Química Orgânica 1 · Estereoquímica das reações de alcenos
Epoxidize a trisubstituted ring alkene with mCPBA
Condições: 1-Methylcyclohexene with meta-chloroperoxybenzoic acid (mCPBA) in CH₂Cl₂ at 0 °C. Ar is the unchanged 3-chlorophenyl ring. Transfer the hydroxyl oxygen of the peracid to the upper (wedge) face of the ring; the lower face is equally accessible. Target the epoxide; no ring opening is performed.
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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.
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Transfer the oxygen in one step
In the butterfly transition state the O–H of the peracid is hydrogen-bonded to its own carbonyl oxygen, so four pairs shift around one cycle with no ions formed. The O–O bond is weak and the resulting carboxylic acid is a stable, resonance-stabilized leaving fragment, which makes the hydroxyl oxygen electrophilic. A trisubstituted alkene is more electron-rich than cyclohexene, so it reacts faster.
As setas curvas
- The alkene π pair forms a bond to the electrophilic hydroxyl oxygen of the peracid.
- The peracid O–H bond pair forms the second epoxide C–O bond as the proton transfers.
- The weak O–O bond pair forms a new carbonyl π bond at the oxygen next to the acyl carbon.
- The original carbonyl π pair forms an O–H bond to the peracid proton.
1-Methylcyclohexene oxide and 3-chlorobenzoic acid
The transferred oxygen now bridges the former alkene carbons on the upper face. Both new C–O bonds were made on the same face, so the epoxide is necessarily cis-fused to the ring. The methyl-bearing carbon is now a quaternary stereocenter, with its methyl on the lower face (hash) rather than in the alkene plane. Transfer to the lower face gives the mirror image at the same rate, so the epoxide is racemic. The peracid proton ends on the original carbonyl oxygen of 3-chlorobenzoic acid; the oxygen originally bonded to both carbon and oxygen becomes its carbonyl oxygen.
O que você vai praticar
Draw the four-pair mCPBA butterfly transfer to 1-methylcyclohexene and explain why the new quaternary stereocenter forms as a racemic cis-fused epoxide.
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