Cyclopropyl carbanions

Acetylene, where the carbon is sp hybridized with 50% s character, is much more acidic than ethylene (sp, 33% s), which in turn is more acidic than ethane, with 25% 5 character. Increased s character means that the electrons are closer to the nucleus and hence of lower energy. As previously mentioned, cyclopropyl carbanions are more stable than methyl, owing to the larger amount of s character as a result of strain (see p. 181). 

The partial loss of configuration often observed during reduction of cyclopropyl halides may actually occur via the corresponding cyclopropyl radicals, which lose configuration rapidly °>. In that event, their behavior would resemble that of vinyl halides, as exemplified by the 3-iodo-3-hexenes 16>. Occasional cases of partial inversion could be associated with shielding of the cyclopropyl carbanion by the electrode surface, with concomitant protonation on the other face of the carbanion S8>. 

Cyclopropyl carbanions are capable of maintaining their configuration whereas the CT-radical has been shown to reach inversion equlibrium with a rate constant of lO" s". ITie cyclopropyl bromide 13, and the corresponding iodide, are reduced in a single two-electron polarographic wave and the S +)-isomer yields the R(-)-hydrocarbon with 26% enantiomeric excess [67, 68]. Such a substantial retention of configuration during reduction of the carbon-bromine bond indicates a very fast second electron transfer process. Results from reduction of the cyclopropyl bro-... 

It has been reported that gem-dihalocyclopropanes, usually undergo electrochemical or metallic reduction with predominant retention of configuration as shown in Scheme 34. 124,126) The stereoselectivity of these reactions may result from the intermediacy of a cyclopropyl carbanion which is unusually stable to racemization compared to other alkyl carbanions. 125>... 

Cyclopropyl carbanions. Attempts to deprotonate cyclopropanes substituted by a electronegative group result in dimerization or trimerization. The desired carbanions can be generated in situ by desilylation of an a-(trimethylsilyl)cyclopropane derivative with BU4NF or C(,H,CH,N(CH,),F and used to effect aldol condensations. 

The cyclopropyl carbanion that presumably is involved in this reaction retains its configuration, probably because inversion causes increased ring strain (see Section VI). 

Table 3 Solvent and Temperature Effect on the Stereochemistry of Cyclopropyl Carbanion...

To explain these results, the authors suggest that the incipient carbanion in the transition state is stabilized by donation of its electrons into the carbon-oxygen o orbital, i.e., that it is a kinetic anomeric effect [29]. A related anomeric stabilization of a sulfonyl carbanion was proposed by Padwa and Wanamaker [30] to explain a strong preference for a syn-peiiplanar arrangement in methoxy-substituted cyclopropyl carbanion. 

Padwa A, Wanamaker MW (1986) Significance of the anomeric effect on the configurational stability of cyclopropyl carbanions. Tetrahedron Lett 27 2555-2558... 

Retention of configuration by a chiral cyclopropyl carbanion. (Adapted from reference 347.)... 

The softness of a carbanion is related to the hybridization of the carbon. Higher p character increases the softness sp > sp > sp) and configurational instability of the carbanion. Reactions involving carbanions formally derived from alkanes often lead to racemization, whereas configuration is retained at the alkenic carbanion center. The fact that cyclopropyl carbanions maintain their original stereochemistry (14) concurs with the above notion, as the s char-... 

Whereas the methyl-substituted cyclopropyl carbanion (861 R = Me) maintains its configuration, the cyanato and acetylenic derivatives (861 R = CNO or C=CR) racemize, an sp carbon atom evidently lowering the barrier to inversion. The effect of an sp nitrogen atom has now been assessed by preparation of the isonitrile (861 R = NC) from an optically active precursor. Its methylation below — 50°C gives (862) in 98 % optical purity. 

Explain why the neutral aziridine ring system is isoelectronic with the cyclopropyl carbanion. 

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