Sommelet-Hauser Rearrangement

Sommelet-Hauser Rearrangement

Sommelet-Hauser Rearrangement

The rearrangement involves the reaction of benzyl quaternary ammonium salts in presence of sodium amide or another alkali metal amide to form N,N-dialkylbenzylamine having a new alkyl group in the aromatic ortho position is known as Sommelet–Hauser rearrangement. The Sommelet-Hauser reaction is highly favored at low temperature and in polar solvents like NH3, DMSO, HMPA.

Since the product formed is benzylic tertiary amine, it can be further alkylated and the product again subjected to rearrangement. This process continues until the ortho position is blocked. This reaction is most carried out with three alkyl groups on nitrogen but other groups can also be used. When the alkyl groups attached to the nitrogen contain a hydrogen atom at their β-position, the Hofmann elimination may compete; cyclic quaternary ammonium salts react by ring-expansion.

Mechanism of Sommelet-Hauser Rearrangement

The reaction begins with deprotonation of the benzylic methylene proton which is acidic to yield benzylic ylide. The ylide formed is in equilibrium with the second ylide which is formed by deprotonation of one of the ammonium methyl groups. The second ylide which is present in much smaller amounts undergoes a [2,3] sigmatropic rearrangement and subsequent aromatization in order to form the final product

Sommelet-Hauser Rearrangement Mechanism

The drawback of Sommelet-Hauser Rearrangement is that it is accompanied by Stevens rearrangement. Low temperatures and polar solvents (NH3, DMSO, HMPA) usually favor the Sommelet-Hauser rearrangement, whereas higher temperatures and nonpolar solvents (hexanes, ether) facilitate the Stevens rearrangement.

Sommelet-Hauser Rearrangement Limitation