Enolate anions, addition reactions acidity

Now this is exactly the same situation we encountered when we compared the reactivity of aldehydes and ketones with that of carboxylic acid derivatives (see Section 7.8). The net result here is acylation of the nucleophile, and in the case of acylation of enolate anions, the reaction is termed a Claisen reaction. It is important not to consider aldol and Claisen reactions separately, but to appreciate that the initial addition is the same, and differences in products merely result from the absence or presence... 

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... 

The Mannich reaction is best discussed via an example. A mixture of dimethylamine, formaldehyde and acetone under mild acidic conditions gives N,N-dimethyl-4-aminobutan-2-one. This is a two-stage process, beginning with the formation of an iminium cation from the amine and the more reactive of the two carbonyl compounds, in this case the aldehyde. This iminium cation then acts as the electrophile for addition of the nucleophile acetone. Now it would be nice if we could use the enolate anion as the nucleophile, as in the other reactions we have looked at, but under the mild acidic conditions we cannot have an anion, and the nucleophile must be portrayed as the enol tautomer of acetone. The addition is then unspectacular, and, after loss of a proton from the carbonyl, we are left with the product. 

Nucleophilic addition of an enolate anion from a carboxylic acid derivative onto an aldehyde or ketone is simply an aldol-type reaction (see Section 10.3). 

The mechanistic steps can be deduced by inspection of structures and conditions. Enolate anion formation from diethyl malonate under basic conditions is indicated, and that this must attack the epoxide in an Sn2 reaction is implicated by the addition of the malonate moiety and disappearance of the epoxide. The subsequent ring formation follows logically from the addition anion, and is analogous to base hydrolysis of an ester. Ester hydrolysis followed by decarboxylation of the P-keto acid is then implicated by the acidic conditions and structural relationships. 

The hydrogen (known as an a-hydrogen) bonded to a carbon adjacent to a carbonyl carbon (called an a-carbon) is acidic enough to be removed by a strong base, usually NaOH, to form an enolate anion. The enolate anion adds to the carbonyl carbon of a second molecule of aldehyde or ketone via nucleophilic addition reaction. 

The enolate anion attacks the carbonyl carbon of a second molecule of ester and gives a P-ketoester. Thus, the Claisen condensation is a nucleophilic acyl substitution reaction. Eor example, two molecules of ethyl acetate condense together to form the enolate of ethyl acetoacetate, which upon addition of an acid produces ethyl acetoacetate (P-ketoester). 

The pathway by which the reactions are considered to occur involves attack of the enolate anion at the 1 0 bond of o-iodosyl benzoic acid followed by reductive elimination of o-iodobenzoic acid upon addition of methoxide to the carbonyl group. Ring opening of the epoxide thus formed yields the hydroxy dimethyl acetal ... 

Nitroalkenes, e.g. 1 -nitrocyclohexene, 1-nitrocycloheptene, 1-nitrocyclooctene and (3-methyl-j8-nitrostyrene, undergo Michael addition with the enolate anions of (3-ketoesters. The resulting acids undergo a Nef reaction to give 1,4-diketones, which yield furans by subsequent ring closure (Scheme 18) (59LA(626)7l). 

One of the simplest biochemical addition reactions is the hydration of carbon dioxide to form carbonic acid, which is released from the zinc-containing carbonic anhydrase (left, Fig. 13-1) as HC03-. Aconitase (center, Fig. 13-4) is shown here removing a water molecule from isocitrate, an intermediate compound in the citric acid cycle. The H20 that is removed will become bonded to an iron atom of the Fe4S4 cluster at the active site as indicated by the black H20. An enolate anion derived from acetyl-CoA adds to the carbonyl group of oxaloacetate to form citrate in the active site of citrate synthase (right, Fig. 13-9) to initiate the citric acid cycle. 

The reactions discussed in this chapter that depend on the formation of enolate anions (i.e., halogenation, aldol addition, and alkylation) often proceed smoothly and under milder conditions with 1,3-diketones than with monoketones. This is because the 1,3-diketones are stronger acids and therefore can form the enolate anions with weaker bases. The principal synthetic methods for preparing 1,3-dicarbonyl compounds will be discussed in Chapter 18. 

Reports of nucleophilic vinylic photosubstitution reactions, which occur via the S l mechanism, are conspicuously scarce. One such example is the cobalt carbonyl catalysed photostimulated carbonylation of vinylic halides339. By this method 1-bromo and 1-chlorocyclohexene are converted into 1-cyclohexenecarboxylic acid in 98 and 97% yield, respectively. In a prototype vinylic S l reaction, of / -bromostyrene with the enolate anion CH2COCMe3, an ionic elimination-addition route seems to be followed along with the S l route340. 

Diesters (malonates and substituted derivatives) combine three useful features in conjugate addition reactions they form stable enolate anions that undergo clean conjugate addition if required, one of the ester groups can be removed by hydrolysis and decarboxylation and, finally, the remaining acid or ester is ideal for conversion into other functional groups,... 

Similar information is available for other bases. Lithium phenoxide (LiOPh) is a tetramer in THF. Lithium 3,5-dimethylphenoxide is a tetramer in ether, but addition of HMPA leads to dissociation to a monomer. Enolate anions are nucleophiles in reactions with alkyl halides (reaction 10-68), with aldehydes and ketones (reactions 16-34, 16-36) and with acid derivatives (reaction 16-85). Enolate anions are also bases, reacting with water, alcohols and other protic solvents, and even the carbonyl precursor to the enolate anion. Enolate anions exist as aggregates, and the effect of solvent on aggregation and reactivity of lithium enolate anions has been studied. The influence of alkyl substitution on the energetics of enolate anions has been studied. ... 

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