Study On The Chiral Recognition Mechanism Of Methyl Mandelate And Methyl α-Cyclohexylmandelate On Permethylated β-Cyclodextrin Column

In this thesis, the deeper studies on the chiral recognition mechanism of methyl mandelate (MMA) and methyl a-cyclohexylmandelate (MCHMA) enatiomers on permethylatedβ-cyclodextrin (PM-β-CD) chiral stationary phase were carried out by experimental and theoretical methods. High performance liquid chromatography (HPLC) and molecular modeling method were adopted. The nature of chiral recognition was illustrated at the molecular level in order to understand the positions and the forces of chiral recognition on the stationary phase.MMA and MCHMA enatiomers were successfully separated on PM-β-CD chiral stationary phase by high performance liquid chromatography. The influence of the frequently-used organic modifiers, such as methanol, ethanol and acetonitrile, along with the influence of the column temperature on separation of the two enantiomers was studied. The results show that good separations of MMA and MCHMA enatiomers were done when the liquid phase was methanol mixed with water. And, the resolution factor increased with the decrease of methanol content. Higher column temperature decreased the retention time, but caused poor separation efficiency. So, the column temperature was set at 20℃. Additionally, the thermodynamic parameters in the process of enantioseparations were determined in order to discuss driven power and chiral discrimination mechanism in the process of enantioseparations. The results indicated that the separation of MMA and MCHMA enantiomers were enthalpy-driven process.The host-guest complexation of PM-(3-CD with MMA and MCHMA enantiomers was successfully simulated by Quantum Mechanics semi-empirical PM3 method for the first time. Chiral recognition mechanism of (R/S)-MMA enantiomers on PM-β-CD column was investigated. The modeling results showed that the stabilization complexation structures which formed with (R/S)-MMA enantiomers and PM-β-CD were different. Although, (R)-MMA and (S)-MMA were located at the wider edge of the PM-β-CD cavity and the carbonyl groups of (R/S)-MMA both pointed to the wider edge of the PM-β-CD cavity, the benzene ring of (R)-MMA locates horizontally approximately on the wider edge of the PM-β-CD cavity while the benzene ring of (S)-MMA was deeply included into the hydrophobic cavity. Therefore, compared to (R)-MMA, the hydrophobic force between (S)-MMA and PM-β-CD was stronger. And, the chiral carbons (C*) in (R/S)-MMA molecules for (R/S)-MMA/PM-β-CD complexes were both close to C(2)and C(3) in glucose unit of PM-β-CD. So, the chiral recognition mechanism was closely related to the chiral environment provided with C(2) and C(3) in the glucose unit of PM-β-CD. At last, the final stabilization structures of (R/S)-MMA/PM-β-CD complexes obtained by PM3 optimization were further studied by NBO analysis, and the result indicated that there were weak hydrogen bondings between (R/S)-MMA enantiomers and PM-β-CD.ONIOM (RB3LYP/6-31G(d):RPM3) method was adopted to further study of the most stable (R/S)-MMA/PM-β-CD complexes obtained by PM3 method. The result of ONIOM was consistent with that of PM3, that the stabilization energy of (S)-MMA/PM-β-CD complex was less than that of (R)-MMA/PM-β-CD. The result of molecular modeling was also consistent with the result of the experiment that the retention time of (S)-MMA on PM-β-CD column was longer than that of (R)-MMA. Additionally, in order to have a better estimation of the chiral recognition between (R/S)-MMA and PM-β-CD, the interaction between host and guest was further studied by NBO analysis. The result show that hydrogen bonding interaction, dipole-dipole interaction, charge-transfer function and hydrophobic force all play key roles in the chiral recognition of ((R/S)-MMA on PM-β-CD chiral column.