The mechanism of the Stevens and Sommelet-Hauser Rearrangements. A Theoretical Study

The [1,2] and [2,3] migration steps in the Stevens and Sommelet-Hauser rearrangements which occur in the ylids of quaternary ammonium salts have been studied M05-2x levels. The Stevens migration has been found to take place through a diradical pathway in several cases (tetramethylammonium, benzyltrimethylammonium, benzylphenacyldimethylammonium ylids). By contrast, in the phenyltrimethylammonium ylid this reaction takes place through a concerted process. The Sommelet-Hauser takes place through a concerted transition structure. The most important factor determining the extent of competition with the Stevens rearrangement is the difference in the reaction energies as the formation of the Sommelet-Hauser intermediate is significantly less endoergic.