Sommelet–Hauser rearrangement

The Sommelet–Hauser rearrangement (named after M. Sommelet^[1] and Charles R. Hauser^[2]) is a rearrangement reaction of certain benzyl quaternary ammonium salts.^[3][4] The reagent is sodium amide or another alkali metal amide and the reaction product a N,N-dialkylbenzylamine with a new alkyl group in the aromatic ortho position. For example, benzyltrimethylammonium iodide, [(C₆H₅CH₂)N(CH₃)₃]I, rearranges in the presence of sodium amide to yield the o-methyl derivative of N,N-dimethylbenzylamine.^[2] Although the appearance is of one of the methyl groups migrating off the trimethylammonium group, the process is actually the a pericyclic reaction in which the nitrogen and its three methyl substituents migrate.

Mechanism

The benzylic methylene proton is acidic and deprotonation takes place to produce the benzylic ylide (1). This ylide is in equilibrium with a second ylide that is formed by deprotonation of one of the ammonium methyl groups (2). Though the second ylide is present in much smaller amounts, it undergoes a 2,3-sigmatropic rearrangement because it is more reactive than the first one and subsequent aromatization to form the final product (3).^[5]

The Stevens rearrangement is a competing reaction.