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
hydrogenated rapidly (within 10 s) to furnish the bridgehead olefins 13b and 12c. The accompanying hydrogenation enthalpies are -220 and -141 kJmol(-1), respectively. Strain energies (SE) and olefinic strains (OS) of a number of bridgehead olefins have been evaluated by DFT calculations; it was concluded that 13b belongs to the class of hyperstableolefins which correlates nicely with its reluctance to undergo