Long-Range ¹H–¹⁵N <i>J</i> Couplings Providing a Method for Direct Studies of the Structure and Azide–Tetrazole Equilibrium in a Series of Azido-1,2,4-triazines and Azidopyrimidines

Abstract

The selectively ¹⁵N labeled azido-1,2,4-triazine <b>2</b>*<b>A</b> and azidopyrimidine <b>4</b>*<b>A</b> were synthesized by treating hydrazinoazines with ¹⁵N-labeled nitrous acid. The synthesized compounds were studied by ¹H, ¹³C, and ¹⁵N NMR spectroscopy in DMSO, TFA, and DMSO/TFA solutions, where the azide–tetrazole equilibrium could lead to the formation of two tetrazoles (<b>T</b>, <b>T′</b>) and one azide (<b>A</b>) isomer for each compound. The incorporation of the ¹⁵N label led to the appearance of long-range ¹H–¹⁵N coupling constants (<i>J</i>_HN), which can be measured easily by using amplitude-modulated 1D ¹H spin–echo experiments with selective inversion of the ¹⁵N nuclei. The observed <i>J</i>_HN patterns enable the unambiguous determination of the mode of fusion between the azole and azine rings in the two groups of tetrazole isomers (<b>2</b>*<b>T′, 4</b>*<b>T′</b> and <b>2</b>*<b>T, 4</b>*<b>T</b>), even for minor isoforms with a low concentration in solution. However, the azide isomers (<b>2</b>*<b>A</b> and <b>4</b>*<b>A</b>) are characterized by the absence of detectable <i>J</i>_HN coupling. The analysis of the <i>J</i>_HN couplings in ¹⁵N-labeled compounds provides a simple and efficient method for direct NMR studies of the azide–tetrazole equilibrium in solution