Translational isomerism in some two- and three-station [2]rotaxanes

The template-directed syntheses of three [2]rotaxanes are described. They all have dumbbell components, with both hydroquinone and resorcinol rings inserted into polyether chains terminated by tetraarylmethane stoppers, that become encircled during the key self-assembly processes by the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), with its two pi-electron deficient bipyridinium units. It has been demonstrated by low-temperature H-1 NMR spectroscopy that the pi-electron deficient tetracationic cyclophane has a remarkably high preference to reside around the hydroquinone ring in these molecular shuttles. This observation illustrates how a very small constitutional difference-hydroquinone versus resorcinol recognition sites-can lead to the overwhelming preference for one translational isomer over another in this particular range of [2]rotaxanes.