Química Orgânica 1 · Estereoquímica das reações de alcenos
trans-2-Methylcyclopentan-1-ol from 1-methylcyclopentene
Condições: BH3·THF; then H2O2, NaOH
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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 8 passos escritos
Add B and H from one face
Borane adds in one four-centre step: boron bonds to the less substituted alkene carbon and hydrogen to the methyl-bearing carbon, both from the same face. Boron goes where steric crowding is lower and partial positive charge is better carried by the tertiary carbon.
As setas curvas
- The alkene π pair fills the empty p orbital of boron at the less substituted carbon.
- The B–H pair delivers hydrogen to the methyl-bearing carbon on the same face.
Regiochemistry
Boron and hydrogen add in one step, but not symmetrically.
Hydroperoxide adds to boron
H2O2 and NaOH give the hydroperoxide anion. Its oxygen lone pair fills the empty p orbital of the six-electron boron, giving a borate.
As setas curvas
- A hydroperoxide lone pair bonds to boron.
Shift the ring carbon from boron to oxygen
The C–B bond pair migrates to the neighbouring oxygen while hydroxide leaves. The carbon keeps its bonding pair throughout, so its configuration is retained: the OH ends up exactly where boron was.
As setas curvas
- The C–B bond pair forms the new C–O bond.
- The weak O–O pair leaves with hydroxide.
Hydroxide adds to the borate ester
Boron is again six-electron and Lewis acidic, so hydroxide adds to it.
As setas curvas
- A hydroxide lone pair bonds to boron.
Release the alkoxide
The B–O bond to the ring oxygen breaks, freeing the alkoxide. The C–O bond is untouched, so the stereocentre is unchanged.
As setas curvas
- The B–O pair leaves with the ring oxygen as an alkoxide.
Protonate the alkoxide
Water protonates the alkoxide to give the neutral alcohol and regenerate hydroxide.
As setas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
trans-2-Methylcyclopentan-1-ol
H and OH were added syn, anti-Markovnikov: the OH is on the former CH carbon and the methyl, which ended up on the face opposite the added hydrogen, is trans to it. Both alkene faces react, so the product is racemic.
O que você vai praticar
Draw syn hydroboration of 1-methylcyclopentene, then HOO⁻ addition to boron, the alkyl shift with retention, and release of trans-2-methylcyclopentan-1-ol.
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