Homolytic Aromatic Substitution

Abstract

Homolytic aromatic substitution (HAS) encompasses a wide range of synthetic transformations based on inter- and intramolecular additions of radicals to arenes. This chapter is intended to provide a description of important aspects and recent advances of these reactions including the principal mechanisms and the synthetic protocols used. An overview of selected examples are shown, which can be used as guides for a planned transformation. The chapter has been organized according to the different routes to generate the intermediate radicals. In the first one, the radicals are formed by homolytic cleavage processes (thermolysis and photolysis). The second route involves the use of peroxides or free radical initiator with tin hydrides and silanes. Finally, the third route involves redox reactions, where radicals can also be generated either by reduction or oxidation of an appropriate precursor. Also, the base-promoted electron transfer to induce the HAS is presented. In each of the procedures, various examples have been selected with the intent of showing which reaction is a suitable alternative to gain access to complex molecules, which makes these processes a powerful synthetic tool.