ZUR STABILISIERUNG VON PARACYCLOPHANEN DURCH SUBSTITUTION AM BENZOLRING

Article abstract of DOI:10.1016/S0040-4020(01)86834-4

A relatively efficient route to substituted <5>paracyclophanes 11 is described.Diels Alder reaction of the pentamethylenecyclobutadienes 3 and 4, prepared in situ, with acetylenes E-C<*>C-E (5, E=electron withdrawing group) gives a mixture of Dewar benzenes, from which the 1,4-bridged isomer 7 is conveniently isolated after aromatization of the other isomers.Irradiation (254 nm) of 7 at 220 K establishes a photo equilibrium between 7 and 11 with 2-15 percent of the latter, depending on E.The aromatic character of 11 is derived from the UV- and 1H-NMR spectra; at 220K, two conformational isomers 11' and 11'' can be discerned.Although most 11 are thermally more stable than the parent compound <5>paracyclophane, they slowly decompose in solution at room temperature.