Polar effects in free-radical reactions. Solvent and isotope effects and effects of base catalysis on the regio- and chemoselectivity of the substitution of protonated heteroaromatic bases by nucleophilic carbon-centered radicals

Abstract

The substitutions of protonated pyridine, quinaldine, lepidine, and 3-cyano- and 4-cyanopyridine by Ph, Me, n-Pr, n-Bu, i-Pr, ţ-Bu, a-tetrahydrofuranyl (a-THF), dioxanyl, and benzyl radicals are affected by the nature of the solvent as concerns the regioselectivity and the relative rates. The isotope effect is negligible with the phenyl radical, but it is significant and solvent-dependent with isopropyl and a-THF radicals. The effect of the solvent increases by increasing the nucleophilic character of the carbon-centered radicals. The results support a strong influence of the reversibility and of the polar effect on the substitutions of protonated heteroaromatic bases by nucleophilic carbon-centered radicals.