12,15-dichloro[3.0]orthometacyclophane: A highly strained biphenylophane

The title compound, 3, in which two methylene groups of the parent compound 4 have been replaced by an ortho-substituted benzene ring, has been synthesized. It exists exclusively in the endo-conformation, in contrast to 4 for which the exo-conformation is preferred. High-level density functional calculations are presented for both compounds. They show that the endo-conformation of 3 is favored over the exo-conformation by 3.3 kcal mol(-1). The incorporation of a benzene ring in the bridge leads to a substantial increase of the strain energy (Delta SE = SE (3) - SE (4) = 4.7 kcal mol(-1)), making 3 the, thus far, smallest and most reactive, yet isolable, [n]metacyclophane.