Chiral mesoporous silica

Using an elegant approach, Che et al. prepared chiral mesoporous silica using bio-inspired surfactants [63]. The trimethylammonium group of the quaternary amine used as a surfactant in the synthesis of MCM-41 (CTAB) was replaced by L-alanine. The chirality of the amino acid in the polar head of the surfactant induces chirality in the micelle used as template (see Figure 3.15). This simple modification in the surfactant allowed the preparation of the first chiral mesoporous silica with tunable pore size and ordered porosity. A key step in this synthesis is the transfer of the chirality from the surfactant to the solid, which was accomplished by electrostatic interaction between the terminal amino acid and the... 

Figure 3.27 Schematic drawing of a structural model of individual particles of chiral mesoporous silicas with an outer helix-like morphology (left), showing three channels inside these particles, which are mnning along the long axis of the particle and are twisted around each other, and (right) the respective cross-sections. (Adapted from Reference 303.)...

T. (2003) Synthesis of anionic-surfectant-templated mesoporous silica using organoalkoxysilane-containing amino groups. Chem, Mater., 15 (24), 4536-4538 (b) Che, S., Liu, Z Ohsuna, T., Sakamoto, K Terasaki, 0 and Tatsumi, T. (2004) Synthesis and characterization of chiral mesoporous silica. Nature, 429 (6989), 281-284. 

K., Terasaki, O., and Tatsumi, T. (2004) Synthesis and characterization of chiral mesoporous silica. Nature, 429, 281-284. 

Control of morphology and helicity of chiral mesoporous silica. Adv. Mater, 18, 593-596. 

Fig. 4.23 Preparation of mesoporous silica materials with chirally twisted rod-like structures by using surfactant with a chiral amino acid moiety as a structure-directing reagent.

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts... 

The dramatically higher surface area of mesoporous silica in comparison to commercially available chromatographic grade silica enhances resolution of molecules by increasing capacity factors to allow effective separations of analytes. In this report, we illustrate the use of these types of silica in normal phase HPLC, reverse phase HPLC, and chiral HPLC. 

With a polymer-bound chiral selenium bromide reagent, the stereoselective activation of olefins has been accomplished (entry 19) 24 Uehlin and Wirth24 found that both mesoporous silica and polystyrene-supported... 

The importance of the rigid aminoindanol backbone in asymmetric catalytic Diels-Alder reactions is a subject of continued interest.16 50 One example immobilized the copper-inda-box complex onto mesoporous silica in the context of continuous large-scale production of chiral compounds.16 Using 10 mol% of this catalyst (Figure 17.5), the Diels-Alder reaction between /V-acryoyloxazo-lidinone and cyclopentadiene proceeded in 99% yield, 17 1 endo. exo selectivity, and 78% ee of endo cycloadduct. The catalyst could easily be recovered and reused several times without significant loss of diastereoselectivity (15 1 endo. exo selectivity after the fifth reuse) or enantioselectivity (72% ee after the fifth reuse).16 The same remarkable reactivity was observed with a number of diene-dienophile partners. 

Johnson BEG, Raynor SA, Shephard DS, Maschmeyer T, Thomas JM, Sankar G, Bromley S, Oldroyd RD, Gladden L, Mantle MD (1999) Superior performance of a chiral catalyst confined within mesoporous silica. Chem Commun 1167... 

Dioxiranes derived from chiral ketones have been used extensively to promote the enantioselective epoxidahon of alkenes [60]. The first examples of immobilized chiral ketones for this reaction were reported by Sartori and Armstrong [61]. A modified racemic hopinone was supported onto amorphous sihca KG-60, mesoporous silica MGM-41, and 2% crosslinked PS, respechvely, to obtain the insoluble precatalysts 38a-c (Scheme 8.18). 

N,N bis(3,5 di tert butylsalicylidene) chiral diamine) catalysts on mesoporous silica... 

The synthesis and characterization of a chiral amino-alcohol ((1R-2S)-ephedrine) immobilized on MCM-41 type mesoporous silicas (MTS Micelle Template Silicas) are described. The activity of these supported catalysts in the enantioselective addition of diethylzinc to benzaldehyde are reported, and compared with those obtained with the corresponding silica gel supported catalysts. The observed differences are discussed in terms of the nature of the grafting which depends on the support structure. 

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