Silyllithium optically active

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... 

The formation of an asymmetric silyllithium reagent by lithium cleavage of the silicon-silicon bond of an optically active disilane 10 (eq. [7]) has been reported (26). Hydrolysis of the silyllithium reagent 11 yielded an optically active silicon hydride. This result demonstrates that silyllithium has considerably enhanced optical stability relative to acyclic alkyllithium compounds. 

The formation of an asymmetric silyllithium reagent from an optically active disilane has been reported92 (equation 25). 

The formation of chiral silyllithium and silyl-Grignard reagents was also observed in the reactions of an optically active silyl-cobalt carbonyl complex93,94. Treatment of the chiral cobalt complex with methyllithium produced optically active silyllithium which hydrolysed to the corresponding hydrosilane (equation 26). 

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. 

The formation of silyllithium is a retention process. The optical purity was kept higher than 85% ee at —78°C even after 5 h. Such optically active silyllithium can be used to synthesize various optically active oligosilanes. The reaction of (5)-chloro-,... 

Scheme 4.4. Synthesis of optically active silyllithium from optically active 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. 

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