Química Orgânica 2 · Rotas de síntese: reagentes, grupos protetores e planejamento em várias etapas
BBr₃ removes the methyl group from 4-methylanisole
Condições: 4-Methylanisole, BBr₃ (1.2 equiv), CH₂Cl₂, −78 °C → 25 °C; then H₂O
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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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The ether oxygen binds boron
Boron in BBr₃ has only six valence electrons and an empty p orbital. A lone pair on the ether oxygen fills it, giving a Lewis acid–base adduct with O⁺ and B⁻.
As setas curvas
- The oxygen lone pair forms the O–B bond.
A bromide leaves boron
One B–Br bond breaks heterolytically, releasing bromide and leaving a neutral three-coordinate boron on the positively charged oxygen.
As setas curvas
- The B–Br pair leaves with bromine as bromide.
Which C–O bond breaks?
The oxonium oxygen is attached to a methyl carbon and an aromatic ring carbon; only one of them can be attacked by bromide.
Bromide attacks the methyl carbon
Bromide performs an SN2 displacement at the methyl carbon. The C–O bond breaks and the pair stays on the oxygen, which is now bonded to the ring and to boron.
As setas curvas
- The bromide lone pair attacks the methyl carbon from the back.
- The C–O pair stays on oxygen.
Water adds to boron during workup
When water is added, a water lone pair fills the empty p orbital of the aryloxydibromoborane.
As setas curvas
- The water lone pair forms the O–B bond.
A second water removes a proton
Another water molecule takes a proton from the boron-bound oxonium oxygen, giving hydronium.
As setas curvas
- The water lone pair takes the proton.
- The O–H pair stays on the boron-bound oxygen.
Hydronium protonates the aryl oxygen
Hydronium gives its proton to the oxygen bonded to the ring, so that this oxygen can leave boron as a neutral phenol.
As setas curvas
- The aryl oxygen lone pair takes the proton.
- The O–H pair stays on oxygen, leaving water.
The B–O bond breaks and the phenol is released
The bond between boron and the protonated aryl oxygen breaks with both electrons going to oxygen, releasing 4-methylphenol and leaving a neutral three-coordinate boron.
As setas curvas
- The O–B pair returns to the aryl oxygen.
4-Methylphenol and bromomethane
The methyl protecting group has left as bromomethane and the free phenol is released. The remaining B–Br bonds are hydrolysed by more water to boric acid and HBr. Methyl ethers survive bases, Grignard reagents, hydrides and oxidants, so BBr₃ is the usual way to unmask the phenol at the end of a route.
O que você vai praticar
Draw the Lewis acid–base adduct of 4-methylanisole with boron tribromide, the bromide SN2 at the methyl carbon, and the aqueous workup that releases 4-methylphenol, the deprotection of a phenol methyl ether.
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