Trialkoxysilanes reactions

UsuaHy, trichlorosilane, trialkoxysilane, methyl dichlorosHane, and methyl dialkoxysHane are used. For example, the reaction of trichlorosilane with aHyl methacrylate is as foHows ... 

Consider a trialkoxysilane as co-precursor for an initial 1 1 mixture of alkoxysilane and organoalkoxysilane, and recall that in the case of ORMOSIL condensation takes place only through elimination of water27 and not by alcohol formation. The mechanism involves hydrolysis and condensation reactions ... 

The trialkoxysilane (1.5 mmol) is added with stirring at room temperature to the ketone and quininium fluoride (0.02 mmol) in THF (2 ml). When the reaction is complete, as shown by TLC analysis, aqueous sodium hydroxide (3M, 5 ml) is added and the mixture is stirred for a further 12 h at room temperature. The mixture is extracted with Et20 (3 x 15 ml) and the extracts are washed well with H20, dried (MgS04), and evaporated to yield the chiral alcohol. 

Figure 7.8 The proposed reaction of a hydrolysed mono-organo trialkoxysilane with cell wall polymers.

In this synthetic strategy, the macrocyclic antibiotic is covalently bonded to the silica matrix in two steps (1) chemical modification of the selector via reaction between suitable groups of the antibiotic and proper groups of the spacer, reacting also as a di- or trialkoxysilane (2) immobilization of the functionalized selector on unmodified silica particles. 

Modifications of monodisperse colloidal silica, of 10 or 500 nm in diameter, were carried out using trialkoxysilane-terminated polymer in a low polar solvent, such as acetone or 1,2-dimethoxyethane. without coagulation during the coupling reaction (35,37-42). In this modification, the hydrophobic polymer can be efficiently bound to hydrophilic colloidal silica surface. The reaction mechanism of the binding... 

All of these reactions have been used to bind organofunctional groups to inorganic surfaces for chromatographic purposes (18, 48, 61, 68, 69). A totally different approach to the use of silica as the support is illustrated in the following reaction in which the functional group, for example, a tertiary phosphine, is built into the trialkoxysilane which is then polymerized to produce a nonlinear polymer based on an —Si—O—Si— backbone (115) ... 

The two most commercially important reactions of the trialkoxysilanes are hydrolysis to silanetriols ... 

Base catalysis—hydrolysis. Pohl studied the hydrolysis in aqueous solutions of a series of trialkoxysilanes of R Si(OCH,CH,OCH,), structures in which R was an alkyl or a substituted alkyl group [42]. The reactions were followed using an extraction/quenching technique. Silanes were studied at concentrations ranging from 0.001 to 0.03 M and pHs adjusted from 7 to 9. The hydrolysis was found to be first order in silane. The order in water was not determined because the reactions were carried out in a large excess of water (water was the solvent). The rate constants for the hydroxide anion catalyzed hydrolysis reactions and reaction half-lives are reported in Table 1. 

For the general case of trialkoxysilanes, although the hydrolysis reaction is reversible, under the conditions employed it may be considered as three consecutive, irreversible pseudo-first-order reactions shown in the following equations ... 

Hydridosilicates are readily formed by the reaction of trialkoxysilanes with alkali-metal alkoxides60 (equation 25). The results are best when potassium alkoxides are used. 

When trialkoxysilanes are reacted with alkali metal hydrides, mixtures of mono- and dihydrosilicates are obtained (equation 25a)59,61. The results of this reaction depend strongly upon several factors the size of the alkoxy group, the solvent, and in particular whether the hydride is used in the presence of 18-crown-6 or not (Table 8). The Table shows that as the bulk of the alkoxy group increases, the ratio of monohydrido to dihydrido products increases. Use of 18-crown-6 makes the reaction less selective, in the sense that both products are obtained, while without the crown ether quantitative yields... 

Both l-(butadien-l, 3 -yl)silatrane (93) and -trialkoxysilane can react by a Diels-Alder-type reaction with tetracyanoethylene (TCNE) or maleic anhydride (MA) to give the corresponding adducts (equations 129 and 130). However, a higher temperature is required for effective conversion of trialkoxysilane355. 

In some cases, the reaction of silicon and methanol has been optimized for formation of (MeO)4Si. As discussed above, thiophene addition favored formation of (MeO SiH. Both thiophene and propyl chloride poison copper copper poisoning seems to favor formation of the trialkoxysilane. High-temperature pretreatment disfavors trialkoxysilane formation copper is formed on the surface of the silicon during pretreatment at 450 °C98. Metallic Cu catalyzes dehydrogenation of alcohols and favors formation of (RO)4Si. Workers from Tonen Corporation reported 50% conversion of silicon to make (MeO Si with 92% selectivity if silicon, methanol and Cu(OMe)2 were pretreated (lower conversion and selectivity without pretreatment) and then reacted at 180 °C and 1 atmosphere99. 

The polymerization of silicon alkoxides, Si(OR)4, to produce inorganic glasses proceeds through these three reactions and is commonly referred to as the sol-gel process. Reactions of trialkoxysilane-terminated molecules R OR 2)3 are similar and give hybrid organic-inorganic networks,... 

The silanization reaction is performed in dioxane/water solvent at reflux temperature, with acid catalysis.80 The reaction of the TES is analogous to other trialkoxysilanes. 

The rather difficult question how many alkoxygroups of one trialkoxysilane molecule participate in the chemical bonding has been studied by high resolution NMR on 29Si and 13C nuclei.92 93,94 Sindorf and Maciel92 concluded that in the case of dehydrated silicas, mainly monodentate and some bidentate species are formed. Reactions involving all three alkoxy groups do no occur on the surface of dry silica. 

Surface modification is possible by essentially all the reactions which have been developed over the past decades for amorphous silica. Excellent compilations of the different reagents and modifications achieved for ordered mesoporous silica are given in reviews by Anwander [4] and by Sayari and Hamoudi [5]. In almost all cases, trialkoxysilanes, trichlorosilanes, or disilazanes are used, which bear the organic functional group that is to be grafted to the surface of the silica. Also, several other review papers on surface modification with organosilicon groups are available [6 - 8]. 

Transition metal carbonyls such as Co2(CO)8 and CoH(CO)4, formed in the reaction of R3SiH with dimer (but also Fe(CO)5 and M3(CO)i2 (M = Fe, Ru, Os)) have been found to be active catalysts for the hydrosilylation of olefins, dienes, unsaturated nitriles, and esters as well as for hydrosilylation C=0 and C=N bonds [56]. Hydrosilylation of phenylthioacetylenes in the presence of this catalyst is extremely regioselective [57]. Cobalt(I) complexes, e. g., CoH(X)2L3 (X = H, N), could be prospective candidates for investigation of the effectiveness of alkene hydrosilylation by trialkoxysilanes as well as dehydro-genative silylation [58]. Direct evidence for the silyl migration mechanism operative in a catalytic hydrosilylation pathway was presented by Brookhart and Grant [59] using the electrophilic Co cationic complex. 

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