Silyl anions configurational stability

When all substituents are different, a chiral center is present on silicon. Enantiomeric silyl anions are generated in three ways, with retention of configuration for the configurational stability of silyllithium, see Section III-B. 

Concerning stereochemistry, reactions at silicon usually proceed with high stereoselectivity, with either retention or inversion of configuration, and only rarely with racemization. Moreover, asymmetric silyl radicals and silyl anions show significant optical stability. 

The preceding experiments provide chemical evidence for significant configurational stability of silyl anions. An NMR study of diisopropylphenylsilyllithium showed that the isopropyl methyl groups are anisochronous up to 185°C, indicating that the barrier to inversion at silicon in the silyl anion is greater than 24 kcal/mol. (30). 

The asymmetric germyl anions appear to have a higher configurational stability than their silyl analogs. This may be due to the tendency of silyl anions to react by electron-transfer processes (as illustrated in eq. [20])... 

Reactions of methyllithium or methyl Grignard reagents resulted in the formation of optically active silyl anions which were characterized after hydrolysis and reaction with allyl bromide (243). The observed overall predominant retention of configuration provides evidence for the optical stability of silyl anions. Their first preparation was reported by Sommer and Mason (245) and they appeared configurationally less stable than the chiral germyl anions (31). 

Silyl anions (silyl-alkali metal derivatives) have been prepared as described in equations (15-17). Reactions of triphenylsilyl anions with electrophiles give the expected products.Silyl anions have a considerable configurational stability (equation 64). 

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