A-Halogenosilanes

The alkyl- or aryl-halogenosilanes are prepared commercially by passing the vapour of an alkyl or an aryl halide over a heated intimate mixture of powdered sihcoii and either copper or silver. 

Three types of halogenosilanes (235, 236 and 237) with pentacoordinate silicon atom are considered. The first comprises anions of type 235, the second is a neutral species containing a Lewis base (D) (236) and the last is a cation formed and stabilized by a chelate ligand of a Lewis base character (237). 

The authors evidenced, using a methoxylated silica as a substrate, that at these temperatures no reaction occurs of the halogenosilanes with the siloxane bridges of the silica. No pretreatment temperature was mentioned, however. Therefore we cannot exclude a small reaction of the bromo- or iodosilanes with the siloxane bridges of the silica, thermally pretreated at high temperatures. 

The first observation of the formation of a silicone oil was reported more than 120 years ago [195], but it was just after the World War II that the first industrial applications started at General Electric and Dow Corning, mainly after the discovery of direct synthesis of organochlorosilanes from halogenosilanes by Rochow [196]. The structure of silicones was first recognized by Robinson and Kipping [197],... 

This type of elimination reaction seems to be limited to halogenosilanes containing a silicon atom exhibiting a strong Lewis acidity. The analogous reaction of CP2M0H2 with several other chlorosilanes only yields the monochloro compound Cp2Mo(H)Cl. 

A related process is implicated in the nucleophilically induced racemization at tetracoordinate halogenosilanes (13), which has been shown to involve two molecules of nucleophilic agent. Racemization arises from an equilibrium between five- and six-coordinate intermediates (eq. [116]). This process is controlled by entropy. It is characterized by a highly negative activation entropy and a low or sometimes negative activation enthalpy. 

Common synthetic pathways to obtain mono-, di-, tri- and tetraalkylsilanes are reactions of the respective halogenosilanes with lithiated alkyl compounds, with alkyl-magnesium halides (Grignard reactions) and with alkyl halides in a Wurtz-Fittig-type reaction, as well as the addition of silanes to alkenes, i.e. hydrosilylation. 

Especially, trimethylazidosilane (TMSA) (452) is an excellent reagent for preparative purposes, since it can be applied to many reactions which otherwise require the application of the explosive hydrogen azide. A number of synthetic methods for the preparation of azidosilanes are known. Starting materials are mostly metal azides, which are applied to halogenosilanes in THF via this approach TMSA (452) is obtained in 81 % yield (equation 243)230, 258 from Me3SiCl. 

Another way to synthesize alkoxyorganylsilanes is the reaction of halogenosilanes with alkyl orthoformate 29 (equations 293)328 in the presence of a Lewis acid. 

For the silylation of polyols, halogenosilanes are rarely applied in order to prevent side-reactions so a variety of other silylating agents are in use. These include hexa-methyldisilazane (405), iV-TMS-acetamide (435), N,0-bis(TMS) acetamide (438) and TMS-diethylamine (463). When Me3SiCl (16) is applied, a base like pyridine as HC1 acceptor has to be used. Thus, ethylene glycol (699) yields l,2-bis(trimethylsiloxy)ethane (705) (equation 358)394 in 90% yield. With 1,5-dihydroxynaphthalene (706) and Me3SiCl/pyridine l,5-bis(trimethylsiloxy)naphthalene (707) is formed (equation 359)395,... 

Ammonium fluoride Halogenosilanes from siloxanes s. 4, 471 s. a. C. Eaborn, Soc. 1949, 2755 NHsF... 

Without additional reagents and sulfuric acid Halogenosilanes from aminosilanes s. 4, 473 m.a.T. and HjSO SiNH2 SiHal... 

Two mechanisms have been proposed for the racemization of optically active halogenosilanes in non-polar solvents, catalysed by nucleophilic species. One involves the formation of a five-co-ordinate intermediate, and the other a six-coordinate species. In an attempt to distinguish between these possibilities the cis-trans isomerization of l-chloro-l,2-dimethylsilacyclobutane (6) and l-chloro-1,2-dimethylsilacyclopentane (7) has been studied. Reaction in carbon tetrachloride is... 

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