Carbanions basicity

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

We have presented evidence that pyrrole-2-carboxylic acid decarboxylates in acid via the addition of water to the carboxyl group, rather than by direct formation of C02.73 This leads to the formation of the conjugate acid of carbonic acid, C(OH)3+, which rapidly dissociates into protonated water and carbon dioxide (Scheme 9). The pKA for protonation of the a-carbon acid of pyrrole is —3.8.74 Although this mechanism of decarboxylation is more complex than the typical dissociative mechanism generating carbon dioxide, the weak carbanion formed will be a poor nucleophile and will not be subject to internal return. However, this leads to a point of interest, in that an enzyme catalyzes the decarboxylation and carboxylation of pyrrole-2-carboxylic acid and pyrrole respectively.75 In the decarboxylation reaction, unlike the case of 2-ketoacids, the enzyme cannot access the potential catalysis available from preventing the internal return from a highly basic carbanion, which could be the reason that the rates of decarboxylation are more comparable to those in solution. Therefore, the enzyme cannot achieve further acceleration of decarboxylation. In the carboxylation of pyrrole, the absence of a reactive carbanion will also make the reaction more difficult however, in this case it occurs more readily than with other aromatic acid decarboxylases. 

B. Aebischer, R. Meuwly, and A. Vasella, Chain elongation of 1-C-nitroglycosyl halides by substitution with some weakly basic carbanions, Helv. Chem. Acta, 67 (1984) 2236-2241. 

Benzyne is an extremely reactive compound. It cannot be isolated and exists only for a very short time before it reacts. Under the strongly nucleophilic conditions of these reactions, a nucleophile adds to the bond to generate a carbanion. The strongly basic carbanion then removes a proton from some weak acid in the reaction mixture to form the final product. 

The strongly basic carbanion removes a proton from the solvent, ammonia, to complete the reaction. 

The high bond energies formed by carbon (owing to sp3 hybridisation) suggest that with basic carbanions ADn-x is always less than AEn5 and hence reversal of the rate order will be observed only with the hardest acids. 

The carbon-metal bond in such compounds can range from an almost completely ionic bond to one that is predominantly covalent. Benzyl-sodium, for example, may be dissolved in ether to yield a conducting solution on the other hand, the lithium-carbon bond in the colorless ethyliithium is quite nonpolar. The chemistry of such compounds, be they ionic or covalent, is best understood by considering them as sources of the highly basic carbanions that would be formed by removal of the metal ion thus the chemistry of benzylsodium is the chemistry of the CeH CH ion, whereas the chemistry of ethyliithium is the chemistry of the ethide ion, C2H Such ions will attack acidic hydrogens to form the parent hydrocarbons, will attack the more positive end of a double bond, and can carry out a number of nucleophilic displacements these reactions are discussed in texts on organic chemistry. 

Treatment of a -unsaturated complex (251) with the basic carbanion LiCH2CN causes a 1,3-migration of the iron... 

Dual electrophile/probase behavior of alkyl iodides has also been explored [72]. Two-electron reduction of, for example, methyl iodide generates a very basic carbanion... 

Solvation of carbanions is complicated by their basicity. Less basic carbanions (P abH 25) can be made in protic media like alcohols that can hydrogen bond. But more basic carbanions will just deprotonate the protic solvent, and be quenched. Therefore nonprotic solvents such as diethyl ether are commonly used as solvents for these very basic carbanions. 

A convenient synthetic method for the conversion of aryl bromides to phenols is the reaction of the corresponding organometalUc reagents with molybdenum peroxide-pyridine-hexamethylphosphoramide (MoOs-Py-HMPA = MoOPH)". This method provides a mild one-pot reaction sequence for the synthesis of phenols under basic conditions. Phenols are obtained in good to excellent yields with several prototype compounds. Other strongly basic carbanions have been hydroxylated with MoOPH, including aryllithium derivatives" . Table 2 shows some examples of this type of reaction" . ... 

Acidity constants for ionization of weak carbon acids in water caimot be determined by direct measurement when the strongly basic carbanion is too unstable to exist in detectable concentrations in this acidic solvent. Substituting dimethyl-sulfoxide (DMSO) for water causes a large decrease in the solvent acidity because, in contrast with water, the aprotic cosolvent DMSO does not provide hydrogenbonding stabilization of hydroxide ion, the conjugate base of water. This allows the determination of the pfC s of a wide range of weak carbon acids in mixed DMSO/water solvents by direct measurement of the relative concentrations of the carbon acid and the carbanion at chemical equilibrium [3, 4]. The pfC s determined for weak carbon acids in this mixed solvent can be used to estimate pfC s in water. 

Trimethylsilyl)methyl anion 328 reacts with N20 to generate Me3SiO 329 and the driving force for reaction 128 is obviously the formation of the strong silicon-oxygen bond and the conversion of the highly basic carbanion to the more stable oxide ion 330. 

Most examples of [1,2]-Wittig rearrangements involve relatively basic carbanions. However, a few reports have demonstrated that enolates derived from a-alkoxy carbonyl compounds can also participate in [1,2]-Wittig rearrangements. For example, a-benzyloxy lactam 34 was converted to 35 in 63% yield upon treatment with LiHMDS. Related enolate Wittig rearrangements have also been used in tandem processes as described below. 

Step 5 Break a bond to give stable molecules or ions. Loss of the stable molecule nitrogen drives formation of a highly basic carbanion. 

Step 6 Add a proton. Protonation of the highly basic carbanion gives the final reduced product in which the carbonyl O atom has been replaced with two H atoms. 

Furans. Timm and coworkers have devised a strategy to synthesize 2,5-disubstituted furans using sulfone carbanion-mediated solid support as a traceless linker (Scheme 12.16). They demonstrated the usefulness of the highly reactive basic carbanionic species of the... 

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