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AdditionIntermediateNobel Prize 1954
Wittig Reaction
Also known as: Wittig Olefination
Discovered by Georg Wittig (1954)
General Equation
R₂C=O + Ph₃P=CHR' → R₂C=CHR' + Ph₃P=O
Description
Converts a C=O to a C=C: the phosphorus ylide reacts with an aldehyde/ketone to give an alkene and triphenylphosphine oxide. The position of the new double bond is unambiguous (unlike elimination). Nobel Prize 1979.
Reaction Scheme
Reactants:
- • Aldehyde or Ketone
Products:
- • Alkene + Ph₃P=O (triphenylphosphine oxide)
Reagents:
- • Phosphonium ylide (Ph₃P=CHR)
Conditions:
- • THF or Et₂O, RT to reflux
Mechanism (3 steps)
1
Ylide preparation: An alkyl halide reacts with PPh₃ to form a phosphonium salt. Treatment with strong base (n-BuLi) generates the ylide (Ph₃P=CHR).
Arrow: curvedPh₃P=CHR (Wittig reagent)
Ylide has carbanion character stabilized by phosphorus
2
The ylide carbon attacks the carbonyl carbon, forming a betaine intermediate that cyclizes to an oxaphosphetane (4-membered ring with O and P).
Arrow: curvedOxaphosphetane (4-membered ring)
Key intermediate — determines E/Z selectivity
3
The oxaphosphetane undergoes retro-[2+2] cycloreversion to give the alkene and triphenylphosphine oxide (strong P=O bond is the driving force).
Arrow: retro
Thermodynamically driven by strong P=O bond
Key Points to Remember
- ✓Creates C=C exactly where C=O was — no ambiguity
- ✓Non-stabilized ylides → Z-alkene (kinetic)
- ✓Stabilized ylides (EWG on α-C) → E-alkene
- ✓HWE modification (phosphonate esters) gives E-alkene selectively
- ✓Complementary to Grignard (which gives alcohols)
- ✓Georg Wittig — Nobel Prize 1979
Examples
Reactant:
Cyclohexanone + Ph₃P=CH₂
Reagent:
THF
Product:
Methylenecyclohexane
Yield: 85%
Reactant:
Benzaldehyde + Ph₃P=CHCO₂Et
Reagent:
THF (stabilized ylide → E)
Product:
Ethyl (E)-cinnamate
Yield: 90%
Synthetic Utility
Essential for total synthesis — places the C=C double bond at an exact, predetermined position. The "disconnection" approach in retrosynthesis relies heavily on Wittig.
Limitations
- ⚠Ph₃P=O byproduct difficult to separate
- ⚠Non-stabilized ylides are moisture-sensitive
- ⚠Sterically hindered ketones may give low yields
- ⚠E/Z selectivity can be unpredictable with some substrates
#C=C formation#olefination#ylide#Nobel#retrosynthesis