Diselenides, oxidation potentials

Diphenyltellurides, 119-121 molecular modeling, 121 Diselenides, oxidation potentials, 135t... 

Table 15 Oxidation potentials and peak potentials of selenides and diselenides...

The reactivity of the diselenide dication (51) towards various nucleophiles and in redox reactions is investigated. The diselenide dication undergoes electrophilic substitution reactions with activated aromatic compounds to give para-substituted selenonium salts (57), but oxidizes benzenethiol into diphenyl disulfide <90CL393>, both of which are analogous chemical behaviors to those of the disulfide dication (36). Thus, the Se-dication can both function as an electrophile and as an oxidant. The oxidation potential of the aromatic compound it reacts with determines the mode of reaction. Indeed, the reaction with phenothiazine, whose oxidation potential is known to be lower than that of the Se-dication (51), generates a cation radical detected by the color of the reaction mixture and the UV-visible absorptions at 436 and 515 nm. 

Cys(StBu) with phosphines only a slight excess of tributylphosphine is required for this purpose. Under these conditions reduction of the diselenide does not occur at all, and subsequent air oxidation of the two cysteine residues at high dilution leads in a highly selective manner to the diselenide- and disulfide-bridged peptides. The selectivity of the disulfide bridging is assured by the complete absence of thiol/diselenide exchange reactions even at alkaline pH values due to the very low reactivity of the diselenide toward mono-thiols as a result of their highly differentiated redox potentials. ... 

Conditions such as pH, oxidation-reduction potential, and the presence of metal oxides affect the partitioning of the various compounds of selenium in the environment. In general, elemental selenium is stable in soils and is found at low levels in water because of its ability to coprecipitate with sediments. The soluble selenates are readily taken up by plants and converted to organic compounds such as selenomethionine, selenocysteine, dimethyl selenide, and dimethyl diselenide. Selenium is bioaccumulated by aquatic organisms. Very low levels of selenium are found in ambient air. 

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