New 2‐substituted pyrroline‐N‐oxides: An EPR solvent study of the radical spin adducts

Abstract

AbstractTen substituted 5,5‐dimethyl‐1‐pyrroline‐N‐oxides as well as the parent nitrene spin trap (DMPO) were prepared: 5,5‐dimethyl‐1‐pyrroline‐N‐oxide, 2,5,5‐trimethyl‐1‐pyrroline‐N‐oxide, 2‐tert‐butyl‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide, 2‐phenyl‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide, 2‐d5‐phenyl‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide, 2‐phenyl‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide‐nitronyl‐13 C, 2‐(4‐fluorophenyl)‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide, 2‐(4‐chlorophenyl)‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide, 2‐(4‐tert‐butylphenyl)‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide, 2‐(4‐methylphenyl‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide) and 2‐(2‐ methylphenyl)‐5,5‐dimethyl‐1‐pyrroline‐N‐oxide. Analytical (i.e. EPR‐grade) samples of these novel cyclic nitrones were obtained and characterized by (among other methods) 1H NMR spectroscopy. Reduction of DMPO and these various 2‐substituted cyclic nitrones gave the corresponding cyclic N,N‐dialkylhydroxylamines, whose structures and conformations were also analyzed by 1H NMR spectroscopy. Air oxidation of these cyclic N,N‐dialkylhydroxylamines provided access to the EPR spectra of the hydrogen, methyl, tert‐butyl, phenyl, d5‐phenyl, nitronyl‐13C‐phenyl, 4‐fluorophenyl, 4‐chlorophenyl, 4‐tert‐butylphenyl, 4‐methylphenyl and 2‐methylphenyl cyclic aminoxyl (pyrrolidine N‐oxyl nitroxide) radical spin adducts of DMPO. The 14N, 13C (where applicable) and 1H hyperfine splitting constants of these aminoxyl adducts in ten solvents of widely different polarities (e.g., hexane to water) were measured and the solvent effect on these parameters was evaluated. It was found that for the various 2‐substituted DMPO‐type spin adducts both the nitrogen and β‐hydrogen EPR hyperfine splittings correlated linearly (r2 ⩾ 0.90) with typical solvent polarity parameters such as ET(30). The correlation between the nitrogen and β‐hydrogen hyperfine splitting constants were even more linear (r2 ⩾ 0.97).