atom of the predominant tautomer of 3, 5-disubstituted 1, 2, 4-triazoles is attached to the nitrogen atom at position 1 or 2 which is closer to the more electron-releasing substituents in the 3 or 5 position. For instance, 3-p-nitrophenyl-, 3-p-chlorophenyl-, and 3-γ-pyridyl-5-methylthio-1, 2, 4-triazole exist predominantly in the 1H form, while 3-p-aminophenyl- and 3-p-methoxyphenyl-5-methylthio-1, 2, 4-triazole exist predominantly in the 2H form. These results are explained in terms of the formation of cyclic dimer-type hydrogen bonds involving the N-1 and N-2 atoms of triazole rings with an exchange of a proton between the molecules. However, 5-methylthio-3-α-pyridyl-1, 2, 4-triazole is exceptional in existing in the 2H form, since this compound can form hydrogen bonds of another type involving the nitrogen atom of the α-pyridyl group.