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 b ipyridinium guests demonstrated. Both crown ethers could be interlocked mec hanically with cyclobis (paraquat-p-phenylene) to afford two chiral [2]cate nanes. Furthermore, these crown ethers were also mechanically interlocked w ith a tetracationic cyclophane incorporating a 2,2'-dihydroxy-1,1'-binaphth yl spacer to afford mixtures of diastereoisomeric [2]catenanes. The composi tion of these mixtures was determined by H-1-NMR-spectroscopic and HPLC ana lyses which revealed that modest diastereoselection (56:44-67:33) occurs du ring the kinetically controlled self-assembly of the catenanes. The free en ergy 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 id entified and the associated free energy barriers (10.3-10.4 kcal mol(-1)) d etermined.