Silyllithium reaction with chlorosilane

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. 

Reaction of Silyllithium with Chlorosilane Halogen-metal exchange and Si-Si Bond Cleavage Reaction... 

It has been reported that in reactions of silyllithium with chlorosilane, the products depend on the mode of addition (26,27), as shown by the following two examples Whereas addition of silyllithium to chlorosilane gives the desired cross-coupled disilane [Eq. (12)], addition of chlorosilane to silyllithium forms a mixture of homo-coupled disilanes [Eq. (13)]. 

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. ... 

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. 

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]. 

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. 

Reaction of a chlorosilane with sodium or potassium. (More severe reaction conditions are often required for silylsodium and silylpotas-sium than for silyllithium.)... 

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