1.2.4- Triazole, 1-phenyl-, pyrolysis

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. 

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>. 

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). 

The cycloadducts of cyclooctatetraene and its substituted derivatives with 4-phenyl- and 4-methyl-37/-triazole-3,5(4//)-dione and diethyl diazenedicarboxylate are intermediates in the synthesis of polycyclic structures5 10 l2 23. Furthermore, pyrolysis of the diethyl diazenedicarboxylate cycloadduct 12 affords diethyl 1.2.4a,8a-tetrahydro-l,2-cinnolinedicarboxylate with undetermined stereochemistry of the ring fusion23. 

Gas-phase pyrolysis of 1,2,3-triazoIe produced vinyl azide and its decomposition products (83JA7681). 4-Diazo-l,2,3-triazole (20) (prepared by dia-zotizing 4-aminotriazole in water), when refluxed in benzene, gave 4-phenyl-1,2,3-triazole (53% yield) and a mixture of cyanobicycloheptatrienes (15%). Photolysis proceeded similarly, except that the latter products preponderated (82TL5115). 

A species which may be described either as an iminocarbene54 or a nitrile ylide (52) is formed by gas-phase pyrolysis of 1-phenyl-l,2,4-triazole (Eq. 14). The rearrangement, leading to isoindole, was rationalized in terms of a 1,5-phenyl shift followed by nitrogen extrusion from the 3ff-triazole 51.54... 

Isoindole has been detected (by trapping it with iV-phenylmaleimide) in the pyrolysis of 1-phenyl-1,2,4-triazole at 800°C (Section III,F).18... 

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