Química Orgánica 2 · Enoles, enolatos y reacciones en alfa
Ethyl 3-oxooctanoate by γ-alkylation of the ethyl acetoacetate dianion
Condiciones: LDA (2 equiv), THF; then CH3CH2CH2CH2Br; then H3O⁺ workup
Dibuja este problema tú mismo
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 6 pasos escritos
First LDA takes the central proton
The CH2 between the two carbonyls (pKa about 11) is by far the most acidic site, so the first equivalent of LDA removes it to give the stabilized monoanion.
Las flechas curvas
- The amide nitrogen lone pair takes the central proton.
- The C–H electrons form a C=C bond toward the ester carbon.
- The ester π electrons move onto the ester oxygen.
Second LDA takes a terminal methyl proton
With the central site already anionic, the next most acidic hydrogens are on the ketone methyl group (pKa about 20). LDA is strong enough to remove one, giving a dianion with an enolate on each side.
Las flechas curvas
- A second LDA nitrogen takes a methyl proton next to the ketone.
- The C–H electrons form a C=C bond to the ketone carbon.
- The ketone π electrons move onto oxygen.
Which carbon reacts?
In a dianion the last-formed, least stabilized carbanion is the most reactive nucleophile.
The γ carbon attacks 1-bromobutane
The ketone enolate oxygen pushes electrons back into the C=O while the terminal C=C π pair attacks the CH2Br carbon in an SN2 reaction. The ester enolate is left untouched.
Las flechas curvas
- The oxygen lone pair re-forms the ketone C=O.
- The terminal C=C π electrons form the new C–C bond.
- Bromide leaves with the C–Br electrons.
Acid workup protonates the ester enolate
Aqueous acid (assumed workup) puts a proton back on the central carbon, giving the neutral β-keto ester.
Las flechas curvas
- An oxygen lone pair re-forms the ester C=O.
- The C=C π electrons take the proton at the central carbon.
- The O–H electrons stay on water.
Ethyl 3-oxooctanoate
The butyl group sits on the former methyl carbon, extending the ketone chain; the central CH2 is unchanged. A one-anion procedure (NaOEt) would have alkylated the central carbon instead.
Qué vas a practicar
Form the dianion of ethyl acetoacetate with two equivalents of LDA, alkylate it at the terminal (γ) carbon with 1-bromobutane, and protonate the remaining enolate.
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