Química Orgânica 2 · Enóis, enolatos e reações em alfa
Ethyl 2-ethyl-2-methyl-3-oxobutanoate by two alkylations of ethyl acetoacetate
Condições: NaOEt, EtOH; CH3I; then NaOEt, CH3CH2Br
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O mecanismo, passo a passo
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Ethoxide removes a doubly activated proton
The CH2 between the ketone and the ester is flanked by two carbonyls, so its pKa (about 11) is far below that of ethanol (about 16): ethoxide deprotonates it essentially completely. The electrons flow into the ketone oxygen to give a stabilized enolate.
As setas curvas
- An ethoxide lone pair takes the acidic proton between the carbonyls.
- The C–H electrons form a C=C bond to the ketone carbon.
- The ketone π electrons move onto oxygen, which carries the negative charge.
Put the charge on carbon
The negative charge is shared by the ketone oxygen, the ester oxygen and the central carbon. The carbon contributor is the one that forms the new C–C bond, so it is drawn next.
As setas curvas
- An oxygen lone pair re-forms the C=O π bond.
- The C=C π electrons move onto the central carbon as a lone pair.
SN2 on methyl iodide
The central carbanion attacks the methyl carbon from the back side and iodide leaves. Methyl iodide is an unhindered SN2 electrophile, so substitution wins over elimination.
As setas curvas
- The carbanion lone pair forms the new C–C bond to the methyl carbon.
- The C–I bond electrons leave with iodide.
Why one carbon only?
After the first alkylation one acidic hydrogen remains on the central carbon, so a second deprotonation is possible at the same site.
Second deprotonation
A second equivalent of ethoxide removes the last hydrogen on the central carbon. The resulting anion is again delocalized over both carbonyls; the carbon lone pair is drawn directly because it is the nucleophilic site.
As setas curvas
- Ethoxide takes the remaining acidic proton.
- The C–H electrons stay on the central carbon as a stabilized carbanion.
SN2 on ethyl bromide
The carbanion attacks the primary carbon of ethyl bromide and bromide leaves, building a quaternary carbon that carries both new groups.
As setas curvas
- The carbanion forms the new C–C bond to the CH2 of ethyl bromide.
- The C–Br electrons leave with bromide.
Ethyl 2-ethyl-2-methyl-3-oxobutanoate
The central carbon now carries a methyl and an ethyl group and has no hydrogen left, so no further alkylation occurs. The new stereocentre forms racemic.
O que você vai praticar
Draw two rounds of ethoxide deprotonation and SN2 alkylation at the carbon between the two carbonyls of ethyl acetoacetate, first with methyl iodide and then with ethyl bromide.
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