Enantioselective, Chirally Templated Sol−Gel Thin Films

Abstract

Enantioselective surfactant-templated thin films were fabricated through the sol-gel (SG) process. The enantioselectivity is general in the sense that it discriminates between pairs of enantiomers not used for the imprinting process. The chiral cationic surfactant (-)-N-dodecyl-N-methylephedrinium bromide (1) was used as the surfactant template, and after its extraction chiral domains were created. The chiral discriminative feature of these films was examined by challenging with pure enantiomer solutions for rebinding. Selective adsorption was shown using (R)- and (S)-propranolol, (R)-2 and (S)-2, respectively, and (R)- and (S)-2,2,2-trifluoro-1-(9-anthryl)ethanol, (R)-3 and (S)-3, respectively, as the chiral probes. The selective adsorption was measured by fluorescence analysis, and the chiral selectivity factors were found to be 1.6 for 2 and 2.25 for 3. In both cases, (R)-enantiomer was adsorbed preferably. The resulting material was characterized by transmission electron microscopy, by diffraction, and by surface area measurements, and was found to be semicrystalline with short-range ordered domains (50 A) of hexagonal symmetry.

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

“…[35] As for our own lab, we have shown that chirally imprinted sol-gel-derived thin films exhibit enantiolelectivity and, for instance, reported recently surfactant-imprinted films that enantioselect between enantiomer pairs not used for the imprinting. [36] Two important potential applications-chiral chromatography [37] and chiral catalysis [38][39][40] of the thin films require the conversion of the thin-films imprint- …”

“…This is particularly so for the newly developed enantioselective surfactant-doped sol-gel materials, where in the case of these films, no enantioselectivity could be observed. [36] We begin with the investigation of the possibility of enantioselectivity of the nonextracted DMB@PSG; if such exists, then its origin might be either due to direct interaction with the entrapped DMB, or (and this is more interesting) due to chiralization of the silica matrix due to interactions with the chiral DMB, affecting perhaps not only the neighboring silica cage framework but also deeper. Furthermore, if there are enough chiral molecules that are homogenously spread in the matrix, the chiral zones could overlap and become dominant.…”

“…[42,43] The potential of simple physical entrapment as an alternative method for the preparation of chiral chromatographic materials is appealing because of its simplicity, with the limitation of not using solvents capable of extracting the doped molecules (methanol in our case). It is interesting to note that in the case of DMB-doped thin films, [36] no enantioselectivity was observed. This is so because thin films have a very low accessible porosity that was opened only after extraction of the dopant.…”

Biomimetic Polymers for Chiral Resolution and Antifreeze Applications

“…110 Chiral imprinting of sol-gel thin films exhibiting enantioselectivity has been developed by David Avnir's group. In a series of articles, Avnir et al [111][112] showed that template molecules such as propranolol, 2,2,2-trifluoro-1-(9-anthryl) ethanol, DOPA, and tyrosine can be used to prepare a chiral imprint sol-gel matrix. The shape of the chiral matrix is maintained when the template molecule is extracted.…”

Forcing a cis‐Product by Matrix Imprinting: Heck Reaction Catalyzed by Palladium Acetate Entrapped within cis‐Imprinted Sol‐Gel Derived Silicates

“…[7] Other shape selective imprintings for synthesis and catalysis are of course known, mainly in the context of "foot-printing" and other types of surface and thin layer catalytic operations. [8][9][10] Only very little had been achieved in attempted shape selective catalysis using bulk imprinted sol-gel materials. [11] When a 4:1 mixture of tetramethoxysilane (TMOS), octy(triethoxy)silane (OTEOS) and palladium acetate was treated with cis-stilbene during the hydrolysis and polycondensation of the silanes, followed by extraction of 96-98% of the imprint from the resulting ceramic sol-gel material with CH 2 Cl 2 , (see Experimental Section) the aforementioned Heck reaction of microemulsified bromobenzene and styrene in water resulted in 86% of cis-stilbene.…”