期刊
DALTON TRANSACTIONS
卷 50, 期 10, 页码 3577-3585出版社
ROYAL SOC CHEMISTRY
DOI: 10.1039/d0dt04436a
关键词
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资金
- U.S. D.O.E. [DE-SC0016359]
- NIH [T32 GM08545]
- NIH Shared Instrumentation Grant [S10OD016360]
- U.S. Department of Energy (DOE) [DE-SC0016359] Funding Source: U.S. Department of Energy (DOE)
High-valent metal-oxo intermediates play a crucial role in oxygen-atom transfer reactions, and changes in metal reduction potential can lead to variations in reaction mechanisms. The rate variations observed for Mn-IV-oxo complexes are primarily controlled by activation enthalpy, indicating a single-step OAT mechanism. This suggests that large variations in sulfoxidation by Mn-IV-oxo centers occur without a change in reaction mechanism.
High-valent metal-oxo intermediates are well known to facilitate oxygen-atom transfer (OAT) reactions both in biological and synthetic systems. These reactions can occur by a single-step OAT mechanism or by a stepwise process initiated by rate-limiting electron transfer between the substrate and the metal-oxo unit. Several recent reports have demonstrated that changes in the metal reduction potential, caused by the addition of Bronsted or Lewis acids, cause a change in sulfoxidation mechanism of Mn-IV-oxo complexes from single-step OAT to the multistep process. In this work, we sought to determine if ca. 4000-fold rate variations observed for sulfoxidation reactions by a series of Mn-IV-oxo complexes supported by neutral, pentadentate ligands could arise from a change in sulfoxidation mechanism. We examined the basis for this rate variation by performing variable-temperature kinetic studies to determine activation parameters for the reactions of the Mn-IV-oxo complexes with thioanisole. These data reveal activation barriers predominantly controlled by activation enthalpy, with unexpectedly small contributions from the activation entropy. We also compared the reactivity of these Mn-IV-oxo complexes by a Hammett analysis using para-substituted thioanisole derivatives. Similar Hammett rho values from this analysis suggest a common sulfoxidation mechanism for these complexes. Because the rates of oxidation of the para-substituted thioanisole derivatives by the Mn-IV-oxo adducts are much faster than that expected from the Marcus theory of outer-sphere electron-transfer, we conclude that these reactions proceed by a single-step OAT mechanism. Thus, large variations in sulfoxidation by this series of Mn-IV-oxo centers occur without a change in reaction mechanism.
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