Electron transfer in the reactions of organic trichloromethyl derivatives with iron(II) chloride

Abstract

Reduction of trichloromethyl derivatives RCCl₃[1, R = Ph; 2, R = PhC(O);3, R = EtOC(O)] with iron(II) chloride in acetonitrile, has been studied in order to examine the mechanism of the electron transfer (ET) process and the reactions of the radicals formed. Substrates 1–3 afforded different product compositions and the cause was identified as differences in the reactivity of radicals which is substantially of two types: reductive coupling and proton abstraction after further reduction to a carbanion coordinated to the metal ion. Compound 1 gave only coupling products, compound 2 only hydrogenated products and compound 3 a mixture of coupling and hydrogenated products depending on experimental conditions. Proton abstraction by the carbanion was found to occur from water molecules, which should be present in the coordination shell of the metal ion, and not from the solvent. The different behaviour of compounds 1–3 is attributed to the presence of substituents which are able to stabilize the radical and carbanionic intermediates. Rate constants at different temperatures were measured and the activation parameters calculated. The three substrates differ only slightly in reaction rates, in the order 1 > 2 > 3. Activation enthalpies are very close to each other and this agrees with the almost equal values of C–Cl bond dissociation energies of compounds 1–3, empirically determined. Large, negative entropies of activation were found, suggesting that an ordered activation complex should be formed in order that electron transfer from the metal ion to the organic halide can take place.