Silanes direct synthesis

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

The hydrosilylation of carbonyl compounds by EtjSiH catalysed by the copper NHC complexes 65 and 66-67 constitutes a convenient method for the direct synthesis of silyl-protected alcohols (silyl ethers). The catalysts can be generated in situ from the corresponding imidazolium salts, base and CuCl or [Cu(MeCN) ]X", respectively. The catalytic reactions usually occur at room tanperature in THE with very good conversions and exhibit good functional group tolerance. Complex 66, which is more active than 65, allows the reactions to be run under lower silane loadings and is preferred for the hydrosilylation of hindered ketones. The wide scope of application of the copper catalyst [dialkyl-, arylalkyl-ketones, aldehydes (even enoUsable) and esters] is evident from some examples compiled in Table 2.3 [51-53],... 

Figure 4. shows the route from the high boiling residue of the direct synthesis to silicon carbo-nitride fibers. Methylchlorodisilanes and trichlorosilanes as additives are mixed in a specific ratio and react with methylamine and a small amount of ammonia to form an aminodisilane/oligosilazane. The subsequent polycondensation reaction of this mixture by heating to 250 °C yields a soluble and melt spinnable polysilazane. In comparision with the polysilane the properties of the polysilazane depend on the ratios of the disilanes/silanes and methylamine/ammonia and also on the reaction conditions. 

A much more general and very simple synthesis requiring a minimum of laboratory manipulation uses a Swern oxidation of the corresponding diols to give the a-ketoacyl silanes directly in useful yields (Scheme 38)7,124. Purification in this case was accomplished in the dark, by chromatography at —78°C or by distillation. a-Ketoacyl silanes appear to be intermediates in the oxidation of silyl acetylenes to a-ketoesters by osmium tetroxide125, and indeed have been isolated from the oxidation of silyl acetylenes by dimethyl dioxirane126. 

Table 6. Thermophysical characteristics of reactors for the direct synthesis of or-ganochloro silane s...

Promoters are normally incorporated in quantities less than 1%. Zinc is known as one of the most effective promoters for the direct synthesis methylchlorosilanes.33 Cadmium is known as one of the most effective promoters for the direct synthesis of tris(sila)alkanes,20 bis(chlorosilyl)methanes,22 and tris(chlorosilyl)methane.23 In the direct reaction of (dichloromethyl)silane 6b, the total amounts (percentages) of (trisilyl)methane.23 In the direct reaction of (dichloromethyl)silane 6b, the total amounts (percentages) of (trisilyl)methane products 7b-10b in the... 

Cadmium was a good promoter for the reaction, and the optimum temperature for this direct synthesis was 280 °C.24 Bis(chlorosilyl)methanes 3 and 4 were the same as those obtained in the direct reaction of silicon with a mixture of (chloromethyl)silanes 1 and hydrogen chloride,21 indicating that some of the starting (dichloromethyl)silanes 6a-d decomposed to... 

The disadvantages of the synthesis routes 1-4 are the application of highly reactive and expensive metals (Li, Na, K, Mg) and the enormous quantity of solvents. Particularly, as result of the dehalocoupling reactions, the polymers are unreactive at room temperature. To overcome these problems we synthesized spinnable reactive poly(silanes/-carbosilanes) via heterogeneous catalytic disproportionation of methylchlorodisilanes which have been wasted as a byproduct of the "Direct synthesis" of methylchlorosilanes so far. 

In order to get some information about the fate of silylenes S-la-d in the presence of a second molecule of methyl halide 6a-d, we checked whether silylenes S-la-d can experience stabilization by donor/acceptor interaction with 6a-d. Indeed, the two partners form weak donor/acceptor complexes 7a-d. That means that if S-la-d is created in a matrix at very low temperature in the presence of an excess of a methyl halide 6a-d, complexes like 7a-d should be present. Such a solvation might be the prerequisite for the direct synthesis of silanes 8a-d. These compounds represent the global minima on the CiHaSiX potential energy surfaces and are expected to be generated fl-om the two educt molecules S-la-d and methyl halide 6a-d in rather exothermic (AH, = 65-95 kcal moP ) processes. In the chlorine series (Scheme 1) the value is 73.9 kcal moP. ... 

Table 4.2-1. Average Composition of a Raw Silane Mixture from Industrial Direct Synthesis.

The phenylchlorosilanes and phenylmethylchlorosilanes also required in the manufacture of silicones, currently only produced to a limited extent by direct synthesis, are preferably produced by the reaction ol chlorobcn/enc with hydrogen-containing silanes according to the equations ... 

Organohydrogensilanes are formed as byproducts in the direct synthesis of dimethyldichlorosilane (see Section 4.2.1) and are important in the manufacture of hydrogen-containing siloxanes and organofunctional silanes (see... 

Systematic studies (41,95) have shown that Mg compounds of the type RiiCsHsSiCILMgX (R = alkyl) are readily obtained. They react slowly with chlorosilanes, however, and give unsatisfactory yields of product. It is frequently possible to obtain better results by replacing the Mg compound by that of Li. Reaction of chloromethyl silanes with Li is greatly dependent on the substituents that are attached to Si. In some cases it was not possible to obtain the Li compound by direct synthesis, in spite of variations in the solvent, temperature, and reaction time. Table VII summarizes qualitatively the behavior of chloromethyl silane derivatives with magnesium and lithium. 

Methylsilyl chloride has acquired considerable industrial importance as a precursor for the preparation of silicon it is prepared directly by reaction of methyl chloride with silicon in the presence of copper as catalyst at high temperatures.317 Details for direct synthesis of methylsilyl chlorides have been given by Rochow.318 The main product is dichloro(dimethyl)silane in accordance with a reaction 2CH3C1 + Si - (CH3)2SiCl2, but trichloro(methyl)-silane, trimethylsilyl chloride, tetramethylsilane, silicon tetrachloride, and other products are also formed. 

Halo(vinyl)silanes can also be obtained by direct synthesis,324 but for them tin325 and nickel326 prove better catalysts than copper. 

It is interesting to comment that benzene is the best choice as the solvent to produce Tg from Ta-tetrol as is the same with the direct synthesis of Tg from phenyltri-(ethoxy)silane in the presence of BzTMAH. Tetrabutylammonium fluoride showed... 

Phosphorus-, tin-, boron- and indium-doped silicon samples give different photo-EMF signals as well as modified chemical reactivity in the direct synthesis. The actual concentrations of n- and p-dopands control the reactivity of the silylene intermediates. Tests on these silicon samples were demonstrated that the electronic state of the educt silicon significantly affects the selectivity. Thus the amounts of disilanes in the crude silane mixture decreased if the n-type semiconductor behavior of the educt silicon was intensified by doping. 

A method for the preparation of halosilyl carbamates and isocyanate involves direct synthesis from amines via halosilyl carbamate intermediates [277]. A primary amine is converted to its carbamic acid salt, which is then treated with a silane containing > 2 halogen atoms bonded to Si. Gentle heating of the resulting halosilyl carbamate gives the isocyanate. In an alternative (exchange) procedure, the carbamic acid salt is treated with any halosilane to form a silyl carbamate, which is trans-silylated. 

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