Steric, Polar, and Resonace Effects in Reactivity and Regioselectivity of Aryl Radical Addition to α,β-Unsaturated Carbonyl Compounds

Article abstract of DOI:10.1021/jo00340a017

Free-radical decomposition of diazonium salts by titanous salts in the presence of olefins conjugated with carbonyl groups leads to reductive arylation or arylation and diazo coupling of the radical adduct, depending on the orientation of the aryl radical addition (β or α).The absolute rate constants of the addition (10⁷ - 10⁸ M^-1s^-1 at 5 deg C) were determined by comparison with the rate of the iodine abstraction by aryl radicals from isopropyl iodide.The obtained data of the addition rates to the β position correlate well with the E_s steric parameters.The influence of the resonace stabilization of the radical adduct on the reactivity can be significant, but the regioselectivity of the addition is mainly determined by steric effects.The different fate of the α- and β-radical adducts are discussed on the basis of their different polar character.