Enolate carbanion

Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

Carbanions are very useful intermediates in the formation of carbon-carbon bonds. This is true both for unstabilized structures found in organometallic reagents and stabilized structures such as enolates. Carbanions can participate as nucleophiles both in addition and in substitution reactions. At this point, we will discuss aspects of the reactions of carbanions as nucleophiles in reactions that proceed by the 8 2 mechanism. Other synthetic aj lications of carbanions will be discussed more completely in Part B. 

On the other hand, at high concentrations of acetaldehyde, when the intermediate enolate carbanion is rapidly captured by another molecule of aldehyde, reverse of the initial parallel proton-abstraction steps is prevented (k3[CH3CHO] k-1 and k 2[BH 1 ]), and the rate of the overall reaction is effectively limited by the initial proton abstractions these then constitute (parallel) rate-limiting steps. The overall process is now first order in acetaldehyde and shows general-base catalysis [5], i.e. the rate law is given by Equation 3.13 ... 

A possible mechanism which accounts for the general features of the reaction in aqueous solution is shown in Scheme 4.7. B represents any base, in addition to OH, present in the aqueous reaction mixture, C032 or an amine, for example and, because acetaldehyde is only very weakly acidic in aqueous solution (pKa = 16.7) [18], the proposed enolate carbanion intermediate can be present at only extremely low concentrations. 

A major structural difference between aromatic aldehydes and most aliphatic analogues is that the former lack an a-hydrogen atom. As a consequence, they are unable to enolize and so enolates/carbanions cannot be generated from them. Nevertheless, aromatic aldehydes can react with carbanions derived from, for example, aldehydes, ketones, esters and anhydrides, and so undergo a range of condensation reactions. 

Ambident nucleophiles are considered to be bidentate molecules whose nucleophilic centres have direct chemical interaction with each other, such as a ketone with its enol-carbanion tautomers. 

Other desulphurization reactions which probably involve ionic intermediates include those of (75) and /S-ketosulphides. The reaction of the latter type of compounds is thought to involve enolate carbanions, and this... 

It has been proposed that some aldolases function by a similar mechanism (5). In these cases metal ion complexes of sugars may act as electrophiles that react with enolate carbanions derived from the other reaction partner (Scheme 3). Alternatively, the metal ion may stabilize the incipient enolate. 

Reduction of a,R-unsaiurated esters, a,(3-Unsaturated esters undergo 1,4-reduction to the enolate carbanion of the saturated ester (a) when treated with this borohydride in THF at low temperatures. When this reduction is carried out in the absence of a proton donor the carbanion undergoes Claisen condensation to give the dimer of the saturated ester as a coproduct. This side... 

In ether and THF, 0-alkylation is a much less serious problem in reactions with alkyl halides. With the possible exception of iodomethane, the use of lithium enolates in ether solvents leads to C-alkylation as the major product in virtually all cases. When the enolate carbanion center is sterically hindered, however, O-alkylation can be a problem, even in THF or ether. There are some reagents that prefer 0-alkylation, with silyl halides, and anhydrides being the most common. Both of these O-alkylation reactions will be discussed in Section 9.3.C. 

Biological synthesis of fatty acids is analogous to the malonate synthesis of carboxylic acids. The enolate carbanion from malonate acts as a nucleophile in a nucleophilic substitution on the acetyl-CE followed by decarboxylation. Each series puts the three-carbon malonate on the ACP and then decarboxylates the substitution product, resulting in lengthening of the carbon chain by two carbons at a time. Naturally occurring fatty acids are even numbered carboxylic acids. 

Such alkali metal ion pairs are capable of two electron transfer from the potassium anion towards a suitable substrate, e.g. p-butyrolactone with formation of a respective carbanion. The strong tendency to two electrons transfer is due to the unusual oxidation state of potassium anion bearing on its outer s orbital a labile electron doublet shielded from the positive potassium nucleus by inner orbitals. Using 5 -enantiometr of P-butyrolactone as a monomer and potassium supramolecular complex as catalyst, enolate carbanion is formed as the first reactive intermediate which induces polymerization, yielding poly-(R)-3-hydroxybutanoate. The resulting biomimetic polyester has the structure similar to native PHB produced in nature, except for acetoxy-end-groups, which are formed instead of the hydroxyl ones typical for natural PHB. 

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