Selective ¹⁵N-Labeling and Analysis of ¹³C−¹⁵N <i>J</i> Couplings as an Effective Tool for Studying the Structure and Azide−Tetrazole Equilibrium in a Series of Tetrazolo[1,5-<i>b</i>][1,2,4]triazines and Tetrazolo[1,5-<i>a</i>]pyrimidines

Two general methods for the selective incorporation of an 15N-label in the azole ring of tetrazolo[1,5-b][1,2,4]triazines and tetrazolo[1,5-a]pyrimidines were developed. The first approach included treatment of azinylhydrazides with 15N-labeled nitrous acid, and the second approach was based on fusion of the azine ring to [2-15N]-5-aminotetrazole. The synthesized compounds were studied by 1H, 13C, and 15N NMR spectroscopy in both DMSO and TFA solution, in which the azide−tetrazole equilibrium is shifted to tetrazole and azide forms, respectively. Incorporation of the 15N-label led to the appearance of 13C−15N J coupling constants (JCN), which can be measured easily using either 1D 13C spectra with selective 15N decoupling or with amplitude modulated 1D 13C spin−echo experiments with selective inversion of the 15N nuclei. The observed JCN patterns permit unambiguous determination of the type of fusion between the azole and azine rings in tetrazolo[1,5-b][1,2,4]triazine derivatives. Joint analysis of JCN patterns and 15N chemical shifts was found to be the most efficient way to study the azido-tetrazole equilibrium.