Bicarbonate Surfoxidants:  Micellar Oxidations of Aryl Sulfides with Bicarbonate-Activated Hydrogen Peroxide

Abstract

The mechanism and kinetics of bicarbonate-catalyzed oxidations of sulfides by H(2)O(2) at the aqueous /cationic micellar interface have been investigated. The general term surfoxidant is introduced to describe the combination of an ionic surfactant with a reactive counterion that is itself an oxidant or activates an oxidant from the bulk solution to form an oxidant counterion. It is shown that the new catalytic cationic surfoxidant CTAHCO(3) (cetyltrimethylammonium bicarbonate) significantly enhances the overall oxidation rates as compared to the addition of bicarbonate salts to CTACl and CTABr, for which the halide counterions must undergo equilibrium displacement by the oxidant anion (peroxymonocarbonate, HCO(4)(-)). General equations based on the classic pseudophase model have been derived to account for the preequilibrium reaction in the aqueous and micellar phases, and the resulting model can be used to describe any micellar reaction with associated preequilibria. Rate constants and relevant equilibrium constants for HCO(4)(-) oxidations of aryl sulfides at micellar surfaces have been estimated for CTAHCO(3), CTACl, and CTABr. The second-order rate constants in the Stern layer (k(2)(m)) for sulfide oxidations by HCO(4)(-) are estimated to be approximately 50-fold (PhSEtOH) and approximately 180-fold (PhSEt) greater than the background rate constant k(m)(0) for oxidation by H(2)O(2) at the micellar surface. The estimated values of k(2)(m) are lower than the corresponding values in water by a factor of 20-70 depending on the substrate, but the high local concentration of the bicarbonate activator in the surfoxidant and the local accumulation of substrate as a result of strong binding to the micelle lead to a net increase in the observed reaction rates. Comparisons of CTAHCO(3)-activated peroxide to other highly reactive oxidants such as peroxymonosulfate (HSO(5)(-)) in aqueous surfactant media suggest a wide variety of potential applications for this green oxidant.

Production of the Carbonate Radical Anion during Xanthine Oxidase Turnover in the Presence of Bicarbonate

“…In this context, the results reported here are particularly relevant, because they demonstrate that xanthine oxidase turnover at physiological pH can produce a strong one-electron oxidant from the biologically ubiquitous bicarbonate-carbon dioxide pair by a mechanism unrelated to classical Fenton chemistry. In addition to indicating that the carbonate radical anion may be an important mediator of the pathophysiological effects of xanthine oxidase, our results add to recent studies demonstrating the importance of the bicarbonate-carbon dioxide pair as a source of biological oxidants (21)(22)(23)(24)(25)(26)(27)(28)(29).…”

“…4) or with the reactivity of the hydroxyl radical toward the bicarbonate/carbonate anion (Reactions 7 and 8) (17,18), but rather, it correlated with the equilibrating concentration of carbon dioxide (Reactions 5 and 6). This result, a faster electron than proton transfer during XO turnover (8), and the recent NMR characterization of the peroxymonocarbonate anion in aqueous mixtures of bicarbonate/hydrogen peroxide (K ϭ 0.33) (28,29) argue for the mechanism shown in Fig. 1.…”

“…This proposition was based on the fact that the electrons coming from the oxidizing substrate flow through the many redox centers of XO, opening the possibility for production/escape of intermediates that are partially reduced/protonated such as the superoxide radical anion and the deprotonated peroxide (8). The latter could react with surrounding carbon dioxide to produce peroxymonocarbonate that either leaves the active site and acts as a two-electron oxidant (26,28,29) or is reduced at the active site to the carbonate radical anion (Fig. 1) (21).…”

Microwave‐Assisted Cross‐Coupling for the Construction of Diaryl Sulfides

“…9 Most importantly, diaryl sulfides serve as useful starting materials for the construction of heterocycles containing sulfur atom and precursors leading to sulfones and sulfoxides. [10][11][12][13] As a result, the preparation of diaryl sulfides has caught a great attention. [14][15][16][17] Various traditional methods such as Chan-Lam coupling, Leuckart thiophenol reaction, Sandmeyer-type reaction through diazonium salts, and Ullmann reaction can form aryl-sulfur bonds.…”

Chemiluminescence Energy Transfer Reaction for the On-Line Preparation of Peroxymonocarbonate and Eu(II)−Dipicolinate Complex

“…This feature made it widely used as an oxidant in many reactions, 10 such as methionine, 11 alkenes, 8,12,13 and sulfide. 6,14 However, the oxidant was unstable and hydrolyzed rapidly, forming HCO 3 -and H 2 O 2 . It was obviously difficult to store the HCO 4 -ions.…”