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

Reactions of 9,10-dibromo- and -diiodo-2-nitro-9,10-ethano-9,10-dihydr oanthracene (10 and 11, respectively) with the tertiary carbanions, 1, 3, 5, and 7-9, proceed exclusively by reduction at the bridgehead wit h no substitution products being observed. It is proposed that the red uction process occurs by a radical chain mechanism including an intram olecular electron transfer step and beta-hydrogen abstraction from alk yl substituents on the participating carbanions. These ethanoanthracen es contain halogens at bridgehead positions that are meta- and para-be nzylic relative to an aromatic nitro group, thus allowing the determin ation of the relative reactivities of the two benzylic sites within th e same molecule. Quantitative studies on the reaction of 11 with sodiu m salts of 2-ethylmalononitrile and diethyl 2-ethylmalonate reveal tha t the reduction process is regioselective, with reduction occurring mo re readily at the benzylic bridgehead position para to the nitro group than at the corresponding meta-benzylic position. The ratio of meta:p ara 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 ord ers of magnitude) determined for other nitrobenzylic systems, known to undergo S(RN)1 substitution reactions with the same nucleophiles. The se differences in the ratio of rate constants of regioselective reduct ion compared with those observed for substitution reactions is discuss ed 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 b y a factor of less than 2.