Silicon with alkyl chlorides

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... 

II. Direct Reaction of Elemental Silicon with Alkyl Chlorides. 148... 

DIRECT REACTION OF ELEMENTAL SILICON WITH ALKYL CHLORIDES... 

In specific reaction conditions, formation of alkyldichlorosilanes with very high selectivity is observed from the reaction of elemental silicon with alkyl chlorides. For example, when a gaseous mixture of w-propyl chloride... 

Although the direct reaction of elemental silicon with alkyl chlorides has been extensively studied and a vast number of reports are available in the literature, the reactions with polychlorinated alkanes such as methylene chloride and chloroform are scarce, possibility due to the intractability of the non-volatile high molecular weight products often obtained. 

Recent studies on the direct reaction of elemental silicon with alkyl chlorides such as methyl chloride, activated alkyl chlorides, polychloro-methanes, (chloromethyl)silanes, (dichloromethyl)silanes, etc. are summarized in this review. In the direct reaction of elemental silicon with activated alkyl chlorides and polychloromethanes, the decomposition of the reactants can be suppressed and the production of polymeric carbosilanes reduced by adding hydrogen chloride to the reactants. These reactions provide a variety of new organosilicon compounds containing Si-H and Si- Cl functionalities, which should find considerable application in the silicone industry. 

The reaction is carried out by first reacting the alkyl or aryl halide with magnesium shavings in an ether suspension and then treating with silicon tetrachloride (prepared by passing chlorine over heated silicon). With methyl chloride the following sequence of reactions occur ... 

Silicon tetrachloride reacts with alkyl chloride and sodium to form the... 

Allyl chloride did not react at temperatures below 220 °C. The total amount of products and the selectivity for allyldichlorosilane gradually increased as the temperature increased to 260 °C, but decreased beyond 280 °C. Reaction temperatures were significantly lower than those for unactivated alkyl chlorides, a reflection of the higher reactivity of allyl chloride. As hydrogen chloride content increased from 50 to 75 mol% in the mixture of allyl chloride and hydrogen chloride, the yield of allyldichlorosilane increased from 31 to 41% and that of diallyldichlorosilane decreased from 1.9 to 0.2%. However, the amount of trichlorosilane, formed from the reaction of elemental silicon with hydrogen chloride, also increased. 

In the alkylation of silicon with methyl chloride, mono-, di-, and trimethyl chlorosilanes are formed. The reaction products must then be fractionated. Because the dimethyl derivative is bifunctional, it produces linear methylsilicone polymers on hydrolysis. 

We should compare the S reaction at silicon with the S 2 reaction at carbon. There are some iportant differences. Alkyl halides are soft electrophiles but silyl halides are hard electrophiles. Alkyl halides react only very slowly with fluoride ion but silyl halides react more rapidly with fluoride [than with any other nucleophile. The best nucleophiles for saturated carbon are neutral and/or based on elements down the periodic table (S, Se, I). The best nucleophiles for silicon are charged and based on highly electronegative atoms (chiefly F, Cl, and O). A familiar example is the reaction of enolates at carbon with alkyl halides but at oxygen with silyl chlorides (Chapter 21). 

Thousands of papers and patents have appeared on the direct synthesis of organochlorosilanes since 1940 when Rochow reported the direct reaction.4 These results are comprehensively accounted for in a number of earlier reviews and books.3 8 The purpose of this review is to describe the recent developing trend of the direct reaction of elemental silicon with activated or unactivated alkyl chlorides, in particular, emphasizing direct reactions with a mixture of hydrogen chloride and activated alkyl chlorides such as polychlorinated methanes, silylmethyl chlorides and dichlorides, allyl chloride, etc. 

Zinc inhibited and cadmium promoted this particular reaction, consistent with the results observed for the direct reaction of elemental silicon with a mixture of alkyl chloride and hydrogen chloride, described above. In addition to allylchlorosilanes, several other compounds were also produced as minor products, including the following 2-methyl-1,1,4,4-tetrachloro-l,4-disilabutane 1,1,5,5-tetrachloro-1,5-disilapentane and 3-(dichlorosilyl)-1,1,5,5-tetrachloro-1,5-disilapentane. 

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