Temperature Dependence of Enantioselectivity in Complexations of Optically Active Phenolic Crown Ethers with Chiral Amines in Solution

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 the1H n.m.r. spectroscopic method in CDCl3. 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 TΔR,SΔS values and the ΔR,SΔH values (TΔR,SΔS=0.812ΔR,SΔH-0.342, R=0.983) shows that the ΔR,SΔH and ΔR,SΔS values compensate each other. Therefore, complexation having a larger ΔR,SΔH value does not always show a higher degree of the enantioselectivity at any temperature than that having a smaller ΔR,SΔH value.