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

Abstract

The combination of (i) an optically active, axially chiral π-electron-deficient tetracationic cyclophane derivative of cyclobis(paraquat-p-phenylene), in which both of the p-phenylene spacers have been replaced by axially-chiral 3,3′-disubstituted binaphthol spacers, and (ii) enantiomeric, π-electron-rich substrates, in which a hydroquinone ring is inserted into the polyether backbone terminated by carboxyl groups and substituted in a C2-symmetric manner by two methyl groups, thus creating two equivalent chiral centers in the substrate, produces in solution 1:1 complexes in which the π-electron-rich substrates are inserted into the π-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 π-electron-rich substrates span the range from 0.1 to 0.7 kcal mol-1. Chiral recognition becomes more effective the closer the chiral centers are to the hydroquinone templating unit. CD spectroscopy reveals 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 has been found to be particularly effective, considering that the chiral centers on the substrates are located on polyether chains which possess a high degree of conformational freedom.