Lecture on Oxymercuration-Demercuration of Alkenes

Introduction

• Oxymercuration-Demercuration: A method to hydrate alkenes by adding water across the double bond.
• Comparison with Acid-Catalyzed Hydration: Unlike acid-catalyzed hydration, this method does not involve carbocation intermediates, avoiding rearrangements.

General Reaction Mechanism

1. Oxymercuration Step:
   • Water is added to the alkene using a mercury catalyst, forming a mercurinium ion intermediate.
2. Demercuration Step:
   • Sodium borohydride reduces the mercury intermediate, yielding the final hydrated alkene product.

• Stereospecificity: Water adds in an 'anti' fashion across the double bond (hydroxyl group on one side, proton on the opposite).

Mechanism Details

• Mercury Catalyst:
  • Structure: Mercury diacetate that loses an acetate group to form the active catalyst.
  • Reaction with Alkene: Mercury cation adds to alkene, forming a cyclic intermediate.

Mechanism Steps

1. Formation of Cyclic Intermediate:
   • The pi bond of the alkene attacks the mercury cation, mercury lone pair forms a back-bond creating a cyclic intermediate.
2. Water Nucleophilic Attack:
   • Water attacks the carbon of the cyclic intermediate, opposite the mercury, leading to stereospecific anti-addition.
3. Deprotonation:
   • A water molecule deprotonates the intermediate to stabilize it.
4. Final Reduction:
   • Sodium borohydride replaces the mercury-carbon bond with a hydrogen-carbon bond.

Example Reaction

• Substrate: Consider an alkene treated with mercury diacetate and sodium borohydride.
• Products:
  • Only the Markovnikov product is formed (more substituted alcohol), avoiding carbocation rearrangement.
• Regiospecificity: Ensures addition to the more substituted carbon.

Detailed Example

• Stereospecific Reaction:
  • Produces specific stereoisomers due to nature of water attack.
  • Only certain enantiomers formed because of anti-addition of hydroxyl and proton.

Mechanism of Stereospecificity

• Formation of Cyclic Intermediates:
  • Mercury can approach from different directions leading to different cyclic intermediates.
• Water Attack:
  • Must attack opposite to mercury, determining the stereochemistry of final product.
• Enantiomers Produced:
  • Two enantiomers are formed; methyl groups are trans.

Conclusion

• The reaction is both regiospecific (yielding one regioisomer) and stereospecific (resulting in select stereoisomers).
• Practical Tip: Understanding the mechanism thoroughly allows prediction of product stereochemistry and regiochemistry.