Silicon alkoxysilanes

The most significant difference between the alkoxysilanes and siUcones is the susceptibiUty of the Si—OR bond to hydrolysis (see Silicon compounds, silicones). The simple alkoxysilanes are often operationally viewed as Hquid sources of siUcon dioxide (see Silica). The hydrolysis reaction, which yields polymers of siUcic acid that can be dehydrated to siUcon dioxide, is of considerable commercial importance. The stoichiometry for hydrolysis for tetraethoxysilane is... 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

Grafting from silica particles, silicon wafers, and related surfaces usually involves attaching a chlorosilanc or alkoxysilane derivative. Thus alkoxyamincs (e.g, 361,744,749 3627 0) and a wide variety of ATRP initiators (e.g. 363751) have been attached directly to surfaces and used to initiate grafting from" processes. 

Alkali-developable silicon-containing positive photoresists, development, 175 Alkoxysilanes, polymerization,... 

It has been well recognized that the hydrolysis of alkoxysilanes and chlorosilanes is effectively catalyzed when fluoride anions are present due to formation of hypercoordinated silicon intermediates.803 More in-depth studies by Bassindale et al. showed that the reaction of PhSi(OEt)3 with stoichiometric amounts of Bu4NF surprisingly yields an encapsulation complex, namely tetrabutylammonium octaphenyloctasilsesquioxane fluoride 830, in which the fluorine atom is situated inside the cubic siloxane cage (Scheme 114). The Si--F distance of average 2.65 A is shorter than the sum of van der Waals radii (3.57 A), which renders the coordination number of the silicon atoms at [4+1]. 

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

Scheme 6.16 Sni reaction at silicon preparation of cyclic alkoxysilanes...

In a typical sol-gel synthesis, the metal or the compounds of main group elements undergo hydrolysis and condensation reactions giving gel materials with extended three-dimensional structures. As shown in Equation 5.32 for silicon, addition of an acid or base catalyst to a solution of an alkoxysilane reagent such as tetrame-thoxysilane (TMOS), water and methanol leads to the hydrolysis of the Si-OMe bonds to form Si-OH functional groups. 

Organic silane derivatives with hydrolyzable groups on silicon (—OR, —Cl, —0C(0)R, etc.) are usually derived from a chlorosilane. Before or after formation of a carbon-silicon bond by hydrosilylation, the chlorosilanes are commonly converted to alkoxysilanes ... 

The alcoholysis reaction was found to be first order in both the silane and the catalysts, and of intermediate order in the alcohol, when catalyzed by carboxylic acids. When catalyzed by dichloro-acetic acid, the reaction has a Hammet p value of +0.43. This is consistent with a concerted displacement reaction between the alkoxysilane and the complex involving the alcohol and the carboxylate anion. The intermediate is a negatively charged intermediate, probably a penta-substituted silicon species. 

We will next deal with the hydrolysis reaction. Traditionally, most industrial processes used to make products containing the siloxane bond, such as silicones and silicates, involve the hydrolysis of chlorosilanes. This causes hydrochloric acid to be produced as a by-product. Hydrochloric acid is, however, an unacceptable by-product in products such as zinc-rich silicate coatings and glasses and glass ceramics made by the sol-gel process. The alternative is to use alkoxysilanes, which when hydrolyzed, have alcohols as a by-product. 

Until 1982, most alkoxysilanes had been produced from chlorosilanes and alcohols. Hydrochloric acid was therefore still a problem. In 1982, a process was developed in which TMOS could be made directly from elemental silicon and methanol [5]. In the production of silicate coatings, TMOS is first converted to TEOS by an alcoholysis reaction with ethanol. This prevents toxic methanol vapors from escaping from the curing coating. The TEOS is partially hydrolyzed with the rest of the hydrolysis occurring at the time of application. This is therefore a way to produce silicates without chlorine. (If a practical method for converting alkoxysilanes to alkylsilanes could be found, there would also be a nonchlorine method of production of silicones.)... 

Interest in this method of preparation of silicates has led us to study the mechanisms involved in the various reactions. In a previous paper we investigated the kinetics and mechanism of the alcoholysis of TMOS [13]. Because alkoxysilanes are important in the formation of silicones and silicates, their hydrolysis and subsequent condensation have been widely studied. Excellent reviews of the field can be found in The Siloxane Bond by Voronkov,... 

OCH3, Cl and CF3. This allows for the analysis of the electronic effect on the silicon atom while minimizing the steric effects. Mono-alkoxysilanes were used to avoid needless complication of the rate expression. 

The kinetics of the acid catalyzed alcoholysis of alkoxysilanes was studied. The influence of attaching a para-substituted phenyl group to the silicon was explored. 

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