Imine synthesis — nitrogen analog of Wittig
Aza-Wittig reaction
How the Aza-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: R–N=PPh₃ (from R–N₃ + PPh₃, Staudinger), aldehyde or ketone. Conditions: THF or toluene, 25–110 °C, anhydrous. Bonds formed: C=N; P=O; bonds broken: C=O π; P=N π. Stereochemistry: Imine E/Z set at ring collapse; no stereocentre needed. Rate and selectivity: Cycloreversion is fast; driven by strong P=O bond (≈ 130 kcal/mol).
At a glance
Step 1: Make the iminophosphorane
An iminophosphorane (R–N=PPh₃, made from an azide + PPh₃) carries a nucleophilic nitrogen, stabilized by adjacent phosphorus — the nitrogen analog of a Wittig ylide's carbon.
Step 2: N attacks C=O: azaoxaphosphetane
That nitrogen attacks a ketone or aldehyde's carbonyl carbon, forming a four-membered azaoxaphosphetane ring — directly parallel to the Wittig's oxaphosphetane.
Step 3: Ring collapses to imine + Ph₃P=O
The ring collapses, ejecting triphenylphosphine oxide and forming a new C=N bond — this installs an imine exactly where a Wittig would install an alkene.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Ph
- Phenyl. –C₆H₅.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- PPh3
- Triphenylphosphine. Nucleophilic phosphine; forms very strong P=O bonds, driving Wittig, Appel and Mitsunobu reactions.
How it compares with related reactions
| Aza-Wittig reaction | Baeyer-Drewson indigo synthesis | Bamford-Stevens reaction | |
|---|---|---|---|
| Reagents | R–N=PPh₃ (from R–N₃ + PPh₃, Staudinger), aldehyde or ketone | o-Nitrobenzaldehyde, acetone, aq. NaOH | Tosylhydrazone, strong base (NaOMe, NaH, or NaOEt) |
| Conditions | THF or toluene, 25–110 °C, anhydrous | H₂O/acetone, 20–25 °C, then air | Aprotic solvent (diglyme, toluene), 100–200 °C |
| Stereochemistry | Imine E/Z set at ring collapse; no stereocentre needed | Product is the E-indigo, planar and strongly H-bonded | Mixture of E/Z; thermodynamic (more substituted) alkene favoured |
| Rate / selectivity | Cycloreversion is fast; driven by strong P=O bond (≈ 130 kcal/mol) | Aldol step fast; indigo precipitates as deep blue solid | Diazo decomposition to carbene is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Aza-Wittig reaction?
R–N=PPh₃ (from R–N₃ + PPh₃, Staudinger), aldehyde or ketone. Conditions: THF or toluene, 25–110 °C, anhydrous.
Which bonds are formed and which are broken?
Formed: C=N; P=O. Broken: C=O π; P=N π.
What is the stereochemical outcome?
Imine E/Z set at ring collapse; no stereocentre needed
What controls the rate or the selectivity?
Cycloreversion is fast; driven by strong P=O bond (≈ 130 kcal/mol)
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