Chiral gels materials

Complexation of (124) and (125) with [ Rh(COD)Cl 2] in the presence of Si(OEt)4, followed by sol-gel hydrolysis condensation, afforded new catalytic chiral hybrid material. The catalytic activities and selectivities of these solid materials have been studied in the asymmetric hydro-gen-transfer reduction of prochiral ketones and compared to that of the homogeneous rhodium complexes containing the same ligands (124) and (125) 307... 

S. Fireman-Shoresh, S. Marx and D. Avnir, Enantioselective Sol-Gel Materials Obtained by Either Doping or Imprinting with a Chiral Surfactant, Adv. Mater., 2001, 19, 2145. 

The use of a chiral starting material (140, R = Me) and a prochiral electrophile, such as pivalaldehyde, gave a ca 1 1 mixture of diastereomers. On the other hand, careful hydrolysis of compounds 142 (aqueous oxalic acid, silica gel) afforded the corresponding functionalized a, -unsaturated cyclohexenones 143 in >95% yield. 

In addition, chiral dendrimers (see Section 4.2) can be resolved with the aid of HPLC into their enantiomers, if the silica gel material used as stationary phase has optically active substances bound to its surface [9]. Since the chiral stationary phase (CSP) [10] undergoes different intensities of interaction with the enantiomeric dendrimers, they are retained to different degrees, and in the ideal case two completely separated (baseline separated) peaks are obtained. This separation technique was successfully applied inter alia to racemic mixtures of planar-chiral dendro[2.2]paracyclophanes, cycloenantiomeric dendro[2] rotaxanes, topologically chiral dendro[2]catenanes [11] as well as topologically chiral, dendritically substituted molecular knots (knotanes) [12] (Section 4.2.3). 

Okamoto, Y., Kawashima, M., Yamamoto, K., and Hatada, K. (1984) Useful chiral packing materials for high-performance liquid chromatographic resolution. Cellulose triacetate and tribenzoate coated on macroporous silica gel, Chem. Lett., 739-742. 

Chiral porous materials have many applications in separation science and catalysis.22 Marx and Avnir have described chirality in sol-gel materials, with a particular focus on their work.31 In this section, an overview of some of this work is given, with emphasis on the use of chiral templates to create materials that demonstrate enantioselectivity. 

We performed the synthesis of a new class of hybrid solids by grafting the coupling CPTMS agent by the sol-gel method rather than the silylation method. Supporting (-)-ephedrine on Al-MTS support with large pores by this new grafting method leads to a chiral hybrid material with a high efficiency in the enantioselective alkylation of benzaldehyde by diethylzinc.15... 

Y. Okamoto, M. Kawashima, and K. Hatada, Chromatographic resolution. 7. Useful chiral packing materials for high-performance hquid chromatographic resolution of enantiomers Phenylcarbamates of polysaccharides coated on sihca gel, J. Am. Chem. Soc. 106 (1984), 5357-5359. 

One of the reasons why gels and self-assembled fibers affracf so much attention is their very high potential for applications. The last section of this chapter addresses applications which specifically make use of chirality, such as those based on enantioselective molecular recognition, or those based on the use of chiral fibers as templates for the generation of helical arrays of proteins or of chiral inorganic materials. 

Ikai T, Yamamoto C, Kamigaito M, Okamoto Y (2007) Immobilization of polysaccharide derivatives onto silica gel Facile synthesis of chiral packing materials by means of intermo-lecular polycondensation of triethoxysilyl groups. J Chromatogr A 1157 151-158... 

Polysaccharides, the most abundant biopolymers in nature, have also been used in chromatography as CS. Although intrinsically chiral, the materials used for this purpose are polysaccharide derivatives. Among them, esters and carbamates of cellulose and carbamates of amylose, in particular the 3,5-dunethylphenylcarbamates of either of them, are the most popular. When the CSPs are constituted by a silica gel matrix onto which the CS is simply coated, the composition of the mobile phase is limited to those solvents that do not solubilize the polysaccharide derivative. Mixtures of a hydrocarbon, hexane, or heptane, with an alcohol, usually 2-propanol, methanol, or ethanol, are common mobile phases. Although aqueous mobile phases can be used together with acetonitrile or methanol, these conditions tend to provide lower enantioselectivity values. ... 

---