Hydroboration

Alkene Reactions: Hydroboration using BH3, H2O2, and NaOH

Anti-Markovnikov alcohols via hydroboration–oxidation (BH₃·THF; then H₂O₂, NaOH)

Borane–tetrahydrofuran (BH₃·THF) hydroborates an alkene syn, placing boron on the less substituted carbon. Basic hydrogen peroxide (H₂O₂/NaOH) then converts the C–B bond into a C–O bond with retention at the migrating carbon. The overall transformation is predictable and closed-shell—no carbocations, no rearrangements—and the syn relationship of the incoming H and OH is preserved in the alcohol product.


Quick Summary

  • Reagents/conditions: Step 1 BH₃·THF (0–25 °C, anhydrous THF). Step 2 H₂O₂ + NaOH(aq) (0–25 °C).
  • Outcome: Anti-Markovnikov hydration; H and OH add to the same face (overall syn addition).
  • Regioselectivity: Boron targets the less substituted carbon; oxidation substitutes B with OH at that carbon.
  • Stereochemistry: Syn hydroboration followed by retentive oxidation → syn H/OH in the product.
  • No rearrangements: Entire pathway is closed-shell—no carbocations or radical “peroxide effect.”


Mechanism (6 Steps)

Class: Concerted hydroboration followed by peroxide oxidation and hydrolysis.

Step 1: syn hydroboration
Step 1 — BH₃ adds across the C=C through a four-center transition state: H → more substituted carbon, B → less substituted carbon (syn).

Hydroboration is stereospecific and repeats up to three times per BH₃ molecule, generating a trialkylborane.

Step 2: hydroperoxide attack
Step 2 — Hydroperoxide anion (HOO⁻), produced under basic conditions, adds to boron to form a peroxyborate.

NaOH activates H₂O₂ to deliver HOO⁻, which coordinates the electron-poor boron center.

Step 3: alkyl migration
Step 3 — An alkyl group migrates from boron to oxygen, displacing hydroxide and retaining configuration at carbon.

The B→O migration keeps the migrating carbon’s stereochemistry, locking in the syn relationship.

Step 4: borate reorganization
Step 4 — Interaction with base rearranges the borate, positioning boron for departure.

Boron remains coordinated while the negatively charged oxygen readies for protonation.

Step 5: water protonation
Step 5 — Water protonates the alkoxide oxygen, beginning liberation of the alcohol.

Proton transfer neutralises the oxygen while boron prepares to exit as borate.

Step 6: hydrolysis
Step 6 — Hydrolysis removes boron as borate, releasing the anti-Markovnikov alcohol with syn H/OH.

The former boron-bearing carbon now bears the OH group, and the hydrogen added in Step 1 remains syn.


Mechanistic Checklist

  • Hydroboration is concerted and syn—do not draw carbocations or radicals.
  • Boron binds the less substituted carbon; oxidation retains configuration at carbon.
  • Each BH₃ can hydroborate three alkenes (trialkylborane stage).
  • Bulky boranes (9-BBN, Sia₂BH) improve terminal selectivity but follow the same pathway.
  • Hydrolysis removes boron completely; no rearrangements occur.


Worked Example

Substrate: 2-methyl-2-pentene
Substrate — 2-methylpent-2-ene
Reagents: BH3·THF, then H2O2, NaOH
BH₃·THF; then H₂O₂, NaOH
Product: 2-methylpentan-3-ol
Product — 2-methylpentan-3-ol (racemic anti-Markovnikov; syn H/OH)


Multiple Alkenes & Selectivity

  • Less hindered, electron-rich alkenes hydroborate fastest; BH₃ often adds to three double bonds before oxidation.
  • Terminal selectivity improves with bulky boranes (9-BBN, Sia₂BH); directing effects (e.g., allylic OH) depend on the specific borane and conditions—no Zn chelation is involved in BH₃·THF hydroboration.


Practical Tips & Pitfalls

  • Safety: BH₃·THF is reactive and fuming—handle cold, under inert atmosphere. H₂O₂ is an oxidizer; add slowly to control exotherms.
  • Order of addition: Complete hydroboration before introducing NaOH/H₂O₂; peroxide without base is sluggish.
  • Workup: Ensure full oxidation to remove boron; borates cling to glassware—rinse promptly.
  • Avoid misconceptions: This is not the radical "peroxide effect" seen with HBr/ROOR; the entire sequence is polar and closed-shell.


Exam-Style Summary

  • Reagents: BH₃·THF, then H₂O₂/NaOH.
  • Mechanism: Syn hydroboration → peroxide attack on boron → retentive alkyl migration → base-assisted collapse → hydrolysis.
  • Outcome: Anti-Markovnikov alcohol (syn H/OH), no rearrangements.
  • Selectivity: B prefers the less substituted carbon; bulky boranes enhance terminal selectivity.


Interactive Toolbox

  • Compare hydroboration–oxidation with acid-catalyzed hydration (Markovnikov, possible rearrangements) in the Reaction Solver.
  • Export the six-step mechanism panels via the Mechanism Explorer.


FAQ / Exam Notes

Why is the addition anti-Markovnikov? The hydroboration transition state positions boron at the less hindered carbon; oxidation then maps B → OH at that carbon.

Is the addition syn or anti? Hydroboration is syn, and the oxidation step retains configuration—H and OH end up syn overall.

Can I use bulky boranes? Yes. 9-BBN or Sia₂BH improves terminal selectivity but proceeds through the same mechanism.

Do rearrangements occur? No. The pathway is closed-shell with no carbocations or radicals, so 1,2-shifts are absent.

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