Silyllithium compounds reactions with

Scheme 6. Reactions of the enantiomerically enriched silyllithium compound 4 with cyclopropylmethyl halides.

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. 

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. ... 

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). 

An enlargement of the silicon skeleton is possible by coupling reactions with lithium metal or silyllithium reagents [4-6]. The homocoupling of tris(diethylamino)-l-chloro-l,2-dimethyldisilane results in the linear hexakis(diethylamino)-l,2,3,4-tetramethyltetrasilane [7]. After treatment with HCl the hexachloro compound could be obtained. By the reaction of aminochlorophenylsilanes with... 

In the reaction of 1 with LiN[(Ar)SiMe3] (Ar = 2,6-Me2C6H3) it was possible to isolate the silyllithium compound 9 ( Si NMR at 8 15.2 and -4.5) (Scheme 4), which is related to the intermediate A in Scheme 3. Compound 9 was not thermally stable. Heating a CeDg solution of 9 for several hours, converted the silyllithium compound into its corresponding lithium amide 10, as shown by H NMR spectra. 

Silyllithium compounds are useful and important reagents for silyl group transfer reactions to organic molecules or organometallic systems [1]. lithiated silanes can be prepared by reaction of lithium metal with chlorosilanes or disilanes. The latter method is limited to systems bearing at least one aryl group [2]. 

Our approach for starting material suitable as a silyl dianion synthon (D) was via the bis(diphenylmethyl)-substituted silane 15. It was selectively cleaved to give the silyllithium compound 16, which resulted, after a trapping reaction with chlorotrimethylsilane, in disilane 17 (Scheme 3 yield of 17 80 %). The isolated and purified disilane 17 then was cleaved at the Si-C bond with lithium metal, resulting in the lithiated silane 18. Another trapping reaction with chlorotrimethylsilane gave the trisilane 19 (yield of 19 78 %). 

Furthermore functionalized and chiral diphenylmethyl-substituted compounds, like rac-20, are potential precursors for enantiomerically enriched silyllithium compounds [7, 8]. They can be transformed selectively and in good yields to systems of type rac-21. A trapping reaction with chlorotrimethylsilane resulted in the products rac-22 (yield of rac-22 89 %) and 14 (Scheme 4). 

Summary The highly enantiomerically enriched silyllithium compound lithiomethylphenyl(l-piperidinylmethyl)silane (2) reacts stereospedfically with chlorosilanes, but over a period of several hours slow racemization in solution at room temperature occurs, which can be supressed by a metathesis reaction with [Mg(thf)4]Br2. Quantum chemical calculations of solvated model systems allow an assessment of possible intermediates during the racemization process. 

Recent experiments revealed dependence between the rate of the racemization process and the concentration of the silyllithium compound 2 in solution. Due to decomposition the reaction rate cannot be determined exactly, but it is not simply first or second order. We believe that a solvated lithium cation plays an important role in the inversion process of 2. Thus, this process can be described by the interaction of the solvated lithium cation with compound 2 (model system 4). 

We were able to prove that it is possible to synthesize the highly enantiomerically enriched silyllithium compound 2 (ee > 98 %) in large amounts and to perform stereospecific reactions with chlorosilanes. Due to the observed racemization of 2 in solution, we believe that a thorough reassessment of previous studies concerning optically active silyllithium species is in order. 

The resulting dark green solution was converted into the expected 1-methyl-1-(trimethylsilyl)-l-silafluorene by reaction with an excess of MesSiCl. The Si NMR chemical shift for 128 (S = —22.09 ppm) is in the range of aryl-substituted silyllithium compounds, e.g. Ph2MeSi Li+, which have no delocalization of the negative charge on silicon to the phenyl substituents. Downfield shifts of the ring carbons are also consistent with a localized silyl anion. 

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. 

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