1-Substituted 1,2,4-triazole 4-oxides

The formula for the parent 2-substituted 1,2,3-triazole 1-oxide 341 is shown in Scheme 100. 

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). 

The resonance structures of the 2-substituted 1,2,3-triazole 1-oxides 341 are discussed in Section 1.1.1. According to IUPAC nomenclature, structure 341 is a 2-substituted 2H-l,2,3-triazole 1-oxide. Other names found in the literature are 2-substituted 1,2,3-triazole 1-oxides. In the present review the most commonly used naming, which is accepted by IUPAC, Chem. Abstr. Autonom., has been adopted calling structure 341 a 2-substituted 1,2,3-triazole 1-oxide. 

The first 1,2,3-triazole 1-oxide 326 was reported in 1898 (1898JPC160, 1898G173). It was obtained in low yield by oxidative cyclization of 2-hydroxyiminophenylhydrazones (2-oxime phenylhydrazones) using HgO as the oxidant. About 285 2-substituted 1,2,3-triazole 1-oxides 326 with a variety of substituents at C3, C4, and C5 have been reported. A review appeared in 1989 (1989CHE113). Examples of the use of 1,2,3-triazole 1-oxides in the synthesis of substituted 1,2,3-triazoles have been discussed (1988BSB573). 

Despite many attempts it has not been possible to oxidize 2-substituted 1,2,3-triazoles 382 to the corresponding 1-oxides 326. Peracetic acid, 3-chloroperbenzoic acid, dichloropermaleic acid, trifluoroperacetic acid, peroxydisulfuric acid, and f-pentyl hydrogen peroxide in the presence of molybdenum pentachloride all failed to oxidize 382 (1981JCS(P1)503). Alkylation of 1-hydroxytriazoles 443 invariantly produced the isomeric 3-substituted 1,2,3-triazole 1-oxides 448 (see Scheme 132). However, the 2-substituted 1,2,3-triazole 1-oxides 326 can be prepared by oxidative cyclization of 2-hydroxyiminohydrazones (1,2-hydrazonooximes, a-hydrazonooximes) 345 or by cyclization of azoxyoximes 169. Additional methods of more limited scope are reaction of nitroisoxazoles 353 with aryl-diazonium ion and base, and reaction of nitroimidazoles 355 with hydroxy-amine- or amine-induced rearrangement of nitro-substituted furoxanes 357. 

Bromine at C5 of 2-substituted 1,2,3-triazole 1-oxides 361 is readily removed without touching the N-oxygen by treatment with aqueous-methanolic Na2S03 (2010UP2). Debromination has also been achieved by metallation followed by protonation as described in Section 4.1.6.4 (Scheme 109). 

These observations lead to the conclusion that 2-substituted 1,2,3-triazole 1-oxides 326 react with AcCl with a regioreactivity that tends to drop in the order C5, C4 > 5-Me > C4 > C2 > 4-Me. 

Substituted 1,2,3-triazole 1-oxides 326 are silylated at the oxygen atom to give moisture sensitive silyloxytriazolium salts, which have been characterized by their NMR spectra (1993JCS(P1)625, 1992JCS(P1)2555). 

Substituted 1,2,3-triazole 1-oxides 346 act as 1,3-dipoles when heated with diethyl acetylene dicarboxylate yielding 440 (1987CC706, 1990JCS(P1)3321) A conceivable mechanism, slightly different from that presented in the original paper, is sketched in Scheme 124. 

The 2-substituted 1,2,3-triazole 1-oxides 326 have been used for synthesis of 2- substituted 1,2,3-triazoles 382 and regioselectively substituted 1-hydroxy-1,2,3-triazoles 443 manipulating the substituent in the N-oxide... 

---