Química Orgánica 2 · Desplazamientos sigmatrópicos, cicloadiciones y reacciones electrocíclicas
Johnson–Claisen rearrangement of allyl alcohol to ethyl pent-4-enoate
Condiciones: CH3C(OEt)3, H⁺ (cat.), Δ
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 10 pasos escritos
Protonation of an ethoxy oxygen
Propanoic acid (assumed catalyst, the usual choice) protonates one ethoxy oxygen of the orthoester so that ethanol can leave.
Las flechas curvas
- The ethoxy oxygen lone pair takes the acid proton.
- The O–H pair stays on the carboxylate oxygen.
Loss of ethanol gives an oxocarbenium ion
A neighbouring ethoxy lone pair pushes out ethanol, giving a dialkoxycarbenium ion; no free carbocation forms because oxygen donates into the empty orbital.
Las flechas curvas
- An ethoxy lone pair forms the C=O+ π bond.
- The C–O bond pair leaves with ethanol.
Allyl alcohol adds to the oxocarbenium ion
The allyl alcohol oxygen attacks the cationic carbon and the C=O+ π pair returns to the ethoxy oxygen.
Las flechas curvas
- The allyl alcohol oxygen lone pair bonds to the cationic carbon.
- The C=O+ π pair returns to the ethoxy oxygen.
Propanoate removes the O–H proton
Propanoate takes the proton from the new oxonium ion, giving a mixed allyloxy orthoester and regenerating propanoic acid.
Las flechas curvas
- The carboxylate oxygen lone pair takes the proton.
- The O–H pair stays on the allyloxy oxygen.
Protonation of a second ethoxy oxygen
The acid protonates another ethoxy oxygen to set up loss of a second ethanol.
Las flechas curvas
- The ethoxy oxygen lone pair takes the acid proton.
- The O–H pair stays on the carboxylate oxygen.
Loss of a second ethanol
The remaining ethoxy lone pair pushes out ethanol, giving a new oxocarbenium ion that carries the allyloxy group.
Las flechas curvas
- The ethoxy lone pair forms the C=O+ π bond.
- The C–O bond pair leaves with ethanol.
Deprotonation gives the ketene acetal
The methyl hydrogens next to the oxocarbenium carbon are acidic; propanoate removes one, giving the ketene acetal (an allyl vinyl ether).
Las flechas curvas
- The carboxylate lone pair takes a methyl proton.
- The C–H pair forms the ketene acetal C=C.
- The C=O+ π pair returns to the ethoxy oxygen.
Why heat and removal of ethanol?
Each exchange is an equilibrium; distilling off ethanol pulls it toward the ketene acetal, which is consumed irreversibly by the [3,3] shift.
[3,3] shift to the γ,δ-unsaturated ester
The ketene acetal C=C–O–CH2–CH=CH2 array rearranges through a chair transition state, making a new C–C bond and the ester C=O.
Las flechas curvas
- The allyl π pair forms the new C–C bond to the ketene acetal carbon.
- The breaking O–CH2 σ pair becomes the new C=C.
- The ketene acetal π pair forms the ester C=O.
Ethyl pent-4-enoate
The allyl group has moved from oxygen to carbon, extending the acetate by three carbons; two ethanol molecules are released and the propanoic acid catalyst is regenerated.
Qué vas a practicar
Draw the acid-catalysed exchange of allyl alcohol into triethyl orthoacetate, the loss of ethanol that gives the ketene acetal, and the [3,3] shift to ethyl pent-4-enoate.
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