Química Orgânica 1 · Adição a alcenos e regioquímica
Hydroboration–oxidation of 2-methylpent-2-ene
Condições: BH3·THF, THF, 0 °C; then H2O2, NaOH, H2O
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Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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Concerted syn hydroboration
The π bond donates to the empty p orbital of boron at the less substituted alkene carbon while a B–H hydrogen moves, with its electron pair, to the more substituted carbon. All bonds change in one four-centre transition state, so H and B add to the same face.
As setas curvas
- The π pair forms the C–B bond at the less substituted carbon.
- The B–H pair moves with the hydrogen to the more substituted carbon.
Which carbon gets boron?
Boron goes to the less hindered alkene carbon (one alkyl group), and the partial positive charge in the transition state sits on the more substituted carbon, which receives the hydride.
Hydroperoxide adds to boron
In basic hydrogen peroxide, the hydroperoxide anion uses an oxygen lone pair to fill the empty p orbital of the alkylborane, giving a tetracoordinate borate.
As setas curvas
- The hydroperoxide oxygen lone pair forms the B–O bond.
Alkyl shift from boron to oxygen
The alkyl group migrates from boron to the adjacent oxygen with its bonding pair as the weak O–O bond breaks and hydroxide leaves. The carbon keeps its configuration, so oxygen ends up exactly where boron was.
As setas curvas
- The C–B pair moves with the alkyl group to oxygen.
- The O–O bond breaks and hydroxide leaves.
Hydroxide adds to boron
Hydroxide fills the empty p orbital of the alkoxyborane, making a borate that can release the alkoxide.
As setas curvas
- The hydroxide lone pair forms a B–O bond.
The alkoxide is released
The B–O bond to the former alkyl oxygen breaks, releasing the alkoxide and a neutral hydroxyborane.
As setas curvas
- The B–O bonding pair stays on the alkoxide oxygen.
Water protonates the alkoxide
Water, the solvent, donates a proton to the alkoxide, giving the alcohol and hydroxide.
As setas curvas
- The alkoxide lone pair bonds to a water hydrogen.
- The O–H pair stays on oxygen as hydroxide.
2-Methylpentan-3-ol
OH sits on the former less substituted alkene carbon and H on the more substituted one: the anti-Markovnikov alcohol.
O que você vai praticar
Draw the concerted addition of H–B across 2-methylpent-2-ene with boron on the less substituted carbon, then the oxidation (hydroperoxide on boron, alkyl shift to oxygen, borate cleavage, protonation) giving 2-methylpentan-3-ol.
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