Silica-Supported Amines

An aminated silica (13) was treated wiA HjPtCls and RhCl(PPh3)3 to give the aminated silica-supported platinum (13-Pt) and rhodium (13-Rh) catalysts, respectively. The metal content in the immobilized catalysts was 0.05 wt% for Pt and 0.35-0.49 wt% for Rh depending on the structure of amine moiety. The hydrosilyiation of 1-alkenes, allyl chloride, and allyl chloroacetate by HSiCl3 and HSifOEtj) was studied 13-Rh (NR2 = morpholino) retains its high catalytic activity even after nine times of use. ... 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

In a facile and rapid stereoselective, three-component, one-pot reaction, a series of cis-isoquinolonic acids 132 were synthesized using silica supported sulfuric acid to catalyze the reaction between homophthalic anhydride 133 with different aldehydes 134 and amines 135. This three-component cyclocondensation offers a variety of advantages including high yields, easy experimental work-up, and the use an inexpensive, non-toxic, readily available, and recyclable catalyst <06JHC187>. 

Figure 3.17 Amine catch and release purification with a silica-supported acid.

Normal phase columns have also been shown to give non-linear van t Hoff plots in many cases. Bidlingmeyer and Henderson [35] found an improved separation of lipophilic amines at high temperature. They were unable to determine whether the lack of linearity was due to absorptive and electrostatic effects or changes in solvation of the silica as a function of temperature. They also noted a degradation of the silica support at elevated temperatures and found it necessary to use a pre-column to pre-saturate the mobile phase with silica. 

Comparable to the other protein-coated supports are the Ultrabiosep and the BioTrap phases. The former are composed of C4, Cg, or Cis reversed-phase silica supports covered with a biological polymer which is not described in the literature (135). The latter are commercially available as Bio Trap Acid or Biotrap Amine precolumns (136). They are Cis-modified silica supports covered with -1-acid glycoprotein as a biocompatible layer. Due to the immobilized protein, this type of reversed-phase material also possesses weak ion-exchange properties. 

Figure 25-7 Tailing of amine bases on silica (a) Type A silica support gives distorted peaks.

Mesoporous silica supported complexes of samarium(II), also having an amine and tetrahydrofuran ligand, were found to incandesce on exposure to air. The initial deposition was from samarium(II) bis(dimethylsilyl)amide, and bis(dimethylsilyl)-amine is itself not a compound likely to be air stable. 

Tanthana J, Chuang SSC (2010) In situ infrared study of the role of PEG in stabilizing silica-supported amines for C02 capture. ChemSusChem 3(8) 957-964... 

Tetrasubstituted pyrroles could be obtained by skeletal rearrangement of 1,3-oxazolidines, a reaction that is substantially accelerated by microwave irradiation. Dielectric heating of a 1,3-oxazolidine 7, absorbed on silica gel (1 g silica gel/mmol) for 5 min in a household MW oven (900 W power) cleanly afforded the 1,2,3,4-tetrasubstituted pyrrole 8 in 78% yield, thus reducing the reaction time from hours to minutes (Scheme 5) [24], 1,3-Oxazolidines are accessible in one-pot, two-step, solvent-free domino processes (see also Sect. 2.6). The first domino process, a multi-component reaction (MCR) between 2 equivalents of alkyl propiolate and 1 equivalent of aldehyde furnished enol ethers 9 (Scheme 5). Subsequent microwave-accelerated solvent-free reactions of enol ethers 9 with primary amines on silica support afforded intermediate 1,3-oxazolidines, which in situ rearranged to the tetrasubstituted pyrroles (2nd domino process). Performed in a one-pot format, these... 

The interaction of amine-modified silica with Cu2+ ion is the most documented.23,24,25 However, the retention of other transition metals as well as precious metals with dedicated modification layers has also been reported. The separation of Pd and Pt from base metals, Ir(III) and Rh(III) was effectuated by using silica-bound thioether sulfur and primary amine groups.26 A review on polymeric as well as modified silica supports for separation and preconcentration of trace metals is presented by Kantipuly.27 This metal immobilization also allows other applications such as metalion chromatography28 and heavy metal catalysis.29... 

Although only a small amount of catalyst is used, recyclability is an issue and several heterogeneous TEMPO systems have been reported. For example, MCM-4112 and silica-supported TEMPO13,14 were applied in oxidation reactions using hypochlorite as the oxidant. The preparation of these catalysts involves initial functionalisation of the support followed by covalent attachment of a 4-substituted TEMPO via an amide, amine or ether linker (figure 2). 

The differences displayed above are probably caused by coordination/bonding of copper to the free silanol groups on the surface of MCM-41 and silica. Besides these free silanol groups, the silica supported TEMPO system reported by Brunei et al.14 also contains unreacted amine linkers, which inactivate the catalyst almost completely (entry 8). Therefore, the activity of the MCM-41 and silica supported TEMPO systems may be improved by blocking the free silanol groups and amine linkers on the surface. 

The amorphous silica-supported amine systems show promising selectivity and recyclability for the heterogeneous catalysis of the Knoevenagel reaction (scheme 1). However they also demonstrate distinct limitations on the choice of solvent for the reaction and moderate turnover numbers.3 Materials prepared via grafting of HMS or in-situ preparation of organo-functionalised HMS will hopefully overcome these limitations. 

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