N-oxide activation — iminium generation
Polonovski reaction
How the Polonovski 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: Tertiary amine N-oxide, Ac₂O (or TFAA); then Nu⁻ (CN⁻, H⁻, enol). Conditions: CH₂Cl₂, 0–25 °C. Bonds formed: N–O–Ac (then C=N⁺); C–Nu; bonds broken: N–O; α C–H. Stereochemistry: Iminium regiochemistry set by the more acidic/accessible α-H. Rate and selectivity: O-acylation fast; elimination is rate-limiting.
At a glance
Step 1: N-oxide O is acylated
A tertiary amine N-oxide reacts with acetic anhydride, converting the N-oxide oxygen into an acyloxy leaving group tethered directly to nitrogen — the same initial activation step used in the Boekelheide rearrangement, just on a simple tertiary amine instead of a heteroaromatic system.
Step 2: Acetate removes α-H; N–O breaks
Loss of that acyloxy leaving group, with simultaneous loss of a proton from the carbon alpha to nitrogen, generates an iminium ion directly — an E1cb-like elimination centered on nitrogen instead of the more familiar carbon-based eliminations covered throughout this library.
Step 3: Nucleophile adds to the iminium
This iminium ion is now available for the same nucleophilic addition chemistry used throughout — attack by cyanide, an enol, or hydride delivers a new substituent alpha to the original amine nitrogen, making this a key ring-functionalization step in numerous alkaloid total syntheses.
Abbreviations and terms
- Nu
- Nucleophile. An electron-rich species that donates an electron pair to form a new bond.
- Ac
- Acetyl. –C(=O)CH₃.
- OAc
- Acetoxy. –OC(=O)CH₃, an acetate group.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- Ac2O
- Acetic anhydride. Acetylating agent.
- E2
- Bimolecular elimination. Concerted base removal of β-H and loss of the leaving group (anti-periplanar).
- E1cB
- Elimination, unimolecular conjugate base. Deprotonation first to a carbanion, then loss of the leaving group.
How it compares with related reactions
| Polonovski reaction | Appel reaction | Bamford-Stevens reaction | |
|---|---|---|---|
| Reagents | Tertiary amine N-oxide, Ac₂O (or TFAA); then Nu⁻ (CN⁻, H⁻, enol) | PPh₃, CBr₄ (or CCl₄) | Tosylhydrazone, strong base (NaOMe, NaH, or NaOEt) |
| Conditions | CH₂Cl₂, 0–25 °C | CH₂Cl₂, 0–25 °C | Aprotic solvent (diglyme, toluene), 100–200 °C |
| Stereochemistry | Iminium regiochemistry set by the more acidic/accessible α-H | Inversion at carbon (SN2) | Mixture of E/Z; thermodynamic (more substituted) alkene favoured |
| Rate / selectivity | O-acylation fast; elimination is rate-limiting | Driven by strong P=O bond; works best on 1° and 2° alcohols | 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 Polonovski reaction?
Tertiary amine N-oxide, Ac₂O (or TFAA); then Nu⁻ (CN⁻, H⁻, enol). Conditions: CH₂Cl₂, 0–25 °C.
Which bonds are formed and which are broken?
Formed: N–O–Ac (then C=N⁺); C–Nu. Broken: N–O; α C–H.
What is the stereochemical outcome?
Iminium regiochemistry set by the more acidic/accessible α-H
What controls the rate or the selectivity?
O-acylation fast; elimination is rate-limiting
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go