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 RO-9,10-DIHYDRO-9,10-ETHANOANTHRACENE-9-CARBOXYLIC AND RO-9,10-DIHYDRO-9,10-ETHANOANTHRACENE-9-CARBOXYLIC ACIDS

The results obtained from variable scan rate cyclic voltammetry (c.v.) on 2-nitro- and ro-9,10-dihydro-9,10-ethanoanthracene-9-carboxylic ac ids [(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. Thi s intramolecular process involves a one-electron reduction of the nitr oaromatic group, followed by a rapid and irreversible pi(ArNO2)(.-) - -> pi(RCO2H)(.-) intramolecular electron transfer to the carboxylic a cid 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 g roups for each of compounds (4) and (5) are dependent on the concentra tions of the reactants. At concentrations of substrate >1 mM, reductio n of the acid moiety is increasingly dependent on complex intermolecul ar processes. These intermolecular processes compete with intramolecul ar electron transfer from the nitroaryl anion to the apical acid group at the benzylic bridgehead position. Digital simulations of the volta mmetric data were confined to substrate concentrations <1 mM, and show that the intramolecular reductions of the apical carboxylic acid prot ons of (4) and (5) are complicated by competing intermolecular electro n transfer and intermolecular self-protonations of the nitro radical a nions. The value of the intramolecular electron transfer rate constant for the meta compound is an order of magnitude larger than that for t he para compound, which is the opposite reactivity pattern to that gen erally found in the S(RN)1 reactions of m- and p-nitrobenzyl halides. This indicates that there is likely to be an important contribution fr om an intramolecular through-space electron transfer mechanism for the former reaction.