Aryl azides ring expansion

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... 

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%. 

Aryl azides also give ring expansion on heating, for example,... 

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

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. 

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>). 

Aryl and heteroaryl azides are known to undergo ring expansion to azepines on photoelimination of nitrogen in the presence of nucleophiles. Evidence for benzazirine and 1-azacycloheptatetraene intermediates has previously been described and both species continue to be invoked to explain transformations of this type. The presence and nature of substituents appears to influence the course of the reaction, with electron-withdrawing substituents promoting azepine formation. o-Substituted aryl azides (102) are converted on irradiation in methanol-tetrahydrofuran into the 3-substituted 2-methoxy-3H-azepines (103). A pathway via the benzazirine (104) is proposed in Scheme 4, although... 

Similar products were obtained from other aryl azides but the reactions, which may be heterogeneous, are complicated by unexpected migrations, fragmentations and ring expansions and a full analysis will require extensive isotopic studies... 

---