Química Orgânica 2 · Éteres e abertura de epóxidos
1-Methoxy-1-methylcyclohexane by alkoxymercuration
Condições: Hg(OAc)2, CH3OH; then NaBH4
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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 6 passos escritos
Mercury(II) adds to the alkene
The cation AcOHg⁺ (from mercury(II) acetate losing one acetate) is the electrophile. The π pair bonds mercury to the ring CH carbon, leaving positive character on the methyl-bearing carbon; in reality this is a bridged mercurinium ion, which is why no rearrangement occurs.
As setas curvas
- The alkene π pair forms the C–Hg bond at the less substituted carbon.
Methanol attacks the more substituted carbon
Methanol attacks the tertiary carbon, which carries most of the positive charge in the mercurinium ion, from the face opposite mercury. This is the Markovnikov orientation.
As setas curvas
- A methanol lone pair forms the new C–O bond.
Acetate removes the O–H proton
The oxonium ion loses its proton to acetate, giving a neutral methyl ether that still carries mercury on the neighbouring carbon.
As setas curvas
- An acetate lone pair takes the proton.
- The O–H bond pair stays on the ether oxygen.
No rearrangement
Compare with acid-catalysed addition of methanol.
NaBH4 replaces mercury with hydrogen
Demercuration: hydride from borohydride ends up on the carbon that held mercury, and mercury is released. The real process goes through radicals; it is drawn here as a single hydride transfer to show where the H ends up.
As setas curvas
- The B–H bond pair delivers hydride to the carbon bearing mercury.
- The C–Hg bond pair moves onto mercury, which leaves.
1-Methoxy-1-methylcyclohexane
1-Methoxy-1-methylcyclohexane forms: the methoxy group sits on the more substituted carbon (Markovnikov) and hydrogen replaced mercury.
O que você vai praticar
Draw mercury(II) adding to 1-methylcyclohexene, methanol attacking the more substituted carbon, loss of the O–H proton, and replacement of mercury by hydrogen with NaBH4.
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