Other Chromatographic and Related Techniques of Resolving Enantiomers in Ligand Exchanging Systems

In gas chromatography, ligand exchange has acquired great importance since Schurig introduced metal complexes of chiral terpeneketonate (acylated camphor, carvone, pulegone, menthone) as additives to stationary phase in capillary GC in 1977. Ni(II), Mn(II), Cu(II), Zn(II), and Rh(I) proved especially useful in this 'complexation' chromatography. The resolution mechanism is ascribed in terms of the interaction between the above sterically or electronically unsaturated metal chelates with volatile ligands which offer lone electron pairs to form a labile mixed-ligand sorption complex. Enantioselectivity of this process is small, but, due to the extremely high plate numbers attainable with capillary GC columns, excellent resolution is achieved for many classes of alkenes, alcohols, ethers, oxirane, sulfur-containing compounds, aziridine, etc. It is important that even compounds containing one single electron-donating atom (O, N, S) are successfully resolved by GC, contrary to LC, where the presence of two donors to form a chelate with the central metal ion is highly desirable.

Similar bonded chiral selectors, e.g. polysiloxane-anchored Ni(II)-bis-[(3-heptafluorobutanoyl)-(1R)-camphorate], Chirasil-Nickel, can also be used in the supercritical fluid chromatography (SFC) mode. Although SFC cannot compete with GC with regard to efficiency, a significant increase in selectivity, due to a substantially lower temperature of the column, can compensate for the loss in plate number. Because of the high solvation strength of supercritical carbon dioxide, a number of low-volatility racemic alcohols, diols and esters have been resolved by ligand exchange SFC at temperatures as low as 40-70┬░C.

In capillary electrophoresis (CE) and other elec-tromigration techniques, chiral ligand exchange can be involved in several ways. Cu(l-His)2 has just been added to the ammonium acetate electrolyte to induce chiral resolution of dansylated amino acids. Analyte enantiomers which are strongly involved in the formation of positively charged mixed complexes with Cu(l-His) move toward the cathode at a higher rate than the uncomplexed ligands. Since the enan-tioselectivity of the ligand exchange reaction in the bulk solution is small, the resolution of enantiomers is also small. It is only slightly better with copper-aspartame as chiral additive. A significant improvement in selectivity can be obtained by adding tetradecyl sulfate, as a micelle-forming surfactant. Obviously, the enantioselectivity increases through the interaction of the diastereomeric ternary complexes

Figure 5 Electrophoregram of the enantiomer separation of DL-Phe, DL-Trp and DL-MeTrp with and without SDS. Conditions: (a) 80 mM L-Hypro, 40 mM Cu(II) sulfate, pH 4.0; (b) 50 mM L-Hypro, 25 mM Cu(II) sulfate, 15 mM SDS, 3 M urea, pH 4.0. Capillary: fused silica, 75 cm x 75 ^m i.d. (effective length, 66 cm) U = 27 kV, UV 208 nm, ambient temperature. (Reproduced with permission from Schmid MG and Guebitz G (1996) Enantiomerl : 23-27. Gordon and Breach Publishers.)

Figure 5 Electrophoregram of the enantiomer separation of DL-Phe, DL-Trp and DL-MeTrp with and without SDS. Conditions: (a) 80 mM L-Hypro, 40 mM Cu(II) sulfate, pH 4.0; (b) 50 mM L-Hypro, 25 mM Cu(II) sulfate, 15 mM SDS, 3 M urea, pH 4.0. Capillary: fused silica, 75 cm x 75 ^m i.d. (effective length, 66 cm) U = 27 kV, UV 208 nm, ambient temperature. (Reproduced with permission from Schmid MG and Guebitz G (1996) Enantiomerl : 23-27. Gordon and Breach Publishers.)

with the surface of the pseudo-stationary micelle phase. Good resolution for dansyl (DNS) amino acids has been obtained with mixed-micelle solutions containing sodium dodecyl sulfate (SDS) and copper complexes of N,N-didecyl-l-alanine. Figure 5 shows an example of CE resolution of underivatized racemic amino acids with Cu(l-Hyp)2 in the support electrolyte, which is facilitated by SDS micelles. Note the inversion of the elution order of the enantiomers and solutes on transition from the homogeneous support electrolyte solution to the pseudo-heterogeneous micellar system.

In addition to paper chromatography which represents the oldest chiral planar chromatography tech nique, ligand exchange opened enormous possibilities for chiral thin-layer chromatography. Macherey-Nagel in cooperation with Degussa (Germany) developed reversed-phase plates coated with copper complexes of N-(2-hydroxydodecyl)-l-hydroxypro-line in the manner described earlier by Davankov for chiral coating of RP-columns. The Chiralplate® has proved to be extremely versatile in the resolution of racemic a-amino acids, their N-methyl-, N-formyl-, a-alkyl-, and halogenated amino acids, dipeptides, a-hydroxy acids, thiazolidine derivatives, anomers of several nucleobases, etc. Copper complexes of N,N-diallky amino acids have also been tested as chiral coatings of plates, but so far have not found broad application.

Finally, enantiomeric separation of phenylalanine and lactic acid by diffusion through a liquid membrane was studied, using a solution of N-decyl-l-hydroxyproline copper complexes in hexanol-decane mixtures as the membrane. The rate of migration of the d-Phe was found to be about 2.4 times faster as compared to those of the l-isomer. Though the productivity potential of the technique is high, the above value of the kinetic enantioselectivity is still insufficient for practical use.

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