Metal-silyl bonds

The alkene inserts either in the metal hydride bond or in the metal silyl bond. The latter reaction leads to alkenylsilyl side products and also alkane formation may occur. Similar reactions have been observed for hydroboration, the addition of R2BH to alkenes. (R2 may be the catechol dianion). 

The X-ray structure of 83 was in accord with the prediction of Scheme 4 in that the Si-Cl bond lies in the niobocene bisecting plane trans to the hydride and is elongated compared with classical complexes of the type [M(SiR2Cl)L ]. There was, however, no good reference system that would allow for the comparison of the Nb-Si bond lengths. The metal-silyl bond lengths are strongly affected by Bent s... 

Insertion of unsaturated molecules into a transition metal-silyl bond has been suggested for the catalytic reactions related to hydrosilylation and silylcarbonylation. However, there is little direct evidence supporting such a process for unsaturated molecules to insert into a metal-silyl bond in organometallic complexes. " Thus, the fact that 108 is readily derived from 11 and 13 demonstrates the participation of this process in the catalytic cycle of silylformylation. 

The reaction mechanism commonly accepted to account for the double silylation of unsaturated substrates involves three key steps. First, the disli-lane undergoes oxidative addition to the metal center, forming a transition metal-bis(silyl) complex. The unsaturated moiety inserts into the metal-silyl bond, followed by Si-C reductive elimination to give the double sily-... 

A mechanistic rationale for the apparent trans -addition in some of these processes is shown in Scheme 6. Initial insertion of the alkyne into the metal-silyl bond yields an intermediate with an unfavorable interaction between the metal and the silyl group. Coordination of the vinyl part to the metal gives formally a metaUacyclopropene. 

The mechanistic scheme presents the conventional oxidative addition— reductive elimination steps to explain the hydrosilylation. The oxidative addition of trisubstituted silanes HSiRs to a metal alkene complex (usually with d and d ° configuration) is followed by migratory insertion of alkene into the M—H bond, and the resulting metal(silyl)-(alkyl) complex undergoes reductive elimination by the Si—C bond formation and regeneration of metal alkene complex in excess of alkene. As the facile reductive elimination of silylalkane from [alkenyl-M]-SiRs species has not been well established in stoichiometric reaction, a modified Chalk-Harrod mechanism has been proposed to explain the formation of unsaturated (vinylsilane) organosilicon product, involving the alkene insertion into the metal-silyl bond followed by C—H reductive elimination (Scheme 2) (38). 

There are many metal-silyl complexes because of the robustness of the metal-silyl bond. - Several reasons explain this stability ... 

Basically the same methods known from the synthesis of classical metal-silyl complexes can also be applied to the preparation of low valent Si compounds. The procedures given here are summarized with the focus on silylene complexes These are a) reactions of appropriate metal anions with halosilanes, which are the most important methods for the formation of M-Si bonds. Alternatively, silyl... 

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

In addition to routine spectroscopic characterization, transition metal silyls have been examined by a variety of physical methods, principally to determine (1) the definite presence of an Si—M bond, (2) the manner in which such a bond is influenced by other ligands, (3) whether such a bond possesses any w-component, and (4) the trans influence of the silyl ligand. 

The above-mentioned results indicate the additive effect of protons. Actually, a catalytic process is formed by protonation of the metal-oxygen bond instead of silylation. 2,6-Lutidine hydrochloride or 2,4,6-collidine hydrochloride serves as a proton source in the Cp2TiCl2-catalyzed pinacol coupling of aromatic aldehydes in the presence of Mn as the stoichiometric reduc-tant [30]. Considering the pKa values, pyridinium hydrochlorides are likely to be an appropriate proton source. Protonation of the titanium-bound oxygen atom permits regeneration of the active catalyst. High diastereoselectivity is attained by this fast protonation. Furthermore, pyridine derivatives can be recovered simply by acid-base extraction or distillation. 

The preparative potential of silyl- or stannyl-substituted polynuclear complexes is currently far from being exploited. Due to the reactivity of silyl ligands, selective cleavage of metal-silicon bonds is possible. In some cases this was observed during the reaction of an anionic silyl complex with metal... 

---