Methyl silyllithium

Tris(trimethylsilyl)silyllithium, which is readily formed upon treatment of tetrakis(trimethylsilyl)silane with methyllithium or triphenylsilyllithium in ether-tetrahydrofuran (63), is useful as the reagent for construction of molecules of an interesting class of highly sterically hindered methyl-polysilanes. The synthesis of hexakis(trimethylsilyl)disilane gives an example (59). 

Preparation of the branched silane began with the reaction of tris(trimethylsilyl)silane with CHC13 (CC14) and MeLi to afford the peralkylated methyl[tris(trimethylsilyl)]-silane. Subsequent treatment with A1C13 and ClSiMe3 gave the trichlorosilane, which was then reacted with tris(trimethylsilyl)silyllithium yielding the final silane dendrimer. 

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]. 

Methoxy-bis[tris(trimethylsilyl)silyl]methane (4) - the first compound bearing two hypersilyl groups at a carbon atom - was synthesized by the reaction of tris(trimethylsilyl)silyllithium (1) with dichloromethyl methyl ether in a yield of 35 % In view of the extreme bulkiness of the two hypersilyl substituents, the ease of the formation of 4 is really surprising. But the reaction pathway is easily understood as a consecutive replacement of the two chlorine atoms of the dichloromethyl methyl ether by the silanide 1 (Eq. 1). 

The intermediate formation of chiral silyllithium and silyl Grignard reagents has also been observed in the reaction of methyllithium (eq. [8]) and methyl-magnesium bromide (eq. [9])... 

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]. 

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. 

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. 

The chiral silyllithium reagent methyl(naphthyl)(phenyl)silyl-lithium is configurationally stable at and below room temperature, and reacts with methyl(naphthyl)(phenyl)silyl chloride with retention at the nucleophilic silyl group and inversion at the electrophilic silyl group... 

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... 

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