ABSTRACT
This chapter describes in general terms the types of reactivity found in the typical six- and five-membered aromatic heterocycles. The study of aromatic heterocyclic reactivity can be said to have begun with the results of electrophilic substitution processes – these were traditionally the means for the introduction of substitutents onto heterocyclic rings. In the heterocyclic field, the displacement of good leaving groups, often halide, by a nucleophile is a very important general process, especially for six-membered electron-poor systems. The reaction of nucleophilic radicals, under acidic conditions, with heterocycles containing a C=N unit, is by far the most important and synthetically useful radical substitution of heterocyclic compounds. Although much less well developed than the Minisci reaction, substitution with electrophilic radicals can be used in some cases to achieve selective reaction in electron-rich heterocycles. The detail of the mechanism of metallation is still under discussion; it may involve a four-centre transition state.
