Silicon—carbon bonds hydrogen halides

The silyl halides can now" be prepared in high purity and high yield by the facile hydrogen halide cleavage of the carbon-silicon bond in arylsilanes. " No catalyst is required, and the use of the hazardous intermediate reagent, silane, is avoided. Bromosilane is prepared by the reaction of hydrogen bromide and phenylsilane. The latter is obtained by lithium hydro-aluminate reduction of the commercially available phenyltri-chlorosilane. lodosilane can be prepared in a similar fashion however, mixtures of iodosilane and benzene are difficult to separate. The preferred alternative procedure described below utilizes an isomeric mixture of 2-, 3-, and 4-chlorophenylsilanes as the intermediate. This intermediate is obtained by the chlorination of phenyltrichlorosilane, and is then reduced to the hydride. 

Polymer Modification. The introduction of functional groups on polysilanes using the alkali metal coupling of dichlorosilanes is extremely difficult to achieve. Some polymers and copolymers with 2-(3-cyclohexenyl)ethyl substituents on silicon have been made, and these undergo hydrogen halide addition to the carbon—carbon double bond (94,98). 

Other polar reagents such as hydrogen halides react analogously with organometallic compounds and in these cases quite small differences in the polarity of the carbon-metal bond have evident effect for example, phenyl groups are removed from silicon by aqueous hydrochloric acid, but methyl groups are unaffected thereby. 

Some reactions of trimethylchlorosilane are summarized in Fig. 4.4. Many of these involve nucleophilic attack at silicon in which chlorine is substituted by another group. Alkoxysilanes are obtained using metal alkoxides or alcohols in the presence of pyridine. If the latter reaction is carried out in a non-polar solvent such as light petroleum, pyridinium hydrochloride is precipitated and may be filtered off, leaving the alkoxysilane in solution. Hydrogen chloride cleaves carbon-oxygen bonds in alkoxysilanes (as it does in ethers also) to form an alkyl halide and a siloxane ... 

Because carbon bonds so readily with itself, there are many hydrocarbons (see Chapter 18). Silicon forms a much smaller number of compounds with hydrogen, called the silanes. The simplest silane is silane itself, SiH4, the analog of methane. Silane is formed by the action of lithium aluminum hydride on silicon halides in ether ... 

You have now seen how enols and enolates react with electrophiles based on hydrogen (deuterium), carbon, halogens, silicon, sulfur, and nitrogen. What remains to be seen is how new carbon-carbon bonds can be formed with alkyl halides and carbonyl compounds in their normal electrophilic mode. These reactions are the subject of Chapters 26-29. We must first look at the ways aromatic compounds react with electrophiles. You will see similarities with the behaviour of enols. 

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