Mechanism for chiral recognition of binaphthyl derivatives by cyclodextrins

Abstract

The mechanism for chiral recognition of 1,1′-binaphthyl-2,2′-diyl hydrogen phosphate (BNP) and 1, 1′binaphthyl-2,2′-dicarboxylic acid (BNC) by β-cyclodextrin (β-CDx) and heptakis(2,3,6-tri-O-methyl)β-CDx (TMe-βCDx) has been investigated by ′H NMR spectroscopy, thermodynamic parameters for complexation and molecular mechanics-molecular dynamics (MM-MD) calculations. TMe-β-CDx recognizes axial chiralities of the BNP anion and undissociated BNC, (S)-BNP and (R)-BNC being the preferable guest enantiomers. The ability of β-CDx to recognize the axial chiralities is inferior to that of TMe-β-CDx. ′H NMR spectra suggest that (S)-BNP is shallowly bound to the wider side of the TMe-β-CDx cavity where a hydrophilic phosphate group is located inside the hydrophobic CDx cavity. Meanwhile, (R)-BNP seems to form the complex of TMe-β-CDx where the C₂ symmetry axis of (R)-BNP is perpendicular to the C₇ symmetry axis of TMe-β-CDx. The complexation of the preferable enantiomer of the guest with TMe-β-CDx is entropically favourable while that of the undesirable enantiomer of the guest is entropically unfavourable. The MM–MD calculations suggest that the orientation of the preferable guest is determined by the dipole-dipole interactions between the host and the guest. Similar orientation in the cases of the undesirable enantiomers may be prevented by steric factors. The positive entropy changes in the complexation of (S)-BNP and (R)-BNC seem to be ascribed to the extended dehydration from both the hydrophilic groups of the guest and the ethereal oxygen atoms of the wider side of the TMe-β-CDx cavity upon inclusion.