Aryl azides isomerization

Wedegaertner and Kattak (27) reported the intermolecular cycloaddition of aryl azides with allenes (Scheme 9.27). Cycloaddition of an aryl azide with the 1,2-propanediene 131 produced a mixmre of the isomeric triazolines 132 and 133, whereas when the cycloaddition was conducted with cyclonona-1,2-diene 134, the triazoline 135 was the sole product. X-ray crystallographic analysis confirmed the stmcture of 135. 

For most aryl azides, the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same. Thus, in all these phenyl-nitrenes cyclization to substituted benzazirines is the rate-limiting step of the process of isomerization to ketenimine, as is the case for the parent phenylnitrene. The only known exception, o-fluorophenylnitrene, will be discussed in the next section. 

Denson (Ref 9) on those a-azidoalkylidenimines which undergo cyclic isomerizations to tet-razoles and Boyer Canter (Ref 15) made a thorough survey of the available information on alkyl and aryl azides. Cirulis Straumanis (Ref 6) prepd a number of new azides of org bases bur none of these azides showed eiqjl props. Schaad (Ref 14) obtd a patent for the manuf of esters of hydrazoic acid based on the reaction of alkenes, cyclic olefins, ary 1-alkenes cycloalkylalkenes with HN3 in the presence of an acid catalyst... 

The exocyciic olefinic bond in strained enamines can undergo cycloaddition with aryl azides to give spirotriazoline adducts that are stable233 or that spontaneously isomerize to triazoles (Section IV,A) (Scheme 56).234... 

Reaction of the ylide with aryl azides leads to enamines (52) as the major product via the diazo ester (51) formed by the spontaneous isomerization of... 

Good yields of l-aryl-4-nitrotriazoles (288, 289) are obtained on cycloaddition of 1-morpholino-2-nitroethylene (287) (review 8ITI453) with aryl azides (66tl6043, 68G949). The Spiro cycloaddition product from (290) and phenyl azide isomerizes to the triazole (291)... 

Early attempts to prepare /i-nitrovinyl azides led to the formation of furoxans.20 The -/i-nitrovinyl azide 54 has however been isolated.56 Its thermolysis in benzene afforded the azirine 57 in 60% yield the same compound was obtained on photolysis, together with a small amount of the furoxan 56, which is ascribed to photochemical -Z isomerism, leading initially to 55 (Eq. 16). In the aromatic series it is known that o-nitro-aryl azides undergo anchimerically assisted nitrogen elimination, giving benzofuroxans.57... 

It now seems incontestable that irradiation of phenyl azide at room temperature gives dehydroazepine. Once formed, dehydroazepine can react with itself and/or phenyl azide to give tarry polymer or with nucleophiles to give substituted 3f/-azepines. The rate of reaction of dehydroazepines with amines depends dramatically on substitution the 5-acetyl substituted compound reacts 10,000 times faster than does the 5-methoxy substituted dehydroazepine. At low concentration of phenyl azide, dehydroazepine itself has a lifetime of approximately 5 ms and, presumably, isomerizes to phenyl nitrene. We will have more to say about this point later. The achievement of positive structural identification of the reactive intermediate formed in the room temperature photolysis of phenyl azide permits the detailed characterization of phenyl azide photochemistry. Further consideration of this analysis will be aided by examination of the results from time-resolved experiments for other aryl azides. 

The insensitivity of allylic azide isomerization rates to substituent effects and solvent ionizing power, and the relatively small value of the negative volume of activation make it very unlikely that ion pair intermediates are involved in these reactions. The negative entropy and volume of activation are compatible with a concerted process involving a cyclic transition state resembling a 1,2,3-triazine. However, the known ability of alkyl and aryl azides to add to olefins - suggests the possibility of an intramolecular addition-elimination mechanism,viz. 

In contrast to the N-MEM (N-alkyl) analogue (see Sect. 2.2.2), N-aryl-aza-fiilleroids, obtained from the thermal reaction of aryl azides with 50, can be photochemically isomerized to azirenofullerenes [51]. This closely resembles the difference in the photochemical behavior of C-aryl- and C-alkyl fulleroids obtained from the reaction of diazo compounds with Qo (e.g. [5,6] CgiHj is photochemically stable). After some initial studies (see for example [52-55]), the addition of azides and nitrenes to C o has been investigated and used for the preparation of a series of functional fullerene derivatives by a number of other investigators, but their work is not relevant in relation to the preparation of aza[60] fullerene. The addition of azides to fullerenes has been brie% reviewed previously [56-58]. 

Wolff (1912) was the first to observe the ring expansion of phenyl azide, on thermolysis in aniline, to a compound he called dibenzamil (45). It was many years before this compound was shown to be 2-anilino-3if-azepine. This reaction was also found to take place on photolysis of aryl azides in primary and secondary aliphatic amines. The mechanism that was proposed and which has since been generally accepted involves nucleophilic attack by solvent on a benzazirine, that is believed to be in equilibrium with singlet arylnitrene. Nmr evidence has been advanced for the intermediacy of a 1/f-azepine (46) in related expansions which, as expected, rapidly isomerizes to the more stable tautomer. Spectroscopic evidence is still lacking for the aziridine intermediate 47, but this could simply be due to its short lifetime. On the other hand, the recent work of Chapman and Le Roux discussed in Section 1.4 has brought into question the intermediacy of benzazirine in phenyl... 

Accordingly, 5-substituted-amino-l,2,3,4-thiatriazoles (Tables III and IV) are formed quite generally from 4-substituted-thiosemicarb-azides. When the substituent is an aryl group these initial products are isomerized to 5-mercaptotetrazoles on treatment with alkali whereas this is not the case when the substituent is an alkyl group. 

The triazoles previously obtained from jS-keto-ylides and acyl azides or ethyl azidoformate are the 2-acyltriazoles (80) formed by isomerization under the basic conditions of the initially formed 1-substituted triazoles (79). The latter can be isolated in some cases if the reactions are interrupted. Aryl mono- and bis-azides have also been used in the preparation of the triazolcs (81). 

The fused tetrazoles 10 containing various aryl groups in position 6 were boiled in acetic acid in the presence of either triphenylphosphine or copper and gave rise to the amino-substituted triazines 12. The reaction proceeds obviously via valence bond isomerization of 10 to the azide 11 and is regarded as a useful synthetic route to the diamino compounds 12. 

The reaction sequence starts from tetrazolyldiazonium salt 42 prepared from aminotetrazole 41 by diazotation. This compound when reacted with arylformylacetonitrile 43 leads to the intermediate formation of the condensation product 44, which easily undergoes ring closure to 45. This tetrazolo[5,l-z][l,2,4]triazine compound, however, forms an equilibrium with the valence bond isomeric azide 46, which can participate in a different ring closure than the reverse route, and yields the tetrazolo[l,5-A][l,2,4]triazine product 47. The reaction was carried out with a series of various aryl derivatives and proceeded in good to excellent yields (68-87%). 

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