SYNTHESIS AND STRUCTURE ELUCIDATION OF A NEW [2]-CATENANE

A supramolecular self-assembly process has lead to the synthesis of new type of [2]-catenane. A 34% yield of catenane was obtained from a one-pot double macrocyclization reaction. Its three-dimensional structure was determined by using two-dimensional H-1 NMR spectroscopy. This structure agrees well with the predictions of molecular mechanics calculations, although there is some ambiguity in the orientation of one of the amide bonds. The molecule is locked into a well-defined conformation by several intermacrocycle H-bonds and pi-pi interactions and it is these interactions that template the assembly of the interlocked ring system. The dynamic properties of the catenane were investigated by variable-temperature H-1 NMR. The ''inside'' and ''outside'' parts of the molecule do not exchange with one another because of bulky cyclohexyl groups, which act as ''spokes'' preventing free rotation of the macrocycles. However, the system does possess some conformational mobility, which can be followed on the H-1 NMR time scale. A chiral NMR shift reagent was used to show that the [2]-catenane adopts a chiral ground state. It switches between two enantiomeric conformations at a rate of 1 s-1 at room temperature.