Analytical Sciences

Temperature Dependence of Enantioselectivity inComplexationsof Optically Active Phenolic Crown Ethers withChiral Amines in Solution

Koichiro NAEMURA * , Kazuhisa MATSUNAGA,Junichi FUJI, Kazuko OGASAHARA,Yasushi NISHIKAWA, KeijiHIROSE and Yoshito TOBE

Department of Chemistry,Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560,Japan

The association constants (K) of complexes of chiral neutral amineswith optically active crown ethers ((S,S)-1,(S,S)-2,(S,S)-4 and (S,S)-5) whichcontain the phenyl chiral barriers and the phenol moiety bearing anadditional para-substituentwere determined at various temperatures by the 1H n.m.r. spectroscopic method in CDCl3. The thermodynamic parametersfor complexation, which were determined from the van't Hoff plots of theK values, suggested that the acidity of thephenolic crown ethers markedly affected the enthalpy and entropy changesupon complexation with neutral amines. Thecrown ethers ((S,S)-2, (S,S)-4and (S,S)-5) having higher acidity showed morenegative DH and DS values and a largerslope in the enthalpy-entropy compensation plot than crown ether(S,S)-1 having a lower acidity. Thetemperature-dependentenantioselectivity in complexation of chiral neutral amines with(S,S,S,S)-6 containing phenylsubstituents andthe cyclohexane moieties as chiral barriers together with thep-(2,4-dinitrophenylazo)phenol moiety was also examined,demonstrating that steric interactions between the bulky steric barriersand the amine induced larger conformationalchanges of the host and guest molecules upon complexation. The data of thetemperature-dependent enantioselectivitydemonstrate a reversal of the relative merits of the enantioselectivitydepending upon the temperature. A linear relatioshipbetween the TDR,SDS values and the DR,SDH values (TDR,SDS=0.812DR,SDH-0.342, R=0.983) showsthat the DR,SDH andDR,SDS values compensateeach other. Therefore, complexation having a larger DR,SDH value does not always show ahigher degree of the enantioselectivity at any temperature than that havinga smaller DR,SDHvalue. (Keywords: Optically active phenolic crown ether, enantioselectivecomplexation, neutral amine, temperature dependentenantioselectivity, thermodynamic parameter, complexation)

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