Química Orgânica 1 · Transformações de alcinos
Disiamylborane hydroboration–oxidation of hex-1-yne
Condições: Sia₂BH, THF; then H₂O₂, NaOH, 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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Concerted syn hydroboration
In one four-centre step the alkyne π pair bonds carbon to boron's empty p orbital while the B–H pair delivers hydride to the other carbon. B and H add to the same face (syn); boron goes to the less hindered carbon and no carbocation forms.
As setas curvas
- The π pair bonds C to the empty orbital on boron.
- The B–H pair forms the new C–H bond on the same face.
Hydroperoxide adds to boron
Under the basic oxidation conditions, HOO⁻ (from H₂O₂ and NaOH) fills boron's empty orbital, giving a negatively charged borate.
As setas curvas
- A lone pair of the negatively charged peroxide oxygen bonds to boron.
Alkenyl group shifts from boron to oxygen
The C–B pair migrates to the adjacent oxygen as the weak O–O bond breaks, expelling hydroxide. The C=C geometry is kept.
As setas curvas
- The C–B bond pair forms the new C–O bond.
- The O–O bond breaks; hydroxide leaves with the pair.
Hydroxide adds to boron
Boron is electron-deficient again, so hydroxide bonds to it, setting up cleavage of the B–O bond to the enol oxygen.
As setas curvas
- A hydroxide lone pair bonds to boron.
Release the enolate
The B–O bond breaks with both electrons going to oxygen, releasing a resonance-stabilized enolate and a neutral boron by-product.
As setas curvas
- The B–O pair moves onto oxygen, forming the enolate.
Why a bulky borane?
Choosing the borane.
Protonate the enolate at carbon
Water protonates the enolate at the α-carbon while the oxygen lone pair forms the C=O bond: base-mediated tautomerization to the carbonyl compound.
As setas curvas
- The oxygen lone pair forms the C=O π bond.
- The C=C π pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Carbonyl product
The keto form is the stable tautomer, so the carbonyl compound is isolated; boron ends up as a borate/boric acid derivative.
O que você vai praticar
Draw syn hydroboration of hex-1-yne by disiamylborane, the peroxide oxidation to the enolate, and its protonation at carbon to hexanal.
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