8.5 Hydration of Alkenes: Addition of H2O by Hydroboration

Hydroboration-oxidation turns alkenes into alcohols in two steps. First, borane adds to the double bond. Then, oxidation with hydrogen peroxide creates the alcohol. This reaction is special because it goes against the usual rules for adding stuff to alkenes.

The alcohol forms on the less crowded carbon, opposite to what you might expect. Also, the reaction keeps the 3D shape of the original alkene. These unique features make hydroboration-oxidation super useful for making specific alcohols from alkenes.

Hydroboration-Oxidation Reaction

Hydroboration-oxidation reaction sequence

Top images from around the web for Hydroboration-oxidation reaction sequence

• 

  Organic chemistry 20: Alkenes - oxymercuration, hydroboration View original

  Is this image relevant?

• 

  Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh ... View original

  Is this image relevant?

• 

  Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh ... View original

  Is this image relevant?

• 

  Organic chemistry 20: Alkenes - oxymercuration, hydroboration View original

  Is this image relevant?

• 

  Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Hydroboration-oxidation reaction sequence

• 

  Organic chemistry 20: Alkenes - oxymercuration, hydroboration View original

  Is this image relevant?

• 

  Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh ... View original

  Is this image relevant?

• 

  Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh ... View original

  Is this image relevant?

• 

  Organic chemistry 20: Alkenes - oxymercuration, hydroboration View original

  Is this image relevant?

• 

  Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh ... View original

  Is this image relevant?

1 of 3

• Two-step reaction sequence converts alkenes to alcohols
  • Step 1: Hydroboration
    • Borane ($BH_3$) adds across the alkene double bond forming a trialkylborane intermediate
    • Boron atom attaches to the less substituted carbon due to steric factors (less hindered)
  • Step 2: Oxidation
    • Trialkylborane intermediate treated with hydrogen peroxide ($H_2O_2$) under basic conditions (usually $NaOH$)
    • Carbon-boron bond replaced by a carbon-oxygen bond forming an alcohol
    • Oxygen atom attaches to the carbon previously bonded to boron

Regiochemistry and stereochemistry of hydroboration-oxidation

• Regiochemistry: anti-Markovnikov addition (exhibits regioselectivity)
  • Hydroxyl group ($-OH$) attaches to the less substituted carbon of the original alkene
  • Contrasts with acid-catalyzed hydration and oxymercuration-reduction (follow Markovnikov's rule)
  • Mechanism explains boron preferentially bonding to less hindered carbon in hydroboration step
• Stereochemistry: syn addition and stereospecific
  • Syn addition: boron and hydrogen add to the same face of the alkene in hydroboration step
    • Contrasts with bromine addition to alkenes (anti addition)
  • Stereospecific: alkene stereochemistry retained in product alcohol
    • Contrasts with acid-catalyzed hydration (not stereospecific)

Product prediction in hydroboration-oxidation

• General rule: $-OH$ group attaches to less substituted carbon of original alkene
• Monosubstituted alkenes (1-butene): $-OH$ group on terminal carbon
• Disubstituted alkenes:
  1. $-OH$ group on less substituted carbon
  2. If carbons equally substituted, steric hindrance determines major product
     • $-OH$ group preferentially forms on carbon with less bulky substituents
• Trisubstituted alkenes: $-OH$ group on only unsubstituted carbon
• Cyclic alkenes follow same rules ($-OH$ group on less substituted carbon)
• Stereochemistry retained:
  • $(E)$-alkenes form anti product
  • $(Z)$-alkenes form syn product

Mechanism and intermediate species

• Hydroboration step:
  • Borane acts as an electrophile, alkene as a nucleophile
  • Forms a four-centered transition state
  • Results in a trialkylborane intermediate
• Oxidation step:
  • Hydroxide ion attacks the boron atom
  • Hydride migrates from boron to carbon
  • Peroxide displaces hydroxide, followed by rearrangement
  • Yields the final alcohol product