Heterocycles 52, 291 (2000)

DOI: 10.3987/COM-99-S23

A theoretical Study of the 1,2,3-Triazolo-Iminodiazomethane Isomerism in the Gas Phase

The ring opening reactions of the 1,2,3-triazolate anion, of the three tautorners of NH-1,2,3-triazole, and of different 1,2,3-triazolium cations, to give the corresponding iminodiazomethanes, have been studied by means of ab initio calculations at the HF/6-31 ** level. The 1,5-electrocyclization reactions of 1,2,3-triazolate and 1H-1,2,3-triazole proceed through planar transition states with high and medium energy barriers, respectively. In both cases the cyclic forms are thermodynamically more stable than the open-ring ones. In contrast, the decomposition of the 1,2-H-1,2,3-triazolium cation into protonated iminodiazo-methane is an exothermic process. In the last case, the preferable way of the reaction proceeds via non-aromatic intermediates (1,4H2- and 1,1H2-1,2,3-triazolium ions) which have a very low activation energy towards the ring opening. It has been found that there is a high ππ-conjugation between diazornethyl and imine fragments in the chain forms.