4 - Intermolecular Radical Addition Reactions

Abstract

This chapter focuses on radical addition reactions and presents various examples illustrating the various methods used. There are two types of radicals: nucleophilic radicals (R˙N, SOMO energy level is high, such as c-C6H˙12 and t-Bu˙) and electrophilic radicals (R˙E, SOMO energy level is low, such as EtO2CCH2˙). The reactivity of these radicals with olefins depends on the electronic character and density of olefins in the addition reactions. Thus, based on the electron density of radicals, either electron-rich olefins (ED: electron-donating group) or electron-deficient olefins (EW: electron withdrawing group) must be selected, to increase SOMO–HOMO orbital interaction or SOMO–LUMO orbital interaction. The yields can be increased when these orbital energy gaps are reduced. Mode A indicates that nucleophilic alkyl radical R˙N prefers to react with electron-deficient olefins, and Mode B indicates that electrophilic alkyl radical R˙E prefers to react with electron-rich olefins. Therefore, the reaction with the opposite combination, such as R˙N with electron-rich olefins, or R˙E with electron-deficient olefins, does not proceed effectively. The most well-known radical allylation reagent is allyltributyltin. Typical radical allylations are carried out by the irradiation of a benzene solution of alkyl bromide or selenide and allyltributyltin in the presence of AIBN with a mercury lamp. Various other reactions are also dealt with in detail in this chapter.