Trigonal-pyramidal geometry, carbanions

Eor an sp hybridized carbanion, one expects that it will prefer a structure where the central carbon adopts a trigonal-pyramidal geometry with a lone-pair occupying... 

In alkyl anions R3C , n = 3 and m 1. The substituents lie in one plane, and the central atom lies above it. The carbanion center has a trigonal pyramidal geometry. The bond angles are similar to the tetrahedral angle (109° 28 ). The geometry can be considered to be tetrahedral if the lone pair is considered to be a substituent. 

With the molecular geometry for a carbanion described as a trigonal pyramid the question is whether or not carbanions can display chirahty. After all, when the activation barrier for inversion of this geometry is too low any attempt at introducing chirality will end in racemization. However, solid evidence exists that carbanions can indeed be chiral, for example, in research carried out with certain organoUthium compounds. 

Most alkyl carbanions undergo facile pyramidal inversion. Cyclopropyl anions are an exception, presumably because the transition state, with a planar trigonal carbon, is more strained than the ground state. The configurational stability of cyclopropyl anions is of value in the synthesis of deuterated cyclopropanes by the Haller-Bauer reaction (see Section II.B). An interesting dilemma arises when a cyclopropyl anion is stabilized by a n-electron acceptor substituent such as a nitrile or an ester. Will the anion then retain its pyramidal equilibrium geometry for the strain reasons alluded to above, or will it become planar in order to maximize overlap of the filled orbital on carbon with the n orbital of the substituent Walborsky and coworkers addressed this question in a series of experiments in which rates of H/D exchange and racemization were compared for an optically active cyclopropane exposed to a base in a deuterated hydroxylic solvent. The outcome can be illustrated with the particular example of 1,1-diphenylcyclopropane-2-... 

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