Química Orgânica 2 · Transformações de álcoois
Epoxide opening with a methyl shift and inversion
Condições: One enantiomer of 2-ethyl-3,3-dimethyl-2-phenyloxirane with catalytic aqueous acid, shown as hydronium. One ring carbon carries phenyl and ethyl, and the other carries two methyls. Follow concerted opening at the phenyl-bearing carbon with methyl migration from its neighbor.
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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
Protonate the oxygen
Draw both arrows for the proton transfer. The water formed here can take back a proton later to regenerate hydronium. No halide is added.
Nota: The methyl group anti to the breaking benzylic C–O bond migrates to the phenyl-bearing carbon. Solvent water accepts the final proton. Hydronium transfers a proton and becomes water. The acid counterion is not drawn.
As setas curvas
- An oxygen lone pair bonds to the hydronium proton, giving this oxygen a positive charge.
- The O–H bond electrons stay on the water oxygen. Hydronium becomes water.
Open the ring as a methyl shifts
Drawn as one concerted event: the methyl migrates anti-periplanar to the breaking C–O bond, so the phenyl-bearing carbon is attacked from the back and its configuration inverts (like SN2). A long-lived free benzylic cation would instead lose the stereochemistry; this depiction assumes the concerted, stereospecific pathway.
As setas curvas
- The benzylic C–O pair leaves with oxygen as the strained ring opens.
- The methyl σ-bond pair forms a bond to the phenyl-bearing carbon, arriving on the face opposite the departing oxygen.
- The oxygen, still bonded to the carbon that loses a methyl group, donates a pair to form the C=O π bond.
Water takes back a proton
Water removes a proton from the product. The product becomes neutral and water becomes hydronium.
As setas curvas
- The water oxygen takes a proton.
- The O–H bond electrons return to the positively charged oxygen, making it neutral.
Products
3-Methyl-3-phenylpentan-2-one: the epoxide oxygen becomes the ketone oxygen at the carbon that loses a methyl group. The migrated methyl bonds to the phenyl-bearing carbon on the face opposite the departing oxygen, with inversion at that carbon. Water regenerates hydronium, so the acid is a catalyst.
O que você vai praticar
Follow acid-activated epoxide opening coupled to a 1,2-methyl shift (a semipinacol, Meinwald-type rearrangement) and follow inversion at the migration terminus.
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