Química Orgânica 1 · Estereoquímica das reações de alcenos
Prilezhaev epoxidation — followed by basic ring opening
Condições: Styrene is epoxidized with peracetic acid. The epoxide is isolated before sodium methoxide in methanol is introduced: the acid coproduct is removed before this step. Determine the favored opening site under these basic conditions. Wedge/hash show one of two mirror-related pathways.
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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 4 passos escritos
Transfer oxygen in one step
Draw the four arrows as one cycle: oxygen transfer happens in one step, without a free carbocation. Swapping the carbon ends of the two C–O arrows gives the same epoxide.
Nota: Use the terminal peroxide oxygen, the one bonded to hydrogen, to make the epoxide. Swapping the two carbon ends of the arrows gives the same ring.
As setas curvas
- The alkene π pair forms one C–O bond of the epoxide.
- The terminal O–H pair forms the other C–O bond. Swapping the carbon ends of these two arrows gives the same epoxide.
- The O–O pair forms the new C=O π bond in the acid.
- The original C=O π pair forms an O–H bond to the peracid proton.
Begin the isolated-epoxide stage
The acid coproduct was separated before base was added; otherwise it would consume methoxide. Sodium and methanol now enter explicitly.
As setas curvas
- Methoxide attacks the less hindered epoxide carbon from the opposite face.
- The breaking C–O bond pair remains on the epoxide oxygen.
Finish the basic opening
The benzylic C–O stereobond does not break.
As setas curvas
- The alkoxide takes a proton from methanol.
- The solvent O–H pair produces methoxide.
Beta-methoxy alcohol and base
New methoxy oxygen is attached to the terminal carbon; original epoxide oxygen becomes the benzylic alcohol. Proton transfer moves the negative charge to solvent methoxide, balanced by sodium. The unbroken benzylic C–O bond retains its configuration.
O que você vai praticar
Carry styrene epoxidation into methoxide opening, selecting the carbon site and preserving the unbroken stereocenter during protonation.
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