Alkene synthesis — ylide chemistry
Wittig reaction: ylide + acetaldehyde → alkene
How the Wittig reaction works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Ph₃P=CH₂ (ylide, from Ph₃P⁺CH₃ Br⁻ + n-BuLi); acetaldehyde. Conditions: THF, 0 °C → 25 °C; anhydrous, inert atmosphere. Bonds formed: C=C π; P=O (very strong, ~130 kcal/mol); bonds broken: C=O π; P–C. Stereochemistry: Unstabilised ylides: Z-alkene; stabilised: E-alkene. Rate and selectivity: Driven by the strong P=O bond of Ph₃P=O.
At a glance
Step 1: Ylide carbon attacks C=O
A phosphorus ylide carries a carbanion-like carbon stabilized by the adjacent, electropositive phosphorus. That carbon is strongly nucleophilic and attacks the electrophilic carbonyl carbon of the aldehyde.
Step 2: Oxaphosphetane forms
This forms a strained four-membered ring — an oxaphosphetane — as the new C–C bond and a P–O bond form together.
Step 3: Retro-[2+2]: alkene + Ph₃P=O
The ring collapses in a retro-[2+2]-like step: the C–C bond stays, a new C=C π bond forms, and triphenylphosphine oxide is ejected. The net effect is installing an alkene exactly where the carbonyl used to be.
Abbreviations and terms
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- n-BuLi
- n-Butyllithium. Very strong base/nucleophile (pKa of butane ≈ 50); deprotonates alkynes and does lithium–halogen exchange.
- PPh3
- Triphenylphosphine. Nucleophilic phosphine; forms very strong P=O bonds, driving Wittig, Appel and Mitsunobu reactions.
- TPPO
- Triphenylphosphine oxide. The thermodynamic sink of Wittig/Mitsunobu/Appel reactions.
How it compares with related reactions
| Wittig reaction | Horner-Wadsworth-Emmons (HWE) reaction | Julia-Kocienski olefination | |
|---|---|---|---|
| Reagents | Ph₃P=CH₂ (ylide, from Ph₃P⁺CH₃ Br⁻ + n-BuLi); acetaldehyde | Phosphonate ester (EtO)₂P(O)CH₂CO₂Et, NaH or NaOMe, aldehyde | 1-phenyl-1H-tetrazol-5-yl (PT) sulfone, KHMDS (or NaHMDS, LiHMDS), aldehyde |
| Conditions | THF, 0 °C → 25 °C; anhydrous, inert atmosphere | THF or DME, 0–25 °C | THF or DME, –78 °C → 25 °C |
| Stereochemistry | Unstabilised ylides: Z-alkene; stabilised: E-alkene | Strong E-selectivity (trans) | Highly E-selective (anti-adduct, then anti elimination) |
| Rate / selectivity | Driven by the strong P=O bond of Ph₃P=O | Aldehyde addition rate-limiting; water-soluble phosphate easily removed | One-pot; Smiles shift and elimination are fast after addition |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Wittig reaction?
Ph₃P=CH₂ (ylide, from Ph₃P⁺CH₃ Br⁻ + n-BuLi); acetaldehyde. Conditions: THF, 0 °C → 25 °C; anhydrous, inert atmosphere.
Which bonds are formed and which are broken?
Formed: C=C π; P=O (very strong, ~130 kcal/mol). Broken: C=O π; P–C.
What is the stereochemical outcome?
Unstabilised ylides: Z-alkene; stabilised: E-alkene
What controls the rate or the selectivity?
Driven by the strong P=O bond of Ph₃P=O
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go