Coupling reactions dehydrocouplings

Because of these limitations, considerable effort has been focused on the development of new synthetic routes to polysilanes. The early transition metal-catalysed dehydrocoupling process discovered in 1985 [eqn (10.8)] is potentially very attractive however, the molecular weights of the polysilanes formed to date are generally fairly low (Mn<8000)." The catalysts used for these coupling reactions are usually titanocene or zirconocene derivatives." " " ... 

Syntheses of Si-B-C-N polymers in which borazine units are directly attached to polysilazanes were investigated in detail by Sneddon et al." " The principle was to join borazine or its derivatives to oligomeric or polymeric silazanes by dehydrocoupling or dehydrosilylative (elimination of HSiRj) coupling reactions. For example, hydridopolysilazane was treated with borazine at temperatures... 

As the bulk of the substituents on silicon increases, the shorter become the chains in the coupled product, to the limit of disilane. This limit occurs for both primary and secondary silanes. One difference between the metallocene (Group 4) promoted dehydrocoupling reactions and that of metals such as Rh or Pt, is the difficulty in the condensation of dialkylsilanes from the former grouping. Dialkylsilanes condense in the presence of metallocene catalysts only if a hydrogen acceptor (such as an olefin) is present. An interesting exception to the condensation of a secondary silane center comes from the reactions of hydrogen terminated disilanes such as 1,2-dimethyldisilane which forms polymers from both metallocene promoted reactions as well as with Rh and Pd complexes. 

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. 

Although for the catalytic transformations of organosilicon compounds only hydrosilylation is well known as industrially important process, in the last 20 years other reactions of silicon compounds catalyzed by transition metal complexes have been discovered and developed. They include double (bis)silylation of alkenes and alkynes, silylative coupling of alkenes and alkynes with vinylsi-lanes, dehydrocoupling of hydrosilanes, silylformylation and silylcarbonylation of unsaturated compounds, and dehydrogenative silylation of alkenes and alkynes with hydrosilanes. Only the latter, as related to hydrosilylation (and very often its side reaction), has been discussed here (13). 

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