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 SRN1 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
Mononitration of 9-substituted ethanoanthracenes, bearing Me, But, F, Br, I, OMe , NO2, CN, CHO or CO2Me substituents at the bridgehead carbon, was found to occur exclusively at the β-positions of the aromatic ring. The mononitro products were isolated, identified by 1H n.m.r. spectroscopy, and their relative proportions were estimated by quantitative g.l.c . and/or by 1H n.m.r. spectroscopy. For all the above substrates the proportion of nitration at the β-position meta to the bridgehead carbon bearing the substituent [to give compounds of the general form (4)] was greater than the proportion of nitration at the corresponding β-position para to the bridgehead substituent [to give compounds of the general form (3)]. Whilst the preferential nitration at the β-positions of the aromatic rings is consistent with the previously reported nitration of 9,10-dihydro-9,10-ethanoanthracene (2a) itself, no observations of this preferential meta attack have been made previously. No correlation could be made of this behaviour with available substituent parameters for the widely sterically and electronically disparate set of substituents used in this study, and the origin of this preferential attack remains unclear. Dinitration in this system was studied only superficially. The influence of the bridgehead substituent together with that of the nitro group already present on one aromatic ring appear to combine with quite unpredictable results in orienting the position of attack of the incoming nitro group onto the other (non-nitrated) aromatic ring.
Cyclic voltammetry and electron paramagnetic resonance spectroscopy were used to examine apical substituent effects on the properties of Me2SO solutions of the radical anions from 9-substituted and 9,10-disubstituted 2- and 3-nitro-9,10-dihydro-9,10-ethanoanthracenes (1)-(24). The reductions of the nitro group are, in general, reversible at 100 mV s-1 and at 20°, except where there are coupled intra-or inter-molecular electron or proton transfer reactions with aliphatic bridgehead substituents, such as a carboxylic acid or iodine. The substituent effects for the meta- and para-nitroethanoanthracene systems are both similar and additive. This similarity in the meta and para polar substituent effects is attributed to the orthogonality of the π*(ArNO2/ArNO2-) orbital with the σ* orbital of the carbon substituent (C-X) groups at the bridgehead positions. Overall, the meta substituent effect was slightly lower than that for corresponding simple meta-nitrobenzyl systems, but the para-nitro systems showed substituent effects that are a factor of 2 smaller than that for corresponding simple para-nitrobenzyl systems. These linear correlations between the substituent effects and redox potentials have been used to estimate the redox potentials of irreversible systems, which are required for digital simulations of reactions involving nitroaryl radical anions. Only small substituent effects are present in hyperfine coupling with the aliphatic and aromatic protons of the nitroaromatic radical anions of ethanoanthracenes (1)-(13), but a clear trend to lower nitrogen hyperfine values was observed with increasing electron-withdrawing ability of the apical substituent. In addition, no spin density was transferred to the benzylic bridgehead substituent in any of the nitro radical anions studied, clearly demonstrating that the bond between the bridgehead substituent and the carbon at a benzylic position is orthogonal to the π-system of the nitroaromatic ring bearing the odd electron.