Shifts Occurring by Addition-Elimination Mechanisms.

The Sommelet-Hauser rearrangement, a reaction that often competes with the Stevens rearrangement, might be classified as a [2,3]-sigmatropic shift. However, it may also be considered an addition-elimination process, as drawn below. A nitrogen ylide, formed by reaction of a quaternary ammonium salt with strong base, is the reactive intermediate. This species may be trapped by an electrophile, but normally rearranges in the fashion shown. The Truce-Smiles rearrangement displayed by clicking on the diagram represents another such aryl relocation, in this case a 1,4-shift.