Silicon, Direct Process Reaction

V. SILICON DIRECT PROCESS REACTIONS WITH REAGENTS OTHER... 

A key report investigated a variety of substrates in their reaction with silicon in an effort to find evidence for silylene intermediates during the silicon direct process reaction. When silicon, copper and methanol were reacted as described above but in the presence of alkenes, alkyldimethoxysilanes and (MeO SiH were formed95-97. The use of allyl propyl ether instead of alkenes gave allyldimethoxysilane, with 38% selectivity. These results and the reaction of silicon with MeCl in the presence of butadiene to give silacyclopent-3-enes indicates intermediate formation of silylenes. 

As previously mentioned, the Direct Process Reaction involves the reaction of (2) with silicon at elevated temperatures to form methyl-chlorosilanes (6-9) ... 

At the turn of the century the cost of elemental silicon was 1600/lb. Improvement of reduction conditions allowed this price to fall to 0.10/lb by 1931 and made the use of the element more practical. Elemental silicon is employed as an additive to iron to give silicon-steels, which are more acid resistant, to alloy copper for production of silicon bronzes and as an additive to aluminum or magnesium to improve the strength of these structural materials and provide resistance to corrosion. The use of silicon as a semiconductor had begun by 1940 and after discovery of the direct process reaction became the necessary starting point for the production of silicone polymers103. 

Compared to the Grignard route, the direct process reaction requires only the reduction of silica to silicon and the production of methyl chloride. A form of the overall reaction (related to equation 2) is shown in equation 3. On the basis of this equation there... 

TABLE 8. Products in the direct process reaction of silicon... 

The best compilation of data on the direct process reaction can be found in R. J. H. Voorhoeve, Organosilanes Per cursors to Silicones, Elsevier, Amsterdam, 1967. 

Two groups have made minor breakthroughs in the non-halogen direct process. In 1978 Malek, Speier and coworkers reported the reaction between dimethyl ether and silicon . Prior to their work there were some attempts to employ dimethyl ether as a substrate. Rochow and Zuckeiman disputed one claim . Later Newton and Rochow discussed the difficulty of using dimethyl ether in a direct process . Malek s and Speier s work showed that when silicon was reacted with MeOMe in the presence of metal catalysts and MeBr, 64.4% Me2Si(OMe)2 could be obtained. The silicon (150 g, 98%, 0.44% Fe, 0.26% Al, 0.058% Ca, 0.005% Sn, 0.04% Mn, 0.032% Ti, 0.051% V, 0.002% B, 0.014% Cr, 0.05% Zn and 0.002% Pb) was ground with steel balls to 2.7 microns for 16 h with Cu powder (12 g). One-hundred parts of silicon, 3.3 parts of copper, 0.3 part of iron, 0.43 part of aluminum and 0.13 part of calcium were combined in an autoclave and MeOMe and MeBr were added (mole ratio of Si MeOMe MeBr 100 350 1) and then heated for 20 h at 258 °C. While this process was not entirely halogen-free, it was the most successful ether silicon direct process to date. 

Silicon—Carbon Bond-Forming Reactions. After the Rochow-MbUer direct process, the hydro silylation reaction (139),... 

Synthesis of Silicone Monomers and Intermediates. Another important reaction for the formation of Si—C bonds, in addition to the direct process and the Grignard reaction, is hydrosdylation (eq. 3), which is used for the formation of monomers for producing a wide range of organomodified sihcones and for cross-linking sihcone polymers (8,52—58). Formation of ether and ester bonds at sihcon is important for the manufacture of curable sihcone materials. Alcoholysis of the Si—Cl bond (eq. 4) is a method for forming silyl ethers. HCl removal is typically accomphshed by the addition of tertiary amines or by using NaOR in place of R OH to form NaCl. 

Organosilanes, especially dimethyldichlorosilane (M2), are important chemicals used in the silicone industries. The direct reaction of silicon with an organic halide to produce the corresponding organosilanes as a gas-solid-solid catalytic reaction was first disclosed by Rochow [1]. In the reaction, a copper-containing precursor first reacts with silicon particles to form the catalytically active component, which is a copper-silicon alloy, the exact state of which is still under discussion. As the reaction proceeds. Si in the alloy is consumed, which is followed by the release of copper. This copper diffuses into the Si lattice to form new reaction centers until deactivation occurs. The main reaction of the direct process is ... 

Reaction (2) was also studied using the different catalysts. Before exposure to CH3CI, the contact masses were subject to XRD analysis. In the XRD patterns of the catalysts with higher activities for the Rochow direct process, the XRD peaks of the Cu-containing species were weaker and broader when normalized to the silicon peaks (silicon was used in excess). This suggests that some undetectable species were formed and the catalytic species were well-dispersed. This agrees well with the view of Lieske and co-workers [5]. 

The "conventional" methods for the preparation of SiC and Si3N4, the high temperature reaction of fine grade sand and coke (with additions of sawdust and NaCl) in an electric furnace (the Acheson process) for the former and usually the direct nitridation of elemental silicon or the reaction of silicon tetrachloride with ammonia (in the gas phase or in solution) for the latter, do not involve soluble or fusible intermediates. For many applications of these materials this is not necessarily a disadvantage (e.g., for the application of SiC as an abrasive), but for some of the more recent desired applications soluble or fusible (i.e., proces-sable) intermediates are required. 

More than 60 years after its simultaneous discovery by Rochow and Muller, the direct reaction of copper-activated silicon with alkyl chlorides is arguably still the most important industrial process for the preparation of basic organosilanes. An inspiring historic account highlighting the significance of this seminal work has been given by Seyferth.12 A comprehensive review on the subject has been written by Jung and Yoo.13 The most recent work associated with the direct process is concerned with the role of metallic promoters, such as Zn and Cd, as well as mechanistic aspects.14... 

The example of the first category is the formation of alkyl- and arylchlorosilanes in the so-called direct process (DP). The process was discovered over 60 years ago by Rochow in the United States, and, independently, by Muller in Germany, and it is still the most important reaction in organosilicon chemistry. In fact, it is at the very basis of the silicone industry, being the primary source of organochlorosilane precursors (mostly methylchlorosilanes, comprising over 90% of the total) in the production of silicone oligomers and polymers. 

There are many examples of the direct reaction of silicon with ethyl chloride, vinyl chloride and chlorobenzene1. Vinyl and allylchlorosilanes76 were first made via a direct process in 1945 as were phenylchlorosilanes77. Jung s group has recently extended the direct reaction of silicon with a variety of substrates including allyl chloride78. Silicon... 

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