Aryl azides nucleophilic aromatic substitution

Scheme 3.28 Aryl azides via nucleophilic aromatic substitution of electron-deficient arenes ...

C. Synthesis of Aromatic Azides by Nucleophilic Substitution I. Aryl azides... 

Some other aromatics azidated in this way included 1,2,3-trimethoxybenzene (at C-5), naphthalene (at C-l), mesitylene, etc. Mechanistic studies have shown that azide reacts as a nucleophile with aryl cation radicals formed through electron abstraction by BTI. With p-alkylanisoles bearing at least one benzylic proton, BTI and Me3SiN3 in acetonitrile gave sp3-C-substitution products, homolytically. Among the R groups were not only alkyls but also cyano, nitro and others [102]. 

In water, N3 is much less reactive in aromatic nucleophilic substitution than expected from its reactivity toward carbocations, that is, its N+value. Ritchie (43) initially developed his N+ scale from nucleophilicities toward preformed carbocations and the scale fits the data for nucleophilicities toward many electrophiles, regardless of their charge. However, in water, and similar hydroxy lie solvents, the nucleophilicity of azide ion, relative to that of other anions, seems to be related to the carbocation-like character of the electrophile. An acyl derivative with its sp2 carbonyl group is somewhat akin to a carbocation stabilized by an alkoxide group, >C=0 <-— >C+-0 , just as a triarylmethyl carbocation is stabilized by electron delocalization into the aryl groups and azide ion is a good nucleophile toward these electrophiles. As compared with anions such as OH- or CN , azide ion, in water, is very reactive toward carbocations and in deacylation but is relatively unreactive toward dinitrohaloarenes (44). 

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