Tetrazole equilibrium with imidoyl azide

The preceding section dealt with imidoyl azides in which the imido function is not part of a heteroaromatic (or otherwise highly conjugated) system. In these cases, the tetrazole turned out to be the thermodynamically favoured isomer. If the imido function is made part of a ring the azide form is often much more stable in many other cases, equilibria between azide and tetrazole forms are established. Lattice energies can make one form exclusive in the crystalline state of compounds, which in solution display an azide-tetrazole equilibrium . 

Azido-tetrazole tautomerism is also observed for partially hydrogenated azoles and even for monocyclic tetrazoles. The latter (258, X = NR ) may exist in equilibrium with an acyclic imidoyl azide... 

In the reaction of imidoyl halides with azide ions, imidoyl azides (31) are obtained, which caused some interest on account of their ring-chain tautomerism (31)/(32) (Scheme 43)." - ° It is not possible, however, to discuss this point in detail here. ° The final elucidation of a series of dubious structural assignments had to wait until the advent of modem spectroscopic methods. As expected, the position of this equilibrium is influenced by the nature of substituents R/R as well as by temperature and solvents. Electron-withdrawing ligands seem to favor the formation of the tetrazole isomer (32). The equilibrium (31)/(32) does not always operate — there are a number of cases known which show a comparatively high activation barrier.The same problems are encountered with the reaction products from 2-halo-azoles and -azines with azide ions. " °... 

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