Dehydrogenative coupling of silanes

The dehydrogenative coupling of silanes is a powerful technique of the formation of Si-Si bonds in disilanes, oligosilanes and long, silicon-based polymers. These reactions are Wurtz-type coupling processes, and they generally must be catalyzed, as the rate-determining step is the kinetically unfavourable dissociation of hydrogen from silicon. Luckily, there are numerous transition metal complexes that can catalyze reactions between a wide variety of silanes, whether they be primary, secondary or tertiary.

Generally, the dehydrogenative coupling of primary silanes (as well as secondary and tertiary) proceeds via a Wurtz-type reaction, where two molecules of RSiH₃ couple to form the corresponding dimer and eliminate one molecule of dihydrogen. Using an effective catalyst, this process can be extended to build long polysilanes.

Some of the catalysts that can be used in the dehydrogenative coupling of primary organosilanes are titanocene, zirconocene and their derivatives. While Wurtz-type coupling reactions are still the most common way to produce high molecular weight polysilanes, reactions using these titanocene and zirconocene catalysts and others like them have been improving and researchers are achieving higher molecular weight polysilanes as a result.

Cp₂Ti(OPh)₂ (Cp = cyclopentadienyl) is a good catalyst for dehydrogenative coupling between primary silanes. This titanium catalyst is advantageous because it is highly active at room temperature, and in addition to silane polymerization, it can functionalize the ends via the hydrosilylation of an olefin, all in one step. For example, phenylsilane can be polymerized in the presence of vinyltrimethoxysilane, which produces a polysilane with a trimethoxysilane group at the end.

While catalysts for dehydrogenative coupling reactions generally tend to be transition metal complexes, there has been some work done using magnesium oxide or calcium oxide for the reaction of phenylsilane and monosubstituted alkenes. This reaction is reported as being selective for only the dehydrogenative coupling and not the hydrosilylation reaction. In addition to high selectivity for dehydrogenative coupling, the reaction also has the advantage of being easy to separate through simple filtration of the reaction mixture to remove the solid magnesium oxide catalyst.

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