Electrochemical and ESR characterization of free radicals from C-4 nitroso phenyl 1,4-dihydropyridines in aprotic media

Abstract

The present paper deals with the electrochemical and electron spin resonance (ESR) characterization of a number of nitroso phenyl 1,4-dihydropyridines in aprotic media on a hanging drop mercury electrode (HDME). The electrochemically generated nitroso radical anions decayed by a second-order reaction. Kinetic rate constants varied between k(2), 4906 +/- 43 (M s)(-1) and 17,955 +/- 159 (M s)(-1) depending on the chemical structures. An increase in the bulk of the alkyl substituents from methyl-to isopropyl in 3- and 5-positions on the dihydropyridine ring stabilized the free radicals. Also, the electrochemically generated nitro radical anions from the parent nitrophenyl 1,4-dihydropyridines (1,4-DHP) derivatives were kinetically characterized and were significantly more stable than nitroso derivatives. The time-course of controlled potential electrolysis of the different compounds was followed electrochemically by differential pulse voltammetry, cyclic voltammetry, and spectroscopically by UV-visible and ESR. There is an intermolecular consecutive reaction during electrolysis, which gives the nitranion products. Formation of the one-electron reaction product, i.e., the nitroso radical anion, was confirmed by ESR experiments, and the corresponding hyperfine coupling constants were calculated for all the nitrosophenyl 1, 4-DHP derivatives studied.