Free Radical Synthesis of Pyridylderivatives: Influence of the Solvent

Abstract

The photochemically induced reactions of the azaheteroaromatic bases are strongly affected by the medium. For example, changing the acidity of the solution results in a change of mechanism of the reaction from that of hydrogen abstraction to the electron-transfer. 2, 4-Dicyanopyridine (1) is a substrate that works only by electrontransfer, but the two charged species, radical anion and radical cation, created by photochemical excitation give rise to very different reactions depending on the polarity of the solvent. In non polar medium, if (1) is photochemically excited in the presence of an alkene, substitution of a cyano group with an allyl group occurs, resulting in the formation of 2-cyano-4-allyl pyridine and 4-allyl-2-cyano pyridine. In polar solvents, however, there is the formation of a new ring between position 5 of the heterocyclic ring and the carbon atom of the cyano group in position 4, resulting in a cyclic imino derivative. An explanation of the two different paths of the reaction are given in terms of the stabilization of the intermediate ionic couple.