Electrophilic Attack on N Aromatics Pyrrole and Pyridine

A DFT study of the reactivity of pyridine and the diazabenzenes towards electrophilic substitution, assuming frontier orbital control of the reactions, predicts their low reactivity as the HOMOs of these substrates are not n-orbitals.5 For pyridine-N-oxide, however, the HOMO is an aromatic orbital. DFT studies giving Fukui indices predict6 the preferred sites of electrophilic attack on pyrrole, furan, and thiophene and calculation of the local softness of the reactive sites rationalizes relative reactivities. 

Imidazole (1) can be considered as having properties similar to both P3nrole and pyridine. In consequence, one would expect that electrophilic reagents would attack the unshared electron pair on N-3, but not that on the pyrrole nitrogen since it is part of the aromatic sextet. The ring carbons, too, are prone to attack by electrophilic, radical and nucleophilic species to a greater or lesser extent substitution reactions which do not destroy the aromatic character are predominant. While the imidazole ring is rather susceptible to electrophilic attack on an annular carbon, it is much less likely to become involved in nucleophilic... 

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