Química Orgânica 2 · Condensação de Claisen e adição conjugada
Crossed Claisen of 4-methylacetophenone with ethyl trifluoroacetate
Condições: 4-Methylacetophenone, ethyl trifluoroacetate (1.2 equiv), NaOEt (1.2 equiv), EtOH, reflux; then aq. HCl workup
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Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 7 passos escritos
Ethoxide forms the ketone enolate
Ethoxide removes a proton from the methyl group next to the ketone carbonyl, giving the enolate.
As setas curvas
- An ethoxide lone pair takes an α-hydrogen.
- The C–H pair forms the C=C π bond of the enolate.
- The carbonyl π pair moves onto oxygen.
The enolate adds to the trifluoroacetyl carbonyl
The enolate carbon attacks the ester carbonyl carbon. Three fluorines pull electron density from that carbon, making it far more electrophilic than an ordinary ester.
As setas curvas
- The oxygen lone pair reforms the ketone C=O.
- The enolate π pair forms the C–C bond to the ester carbonyl carbon.
- The ester C=O π pair moves onto oxygen.
Why no ester self-condensation?
Ethyl trifluoroacetate has no α-hydrogens, so it can only be the electrophile; its CF3 group also makes it the most electrophilic carbonyl present.
The tetrahedral intermediate expels ethoxide
The alkoxide re-forms the carbonyl and ethoxide leaves, giving the trifluoromethyl 1,3-diketone.
As setas curvas
- The oxygen lone pair reforms the C=O bond.
- The C–O bond breaks and ethoxide leaves.
Ethoxide deprotonates the diketone
The CH2 between the two carbonyls, made even more acidic by the CF3 group, is removed by ethoxide. Forming this stabilised enolate is effectively irreversible and drives the condensation.
As setas curvas
- An ethoxide lone pair takes the doubly activated proton.
- The C–H pair forms a C=C π bond toward the trifluoroacetyl carbon.
- The C=O π pair moves onto oxygen.
Acid workup gives the neutral diketone
Hydronium protonates the enolate at the central carbon as the oxygen lone pair re-forms the C=O bond.
As setas curvas
- The oxygen lone pair re-forms the carbonyl.
- The C=C π pair takes a proton from hydronium at the central carbon.
- The O–H pair stays on water.
4,4,4-Trifluoro-1-(4-methylphenyl)butane-1,3-dione
The trifluoroacetyl group has been transferred to the methyl ketone carbon, giving the fluorinated 1,3-diketone.
O que você vai praticar
Draw how the enolate of 4-methylacetophenone is trifluoroacetylated by ethyl trifluoroacetate, including the deprotonation that drives the reaction and the acid workup to 4,4,4-trifluoro-1-(4-methylphenyl)butane-1,3-dione.
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