The association constants (K) of complexes of chiral neutral amines with optically active crown ethers ((S,S)-1, (S,S)-2, (S,S)-4 and (S,S)-5) which contain the phenyl chiral barriers and the phenol moiety bearing an additional para-substituent were determined at various temperatures by the 1 H n.m.r. spectroscopic method in CDCl 3 . The thermodynamic parameters for complexation, which were determined from the van't Hoff plots of the K values, suggested that the acidity of the phenolic crown ethers markedly affected the enthalpy and entropy changes upon complexation with neutral amines. The crown ethers ((S,S)-2, (S,S)-4 and (S,S)-5) having higher acidity showed more negative ΔH and ΔS values and a larger slope in the enthalpy-entropy compensation plot than crown ether (S,S)-1 having a lower acidity. The temperature-dependent enantioselectivity in complexation of chiral neutral amines with (S,S,S,S)-6 containing phenyl substituents and the cyclohexane moieties as chiral barriers together with the p-(2,4-dinitrophenylazo)phenol moiety was also examined, demonstrating that steric interactions between the bulky steric barriers and the amine induced larger conformational changes of the host and guest molecules upon complexation. The data of the temperature-dependent enantioselectivity demonstrate a reversal of the relative merits of the enantioselectivity depending upon the temperature. A linear relatioship between the TΔ R,S ΔS values and the Δ R,S ΔH values (TΔ R,S ΔS=0.812Δ R,S ΔH-0.342, R=0.983) shows that the Δ R,S ΔH and Δ R,S ΔS values compensate each other. Therefore, complexation having a larger Δ R,S ΔH value does not always show a higher degree of the enantioselectivity at any temperature than that having a smaller Δ R,S ΔH value.
- Enantioselective complexation
- Neutral amine
- Optically active phenolic crown ether
- Temperature dependent enantioselectivity
- Thermodynamic parameter