Molecular recognition study on supramolecular systems. Part 19.† Circular dichroism studies of inclusion complexation of aliphatic alcohols by organoselenium modified β-cyclodextrins
Complex stability constants for the stoichiometric 1∶1 inclusion complexation of various aliphatic alcohols with mono[6-(benzylseleno)-6-deoxy]-β-cyclodextrin (1), mono[6-(phenylseleno)-6-deoxy]-β-cyclodextrin (2), and mono[6-(o-, m-, or p-tolylseleno)-6-deoxy]-β-cyclodextrin (3–5) have been obtained by spectrophotometric titrations at 25 °C in phosphate buffer solution (pH = 7.2). The Cotton effects observed indicate that the aromatic moiety penetrates shallowly into the hydrophobic cavity of cyclodextrin. Therefore, the aromatic moiety can be taken as an induced circular dichroism (ICD) probe to investigate the inclusion phenomena. The results obtained demonstrate that the modified β-cyclodextrin (1) is highly sensitive to the size/shape and conformational rigidity of guest molecules, giving fairly good molecular selectivity up to 114 for adamantan-1-ol/cyclopentanol and relatively high E/Z selectivity up to 2.7 for geraniol/nerol. Interestingly, all of the modified β-cyclodextrins employed displayed relatively good enantioselectivity for (+)-enantiomers of borneol and menthol. The molecular recognition ability and enantioselectivity for aliphatic alcohols of the modified β-cyclodextrins (1–5) are discussed from the viewpoints of the size/shape–fit relationship and the multipoint recognition mechanism.