Photolysis of 1,2,3-triazoles

Other methods for the synthesis of l,4-dihydro-l,2,4,5-tetrazines 9 which can be explained by intermediate formation of nitrile imines 8 are the photolysis of 1,2,3-triazoles 2181 and sydnones 3,182 reaction of tetrazoles 4180 with tosyl chloride, elimination of pyridine from the pyridinium compounds 5,189-190 of hydrogen chloride from the hydrazonic acid chlorides 1 s.3-i88 or elimination 0f sulfinate from the a-sulfonyl hydrazone 7.189... 

Nitrogen extrusion has been used to make fragile molecules 2-thiirene (99) has been obtained by matrix photolysis of 1,2,3-thiadiazole (98), and azirines from 4//-triazoles (100 — 101). 

While the photolysis and thermolysis of 1,2,3-triazoles are of considerable interest with regard to mechanistic investigations and in the search for highly reactive intermediates, the elimination of nitrogen from suitable triazolines provides an excellent synthetic entry to aziridines cf. Section 4.11.6). Several substantial reviews have been published on this topic (B-76PH403, B-70MI41101, B-67MI41101). 

Other evidence to support nitrene formation is the identity of the reaction products obtained when nitrenes are generated by independent methods. For instance, N-nitrenes of 1,2,3-triazoles can be generated by oxidation of 1-amino-u-triazoles (88M1041), on photolysis of their potassium tosylates (134) (64AG144), or by decomposition of unstable lithium salts of l-(l,2,3-triazole-l-yl)-4-p-tolylsulfonyl-tetrazenes[72JCS(Pl)1315] [Eq. 

Photolysis of 2,4,5-triphenyl-1,2,3-triazole 1-oxide gives the triphenyl-1,3,4,5-oxatriazine shown in Scheme 61 (80AJC2447). 

Photolysis of , 4,5-triphenyl-1,2,3-triazole 1-oxide gives the 1,3,4,5-oxatriazine (13) (80AJC2447). This is the only reported example of this monocyclic ring system, and the mechanism of its formation (Scheme 35) probably involves the intermediacy of an oxaziridine and an oxygen walk process not uncommon in the photochemistry of heterocyclic iV-oxides. 

The photolysis of 1,2,4-oxadiazoles in the presence of sulfur nucleophiles has been shown to afford 1,2,4-thiadiazoles. N—S bond formation between the ring species and the sulfur nucleophile is thought to account for the observed products.96 A review has appeared which includes an account of the rearrangement of 1,2,3-thiadiazoles to other heterocycles such as 1,2,3-triazoles and 1,2,3,4-thiatriazoles.97... 

Intramolecular carbenoid and nitrenoid insertions are also quite effective for the preparation of peri-condensed heterocycles. Thus, photolysis of 1-naphthyl-1,2,3-triazoles 113 leads to bcnzo[d,e quinolines 115, possibly via carbene intermediate 114 (Scheme 55) <1987J(P1)413>. Similarly, on photolysis or thermolysis of 8-azido-l-arylazonaphtha-lenes 116 naphtho[l,8-<7, ]triazine derivatives 117 are formed along with A-aryhminobenzo[/y/]indazoles 118 (Scheme 56) <1978JOC2508, 1982JOC1996>. 

Irradiation of 2H-1,2,3-triazoles (91) in ether gives 22% MeCN, and in the presence of excess cyclopentene it affords the adduct (92) (68CC977). 4H-Triazolines (93) bearing methoxycarbonyl and alkyl groups at the j3 -position give, on thermolysis, 2H- 1,2,3-triazoles (94) and/or 3-amino-2i/-azirines on photolysis, only azirines are formed (80CC940). 

Unusual pyrrole derivatives are formed by the photolysis of l-vinyl-4,5-dihydro-l//-1,2,3-triazoles (8ICCIS19). 

Insensitivity to photolysis is suggested by the formation of 1,2,4-triazoles from some other heterocycles. However, mesoionic 4-aryl-l-phenyl-l,2,4-triazolin-3-ones (44) are decomposed on photolysis as in Scheme 12 (72CC498) triazolinethiones lose sulfur only <70AJC631>. 

Cyclooctyne (75) can be prepared according to the usual rhethods of synthesis of acetylenic compounds. For instance, oxidation of the bishydrazone of cyclooctane-1,2-dione (74) " " or debromination of 1,2-dibromocyclooctene (76) with magnesium or the photolysis of the anion of I-tosyIamino-1,2,3-triazole derivative (77) gave cyclooctyne (75). 

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). 

Triazoles and Benzotriazoles. - The alkenediazonium salt (431 Ar = P-NO2C6H4) is converted into the triazoles (432 R = H or alkyl) by the action of amines. Anodic oxidation of the oxime phenylhydrazone of benzil, HON=CPhCPh=NNHPh, gives the AA-oxide (433). The alkenyl-triazolines (434 R = H or alkyl) afford the aziridines (435) on flash vacuum py roly sis.Photolysis of the 4-alkylaminotriazoles (436 R = alkyl) yields 2-alkylpyrroles rather than the expected 3-alkyIpyrroles. The... 

Pyrolysis or photolysis of 1-unsubstituted 1,2,3-triazoles yields nitriles ... 

When treated with heat or light nitrogen, can be eliminated from tetrazoles and 1,2,3-triazoles. Nitrile imides can be formed by photolysis or thermolysis of 2,5-disubstituted tetrazoles. The nitrile imides can then react... 

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