Wittig Reaction Lecture Notes

Introduction

• Wittig reaction: converts ketones into alkenes.
• Reactants: ketone and phosphonium ylide.
• Major Product: Alkene.

Mechanism Overview

1. Ketone Reaction:

   • Rotate ketone so carbonyl group faces CH2 group.
   • Replace oxygen with CH2 (or attached group).

2. Example 1:

   • Given: Ketone + phosphorus ylide.
   • Product: Alkene (replace O with ylide's group).
   • Can draw product in different orientations.

3. Example 2:

   • Given: Cyclohexanone + ylide (different representation).
   • Product: No cis/trans isomers due to symmetry.

Detailed Mechanism

1. Starting Materials:

   • Begin with triphenylphosphine (P attached to three benzene rings).
   • Phosphorus atom has a lone pair (Ph₃P).
   • Reaction with alkyl halide (methyl or primary preferred for SN2 reaction).
   • Phosphorus alkylated (P-ch2-ch3), gains positive charge.

2. Reacting with Butyl Lithium:

   • Abstracts proton from P-attached carbon (forms ylide).
   • Ylide has two resonance forms: (one with negative charge on carbon, one forming pi bond).
   • Reacts with ketone or aldehyde (e.g., acetaldehyde).

3. Cycle Formation:

   • Ylide carbon attacks carbonyl carbon, forms 4-membered ring (oxa-phosphetane).

4. Alkene Formation:

   • Bonds break: electrons go to more electronegative atoms (P and O).
   • By-product: Triphenylphosphine oxide.
   • Main product: Alkene (cis and trans isomers, trans often more stable).

Summary

• The Wittig reaction is used to synthesize alkenes from ketones/aldehydes using phosphorus ylides.
• Detailed understanding of electron movement key to predicting products.
• Typically forms a mixture of isomers, with trans being more stable.