Molecular Meccano, 51. - Diastereoselective Self-Assembly of [2]Catenanes

Two chiral pi-electron-rich crown ethers incorporating either a binaphthol or two D-mannitol units have been synthesized and their abilities to bind bipyridinium guests demonstrated. Both crown ethers could be interlocked mechanically with cyclobis (paraquat-p-phenylene) to afford two chiral [2]catenanes. Furthermore, these crown ethers were also mechanically interlocked with a tetracationic cyclophane incorporating a 2,2'-dihydroxy-1,1'-binaphthyl spacer to afford mixtures of diastereoisomeric [2]catenanes. The composition of these mixtures was determined by H-1-NMR-spectroscopic and HPLC analyses which revealed that modest diastereoselection (56:44-67:33) occurs during the kinetically controlled self-assembly of the catenanes. The free energy barriers (12.8-16.8 kcal mol(-1)) associated with the circurmrotation of one macrocyclic component through the cavity of the other and vice versa were determined by variable-temperature H-1-NMR spectroscopy. In addition, another dynamic process involving the "rocking" of the mean planes of the mechanically interlocked macrocycles with respect to each other was also identified and the associated free energy barriers (10.3-10.4 kcal mol(-1)) determined.