Estimation of the standard reduction potentials of some 1-arylethyl radicals in acetonitrile

The redox properties of some arylethyl radicals, which are involved in the electrochemical synthesis of important anti-inflammatory agents, have been investigated in CH₃CN by an indirect electrochemical method based on the catalytic reduction of the corresponding arylethyl halides (RX) by electrogenerated aromatic or heteroaromatic anion radicals (D⁻). The reaction between RX and D⁻ leads to a radical R, which can react with D⁻ either by radical coupling (k₃) or by electron transfer (k₄). Examination of the competition between these reactions, which can be expressed by a dimensionless parameter q=k₄/(k₃+k₄), as a function of E⁰_D/D⁻ allows estimation of the reduction potentials of the arylethyl radicals. The standard reduction potentials obtained for the radicals 1-(4-isobutylphenyl)ethyl, 1-(6-methoxy-2-naphthyl)ethyl, and 1-(4-biphenyl)ethyl are −1.64, −1.62, and −1.15 V vs. SCE, respectively.