1.2.3- Triazoles pyrolysis

The intermediacy of N-arylbenzotriazoles in the formation of carbazoles from o-anilinobenzenediazonium salts has already been mentioned in Section 3.03.2.3. The parallel conversion of 1,4- and 1,5-diphenyl-l,2,3-triazoles to 3-phenylindole with minor amounts of the 2-isomer has been effected by flash vacuum pyrolysis (Scheme 106a) (75JCS Pl)l). Similar treatment of 1,3,5- or 3,4,5-triphenyl-1,2,4-triazole provides 1,3-diphenylisoindole (Scheme 106b) <75JCS(P1)12>. 

The 1-azirines obtained from the vapor phase pyrolysis of 4,5-disubstituted 1-phthalimido-1,2,3-triazoles (157) have been found to undergo further thermal reactions (71CC1S18). Those azirines which contain a methyl group in the 2-position of the ring are cleaved to nitriles and phthalimidocarbenes, whereas those azirines which possess a phenyl substituent in the 2-position rearrange to indoles. 

The addition of phthalimidylnitrene (374) to simple alkynes affords 1-azirines in yields of 1-15% (Scheme 10). In this reaction, which is of no real preparative value, the symmetrical 2-azirines (375) were suggested as the most plausible intermediates and unequivocal proof of the existence of such species was demonstrated from a series of 1,2,3-triazole pyrolysis reactions <71CC1518). Extrusion of nitrogen from the regioisomeric 4,5-disubstituted 1,2,3-triazoles (376) during flash vacuum pyrolysis furnished identical product mixtures which included both regioisomeric 1-azirines (377). 

Although by no means a preparative route to either 1- or 2-azirines, the elimination of nitrogen (by flash vacuum pyrolysis at 400 °C) from the regioisomeric 4,5-disubstituted IH-1,2,3-triazoles (376) leads to similarly regioisomeric 1-azirines (377) <73JCS(P1)550). 

Triazole, 1-phenyl-derivatives, 5, 752 polymeric complexes, 5, 790 pyrolysis, 5, 108... 

A variety of acyclic and cyclic S-N compounds decompose at moderate temperatures (100-150 °C) with the formal loss of a symmetrical NSN fragment, but this molecule has never been detected. The lowest energy isomer, linear NNS, is generated by flash vacuum pyrolysis of 5-phenyl-l,2,3,4-thia-triazole.40 Ab initio molecular orbital calculations indicate that the resonance structure N = N+-S is dominant.41... 

Photolysis and p5Tolysis of 4-phenyltriazole gives mainly phenyl-acetonitrile pyrolysis of 4,5-diphenyltriazole in solution gives 2,3,5,6-tetraphenylpyrazine, which is formally derived from the triazole by loss of nitrogen, dimerization, and oxidation (Scheme 58). ... 

The vapor-phase pyrolysis of 4-hydroxy-1,2,3-triazole and its iV-methyl derivative affords methan-imine and its A-methyl analog. Analysis of the reaction path by the MNDO method shows the presence of two stable or metastable isomers, (liif)-4-hydroxy-l,2,3-triazole and its ketone protomer <89NJC551>. 4-Diazo-1,2,3-triazoles (122) thermolyze or photolyze in benzene to 4//-l,2,3-tri-azolylidenes (123) which convert benzene to 4-phenyl-1,2,3-triazoles and/or isomerize to a-diazo-nitriles (124). Intermediates (124) react with benzene via a carbene to give addition, ring expansion or substitution products (Scheme 17) <82TL5115>. The similar thermolysis of diazotriazoles in substituted benzene gives complex mixtures in which all of the components are sometimes impossible to isolate and identify <90AHC(48)65>. 

Mesomalonodialdehyde 1,3-dioxime is treated with arylhydrazines to give the mesoaldehyde 1,3-dioxime-2-arylhydrazones (685), which are O-acetylated with acetic anhydride to afford the mono-and di-acetyl derivatives (Scheme 139). Both intermediates are cyclized to 2-aryl-1,2,3-triazole-4-carbaldehyde oximes (686) and (687) with caesium carbonate. Triazoles (687) are converted to 4-cyanotriazoles by refluxing with acetic anhydride followed by pyrolysis <83JHC1561>. 

Reaction of the dicyanoindenone 231 with pyrazole or 1,2,3-triazole diazonium salts gives deeply colored amino-triazines 232 and 233, which cyclize upon pyrolysis to give the pentacyclic compounds 234 and 235 containing the pyrido[2,3-f]-l,2,4-triazine nucleus (Scheme 36) <2003JRM1161>. 

On pyrolysis, 1-arylimidazoles rearrange to 2-arylimidazoles. In other systems pyrolysis causes more deep-seated changes. 1-Arylbenzotriazoles on pyrolysis or photolysis give carbazoles via intermediate nitrenes (see Section 3.4.1.2.1). 1-Phenyl-1,2,4-triazole (714) is pyrolyzed to isoindole... 

Synthesis of benzo[c]furans and isoindoles (181) is also possible by the addition of benzyne to the respective monocycles (178), followed by reduction (179 — 180) and pyrolysis. In an alternative procedure, (179) is reacted with 3,6-bis(2-pyridyl)-l,2,4,5-tetrazine, which affords (181) under far less vigorous conditions via a retro Diels-Alder reaction of the intermediate (182). 4-Phenyl-1,2,4-triazoles pyrolyze to form isoindoles (Section 3.4.3.12.2). 

In the photolysis of methyl-(3,5,5-trialkyl-3,5-dihydro-[l,2,3]triazol-4-ylidene)-amine (34) in solution, aziridine imines are formed quantitatively, about 85% on the non-least-motion path and 15% on the least-motion path (90CB2195). The gas-phase pyrolysis of 34 affords only the product of the former path (93CB2683). 

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