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  Alkylation of Enolate Anions Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Alkylation of Enolate Anions

The reaction of alkyl halides with enolate anions shows the similar problem of competing SN2 and E2 reaction paths that was come across previous in the alkyl halide chapter. Because enolate anions are extremely strong bases, they will generally cause elimination when reacted with 2º and 3º-halides. Halides that are not capable of elimination and/or have improved SN2 reactivity are the best electrophilic reactants for this reason. Four instances of the C-alkylation of enolate anions in synthesis are shown in the diagram. The first two use the versatile strong base LDA that is the reagent of choice for mainly intermolecular alkylations of simple carbonyl compounds. The dichloro alkylating agent employed in reaction no.1 nicely demonstrates the high reactivity of allylic halides and the unreactive nature of vinylic halides in SN2 reactions.

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The additive influence of carbonyl groups on alpha-hydrogen acidity is illustrated by reaction no.3. Here the two hydrogen atoms activated by both carbonyl groups are over 1010 times more acidic than the methyl hydrogens on the ends of the carbon chain. Certainly, they are adequately acidic (pKa = 9) to allow whole conversion to the enolate anion in aqueous or alcoholic solutions. As displayed (in blue color), the negative charge of the enolate anion is delocalized above both oxygen atoms and the central carbon. Oxygens are the hydrogen bonded to solvent molecules; therefore the kinetically favored SN2 reaction takes place at the carbon. The monoalkylated result displayed in the equation still has an acidic hydrogen on the central carbon and another alkyl group might be attached there by repeating this sequence.
The last instance (reaction no.4) is an interesting case of intramolecular alkylation of an enolate anion. Because alkylation reactions are irreversible, if the reactive sites are in close proximity, it is probable to form small highly strained rings. Reversible bond-forming reactions like the aldol reaction cannot be used for this purpose. In this reaction, the use of aqueous base is also remarkable, in sight of the very low enolate anion concentration noted previous for such type of systems. It is the quick intramolecular nature of the alkylation that allows these unfavorable conditions to be employed.

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The five-carbon chain of the dichloroketone can take several conformations, two of them are approximated in the preceding picture. Even though conformer II of the enolate anion could produce a stable five-membered ring by an intramolecular SN2 reaction, assuming correct orientation of the α and γ' carbon atoms, the concentration of this perfectly coiled structure will be very low. In this example O-alkylation of the enolate anion, rather than the C-alkylation is preferred from stereoelectronic arguments. Alternatively, conformations in which the α and γ-carbons are correctly aligned for three-membered ring formation are much more numerous, the result being that as quick as the enolate base is formed it goes through fast and irreversible cyclization.
Ring closures to four, five, six and seven-membered are also probable by intramolecular enolate alkylation, as demonstrated by the following instance. Generally, five and six-membered rings are thermodynamically most stable, where three-membered ring formation is favored kinetically.

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