3- Substituted triazoles, tautomeric structure

This section covers ligands containing the 1,2,3-triazole ring system. These include, in addition to the parent triazole, various N- and/or C-substituted triazoles, benzotri azole, and a number of 8-azapurines. The coordination chemistry of 5-thio-l,2,3,4-thiatriazole is selectively reviewed. All of these molecules, with the exception of the N-substituted triazoles, are capable of coordinating in anionic as well as neutral form. 1,2,3-Triazole, first prepared by von Pechmann in 1888 (215), is a weak acid (p/ a = 9.26) (88) and exists as a mixture of the tautomeric forms (structures la and lb). Benzotriazole (2), first correctly formulated... 

Hydrazidoyl halides react with 2-amino[l,2,4]triazole to give 5-substituted 6-arylhydrazono-6//-imidazo[l,2-i][l,2,4]triazoles (350) and 6-arylhydrazono-5,6-dihydro-4//-imidazo[l,2-/>][ 1,2,4]-triazol-5-ones (351), respectively the latter products are not determined with respect to the tautomeric structures of the H-, 3H-, or 4/7-form (Scheme 32) <83JHC639>. 

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. 

Hydroxy-1,2,3-triazoles are known, but their detailed structure apparently has not as yet been investigated with respect to theit existence in hydroxy or oxo forms. Suitably substituted hydroxy-1,2,3-triazoles (120) can undergo another type of tautomerism to give the... 

The aromatic 2-substituted 1,2,3-triazole 1-oxides are represented by the structure 341. The parent compound 342 is one of the three possible tautomeric forms of 1-hydroxy-l,2,3-triazole 343. 1-Hydroxy-l,2,3-triazoles 343 constitute a separate group of compounds, which are not included in the present review (Scheme 101). 

In the reaction of 3-nitro-177-1,2,4-triazole with benzyl chloride it was possible to obtain l-benzyl-3-nitro-177-l,2,4-triazole (60%), l-benzyl-5-nitro-17/-l,2,4-triazole (10%), and 4-benzyl-3-nitro-177-l,2,4-triazole (4%), whose structures were determined by H and 13C NMR with homonuclear NOE difference spectroscopy (Table 3.21) [559], The tautomerism of C-nitro-substituted 1,2,4-triazoles was studied by NMR and IR spectroscopy [582], 31P NMR spectroscopy was employed for the identification of nitrated triazolyl-1 phosphates [583, 584],... 

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