Rate-limiting deprotonation of (arylsulfonyl)methyl perchlorates in dichloromethane. Evidence for proton tunneling and the high dynamic basicity of HMPA in a nonaqueous solvent
✍ Scribed by Maarten A. van Andel; Jan B. F. N. Engberts
- Publisher
- Elsevier Science
- Year
- 2010
- Tongue
- English
- Weight
- 380 KB
- Volume
- 109
- Category
- Article
- ISSN
- 0165-0513
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✦ Synopsis
Abstract
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 Brönsted 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 (k~H~/k~D~ = 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 (A~H~/A~D~ = 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.