Enantioselective Sol–Gel Materials Obtained by Either Doping or Imprinting with a Chiral Surfactant

Abstract

The formation of chiral porous materials is an on-going quest because of the numerous applications of such materials in separation science, chiral catalysis, and in pharmaceutics and so forth. Here, we describe two new methods for preparing bulk chiral silica materials, based on the use of a chiral surfactant. By one method the surfactant is used for the imprinting of chiral porosity, and by the second method we use the entrapped surfactant molecules as chiral centers within the silica. The chiral entity in the two types of matrices is different and therefore the enantioselective preferences before and after surfactant removal are not necessarily the same; indeed it was found that chiral discrimination changes before and after surfactant removal. Notably, these adsorbents are capable of recognizing chirality in molecules that have no structural similarity to the imprinting molecule. An additional attractive feature of these materials is their ability to enantioselect in both aqueous media and organic solvents. Specifically, phenylated silica sol-gel (PSG) matrices were doped with the chiral cationic surfactant, (-)-N-dodecyl-N-methylephedrinium bromide (DMB, 1, Scheme 1), and then the DMB was extracted with methanol. The powdered silica matrix was thus rendered chiral both before and after surfactant removal: before, by virtue of the entrapped chiral surfactant within the matrix; and after surfactant removal by forming chiral cavities. Both types of matrices showed general enantioselectivity by preferential adsorption of one enantiomer over several other molecules which are different from the surfactant. The pairs of enantiomers included (R)-and (S)-propranolol (2), (R)-and (S)-binaphthyl-2,2-diyl hydrogenphosphate (BINAP) (3), and (R)-and (S)-naproxen (4). In all cases good enantioselectivities were observed, both for the chirally imprinted powdered monoliths and for the chirally doped ones, with discrimination ratios in the range of 1.22-1.34. The chiral entity in the two types of matrices is different and therefore the enantioselective preferences before and after surfactant removal are not necessarily the same, as indeed was found to be the case. The fact that the discrimination changes before and after surfactant removal attests to the authenticity of the observed enantioselectivity in the two types of chiral materials.It is in order to cite some relevant background references now: Enantioselectivity towards the enantiomers of the imprinting molecule is more common [1] than enantioselectivity towards molecules different than the imprinting one, but which are still related to it. Thus, enantioselectivity towards morphine derivatives different than the imprinting derivative was demonstrated by Becket et al. [2] and by Bartels et al. [3] for silica prepared from sodium silicate. Enantioselective recognition of structurally related enantiomers is also known for organic polymers. We reiterate here that the enantioselectivity shown in this report is towards unrelated molecules. In another type of study Izutsu et a...

Imidazolium-based functional monomers for the imprinting of the anti-inflammatory drug naproxen: Comparison of acrylic and sol–gel approaches

“…Sol-gel imprinting is one possible alternative for achieving molecular recognition of hydrophilic targets, given the ease of preparation of sol-gels and their compatibility with polar environments [12]. Sol-gel imprinting is straightforward and provides an efficient way for preparing hybrid matrices through incorporation of organic components into inorganic polymers under mild thermal conditions, while controlling MIP thickness, porosity and surface area.…”

One‐Pot Synthesis of Functional Helicoidal Hybrid Organic–Inorganic Nanofibers with Periodically Organized Mesoporosity

“…Recently, different authors reported the synthesis of mesoporous silica materials with chiral channels and helical morphology by using chiral or achiral surfactants as templates. [27][28][29][30][31][32][33][34] Indeed, chiral hybrid nanomaterials should present an additional interest for enantioselective catalytic properties, nonlinear optics, selective adsorption, chromatography, pharmaceutics, chiral sensing, and so forth [34,[41][42][43][44][45][46][47] Some works have described the use of achiral surfactants such as aminosilane, [30] ethyl acetate, [29] or perfluorinated acids [32] as structure-directing agents to prepare helical nanofibers. In particular, the synthesis of helical mesoporous silica nanorods in the presence of perfluorooctanoic acid (PFOA) and cetyltrimethylammonium bromide (CTAB) has been recently reported.…”

“…For this system, to get helical organization of the micelles it is crucial to introduce MPTMS early enough to allow thermodynamic stabilization before kinetic silica condensation. [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][54][55][56][57] …”

Silica-based organic–inorganic hybrid materials prepared from chiral precursors

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