Mercaptopropyl-silica

Although some applications for preparative-scale separations have already been reported [132] and the first commercial systems are being developed [137, 138], examples in the field of the resolution of enantiomers are still rare. The first preparative chiral separation published was performed with a CSP derived from (S -N-(3,5-dinitrobenzoyl)tyrosine covalently bonded to y-mercaptopropyl silica gel [21]. A productivity of 510 mg/h with an enantiomeric excess higher than 95% was achieved for 6 (Fig. 1-3). 

Oxazoborolidinone 8 is an example of catalyst supported on silica gel. It is prepared by immobilizing the N-tosyl-0-allyl-(S)-tyrosine with mercaptopropyl silica and treatment with BF3 and has been used to catalyze the Diels-Alder reaction of methacrolein with cyclopentadiene [17] (Equation 4.2). The cycloaddition occurs with good diastereoselectivity but with low enantioselec-tivity. 

Duval [2] prepared three-dimensional chromatographic polysaccharide supports for use in asymmetric synthesis by crosslinking cellulose-g-(4-alloxy-phenyl-carbamate), (II), with mercaptopropyl silica, (III). 

Figure 6-13. S 2p XPS spectrafrom mercaptopropyl silica before and after oxidation. The inset shows the suiface S cintent varies with bulk S content (Wilson etoL, 20002).

NPs nanoparticles, ITO indium tin oxide, MPS mercaptopropyl-silica. All the other abbreviations are as listed in Table 1... 

Recently, Suzuki-type reactions in air and water have also been studied, first by Li and co-workers.117 They found that the Suzuki reaction proceeded smoothly in water under an atmosphere of air with either Pd(OAc)2 or Pd/C as catalyst (Eq. 6.36). Interestingly, the presence of phosphine ligands prevented the reaction. Subsequently, Suzuki-type reactions in air and water have been investigated under a variety of systems. These include the use of oxime-derived palladacycles118 and tuned catalysts (TunaCat).119 A preformed oxime-carbapalladacycle complex covalently anchored onto mercaptopropyl-modified silica is highly active (>99%) for the Suzuki reaction of p-chloroacetophenone and phenylboronic acid in water no leaching occurs and the same catalyst sample can be reused eight times without decreased activity.120... 

The KG-60 silica (5g) was treated with 3-mercaptopropyl trimethoxysilane (45 mmol, 8.5 mL) in dry toluene (75 mL) at reflux for 24 h. After coohng the mixture was filtered on Buchner funnel and the solid washed with Soxhlet apparatus for 16 h with a mixture of dichloromethane/diethyl ether (50 mL, 1/1 v/v). 

A reactor containing 10 ml of toluene and 10 ml of aqueous Na2C03 was treated with the step 2 product (1.0 mmol), 2,7-dipinacol boron-9,9-di-octyl-fluoiene (1.0 mmol), tetrakis triphenylphosphine palladium (0.01 mmol), and 0.16 ml of tricaprylmethyl-ammonium chloride. The mixture was then treated with a few drops of bromobenzene and then refluxed for 1 hour and treated with a few drop of phenyl boronic acid and then further refluxed 15 hours and cooled. The mixture was diluted with toluene and isolated toluene layer washed with water. The mixture was filtered after adding 40 mg of the palladium scavenger 3-mercaptopropyl modified silica gel. The solution was then poured into methanol and a yellow polymer isolated. The polymer was redissolved in toluene and then purified using a short column of silica gel. The solution was rewashed with water, reprecipitated in ethanol, and 0.80 g of polymer isolated having an Mn of 32,800 Da. 

Attachment of suitable linkers to the surface of silica can be achieved by transesterification with (3-aminopropyl)triethoxysilane, which leads to the support 2 (Figure 2.8) [198-200]. Alternatively, silica can be functionalized by reaction with alkyltri-chlorosilanes [201]. For the solid-phase synthesis of oligonucleotides, supports with a longer spacer, such as that in 3, have proven more convenient than 2 [202-206]. Supports 3, so-called LCAA-CPG (long chain alkylamine CPG [194,195]), are commercially available (typical loading 0.1 mmol/g) and are currently the most commonly used supports for the synthesis of oligonucleotides. For this purpose, protected nucleosides are converted into succinic acid monoesters, and then coupled to LCAA-CPG. CPG functionalized with a 3-mercaptopropyl linker has been used for the solid-phase synthesis of oligosaccharides [207]. 

The Step 2 product dissolved in 30 ml CH2C12 was treated with (3-mercaptopropyl-sulfanyl)acetic acid methyl ester (10.7 mmol), then cooled to 0°C, and basic alumina (11.9 g) added. The mixture was then stirred 18 hours at ambient temperature, filtered through celite, and concentrated. The residue was purified by flash chromatography with silica gel using hexane/EtOAc, 6 1, and the product isolated in 80% yield. 

Silsesquioxanes of Mixed Functionality -Octa[(3-chloropropyl)-/ipropyl-silsesquioxanes] and Octa[(3-mercaptopropyl)- propyl-silsesquioxanes] as Models of Organomodified Silica Surfaces... 

This compound and octa[mono(3-mercaptopropyl)-hepta( propyl)-silsesquioxane] [HS(CH2)3][H(CH2)3]7[SigO,2] 2.1 have been used as ligands in Rh(I) and Pt(II) transition metal complexes for modeling silica supported catalysts. 

Fig. 7.2. Functionalization of a silica surface using 3-mercaptopropyl trimethoxysilane (MTS) and the bifunctional agent JV-(g-maleimidobutyryloxy)-succinimide ester (GMBS) and immobilization of biomolecules (BM).

Synthesized probe DNA was immobilized on the interior surface of silica capillary tubes using the method of Kumar [17]. The silanized probe, obtained by reacting (3-mercaptopropyl) trimethoxysilane with a 5 -thiol-labeled oligonucleotide probe... 

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