Mechanism of Buffer Catalysis in the Iodine Oxidation of N-Acetylmethionine Methyl Ester

Article abstract of DOI:10.1021/ja00370a027

The iodine oxidation of N-acetylmethionine methyl ester is catalyzed by carboxylic acid buffers.At very low concentrations, the reaction is first order with respect to buffer and follows an iodide dependence that can be described as inverse squared, changing to invers cubed as the iodide concentration is increased.This iodide dependence is observed at all buffer concentrations examined, although the transition from inverse squared to inverse cubed occurs at higher iodide concentrations as the buffer concentration is increasd.As the buffer concentration is increased, the observed rate constants become dependent upon ² and at "high" buffer (typically > 0.3 M) the reaction again becomes first order with respect to buffer.Broensted coefficients, based on four carboxylic acid buffers, are 1.0 and 1.5 for the first- and second-order terms, respectively.In the reaction catalyzed by acetate, acetic anhydride is produced in an amount equal to the concentraion of sulfide that is oxidized.The data are rationalized in terms of a mechanism involving the rapid formation of an iodosulfonium ion which is attacked by buffer to give an intermadiate O-acylsulfoxide.The O-acylsulfoxide can partition by back-reaction with iodide or by attack of buffer or water at the acyl carbon to give either unhydride or free acid.The β_eq for O-acylsulfoxide formation is estimated to be 1.5.Possible transition states for O-acylsulfoxides formation and the possible roles of sulfurane intermediates in these reactions are discussed.