Química Orgânica 1 · Transformações de alcinos
Hydroborate and oxidize a terminal alkyne
Condições: Hex-1-yne with disiamylborane (Sia₂BH, drawn as R₂BH with R = 1,2-dimethylpropyl) in THF, then aqueous NaOH and H₂O₂. The bulky borane adds only once. Hydroperoxide ion and water enter at the oxidation stage; Na⁺ is omitted. Stop at the enol; its tautomerization to hexanal is a separate exercise.
Desenhe este problema você mesmo
Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 7 passos escritos
Concerted syn hydroboration
Both bonds form in one four-center step, so B and H add to the same side (syn). Boron goes to the less hindered terminal carbon, especially with the bulky R groups; no carbocation forms.
As setas curvas
- One alkyne π pair forms a bond from the terminal carbon to boron's empty orbital.
- The B–H pair forms a C–H bond at the more substituted carbon on the same side as the new C–B bond.
Hydroperoxide adds to boron
Boron in the alkenylborane has only six valence electrons. Adding HOO⁻ gives a tetracoordinate borate with a negative formal charge on boron.
As setas curvas
- A lone pair on the negatively charged hydroperoxide oxygen bonds to the empty orbital on boron.
Shift the alkenyl group to oxygen
The C–B bond electrons form a C–O bond, keeping the alkene geometry. Hydroxide is the leaving group, and boron is neutral again.
As setas curvas
- The C–B pair forms a bond from the terminal carbon to the adjacent peroxide oxygen.
- The weak O–O bond breaks; hydroxide leaves with the pair.
Hydroxide adds to boron
The hydrolysis of the B–O bond to the alkenyl-bound oxygen begins the same way as the oxidation: a nucleophile fills the empty boron orbital.
As setas curvas
- A hydroxide lone pair bonds to boron.
Release the enolate
Breaking this B–O bond releases an enolate ion, which is stabilized by resonance with the C=C. The boron fragment becomes neutral R₂B–OH.
As setas curvas
- The B–O pair moves onto the alkenyl-bound oxygen.
Protonate the oxygen
Water protonates the enolate oxygen, giving the neutral enol and regenerating hydroxide.
As setas curvas
- An enolate oxygen lone pair takes a proton from water.
- The water O–H pair stays on oxygen as hydroxide.
Hex-1-en-1-ol (enol)
OH is at the terminal carbon and the added hydrogen is at the more substituted carbon: anti-Markovnikov hydration. The added H and OH are cis because B and H added syn and the shift kept the geometry, so the E enol forms. The enol tautomerizes to hexanal, an aldehyde, whereas Hg²⁺-catalyzed hydration of the same alkyne gives hexan-2-one.
O que você vai praticar
Follow concerted syn hydroboration of a terminal alkyne by a bulky borane and the peroxide-driven boron-to-oxygen shift to the anti-Markovnikov enol.
Você acabou de desenhar cada seta deste problema.
Cada um dos 5349 problemas de Orgânica 1 e 2 funciona assim: feedback em cada seta, dicas quando você travar e um modo prova sem dicas. Grátis com uma conta, sem cartão.
Travou na sua própria lição?
Tire uma foto: o primeiro scan é grátis, sem cadastro
Você escaneia toda semana? Scan Standard: por cerca de problemas