Ring expansion, of aryl azides

Early efforts to effect the photoinduced ring expansion of aryl azides to 3H-azepines in the presence of other nucleophiles met with only limited success. For example, irradiation of phenyl azide in hydrogen sulfide-diethyl ether, or in methanol, gave 17/-azepine-2(3//)-thione35 (5% mp 106—107 " O and 2-methoxy-3//-azepine (11 %),2 3 respectively. Later workers194 failed to reproduce this latter result, but found that in strongly basic media (3 M potassium hydroxide in methanol/dioxane) and in the presence of 18-crown-6, 17/-azepin-2(3//)-one was produced in 48% yield. In the absence of the crown ether the yield of azepinone falls to 35%. 

Lamara, K. and Smalley, R.K. (1991) 3H-Azepines and related systems. Part 4. Preparation of 3 H-azepin-2-ones and 6 H-azepino[2,l-b]quinazolin-12-ones by photo-induced ring expansion of aryl azides. Tetrahedron, 47 (12/13), 2277-2290. 

Thermal ring expansion of cyclopropyl azides provides a general route (95 — 96) to alkyl- and aryl-azetines (79CB3914). 

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... 

Ring expansion of the benzene ring of a calix[6]arene to a 1 //-azepine in 14% yield by photolysis of an aryl azide confined in the calix structure has been reported.294... 

In contrast to the acyl- and sulfonylnitrenes described in this section, arylnitrenes produced thermally or photolytically from aryl azides, including those bearing strongly electron-withdrawing substituents (e.g., CN, N02, CF3), fail to promote ring expansion of arenes to 1H-azepines, although intermolecular substitution of electron-rich substrates, e.g. mesitylene and A.TV-dimethylaniline, have been noted.167... 

Several [ 1,2,4]triazolobenzothiadiazocin-l 1-ones 174 were prepared via ring expansion of [l,2,4]triazolo[3,2-A -[2,4]benzothiazepin-10(5//)-ones in presence of sodium azide. The intermediate aryl isocyanate 173, formed as a result of Curtius rearrangement, was isolated and characterized by elemental analysis, IR, H NMR, and mass spectroscopies (Scheme 45 <2002PS2303>). 

The photodecomposition of aryl azides continues to attract most attention in this area although few additional details concerning the mechanism have been reported, particularly with reference to ring expansion via 1,2-didehydroazepines. Ring... 

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