2 - Functional Group Conversion

Abstract

Since activation energy of the coupling reaction of radicals is nearly zero, the reaction rate for the coupling reaction of carbon-centered radicals is extremely fast, and almost reaches diffusion control rate. Thus, an increase of the concentration of carbon-centered radicals induces the coupling reaction with ease. The pinacol coupling reaction of aldehydes or ketones has, therefore, been well utilized. Generally, radicals are very reactive species, so coupling reactions, abstraction of a hydrogen atom from the solvents or reagents, and reactions with molecular oxygen in the solution occur rapidly. The rate constants for the coupling reaction of carbon-centered radicals are also provided in this chapter. One typical radical reaction is a coupling reaction. Oxidative decarboxylation coupling reaction of carboxylic acids by electrolysis (Kolbe electrolysis), intramolecular coupling reaction of diesters with Na (acyloin condensation), and formation of pinacols from ketones or aldehydes with Na or Mg are well-known classical methods. Radical reduction of alkyl and aryl halides is a fundamental and important reaction in organic synthesis, and has been extensively used. On the other hand, the conversion of halides to alcohols is a typical SN1 or SN2 reaction in the polar reaction method, and generally the reactions require basic conditions. However, the conversion of halides to alcohols by the radical reaction method can be carried out under neutral conditions.