Halides silyl anions

Reactions of Silyl Anions with Metal Halides... 

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]. 

An extensive study (6) of Sn displacement reactions of allyl halides using silyl anions/anionoids has provided the following regioselective alternatives ... 

Germyl, Stannyl, and Plumbyl Anions The preparative methods for the synthesis of the germyl, stannyl, and plumbyl anions are essentially the same as those mentioned above for the silyl anions. The most widely used methods are (1) reduction of halides R3EX (R = alkyl, aryl E = Ge, Sn, Pb X = Cl, Br) with alkali metals and (2) reductive cleavage of the E-E bond of R3E-ER3 (R = alkyl, aryl E = Ge, Sn, Pb) with alkali metals or organolithium reagents. Due to the favorable polarization of the (E = Ge, Sn, Pb) bond, the direct metalation... 

Vinylsilanes TASF is superior to Bu4NF for cleavage of (CH3)3Si-—Si(CH3)3 to a silyl anion species that reacts with vinyl halides in the presence of a Pd(0)... 

The first evidence for the existence of short-lived (alkoxy)silyl anions (e.g. 32) was obtained by Watanabe and coworkers in the reaction of NaOMe with the corresponding disilanes (31), which yielded in the presence of alkyl halides the expected trapping product of the intermediate (alkoxy)silyl sodium compounds 32 (equation 43)93. 

Silyl complexes also result from the reaction between a silyl anion and a metal halide, for example, in the formation of (26), which has an Si-H stretching frequency of 2103 cm (equation 41). ... 

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. 

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. 

There are numerous ways of forming bonds between two group 14 atoms. An important one, discussed above, involves the nucleophilic attack of a silyl anion on a silyl halide. Another method, called hydrostannolysis, involves condensation of a Sn-H bond and a Sn-NMe2 linkage, with the elimination of HNMe2. Th following reaction leads to a branched Sn4... 

Although the reduction potentials of aUcenes are too high to permit reduction, it is possible to reduce polycyclic aromatic hydrocarbons and alkenes bearing groups that can stabilize anions in aprotic media to afford the corresponding anion radicals and dianions. These species are good nucleophiles and can be captured by added electrophiles such as protons, alkyl halides, silyl halides, and carbon dioxide (Fig. 7) [11],... 

A disadvantage is that solutions of silyl anions prepared from the halosilanes by one of these methods inevitably contain metal halides as by-product. Salt-free perarylated silyl anions can be prepared by the reaction of a disilane with lithium, sodium or potassium in polar solvents such as THF or DME (Table i)28b,29 Alternative procedures for the preparation of halide-free Ph3SiK utilize the reductive cleavage of the Si—C bond of Ph3Si—CMe2Ph by a sodium-potassium alloy in Et20 (equation 17), as well as via the reaction of PhsSiH with KH in DME . 

Methods to attach oligosilanyl unity to other organic or inorganic molecular entities are mostly confined to reactions of silyl anions or halides depending on the nucleophilic or electrophilic nature of the other fragment. Especially for addition reactions to unsaturated molecules also silyl radicals or silyl hydrides are common reagents. 

Formation of silyl hydrides is mainly achieved either by hydrolysis of silyl anions or be the treatment of silyl halides with hydride donors (especially L1A1H4 and DIBALH are well suited). Also addition reactions of HX to unsamrated silicon compounds such as disilenes or silylenes can lead to Si-H bond formation. 

Oligosilanyl halides are also very abundant and their treatment as a separate compound class would thus be too exhaustive. Silyl halides are of major importance for the generation of numerous silicon species. By reaction with nucleophiles including lithium alkyls, silyl anions, amines, amides, alcohols, and alkoxides, Si-X bonds are easily formed. They also serve as starting materials for Si-Si bond formation by Wurtz coupling or for the generation of unsaturated compounds such as disilenes, disilynes, or silylenes. 

Recent years have seen a remarkable development of the preparation, isolation, and characterization of stable silyl radicals. Among these (Me3Si)3Si has gained particular prominence as a popular reagent in organic synthesis [20-26]. In this connection related radicals have been studied to some extent. Following seminal contributions by Matsumoto and coworkers [363-365] key achievements in this development have been made by Sekiguchi and coworkers, which were reviewed only recently [8, 366, 367]. Most stable silyl radicals are trisilylated ones and are commonly prepared either from the respective hydrosilanes, from silyl mercury compounds, by oxidation of silyl anions, or by reduction of silyl halides (Scheme 33) [8]. 

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