Halogenosilane

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],... 

Most studies of halogenosilanes have been concerned with the methylsilanes (Section ll.C). The data that have been collected for the mixed silicon halides Y4 SiX (26,57) are presented graphically in Fig. 12. In contrast to the methylhalogenosilanes (Me4 SiF is shown in Fig. 12 for comparison), the sagging" pattern, which is apparently... 

The 1,4-dihydropyrazine system has been prepared by reductive silylation of pyrazine with alkali metals and halogenosilanes. Thus pyrazine with lithium and trimethylchlorosilane gives l,4-bis(trimethylsilyl)-l,4-dihydropyrazine (623). Reduction of pyrazine with sodium in ethanol gives piperazine (22). 

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. 

Evidence for the foregoing postulate is provided by the observations that such reactions take place only in the presence of trace quantities of free amine and are inhibited by addition of halogenosilanes (which react with amine) furthermore, reactions of chloro-aminosilanes with CS2 are slow and ill-defined. Similar catalysis is found in some related reactions involving B-N or Si-N cleavage, but this is not the case with amides of class b metals such as Sn, where insertion proceeds even in the presence of MesSnCl,... 

Functional silicon compounds R3SiX are also optically stable. Racemization is observed only in the presence of nucleophilic agents. Kinetic studies of the nucleophilically assisted racemizations of halogenosilanes, germanes, and stannanes have revealed that they involve two molecules of nucleophile in the rate-... 

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. 

The main synthetic routes to allylsilanes are Grignard reactions, reductive silylation, and Wittig-type reactions with halogenosilanes and hydrosilylation of dienes. 

The synthesis of aminosilanes from halogenosilanes and amines can usually be accomplished in an inert solvent like diethyl ether, benzene or carbon tetrachloride. Mostly, the reaction is carried out at room temperature and, if necessary, with cooling an alternative approach is to conduct the reaction in liquid ammonia. 

An alternative route to aminosilanes is the reaction of halogenosilanes with metallated amides and amines. Thus, one can synthesize, e.g. t-butyl-t-butylaminofluorophenylsil-ane (413) in 91% yield from phenyl-t-butyldifluorosilane (411) and lithio-t-butylamide (412) (equation 188)212. Analogously t-butyl-(l-methylprop-l-yl)-phenylamino-fluorosilane (416) can be obtained from lithiated aniline 415 with the respective fluorosil-ane 414 (equation 189)213. 

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. 

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