Mechanism of enantioseparation on zeolite-supported chiral stationary phases in the presence of lipophilic buffer

Abstract

Key WordsChiral stationary phase (brush type) Hydrophobic interaction Ion-pairing Lipophilic buffer Zeolite support SummaryChiral stationary phases have been prepared on a zeolite A support. In addition to polar hydrophilic chiral selector molecules derived from DNB-L-Leu and DNB-L-Phe amides, dynamic modification of the underivatized free surface hydroxyl (silanol) groups of the zeolite can be used to increase the overall hydrophobic character. On these non-capped chiral stationary phases, in the presence of lipophilic buffer salts in the aqueous organic phase, the enantioseparation of ion-paired D,Lmandelic acid was facilitated and was studied in more detail. The enantioseparation might be based on hydrogen bonding and n-n interaction supported by simultaneous hydrophobic interaction.

Performance of Chiral Zeolites for Enantiomeric Separation Revealed by Molecular Simulation

“…15,16 It has also been demonstrated that zeolite defects in the form of silanol groups can improve enantioselectivity in chirally modified zeolites. 17 Chiral catalysis is also possible in mesoporous materials such as MCM-41 and SBA-15. 18 Only a few as-synthesized zeolite-like materials were identified as chiral, namely, BEA, 19 CZP, 20 GOO, 21 OSO, 22 and BSV.…”

“…Zeolites can be modified to act as asymmetric catalysts. The most common methods to induce a chiral reaction environment in the zeolite are: (1) adsorption of chiral molecules in an achiral zeolite (chiral inductor method); , (2) covalent bonding of chiral molecules to the achiral zeolite structure (chiral auxiliary method); , and (3) a combination of the chiral inductor and the chiral auxiliary methods. , The confined space and the nonframework cations present in the zeolite are believed to be responsible for the chiral induction. , It has also been demonstrated that zeolite defects in the form of silanol groups can improve enantioselectivity in chirally modified zeolites . Chiral catalysis is also possible in mesoporous materials such as MCM-41 and SBA-15 …”

Investigation of Enantiomer Bonding on a Chiral Stationary Phase by FT-Raman Spectrometry

“…The assignments of the bands are summarized in Table . To extract spectral information, a Savitzky−Golay filtration was performed in the 3375−2800 and the 1700−1480 cm -1 ranges followed by a noise minimization between the original and the filtered spectra (after Iwata et al. − ) using the modified NIPALS (nonlinear iterative partial least squares) method. Bond resolution in the ν NH range was performed by the Jandel Scientific Peakfit program package (version 3.18).…”

“…According to the chromatographic literature, hydrogen bonds of different strengths cannot be responsible for such a difference in the retention behavior other than the one observed in this system between the l and d forms of mandelic acid. On a Pirkle type chromatographic stationary phase (i.e., with π-stacking interactions), retention is mainly due to the interaction between the electron-rich benzene ring of the enantiomer (π-base) and the electron-deficient benzene ring of the phase (π-acid). ,, A spectral evidence of this supposition can be found in the ν CH (aromatic) range of the spectrum (Figure ). The ν CH (aromatic) band intensities of the stationary phase were comparable with the noise level.…”