Alkylsilanes alkylation

This reaction, cataly2ed by uv radiation, peroxides, and some metal catalysts, eg, platinum, led to the production of a broad range of alkyl and functional alkyl trihalosilanes. These alkylsilanes have important commercial value as monomers and are also used in the production of sihcon fluids and resins. Additional information on the chemistry of sihcon hahdes is available (19,21—24). 

The alkylation of halogen-substituted benzenes such as fluorobenzene and dichlorobenzenes with other (dichloroalkyl)silanes in the presence of aluminum chloride catalyst afforded isomeric mixtures of the corresponding (dihalogen-substituted phenyl)alkylsilanes in moderate yields (Eq. (13)). These results are summarized in Table Xll. 

The chemistry of silicone halides was recently reviewed by Collins.13 The primary use for SiCU is in the manufacturing of fumed silica, but it is also used in the manufacture of polycrystalline silicon for the semiconductor industry. It is also commonly used in the synthesis of silicate esters. T richlorosilane (another important product of the reaction of silicon or silicon alloys with chlorine) is primarily used in the manufacture of semiconductor-grade silicon, and in the synthesis of organotrichlorosilane by the hydrosilylation reactions. The silicon halohydrides are particularly useful intermediate chemicals because of their ability to add to alkenes, allowing the production of a broad range of alkyl- and functional alkyltrihalosilanes. These alkylsilanes have important commercial value as monomers, and are also used in the production of silicon fluids and resins. On the other hand, trichlorosilane is a basic precursor to the synthesis of functional silsesquioxanes and other highly branched siloxane structures. 

Modification of the mesoporous silica with alkylchlorosilanes resulted in the increase of the weight loss on the TGA curves (Fig. 5). Thermogravimetric studies revealed a great similarity of the samples modified via conventional and one-step procedures. As can be seen from Fig. 5 the sample with attached longer alkylsilane prepared via one-step procedure has much higher surface coverage than that obtained via conventional modification. A notable difference in the shape of the DTG curves for the MCM-UO and MCM-CO samples could be attributed to the presence of the different conformations of the attached alkyl groups [12]. 

Acid catalysis—hydrolysis. Several series of alkylsilane esters were studied to determine the effect of silane structure on the hydronium ion catalyzed hydrolysis reaction. The hydronium ion catalyzed hydrolysis rate constants for a series of alkyl tris-(2-methoxyethoxy)silanes in aqueous solution were used to define the modified Taft equation log(A /Ah ) = 0.39a + 1.06ES, where Ho is the rate of hydrolysis for methyl tris-2-(methoxyethoxy)silane [42], The hydronium ion catalyzed hydrolysis rate constants and the reaction half-lives are reported in Table 2. In a similar manner, the hydronium ion catalyzed hydrolysis rate constants for a series of trialkylalkoxysilanes in 55% aqueous acetone were used to obtain the modified Taft equation log(/cH//cHo) = -0.37 a + 2.48 E where kHo is the rate of hydrolysis for trimethylalkoxy-silane. 

Alkyl migration, aluminum-mediated, 3, 279 Alkylphosphines, in Rh-catalyzed hydroformylations dirhodium tetraphosphine, 7, 252 monophosphines, 7, 252 Alkylsilanes... 

Many investigations of alkyl and aryl chlorosilanes have been reported (125, 220, 225, 254). The probability of cleavage of the R—Si bond is found to decrease with increase in the size of R (220). Methylsilanes (143) and hydrocarbons with trialkylsilyl groups (145) are discussed and extensive studies of linear and cyclic silicon-methylene compounds are also available (22-24, 105-107, 225). Russian workers have looked at alkylsilanes (68), silacycloalkanes (69), and silylvinylacetylenes (144, 195). Silicon-carbon... 

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. 

Apolar stationary phases suffer from hydrolytic instability at pH extremes. The use of mixed phases of long (Cg, Clg) and short (C, C3) chain alkyls produces stationary phases with increased hydrolytic stability.7,8 Crowding of the long alkyl chains does not allow the alkylsilane molecules to deposit in close packing on a smooth or flat surface. Silane molecules polymerize in vertical direction, loosing contact with the silica surface. The insertion of short chain alkyls allows horizontal polymerization of the silane molecules. Thus, alkyl chains are aligned in a parallel way. The stability of the silane layer is increased consequently (figure 8.1). 

The mobility of the alkylchain,4,11 as well as the accessibility of the functional group control the separation efficiency. Gilpin12 found no difference in chromatographic performance in comparing a bidentate alkylsilane modified silica to its monodentate analogue. This indicates that chain mobility only has a minor influence on performance, for n-alkyl ligands. 

Carlier32,33 used various functional alkylsilane groups on silica as co-monomer, transfer agent or initiator for grafting of a functional polymer. These functional polymers may be used to anchor a catalyst. The polymer polyphenylsilsesquioxane was grafted onto porous silica and sulfonated, to obtain catalysts of high stability with enhanced site accessibility and increased number of sites, as well as high acidity level.34 This catalyst is used for esterification and phenol alkylation. Other catalysts have been reviewed by Pinnavaia,35 and are summarized in table 8.5. 

Alkyl chains are generally deposited on the silica surface using the corresponding alkylsilane. The hydride intermediate route, developed by Sandoval and Pesek78,79 gives an alternative for this method. The one-step alkylsilane route, is replaced by a two-step silanization-alkylation procedure. The loss of process simplicity is compensated by the advantages of an increased alkyl density and enhanced coating stability. 

Figure 3.29 Synthesis of a PMO derivative with the use of a siloxane-based oligomer that consists of an alkylsilane nucleus (white) and three branching trimethyoxysilyl groups (light grey) This precursor acts simultaneously both as organosilica source and surfactant. The alkyl chain length (n) determined the phase of the composite for n = 10a2Dhexagonalphaseisformed for n = 16, a lamellar phase is obtained. This approach can also be employed as co-condensation reaction together with TMOS.

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