Silyllithium compound

Kawakami et al. have prepared optically active bifunctional l,3-dimethyl-l,3-diphenyldisiloxanes.158,159 Strohmann et al. have prepared enantiomerically enriched Si-centered silyllithium compounds, which react stereo-specifically with triorganochlorosilanes.160-162 In solution, slow racemization of the silyllithium compounds takes place, which, however, can be circumvented by transmetallation with MgBr2. Oestreich et al. prepared new Si-centered cyclic silanes adopting the strategies developed by Corriu and Sommer.163 Bienz et al. have developed enantioselective routes for the preparation of C-centered chiral allenylsilanes.156,164-166... 

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. 

This chapter will concentrate mainly on the chemistry of silyllithium compounds, i.e. neutral tetravalent silyl compounds bearing one or more lithium substituents at silicon. In addition, the chemistry of other organosilicon compounds containing alkaline and alkaline earth metals will be outlined in this chapter. 

Silenolates 26, i.e. silicon analogues of enolates, are formed, as shown by Ishikawa and coworkers, when sterically congested tris(trimethylsilyl)acylsilanes 25 are treated with silyllithium compounds (Scheme 10)87a b c The silyllithium reagent does not add to the C=0 bond but exclusively cleaves a Si—Si bond, yielding the corresponding silenolate 26. 

Amino-substituted silyllithium compounds have been prepared by Tamao and coworkers from the corresponding chlorosilanes by treatment with either a dispersion of lithium metal... 

Amino)silyllithium compounds can be obtained alternatively by transmetalation of stannylated aminosilanes with n-BuLi or t-BuLi (equation 41)90. 

Other alkoxy-substituted silyllithium compounds such as (i-PrO)Ph2SiLi and (MeO)Ph2SiLi have been obtained by reduction of the corresponding chlorosilanes with LDMAN at -78 °C (equation 46)45. 

The reactivity difference of the silyllithium reagents raises some questions concerning the mechanism. To clarify the mechanistic pathway, it was examined by semiempirical calculations using hydrosilanes instead of silyllithium compounds (Table 5)25. As deduced from the HOMO energies of Table 5, electron-releasing substituents favor the formation of adducts 35 and 36, which indicate that their formation should involve a radical reaction proceeding via electron transfer from the silyllithium to Cgo. 

Silyllithium compounds may also be prepared by the transmetalation reaction between silylmercury and Li. The requisite silylmercury may be prepared from the corresponding chlorosilane or hydrosilane. The latter can be applied to the synthesis of polysilanyllithium. Equation (17) is an example. ... 

Keywords benzyl halide, density functional calculation, organosilane, silyllithium compound, stereochemistry... 

Scheme 1. Known enantiomerically enriched silyllithium compounds [1-4].

The reaction of silyllithium compound (jR)-4 with benzyl bromide, however, runs with inversion of configuration at the stereogenic silicon center. A bromine-lithium exchange, by way of the ate-complex like transition state (iS)-8, gives the bromosilane (5)-9 (with retention of configuration) and the lithium alkyl 10. Then an addition-elimination reaction proceeds, involving the pentacoordinate intermediate 11 which inverts the configuration at the silicon (Scheme 5). 

The reaction of the enantiomerically enriched silyllithium compound 4 [prepared from disilane (/J)-12] with cyclopropylmethyl chloride occurs with retention of the configuration [(/ )-14], while, for cyclopropylmethyl bromide and iodide, mainly Inversion of the configuration [(S)-14] was observed. The products of the radical reaction (15) indicate a racemization at the silicon center. 

Reaction of a chlorosilane with elemental lithium, normally in an ether solvent, provides a solution of the silyllithium compound with lithium chloride as a by-product. ... 

During the past 20 years structural studies of carbon-based organolithium compounds have led to a solid understanding of a complicated field. These materials are often oligomeric, and the structures vary with the solvent. Similar studies of silyllithium compounds are only embryonic. Very little is known about the degree of aggregation of such species, for example, and it is commonly assumed that they are monomeric. 

The silicon-silicon bond in polysilanes is broken by alcoholic or aqueous alkali (the latter in a cosolvent) with the formation of Si-OR or Si-OH bonds and liberation of dihydrogen. Cleavage of the Si-Si bond by alkali metals is also observed, and was described in Section II.A for perphenylcyclosilanes. An important example is the reaction of tetrakis(trimethylsilyl)silane to give the silyllithium compound 1474 (equation 29). 

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