5-Substituted tetrazoles, annular tautomerism

Annular tautomerism (e.g. 133 134) involves the movement of a proton between two annular nitrogen atoms. For unsubstituted imidazole (133 R = H) and pyrazole (135 R = H) the two tautomers are identical, but this does not apply to substituted derivatives. For triazoles and tetrazoles, even the unsubstituted parent compounds show two distinct tautomers. Flowever, interconversion occurs readily and such tautomers cannot be separated. Sometimes one tautomeric form predominates. Thus the mesomerism of the benzene ring is greater in (136) than in (137), and UV spectral comparisons show that benzotriazole exists predominantly as (136). 

Most of the experimental work on the annular tautomerism of 5-substituted tetrazoles has been carried out on solutions and a number of major studies are described in CHEC-I <84CHEC-I(5)79l>. Nitrogen-15 NMR spectra of 5-substituted tetrazoles and their N-1 and N-2-methyl isomers show... 

Systematic replacement of CH in pyrrole 1 (Chapter 2.3) by N leads to nine additional monocyclic heteroaromatic nitrogen systems 2-10 (Figure 1), which are known collectively as azoles. Annular tautomerism is an important feature of all azoles having an NH function. For example, the triazoles 4 and 6, the triazoles 5 and 7, and the tetrazoles 8 and 9 can equilibrate by proton transfer (see Section 2.4.5). N-Substituted derivatives cannot equilibrate. Tautomers of the parent ring systems of all the azoles except pentazole 10 are known TV-aryl derivatives of pentazole have been characterized . 

Measured dipole moments for a series of substituted tetrazoles gave interesting comparisons (56JA4197). The moments for 1-ethyltetrazole (5.46 D) and 2-ethyl tetrazole (2.65 D) were at either side of the mesoionic compound (7) (4.02 D) and the dipole moments could not be used to identify mesoionic structures. The dipole moments for the 1,5-disubstituted tetrazole structure were consistently high, >5.3 D, while those of the 2,5-disubstituted structure were low, <2.65 D theoretical calculations support this (61T237). Recently, dipole moment measurements were made on 5-(p-tolyl)tetrazole in dioxane (fio 4.99), its 1-methyl isomer 6.03) and its 2-methyl isomer (jUd 2.41) (80JHC1374). These results, when applied to the annular tautomerism of 5-( p-tolyl)tetrazole, suggested 60 10% of the 1-NH form in this medium. 

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