Carbanions tetrahedral configuration

While a planar configuration characterizes the last monomeric unit of a polymeric chain growing by a radical or carbonium ion mechanism, a tetrahedral configuration should be attributed to the end of a growing polymeric carbanion. Hence an isotactic or a... 

The tetrahedral configuration is further stabilized if the gegen ion is close to the carbanion. 

Many such examples are known. In most cases where the stereochemistry has been investigated, retention of configuration is observed,225 but stereoconvergence (the same product mixture from an E or Z substrate) has also been observed,226 especially where the carbanionic carbon bears two electron-withdrawing groups. It is not immediately apparent why the tetrahedral mechanism should lead to retention, but this behavior has been ascribed, on the basis of molecular orbital calculations, to hyperconjugation involving the carbanionic electron pair and the substituents on the adjacent carbon.227... 

The tetrahedral carbanion agrees well with the experimental results that optically active cyclopropyl-nitrile carbanions can undergo reprotonation with retention of configuration under certain conditions. 

The factors that affect the rate of the reaction are easily predictable, namely ones that are similar to those that affect the rate of an SN1 reaction. However, the stereochemical consequences of this pathway are slightly more difficult to predict. The simple carbanion would be expected to adopt a tetrahedral conformation, which is similar to that which a nitrogen atom adopts in a tertiary amine. Consider whether tertiary amines may be resolved into their enantiomers, and then by analogy, suggest what would be the stereochemical consequences of the SE1 mechanism on the configuration at the substituted centre. Hence, suggest what would be the structure of a simple carbanion intermediate. 

Carbanions are tetrahedral, and inversion of configuration can be expected to be easy—since the LUMOs, the cr CH orbitals, are high in energy, the HOMO-LUMO gap is not small as it is in a phosphine. However, carbanions in the form of compounds containing C—Li bonds can often be configurationally stable by virtue of the <7 bond, which would have to be broken to allow inversion. 

An aldol reaction can be completed enantioselectively with an a-carbanion of the chiral carbonyl compound reacting as chiral enolate. Since the a-C atom in chiral enolate is planar, alkylation is diastereoselective because of the recognition of diastereotopic faces by the carbonyl counterpart. Alkylation of the configurationally stable tetrahedral carbanion on the a-C atom is a relatively rare case and is known as the memory of chirality. 

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