Steric, polar, and resonance effects in reactivity and regioselectivity of aryl radical addition to α,β-unsaturated carbonyl compounds

Free-radical decompn. 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 addn. (β or α). The abs. rate consts. of the addn. (107-108 M-1 s-1 at 5°) were detd. by comparison with the rate of the iodine abstraction by aryl radicals from iso-Pr iodide. The addn. rates to the β position correlate well with the Es steric parameters. The influence of resonance stabilization of the radical adduct on the reactivity can be significant, but the regioselectivity of the addn. is mainly detd. by steric effects. The different fate of the α- and β-radical adducts are discussed on the basis of their different polar character