Sommelet-Hauser Rearrangement
The Sommelet–Hauser rearrangement involves the reaction of benzyl quaternary ammonium salts in the presence of sodium amide ($\text{NaNH}_2$) or another alkali metal amide base. The base converts the salt into an intermediate ylide which rearranges to form an $N,N$-dialkylbenzylamine containing a newly substituted alkyl group at the aromatic ortho position. This synthetic pathway is highly favored at low temperatures and inside polar aprotic solvents such as liquid ammonia ($\text{NH}_3$), dimethyl sulfoxide ($\text{DMSO}$), or hexamethylphosphoramide ($\text{HMPA}$).
Because the resulting product is a benzylic tertiary amine, it remains capable of undergoing subsequent alkylations and repeating the rearrangement loop. This cycle can be strategically continued until the active ortho positions on the ring become blocked. While most commonly executed utilizing three methyl configurations on the central nitrogen, alternative alkyl extensions are fully applicable. However, when the groups attached to the nitrogen center harbor hydrogen elements at their $\beta$-position, competing Hofmann elimination paths can occur; cyclic quaternary ammonium structural analogs generally respond by expanding their ring size.
Mechanism of Sommelet-Hauser Rearrangement
The reaction cascades by deprotonating the benzylic methylene proton, which is highly acidic, to generate a highly responsive benzylic ylide. Once produced, this molecule establishes an active equilibrium with a transient secondary ylide that is generated via the deprotonation of one of the ammonium-attached methyl carbon chains. This alternate ylide, though typically present in much smaller microscopic equilibrium quantities, rapidly undergoes a concerted thermodynamic $[2,3]$ sigmatropic rearrangement. The system subsequently yields to an internal proton-shift driven aromatization to finalize the stable ortho-substituted product.
A primary drawback of the Sommelet-Hauser rearrangement is that it is often accompanied by the competitive Stevens rearrangement. Reaction conditions can be altered to select for the desired pathway: lower operational temperatures paired with polar solvents ($\text{NH}_3$, $\text{DMSO}$, $\text{HMPA}$) predominantly stabilize the sigmatropic Sommelet-Hauser product, whereas high thermal ranges and nonpolar solvents like hexanes or diethyl ether accelerate the free-radical/ion-pair Stevens rearrangement process.
