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 h ave been synthesized utilizing low-valent-titanium deoxygenation as th e 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-en e (4) have been studied by variable temperature NMR spectrometry. The results of this study, together with those reported for [2.2]metaparac yclophane (1) and [2.2]metaparacyclophane-1,9-diene (2), show that the free energy of activation for the flipping process (DELTAG(c)double d agger) at the coalescence temperature (T(c)) is closely related to the bond types of the bridges connecting the meta-bridged benzenes and th e para-linked ones. In addition to these structural effects, the nonbo nded interactions of H-1 and H15, and that of H12 with H13 in 5 also i nfluence such conformational behavior, as can be unequivocally substan tiated by the synthesis and study of 6. An X-ray crystallographic stud y has shown that 5 conforms closely to idealized C(s) molecular symmet ry with a dihedral angle of 147.2-degrees between the pair of orthoben zenes and a pronounced boat conformation for the para-bridged one.