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 ylides of quaternary ammonium salts have been studied at M05-2x levels. The Stevens migration has been found to take place through a diradical pathway in several cases (tetramethylammonium, benzyltrimethylammonium, benzylphenacyldimethylammonium ylides). By contrast, in the phenyltrimethylammonium ylide this reaction takes place through a concerted process. The Sommelet-Hauser rearrangement 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.
Ghigo
Giovanni
G
Dipartimento di Chimica Generale e Chimica Organica, C. so M. d'Azeglio 48, I-10125 Torino, Italy. giovanni.ghigo@unito.it
Cagnina
Stefania
S
Maranzana
Andrea
A
Tonachini
Glauco
G