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The Stevens rearrangement in Organic Chemistry is an Organic Reaction converting Quaternary Ammonium Salt s and Sulfonium Salt s to the corresponding Amine s or Sulfide s in presence of a strong Base in a 1,2-rearrangement . 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 T.S. Stevens ''et al.'' J. Chem. Soc., 1928, 3193 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 T.S. Stevens ''et al.'' J. Chem. Soc., 1932, 1926 described the corresponding sulfur reaction. REACTION MECHANISM Key in the Reaction Mechanism M B Smith, J March. ''March's Advanced Organic Chemistry'' (Wiley, 2001) (ISBN 0-471-58589-0) ''Strategic Applications of Named Reactions in Organic Synthesis'' Laszlo Kurti, Barbara Czako Academic Press (March 4, 2005) ISBN 0-12-429785-4 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 ''Macrocycle Ring Expansion by Double Stevens Rearrangement''Keisha K. Ellis-Holder, Brian P. Peppers, Andrei Yu. Kovalevsky, and Steven T. Diver Org. Lett.; 2006; 8(12) pp 2511 - 2514; (Letter) . The ylide is prepared In Situ by reaction of the Diazo compound ''ethyl diazomalonate'' with a Sulfide catalyzed by dirhodium tetraacetate in Reflux ing Xylene . REFERENCES |
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