Pyrolysis and Isomerization Reactions of Multiply‐bridged Cyclophanes

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Abstract
When subjected to pyrolysis in para‐di‐isopropylbenzene (270°C, 40h) [2.2.2] (1,2,4)cyclophane (5 ) is cleaved to 2,9‐dimethyldibenzo[a,e]cyclooctadiene (8 ). Under comparable conditions (290°C, 65h), [2.2.2.2] (1,2,3,5)cyclophane (6 ) is converted to 6,11‐dimethyl[2.2.2] (1,2,3)cyclophane (9 ), the first derivative of a symmetrical [2n]cyclophane carrying three consecutive ethano bridges. The isomeric [2.2.2.2]‐ (1,2,3,4)cyclophane (7 ), on the other hand, cannot be induced to undergo carbon‐carbon‐bond cleavage followed by hydrogen abstraction. Under the influence of aluminiumtrichloride/hydrogenchloride hydrocarbon 5 isomerizes to the unsymmetrical [2.2.2] (1,2,4) (1,2,5)cyclophane (10 ), whereas 6 and 7 are stable under these conditions. The thermal stability and propensity of multiply‐bridged cyclophanes to undergo isomerization reactions is discussed.

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