Medium-Sized Cyclophanes. IV. The Halogenation Reactions of [2.2]Metacyclophane

It has been postulated that, with proper control and choice of reaction conditions, the halogenation reactions of [2.2]metacyclophane might be directed in either of two ways: (A) the addition-oxidation path or (B) the addition-elimination path. An attempted iodination (iodine-silver perchlorate or iodine chloride) or bromination (bromine-iron catalyst) of [2.2]-metacyclophane resulted in the formation of 4,5,9,10-tetrahydropyrene according to the path B reaction. On the other hand, iodination reaction in the presence of nitric acid has been shown to afford 2-iodo-4,5,9,10-tetrahydropyrene, after path A. Both the reactions have been explained by assuming a common intermediate, which is formed by the attack of the halogenonium ion on one ring, accompanied by a simultaneous transannular attack of the generated phenonium ion on the second ring.