Hydroboration-Oxidation
Hydroboration-Oxidation: Boron and hydrogen add across the double bond in a single concerted step (syn addition) with boron bonding to the less substituted…
What happens
Boron and hydrogen add across the double bond in a single concerted step (syn addition) with boron bonding to the less substituted carbon; oxidative workup then replaces C–B with C–OH with retention of configuration, giving the anti-Markovnikov alcohol.
Reaction summary
Propene + BH₃ → trialkylborane → (H₂O₂/NaOH) → 1-propanol
Reagents and conditions
- Starting material: Propene
- Reagents: Borane (BH₃·THF); Hydrogen peroxide / NaOH (oxidative workup)
- Conditions: THF solvent; then H₂O₂ / NaOH
- Products: 1-Propanol (anti-Markovnikov)
Mechanism
- Concerted syn addition: BH₃ adds across the alkene in a four-centered transition state: boron (electrophilic, empty p-orbital) bonds to the less hindered carbon while hydrogen delivers to the more substituted carbon, both on the same face.
- Trialkylborane intermediate: After repeated additions, boron carries alkyl groups in place of its original hydrogens.
- Oxidation (H₂O₂/NaOH): Hydroperoxide anion attacks boron; the alkyl group migrates from boron to oxygen with retention of configuration, then hydrolysis releases the alcohol.
Selectivity
- Regioselectivity: Anti-Markovnikov: boron (and eventually OH) ends up on the less substituted carbon, opposite to acid-catalyzed hydration.
- Stereoselectivity: Syn addition, H and B add to the same face of the alkene, and configuration is retained during oxidation.
Common mistakes
- Confusing this with acid-catalyzed hydration, hydroboration is anti-Markovnikov and syn; oxymercuration/acid hydration are Markovnikov.
- Forgetting the oxidation step retains stereochemistry, no carbocation forms, so no rearrangement is possible.
Memory trick
"Boron goes where hydrogen would rather not": B (and later OH) lands on the less crowded carbon, anti-Markovnikov, syn, no rearrangements.