Five-membered rings, electrophilic fragmentations

The existence of an annelated benzene ring in the benzazole molecule influences much its ability for electrophilic substitution. All benzazoles are more easily nitrated than their five-membered analogs, and the nitro group is generally introduced into the arylene fragment of the molecule. 

When the nucleophilic and the electrophilic positions of the reagent confronted to the aryne are not c-bonded, a cascade intermolecular nucleophilic addition-intramolecular electrophilic cycUzation of arynes can take place. The fragmentation step, which is cmcial for the insertion reaction of arynes into a-bonds, is not involved in annulation processes because the intermediate obtained from the cyclization is usually a stable five- or six-membered ring system. 

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