DODECAMETHOXYORTHOCYCLOPHANE - CONFORMATIONAL AND DYNAMIC PROPERTIES STUDIED BY PROTON-2D EXCHANGE NMR

A new member of the orthocyclophane series, i.e., 3,4,5,10,11,12,17,18,19,24,25,26-dodecamethoxy [1.1.1.1]orthocyclophane (DCP), has been synthesized and its conformational and dynamic properties studied by 1D and 2D proton NMR spectroscopy. Its proton NMR exhibits in solutions two subspectra due to two conformers. Molecular mechanics calculations and NMR chemical shift data indicate that these conformers correspond to the sofa and boat forms, respectively. The equilibrium constant between the two forms was determined at room temperature in a number of solvents. In all cases the ratio [sofa]/[boat] was larger than unity, but in general this ratio decreased with increasing the polarity of the solvent. Two-dimensional exchange NMR experiments were performed in order to investigate the various rearrangement mechanisms involving the two conformers. The results at 30 °C in a nitrobenzene solution show that the direct sofa-sofa pseudorotation is most rapid (k1 = 6.8 s-1), while pseudorotation of the boat is slow and proceeds mainly indirectly via the sofa form. There are two distinct sofa-boat interconversion processes with comparable rate constants (k3 = 4.5 and k4 = 4.7 s-1). X-ray diffraction measurements indicate that DCP crystals are monoclinic and belong to the space group P21/a. There are two symmetry-related molecules per unit cell located at points of inversion symmetry, with geometry corresponding to that of the sofa conformation. © 1990, American Chemical Society. All rights reserved.