Polonovski Reaction

What is Polonovski reaction?

Polonovski reaction is organic rearrangement type reaction used for the N-demethylation of tert-amines. The activation of tertiary amine N-oxides with anhydrides or acyl halides to prepare the corresponding intermediates (iminium ion). Polonovski Reaction known as n-oxide rearrangement.

Polonovski reported that the alkaloid N-oxides treatment with acetyl chloride or acetic anhydride underwent a rearrangement (polonovski rearrangement) in which one of the alkyl groups linked to the nitrogen was cleaved (pyrolysis of amine oxides) the N-acetyl derivative of the alkaloid was obtained.

R₁ = H, alkyl, aryl, heteroaryl; R_2-3 = CH₃, alkyl, aryl; R₄ = CH₃, CF₃ (Polonovski-Potier rxn); X = Cl, Br, OCOR₄, BF₄ ^– , ClO₄ ^– , SbF₆ ^–

Polonovski Reaction Mechanism :

The conversion of the O-acylimonium salt to the imine proceeds via an E2-type elimination. The hydrogen that is
antiperiplanar to the N-O bond is usually removed preferentially. When the N-oxide is activated with iron salts, a SET mechanism is operational, while with SO₂ an intramolecular ionic mechanism is most likely.

Related reactions:

1. Beckmann Rearrangement
2. Benzilic Acid Rearrangement
3. Brook Rearrangement
4. Favorskii Rearrangement
5. Lossen Rearrangement
6. Prins-Pinacol Rearrangement
7. Pummerer Rearrangement

Reference:

1. Ferris, J. P. etal. J. Am. Chem. Soc. 1967, 89, 5270-5275.
2. Ziegler,etal J. Org. Chem. 1997, 62, 1083-1094.