Autoxidation of thiols

Thiols are susceptible to oxidation by peroxides, molecular oxygen, and other oxidizing processes (e.g., radical-catalyzed oxidation) (Fig. 67). Because thiols easily complex with transition metals, it is believed that most thiol autoxidation reactions are metal-catalyzed (108). Autoxidation of thiols is enhanced by deprotonation of the thiol to the thiolate anion. Thiol oxidation commonly leads to disulfides, although further autoxidation to the sulfinic and, ultimately, sulfonic acid can be accomplished under basic conditions. Disulfides can be reduced back to the thiol (e.g., upon addition of a reducing agent such as dithiothreitol). Thiols are nucleophilic and will readily react with available electrophilic sites. For a more thorough discussion, see Hovorka and Schoneich (108) and Luo et al. (200). 

Scheme 10.5 Autoxidation of thiols in a basic medium with dioxygen and CoPc(Y).

Sulphydryl oxidase, an indigenous milk enzyme, has been proposed for the oxidation of thiols in UHT milk to reduce cooked flavor and also thereby to serve as an antioxidant, in conjunction with lactoperoxidase (to destroy the resultant H2O2), by obviating pro-oxidants resulting from autoxidation of thiols (Swaisgood and Abraham, 1980). 

Disulfides form spontaneously by autoxidation of thiols upon exposure to air. This process is, however, relatively slow and not useful for most microgel preparation techniques. Thus, oxidizing catalysts such as diamide [21], peroxides [22], and sodium tetrathionate [23], or a catalyst based on Fenton chemistry [24], are often applied in order to shorten the oxidation time. 

Wallace and Schriesheim dried the solvent by distillation under nitrogen over Linde I3X molecular sieve (calcined under N at 350° for 4 hrs.) and stored it in a dry box equipped with a moisture conductivity cell. They found it superior to DMF or tetramethylurea as solvent for base-catalyzed (KOH or NaOH) autoxidation of thiols and disulfides to sulfonic acids. It is superior also for base-catalyzed autoxidation of alkylthiophenes and toluene, and of 5-0,2 cyclic ketones to the corresponding dibasic acids."... 

For example, the cobalt(II) complex for phthalocyanine tetrasodium sulfonate (PcTs) catalyzes the autoxidation of thiols, such as 2-mercaptoethanol (Eq. 1) [4] and 2,6-di(t-butyl)phenol (Eq. 2) [5]. In the first example the substrate and product were water-soluble whereas the second reaction involved an aqueous suspension. In both cases the activity of the Co(PcTs) was enhanced by binding it to an insoluble polymer, e.g., polyvinylamine [4] or a styrene - divinylbenzene copolymer substituted with quaternary ammonium ions [5]. This enhancement of activity was attributed to inhibition of aggregation of the Co(PcTs) which is known to occur in water, by the polymer network. Hence, in the polymeric form more of the Co(PcTs) will exist in an active monomeric form. In Eq. (2) the polymer-bound Co(PcTs) gave the diphenoquinone (1) with 100% selectivity whereas with soluble Co(PcTs) small amounts of the benzoquinone (2) were also formed. Both reactions involve one-electron oxidations by Co(III) followed by dimerization of the intermediate radical (RS or ArO ). 

The CoPc based materials are the established catalysts for the autoxidation of thiols to disulfides in basic aqueous medium [23], First thiol is deprotonated in the reaction medium. Then the deprotonated thiol coordinates to the Co-macrocycle. When the thiol and dioxygen are coordinated in trans to the MePc, Co mediates the transfer of an electron from RS" to coordinated O2 ... 

The copper ion catalyzed autoxidation of thiols has long been believed to occur via a redox mechanism [51-57] involving free thiyl and superoxide radicals, the catalytic effect of copper being due to the redox couple Cu(I)/Cu(II). The function of 0 would merely be the... 

Reaction [215] represents the initial event in thiyl radical formation as proposed for the so-called spontaneous autoxidation of thiols catalysed by metal ion complexes (Misra 1974, Saez et al. 1982, Harman et al. 1984). The propagation (reactions [216] and [217]) leads to thiol oxidation and contemporary superoxide radical formation. The latter might be reduced to hydrogen peroxide by reacting with (EDTA)2-Mn " to yield back the oxidised complex and close the catalytic cycle. 

Reactions of Thiols with Organophosphonis Compounds.—Autoxidation of thiols can be prevented by addition of tri-n-butylphosphine when the thiol solution is to be used, the phosphine is removed by extraction with a dilute solution of sulphur in CHCla. ... 

Reduction of disulphides to component thiols remains an important topic for study, and PhaP " or BusP " are suitable in aqueous media for reduction of disulphides and for preventing autoxidation of thiols on storage. Quantitative reduction of disulphides by an insolubilized thioP is an ingenious procedure likely to be much used in biochemistry. 

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