Chemical, electrochemical and spectroelectrochemical behaviour of polycyclic nitroaromatics

A range of dinitro-9,10-ethano-9,10-dihydroanthracenes were prepared in order to determine the degree of delocalisation of electron density in their radical anions. Electrochemical and spectroelectrochemical studies of the radical anions formed from these dinitro compounds showed that the isolated nitroaromatic groups are communicating electronically. The greatest communication occurred when the bridgehead group was electron withdrawing in nature and when the nitro groups and the bridgehead substituent were in the 2, 7 and 9 configurations, respectively. The series of substitution reactions between 2-nitro-9,10-diiodo-9,10-ethano9,10-dihydroanthracene and the sodium salt of p-toluenethiol indicated that this reaction was taking place by the SRNt mechanism. The iodine atom at the bridgehead position para to the nitro group was more readily replaced with p-tolylthio than the iodine atom at the bridgehead position meta to the nitro group. This result is in contrast with that obtained for the dibromo analogue which reacts with sodium p-toluenethiolate to give substitution products with the opposite regiochemical outcome. Further, the rate constants for these reactions are significantly lower than those determined for all other nitrobenzylic compounds. The reaction of 2—nitro-9,10-diiodo-9,10-ethano-9,10—dihydroanthracene with various carbanions gave only 'reduced' products. In these reactions it was apparent that reduction occurred more readily at a bridgehead position para to a nitro group. Further, the reaction of the diiodide with the sodium salt of 2-(1,1,1-trideuteromethyl) malononitrile showed that reduction occurred by a hydrogen abstraction mechanism, and that the B-hydrogen in the alkyl carbanions was the source of hydrogen(s) in the reaction. Estimates of the rate constants for intramolecular electron transfer from the nitroaromatic group to the bridgehead iodo group in the iodo series of compounds showed that ket (para) was greater than k9, (meta). This result was consistent with the chemical reactivity observed in substitution and reduction reactions involving the diiodo compound. Irreversible intramolecular electron transfer reactions were also observed in the nitroaromatic compounds with 9-carboxylic acid or 9-nitro substituents