A Conformational Study of [3.3]Metacyclophanes through Variable-Temperature ¹H 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) ¹H NMR method. In order to simplify the interpretation of the 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 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 °C. This result as well as the fact that the energy barrier (ΔG*) 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 (ΔG* = 11-12 keal/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 ¹H NMR data.