Nucleophilic substitution of hydrogen in electron-deficient arenes, a general process of great practical value.

The aim of this tutorial review is to present two main messages. First, addition of nucleophilic agents to electron-deficient arenes proceeds faster in positions occupied by hydrogen than in those, equally activated, occupied by halogens or other nucleofugal groups. Thanks to numerous ways of further, fast conversion of the produced sigma(H) adducts into products of nucleophilic substitution of hydrogen, this is the main primary reaction between nucleophiles and electron-deficient arenes. Conventional nucleophilic substitution of halogen, S(N)Ar reaction, is a secondary process that takes place when ways for fast further conversion of sigma(H) adducts are not available. The second message is that nucleophilic substitution of hydrogen is an efficient tool in organic synthesis. In order to stress the preference for nucleophilic substitution of hydrogen, halonitroarenes are chosen as examples of reacting electron-deficient arenes, but it is obvious that the presence of halogen is not necessary for substitution of hydrogen.