POLAR AND ENTHALPIC EFFECTS IN FREE-RADICAL REACTIONS - FREE-RADICAL DIAZO COUPLING AND REACTIVITY OF CARBOHYDRATE RADICALS

The combination of kinetic data and qualitative concepts of the free-radical reactivity (polar and enthalpic effects) contributed to the development of a variety of new free-radical reactions of synthetic value, including the diazocoupling reaction and the selective functionalization of carbohydrates. Several sources of carbon-centred radicals have been utilized in the diazo coupling reaction: i) alkyl radicals from alkyl iodides, H2O2, Fe(II) salt and DMSO; ii) alkyl radicals from vinyl monomers; iii) aryl radicals from diazonium salts, also in relation to the fact that the reaction products (azoarenes and biaryls) are often formed as side products in classical organic reactions, such as the Sandmeyer, Meerwein and Pschorr reactions. Inductive and field polar effects dramatically influence the reactivity of carbohydrate radicals and the first examples of aromatic substitutions (homocyclic and heterocyclic) by these radicals with complete retention of configuration, discovered by the authors, are discussed.