Polysilane formation reaction products

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

The most important process so far has been the reductive elimination of halogens with the formation of Si-Si bonds. Kipping used this reaction and discovered the first perphenylated cyclosilanes, yielding polysilanes as a by-product [8]. Similarly dodecamethylcyclohexasilane was found using dimethyldichlorosilane as a starting material for this reaction by Burkhardt in 1949, but 90% of the yield appeared as poly silane by-products [9]. 

Summary The formation of reactive intermediates via dehalogenation of chlorosilanes was investigated by using lithium powder and sonication. Whereas in the absence of a diene substrate mainly polysilanes are obtained, reactions with, e.g., dimethylbutadiene, yield the corresponding cycloaddition products, indicating silylenes and silaethenes as intermediates. 

Alternatively, polysilanes can be obtained by transition metal-promoted dehydrocoupling reactions of hydridosilanes. In contrast to reductive coupling of chlorosilanes, no solid by-products form. The only by-product is hydrogen. Thus purification steps are not required. It was shown by several research groups that group 4 metallocenes (Equation 18.4) are uniquely active catalysts for the formation of Si-Si bonds by this procedure. 

Other workers have reported that prolonged sonolysis of organolithiums derived from organic halides [94], chlorosilanes and chlorostannanes [95] results in Wurtz-type coupling. Yields are moderate and the reactions are of little synthetic interest. However, coupling of dichlorosilanes and stannanes produces a novel route to the cyclic polysilanes (9) and (10) (Scheme 37). The product obtained is determined by the steric bulk of the alkyl groups and only low levels of contamination by other silanes is observed [95]. Silylene intermediates did not appear to be involved. However, Boudjouk et al. [101] later reported formation of the tetramesityl silylene (11) which had previously been made by photolysis of (Mes)2Si(TMS)2 [102] (Scheme 38). 

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