Enantioseparation Of Aromatic Acid Enantiomers In The Ionic Liquids-water Two-phase Extraction System

In the past decade, ionic liquids have received great attention owing to their potential as green solvent alternatives to conventional organic solvents. In this work, ionic liquids-water two-phase extraction system (ILWTPES) was developed with hydrophobic ionic liquids, and it was applied in the enantioseparation of aromatic acid enantiomers. Mandelic acid (MA) enantiomers and tryptophan (Try) enantiomers were separated successfully. The quantitative analysis of enantioseparation was carried out on high-performance liquid chromatography (HPLC) using a chiral mobile phase.1. The optimizations of HPLC analysis conditions for MA enantiomers and Try enantiomers were studied. The mobile phase was water/methanol containing3mmol·L-1CuSO4and6mmol·L-L-phenylalanine,and the metal complex formed with Cu2+and L-phenylalanine was used as chiral selector. The concentration of chiral selector, the percentage of methanol in the mobile phase, flow rate and column temperature were studied. The optimal HPLC conditions for MA enantiomers were that:3mmol·L-1chiral selector, methanol percentage of15%, flow rate of1mL·min-1and constant column temperature of22℃.The optimal HPLC conditions for Try enantiomers were that:3mmol·L-1chiral selector, methanol percentage of25%, flow rate of1mL·min-1and constant column temperature of28℃.2. Enantioseparation of MA enantiomers in ILWTPES was studied. With β-cyclodextrin (β-CD) derivatives as hydrophilic chiral selectors, R-MA and S-MA were separated in ILWTPES owing to the different stabilities of two diastereomeric complexes formed with R/S-MA and β-CD derivatives. Factors affecting the separation efficiency were analyzed, namely the types of extracting solvents and β-CD derivatives, the concentrations of β-CD derivatives and MA enantiomers, pH, and temperature. Excellent enantioseparation of MA enantiomers was achieved in ILWTPES under the optimal conditions of [bmim]PF6used as extracting solvents, pH=2.5,[HP-β-CD]=0.15g·mL-1, and temperature of 5℃with a maximum enantioselectivity (a) of1.740. The experimental results demonstrated that ILWTPES with β-CD derivatives as chiral selectors had a good chiral recognition ability.3. Enantioseparation of Try enantiomers in ILWTPES was studied. With metal complex formed with central ion Cu(Ⅱ) and chiral ligand L-proline as chiral selector, D-Try and L-Try were separated in ILWTPES owing to the different exchange capacity between D/L-Try and chiral ligand. Factors affecting the separation efficiency were analyzed, namely the types of extracting solvents, the concentrations of chiral selector and Try enantiomers, molar ratio of Cu(Ⅱ) to L-proline, pH, and temperature. Excellent enantioseparation of Try enantiomers was achieved in ILWTPES under the optimal conditions of [omim]BF4used as extracting solvents, molar ratio of2:1of Cu(Ⅱ) to L-proline, pH=5.5,[Try]=lmmol/L, and chiral selector of5mmol/L with a maximum enantioselectivity (a) of1.426. The experimental results demonstrated that ILWTPES with metal complex as chiral selector by chiral ligand exchange had a good chiral recognition ability.