Resonance stabilization carbanions

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... 

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

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. 

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. 

This type of process is analogous to the nucleophilic allylic rearrangements discussed in Chapter 10 (p. 420). There are two principal pathways. The first of these is analogous to the SeI mechanism in that the leaving group is first removed, giving a resonance-stabilized allylic carbanion, and then the electrophile attacks. 

This reaction, for which the termprototmpic rearrangement is sometimes used, is an example of electrophilic substitution with accompanying allylic rearrangement. The mechanism involves abstraction by the base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more... 

Protonation of the enolate ion is chiefly at the oxygen, which is more negative than the carbon, but this produces the enol, which tautomerizes. So, although the net result of the reaction is addition to a carbon-carbon double bond, the mechanism is 1,4 nucleophilic addition to the C=C—C=0 (or similar) system and is thus very similar to the mechanism of addition to carbon-oxygen double and similar bonds (see Chapter 16). When Z is CN or a C=0 group, it is also possible for Y to attack at this carbon, and this reaction sometimes competes. When it happens, it is called 1,2 addition. 1,4 Addition to these substrates is also known as conjugate addition. The Y ion almost never attacks at the 3 position, since the resulting carbanion would have no resonance stabilization " ... 

Another linear correlation between A// values and between AG values has been proposed to correlate the heats of heterolysis for the carbon-carbon <7 bond with p Cr+ values of the cations and values of the conjugate acids of the anions by Arnett et al. (1987a, 1990a). From the results of calorimetry for the coordination of resonance-stabilized carbo-cations and carbanions in sulfolane or acetonitrile, these workers demonstrated that (28) and (29), for secondary and tertitu cations, respectively, can be used for predicting heats of heterolysis of the carbon-carbon a bond. 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

Arnett and colleagues [219,220] measured the enthalpies of a considerable number of processes where a resonance-stabilized carbenium ion R was reacted with a resonance stabilized carbanion, oxanion, thioanion, or nitroanion, R(T, in mixtures of sulfolane (95%) and 3-methylsulfolane (5%), at 298.15 K ... 

An example where the presence of a counterion makes a difference between the gas phase and solution phase pathways involves the intriguing carbanion produced on deprotonation of 1,3-dithiane at C-2. In solution, this species, almost invariably produced by reaction of the dithiane with butyllithium, is widely used as an acyl anion equivalent in synthetic chemistry. Its importance for the present work is that this is a configurationally stable lithiated species in solution the carbanion stays sp -hybridized, and the lithium prefers the equatorial position, even to the extent of driving a terr-butyl group on the same acidic C-2 carbanion to the axial position in the lithiocarbon species. The carbanion is thought to be stabilized primarily by orbital overlap with the C-S antibonding orbitals, as opposed to more conventional polar and 7t-resonance stabilization. ... 

The stereoselectivity of the catalyst results from the rapid conversion of 1-butene to cfs-2-butene. Therefore butene molecules must be reacting more rapidly with a cfs-butenyl carbanion, or the cfs-butenyl carbanion is present in greater concentrations. The latter interpretation has been favored, and an additional resonance-stabilized structure may account for the preferential formation of the cfs-butenyl carbanion (11). 

This reaction is favored because a resonance-stabilized anion (III) is formed, whereas with a simple cyclic monoolefin, such as methylcyclo-hexene, a nonresonance stabilized tertiary carbanion would have to be formed. 

This addition is energetically favored because a more stable primary carb-anion is formed from a less stable tertiary carbanion. The addition step in the side-chain alkylation reaction is probably not energetically favored because a primary carbanion is formed from a resonance-stabilized benzylic carbanion. However, as the rapid and energetically favored transmetalation reaction following it restores the benzylic carbanion, the over-all process takes place readily. An alternative radical combination mechanism has also been proposed by Morton and Ward 39) for the alkylation reaction. 

The side-chain alkylation reaction of aromatic hydrocarbons has also been studied using unsaturated aromatic olefins, especially styrene. Pines and Wunderlich 43) found that phenylethylated aromatics resulted from the reaction of styrenes with arylalkanes at 80-125° in the presence of sodium with a promoter. The mechanism of reaction is similar to that suggested for monoolefins, but addition does not take place to yield a primary carbanion a resonance stabilized benzylic carbanion is formed [Reaction (23a, b)j. 

Carbanions of the type [HjCR ], [HCR R ] and [CR R R ] (R = NO and R R = CN, NO, NO2) can be considered to be resonance-stabilized, nonlinear pseudohalides. All experimentally known resonance-stabilized methanides are reported to be planar or nearly planar (Table 1). While the parent ion, the methanide anion HsC, adopts a pyramidal structure [Afipianar-pyramidai = 9.8 kJmol rf(CH) = 1.099 A, <(HCH) = 109.7° cf. rf(CH) = 1.093 A, <(HCH) = 109.6°] due to the lack of delocalization (no resonance for the p-AO-type lone pair possible) , substitution of one H atom by NO results in a planar anion since the empty jr -orbitals of the NO group are perfectly suitable to delocalize the carbon lone pair. Further substitution of the second H atom again results in planar anions, and the same holds for the third substitution in case of R = CN. In case of R = NO and NO2, the third substitution leads either to a propeller-type structure with only a small distortion from planarity or one NO2 group is twisted by 90°, nevertheless leaving the central carbon in an almost trigonal planar environment . ... 

Monomer reactivities in anionic copolymerization are the opposite of those in cationic copolymerization. Reactivity is enhanced by electron-withdrawing substituents that decrease the electron density on the double bond and resonance stabilize the carbanion formed. Although the available data are rather limited [Bywater, 1976 Morton, 1983 Szwarc, 1968], reactivity is generally increased by substituents in the order... 

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