Tetrazole relative aromaticity

According to the indices, pyrazole is more aromatic than imidazole. The stability of azoles generally increases with an increasing number of aza-groups, though some exceptions are known. The relative aromaticities of triazoles and tetrazole are questionable. 2H-1,2,3-Triazole (/= 88%) which is the more stable in the gas phase reveals more bond levelling than 1//-1,2,3-triazole (1=13%). 

Table 42 gives an overview of annular tautomerism data for azoles in the gas phase and in solution or crystals. In the gas phase the stability of alternative tautomers largely depends on their relative aromaticities. In Section 2 A.4.2.2 it was noted that 1,2-relationships between pyrrole- and pyridine-type nitrogen atoms favor aromaticity (Figure 21) and this is consistent with the relative stabilities of triazole and tetrazole tautomers in the gas phase (Table 42) <2010T2695>. In solution (and crystals) other factors such as solvent polarity, hydrogen bonding, and temperature become important and the relative stabilities can be reversed. Polar solvents tend to stabilize the tautomer with the largest dipole moment and this probably accounts for the observation of both 2H-1,2,3-triazole (p = 0.12D) and H-1,2,3-triazole (p = 4.55D) in...

Discussion of these compounds is divided into isomers of aromatic compounds, and dihydro and tetrahydro derivatives. The isomers of aromatic azoles are a relatively little-studied class of compounds. Dihydro and tetrahydro derivatives with two heteroatoms are quite well-studied, but such compounds become more obscure and elusive as the number of heteroatoms increases. Thus dihydrotriazoles are rare dihydrotetrazoles and tetrahydro-triazoles and -tetrazoles are unknown unless they contain doubly bonded exocyclic substituents. 

Azoloazoles represent interesting objects for study. Most of these structures are very unstable and can be regarded only as intermediates <1998JPR687>. Nonetheless, some, for instance pyrrolotetrazole and its derivatives, are relatively stable and can exist as the tautomeric forms 17 and 18 (Equation 2). In these structures only one of the heterocycles remains aromatic, for example, it is the tetrazole ring in the 5//-tautomer 18 and the pyrrole ring in the 1/7-tautomer 17. 

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