Química Orgânica 2 · Deslocamentos sigmatrópicos, cicloadições e reações eletrocíclicas
Para-Claisen rearrangement of allyl 2,6-dimethylphenyl ether
Condições: Δ (neat, about 200 °C)
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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.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 6 passos escritos
Claisen [3,3] shift to the ortho carbon
The allyl terminus bonds to an ortho carbon while the O–CH2 bond breaks and the ring C=C next to oxygen becomes C=O. That ortho carbon already carries a methyl, so the dienone cannot tautomerise back to a phenol.
As setas curvas
- The ring π pair between the ipso and ortho carbons forms the C=O π bond.
- The breaking O–CH2 σ pair becomes the new allyl C=C.
- The allyl π pair forms the C–C bond to the methyl-bearing ortho carbon.
Why does the allyl group move on?
The ortho dienone has a quaternary carbon with no hydrogen, so it cannot aromatise; a second [3,3] shift reaches the para carbon, which has one.
Cope [3,3] shift to the para carbon
The allyl group and the cross-conjugated dienone form a new 1,5-diene: the allyl CH2 end bonds to the para carbon while the bond to the quaternary ortho carbon breaks. The allyl group is inverted again, so the original O–CH2 carbon is again attached to the ring.
As setas curvas
- The terminal allyl π pair forms the new C–C bond to the para carbon.
- The ring π pair shifts toward the ortho carbon, making a new ring C=C.
- The breaking ortho C–CH2 σ pair becomes the allyl C=C.
Phenoxide removes the para C–H
A trace of phenoxide (assumed shuttle base) removes the hydrogen on the sp3 para carbon. The C–H pair and the ring π pairs move toward the carbonyl, which becomes a phenoxide: the ring is aromatic again.
As setas curvas
- The phenoxide oxygen lone pair takes the para hydrogen.
- The C–H pair forms a ring C=C.
- The neighbouring ring π pair shifts toward the carbonyl carbon.
- The C=O π pair moves onto oxygen, giving the aromatic phenoxide.
Proton transfer back to the product oxygen
The phenol formed in the previous step returns its proton to the new phenoxide oxygen, giving neutral 4-allyl-2,6-dimethylphenol and regenerating the shuttle.
As setas curvas
- The product phenoxide lone pair takes the phenol proton.
- The O–H pair stays on the shuttle oxygen.
4-Allyl-2,6-dimethylphenol
The allyl group sits para to the OH with its original O-bonded CH2 attached to the ring, because two [3,3] shifts each invert the allyl unit. Regaining aromaticity drives the sequence.
O que você vai praticar
Draw the Claisen shift to the methyl-blocked ortho carbon, the Cope shift that carries the allyl group to the para carbon, and the proton transfers that give 4-allyl-2,6-dimethylphenol.
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