Abstract
A pH-dependence of the Brønsted acid-catalyzed oxidation of sulfides to the corresponding sulfoxides with H2O2 is reported for the first time based on our systematic investigation of the catalytic performance of a series of Brønsted acids. For all of the Brønsted acids investigated, the catalytic performances do not depend on the catalyst loading (mol ratio of Brønsted acid to substrate), but rather depend on the pH value of the aqueous reaction solution. All of them can give more than 98% conversion and selectivity in their aqueous solution at pH 1.30, no matter how much the catalyst loading is and what the Brønsted acid is. This pH-dependence principle is a very novel perspective to understand the Brønsted-acid catalysis system compared with our common understanding of the subject.
Highlights
The chemoselective oxidation of sulfides to the corresponding sulfoxides is a very important transformation in organic synthesis, since the obtained sulfoxides are important intermediates or building blocks in the construction of pharmaceuticals, agrochemicals, and other valuable fine chemicals [1,2,3,4,5,6,7]
In our screening of modifying groups for β-cyclodextrin to construct artificial enzymes [72,73,74], especially sulfide-oxidase, we found that the conversion, yield and chemoselectivity in the Brønsted acid-catalyzed oxidation of sulfides seemed to be dependent on the catalyst loading, which meant that different Brønsted acids could exhibit their optimal catalytic activity by optimizing its catalyst loading for most cases, but they depended on the concentration of the Brønsted acid in the aqueous phase, no matter how much the catalyst loading was
To the best of our knowledge, our study reveals a commonness in the Brønsted acid-catalyzed oxidation of sulfides with H2O2 for the first time, so these attempts to explore the commonness in the different Brønsted acid-catalyzed chemoselective oxidation of sulfides with H2O2 will provide a summary of the abundant previous studies, and an important reference for further studies of the Brønsted acid-catalyzed oxidation of sulfides, and even other Brønsted-acid catalysis systems
Summary
The chemoselective oxidation of sulfides to the corresponding sulfoxides is a very important transformation in organic synthesis, since the obtained sulfoxides are important intermediates or building blocks in the construction of pharmaceuticals, agrochemicals, and other valuable fine chemicals [1,2,3,4,5,6,7] To realize this extremely useful transformation, several oxidants can be employed, such as hydrogen peroxide [8,9,10,11,12,13], molecular oxygen [14,15,16,17,18,19], tert-butyl hydroperoxide [20,21,22,23], cumene hydroperoxide [24,25,26,27], iodobenzene diacetate [28,29,30], sodium hypochlorite [31,32,33], and oxone [34]. Considering the high cost of the metal catalysts, inevitable metal residue contamination of the products, and the toxicity of the transition metal complexes, Brønsted acids and organocatalysts seem to be the more attractive choices, as well as for the high catalytic activity and selectivity seen in Brønsted acids and organocatalysts catalysis
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