Molecular recognition studies on supramolecular systems. 22. Size, shape, and chiral recognition of aliphatic alcohols by organoselenium- modified cyclodextrins

A novel cyclodextrin derivative, mono[6-(p-methoxyphenylseleno)-6- deoxy]-β-cyclodextrin (2), has been synthesized and characterized by elemental analysis and mass, FT-IR, and ¹H NMR spectroscopy. The stability constants (K(s)) of the inclusion complexation of 2 and mono[6-(p- tolylseleno)-6-deoxy]-β-cyclodextrin (3) with a series acyclic and cyclic alcohols have been determined in phosphate buffer solution (pH 7.20) at 25 °C by using the circular dichroism spectral titration method. Although the stability constants obtained for the inclusion complexation of 2 and 3 with aliphatic alcohols are generally smaller than those for native β- cyclodextrin, the modified cyclodextrins can recognize both the size and chirality of the guest molecules. Interestingly, the complex stability constants (log K(s)), or the Gibbs free energy change (-ΔG°), increase linearly with increasing number of carbon atoms in the guest molecule (N(c)), irrespective of the topological differences of acyclic, cyclic, and bicyclic guests. The unit increment of complex stability per methylene (-dΔG°/dN(c)) is not appreciably affected by the difference of the host's substituent but is a critical function of the guest topology, affording distinctly different -dΔG°/dN(c) values of 2.4 and 2.9 kJ mol^-1 for alkanols and cycloalkanols, respectively. In the complexation of chiral guests with 2 and 3, the observed enantioselectivities, as measured by the stability difference (ΔΔG°), are mostly in the range of 1-2 kJ mol^-1.