A CONFORMATIONAL STUDY OF [3.3]METACYCLOPHANES THROUGH VARIABLE-TEMPERATURE H-1-NMR AND OPTICAL-ROTATION

Conformational behavior of 2,2,11,11-tetradeuterio[3.3]metacyclophanes 1-d4 and 2-d4 has been studied by a variable-temperature (VT) H-1 NMR method. In order to simplify the interpretation of the H-1 NMR spectrum, four deuteriums were introduced at C-2 and C-11 positions of the trimethylene bridges of [3.3]metacyclophanes by reductive desulfurization of 2,11-bis(1,4-dithiabutane-1,4-diyl)[3.3]metacyclophanes 4 and 5 with tri-n-butyltin deuteride. Our previous conformational study of tetradeuterio-1,4-dioxa[4.3.3]cyclophane (3-d4) revealed that the temperature-dependent phenomenon in the H-1 NMR spectrum of 1 was ascribed to the inversion the trimethylene bridges. The work also suggested the presence of benzene ring inversion. To confirm this, optically active [3.3]metacyclophanechromium tricarbonyl complexes (-)- and (+)-17 were prepared by means of the HPLC separation of racemic complex (+/-)-17 using a chiral stationary phase. Racemization occurred when (-)- and (+)-17 were decomplexed at 20-degrees-C. This result as well as the fact that the energy barrier (DELTAG(double dagger)) for the benzene ring inversion could not be detected by the VT NMR method indicated that the barrier is much lower than that of trimethylene bridge inversion (DELTAG(double dagger) = 11-12 kcal/mol). The most stable conformer of 1-d4 and 2-d4 is a syn(chair-chair), and the less stable conformer is estimated to be a syn(chair-boat) on the basis of the H-1 NMR data.