Tautomerism of tetrazole

Extensive studies of the annular tautomerism of tetrazoles have been reported. These can be broadly divided between the gaseous, liquid solution and solid states. 

Chermahini AN, Nasr-Esfahani M, Dalimasab Z, Dabbagh HA, Teimouri A (2007) Theoretical studies on tautomerism of tetrazole derivatives by polarisable continuum method (PCM). J Mol Struct (Theochem) 820 7-11... 

Other authors have used dipole moments as an auxiliary technique to other physicochemical methods. Thus, Lumbroso has studied the tautomerism of 5-(p-aryl)tetrazoles in function of the substituent at the para position and compared the results with those obtained by NMR spectroscopy (Section VT,C) (80JHC1373). For a detailed description of Lum-broso s technique see [81JST(77)239]. 

Four pairwise degenerate tautomeric forms referred to as the IH (27a and 27d) and IH (27b and 27c) tautomers are needed to describe the annular prototropic tautomerism of unsubstituted tetrazole 27 (R = H) (Scheme 14). 

Valence tautomerism of heterocyclic systems containing tetrazole rings has received much attention [76AHC(S1), p. 498 84CHEC-I(5)791 96CHEC-1I(4)621], The latest contributions to this area are considered in Section IV,C, which is concerned with the topic of azido-tetrazole tautomerism. 

Azide-tetrazole isomerism, or valence tautomerism, was not discussed for [5.6]bicyclic systems in the previous survey (76AHCS1). During recent years, this type of ring-chain tautomerism has been extensively studied for both six- and hve-membered heterocyclic azides. Tire tautomerism of [5.5]bicyclic tetrazole systems is covered in Section II,C. We discuss the tautomerism of the six-membered heterocyclic azides in this section. 

All triazoles, tetrazoles, and unsymmetrically substituted imidazoles and pyrazoles can exist in two tautomeric forms, e.g., 1 2 and 3 4. However, attempts to isolate the individual tautomers have been unsuccessful, always leading to one isomer (for summaries of this aspect of the tautomerism of imidazoles, see references 1 and 2). Although the isolation of both tautomers of a number of com-... 

Proton and earbon NMR studies of interannular conjugation effects in aryl-substituted tetrazoles have been discussed <84CHEC-I(5)791>. The use of NMR spectroseopy to study tautomerism in tetrazole systems is eovered in Seetion 4.17.4. 

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... 

When dealing with tautomeric studies based on pK (AG°) values (proton gain), it is important whether both tautomers form the same cation when protonated (see Section V). For instance, the two tautomers of tetrazole, 11a and 11b, form two different cations (80KGS665 84CS84) and thus the basic pKaS of tetrazoles cannot be used to determine their tautomeric composition. 

A detailed analysis of substituent effects on the pKa values of imidazoles and tetrazoles as well as benzimidazoles and naphthimidazoles has been made.308 The ortho effect is shown to parallel that of 2-substituted pyridines and quinolines47 190 and application of the Hammett equation to the tautomerism of these systems is also considered. The equation also satisfies the effect of substituents on the basicity of the nitrogen in the 1-position for 1-pyrazolines.309... 

Within the last decade, ab initio and hybrid quantum-chemical methods were in considerable use in tetrazole chemistry, and the level of calculations significantly improved with extended basis sets used for quite complex polyatomic molecules. During this time, theoretical methods were exploited in the study of several fundamental properties of the terazole ring, such as aromaticity and capability to be involved in various kinds of tautomerism, including the effects of substituents and media on these parameters. It was demonstrated that many physical and physicochemical characteristics of tetrazoles could be successfully estimated by these methods not only for the gas phase but also for the condensed state (solvents, crystals). 

Annular tautomerism inherent to NH-unsubstituted tetrazoles and tetrazolium ions governs many chemical and physicochemical properties of these compounds. Since the 1950s, this type of protolytic equilibrium has been investigated by various teams. In this section, we analyze the results of experimental and theoretical studies on the annular tautomerism of NH-tetrazoles and their conjugate acids. The results of investigations published within the last decade are considered from the viewpoint of the classical concepts based on earlier research. 

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