Tetraalkoxysilanes

Nonhydrolytic methods for the formation of siUcon dioxide from tetraalkoxysilanes have been reported (14—15). Others have been reinvestigated... 

Part of this symposium was directed to the synthesis, properties and applications of inorganic and organometallic macromolecules with network structures. The section on organo-oxo macromolecules relevant to sol-gel processing addresses the interesting synthesis and challenging characterization efforts in this area. Brinker (p. 314) outlines the complex chemical and physical factors which affect network formation and structure resulting from the hydrolysis of a tetraalkoxysilane. 

Several classes of silyl ethers have been shown to be excellent substrates for the C-H insertion chemistry of donor/ acceptor-substituted carbenoids.81 Effective C-H insertions predominantly occur at methylene sites. Primary sites are not sufficiently activated electronically while tertiary sites are sterically too crowded. Rl -DOSP -catalyzed functionalization of the allyl silyl ether 3 resulted in a highly diastereoselective transformation, leading to the formation of the /3-hydroxyester 4 in 94% yield and 82% ee (Equation (17)).81 This example illustrates the regioselectivity of this chemistry, because 3 contains two allylic sites but only the methylene site adjacent to the siloxy group was functionalized. Even better substrates are the commercially available tetraalkoxysilanes such as 5, because with these substrates, the high diastereoselectivity was retained while the enantioselectivity was increased (Equation (18)).81... 

Available literature on TBOS and TKEBS mainly focuses on their thermal properties [24]. Specific research work related to the transformation of these compounds under environmental conditions is limited, and biological degradation of these compounds has not been investigated [423]. However, numerous hydrolysis studies have been conducted on the lower homologues of the tetraalkoxysilanes such as tetramethoxysilane and tetraethoxysilane [229,423]. These compounds hydrolyze abiotically to give the corresponding alcohols and silicic acid [424]. 

Entrapment in polymeric matrices is a variation of noncovalent attachment where the support is instead generated in the presence of the enzyme. A particularly popular entrapment technique is sol-gel encapsulation, where the enzyme is trapped within an Si02 matrix formed by acid- or base-catalysed hydrolysis of tetraalkoxysilanes in the presence of enzyme. ° The technique can be tuned to provide the appropriate microenvironment for each enzyme in much the same way as can be done with other immobihzation methods. ... 

Extending the reaction to acyclic trans-allyl silyl ethers 220 results in the highly diaster-eoselective formation of syn-aldol products 221 (Eq. 30) [143]. Even higher stereoselectivity can be achieved with the tetraalkoxysilanes 222, where both the diastereo- and en-antioselectivity for the formation of 223 are exceptional (Eq. 31) [144]. C-H insertion into tetraethoxysilane 222 generates the syn-aldol product 223 in 70% yield (>94% de) with 95% enantiomeric excess. 

Tetraalkoxysilanes (usually tetramethoxysilane, TMOS, bp 121 °C, or tetra-ethoxysilane, TEOS, bp 169 °C) for hydrolytic gelling can be made by reaction of SiCl4 (Section 17.8.2) with the relevant alcohol ... 

Hydrolysis of tetraalkoxysilanes in pure water is usually incomplete, but it is more effectively carried out in an alcohol-water mixture and can be catalyzed by H+ or weak bases such as ammonia. Polymerization occurs in the range pH 2-7 under neutral conditions, the rate of polymerization is limited by the slowness of hydrolysis, whereas in acidic media hydrolysis is complete before polymerization begins. The nanostructure of the gel (density of particles, fractal dimensions of particle clusters, and degree of cross-linking of particles to form a network) is controlled by these conditions. 

CsF in the presence ol tetraalkoxysilanes also effects Michael addition of ketones lo a,/ -unsaturaied ketones, esters, and nitriles. Presumably the enolate is generated and is converted by Si(OR)a into the silyl enol ether, which reacts in situ.2 Examples ... 

Although tetraalkoxysilanes are usually applied as silica precursors in sol-gel procedures, the oligomeric type precursors can also be used instead. Ethyl silicate 40 (ES 40) is the most common commercial form of ethoxypolysiloxane available at prices competitive to those of TEOS/TMOS. Hence its application in the synthesis of composite adsorbents could be economically attractive provided that this would has little or no adverse effect on their final properties. The objective of this work was to test an alternative, cost-effective preparation procedure making use of the low-cost reagents ES 40 and 95wt% EtOH and to compare the properties of the adsorbents obtained with those of their counterparts similar in composition, yet synthesized with the use of TEOS. 

Sol-gel technology - [ALUMINUMCOMPOUNDS - ALUMINIUMOXIDE(ALUMINA) - ACTIVATED] (Vol 2) - [SOL-GEL TECHNOLOGY] (Vol 22) -hydrolysis of tetraalkoxysilanes [SILICON COMPOUNDS - SILICONESTERS] (Vol 22) -for thin films [IHIN FILMS - FILM FORMATION TECHNIQUES] (Vol 23) -htanates for [TITANIUM COMPOUNDS - ORGANIC] (Vol 24) -for vitreous silica [SILICA - VITREOUS SILICA] (Vol 21)... 

Cesium fluoride-Tetraalkoxysilanes, 69 Hexamethylphosphoric triamide, 142 Methyl acrylate, 183 a-Methylbenzylamine, 185 Methyl vinyl ketone, 193 Potassium t-butoxide, 252 Potassium f-butoxide-Xonotlite, 254 Potassium fluoride-Alumina, 254 Tin(II) trifluoromethanesulfonate, 301 Titanium(IV) chloride, 304 Trityl perchlorate, 339 Vinyl(triphenyl)phosphonium bromide, 343... 

Allyltributyltin, 10 Aluminum chloride, 15 (1S, 2S)-2-Amino-3-methoxy-1-phenyl-1-propanol, 17 Cesium fluoride-Tetraalkoxysilanes, 69... 

Cesium fluoride-Tetraalkoxysilanes, 69 Chlorodiisopropylsilane, 72 Chlorodimethylsilane, 73 Chlorodimethylthexylsilane, 74 Chlorodiphenylsilane, polymeric, 74 Chloromethyldiphenylsilane, 74 Chloromethyltrimethylsilane, 76 Chlorotriethylsilane, 165, 323 Chlorotrimethylsilane, 165, 168 Chlorotrimethylsilane-Lithium, 81 Chlorotrimethylsilane-Sodium iodide, 81... 

A plot of the ln[silane] vs. time (Fig. 1) exhibits the similar phenomenon found in our earlier work [13] with the tetraalkoxysilanes. The reverse reaction quickly becomes important and one must correct for it, though the earlier work had an additional complication of successive substitution reactions. As in the earlier paper, this could take the form ... 

Since the early studies with tetrahydrofurans, new substrates have been explored that are capable of highly diastereoselective reactions. trans-Allyl si-lyl ethers are exceptional substrates as illustrated by the conversion of 23 to 24 [Eq. (11)] [24], The reactions are highly diastereoselective (96-98% de) and in many respects outperform the standard aldol reaction. Even better substrates are tetraalkoxysilanes as shown for the reaction of tetraethoxysilane (25) [25]. In this case, both the enantioselectivity and the diastereoselectivity for the formation of C-H activation product 26 are >90% de and ee. 

From a tri- or tetraalkoxysilane by the reaction with 1,2-diols (catechols and glycols) in the presence of a tertiary amine (for example, equation 6)32b. 

Alcoholysis of 1-alkoxysilatranes proceeds more slowly than that of tetraalkoxysilanes due to the bulk of the silatranyl group. The exchange rate falls with the decreasing acidity of the hydroxylic compound, suggesting an initial protonation of the EtO oxygen. Therefore, a Lewis acid catalyst such as Zn(OAc)2 is required for effective silanolysis (reaction 99, R = Ph3Si)69. 

In this section we take a brief look at inorganic polymer networks produced by the acid-catalyzed hydrolytic condensation of tetraalkoxysilanes e.g., tetraethoxysilane (TEOS). The hydrolytic condensation may be represented by the following reactions ... 

Stationary phase technology has also seen significant improvements over the past years. The silica base material is nowadays often a hybrid material, synthesized from tetraalkoxysilanes and functionalized trialkoxysilanes, for example, methyl-trimethoxysilane (MTMS). The introduction of alkyl-trialkoxysilanes into the silica backbone makes the material more resistant to hydrolytic attack and also improves their separation behavior for basic analytes.30 C18 (= octadecylsilane) stationary phases are still the materials typically used in environmental analysis, and the enormous choice of materials with gradually different properties allows columns to be selected that are particularly well suited to a given separation task.31 Reversed phase separations with materials of shorter alkylsilane chain length (C8, C4, and Cl) are less frequently used. 

An alternative method to synthesize organically functionalized mesoporous silica phases is the co-condensation method (also called one-pot synthesis or direct synthesis). Historically, the classical co-condensation reaction involved tetraalkoxysilane [(RO Si (predominantly TEOS or TMOS)] and a terminal organotrialkoxysilane of the type (R 0)3Si-R in the presence of structure-directing agents (Fig. 3.9, top). Meanwhile, not only bis- and multi-silylated precursors are used, that is, precursors of the type [(R 0)3Si]mR, m > 2 (Fig. 3.9, bottom), together with TMOS/TEOS, but... 

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