Selenenic acid

Scheme 15.1 Catalytic cycle for the conversion of a diselenide to a selenenic acid...

In the second step of this reaction the O—O bond of H2O2 is cleaved. During this process, one hydroxyl fragment (0 11) is transferred to the selenolate anion (R—Se-) to form selenenic acid (R—SeO1 II), while simultaneously the second... 

M. T. Molina, M. Ydnez, O. M6, R. Notario and J.-L. M. Abboud the thial along with selenenic acid 67. 

Selenides may be oxidized by various reagents to selenoxides. When the lesulting selenoxides bear a -hydrogen atom syn elimination giving alkenes occurs readily at room temperature widi formation of selenenic acid by-products (Scheme 13). For allylic selenides, the oxidation does not lead te conjugated... 

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... 

When selenenic acids are generated in the presence of alkenes, addition reactions similar to those of selenenyl halides occur. Oxidation and syn elimination of the adducts (42) provide a convenient synthesis of allylic alcohols (equation... 

There has been recent controversy over the reduction of seleninic acids (RSe02H) to selenenic acids (RSeOH). Reich et al and Kice et a/. have independently concluded that earlier claims for the isolation of stable selenenic acids are probably in error. While selenenic acids are stable in dilute solution, they show a pronounced tendency to form the corresponding anhydrides in the absence of water. 

When the concentration of hydroperoxide is high, ebselen 3a is oxidized to selenoxide 11a which reacts with one molecule of the thiol to give selenoxysulfide 36. Intermediate 36 and a second molecule of thiol produce disulfide, while the selenenic acid 37 formed is converted back to ebselen. In biological systems, where the concentration of hydroperoxide is low, ebselen and the thiol give selenosulfide 38 which disproportionates into the disulfide and diselenide 39, subsequently oxidized to selenenic anhydride 40 and finally converted into ebselen. 

Hydroxyalkyl selenoxides directly available firom a-lithioalkyl selenoxides or by oxidation of p-hy-droxyalkyl selenides are valuable precursors of allyl alcohols (Schemes SI and S2 Scheme S4, a Scheme lOS Scheme 161, e Scheme 162, b Scheme 164, b Scheme 166, b Schemes 173 and 7,40,42,48.49.97.98,120.127,128,136.188,189,193,233,261 n t)ie former casc thc reaction is usually performed on thermolysis at around 70 C in carbon tetrachloride and in the presence of an amine able to trap the selenenic acid concomitantly produced. 

This reaction, which is related to the well-known amine oxide pyrolysis, involves a syn elimination of the selenenic acid, but proceeds under milder conditions (20 C to 60 C instead of 400 C). It is more regioselective, takes place away from the oxygen and leads to allyl alcohols instead of enols, except when the only available hydrogen is the one a to the hydroxy group (Scheme 173, b). ... 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

Scheme 23. " ° To avoid these side reactions an excess of oxidant is frequently used to oxidize the se-lenenic acid as quickly as it is formed to the corresponding seleninic acid. Alternatively an amine can be added to convert the selenenic acid into the selenenamide. ...

Selenium dioxide (Se02) is commonly used to hydroxylate alkenes in the al-lylic position. The mechanism of this transformation involves two sequential peri-cyclic reactions. The first reaction is an ene reaction. It gives a selenenic acid. Then a [2,3] sigmatropic rearrangement occurs, and the intermediate loses SeO to give the observed product. Note how two allylic transpositions result in no al-lylic transposition at all ... 

Elimination reactions leading to olefins are usually performed on the corresponding selenoxides 3 9,1 12) (Scheme 2 a). These are often unstable and decompose at room temperature to olefins and selenenic acid (further oxidized to the more stable seleninic acid by excess of oxidant). Hydrogen peroxide in water-THF, ozone and further treatment with an amine or tert-butyl hydroperoxide without or with alumina proved to be the method of choice for such a synthesis of olefins. The reaction is reminiscent of the one already described with aminoxides or sulfoxides 22) and occurs via a syn elimination of the seleninyl moiety and the hydrogen attached to the P-carbon atom. However it takes place under smoother conditions. 

The first stable arylselenenic acid, Ar-Se-OH (109) as a bridged calix [6]arene, was synthesized and characterized by an X-ray study (Scheme 17) Se chemical shifts of selenenic acids seem to be comparable to those of selenonic acids. This is confirmed by the data of the selenoperacetate 110 (same 5-value for the selenoperbenzoate) and the selenenic anhydride 111. 

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