Planar carbanion

Ellison, G.B. Engelking, PC. Lineberger, W.C. J. Am. Chem. Soc., 1978, 100, 2556. Retention of configuration has never been observed with simple carbanions. Cram has obtained retention with carbanions stabilized by resonance. However, these carbanions are known to be planar or nearly planar, and retention was caused by asymmetric solvation of the planar carbanions (see p. 764). 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

However, even planar carbanions need not give racemization. Cram found that retention and even inversion can occur in the alkoxide cleavage reaction (12-39) ... 

Given that fluorine has a destabilizing influence upon planar carbanions, geometry considerations favor the formation of pyramidal anions.69 71 The influence of /(-fluorination on carbanion stability has been described in a variety of ways. In early work the role of negative hyperconjugation was proposed to account for these effects.72... 

Ab initio study of the potential energy surface for P—C bond rotation in the 2-oxo-and 2-thioxo-2-methyl-l,3,2-diazaphosphorinane and -1,3,2-diazaphosphosolidine anions has established that in each case the lowest energy six-membered ring structure features a near planar carbanion with its substituent parallel to the P=X axis (X = 0, S).78... 

Moreover, low temperature NMR spectra indicate a pyramidal structure of the carbanion in triphenylphosphonium cyclopropylid 6, and an X-ray diffraction analysis of the crystalline solid confirms this result (14). Contrary to earlier predictions, cyclo-propylids are thus found to be the first class of ylids to contain non-planar carbanions with an unusual ylidic bonding (Fig.2). They form stable metal complexes, as illustrated by a gold complex (7) +... 

Racemization is the result of heat and alkaline treatment of food proteins. The amino acids present in proteins are of the L-series. The racemization reaction starts with the abstraction of an a-proton from an amino acid residue to give a negatively charged planar carbanion. When a proton is added back to this optically inactive intermediate, either a D- or L-enantiomer may be formed (Masters and Friedman 1980). Racemization leads to reduced digestibility and protein quality. 

Racemization is thought to proceed by abstraction of the ct-proton from an amino acid or amino acid residue in a peptide or protein to give a negatively charged planar carbanion (11 Figure 1). A proton can then be added back to either side of this optically inactive intermediate, thus regenerating the L-form or producing the D-enanticmer. The reaction can be written as... 

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