Azide-tetrazole ring-chain isomerism in polyazido-1,3,5-triazines, triazido-s-heptazine, and diazidotetrazines

The azide-tetrazole isomerism in several polyazido-1,3,5-triazines, triazido-sym-heptazine, and some diazido-1,2,4,5-tetrazines was investigated by ab initio quantum chemical methods in order to determine whether the polyazides are suitable starting materials for the synthesis of the isomeric tetrazoles. The effects of solvation in CCl4, DMSO and water on this isomerism were included using the self consistent reaction field (SCRF) method. The effect of amino- and nitro-substituents on the azide-tetrazole isomerism was also examined. In the gas phase all investigated polyazidoheterocycles do not cyclize to form tetrazoles. An electron-donating amino group favors the ring closure to tetrazoles, whereas an electron-withdrawing nitro group favors the azides. Solvation in polar solvents favors the formation of a tetrazole ring system due to higher charge separation in the tetrazole ring system, but for all polyazido-1,3,5-triazines, including triazido-s-heptazine, the effects of solvation are not strong enough to shift the equilibrium to the tetrazole side, which explains why several attempts to detect these compounds have failed. The monotetetrazoles of diazidotetrazine and bis(azido)azo-1,2,4,5-tetrazine and the ditetrazole of bis(azido)hydrazo1,2,4,5-tetrazine are the minimum energy species in DMSO and water. Thus we predict that the diazidoazo- and hydrazotetrazines will readily cyclize to the tetrazoles in polar solvents.