Silica, functionalization

Methyltrioxorhenium, supported on silica functionalized with polyether tethers, catalyzed the epoxidation of alkenes with 30% aq H2O2 in high selectivity compared to the ring opening products observed in homogeneous media in the absence of an organic solvent.46... 

Compared to scavenging using the bulk product, scavenging of HOBt over a column packed with silica-functionalized carbonate (SiliaPrep Carbonate) enables optimal sequestration with only 5 ppm of residual peroxide from a 5000 ppm HOBt solution (Table 3.2) 4... 

Similarly, heterogeneous amide synthesis can be elegantly achieved in cartridges filled with silica functionalized with carbodiimide. The process outlined in Figure 3.16 yields quantitative conversion and 98.8% purity by reacting benzoic acid with phenylethylamine. 

Attempts have recently been made to prepare solid acids by loading triflic acid into various inert oxides including silica,184 titania,185,186 and zirconia.187,188 Silica functionalized with anchored aminopropyl groups was also used to immobilize triflic acid.189 These new catalysts have been tested in a variety of organic transformations, such as alkane-alkene alkylation, Friedel-Crafts acylation, alkene dimerization, and acetalization. Silica nanoboxes prepared by dealumination of Na-X- and Ca-A-type zeolites were also loaded with triflic acid up to 32 wt%.190 The materials were thoroughly characterized but have not been tested as catalysts. 

The previous example took inspiration from earlier work of Lin et al.18 In their seminal work, Lin and coworkers functionalized the inner pores of MCM-41 with o-phthalic hemithioacetal moieties that are able to react with amines to produce a highly fluorescent isoindole derivative. In order to enhance selectivity in the sense we have discussed above, the solids were also hydrophobized with different groups such as propyl, phenyl, and pentafluorophenyl in a second step. Interestingly, some of these solids displayed a remarkably selective and differentiable response to dopamine versus the less lipophilic glucosamine. The authors also demonstrated that this selectivity was not observed when amorphous (nonporous) silica functionalized with the same organic groups was used, stressing the importance of the 3D... 

Cauvel, A., Renard, G. and Brunei, D. Monoglyceride synthesis by heterogeneous catalysis using mcm-41 type silicas functionalized with amino groups. J. Org. Chem., 1997, 62, 749-751. 

Silica gel is also the support of choice for the activation of V-halosuccinimides. The silica functions both as a proton donor which increases the electrophilic nature of the reagent, and as a support with geometrical constraints which contributes to the stereoselectivity. Alkyl and aryl sulfoxides are readily halogenated at the a position with yields of48-80%91. The reactions are carried in the solid state on the surface of TLC plates. The conversion of the optically active alkyl 4-methylphenyl sulfoxide into 1-haloalkyl 4-methylphenyl sulfoxide is accompanied by inversion of configuration at the S-atom. The stereoselectivity in these reactions is much higher than that observed in liquid-phase halogenation. 

R. Metivier, I. Leray, B. Lebeau, and B. Valeur, A mesoporous silica functionalized by a covalently bound calixarene-based fluoroionophore for selective optical sensing of mercury(ii) in water, J. Mater. Chem. 15, 2965-2973 (2005). 

FIGURE 33 Silica functionalized with the binding motif shown in Figure 32 and two catalysts with the complementary binding motif (728). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

INFRARED AND RAMAN SPECTROSCOPY ° ° 21-138 Applications The following use was made of infrared and Raman spectroscopy identification of surface groups on treated and untreated fumed silica, tion of silica functional groups and coatings by Raman spectroscopy. 

MTS silicas functionalized with amino groups exhibit interesting base catalytic properties in condensation reaction which are well-correlated with their physico-chemical characteristics. The greater activity of the immobilized primary amine sites compare to the tertiary amine sites in the Knoevenagel condensation is explained by the imine intermediate formation which is much more reactive than the parent carbonyl group of the reactant. 

Methyltrioxorhenium (MTO) supported on silica functionalized with polyether tethers (prepared by a sol-gel procedure) was shown to catalyze epoxidations with 30% H2O2 in the absence of solvent [69]. The silica-attached polyether phase is assumed to act as a solvent for the MTO (see Figure 3). The hydropho-... 

Synthesis, anatase nanocrystals 22-P-13 Synthesis, mesoporous silica, functionalized ... 

Mesoporous silica functionalized with quinoline high homogeneity and better... 

Gao, L. Sun, J. Li, Y. Zhang, L., Bimodal Mesoporous Silicas Functionalized with Different Level and Species of Amino Groups for Adsorption and Controlled Release of Aspirin. J. Nanosci. Nanotech. 2011,11,6692-6697. 

Shamim, T. and Paul, S. 2010. Silica functionalized Cu(I) as a green and recyclable heterogeneous catalyst for the Huisgen 1,3-dipolar cycloaddition in water at room temperature. Catal. Lett. 136(3 ) 260-265. 

They can exploit the peculiar reactivity of silica functionalities described above. 

Scheme 1.7 Silica functionalization with organosilanes and chemical modification...

Fig. 23.3 Silica functionalities as they relate to excipient applications and interactions with active pharmaceutical ingredients...

N. Hewitt, Proceedings of Silane Coupling of Precipitated Silica Functional Tire Fillers 2001, Intertech, Fort Lauderdale, FL, USA, 2001, Paper No.6. 

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