Benzotriazole, 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). 

To illustrate the results summarized in Table IX, some examples are discussed in detail. The large number of papers discussing the annular tautomerism of azoles (first column) is exemplified by the studies concerning benzotriazole (57). Wofford, Forkey, and Russell, using pyrazoles and imi-... 

Table 17 Annular Tautomerism of 1,2,3-TriazoIe and Benzotriazole (76AHC(S1)296)...

The annular prototropic tautomerism of the parent 1,2,3-triazole and benzotriazole are described in CHEC-I <84CHEC-I(5)691> and was reviewed earlier <76AHCS295>. 

The understanding and discussion of pK (AG°) values needs knowledge of the structure of the species involved in the equilibrium. The problem of the tautomerism of neutral species is well known [Section IV,G and reference (76MI3)]. However, it is often not recognized that a similar problem arises with charged species, e.g., in cations formed on protonation of neutral molecules having two or more basic sites. This tautomerism can be classified as annular, as in benzotriazole (10) protonated forms, or as functional, as in 1-methyl-4-aminopyrazole (529) cations. 

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