Resonance carbanions

The resonance structure having the charge on the oxygen atom (an enolate ion) is more stable than the original carbanion resonance structure. Therefore, the enolate ion will predominate over the carbanion. 

As indicated vide supra), the classical mechanism for the Wittig reaction involves the initial nucleophilic addition of the ylide carbanion (resonance structures 1 and 2) to the electrophilic carbon of the carbonyl in 3 to afford betaine intermediate 4. Rotation about the central C-C bond provides for ring closure to 1,2-oxaphosphetane 5. These species are thermally unstable and readily decompose, via a concerted electrocyclic process, to generate the corresponding phosphine oxide 8 and the ( )-alkene 6 or (Z)-alkene 7. 

The Claisen condensation is initiated by deprotonation of an ester molecule by sodium ethanolate to give a carbanion that is stabilized, mostly by resonance, as an enolate. This carbanion makes a nucleophilic attack at the partially positively charged carbon atom of the e.ster group, leading to the formation of a C-C bond and the elimination ofan ethanolate ion, This Claisen condensation only proceeds in strongly basic conditions with a pH of about 14. 

CjHsOOC)—CH—COOCjHj (I) + HOC Hj The carbanion (I) is a resonance hybrid (mesomeric anion) to which there are contributions carrying the negative charge on either carbon or oxygen ... 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

Although most of the reactions of preparative importance involving the a-alkyl carbanions are usually carried out under controlled conditions with NHa /NHs being used as the base, a number of reactions using less severe conditions are known, both in the pyrazine and quinoxaline series. In the case of alkylquinoxalines, where an increased number of resonance possibilities exist, mildly basic conditions are usually employed in condensation reactions. 

Carbanions derived from carbonyl compoimds are often referred to as etiolates. This name is derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl... 

Protonation of the dienyl carbanion formed by reduction of a linear dienone may occur at a carbanion center either p or S to the carbonyl group as is implied by the resonance structures shown below ... 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures... 

FIGURE 24.20 The methylmalonyl-CoA epimerase mechanism involves a resonance-stabilized carbanion at the oj-position. 

Since the final proton transfer is essential for a successful condensation, it is important to understand what factors drive the proton transfer. Examine the electrostatic potential map of the carbanion, and draw all of the resonance contributors that are needed to describe this ion. How does this ion differ from the others Which product, if either, would be expected from the following condensation Explain. 

Mechanism of nucleophilic aro-malic substitution. The reaction occurs in two steps and involves a resonance-stabilized carbanion intermediate. 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

In addition reactions to chiral carbonyl compounds, the stereochemical course taken by resonance-stabilized alkali metals or magnesium benzyl anions resembles that taken by localized carbanions of similar bulk. Thus, conditions can be delineated which lead to either the steric approach or chelation control the following serve as examples. 

D. II. O Brien in The Nuclear Magnetic Resonance of Carbanions m Comprehensive Carbanion Chemistry. Part A, E. Buncel, T. Durst, Eds., Vol. 5 A, Chapter 6, pp 271-322, Elsevier. Amsterdam 1980. 

In cases where a double or triple bond is located a to the carbanionic carbon, the ion is stabilized by resonance in which the unshared pair overlaps with the 71 electrons of the double bond. This factor is responsible for the stability of... 

However, there is evidence against d-orbital overlap and the stabilizing effects have been attributed to other causes. In the case of a PhS substituent, carbanion stabilization is thought to be due to a combination of the inductive and polarizability effects of the group, and d-pn resonance and... 

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