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Química Orgânica 2 · Enolatos: alquilação, condensações e adições conjugadas

Robinson annulation of 2-methylcyclohexanone with methyl vinyl ketone

Desenhe o mecanismo · Desafio

Condições: 2-Methylcyclohexanone, but-3-en-2-one (methyl vinyl ketone), NaOEt (catalytic), EtOH, then reflux

Desenhe este problema você mesmo

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.

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O mecanismo, passo a passo

Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.

Ler todos os 10 passos escritos

  1. Ethoxide forms the more substituted enolate

    Ethoxide removes the α-hydrogen on the carbon that carries the methyl group. Proton transfers with ethoxide are reversible, so the two possible enolates equilibrate and the more substituted one is the more abundant.

    As setas curvas

    • The ethoxide lone pair takes the α-hydrogen next to the methyl group.
    • The C–H pair forms the tetrasubstituted enolate C=C bond.
    • The C=O π pair moves onto oxygen.

  2. Which enolate adds?

    The base and temperature decide whether the kinetic or the thermodynamic enolate reacts.

  3. Michael addition to methyl vinyl ketone

    The enolate carbon attacks the terminal CH₂ (the β-carbon) of methyl vinyl ketone, creating a quaternary carbon and a new enolate on the side chain.

    As setas curvas

    • The oxygen lone pair re-forms the ring C=O.
    • The enolate π pair forms the C–C bond to the β-carbon of the enone.
    • The enone C=C π pair shifts toward the carbonyl carbon.
    • The side-chain C=O π pair moves onto oxygen.

  4. Ethanol protonates the side-chain enolate

    Ethanol gives its proton to the enolate carbon, giving the 1,5-diketone and regenerating ethoxide.

    As setas curvas

    • The oxygen lone pair re-forms the C=O bond.
    • The enolate π pair takes the ethanol proton.
    • The O–H pair stays on oxygen as ethoxide.

  5. Ethoxide forms the methyl-ketone enolate

    Ethoxide removes a proton from the terminal methyl group of the side-chain ketone. Bonding this enolate carbon to the ring carbonyl carbon would close a ring of six atoms; the other side-chain enolate could only close a strained ring of four.

    As setas curvas

    • The ethoxide lone pair takes a methyl hydrogen.
    • The C–H pair forms the enolate C=C π bond.
    • The C=O π pair moves onto oxygen.

  6. Intramolecular aldol addition closes the ring

    The enolate carbon attacks the ring carbonyl carbon. The new ring contains the former methyl carbon, the side-chain carbonyl carbon, two CH₂ groups, the quaternary carbon and the ring carbonyl carbon: six atoms.

    As setas curvas

    • The oxygen lone pair re-forms the side-chain C=O.
    • The enolate π pair forms the ring-closing C–C bond to the ring carbonyl carbon.
    • The ring C=O π pair moves onto oxygen, giving an alkoxide.

  7. Ethanol protonates the alkoxide

    Ethanol gives a proton to the tertiary alkoxide, giving the ketol and regenerating ethoxide.

    As setas curvas

    • The alkoxide lone pair takes the ethanol proton.
    • The O–H pair stays on oxygen as ethoxide.

  8. Ethoxide forms the enolate next to the C–OH carbon

    On heating, ethoxide removes the α-hydrogen on the CH₂ between the ketone and the carbon carrying the OH group.

    As setas curvas

    • The ethoxide lone pair takes the α-hydrogen.
    • The C–H pair forms the enolate C=C π bond.
    • The C=O π pair moves onto oxygen.

  9. E1cB loss of hydroxide

    The enolate re-forms the C=O bond and expels hydroxide from the ring-fusion carbon. The new C=C bond is conjugated with the ketone, which drives the dehydration.

    As setas curvas

    • The oxygen lone pair re-forms the C=O bond.
    • The C=C π pair shifts to the bond toward the ring-fusion carbon.
    • The C–O bond breaks and hydroxide leaves.

  10. 4a-Methyl-4,4a,5,6,7,8-hexahydronaphthalen-2(3H)-one

    A new six-membered ring containing a conjugated enone is fused to the original ring, and the methyl group sits on the ring-fusion carbon. All four carbons of methyl vinyl ketone are in the new ring.

O que você vai praticar

Draw the Michael addition of the more substituted 2-methylcyclohexanone enolate to methyl vinyl ketone, the intramolecular aldol addition that closes a six-membered ring, and the E1cB dehydration to the bicyclic enone.

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