Reactivity and Mechanism in the Hydrolysis of β-Sultams

-Sultams show extraordinary rate enhancements of 109- and 107-fold, respectively, compared with the acid- and base-catalyzed hydrolysis of corresponding acyclic sulfonamides. They are about 103-fold more reactive than analogous -lactams. The alkaline hydrolysis of some -sultams shows a rate term that is second-order in hydroxide ion concentration, which is indicative of a stepwise mechanism involving a trigonal bipyramidal intermediate (TBPI). The Brnsted lg value for the alkaline hydrolysis of N-aryl--sultams is -0.58 and the kinetic solvent isotope effect / is 0.60, compatible with rate-limiting formation of the TBPI. Conversely, / for N-alkyl--sultams is 1.55, indicative of rate-limiting breakdown of the TBPI. The acid-catalyzed hydrolysis of -sultams is strongly retarded by electron-withdrawing groups to the sulfonyl group, and it is suggested that the mechanism may involve unimolecular ring opening to generate a sulfonylium ion. The Brnsted lg value for the acid-catalyzed hydrolysis of N-benzyl--sultams is 0.32. The general-acid-catalyzed hydrolysis of N-benzyl--sultam by carboxylic acids shows a Brnsted value of 0.67 and is attributed to a specific acid-nucleophilic mechanism with the formation of a mixed-anhydride intermediate.