MECHANISM FOR CHIRAL RECOGNITION OF BINAPHTHYL DERIVATIVES BY CYCLODEXTRINS

The mechanism for chiral recognition of 1,1'-binaphthyl-2,2'-diyl hydr ogen phosphate (BNP) and 1,1'-binaphthyl-2,2'-dicarboxylic acid (BNC) by beta-cyclodextrin (beta-CDx) and heptakis(2,3,6-tri-O-methyl)-beta- CDx (TMe-beta-CDx) has been investigated by H-1 NMR spectroscopy, ther modynamic parameters for complexation and molecular mechanics-molecula r dynamics (MM-MD) calculations, TMe-beta-CDx recognizes axial chirali ties of the BNP anion and undissociated BNC, (S)-BNP and (R)-BNC being the preferable guest enantiomers. The ability of beta-CDx to recogniz e the axial chiralities is inferior to that of TMe-beta-CDx. H-1 NMR s pectra suggest that (S)-BNP is shallowly bound to the wider side of th e TMe-beta-CDx cavity where a hydrophilic phosphate group is located i nside the hydrophobic CDx cavity, Meanwhile, (R)-BNP seems to form the complex of TMe-beta-CDx where the C-2 symmetry axis of (R)-BNP is per pendicular to the C-7 symmetry axis of TMe-beta-CDx. The complexation of the preferable enantiomer of the guest with TMe-beta-CDx is entropi cally 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-dipol e interactions between the host and the guest. Similar orientation in the cases of the undesirable enantiomers may be prevented by steric fa ctors. The positive entropy changes in the complexation of (S)-BNP and (II)-BNC seem to be ascribed to the extended dehydration from both th e hydrophilic groups of the guest and the ethereal oxygen atoms of the wider side of the TMe-beta-CDx cavity upon inclusion.