Carbanion carbon dioxide

Carboxyl and nitrile groups are usually introduced in synthesis with commercial carboxylic acid derivatives, nitriles, or cyanide anion. Carbanions can be carboxylated with carbon dioxide (H.F. Ebel, 1970) or dialkyl carbonate (J. Schmidlin, 1957). 

Because of thetr electron deficient nature, fluoroolefms are often nucleophihcally attacked by alcohols and alkoxides Ethers are commonly produced by these addition and addition-elimination reactions The wide availability of alcohols and fliioroolefins has established the generality of the nucleophilic addition reactions The mechanism of the addition reaction is generally believed to proceed by attack at a vinylic carbon to produce an intermediate fluorocarbanion as the rate-determining slow step The intermediate carbanion may react with a proton source to yield the saturated addition product Alternatively, the intermediate carbanion may, by elimination of P-halogen, lead to an unsaturated ether, often an enol or vinylic ether These addition and addition-elimination reactions have been previously reviewed [1, 2] The intermediate carbanions resulting from nucleophilic attack on fluoroolefins have also been trapped in situ with carbon dioxide, carbonates, and esters of fluorinated acids [3, 4, 5] (equations 1 and 2)... 

The nucleophilic attack of nitrogen bases leads to a variety of products as the result of addition or addition-elimination reactions The regioselectivity resembles that of attack by alcohols and alkoxides an intermediate carbanion is believed to be involved In the absence of protic reagents, the fluorocarbanion generated by the addition of sodium azide to polyfluonnated olefins can be captured by carbon dioxide or esters of fluonnated acids [J 2, 3] (equation I)... 

Carboxylation (Section 19.11) In the preparation of a carboxylic acid, the reaction of a carbanion with carbon dioxide. Typically, the carbanion source is a Grignard reagent. 

The carbanion is found to react with esters, ketones or aldehydes21 with retention, whereas with carbon dioxide or carbonyl chlorides19,19 a inversion is observed. 

In the case of carbanion and radical intermediates the solvent is less important but the products are partially determined by the resistance of the medium to proton or hydrogen atom abstraction respectively. The increased stability of these intermediates compared with carbonium ions allows the reaction mechanism to be more readily modified by the addition of trapping agents. For example, carbanions are trapped in high yields by the presence of carbon dioxide in the electrolysis medium (Wawzonek and Wearring, 1959 Wawzonek et al., 1955). 

The electrochemical results suggested to explore the possibility of creating a C-C bond between the electrogenerated a-carbanion fi and carbon nucleophiles. Results of practical importance have hitherto been obtained upon electroreduction of 2-bromoisobutyramides in acetonitrile at Hg or Pt cathodes, in the presence of carbon dioxide and an alkylating agent. The enolate-amide fi undergoes quantitative carboxy-alkylation, to yield ester amides of 2,2-dimethylmalonic acid (ref. 16). 

Decarboxylations can be regarded as reversals of the addition of carbanions to carbon dioxide (16-33), but free carbanions are not always involved. When the carboxylate ion is decarboxylated, the mechanism can be either Sgl or Se2. In the... 

Vitamin K is the cofactor for the carboxylation of glutamate residues in the post-synthetic modification of proteins to form the unusual amino acid y-carboxygluta-mate (Gla), which chelates the calcium ion. Initially, vitamin K hydroquinone is oxidized to the epoxide (Figure 45-8), which activates a glutamate residue in the protein substrate to a carbanion, that reacts non-enzymically with carbon dioxide to form y-carboxyglut-amate. Vitamin K epoxide is reduced to the quinone by a warfarin-sensitive reductase, and the quinone is reduced to the active hydroquinone by either the same warfarin-sensitive reductase or a warfarin-insensitive... 

Mechanisms depending on carbanionic propagating centers for these polymerizations are indicated by various pieces of evidence (1) the nature of the catalysts which are effective, (2) the intense colors that often develop during polymerization, (3) the prompt cessation of sodium-catalyzed polymerization upon the introduction of carbon dioxide and the failure of -butylcatechol to cause inhibition, (4) the conversion of triphenylmethane to triphenylmethylsodium in the zone of polymerization of isoprene under the influence of metallic sodium, (5) the structures of the diene polymers obtained (see Chap. VI), which differ. both from the radical and the cationic polymers, and (6)... 

In the case of sodium tris-(/>-nitrophenyl)-methide the carboxylation reaction with carbon dioxide to give the acid fails to take place, although less stable carbanions are readily carbonated.410... 

Most aliphatic ketones can lose a proton from either of two carbon atoms adjacent to the carbonyl. The question of which of the possible carbanions or salts is the effective reagent in a given base-catalyzed reaction depends on the nature of the electrophilic reagent with which the ion subsequently reacts. Thus alkyl methyl ketones lose a primary proton in their reactions with alkali and iodine, alkali and an aldehyde, or alkali and carbon dioxide, but lose a secondary proton in certain other reactions. 

The radical-anions from from alkenes with electron withdrawing substituents will add to carbon dioxide [28]. This process involves the alkene radical-anion, which transfers an electron to carbon dioxide for which E° = -2.21 V vs. see [29]. Further reaction then occurs by combination of carbon dioxide and alkene radcal-anions [30]. The carbanion centre formed in this union may either be protonated or react with another molecule of carbon dioxide. If there is a suitable Michael acceptor group present, this carbanion undergoes an intramolecular addition reaction... 

The addition of carbanions, generated electrochemically by reduction of the carbon-halogen bond, to carbon dioxide has been examined under a variety of experimental conditions. Direct electrosynthesis of carboxylic acids in a divided cell using an aprotic solvent and a tetraalkylammonium salt as electrolyte is most sue-... 

Further data from the polarography and cyclic voltammetry in dimethylformamide are given in Table 5.1 for a series of overall two-electron processes leading to cleavage of a benzyl-heteroatom bond. The first electron transfer step is of the dissociative electron transfer type leading to a benzyl radical. This radical is reduced firrther, at the working potential, to the benzyl carbanion. The carbanion fi om benzyl chlorides, esters, ethers, sulphides, sulphones and quaternary ammonium salts can be trapped by carbon dioxide to form phenylacetic acid [2]. Reac-... 

The radical-anion intermediates derived from aromatic imines behave as nucleophiles towards carbon dioxide, as with 48 [190,191]. Ibis nudeophic character is enhanced by reduction in the presence of chlorotrimethylsilane. A carbanion... 

Electrophiles, which lead to high yields, are methyl iodide, trialkyltin- and trialkylsUyl chlorides, diphenylphosphinyl chloride, acid chlorides, aldehydes and carbon dioxide. Remarkably, though highly acidic ketones are formed on acylation, no deprotonation or racemization by excess of carbanionic species occurs. Other alkyl halides than methyl iodide react very sluggishly with low yields. Benzylic and aUylic halides lead to partial racemization, presumably due to single-electron transfer (SET) in the alkylation step. As very recently found by Papillon and Taylor, racemization of 42 can be suppressed by copper-zinc-lithium exchange before alkylation to 43 via the Knochel cuprates (equation 7) °. 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

The most important reactions of alkyl substituents a and y to the ring heteroatom are those which proceed via base-catalyzed deprotonation. Treatment of 2- and 4-alkyl heterocycles with strong bases such as sodamide and liquid ammonia, alkyllithiums, LDA, etc., results in an essentially quantitative deprotonation and formation of the corresponding carbanions. These then react normally with a wide range of electrophiles such as alkyl halides and tosylates, acyl halides, carbon dioxide, aldehydes, ketones, formal-dehyde/dimethylamine, etc., to give the expected condensation products. Typical examples of these transformations are shown in Scheme 17. Deprotonation of alkyl groups by the use of either aqueous or alcoholic bases can also be readily demonstrated by NMR spectroscopy, and while the amount of deprotonation under these conditions is normally very small, under the appropriate conditions condensations with electrophiles proceed normally (Scheme 18). 

Decarboxylations can be regarded as reversals of the addition of carbanions to carbon dioxide (6-32), but free carbanions are not always involved.471 When the carboxylate ion is decarboxylated, the mechanism can be either SeI or Se2. In the case of the SeI mechanism, the reaction is of course aided by the presence of electron-withdrawing groups, which stabilize the carbanion.472 Decarboxylations of carboxylate ions can be accelerated by the addition of a suitable crown ether, which in effect removes the metallic ion.473 The reaction without the metallic ion has also been performed in the gas phase.474 But some acids can also be decarboxylated directly and, in most of these cases, there is a cyclic, six-center mechanism ... 

Living polymers usually require special reagents to achieve termination. Any electrophilic reagent that reacts with the carbanion active center and also allows preparation of polymers with desired terminal functionalities can be used for this purpose.168,174,181 Hydrogen-terminated polymers can be produced by proton donors, whereas carbon dioxide results in a carboxylate end group. Terminal alcohol functionalities can be formed through reaction with ethylene oxide and carbonyl compounds. 

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