Silyl effects Carbanion stabilization

It has been pointed out previously that silylation of ylides leads to stabilized products and that this is only one example of the very general phenomenon of carbanion stabilization through silicon (34, 61, 72). This effect was also found for arsenic ylides (34, 73), and is the basis for the preparation of other compounds of this series. The influence of silicon is by no means solely an electronic effect. In many cases, where alkylsilyl substituents are introduced, a steric effect may well dominate, which may reduce lattice energies for salts in transylidation reactions, preventing intermolecular contacts in decomposition processes, and rendering the formation of salt adducts unfavorable. This steric effect is reduced to a minimum, but not eliminated, if simple SiH3 groups are employed (61). Even then, however, a pronounced silicon effect is found, which must be based on electronic influences (49, 60, 61). 

Keywords Carbanion Stabilization / Phosphorus Ylides / Structure Determinations / Silyl Group Effects... 

Generally, in organic chemistry the stabilizing effects of silyl- and phosphino-substituents on carbanions are well-known. From electron affinity and gas-phase measurements it is concluded that a-silyl-substituents stabilize carbanionic species by about 20 kcal/mol [4, 5] compaied to simple carbanions. Furthermore, the electron affinity of doubly a-silyl-substituted carbanions is additionally increased by about 16 kcal/mol relative to the singly substituted compound [6]. In conclusion, the stabilizing effect of two a-silyl substituents to a carbanionic center is about 36 kcal/mol as compared with simple carbanions. 

Another common a-silyl anion is produced by die halogen exchange from a methyl (but not odier group) attached to silicon. Odier a-silyl carbanions can be generated by other processes. Such anions lack the resonance stabilization of an ester group seen in the previous example. They are consequently less stable and must be generated under carefully controlled conditions. They are good nucleophiles and add effectively to aldehydes and ketones. 

Generally, the direct metallation of unactivated alkyl groups is not a synthetically useful reaction. However, under certain circumstances unactivated a-silylcarbanion formation has been reported. Treatment of orf/zo-silylated benzamides 149 with LDA gave an a-silyl carbanion 150, stabilized by a complex-induced proximity effect, which then underwent... 

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