Wittig Reaction
Wittig Reaction: The nucleophilic ylide carbon attacks the carbonyl carbon, forming a four-membered oxaphosphetane ring; this ring then collapses (a…
What happens
The nucleophilic ylide carbon attacks the carbonyl carbon, forming a four-membered oxaphosphetane ring; this ring then collapses (a retro-[2+2]), forming the new C=C double bond and the very strong P=O bond that drives the whole reaction forward.
Reaction summary
PhCHO + Ph₃P=CH₂ → PhCH=CH₂ (styrene) + Ph₃P=O
Reagents and conditions
- Starting material: Benzaldehyde
- Reagents: Methylenetriphenylphosphorane (Wittig ylide)
- Products: Styrene; Triphenylphosphine oxide (byproduct)
Mechanism
- Ylide addition: The ylide's carbanion-like carbon attacks the aldehyde's carbonyl carbon, forming a four-membered oxaphosphetane ring.
- Oxaphosphetane intermediate: A strained four-membered ring containing both the new C–C bond and a C–O–P linkage.
- Retro-[2+2] collapse: The ring collapses, breaking the C–O and P–C bonds and forming the new alkene plus the very stable P=O bond of triphenylphosphine oxide, this thermodynamic driving force is why the Wittig reaction works so reliably.
Selectivity
- Stereoselectivity: Non-stabilized ylides tend to give (Z)-alkenes; stabilized ylides (with an adjacent ester/carbonyl) tend to give (E)-alkenes.
Common mistakes
- Forgetting the ylide must be pre-formed from a phosphonium salt with a strong base before it can react.
- Trying to use a Wittig on a substrate with an acidic proton, the ylide (itself a strong base) would just deprotonate it instead.
Memory trick
"Wittig swaps C=O for C=C": the carbonyl oxygen leaves as part of Ph₃P=O, and a new alkene takes its place.