1,2,4-Triazole, aromaticity

The Groebke-type multi-component reaction between 3-amino-l,2,4-triazole, aromatic aldehydes and benzylic isonitriles afforded iV-alkylidene-4//-imidazo[l,2-A [l,2,4]triazol-6-amines in moderate to good yields <2006TL6891>. 

Primary aromatic diamines cannot be diazotised (tetrazotised) and coupled normally. Thus o-])henylenediamiiie yields a triazole derivative and m-phenylenediamine gives an azo dye (Bismarck brown) by selfcoupling. 

Many chemical compounds have been described in the Hterature as fluorescent, and since the 1950s intensive research has yielded many fluorescent compounds that provide a suitable whitening effect however, only a small number of these compounds have found practical uses. Collectively these materials are aromatic or heterocycHc compounds many of them contain condensed ring systems. An important feature of these compounds is the presence of an unintermpted chain of conjugated double bonds, the number of which is dependent on substituents as well as the planarity of the fluorescent part of the molecule. Almost all of these compounds ate derivatives of stilbene [588-59-0] or 4,4 -diaminostilbene biphenyl 5-membeted heterocycles such as triazoles, oxazoles, imidazoles, etc or 6-membeted heterocycles, eg, coumarins, naphthaUmide, t-triazine, etc. 

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. 

The aromatic character is critically dependent upon the position of the heteroatoms in the ring, and oxygenated compounds have marked diene character. Various ERE determinations of 1,2,4-triazole have given values ranging between 83.7 and 205.8 kJ moP (Table 35). LCAO-SCF calculations, however, suggest that the ring is substantially less stable than the diazoles but more stable than tetrazole. 

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. 

The pyrazole ring is particularly difficult to cleave and, amongst the azoles, pyrazoles together with the 1,2,4-triazoles are the most stable and easiest to work with. This qualitative description of pyrazole stability covers the neutral, anionic and cationic aromatic species. On the other hand, the saturated or partially saturated derivatives can be considered as hydrazine derivatives their ring opening reactions usually involve cleavage of the N—C bond and seldom cleavage of the N—N bond. It should be noted, however, that upon irradiation or electron impact the N—N bond of pyrazoles can be broken. 

The mesomeric effect of the C=S linkage is very pronounced and is responsible for the facile quaternization of heterocyclic N-alkylated thiones (159) this effect is operative even when such a shift does not increase the aromaticity of the ring. Thione derivatives of pyridazine, benzothiazole, quinazoline, 1,3-thiazine, triazole,and isoindole are examples of compounds which readily form quaternary salts. 

The classical age of preparative organic chemistry saw the exploration of the extensive field of five-membered heterocyclic aromatic systems. The stability of these systems, in contrast to saturated systems, is not necessarily affected by the accumulation of neighboring heteroatoms. In the series pyrrole, pyrazole, triazole, and tetrazole an increasing stability is observed in the presence of electrophiles and oxidants, and a natural next step was to attempt the synthesis of pentazole (1). However, pentazole has eluded the manifold and continual efforts to synthesize and isolate it. 

More recently, activity in the held of the preparation of phthalocyanine-like compounds useful in material science concentrated on compounds containing only one triazole subunit (triazolophthalocyanines) 89 [94JCS(CC)1525 95ICA(230)153].These aromatic compounds (without or with metals in the cavity) present a problem of annular tautomerism of triazoles, but as yet it is known only that the NH is outside the cavity. 

Cyclization of the 3-(2-aminoethyl)-l,2,4-triazoles 60 with aromatic aldehydes gave a mixture of the corresponding Schiff bases 61 and the 5,6,7,8-tetrahydro-l,2,4-triazolo[l,5-c]pyrimidines 62. Cyclization of 60 with car-bonyl-1,1 -diimidazole (CDI) afforded the 5-oxo analogs 63 (92JPR630) (Scheme 22). 

Attempted dehydrocyclization of the 6-acylhydrazinopyrimidine 65 by heating with polyphosphoric acid led, instead, to pyrimidine ring rupture, yielding the l,l-diamino-2-nitro-2-(3-phenyl-l,2,4-triazol-5-yl)ethene 66. Cyclocondensation of the latter with triethyl orthoformate gave the fully aromatic triazolopyrimidine 67 (94JHC1171) (Scheme 23). 

The interaction of l-methoxybut-l-en-3-yne with aromatic azides proceeds at the unsubstituted acetylenic bond to furnish two structural isomeric triazoles, 166 and 167 (4 1 ratio), due to the different 1,3-dipole orientations (83DIS). 

Ribavirin, an antiviral agent used against hepatitis C and viral pneumonia, contains a 1,2,4-triazole ring. Why is the ring aromatic ... 

The diazotization of heteroaromatic amines is basically analogous to that of aromatic amines. Among the five-membered systems the amino-azoles (pyrroles, diazoles, triazoles, tetrazoles, oxazoles, isooxazoles, thia-, selena-, and dithiazoles) have all been diazotized. In general, diazotization in dilute mineral acid is possible, but diazotization in concentrated sulfuric acid (nitrosylsulfuric acid, see Sec. 2.2) or in organic solvents using an ester of nitrous acid (ethyl or isopentyl nitrite) is often preferable. Amino derivatives of aromatic heterocycles without ring nitrogen (furan and thiophene) can also be diazotized. 

Benzotriazole can exist in two tautomeric forms, l-//-benzotriazole (6.46, R = H) and 2-/f-benzotriazole. If the aromatic ring contains a substituent, the 1- and 3-nitrogen atoms of the triazole are not equivalent, and therefore a 3-//-benzotri-azole derivative can also exist. The equilibrium between the 1 -H and 2-H tautomers of benzotriazoles is strongly on the side of the 1 -H tautomer, in contrast to triazole where the 2-H tautomer is dominant. Tomas et al. (1989) compared experimental data (enthalpies of solution, vaporization, sublimation, and solvation in water, methanol, and dimethylsulfoxide) with the results of ab initio theoretical calculations at the 6-31G level. 

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). 

Regarding the series of hetero aromatic pentacyclic compounds with three heteroatoms, an accelerated synthesis of 3,5-disubstituted 4-amino-1,2,4-triazoles 66 under microwave irradiation has been reported by thermic rearrangement of dihydro-1,2,4,5 tetrazine 65 (Scheme 22). This product was obtained by reaction of aromatic nitriles with hydrazine under microwave irradiation [53]. The main limitation of the method is that exclusively symmetrically 3,5-disubstituted (aromatic) triazoles can be obtained. 

Another important click reaction is the cycloaddition of azides. The addition of sodium azide to nitriles to give l//-tetrazoles is shown to proceed readily in water with zinc salts as catalysts (Eq. 11.71).122 The scope of the reaction is quite broad a variety of aromatic nitriles, activated and nonactivated alkyl nitriles, substituted vinyl nitriles, thiocyanates, and cyanamides have all been shown to be viable substrates for this reaction. The reaction of an arylacetylene with an azide in hot water gave 1,4-disubstituted 1,2,3-triazoles in high yields,123 while a similar reaction between a terminal aliphatic alkyne and an azide (except 111 - nitroazidobenzcnc) afforded a mixture of regioisomers with... 

If CDI is employed as imidazolide, methyl 2,4,6-trihydroxybenzoate is obtained in 47% yield. Propionyl-1,2,4-triazole behaves in the same way as the imidazolides, giving similar yields (50%), but the benzotriazolides and benzimidazolides were not as effective. The o-nitrophenyl and p-chlorophenyl esters of propionic acid did not lead to any aromatic products.11185 Similar 5C + 1C condensation reactions are described in references [119] and [120],... 

Aliphatic and aromatic carboxamides, with the exception of p-nitrobenzamide, are dehydrated in this way in high yield. Acid-labile protective groups such as tetra-hydropyranyl and tert-butyldimethylsilyl ether and base-sensitive compounds are not attacked. A,A -Sulfinyldi-1,2,4-triazole, easily prepared from thionylchloride and triazole [THF, (C2H5)3N, 0 °C, 1 h] in 85-95% yield, was used without further purification. 

A different result was obtained in the cycloaddition to methylenecyclo-propanes 216-218 tearing alkoxycarbonyl substituents on the cyclopropyl ring. In this instance, 1,2,3-triazoles 220 isomeric with the triazolines 219 were formed in the reaction [57]. The formation of triazoles 220 is rationalised by the intermediate formation of triazolines 219, which are unstable under the reaction conditions and undergo a rearrangement to the aromatic triazoles via a hydrogen transfer that probably occurs with the assistance of the proximal ester carbonyl (Scheme 35). The formation of triazoles 220 also confirms the regio-chemistry of the cycloaddition for the methylene unsubstituted methylene-cyclopropanes, still leaving some doubt for the substituted ones 156 and 157. 

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