Halides metal silyls

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]. 

Salt Elimination Reaction of an Alkali Metal Silyl with a Transition Metal Halide... 

Strictly speaking, a catalyst is some species directly involved in the catalytic cycle and, in the reactions discussed here, these species are usually low-valent, coordinatively unsaturated transition metal complexes. Metal halides, e-.g., chloroplatinic acid, PdCl, etc., although often claimed as catalysts are more properly catalyst precursors, since in the presence of silyl hydrides the metal halides are reduced. If no stabilizing ligands, e.g., olefins, phosphines, etc. are present, the reduction normally proceeds to a finely divided form of the metal or to insoluble metal silyl/hydride clusters which may act as heterogeneous catalysts. 

Trialkylsilyl- or stannyl-substituted carbenes 2 and 3 can be generated from the title compounds by a-elimination of hydrogen halide (HX) or metal halide, and silyl- or stannylcyclopropanes can be formed by intra- or intermolecular carbene transfer to C C bonds. To this end, 1-haloalkyl(trialkyl)metal compounds can be treated with sodium (Method A, metal halide exchange followed by base-induced a-elimination of HX) or with a strong base (Method B, a-elimination of HX), and l,l-dihaloalkyl(trialkyl)metal compounds can be treated with an al-kyllithium compound (Method C, metal/halide exchange and subsequent a-elimination of a metal halide). 

Use of main-group metal silyls to prepare transition-metal silyls appears to be a generally applicable method that is primarily limited by the availability of suitable starting materials, since these silyl anion reagents are sometimes rather difficult to obtain. Typically an alkali-metal silyl is generated in solution and then treated with the appropriate transition-metal halide. Displacement of halide by the silyl anion, with salt elimination, then leads to product (equations 17-19)46 49. The lithium silyl in equation... 

Early transition-metal silyl compounds have received much less attention, largely due to the fact that general, straightforward synthetic routes have not been available. Syntheses based on oxidative additions are less applicable, given the more electropositive character of the early metals. Particularly for d° silyl complexes, most syntheses are based on nucleophilic displacement of halide by a silyl anion reagent, usually an alkali metal derivative. 

Halide ligands in early transition-metal silyl complexes are readily displaced with nucleophilic reagents (equation 39) to introduce a variety of functionalities cis to the silyl... 

Beryllium-silicon bonds are a recent development. They are likewise prepared by the reaction of the metal halide with alkali metal silyls, but unlike the Mg analogues, both of the well-characterized Be examples are base-free. While [(te/T-Bu3Si)BeCl] is oligomeric, the molecular structure of (tert-Bu3Si)2Be features a perfectly linear Si-Be-Si unit. CpBe(SiMe3), prepared from CpBeCl and trimethylsilyllithium in pentane, exhibits an intriguing thermal chemistry. The identity of the primary thermolysis products, Scheme 14, implies the participation of silylene intermediates, a contention supported by DFT/NBO (density functional theory/natural bond order) analysis. ... 

Besides the metallocene group 4 metal silyl compounds already discussed, substances which do not contain the cyclopentadienyl ligand are also of interest [7]. Remaricably, the seemingly most straightforward synthetic route, namely the reaction of silyl anions with group 4 tetrahalides, has not been reported so far. The reason for this seems to be connected to the highly Lewis acidic character of the metal halides, which causes ether cleavage and other side reactions. 

Silylene complexes have been prepared by coordination of free silylene, extrusion of dihydrogen from a dihydrosilane, a-hydrogen elimination from a hydrosilyl complex, extraction of hydride, halide, or pseudo-haUde from a metal-silyl complex, and photoextrusion of silylene from a disilanyl complex. Examples of these reactions are shown in Equations 13.39-13.45. 

The chemistry of heavier oligosilanyl chalcogen derivatives was studied in much detail by Herzog and coworkers [335-353]. Their major strategies to obtain these compounds were either reactions of silyl halides with the respective REH (E = S, Se, Te) species in the presence of base or, alternatively, reaction of alkali metal silyls with elemental chalcogenes, which causes an insertion reaction of the respective chalcogen into the Si-metal bond (Scheme 32). 

Variations and Improvements on Alkylations of Chiral OxazoUnes Metalated chiral oxazolines can be trapped with a variety of different electrophiles including alkyl halides, aldehydes,and epoxides to afford useful products. For example, treatment of oxazoline 20 with -BuLi followed by addition of ethylene oxide and chlorotrimethylsilane yields silyl ether 21. A second metalation/alkylation followed by acidic hydrolysis provides chiral lactone 22 in 54% yield and 86% ee. A similar... 

Reactions of Silyl Anions with Metal Halides... 

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