REDUCTION OF BRIDGEHEAD HALOGENS BY AN INTRAMOLECULAR ELECTRON-TRANSFER RADICAL MECHANISM

Reactions of 9,10-dibromo- and 9,10-diiodo-2-nitro-9,10-ethano-9,10-dihydroanthracene (10 and 11, respectively) with the tertiary carbanions, 1, 3, 5, and 7-9, proceed exclusively by reduction at the bridgehead with no substitution products being observed. It is proposed that the reduction process occurs by a radical chain mechanism including an intramolecular electron transfer step and beta-hydrogen abstraction from alkyl substituents on the participating carbanions. These ethanoanthracenes contain halogens at bridgehead positions that are meta- and para-benzylic relative to an aromatic nitro group, thus allowing the determination of the relative reactivities of the two benzylic sites within the same molecule. Quantitative studies on the reaction of 11 with sodium salts of 2-ethylmalononitrile and diethyl 2-ethylmalonate reveal that the reduction process is regioselective, with reduction occurring more readily at the benzylic bridgehead position para to the nitro group than at the corresponding meta-benzylic position. The ratio of meta:para reduction products, determined for the reaction of the diiodide 11 with several carbanions, was in the range 1:(1.6 +/- 0.2). This ratio contrasts with the differences in rate constants (approximately 2 orders of magnitude) determined for other nitrobenzylic systems, known to undergo S(RN)1 substitution reactions with the same nucleophiles. These differences in the ratio of rate constants of regioselective reduction compared with those observed for substitution reactions is discussed in terms of the C-X bond at a bridgehead position lying orthogonal to the plane of the nitroaryl group. As a result of this geometry, the rate of intramolecular electron transfer is significantly reduced and the ratio of para-benzylic to meta-benzylic reactivity differs only by a factor of less than 2.