β-Cyclodextrin (β-CD) is negligibly soluble in organic liquids and can be modified to achieve a higher solubility in water. In this paper, racemic α-cyclohexyl-mandelic acid (α-CHMA) was separated by chiral reactive extraction with aqueous β-cyclodextrin derivatives. Hydroxypropyl-β-cyclodextrin (HP-β-CD), hydroxyethyl-β-cyclodextrin (HE-β-CD), and methyl-β-cyclodextrin (Me-β-CD) were selected as chiral selectors for reactive extraction of α-CHMA enantiomers from organic phase to aqueous phase. Factors affecting the extraction efficiency were investigated, including the types of organic solvents and β-CD derivatives, the concentrations of the chiral selector and α-CHMA enantiomers, pH and temperature. The experimental results demonstrate that HP-β-CD, HE-β-CD, and Me-β-CD have stronger recognition abilities for S-α-CHMA than for R-α-CHMA. Among the three derivatives, HP-β-CD shows the strongest separation factor for α-CHMA enantiomers. A high enantioseparation efficiency with a maximum separation factor (α) of 2.02 is observed at pH 2.5 and 5 ℃.
The kinetics of the extraction of phenylsuccinic acid (PSA) enantiomers by hydroxypropyl--cyclodextrin (HP--CD) in a modified Lewis cell was studied, in which HP--CD dissolved in 0.1 mol L 1 NaH 2 PO 4 /H 3 PO 4 buffer solution (pH = 2.5) was selected as the chiral extractant. PSA enantiomers were extracted from organic phase to aqueous phase in the extraction module. The theory of extraction accompanied by a chemical reaction has been used to obtain the intrinsic kinetics of this extraction module. The different parameters affecting the extraction rate such as agitation speed, interfacial area, initial concentration of PSA enantiomers in organic phase as well as HP--CD concentration in aqueous phase were separately studied. The experimental results demonstrate that the extraction reactions are fast. The reactions were found to be first order with respect to PSA and second order with respect to HP--CD with forward rate constants of 3.4 × 10 2 m 6 mol 2 s 1 for R-PSA and 9.96 × 10 3 m 6 mol 2 s 1 for S-PSA. These data will be useful in the design of extraction processes.
Enantioselective partitioning of ibuprofen enantiomers in a biphasic recognition chiral extraction system was studied. A combination of hydrophobic L-isobutyl tartrate in organic phase and hydrophilic β-cyclodextrin derivative in aqueous phase is necessary to establish a biphasic recognition chiral extraction system. The studies performed involve an enantioselective extraction in a biphasic system, where ibuprofen enantiomers form four complexes with the β-cyclodextrin derivative in aqueous phase and the D(L)-isobutyl tartrate in organic phase, respectively. In these biphasic resolutions, the types and the concentrations of the extractants, pH and temperature all exert a considerable influence on the biphasic recognition process. Good enantioselectivities for ibuprofen enantiomers were obtained at pH≤2.5 and a ratio of 2∶1 of [L-isobutyl tartrate] to [HP-β-CD]. Biphasic recognition chiral extraction is of strong chiral separation ability, and may be very helpful to optimize the extraction systems and realize the large-scale production of enantiomers.
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