Química Orgânica 2 · Deslocamentos sigmatrópicos, cicloadições e reações eletrocíclicas
Ireland–Claisen rearrangement of allyl acetate to pent-4-enoic acid
Condições: LDA, THF, −78 °C; TMSCl; Δ; then H3O⁺
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LDA forms the ester enolate
LDA removes a proton from the acetyl methyl group, which is acidic because the carbanion is delocalised onto the carbonyl oxygen. The bulky amide does not add to the ester carbonyl at −78 °C.
As setas curvas
- The amide nitrogen lone pair takes an α-proton.
- The C–H pair forms the enolate C=C.
- The C=O π pair moves onto oxygen as the enolate oxide.
O-Silylation gives the silyl ketene acetal
Chlorotrimethylsilane traps the enolate on oxygen, the hard nucleophilic site, because the Si–O bond is very strong.
As setas curvas
- The enolate oxygen lone pair attacks silicon.
- The Si–Cl pair leaves as chloride.
[3,3] shift of the silyl ketene acetal
On warming, the C=C–O–CH2–CH=CH2 array rearranges through a chair transition state: the ketene acetal carbon bonds to the allyl terminus and the O–allyl bond breaks, forming the ester C=O.
As setas 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.
Why silylate first?
The silyl ketene acetal is a neutral, isolable 1,5-diene equivalent that rearranges near room temperature; it also fixes the enolate on oxygen.
Acid protonates the silyl ester
In the aqueous acid workup hydronium protonates the ester carbonyl oxygen, making the silyl oxygen a better leaving group from silicon.
As setas curvas
- The carbonyl oxygen lone pair takes a proton from hydronium.
- The O–H pair stays on water.
Water attacks silicon
Water bonds to silicon as the Si–O bond pair moves toward carbon, giving the carboxylic acid and a protonated silanol.
As setas curvas
- A water lone pair attacks silicon.
- The Si–O pair forms the C=O of the acid.
- The protonated C=O π pair returns to oxygen, which becomes the acid OH.
Water removes the silanol proton
A second water deprotonates the silyloxonium ion, giving neutral trimethylsilanol and regenerating hydronium.
As setas curvas
- A water lone pair takes the proton.
- The O–H pair stays on the silanol oxygen.
Pent-4-enoic acid
Two carbons of the allyl group became the new C–C bond and chain; the acid is pent-4-enoic acid, with trimethylsilanol and diisopropylammonium salts as by-products.
O que você vai praticar
Draw the enolate formation, O-silylation, [3,3] shift of the silyl ketene acetal and the acidic hydrolysis that give pent-4-enoic acid from allyl acetate.
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