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 ͑ESR͒ characterization of a number of nitroso phenyl 1,4-dihydropyridines in aprotic media on a hanging drop mercury electrode ͑HDME͒. The electrochemically generated nitroso radical anions decayed by a second-order reaction. Kinetic rate constants varied between k 2 , 4906 ± 43 ͑M s͒ −1 and 17,955 ± 159 ͑M s͒ −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-DHP͒ 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. Universidad de Chile assisted in meeting the publication costs of this article.