Some Investigations on [6]Metacyclophanes

Abstract

[6]Metacyclophanes 2 with chlorine or fluorine substituents have been synthesized by a novel, convenient version of the base-catalyzed double elimination of hydrogen chloride from the corresponding halopropellanes 7. Conformational analysis by 1H and 13C NMR spectroscopy revealed remarkable analogies of certain partial structures of the hexamethylene bridge of 2 with the known exo and endo conformers of [5]metacyclophanes 1. The Diels−Alder reactivity of 2 towards tetracyanoethene was investigated in order to clarify seemingly contradictory reports in the literature. As anticipated, it was observed that the reactivity of 2 is strongly reduced compared to that of the more strained lower homologues 1, but there is a subtle fine tuning: While the unsubstituted parent compound 2a still forms adducts with strong dienophiles, chlorine substitution at the aromatic ring completely blocks the Diels−Alder reactions.