Silyllithium derivatives

The question whether organolithium compounds are essentially ionic, covalent, or intermediate has been vigorously debated [17] and similar considerations clearly apply to silyllithium derivatives. 

Optically active silyllithium derivatives will make it possible to synthesize silicon-carbon bonds in which asymmetry is introduced at both silicon and carbon centers, which cannot be achieved by the substitution reaction by a carbanion at the asymmetric silicon center. The disilane and silylstannane derivatives were obtained by the reaction of chlorosilane with silyllithium or stannyllithium as shown in Scheme 4.4. 

Tetraalkyl- or tetrasilyltetragallium(I) compounds were also obtained by the reactions of the dioxane adducts of Ga2X4 (X = Cl, Br) with bulky alkyl- or silyllithium compounds [Eq. (5)], which were accompanied by disproportionation of Ga(+2) to Ga(+1) and Ga(+3) [44, 45], In particular the yield of the alkyl derivative 21 was very poor and several unknown byproducts were detected by NMR spectroscopy. Furthermore, the reaction requires the employment of a solvent-free lithium compound, which is not readily available. The reaction of tris(trimethylsilyl)silyl lithium yielded the expected product of the disproportionation [(Me3Si)3Si]2GaCl2Li-(THF)2 besides compound 11. 

There is also a series of anions of the type ERj. The best studied are the silyl anionsespecially the silyllithiums. For example, Si(SiMe3)3 is a versatile reagent for the synthesis of a great variety of polysilyl derivatives. It can be prepared by the reaction of Si(SiMe3)4 and methyllithium97 according to... 

Tris(trimethylsilyl)silyllithium, (Me3Si)3SiLi, and its derivatives that contain branched silicon chains are the most extensively investigated species among polysilanyllithiums. 

Caution. Phosphines, and their chloro-substituted derivatives, are toxic and air-sensitive. Silyllithium phosphides, (naphthalene )sodium, and alkyl-lithium reagents are very sensitive to oxygen and moisture. All operations involving these materials should be carried out under an inert atmosphere and in a well-ventilated fume hood. 

Caution. Phosphines and their chloro-substituted derivatives are toxic and air sensitive. Silyllithium phosphides, (naphthalene)sodium, and alkyllithium... 

The Si-Ph bond in the disilabicyclo[4.4.0]decane derivative 58 can be cleaved by metallic lithium to give a silyllithium reagent which retains its Si-Si bond largely intact. This can then be quenched with halosilanes to give polysilyl species 59 and 60 (Scheme 20) <2001CHE1369>. 

The preparation of a variety of higher polysilanes using silylmetallic reagents has been described (4, 5, 14). More recently, silylmetallics have been successfully employed in the synthesis of branched polysilanes. Treatment of silicochloroform with either triphenyl- or dimethylphenyl-silyllithium gives the corresponding tris derivatives. 

Summary The synthesis of derivatives of the type [(Me3Si)3Si]2CHR (R = OH, OCH3, 0C(0)H, 0C(0)Me, NMe ) proved to be surprisingly facile and was achieved by the reaction of tris(trimethylsilyl)silyllithium (1) with suitable Ci building blocks. The results of the X-ray analyses and reactions of these di(hypersilyl) compounds are described in this paper. 

Reactions of a silyllithium reagent with carboxylic acid derivatives, including esters, nitriles, acid chlorides, and thioamides, have been extensively investigated by Fleming. He found that the reaction of acid chlorides with 2 equiv. of PhMe2SiLi affords 1,1-disilylalkoxide 80, in... 

Other unsaturated organometallics such as those derived from sodium, lithium, aluminum, " gallium, indium, " zinc, silver, and silyllithium readily give cyclized compounds. Although the cyclization is easier in the Cy5/Cy6 case and gives the (Cy5) compound, a concerted rather than an homolytic pathway is probably involved. Nevertheless, it must be noted that l-methyl-5-hexenylaluminum gives the 1,2-dimethylcyclopentane with a cis/trans ratio of 2.9 1, similar to the free radical cyclization ratio. 

As might be expected in such types, the compounds tend to be high-melting, with high volatility, and enhanced thermal stability. A novel and very reactive type directly derivable from the parent type, is tris(trimethyl-silyl)silyllithium. 

Organosilicon derivatives of fullerenes TABLE 4. Reactions of silyllithium reagents with Cgo... 

Chemically, Compound C is quite inert. In addition to Kipping s observations, it was also found to be unreactive with bromine in refluxing benzene, mercuric chloride in benzene or tetrahydrofuran, and methyl-lithium in tetrahydrofuran-ether mixtures, all of which react readily with octaphenylcyclotetrasilane. Compound C has been found to react with lithium in tetrahydrofuran to give a mixture of silyllithium compounds, of which the 1,2- and 1,3-derivatives have been identified by means of their... 

Octaphenylcyclotetrasilane reacts readily with lithium metal in tetra-hydrofuran to give a mixture of silyllithium compounds. No starting material is usually recovered however, treatment of such solutions with trimethyl phosphate provided only 27% of 1,4-dimethyloctaphenyl-tetrasilane (34), indicating a fairly low yield of 1,4-dilithiooctaphenyl-tetrasilane. The 1,4-dimethyl derivative (34) was also obtained from the reaction of methyldiphenylsilyllithium with 1,2-dichlorotetraphenyldi-silane this reaction provided a structure proof for the dimethyl compound. 

On the other hand, silyllithium compounds have so far proved to be useful organosilicon reagents that can act as nucleophiles for introducing a silyl group into a wide range of organic structures (3). However, examples of organosilicon compoimds with an interaction between alkali metals and tu electrons are limited. Examples of such compounds include the silacyclopentadienide derivatives (4) and lithium cyclotetrasilenide (5). 

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