Silyllithium compounds from Compound

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

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. ... 

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. 

D. Auer, J. Hbmig, C. Strohmann, Synthesis of a Highly Enantiomerically Enriched Silyllithium Compound, in Organosilicon Chemistry V From Molecules to Materials (Eds. N. Auner, J. Weis), Wiley-VCH, Weinheim, 2003, p. 167... 

Quantum chemical calculations allow an assessment of possible intermediates during the racemization process, since the results can be correlated with experimental observables. Starting from model system 4, it is possible to locate transition state TS-4 for the inversion at the silicon center (Fig. 1). The calculated barrier (159 kJ/mol for the inversion) is decreased drastically if the methyl groups at the silicon center are exchanged by phenyl groups, because these substituents can stabilize the transition state. These results prove once more the importance of the presence of solvated molecules in calculations in order to obtain the sufficient description of inversion processes and barriers, which can be compared with experimental results (inversion barrier for MesSi 199 kJ/mol). Nevertheless, when calculations are considered in the present literature, free silyl anions and unsolvated silyllithium compounds are still discussed as appropriate model systems [14]. 

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. 

The variety of products isolated from such reactions can be explained by the ready reaction of octaphenylcyclotetrasilane with silyllithium compounds. Thus, 1,4-dilithiooctaphenyltetrasilane formed by the initial lithium cleavage of octaphenylcyclotetrasilane is able to cleave more octaphenylcyclotetrasilane. Also, cleavage of the a,oo-dilithio compounds by lithium can increase the variety of disilanyllithium compounds, which can in turn cleave octaphenylcyclotetrasilane, leading to a wide distribution of... 

In the reaction between (chlorodimesitylsilyl)diarylgermanes and t-BuLi in THF, new germyllithium compounds, resulting from an intramolecular hydrogermolysis of an intermediate silyllithium (Scheme 6), were characterized26. The silylated germylanion was characterized by deuteriolysis and alkylation. 

Trans-l,2-bis[tris(trimethylsilyl)silyl]ethylene 5 was obtained from the reaction of tris(tri-methylsilyl)silyllithium with formic acid methyl ester [7]. The corresponding dianion 6 was obtained as the only reaction product in a fast reaction of 5 with two equivalents of potassium tert-butoxide in the presence of two equivalents of crown ether as the first example of a vinylidene-bridged oligosilyl a,co-dianion (Eq. 3), which is in good analogy with our previously synthesized alkynylidene- and alkylidene-bridged compounds [3,4, 8]. 

Since the preparation of anionic species or organosilicon compounds is largely restricted by its reaction conditions, the synthetic utility of silyl anions for the synthesis of organosilicon compounds is limited. However silyllithiums possessing one or more phenyl groups on a silicon atom are readily prepared by the reductive metallation of the corresponding halosilanes with lithium. Allylsilanes are synthesized by the reaction of allyl acetates with the cuprate prepared from such a reagent (eq (47)) [43]. 

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. 

As an analog of alkyl zincate, 5i -zincate was synthesized as early as in 1985 by Oshima and co-workers, using ZnR2 and silyllithium instead of alkyllithium (Eq. 9) [35]. In 1999, Krief and co-workers reported the synthesis of 5n-zincates from stannyllithium in a similar way [36]. Such Si- and Sn-zincates undergo silylzincation or silyl/stannyl Michael addition to unsaturated compounds. The Michael addition employing zincates has been well reviewed elsewhere, so it will not be considered in detail here [8]. Another application of S/-zincate, silylzincation, will be discussed later. 

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