Resonance-stabilized anion

NaOCHjCHa. White solid (Na in EtOH). Decomposed by water, gives ethers with alkyl halides reacts with esters. Used in organic syntheses particularly as a base to remove protons adjacent to carbonyl or sulphonyl groups to give resonance-stabilized anions. 

These acids (51) are organic molecules that contain a plurality of cyano groups and are readily ionized to hydrogen ions and resonance-stabilized anions. Typical cyanocarbon acids are cyanoform, methanetricarbonitrile (5) 1,1,3,3-tetracyanopropene [32019-26-4] l-propene-l,l,3,3-tetracarbonitrile (52) 1,1,2,3,3-pentacyanopropene [45078-17-9], l-propene-l,l,2,3,3-pentacarbonitrile (51) l,l,2,6,7,7-hexacyano-l,3,5-heptatriene [69239-39-0] (53) 2-dicyanomethylene-l,l,3,3-tetracyanopropane [32019-27-5] (51) and l,3-cyclopentadiene-l,2,3,4,5-pentacarbonitrile [69239-40-3] (54,55). Many of these acids rival mineral acids in strength (56) and are usually isolable only as salts with metal or ammonium ions. The remarkable strength of these acids results from resonance stabilization in the anions that is not possible in the protonated forms. 

The reactivity of the 1-methyl group and of corresponding positions (i.e., a-carbon atoms) in other l-alkyl-j8-carbolines, analogous to that in a-picoline, quinaldine, and isoquinaldine, is due to the acidity of this center. Deprotonation yields a resonance-stabilized anion (288) which reacts readily with electrophilic reagents. Metallation with phenyl-lithium of the 1-methyl group of a l-methyl-j8-carboline derivative in which the indole nitrogen is protected, first described by Woodward... 

In strong alkaline solution pteridine behaves as a weak acid with a piCa value of 11.21. To explain this property, the resonance-stabilized anion 21 was derived from the hydrate 16. 

Considerable deuteriation occurs at the C-methyl group when the salt (107) is treated with methan[2H]ol-sodium methoxide, indicating that the resonance-stabilized anion (108) is formed as well as the ylide (109),... 

Methyl-4-phenyl-l,2,5-thiadiazole 1,1-dioxide 21 suffers proton abstraction in basic nonaqueous media to give a resonance stabilized anion 43, neutralization of which using anhydrous TFA gives the orange tautomer 4-methylene-3-phenyl-l,2,5-thiadiazoline 1,1-dioxide 44 (Scheme 3) <2001JP0217>. The tautomeric equilibrium is practically displaced toward 21 in acetonitrile and toward 44 in DMF. 

The facile phase-transfer catalysed N-alkylation of phenylhydrazones provides an effective route to A-alkyl-A-phenylhydrazines, as shown in Scheme 5.5 [27]. Deprotonation of both the hydrazones and the triazenes leads to resonance stabilized anions. It is therefore highly probable that the alkylation occurs on the initially formed anions, instead of the neutral species, as indicated by the red colour imparted to the organic phase in the reactions of the triazenes, which results from the formation of the ion-pair [Q ArN=N-NAr]. 

The reaction of methylenesulphones with allyl halides in the presence of quaternary ammonium salts produces the 1-allyl derivatives [52], unlike the corresponding reaction in the absence of the catalyst in which the SN- product is formed (Scheme 6.5). In contrast, alkylation of resonance stabilized anions derived from allyl sulphones produces complex mixtures [51] (Scheme 6.6). Encumbered allyl sulphones (e.g. 2-methylprop-2-enyl sulphones) tend to give the normal monoalkyl-ated products. Methylene groups, which are activated by two benzenesulphonyl substituents, are readily monoalkylated hydride reduction leads to the dithioacetal and subsequent hydrolysis affords the aldehyde [61]. 

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. 

Vitamin C (ascorbic acid) is also a well-known antioxidant. It can readily lose a hydrogen atom from one of its enolic hydroxyls, leading to a resonance-stabilized radical. Vitamin C is acidic (hence ascorbic acid) because loss of a proton from the same hydroxyl leads to a resonance-stabilized anion (see Box 12.8). However, it appears that vitamin C does not act as an antioxidant in quite the same way as the other compounds mentioned above. 

All nitrosomethanides represent resonance-stabilized anions, which are stable at ambient temperatures. However, their alkali and silver salts are energetic materials and hence most of them are explosive. The reason for this thermodynamic instability lies in the smaller CN and NO bond energies compared to those of N2 and CO (in CO2). 

The extensive use of alkyllithium initiators is due to their solubility in hydrocarbon solvents. Alkyls or aryls of the heavier alkali metals are poorly soluble in hydrocarbons, a consequence of their more ionic nature. The heavier alkali metal compounds, as well as alkyllithiums, are soluble in more polar solvents such as ethers. The use of most of the alkali metal compounds, especially, the more ionic ones, in ether solvents is somewhat limited by their reactivity toward ethers. The problem is overcome by working below ambient temperatures and/or using less reactive (i.e., resonance-stabilized) anions as in benzylpotassium, cumylcesium and diphenylmethyllithium. 

Like the corresponding methylpyridines, 2- and 4-methylquinolines can be deprotonated by a base, such as sodium methoxide, forming resonance-stabilized anions (Scheme 3.9). The latter are useful in synthesis, providing nucleophilic reagents that allow extension of quinoline side chains through reactions with appropriate electrophiles. Activation of the 2-methyl group can also be achieved by the use of acetic anhydride (the same type of process occurs with 2-methylpyridine, Section 2.7.1, Worked Problem 2.3). 

They react with strong bases to form resonance-stabilized anions with - on N. 

The mechanism Favorskii envisioned involved the initial attack of the ethoxy anion on the triple bond to form a vinyl ether. The now accepted carbanionic mechanism assumes the formation of resonance-stabilized anions, allowing the stepwise interconversion of 1- and 2-alkynes, and allenes143 147 (Scheme 4.9). 

Eq. (5.63)] initiates the transformation.235 The next step, the attack of anion 36 on the alkene [Eq. (5.64)], is the rate-determining step since it results in the transformation of a resonance-stabilized anion (36) into anions (37 and 38) that are not stabilized. Transmetalation of 37 and 38 forms the end products and restores the benzylic carbanion [Eqs. (5.65) and (5.66)] ... 

The higher stability of primary anion 37 as compared to secondary anion 38 explains the predominant formation of branched isomers. The high reactivity of conjugated dienes and styrenes compared with that of monoolefins is accounted for by the formation of new resonance-stabilized anions (39 and 40). Base-catalyzed alkylation with conjugated dienes may be accompanied by telomerization. The reason for this is that the addition of a second molecule of diene to the 39 monoadduct anion competes with transmetallation, especially at lower... 

Resonance stabilized anion less susceptible to nucleophilic attack. Can act as a nucleophile... 

Anilide 4 is lithiated selectively in ortho-position to the pivaloyl amide group.4 5 The organolithium species is generated by reaction of 4 with two equivalents of n-butyllithium below 5 °C in MTBE, since the amide proton is also acidic and is deprotonated to yield resonance-stabilized anion IS before the ort/zo-lithiation of the aromatic system with the second equivalent of n-butyllithium takes place. The resulting organolithium species 16 then undergoes nucleophilic attack of ester 176 to give dianion 18. 

Alkyl groups at the 2- and 4-positions of the pyridine ring are electron deficient due to loss of electron density to the ring nitrogen by resonance and induction. A consequence of this effect is the relative ease of deprotonation at the 2- and 4-methyl position to give resonance-stabilized anions (Equations 26 and 27). 

The normal addition process is identical to the other reactions that have been encountered so far in this chapter The nucleophile bonds to the carbonyl carbon and the electrophile bonds to the oxygen of the carbonyl group. In a conjugate addition the nucleophile bonds to the /3-carbon. The electrophile, a proton, can bond to either the a-carbon or the oxygen of the resonance stabilized anion. It actually reacts faster at the oxygen, producing an enol in an overall 1,4-addition. However, as discussed in Section 11.6, ends are less stable than the carbonyl tautomers, so the product that is isolated contains the carbonyl group. 

Alkyl substituents in aromatic azoloazines are reactive towards electrophilic reagents in basic media. Basic reagents readily abstract protons from such alkyl groups yielding resonance stabilized carbanions. Thus, treatment of the methyl derivatives (243) with aldehydes gives the alkenes (245) (Scheme 21) <84H(22)174i). Ready formation of the resonance stabilized anions (244) is behind the activity of the methyl group. 

The chemistry of oxazolones, particularly that of 5(AH)-oxazolones (104), is full of interest. These compounds are attacked by some nucleophiles at C(2) (c/. 105), but fission of the carbonyl-oxygen bond (c/. 106), leading to ar-amino acids or their derivatives, is more usual. 5(4H)-Oxazolones react with electrophiles at C(4) or, less commonly, at C(2) by way of the resonance-stabilized anions (107), and they can function as tautomeric 1,3-dipoles (108) in cycloaddition reactions. 

Copolymerization of styrene with diolefins provides further support that monomer coordinates with the cationic site prior to addition. Korotkov (218) showed that in homopolymerizations styrene is more reactive than butadiene, but in copolymerization the butadiene reacted first at its homopolymerization rate and when it was exhausted the styrene reacted at its homopolymerization rate. This interesting result has been duplicated by Kuntz (245) and analogous results have been obtained by Spirin and coworkers (237) for the styrene-isoprene system. Presumably, the diene complexes more strongly than styrene with the lithium and excludes styrene from the site. That the complex occurs at a cationic site, rather than at the anion or the metal-carbon bond, is indicated by the fact that dienes form more stable complexes than styrene with Lewis acids (246). It should be emphasized that selective monomer coordination is not the only factor influencing reactivities in copolymerizations. Of greatest importance are the relative reactivities of the different polymer anions. The more resonance-stabilized anion is more readily formed and is less reactive for polymerizing the co-monomer. 

The resonance-stabilized anion formed by deprotonating the carbon atom next to a carbonyl group, (p. 1046)... 

Ninhydrin is a common reagent for visualizing spots or bands of amino acids that have been separated by chromatography or electrophoresis. When ninhydrin reacts with an amino acid, one of the products is a deep violet, resonance-stabilized anion called Ruhemann s purple. Ninhydrin produces this same purple dye regardless of the structure of the original amino acid. The side chain of the amino acid is lost as an aldehyde. 

The relative reactivities of these enamines towards / -nitrobenzyl radical were found by carrying out competitive studies in the presence of the resonance stabilized anion Me2C=N02 Li+, which is known to trap the / -nitrobenzyl radical to form / -02N-C6H4CH2CMe2N02 by an SRN1 process28. Enamine 9 is more reactive than 10 and 11, while 9a and 9c are slightly more reactive than 9b. 

The cyclopentadienyl anion 13 is an efficiently resonance-stabilized anion in which all the carbon-carbon bond lengths are equal (Figure 1.9). It forms stable compounds, of which ferrocene (14) is an example, which undergo aromatic substitution reactions such as sulfonation and acetylation. 

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