12,15-DICHLORO[3.0]ORTHOMETACYCLOPHANE - A HIGHLY STRAINED BIPHENYLOPHANE

The title compound, 3, in which two methylene groups of the parent com pound 4 have been replaced by an ortho-substituted benzene ring, has b een synthesized. It exists exclusively in the endo-conformation, in co ntrast to 4 for which the exo-conformation is preferred. High-level de nsity functional calculations are presented for both compounds. They s how that the endo-conformation of 3 is favored over the exo-conformati on by 3.3 kcal mol(-1). The incorporation of a benzene ring in the bri dge leads to a substantial increase of the strain energy (Delta SE = S E (3) - SE (4) = 4.7 kcal mol(-1)), making 3 the, thus far, smallest a nd most reactive, yet isolable, [n]metacyclophane.