Chiral recognition of an anionic tetrahelicene by native cyclodextrins. Enantioselectivity dominated by location of a hydrophilic group of the guest in a cyclodextrin cavity

Abstract

Chiral recognition of 1,12-dimethylbenzo[c]phenanthrene-5,8-dicarboxylic acid (1) by native β- (β-CD) and γ-cyclodextrins (γ-CD) has been studied by means of ¹H NMR spectroscopy. The binding constant (K) for complexation of (M)-1 with β-CD (18700 ± 1700 dm³ mol^–1) is much larger than that for (P)-1 (2200 ± 100 dm³ mol^–1), ΔΔG being 5.2 kJ mol^–1. γ-CD forms less stable inclusion complexes (K = 3100 ± 100 and 690 ± 20 dm³ mol^–1 for (M)-1 and (P)-1, respectively). The 2D ROESY spectra indicate that both CO₂^– groups of 1 are placed near the rim of the secondary OH group side of β-CD though the (P)-1 molecule penetrates into the host cavity somewhat more deeply than the (M)-1 molecule. The deeper penetration of the (P)-1 molecule seems to be an enthalpically unfavourable but entropically favourable process because such a complexation needs dehydration from the CO₂^– group(s) of 1. The enantioselectivity of β-CD toward 1 is dominated by the difference in the enthalpy changes due to the difference in the extent of penetration between the enantiomers of 1. The 2D NMR spectra clearly indicate that at least one CO₂^– group of 1 is located inside of the γ-CD cavity resulting in extensive dehydration from the guest molecule. Such an endothermic process reduces the K value for the 1-γ-CD complex. The difference in the structures of the complexes between the guest enantiomers might be ascribed to the chiral helix-structure of the CD taken upon complexation in water.