Enantioselective discrimination in the self-assembly of [2]pseudorotaxanes

The combination of (i) an optically active, axially chiral p-electron-deficient tetracationic cyclophane deriv. of cyclobis(paraquat-1,4-phenylene), in which both of the 1,4-phenylene spacers were replaced by axially-chiral 3,3'-disubstituted binaphthol spacers, and (ii) enantiomeric, p-electron-rich substrates, in which a hydroquinone ring is inserted into the polyether backbone terminated by carboxyl groups and substituted in a C2-sym. manner by two Me groups, thus creating two equiv. chiral centers in the substrate, produces in soln. 1:1 complexes in which the p-electron-rich substrates are inserted into the p-electron-deficient cavities of the cyclophanes in a pseudorotaxane-like manner. The differences in the free energies of complexation for (RR) and (SS) enantiomers of the p-electron-rich substrates span the range from 0.1 to 0.7 kcal/mol. Chiral recognition becomes more effective the closer the chiral centers are to the hydroquinone templating unit. CD spectra reveal that the different modes of binding of the enantiomeric substrates by the axially chiral tetracationic cyclophane are not accompanied by drastically different core geometries for the [2]pseudorotaxanes. Thus, the chirality of the complex is governed primarily by the properties of the rigid receptor. The combination of the D2 symmetry of the receptor with the C2 symmetry of the substrates is particularly effective, considering that the chiral centers on the substrates are located on polyether chains which possess a high degree of conformational freedom