Química Orgânica 2 · Enóis, enolatos e reações em alfa
2-Hydroxy-6-methylcyclohexan-1-one by Rubottom oxidation
Condições: mCPBA, CH2Cl2; then H3O⁺
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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 7 passos escritos
mCPBA epoxidizes the electron-rich alkene
The silyloxy group makes the C=C very electron-rich. In one concerted step the π bond attacks the terminal peroxy oxygen, the O–O bond breaks, and the peracid proton moves to its carbonyl oxygen.
As setas curvas
- The alkene π electrons attack the outer peroxy oxygen.
- The weak O–O bond breaks toward the carbonyl carbon, forming a C=O.
- The original C=O π electrons pick up the peracid proton.
- The O–H electrons form the second C–O bond of the epoxide.
Acid protonates the epoxide oxygen
In the acidic workup hydronium protonates the epoxide oxygen, turning it into a good leaving group.
As setas curvas
- The epoxide oxygen lone pair takes the proton.
- The O–H electrons stay on water.
The silyloxy oxygen opens the epoxide
The silyloxy lone pair pushes into the ring carbon it is attached to, breaking that C–O bond. The cation formed is an oxocarbenium ion, and the other carbon keeps the new OH.
As setas curvas
- The silyloxy lone pair forms a C=O⁺ π bond.
- The C–O bond of the protonated epoxide breaks onto oxygen.
Why the oxygen ends up on that carbon
The epoxide oxygen is delivered to the carbon that was the β-carbon of the enol ether.
Water removes the silyl group
Water attacks silicon, and the Si–O bond breaks onto the oxocarbenium oxygen, releasing the neutral ketone.
As setas curvas
- A water lone pair bonds to silicon.
- The Si–O electrons move onto the carbonyl oxygen.
Water deprotonates the silyloxonium
A second water removes the proton from the positive oxygen on silicon, giving trimethylsilanol and regenerating hydronium.
As setas curvas
- A water lone pair takes the proton.
- The O–H electrons stay on oxygen.
2-Hydroxy-6-methylcyclohexan-1-one
The α-carbon of the former enol ether now carries an OH and the silyloxy carbon is a ketone. The silyl group leaves as trimethylsilanol and mCPBA becomes 3-chlorobenzoic acid.
O que você vai praticar
Draw epoxidation of the silyl enol ether by mCPBA, acid opening of the siloxy epoxide and removal of the silyl group to give the α-hydroxy ketone.
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