Chlorosilanes with alcohols

The etherification of chlorosilane with alcohols is a major industrial method for ethoxysilane production (Eqs. 1 and 2). 

Alkoxysilanes are easily prepared by the reaction of chlorosilanes with alcohols. When alkoxysilanes contact inorganic materials, they are hydrolyzed by the moisture absorbed on the surface to give the silanol (R Si(OH)4 ). Further, the... 

The most common tetraalkoxysilanes used in the sol-gel process are tetra-ethoxysilane (Si(OC2H5)4) and tetramethoxysilane (Si(OCHj)4), which are abbreviated in the literature as TEOS and TMOS, respectively. The traditional method of preparing tetraalkoxysilanes is by reacting tetra-chlorosilane with alcohol [3]. When anhydrous ethanol is used, the product is TEOS with hydrogen chloride as a by-product ... 

This method is also used with alcohols of the stmcture Cl(CH2) OH (114). HaloaLkyl chlorosulfates are likewise obtained from the reaction of halogenated alkanes with sulfur trioxide or from the chlorination of cycHc sulfites (115,116). Chlorosilanes form chlorosulfate esters when treated with sulfur trioxide or chlorosulfuric acid (117). Another approach to halosulfates is based on the addition of chlorosulfuric or fluorosulfuric acid to alkenes in nonpolar solvents (118). 

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.)... 

Recently silylboronates functionalized on silicon were reported by Suginome and coworkers via the reaction of silyl lithium 379 with borate ester 380 to yield the borosilamide 381. Further transformations on the amine with HC1 furnish the chlorosilane 382, and reaction of SbFs with 382 provide the florosilane 383. The chlorosilanes 382 also react with alcohols or amines to provide the alkoxy or amine substituted silylboronates 384 and 385 respectively (Scheme 61) <20070M1291>. 

There are limitations to the use of DCC yields are variable and A -acylureas are side products. Many other dehydrating agents " have been used, including DCC and an aminopyridine, Amberlyst-15, chlorosilanes, MeS02Cl-Et3N, and AA -carbonyldiimidazole(99). In the latter case, imidazolides (100) are intermediates that readily react with alcohols. 

A proposed transition structure for the chiral formamide-promoted reaction of 2-butenyltrichIorosiIanes with aldehydes has been put forward by Iseki (Fig. 10-1) [56d]. The hexacoordinate silicon atom is bound to the oxygen atoms of the aldehyde, the catalyst, and HMPA. The chiral formamide-promoted reaction of allyltri-chlorosilane with aliphatic aldehydes proceeds in high yield and excellent enan-tioselectivity to produce homoallylic alcohols. When either or (Z)-71 was... 

The coupling reaction can also be realized by using a chlorosilane as an intermediate to "activate" the support surface. Chlorosilanes react easily with the active sites on the support. When trichloromethylsilane is used, always at least one active chlorine group is left, which in its turn can react with alcohols, such as polyethylene glycols. In this way, Mori prepared Chromosorb W (AW) with chemically bonded PEG, resulting in a loading of 4.2 with PEG 20M and of 2.0 % with PEG 3000. 

Other functional silanes also can be hydrolyzed or reacted by nonhydro-lytic techniques to give polyorganosiloxanes. For example, the reactions of chlorosilanes or alkoxy silanes with alcohols, esters, or hydroxyl groups are examples of the nonhydrolytic process [Eq. (17)] [42,43]. 

Chlorosilanes can be transformed into alkoxysilanes with alcohol and amine. Triethylamine, pyridine, and ammonia are usually used as amines. In the next example, imidazole, a relatively strong base, and Ai,Ai-dimethylformamide (DMF) were used to promote alkoxylation of tetraisopropyl-substituted dichlorodisilane. 

TiF is a colorless, very hygroscopic soHd and is classified as a soft fluorinating reagent (4), fluorinating chlorosilanes to fluorosilanes at 100°C. It also forms adducts, some of them quite stable, with ammonia, pyridine, and ethanol. TiF sublimes at 285.5°C, and melts at temperatures >400° C. It is soluble in water, alcohol, and pyridine, hydroly2ing in the former, and has a density of 2.79 g/mL. 

A mixture of 40 mmol or the a-nitro alcohol 1, 48 mmol of (terf-butyldimethyl)chlorosilane or chlorotri-mcthylsilanc, 6.8 g (100 mmol) of I /(-imidazole and 10 mL of DMF is stirred at 20 C for 12 h. Water is added and the mixtnre is extracted with hexane. The combined extract is washed wilh water and sat. aq NaCl and dried over MgS04. Exaporation gives the O-silyl nitro alcohol as an oil which is distilled (bulb-to-bulb) yield 45 - 97%. If unreacled starting material is present in the distillate, it can be removed by chromatography over silica gel (30 g/ l g of product) with l,t,0/pcntanc 1 9 as eluant. 

The chlorosilanes are available from classical synthetic routes [108]. In most cases the products can be obtained from SiCl4 and the lithium salts of the introduced substituents with sufficient selectivity and in high yield. This is particularly true for the alcoholates and the thiolates. The exchange problems R vs. Cl which arise with the t-butylthiochlorosilanes will not be discussed in detail here. These problems are overcome by an appropriate choice of the reaction conditions. 

Silyl protecting groups are the gold standard for the protection of alcohols.234 Novel photochemically removable protection groups for alcohols have been developed by Brook et a/.23S and Pirrung et al,236 For instance, cyclo-pentanol can be reacted with tris(trimethylsilyl)chlorosilane 53 in the presence of a mild base to yield the protected silyl ether 54. The protection group can be removed conveniently upon UV irradiation or by the use of Bu4NF (Scheme 12). 

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. 

Carbosiloxane dendrimers with SiH end groups (Fig. 4.48) [92] are generally accessible by hydrosilylation of MeCOCH2CH2CH=CH2 with chlorosilanes to yield Si-Cl terminated compounds and subsequent reduction with lithium aluminium hydride to form the corresponding alcohol with terminal SiH groups. 

Alkyltrichlorosilanes are etherified when the weight ratio of alkyltri-chlorosilane and ethyl alcohol is 1 (3.05-K3.1) the etherification is accompanied by gradual replacement of chlorine atoms in alkyltrichlorosilane with ethoxyl groups ... 

The strategy of incorporating silicon as a reactive component in the polymeric system to attain flame retardancy has been explored. For example, Ebdon et al. carried out silylation to the polystyrene using //-butyl lithium in the presence of tetramethylethylenediamine, followed by reaction with trimethylchlorosilane, dichlorodimethylsilane, or trichloromethylsilane, as shown in Scheme 8.1. Poly(vinyl alcohol) films have also been modified with chlorosilanes (Scheme 8.1). 

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