Reactivity and mechanism in the hydrolysis of β-Sultams

β-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 β-lactams. The alkaline hydrolysis of some β-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 Bronsted β(Ig) value for the alkaline hydrolysis of N-aryl-β-sultams is -0.58 and the kinetic solvent isotope effect k(OH)/(H₂O)/k(OD)/(D₂O) is 0.60 compatible with rate-limiting formation of the TBPI. Conversely, k(OH)/(H₂O)/k(OD)/(D₂O) for N-alkyl-β-sultams is 1.55, indicative of rate-limiting breakdown of the TBPI. The acid-catalyzed hydrolysis of β-sultams is strongly retarded by electron-withdrawing groups α to the sulfonyl group, and it is suggested that the mechanism may involve unimolecular ring opening to generate a sulfonylium ion. The Bronsted β(Ig) value for the acid-catalyzed hydrolysis of N-benzyl-β-sultams is 0.32. The general-acid-catalyzed hydrolysis of N-benzyl-β-sultam by carboxylic acids shows a Bronsted α value of 0.67 and is attributed to a specific acid- nucleophilic mechanism with the formation of a mixed-anhydride intermediate.