Electrophilic Substitutions of Five-Membered Rings

TL;DR: In this article, the authors provide a sufficiently complete summary of all the quantitative and semiquantitative information available concerning electrophilic substitution in five-membered heteroaromatic rings.

Abstract: Publisher Summary The chapter attempts to provide a sufficiently complete summary of all the quantitative and semiquantitative information available concerning electrophilic substitution in five-membered heteroaromatic rings. The chapter is organized in the following manner. After a preliminary introduction to the “aromaticity” of the five-membered rings, the discussion is centered mainly on the substitution process itself, and the mechanism of the various electrophilic substitutions is considered separately. Afterward attention is shifted from the electrophile to the heteroaromatic substrate, and the relative reactivity of different rings and ring positions is widely examined. Finally, the problem of the transmission of electronic effects of substituents across these rings and the application of Kammett-type correlations is discussed. Systems considered are the parent heterocycles with one heteroatom (furan, thiophene, selenophene, and pyrrole), their monoaza and polyaza derivatives, and the monobenzo derivatives of the above systems. General surveys on the chemistry of furan,l thiophene,2 pyrrle, benz∼thiophene, an d selenophene4 have been published in recent years; reference is made to them, whenever necessary.