1,2,3 -Triazole anions, tautomers

Our contribution to the reactivity of aromatic heterocycles involves only two papers [136], In the first one, we carried out IRC calculations (HF/6-31G ) to confirm the connecting structures of each transition state for the 1,2,3-triazole/iminodia-zomethane isomerizations. For the triazolate anion, both tautomers of 1,2,3-triazole and several 1,2,3-triazolium cations were studied. The below structure reports the case of the neutral molecules 30a and 30b. 

There are two triazoles, and each has one pyrrole-like nitrogen and two pyridine-like nitrogens. Both triazoles have the possibility of tautomerism (in 1,2,3-triazole the tautomers are identical) and both give rise to a single anion. 

An intramolecular counterpart of this reaction is the cyclization (Scheme 17) of anions derived from vinyl azides the neutral azides fail to cyclize, presumably because the cyclic tautomer (the triazole) is not aromatic, whereas the anion is. 

Firstly, the nondegenerated tautomers of 3-nitro-l,2,4-triazole-5-one (NTO) radical anions were investigated by ESR method during electrochemical reduction of NTO in aprotic medium at different temperatures. It was shown that observed reversible temperature variations in the ESR spectra of radical anions were caused by tautomerism. Quantum chemical calculations evidence that l,4-/f-tautomer of radical anion is most preferable [881],... 

Triazole is fairly resistant to A -alkylation under neutral conditions, however both acylations and alkylations involving iV-anions occur readily, but mixtures of 1- and 2-substituted products are often obtained/ Trimethylsilylation produces 2-trimethylsilyl-1,2,3-triazole, and this can be alkylated unambiguously at N-1, with concomitant loss of silicon/ An equilibrium mixture of A-acetyl-1,2,3-triazoles contains predominantly the 2-acetyl isomer. The 2//-tautomer is also dominant in 1,2,3-triazole itself, which may reflect unfavourable adjacent imine lone-pair/lone-pair interactions in the 1//-isomer and is in agreement with calculations that suggest that the 2//-isomer is more aromatic." Heating in sulfolane at 150 °C converts the A-acyl compounds into oxazoles in a synthetically useful transformation. ... 

In marked contrast to the anion (40), which equilibrates with its (A) tautomer (Table 6), the p-triazole congener (41), once formed by cyclization of the respective azide (A), undergoes rapid... 

Similar analyses carried out for CS-substituted 1,2,4-triazoles (2004 JMST(680)107) revealed that in these derivatives electron-withdrawing substituents and the CS-substituted anion stabilize the IH-tautomer, whereas electron-donating substituents favor the 2H- tautomer (Scheme 4). The 4H- form of the C5-substituted molecules is the least stable tautomer. 

---