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

Introduction to Ambident Enolate Anions

Because the negative charge of an enolate anion is delocalized over the alpha-carbon and the oxygen, as displayed previous, electrophiles might bond to either atom. Ambident are the reactants that having two or more reactive sites, so this word is correctly applied to enolate anions. Modestly electrophilic reactants like alkyl halides are not adequately reactive to combine with neutral enol tautomers, but the increased nucleophilicity of enolate anion conjugate base allows such type of reactions to occur. Since alkylations are generally irreversible, their results should reflect the inherent (kinetic) reactivity of the distinct nucleophilic sites.

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If an alkyl halide goes through an SN2 reaction at the carbon atom of an enolate anion the result is an alkylated aldehyde or ketone. Alternatively, if the SN2 reaction takes place at oxygen the result is an ether derivative of the enol tautomer; such type of compounds are stable in the nonexistence of acid and may be isolated and characterized. These alkylations that are displayed above are irreversible under the circumstances generally employed for SN2 reactions; therefore the result composition should give a measure of the comparative rates of substitution at carbon versus oxygen. It has been establish that this competition is sensitive to a number of factors, that including negative charge density, solvation, product stability and cation coordination.
For alkylation reactions of enolate anions to be helpful, these intermediates might be generated in high concentration in the nonexistence of other strong bases and nucleophiles. The aqueous base circumstances employed for the aldol condensation are not appropriate because the enolate anions of simple carbonyl compounds are created in very low concentration, and alkoxide or hydroxide bases induce competing SN2 and E2 reactions of alkyl halides. So, It is essential, to get complete conversion of aldehyde or ketone reactants to their enolate conjugate bases by treatment with a very strong base (pKa > 25) in a non-hydroxylic solvent before any alkyl halides are added to the reaction system. A number of bases having pKa's greater than 30 were explained previous, and some others that have been employed for enolate anion formation are: NaH (sodium hydride, pKa > 45), NaNH2 (sodium amide, pKa = 34), and (C6H5)3CNa (trityl sodium, pKa = 32). Ether solvents such as tetrahydrofuran (THF) are generally employed for enolate anion formation. With exception of the sodium hydride and sodium amide, several of these bases are soluble in THF. Certain other strong bases, like alkyl lithium and Grignard reagents, cannot be employed to make use of the enolate anions because they quickly and irreversibly add to carbonyl groups. Though, these very strong bases are helpful in making soluble amide bases. In the preparation of the lithium diisopropylamide (LDA), for instance, the only other result is the gaseous alkane butane.

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Due to its solubility in THF, LDA is a extensively used base for enolate anion formation. In this application one corresponding of diisopropylamine is formed along with the lithium enolate, but this generally does not interfere with the enolate reactions and is simply removed from the results by washing with aqueous acid. Even though the reaction of carbonyl compounds with sodium hydride is slow and heterogeneous, sodium enolates are created with the loss of hydrogen, and no other organic compounds are formed. The following equation provides instances of electrophilic substitution at both carbon and oxygen for the enolate anion derived from cyclohexanone.

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A full analysis of the issues that direct substitution of enolate anions to carbon or oxygen is out of the scope of this text. Though, an outline of some important features that effect the two reactions displayed above is illustrative.

Reactant

Important Factors

CH3-I

The negative charge density is greatest at the oxygen atom (greater electronegativity), and coordination with the sodium cation is stronger there. Because methyl iodide is only a modest electrophile, the SN2 transition state resembles the products more than the reactants. Since the C-alkylation product is thermodynamically more stable than the O-alkylated enol ether, this is reflected in the transition state energies.

(CH3)3Si-Cl

Trimethylsilyl chloride is a stronger electrophile than methyl iodide (note the electronegativity difference between silicon and chlorine). Relative to the methylation reaction, the SN2 transition state will resemble the reactants more than the products. Consequently, reaction at the site of greatest negative charge (oxygen) will be favored. Also, the high Si-O bond energy (over 25 kcal/mole greater than Si-C) thermodynamically favors the silyl enol ether product.

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