SYNTHESIS AND CONFORMATIONAL BEHAVIOR OF BENZO[2.2]METAPARACYCLOPHAN-9-ENE, DIBENZO[2.2]METAPARACYCLOPHANE AND 1,4-DIMETHYLDIBENZO[2.2]METAPARACYCLOPHANE - X-RAY CRYSTAL-STRUCTURE OF DIBENZO[2.2]METAPARACYCLOPHANE

Dibenzo[2.2]metaparacyclophane (5) and its 1,4-dimethyl derivative 6 have been synthesized utilizing low-valent-titanium deoxygenation as the key step. The conformational mobility of 5 and 6, as well as that of benzo[2.2]metaparacyclophane (3) and benzo[2.2]metaparacyclophan-9-ene (4) have been studied by variable temperature NMR spectrometry. The results of this study, together with those reported for [2.2]metaparacyclophane (1) and [2.2]metaparacyclophane-1,9-diene (2), show that the free energy of activation for the flipping process (DELTAG(c)double dagger) at the coalescence temperature (T(c)) is closely related to the bond types of the bridges connecting the meta-bridged benzenes and the para-linked ones. In addition to these structural effects, the nonbonded interactions of H-1 and H15, and that of H12 with H13 in 5 also influence such conformational behavior, as can be unequivocally substantiated by the synthesis and study of 6. An X-ray crystallographic study has shown that 5 conforms closely to idealized C(s) molecular symmetry with a dihedral angle of 147.2-degrees between the pair of orthobenzenes and a pronounced boat conformation for the para-bridged one.