Sylilene Catalyzed Hydosilylation Reactions

Silylene complexes have been proposed to be key intermediates in many catalytic processes like Rochow`s direct process, the redistribution of silanes, silylene transfer reactions or dehydrogenative coupling reactions. The first stable examples of donor-stabilized silylene complexes were reported only in 1987. Three year later the first stable donor-free silylene complex was synthesized. Now several routes to silylene complexes have been developed, but the three main synthesis strategies to transition-metal silylene complexes are: (1) anionic substituent abstraction, (2) coordination of a free silylene to a metal fragment and (3) silylene complexes by a-migration. With the synthesis of stable, isolable silylene complexes it became possible to explore their reactivity. Silylene complexes have often been invoked to account for the ability of transition-metal complexes to catalyze redistribution at the silicon. For this reaction, mechanistic speculation has centered on intermediate silyl(silylene) complexes of the type Ln M=SiR 2 (SiR` 3 ), in which groups at silicon are thought to exchange via 1,2-shifts between the metal and silicon or via 1,3-shifts between silicon centers. But until now only one catalytic reaction is known where a silylene complex`s involvement has been proven. This reaction is a novel hydrosilylation reaction displaying unusual selectivity. In the proposed work the mechanism of this reaction should be investigated closer. In the course of theses studies a general synthetic route to silylene complexes with additional Si-H bonds, like the catalytic active species, by ?- migration should be established. Here, silylene complexes with several different transition metals should be synthesized. The importance of steric and electronic factors in the complex needs to be investigated. Especially the role of ligands and the influence of the electronic situation (charge, oxidation state) should be studied. Also a closer look at the key step of the new hydrosilylation mechanism, the concerted addition of an alkene across a Si-H bond of the silylene complex, should be done. Here not only non-polar compounds should be studied but also the reaction with polar compounds (aldehyds, isocyanates, nitriles, ) with silylene complexes might be interesting.