Seleninic Acid Oxidation

Diorgano diselenides have been used extensively as precursors to seleninic acids in the presence of hydrogen peroxide.The catalytic activity of preformed seleninic acids and seleninic acids generated in situ are identical. Diorgano ditellurides have also been used as catalysts in thiolperoxidase-like reactions for the oxidation of thiols with various peroxides. However, tellurinic acids are not thought to be involved even though RTe(=0)SPh types of structures are proposed as intermediates. 

Dibenzyl diselenide crystallises from alcohol in yellow needles, which are slightly deeper in colour than those of the p-nitrobenzyl compound, and melt at 92° to 93° C. Exposure to light for an hour or so causes the crystals to turn red. The selenide readily dissolves in hot alcohol, but is only sparingly soluble in ether, insoluble in water. Oxidation with fuming nitric acid converts it into benzyl seleninic acid, and boiling with copper or mercury in suspension precipitates selenium. Boiling with iodine in chloroform solution gives the tetra-iodide, M.pt. 98° C. the tetrabromide melts at 137° C.5... 

Methyl-j8-naphthylselenone, C10H7.SeO2.CH3, occurs when the sodium salt of naphthalene-j8-seleninic acid in methyl alcohol is boiled with methyl iodide for a long period. After treatment with water and extraction with ether, removal of the latter and crystallisation from alcohol gives a 48 per cent, yield as golden-yellow crystals, M.pt. 136° C. The selenone may also be obtained by the oxidation of methyl-j3-naphthyl selenide with permanganate. 

It has already been pointed out that this compound may be prepared by hydrolysis of the selenocyanate it may also be obtained by the interaction of sodium selenide and 1-chloroanthraquinone. By the latter process orange-red plates, M.pt. 212° C., result. If 2-chloroanthra-quinone be used in the reaction, 2-anihraquinone selenophenol is obtained as an orange-yellow product, which gives a violet-red solution in sulphuric acid. Concentrated nitric acid causes oxidation to the seleninic acid. 

Stoichiometric oxidation with Se02 became more attractive after Sharpless showed that the reaction could be carried out with catalytic amounts of Se02 and TBHP as the (re)oxidant [106]. The reaction involves an oxometal mechanism (see Fig. 4.39). The use of fluorous seleninic acids with iodoxybenzene as oxidant introduces the possibility of recycling the catalyst [107]. 

The mechanism of Se02 oxidation involves the formation of seleninic acid A. The [2,3]-sigmatropic rearrangement of A gives B, which is then converted into allylic alcohol and Se(II) by-products (Scheme 7.33). 

As shown in Scheme 1, the reduction of diselenides generates selenols and selenolates, while their oxidation results in the formation of selenenic and seleninic acids and related compounds (see Section 7). Halogenation of diselenides with... 

The partial oxidation of diselenides with appropriate oxidants such as hydrogen peroxide initially generates the corresponding selenenic acids (29) or anhydrides (28). Further oxidation leads to the corresponding seleninic acids (31) or... 

Selenonic acids (45) have been little studied but can be produced by the further oxidation of seleninic acids with potassium permanganate. 

Oxidation of diselenides with Br2, H202,23s-23 HN03,2 -2 i ozone, 2 TBHP242-244 nd MCPBA affords seleninic acids (30) and/or their anhydrides (31 equation 40), both of which are known as useful oxidants of various organic compounds. The oxidation has been suggested to proceed as shown in Scheme 11 and as evidence intermediate compounds were isolated m the MCPBA oxidation of... 

It should be mentioned that derivatives of acetophenone react much more slowly than the corresponding benzaldehyde derivatives. Therefore, to complete the reaction in a reasonable time, 90% hydrogen peroxide should be used. The nitro-substituted seleninic acids 103 a,b also manifested remarkable catalytic activity in the hydrogen peroxide oxidation of styrene to phenyloxi-rane [51]. 

The nitro-substituted seleninic acid 103a was also found [531 to be an effective catalyst for the hydrogen peroxide mediated oxidative conversion of aromatic hydrazones 107 into nitriles 108 (Eq. 21). However, oxidation of hexanal dimethylhydrazone with this system gave only the parent aldehyde. 

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