Resonance carbanion stabilization

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... 

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 intermediate corresponding to 64 in the case of 58 is a symmetrical compound, and the three-membered ring can be opened with equal probability on either side of the carbonyl, accounting for the results with In the general case, 64 is not symmetrical and should open on the side that gives the more stable carbanion. This accounts for the fact that 60 and 61 give the same product. The intermediate in both cases is 63, which always opens to give the carbanion stabilized by resonance. The cyciopropanone intermediate (64) has been isolated in the case where R2=R5 f.Bu and R =R =H,... 

Heterocyclic carbanions stabilized by ylid formation, or by resonance that places the negative charge on a heteroatom, are specifically excluded. In addition, heterocyclic systems that do not depend on additional stabilization factors for their initial deprotonation, continued existence, or subsequent reaction with electrophilic substrates are discussed in less detail. 

Pyridoxal phosphate forms a Schiff base (imine) with the glycine. A carbon-bound hydrogen is labile, and the resulting carbanion stabilized by resonance back into the pyridoxal phosphate. The carbanion approaches the carbonyl carbon of the succinyl-CoA. Following the elimination of the CoASH, the intermediate shown in figure 22.13 is formed. The intermediate then loses a C02, forming a carbanion that is resonance stabilized back into the pyridoxal phosphate. 

FIGURE 9.37 Detailed mechanism of aminotransferase reaction. All of the steps are reversible. Double arrows are shown only at Steps 3 and 8 to emphasize the importance of the resonance that stabilizes the carbanion, that is, allows a negative charge to develop on the a-carbon of the substrate. 

Aliphatic nitro compounds and aromatic compounds containing methyl group ortho or para to a nitro group also undergo reactions which appear to involve carbanions stabilized by resonance (p. 23) ... 

To determine the reactivity of an organometallic on the basis of R when the metal is the same, compare the carbanion stability (Section 3.4.1). As the number of electron-donating alkyl groups on the carbanion increases, the carbanion gets less stable and more reactive. The alkyllithium general reactivity trend is tertiary > secondary > primary > methyl. The less stable the carbanion, the more reactive it is as an electron source. For carbanions that are resonance-stabilized by electron-withdrawing groups, the identity of the metal is of less importance than the stabilization of the carbanion. 

Comparison of the resultant data reveals that the a halogen substituents facilitate carbanion formation in the order I Br > Cl > F, which is almost the reverse of that expected from the inductive effect. d-Orbital resonance and polarizability increase in the order F < Cl < Br < I, and it is difficult to assess their relative importance in carbanion stabilization . 

The effect was attributed to a decrease in carbanion stabilization arising from the greater electronegativity of the sp hybrid orbital used for C-F bonding by Hine, J. Mahone, L. G. Liotta, C.L.f. Am. Chem. Soc. 1967,89,5911 and to an electron-donating resonance effect by Streitwieser and Mares (reference 329). 

In contrast, substituents that accept electron density by resonance can stabilize carbanionic centers, thus lessening the interaction with metal ions and facilitating racemization. The 1-lithio derivative of (-)-(R)-l-cyano-2,2-diphe-nylcyclopropanecan be alkylated withmethyl iodide to yield racemic-l-methyl-l-cyano-2,2-diphenylcyclopropane, indicating racemization at the carbanionic center. This result could be ascribed either to a planar carbanion with appreciable C=C=N character or to a rapidly inverting tetrahedral carbanion. The X-ray crystal structure of l-cyano-2,2-dimethylcyclopropyIlithium... 

Carbanions stabilized by resonance interaction with carbonyl, nitro, or cyano groups would be expected to adopt planar geometry to permit maximum delocalization of the negative charge. This expectation is confirmed by studies that have compared the rate of racemization at chiral centers adjacent to a carbonyl group with... 

There is a wealth of information on gas phase ion thermodynamics because of the power of mass spectrometry and ion cyclotron resonance techniques. Before we discuss carboca-tion, and subsequently carbanion, stabilities, keep in mind that ionic structures are much more sensitive to environmental influences than radicals. The polarity, nucleophilicity, and hydrogen bonding ability of the solvent are important influences, as are the nature of the counterion. As such, thermodynamic information is a less reliable predictor of reactivity for carbocations and carbanions than it is for radicals. Nevertheless, gas phase thermodynamics is an excellent starting point, defining the intrinsic stabilities of ions. Any deviation in trends between gas phase and solution studies is likely a consequence of solvation effects, a theme we will visit many times throughout this book. 

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. 

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. 

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... 

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. 

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