Monocyclic 2-substituted 1,2,4-triazole

Monocyclic 2-substituted 1,2,4-triazole 1-oxides derived from 522 have been reported (Scheme 153). The structure of the /V-oxide fragment was proved by X-ray crystallography (2010LOCEC377). IR, H NMR, C NMR, and MS data have been published (1976JCS(P1)2166). 2-Substituted... 

We do not discuss in detail the cases of tautomerism of heterocycles embedded in supramolecular structures, such as crown ethers, cryptands, and heterophanes, because such tautomerism is similar in most aspects to that displayed by the analogous monocyclic heterocycles. We concentrate here on modifications that can be induced by the macrocyclic cavity. Tire so-called proton-ionizable crown ethers have been discussed in several comprehensive reviews by Bradshaw et al. [90H665 96CSC(1)35 97ACR338, 97JIP221J. Tire compounds considered include tautomerizable compounds such as 4(5)-substituted imidazoles 1///4//-1,2,4-triazoles 3-hydroxy-pyridines and 4-pyridones. 

Most of the reported 1,2,5,6-tetrazocine systems probably do not exist in the monocyclic form but rather as the tetraazapentalene betaine structure (see Introduction) 2 3 7"13 for the tetra-phenyl-substituted system, the compound does actually exist as the mesoionic tetraphenyl-[1,2.3]triazolo[l,2-h][l,2,3]triazole, as determined by H and 13C NMR spectroscopy.15 The following syntheses arc therefore questionable. 

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 . 

The intermediate bicyclic vinyl dihydro triazoles were generally not isolated, as they are unstable. By thermal decomposition, the vinylaziridines were converted to regioisomeric bicyclic pyrrolines 1 and/or 2, with different substitution patterns, depending on the nature of the substituents. Under the same conditions a 1,5-homodienyl shift can take place to give a monocyclic pyrroline 3. Nucleophilic displacement by iodide ion, however, regioselectively cleaves the C-N bond of the aziridine ring, giving 2 exclusively. 

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