Rate‐limiting deprotonation of (arylsulfonyl)methyl perchlorates in dichloromethane. Evidence for proton tunneling and the high dynamic basicity of HMPA in a nonaqueous solvent

Kinetic data are reported for the water-catalyzed hydrolysis of several (arylsulfonyl)-methyl perchlorates in water and for the HMPA-catalyzed decomposition of the same substrates in dichloromethane. Under both conditions, the reaction exhibits general base catalysis and the covalent perchlorates react via rate-limiting deprotonation at the α-sulfonyl methylene moiety. Primary kinetic deuterium isotope effects (KDIE's) in water are indicative for a complete loss of zero-point energy in the transition state. The high symmetry of the transition state is supported by the Bronsted β value of ca. 0.5. HMPA was found to be an effective general base catalyst in dichloromethane as the solvent. This reaction shows primary KDIE's (kH/kD = 9.3 at 25°C) which are about 1.5 times higher than the (semi)classical limit. In addition, the ratio of the pre-exponential Arrhenius factors is abnormally small (AH/AD = 0.14). These classically anomalous results are interpreted in terms of a contribution of quantum-mechanical tunneling to the transition state for HMPA-induced deprotonation. Possible reasons for the different kinetic characteristics of the deprotonation reaction in the aqueous and nonaqueous medium are given.