Química Orgánica 1 · Repaso de estereoquímica para el examen
Hydroboration–oxidation of 1-ethylcyclohexene gives trans-2-ethylcyclohexan-1-ol
Condiciones: BH3·THF; then H2O2, NaOH
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 8 pasos escritos
Syn addition of B–H across the alkene
Boron, the electrophile, bonds to the less substituted ring CH while the hydride is delivered to the ethyl-bearing carbon in the same four-centre transition state. H and B add to the same face, so the new hydrogen and boron are cis.
Las flechas curvas
- The π pair bonds to the empty orbital of boron at the less hindered carbon.
- The B–H pair delivers the hydride to the more substituted carbon.
Where do H and B go?
Hydroboration is concerted, so there is no carbocation and no rearrangement.
Hydroxide deprotonates hydrogen peroxide
In the oxidation, hydroxide removes a proton from hydrogen peroxide to give the hydroperoxide anion, the nucleophile that attacks boron.
Las flechas curvas
- Hydroxide takes the O–H proton of hydrogen peroxide.
- The O–H pair stays on oxygen, giving the hydroperoxide anion.
Hydroperoxide adds to boron
The hydroperoxide anion bonds to the empty p orbital of boron, giving a tetrahedral borate.
Las flechas curvas
- A lone pair on the hydroperoxide oxygen fills the empty orbital on boron.
Carbon migrates from boron to oxygen
The ring carbon moves with its bonding pair from boron to the adjacent oxygen as hydroxide leaves; the carbon keeps its configuration, so oxygen ends up exactly where boron was.
Las flechas curvas
- The C–B bond pair forms the new C–O bond.
- The O–O bond breaks and hydroxide leaves.
Hydroxide cleaves the B–O bond
Hydroxide adds to boron and the B–O bond breaks, releasing the alkoxide of the product.
Las flechas curvas
- A hydroxide lone pair bonds to boron.
- The B–O pair moves onto the alkoxide oxygen.
Water protonates the alkoxide
The alkoxide takes a proton from water to give the neutral alcohol.
Las flechas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
trans-2-Ethylcyclohexan-1-ol
The OH is on the less substituted carbon (anti-Markovnikov) and trans to the ethyl group. Addition to the other face gives the enantiomer, so the trans alcohol is racemic.
Qué vas a practicar
Draw the syn addition of borane to 1-ethylcyclohexene and the oxidation with basic hydrogen peroxide, and explain why the alcohol is trans to the ethyl group.
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