2H-1,2,3-Triazol

D-arab/no-Hexos-2-ulose phenylosotriazole or (1 fl)-1-(2-phenyl-2H-1,2,3-triazol-4-yl))-D-erythritol or 2-phenyl-4-(D-arab/no-1,2,3,4-tetrahydroxybutyl)-2H-1,2,3-triazole... 

Other heterocyclic N-oxides have been found to display biological activities that may depend on their proven or hypothesised ability to release NO these include 4/ /-pyrazol-4-oTie 1,2-dioxides 71, 2H-1,2,3-triazole 1-oxides 72, benzotetrazine 1,3-dioxides 73 and 1,2,3-benzotriazine 3-oxides 74. These systems have been less extensively studied than the furoxan and 1,2-diazetine dioxide systems discussed above. 

A number of 2H-1,2,3-triazole 1-oxides 72 were prepared by chemists at the Cassella Company as potential NO-donors in view of their formal structural similarity with furoxan derivatives [18]. Derivative 72a was studied in depth. It was obtained by cupric sulfate oxidation of intermediate 79, derived from the action of the substituted phenylhydrazine 78 on the oximino acetoacetic acid amide 77 (Scheme 6.13). 

Oxazole Isoxazole 1,2,4-Triazole 4H-1,2,4-T riazole 1H-1,2,3-T riazole 2H-1,2,3-Triazole... 

The aromaticity of triazole tautomers was assessed by the Bird indices 122 2H-1,2,3-triazole (91B, 1= 88) was found to be slightly more aromatic than its 1H-isomer (91A, 1= 73). The small difference in Bird indices supports only a weak influence of the aromaticity, and the lower stability of the 1 //-isomer was explained by the nitrogen lone-pair repulsion that destabilizes cyclic azo derivatives. 

There is an excellent correlation between the experimental microwave dipole moments of a variety of azoles and those calculated by the ab initio method at the 6-31G //6-31G level p,exp = 0.942p,ca, + 0.008 (86JPC5597). With this equation, the experimental values for 1H- and 2H-1,2,3-triazole are predicted as 4.35 D and 0.32 D, respectively (89JCO(10)426). Thus, quantum chemical calculations are a valuable aid in estimating tautomeric equilibria by dipole moment studies (90ZN(A)1328). 

According to the indices, pyrazole is more aromatic than imidazole. The stability of azoles generally increases with an increasing number of aza-groups, though some exceptions are known. The relative aromaticities of triazoles and tetrazole are questionable. 2H-1,2,3-Triazole (/= 88%) which is the more stable in the gas phase reveals more bond levelling than 1//-1,2,3-triazole (1=13%). 

Reaction of p-tolyl substituted 2H-1,2,3-triazoles with anils of aromatic aldehydes was originally confined to the 2-(p-tolyl)-4-phenyl-and 2-(/)-tolyl)-4,5-diphenyl-2//-l,2,3-triazoles (134a and b), which, with 4-phenylbenzalaniIine, gave the styryl derivatives 135a and b, respectively.57... 

CA Registry No [288-36-8]. This structure also exists as the tautomeric forms, 2H-1,2,3-Triazole with a CA Registry No [288-35-7], and in a 4H-form, which is relatively rare... 

Table 42 gives an overview of annular tautomerism data for azoles in the gas phase and in solution or crystals. In the gas phase the stability of alternative tautomers largely depends on their relative aromaticities. In Section 2 A.4.2.2 it was noted that 1,2-relationships between pyrrole- and pyridine-type nitrogen atoms favor aromaticity (Figure 21) and this is consistent with the relative stabilities of triazole and tetrazole tautomers in the gas phase (Table 42) <2010T2695>. In solution (and crystals) other factors such as solvent polarity, hydrogen bonding, and temperature become important and the relative stabilities can be reversed. Polar solvents tend to stabilize the tautomer with the largest dipole moment and this probably accounts for the observation of both 2H-1,2,3-triazole (p = 0.12D) and H-1,2,3-triazole (p = 4.55D) in...

H-1, 2,3-Triazol 4-Hydroxy-2-phenyl- E8d, 357 (1-Oxid-Red.) 111-1,2,4-Triazol 5-Oxo-3-phenyl-4,5-dihydro- X/2, 159... 

H-1,2,3-Triazol 2,4,5-Tris-[trime-thylsilyl]- E8d, 316 (R3Si-C = C—SiR3 + RjSi-N3)... 

Several types of molecular orbital calculation have been carried out on IH- and 2H- 1,2,3-triazole structures, often in combination with predictions of the most reactive sites, and studies of dipole moments, H, and NMR chemical shifts, proton exchange... 

Using CNDO/2 calculations, relative base strengths for IH- and 2H-1,2,3-triazole, as well as for and lH,3//-triazolium ions, have been obtained and compared with... 

In turn, from the total energy Et it now follows that 2H-1,2,3-triazoles are more stable than the IH-tautomers, where ET = E .+E +E - -E uaeus- This last calculation was confirmed recently by PES spectra (see Section 4.11.3.2.10) and dipole moment measurements (see Section 4.11.3.2.2), comparing them with the corresponding AT-methyl derivatives. A refined and optimized molecular structme has also been given for both the IH-and 2H-tautomers (Figure 2) (81ZN(A)1246). 

Gas phase pyrolysis of the unsubstituted IH- and 2H-1,2,3-triazole mixture resulted in the formation of acetonitrile as the main product, and HCN and NHs occur as by-products. At about 850 °C, an unexpectedly high pyrolysis temperature, 1,2,3-triazole was decomposed quantitatively. This is explained by the predominance of the 2//-form in the gas phase (78JCR(S)298). 

Irradiation of 2H-1,2,3-triazoles (91) in ether gives 22% MeCN, and in the presence of excess cyclopentene it affords the adduct (92) (68CC977). 4H-Triazolines (93) bearing methoxycarbonyl and alkyl groups at the j3 -position give, on thermolysis, 2H- 1,2,3-triazoles (94) and/or 3-amino-2i/-azirines on photolysis, only azirines are formed (80CC940). 

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