Electrochemical Studies on the Competition Between Intramolecular Electron Transfer and Intermolecular Protonation of Nitroaryl Radical Anions in the Reductions of Apical Carboxylic Acid Substituents of 2- and 3-Nitro- 9,10-dihydro-9,10-ethanoanthracene-9-carboxylic Acids

Abstract

The results obtained from variable scan rate cyclic voltammetry (c.v.) on 2-nitro- and 3-nitro-9,10- dihydro-9,10-ethanoanthracene-9-carboxylic acids [(4) and (5), respectively], combined with simulations of various c.v. responses, are consistent with reduction of a benzylic acid group being facilitated by an intramolecular electron transfer process. This intramolecular process involves a one-electron reduction of the nitroaromatic group, followed by a rapid and irreversible π*(ArNO₂)^•- → π*(RCO₂H)^•- intramolecular electron transfer to the carboxylic acid group at a benzylic bridgehead position of the acids (4) and (5). The reduction potentials of the acid groups are shifted more than 0·3 V to positive potentials at slow scan rates (20-100 mV s^-1) compared with the unnitrated acid derivative (6). The reduction potentials and the relative peak currents for the reductions of the nitro and acid groups for each of compounds (4) and (5) are dependent on the concentrations of the reactants. At concentrations of substrate >1 mM, reduction of the acid moiety is increasingly dependent on complex intermolecular processes. These intermolecular processes compete with intramolecular electron transfer from the nitroaryl anion to the apical acid group at the benzylic bridgehead position. Digital simulations of the voltammetric data were confined to substrate concentrations <1 mM, and show that the intramolecular reductions of the apical carboxylic acid protons of (4) and (5) are complicated by competing intermolecular electron transfer and intermolecular self-protonations of the nitro radical anions. The value of the intramolecular electron transfer rate constant for the meta compound is an order of magnitude larger than that for the para compound, which is the opposite reactivity pattern to that generally found in the S_RN1 reactions of m- and p-nitrobenzyl halides. This indicates that there is likely to be an important contribution from an intramolecular through-space electron transfer mechanism for the former reaction