Heteroaromatic substitution, nucleophilic review

One of the merits of the above treatment, which justifies its inclusion in this review, is that it allows a quantitative comparison of the selectivity of nucleophilic heteroaromatic substitution (expressed by the reaction constant) with that for the analogous reaction with nitro-activated systems. Values for the latter are in the range 3.6 to 6.0. The fact that in both cases high p-values of similar magnitude are found is consistent with the hypothesis of similar mechanisms for both classes of compounds. 

All the reactions discussed in this review are aromatic nucleophilic substitutions in the ordinary sense. These reactions are briefiy described in the following sections with respect to their general kinetic features and mainly involve aza-activated six-membered ring systems, although a few studies of other heteroaromatic compounds are also available. 

Thus the overall picture of homolytic substitution of heteroaromatic compounds has undergone only minor modification as regards arylation, but very great modification as regards substitution with nucleophilic radicals, since Norman and Radda reviewed the field in these Advances. 

The examples of nucleophilic substitution discussed in Sect. 3.4 belong to the ipso-type [185]. In c/ne-substitution, the leaving group departs from the position adjacent to the nucleophile addition site, and in teZe-substitution, the leaving group departs from a more remote position on the ring or even from a side-chain [159]. cine- and feZe-substitutions of various nonaromatic, aromatic and heteroaromatic compounds, including thiophene, have been reviewed [185]. 

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