Química Orgânica 2 · Enolatos: alquilação, condensações e adições conjugadas
Crossed Claisen of cyclohexanone with diethyl oxalate
Condições: Cyclohexanone, diethyl oxalate (1.1 equiv), NaOEt (1.1 equiv), EtOH, 0 °C to room temperature; then dilute aqueous HCl
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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 6 passos escritos
Ethoxide forms the cyclohexanone enolate
Ethoxide removes an α-hydrogen of cyclohexanone. Diethyl oxalate has no α-hydrogens, so the ketone is the only possible enolate donor.
As setas curvas
- The ethoxide lone pair takes an α-hydrogen.
- The C–H pair forms the enolate C=C π bond.
- The C=O π pair moves onto oxygen.
The enolate adds to an oxalate carbonyl
The enolate carbon attacks one carbonyl carbon of diethyl oxalate. The adjacent ester group withdraws electrons and makes this carbonyl unusually electrophilic.
As setas curvas
- The oxygen lone pair re-forms the ketone C=O.
- The enolate π pair forms the C–C bond to the oxalate carbonyl carbon.
- The C=O π pair moves onto oxygen.
Ethoxide is expelled
The tetrahedral alkoxide re-forms its C=O bond and pushes out ethoxide, giving a ketone joined to an α-keto ester.
As setas curvas
- The oxygen lone pair re-forms the C=O bond.
- The C–O bond breaks and ethoxide leaves.
Ethoxide removes the doubly activated proton
The ring CH now sits between two carbonyl groups and is much more acidic than ethanol. Its deprotonation is the favourable step that pulls the whole sequence forward, so a full equivalent of ethoxide is used.
As setas curvas
- The ethoxide lone pair takes the proton between the two carbonyls.
- The C–H pair forms a C=C π bond toward the ring ketone carbon.
- The ring C=O π pair moves onto oxygen.
Acid workup
Hydronium, added after the reaction, protonates the stabilised enolate at the ring carbon, giving the neutral dicarbonyl product.
As setas curvas
- The oxygen lone pair re-forms the ring C=O.
- The C=C π pair takes a proton from hydronium.
- The O–H pair stays on oxygen as water.
Ethyl 2-oxo-2-(2-oxocyclohexyl)acetate
An ethoxalyl group, –CO–CO₂Et, has replaced one α-hydrogen of cyclohexanone. The second ester of diethyl oxalate is untouched.
O que você vai praticar
Draw the ethoxide-mediated acylation of cyclohexanone by diethyl oxalate, the deprotonation of the 1,3-dicarbonyl product that drives the reaction, and the acid workup.
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