The Stevens rearrangement in organic chemistry is an organic reaction converting quaternary ammonium salts and sulfonium salts to the corresponding amines or sulfides in presence of a strong base in a 1,2-rearrangement.[1]
The reactants can be obtained by alkylation of the corresponding amines and sulfides. The substituent R next the amine methylene group is an electron-withdrawing group.
The original 1928 publication by T.S. Stevens [2] concerned the reaction of 1-phenyl-2- (N, N-dimethyl) ethanone with benzyl bromide to the ammonium salt followed by the rearrangement reaction with sodium hydroxide in water to the rearranged amine.
A 1932 publication [3] described the corresponding sulfur reaction.
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Key in the reaction mechanism [4][5] for the Stevens rearrangement (explained for the nitrogen reaction) is the formation of an ylide after deprotonation of the ammonium salt by a strong base. Deprotonation is aided by electron-withdrawing properties of substituent R. Several reaction modes exist for the actual rearrangement reaction.
A concerted reaction requires an antarafacial reaction mode but since the migrating group displays retention of configuration this mechanism is unlikely.
In an alternative reaction mechanism the N-C bond of the leaving group is homolytically cleaved to form a di-radical pair (3a). In order to explain the observed retention of configuration, the presence of a solvent cage is invoked. Another possibility is the formation of a cation-anion pair (3b), also in a solvent cage.
Competing reactions are the Sommelet-Hauser rearrangement and the Hofmann elimination.
In one application a double-Stevens rearrangement expands a cyclophane ring [6]. The ylide is prepared in situ by reaction of the diazo compound ethyl diazomalonate with a sulfide catalyzed by dirhodium tetraacetate in refluxing xylene.