Mechanism and Kinetics of One-electron Oxidation of Methanesulfinate and Two-electron Oxidation of Thiols and Disulfides

Abstract

One-electron oxidation of alkylsulfinic acids has been of great interest for years as the reaction mechanisms are known to be quite elusive. We herein investigate the kinetics and mechanism of oxidation of an alkylsulfinic acid, CH3SO2H (MSA) by two types of one-electron oxidants, hexachloroiridate(IV), Ir(IV), and bis(1,4,7-triazacyclononane)nickel(III), Ni(III), respectively, in aqueous media at room temperature under anaerobic conditions, with the aim to provide in-depth knowledge of such reactions. The oxidation of MSA produces a sulfonyl radical as a transient intermediate through outer-sphere electron transfer pathway, and methanesulfonate (CH3SO3H) as the final product for both reactions. For the MSA-Ir(IV) reaction, a reactive intermediate of the metal complex is an iridium-bound sulfonyl chloride from the inner-sphere oxidation of sulfonyl radical by Ir(IV). It undergoes dissociation to form methanesulfonyl chloride, followed by its hydrolysis to release CH3SO3H. The reaction rate is revealed to be first-order in [MSA] and second-order in [Ir(IV)]. This reaction is inhibited by strong acid but independent of pH between 3 and 6. Inhibition by the product Ir(III) is significant. Notably, this work provides the first good evidence of the strong chlorine atom affinity of a sulfonyl radical. The rate law of the overall reaction was determined at μ=0.1 M (NaClO4). For the MSA-Ni(III) reaction, the reaction kinetics is found to have a second-order dependence on [Ni(III)] and first-order on [MSA]. Ni(II) is proved to be a strong inhibitor for the reaction rate. The trapping of the radical by Ni(III) yields a complex that decomposes slowly to release Ni-tacn, followed by its dissociation to product Ni2+ ion and the ligand, tacn. The presence of the sulfonyl radical was confirmed by the scavenging competition between PBN and Ni(III). The intermediates appeared in the system were characterized through NMR and UV-Vis. As a result, a general rate law is formulated as a function of [Ni(III)], [MSA], [Ni(II)] and pH at μ=0.1 M (NaCl). Two-electron oxidation of two thiols (RSH) including 2-mercaptoethansulfonate (MESNA) and mercaptoethanol (BME), and several disulfides (RSSR) by iodine/oxy-iodine species are also investigated extensively in this work on account of the biological significance of these species. Iodometric titrations are used to determine the reaction stoichiometry of RSH/iodate reaction, and with excess of IO3-, the overall reaction is described as 10RSH+2IO3-+2H+→5RSSR+I2+6H2O. The equilibrium and kinetic studies between RSH and iodine/iodate was evaluated through instrumental methods and kinetic fitting to understand the intermediates such as sulfenyl iodide (RSI) and sulfenic acid (RSOH). The ultimate products are characterized by 1H-NMR, UV-Vis, and Mass Spectrometry. This reaction proceeds relatively quickly, depending on the concentration of the reactants, iodide, and pH. The complex reaction dynamics can be interpreted from 2 processes: clock reaction and rapid formation of disulfide. The detailed reaction pathways and relevant kinetic coefficients are elaborated and simulated through a mechanism network of 15 step reactions. As a follow-up, the reaction between the disulfides of MESNA, BME and 3-mercaptopropionic acid and iodine/triiodide are explored as well. These three disulfides behave differently while reacting with iodine. Rate laws regarding various factors such as [RSSR], [iodide], [iodine], and pH are derived to explain the reaction kinetics and mechanism. The final products of the reactions are identified and found to be thiosulfinate, thiosulfonate and sulfonic acid, respectively.