Nucleophilic Character of the Alkyl Radicals. 19. Absolute Rate Constants in the Homolytic Alkylation of Protonated Heteroaromatic Bases by n-Butyl and tert-Butyl Radicals

Article abstract of DOI:10.1021/jo01311a039

The rate constants for the homolytic alkylation of protonated heteroaromatic bases (quinoline, pyridine, and 4-cyano-, 4-acetyl-, 4-methyl-, and 4-methoxypyridine) by n-butyl and tert-butyl radicals were measured at 57 deg C by competition of the aromatic addition with the alkyl radical oxidation by Cu²⁺ salts (for which the rates are known).With the more activated substrates (quinoline and 4-cyano- and 4-acetylpyridine) the tert-butyl radical is significantly more reactive then the n-butyl radical, clearly showing that polar effects are more important than steric and enthalpic effects in determining the reaction rates.The reversibility of the alkylation by the tert-butyl radical is discussed.