2//-1,2,3-Triazole, relative aromaticity

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%). 

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...

Copper Corrosion Inhibitors. The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole (BZT) and tolyltriazole (TTA). These compounds bond direcdy with cuprous oxide (CU2O) at the metal surface, forming a "chemisorbed" film. The plane of the triazole Hes parallel to the metal surface, thus each molecule covers a relatively large surface area. The exact mechanism of inhibition is unknown. Various studies indicate anodic inhibition, cathodic inhibition, or a combination of the two. Other studies indicate the formation of an insulating layer between the water surface and the metal surface. A recent study supports the idea of an electronic stabilization mechanism. The protective cuprous oxide layer is prevented from oxidizing to the nonprotective cupric oxide. This is an anodic mechanism. However, the triazole film exhibits some cathodic properties as well. 

Discussion of these compounds is divided into isomers of aromatic compounds, and dihydro and tetrahydro derivatives. The isomers of aromatic azoles are a relatively little-studied class of compounds. Dihydro and tetrahydro derivatives with two heteroatoms are quite well-studied, but such compounds become more obscure and elusive as the number of heteroatoms increases. Thus dihydrotriazoles are rare dihydrotetrazoles and tetrahydro-triazoles and -tetrazoles are unknown unless they contain doubly bonded exocyclic substituents. 

In studies aimed at understanding the influence of structure on the reactivity of diazonium ions, Diener and Zollinger (1986) found that the NMR chemical shifts of the aromatic or heteroaromatic parent compounds provided a novel probe. This method can be applied both to substituted benzenediazonium ions and to various heteroaromatic diazonium ions, and it also provides semiquantitative information on the relative reactivities of the l,3,4-triazole-2-diazonium ion (12.5) and its deprotonated zwitterion (12.6). 

The relative stability of 1,2,3-triazole and benzotriazole tautomers can be rationalized based on the concept of aromaticity. The aromaticity of both 1,2,3-triazole tautomers probably being similar, the lone-lone pair repulsion accounts for the lower stability of 1//-1,2,3-triazole. In the case of benzotriazole, the aromaticity of the benzenoid 1//-benzotriazole has been considered greater than that of the quinonoid 2//-benzotriazole <89JA7348>. However, great care must be taken in measuring the difference in aromaticity between the two tautomers because this will depend strongly on the dielectric constant of the medium. 

Partially or fully reduced triazoles are relatively unstable and have not aroused great interest except for those with exocyclic double bonds, which retain the potential aromatic character of the parent triazole. Compounds such as triazolinone (2) and 3,5-dioxo-l,2,4-triazolidine (urazole) (3) will be treated as triazoles, since they can be written in the tautomeric triazole forms (2a) and (3a) (Equations (2) and (3)). 

We have devoted three papers explicitly to the relationships between aromaticity and tautomerism the first to the tautomerism of 1,2,3-triazole (30) and benzotria-zole (31) [46], If, in the first case, the relative stabilities are determined by the lone-pair/lone-pair repulsion of the adjacent lone pairs that destabilize 30a, in the second case this is partly compensated by the greater aromaticity of the benzenoid structure 31a. In the second paper, we discuss the aromaticity of formal 47i-electron antiaromatic 17/-2-azirine (32), 671-electron aromatic l,27/-3-diazetine (33), pyrrole (34), and 1,2-dihydropyridazine (35) [47], Compounds 33 and 35 are not planar and not aromatic. 

The parent 1,2,3,5-thiatriazole and its derivatives are even less stable than the isomeric 1,2,3,4-triazoles (see Section 6.09.5). Although according to theoretical calculations the former should be more aromatic than the latter (see Section 6.10.2.2), they are unstable and decompose upon preparation. 1,2,3,5-Thiatriazole-l-oxides and 1,1-dioxides are relatively stable solid compounds with melting points in the range of 100-200 °C <2004HOLJ833>. 

Given the aromatic nature and small size of the 1,2,4-triazole ring, the relatively few measurements on which the accepted notion of its coplanarity rests need not be questioned. If puckering could be induced through the bonding or steric repulsion of substituents, one would expect optical activity which has not been observed as yet. Triazole derivatives with asymmetric substituents (22) can be obtained in optically active form and as a stereospecific dienophile it has found extensive use in the resolution of homocyclic dienes (80JOC5105). A little information is available on the conformation of some triazoles (siRCRSse). 

Chemical cellulose esters are relatively stable to UV radiation since they lack aromatic chromophores. Even so, exposure to UV radiation may cause some chain scission and loss of physical properties in cellulose esters exposed to outdoor environments esters formulated for such use must be stabilized accordingly. Some resorcinol and benzophenone derivatives, such as resorcinol monobenzoate and 2-hydroxy-4-methoxybenzophenone, are reportedly excellent UV-light stabilizers for cellulose esters (56,57). Other stabilizers include piperidine derivatives (58) and substituted triazole compounds alone (59) and in combination with resorcinol monobenzoate (60). 

Hydrogen borrowing and dehydrogenative condensations provide new opportunities for the preparation of both saturated and aromatic heterocycles respectively. The ability to directly access azacycles from stable species such as alcohols and amines allows chemists to circumvent the preparation and use of relatively unstable carbonyls and alkyl halides that conventional methods require. Pyridines, pyrazines, pyrroles, as well as fused bicyclic heteroaromatics, may all be prepared by dehydrogenative condensation this reactivity will likely be extended to pyrimidines, imidazoles, pyrazoles, and triazoles in the near future. Continuous advances in scope and scalability will expand the role of hydrogen transfer in the discovery and production of small molecule therapeutics. 

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