A-Alkoxysilanes

Similar reactions can be performed using the less toxic a-alkoxysilanes as starting materials ... 

HOMO in 250 by ca. 0.4 V (the difference between the oxidation potentials of 250 and 251) this is a much smaller effect than the corresponding ac s,- 0 interaction, which lowers the oxidation potential of a-alkoxysilanes by as much as 0.9 V. This is consistent with the better energy match between the ac Sl and n0 orbitals. 

High-yield anodic oxidation of a-alkoxysilanes (XCV) to )5-hydroxy aldehyde acetals was used as the key step in an iterative preparation of optically active polyols [Eq. (59)]. The reaction apparently involves formation of the ether radical cation, followed by rapid C-Si bond scission and nucleophilic trapping by solvent [142]. 

Since the orbital interaction plays a major role, the oxidation potentials of a-het-eroatom-substituted organosilicon compounds depend on the geometry. As a matter of fact, the oxidation potentials of a-alkoxysilanes in which the rotation around the C-O bond is restricted vary dramatically with the torsion angle of Si-C-O-C (Table 4) [115]. 

Maleczka, R. E., Jr., Geng, F. Methyllithium-Promoted Wittig Rearrangements of a-Alkoxysilanes. Org. Lett. 1999,1, 1115-1118. 

Acylsilanes are reduced to a-alkoxysilanes by a variety of metal hydrides. Reduction with (—)-chlorodiisopinocamphenylborane provides (R)-alcohols in >80% ee and in 30-65% yield. ... 

A new iterative route to enantio-pure polyols using a-alkoxysilanes has appeared (Scheme 3). Thus, electrochemical oxidation of the a-methoxysilane followed by protection of the hydroxyl gave the dimethyl acetal intermediate as shown. Subsequent olefination afforded the vinyl silane which was further converted as indicated into the homologated P-hydroxy-a-methoxysilane. Further chain extension can be obtained by repeating the cycle. 

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

The successfiil synthesis of a transparent soHd polymer electrolyte (SPE) based on PEO and alkoxysilanes has been reported (41). The material possessed good mechanical properties and high electrical conductivity (around 1.8 x 10 S/cm at 25°C) dependent on the organic—inorganic ratio and PEO chain length. 

The physical properties of commercial alkoxysilanes are provided in Table 1. Two classes of silane esters have very distinct properties and are generally considered apart from alkoxysilanes. Sdatranes are compounds derived from trialkanolamines and have siHcon—nitrogen coordination. These are generally hydrolytically stable and have unique physiological properties (3). A second special class of monomeric esters are cycHc diesters of polyethyleneoxide glycols designated sila-crowns, which have appHcation as catalysts (4). Neither silatranes nor sila-crowns are considered herein. 

The synthesis of triethoxysilane (28) and trimethoxysilane (29) has also been achieved by direct process. In 1980 there were no direct processes for the production of alkoxysilanes. In 1995 Silbond in Weston, Michigan, and Carboline in St. Louis, Missouri, operated processes for the production of tetraethoxysilane in the United States, and OSi/Witco announced start-up of a process to produce triethoxysilane and tetraethoxysilane in TermoH, Italy. 

Apart from the direct action of an alcohol on a chlorosilane or siUcon, the only other commercial method used to prepare alkoxysilanes is transesterification. 

The analogous reaction between anhydrides and alkoxysilanes also produces acyloxysilanes. The direct reaction of acids with chlorosilanes does not cleanly lead to full substitution. Commercial production of methyltriacetoxysilane direcdy from methyltrichlorosilane and acetic acid has been made possible by the addition of small amounts of acetic anhydride or EDTA, or acceptance of dimethyltetraacetoxydisiloxane in the final room temperature vulcanising (RTV) appHcation (41—43). A reaction that leads to the formation of acyloxysilanes is the interaction of acid chlorides with silylamides. 

Glass Frosting. Deposition of siUcon dioxide is used to impart a translucent coating on glass (qv) (50). The surfaces are either exposed to tetraethoxysilane or tetramethoxysilane under high moisture conditions, or the alkoxysilanes are ignited and the resulting powder is appHed to the surface. 

Relative hydrolysis and condensation rate studies of multifunctional silanes, Si(OR), under acidic and basic catalysis showed that the first (OR) group hydroly2es much more readily than subsequent groups (195). Sdanol—sdanol condensation is much slower than sdanol—alkoxysilane condensation, even if the alkoxysilane is monofunctional, thus suggesting that chain extension is insignificant ia the presence of a cross-linker (196—199). 

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)... 

The general mechanism of how alkoxysilanes form bonds with a fiber surface that contains hydroxyl groups is depicted in Figure 9. 

Alkoxysilanes undergo hydrolysis, condensation (catalysts for alkoxysilane hydrolysis are usually catalysts for condensation), and a bond formation stage under base as well as under acid catalyzed mechanisms. In addition to this reaction of silanols with hydroxyls of the fiber surface, the formation of polysiloxane structures also can take place. 

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. 

The brush phases are made from the mono-chlorsilanes or the mono-alkoxysilanes which attach a single organic moiety to each reacted hydroxyl group. For example using octyldimethylchlorsilane, dimethyloctyl chains would be attached to the surface like bristles of a brush, hence the term brush phase. In all bonding processes, some hydroxyl groups remain unreacted which would allow the polar character of the silica play a part in retention and also render the surface liable to dissolution in water and dilute acids. For this reason the material is capped by treatment with either trimethylchlorsilane or hexamethyldisilazane to eliminate any unreacted or stearically... 

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