Stevens rearrangement

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.

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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 ^[1] concerned the reaction of 1-phenyl-1- (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 ^[2] described the corresponding sulfur reaction.

Reaction mechanism

Key in the reaction mechanism ^{[3] [4]} 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.

Scope

Competing reactions are the Sommelet reaction and the Hofmann elimination.

In one application a double-Stevens rearrangement expands a cyclophane ring ^[5]. The ylide is prepared in situ by reaction of the diazo compound ethyl diazomalonate with a sulfide catalyzed by dirhodium tetraacetate in refluxing xylene.

References

1. ^ T.S. Stevens et al. J. Chem. Soc., 1928, 3193
2. ^ T.S. Stevens et al. J. Chem. Soc., 1932, 1926
3. ^ M B Smith, J March. March's Advanced Organic Chemistry (Wiley, 2001) (ISBN 0-471-58589-0)
4. ^ Strategic Applications of Named Reactions in Organic Synthesis Laszlo Kurti, Barbara Czako Academic Press (March 4, 2005) ISBN 0-12-429785-4
5. ^ Macrocycle Ring Expansion by Double Stevens RearrangementKeisha K. Ellis-Holder, Brian P. Peppers, Andrei Yu. Kovalevsky, and Steven T. Diver Org. Lett.; 2006; 8(12) pp 2511 - 2514; (Letter) doi:10.1021/ol060657a