Cyclopropyl anions configurational stability

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-... 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

Merle A. Battiste and James M. Coxon TABLE 3. Configurational stability of some cyclopropyl anions ... 

No thermal ring openings are observed for conventional cyclopropyl anions, even if they carry two phenyl groups to provide stabilization of the potential terminus of the allyl anion. CH acidic optically active precursors of cyclopropyl anions exhibit a very high degree of retention of configuration when treated with bases in polar protic solvents and in most cases in aprotic solvents, too. - ... 

A similar situation is observed in cyclopropyl(l-lithiocyclopropyl)acetylene (298). The compound has a pyramidal configuration at the anionic carbon atom but isomerization occurs easily at a rate which is dependent on the composition of the benzene/THF mixture. In the case of 297 the phenyl substituent should stabilize the negative charge mostly by delocalization while the acetylenic group in 298 should operate more inductively. 

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