Journal of Electroanalytical Chemistry, Vol.567, No.1, 67-75, 2004
Electroreduction of halogenated fumaric esters: a combined electrochemical and computational investigation
Diethyl fumarate and related halogenated derivatives have been studied by cyclic voltammetry and electrolysis, on mercury and glassy carbon electrodes, in DMF + 0.1 mol L-1 TBAP and in acetonitrile + water (3:4) with 0.1 mol L-1 NaCl or 0.1 mol L-1 TEAP. For compounds with two reducible functionalities, the electron-deficient olefin and the C-X group, the grade of substitution on the halogenated carbon causes differences on the site involved in the first electron transfer thus determining the chemical reactivity. Enediester compounds with -CH2-groups insulating the gem-trihalide groups (-CX3, X = Br and Cl) suffer reduction, and the C-X cleavage is the reaction of choice. The reaction pathway also depends on the nature of the electrode and significant positive potentials shifts for compounds with a gem-tribromide group are evident in the cyclic voltammogram on a mercury electrode, in relation to results on a glassy carbon electrode. The formation of easily reduced organomercurials is the main reason for this intense positive shift in the first electron transfer. On a vitreous carbon electrode, factors directly related to the strength of the C-X bond play a determinant role. Concerning electrolysis, the gem-tribromo derivative furnishes H2C=CBr2 and ethyl hydrogenfumarate, through hydrolysis produced by initial cleavage of the C-Br. For the other polyfunctional compounds (-CHX2, -CH2X), the olefin is reduced first to yield dimers and/or hydrogenated products depending upon the reaction conditions. Quantum chemical calculations performed on the anion radical of these compounds yield spin densities located on the -CBr3/-CCl3 group and on the enediester group for the -CCl2/-CBr2, -CH2Br/-CH2Cl derived compounds. Upon geometry optimisation, the C-Br bond in compounds containing -CBr3 groups are longer, suggesting that this bond can easily be cleaved, yielding organomercurial compounds, in the case of the use of a mercury electrode or leading to bond cleavage, for inert electrodes. For the other substituents, the optimisation basically does not affect the geometry of the radical anion compared to the neutral substrate. These results have been confirmed by the calculated dissociation energy of the C-Br bond, which for the -CBr3 group is approximately 100 kJ mol(-1) as against 240 kJ mol(-1) for the same bond in the -CH2Br group. The coherence between the experimental and calculated data reinforces the usefulness of these computational tools in rationalising the reactivity of electrogenerated species as well as in predicting the electrochemical reaction outcome. (C) 2003 Elsevier B.V. All rights reserved.
Keywords:electron-transfer;halogenated fumaric esters;effect of the electrode nature;cyclic voltammetry;electrosynthesis;computational investigation