Química Orgánica 1 · Estereoquímica de las reacciones de alquenos
trans-2-Methylcyclopentan-1-ol from 1-methylcyclopentene
Condiciones: BH3·THF; then H2O2, NaOH
Dibuja este problema tú mismo
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
Add B and H from one face
Borane adds in one four-centre step: boron bonds to the less substituted alkene carbon and hydrogen to the methyl-bearing carbon, both from the same face. Boron goes where steric crowding is lower and partial positive charge is better carried by the tertiary carbon.
Las flechas curvas
- The alkene π pair fills the empty p orbital of boron at the less substituted carbon.
- The B–H pair delivers hydrogen to the methyl-bearing carbon on the same face.
Regiochemistry
Boron and hydrogen add in one step, but not symmetrically.
Hydroperoxide adds to boron
H2O2 and NaOH give the hydroperoxide anion. Its oxygen lone pair fills the empty p orbital of the six-electron boron, giving a borate.
Las flechas curvas
- A hydroperoxide lone pair bonds to boron.
Shift the ring carbon from boron to oxygen
The C–B bond pair migrates to the neighbouring oxygen while hydroxide leaves. The carbon keeps its bonding pair throughout, so its configuration is retained: the OH ends up exactly where boron was.
Las flechas curvas
- The C–B bond pair forms the new C–O bond.
- The weak O–O pair leaves with hydroxide.
Hydroxide adds to the borate ester
Boron is again six-electron and Lewis acidic, so hydroxide adds to it.
Las flechas curvas
- A hydroxide lone pair bonds to boron.
Release the alkoxide
The B–O bond to the ring oxygen breaks, freeing the alkoxide. The C–O bond is untouched, so the stereocentre is unchanged.
Las flechas curvas
- The B–O pair leaves with the ring oxygen as an alkoxide.
Protonate the alkoxide
Water protonates the alkoxide to give the neutral alcohol and regenerate hydroxide.
Las flechas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
trans-2-Methylcyclopentan-1-ol
H and OH were added syn, anti-Markovnikov: the OH is on the former CH carbon and the methyl, which ended up on the face opposite the added hydrogen, is trans to it. Both alkene faces react, so the product is racemic.
Qué vas a practicar
Draw syn hydroboration of 1-methylcyclopentene, then HOO⁻ addition to boron, the alkyl shift with retention, and release of trans-2-methylcyclopentan-1-ol.
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