Chlorosilanes, coupling reactions

The coupling reaction of aromatic aldehydes proceeds well only in water by using VCI3 as a catalyst in the presence of A1 (Scheme 9). It should be noted that this method does not require a chlorosilane as an additive [31]. 

The pioneering work of Hiyama has demonstrated that organosilanes (suitably functionalized) in the presence of a nucleophilic activator can undergo Pd-catalyzed cross-coupling reactions. The chlorosilanes, fluorosilanes and alkoxysilanes are used to couple with a variety of electrophiles. 

Wurtz coupling reactions of chlorosilanes are the main route to the silicon-silicon bonded compounds. For example, hexamethyldisilane can be prepared by refluxing trimethylchlorosilane with lithium sand in THF (97%). Lithium may be substituted by sodium by using a mixture of HMPA-THF as the solvent. Linear and branched oligosilanes can be prepared by the same method (equations 55-57). 

The coupling reaction can also be realized by using a chlorosilane as an intermediate to "activate" the support surface. Chlorosilanes react easily with the active sites on the support. When trichloromethylsilane is used, always at least one active chlorine group is left, which in its turn can react with alcohols, such as polyethylene glycols. In this way, Mori prepared Chromosorb W (AW) with chemically bonded PEG, resulting in a loading of 4.2 with PEG 20M and of 2.0 % with PEG 3000. 

Aryl chlorosilanes were found to undergo palladium-catalyzed cross coupling reaction with aryl chlorides in the presence of a fluoride salt giving unsymmetrical biaryls (see Scheme 36 [129]). A variety of functional groups (cyano, acetyl, fluoro, trifluoromethyl) are tolerated in this reaction. While the... 

One of the first recommendations [50] of the Barbier reaction as the more favourable synthetic route over the two-step procedure via a preformed Grig-nard reagent was based on researches [93] of one-step coupling reactions of chlorosilanes and benzyl chloride (see p. 50, Sect. 2.4.4)... 

Numerous coupling reactions with chlorosilanes as well as with other halides of various different elements have, with great success, been applied in one-step procedures. 

Also aromatic organo halides were used in coupling reactions in the following example [106] a rather complicated chlorosilane was involved in reactions which gave fair yields ... 

Compared to the Wurtz method, electrochemical reduction with alkaline-earth metals is a milder alternative coupling reaction with the corresponding organochlorosilane. The silanes that can react with Mg are not preferred for this approach. Regardless of this limitation, the electrochemical method is applicable to phenyl-containing chlorosilane because Mg and Mg-Cu are not reactive when in contact with water and air. The water- and/or air-sensitive reactions are not welcome for technical reasons. 

The reaction scheme requires that the end block be polymerized first in cyclohexane and then transfered as living polymer to a isopentane solution to make the second block of pure cis 1,4 polyisoprene. To complete the systhesis the di-block polymer is coupled with a di-ester or di-functional chlorosilane coupling agent to obtain the final linear tri-block copolymer. The series of reactions are as follows ... 

Chlorosilane produces the corrosive by-product hydrogen chloride in the coupling reaction process and thus should be used judiciously. 

It seems that the ideal Wurtz-type coupling reaction of chlorosilanes combines all recent scientific results, using an optical active solvent at low temperatures and additives that increase the yield of desired polymer. To what extent the combination of these promising synthetic methods is able to deliver a high molecular weight monodisperse polysilane in high yields remains to be determined. 

Silacycles are coupling products of the reaction of dichlorodiorganosilanes with alkali metals [42]. This resembles the Wurtz-type coupling reaction of chlorosilanes in educts and reaction conditions and thus tolerates the same narrow range of functional groups. For this reason, it is mainly the methyl and phenyl substituted silanes which have been investigated. 

AC2O or AcCl can be utilized instead of a chlorosilane in the VOCU-catalyzed pinacol coupling reaction of aromatic aldehyde to give the diacetate 32 (Scheme 2.22) [70]. As mentioned above, a low-valent vanadium catalyst appears... 

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