Selenides nucleophile oxidation

An isocyanate intermediate may also be involved in the selenium-catalyzed process, which starts with the formation of carbonyl selenide from the reaction between selenium and CO, followed by nucleophilic attack by NuH (Scheme 28). When NuH = primary amine, the resulting RNH(CO)SeH intermediate may eliminate H2Se to give the isocyanate, which then reacts with Nu H to give the final product (Scheme 28, path a). Alternatively, oxidation of Nu(CO)SeH by 02 may lead to a bis(carbamoyl)diselenide species, which is attacked by NuH (Scheme 28, path b). 

The absence of dimer radical cation formation by diphenyl selenide under the pulse radiolysis conditions is in contrast to bimolecular reactions believed to occur under electrochemical conditions/ In these experiments, a rotating disk electrode was used in combination with commutative voltammetry under anhydrous conditions. The results led to the conclusion that reversible one-electron oxidation is followed by disproportionation, then reaction of the resulting dication with diphenyl selenide or an external nucleophile, with the likely intermediacy of the dimer dication (Fig. 33). As expected, the dihydroxy selenane is formed when water is present. Based on the kinetics of the electrochemical reaction, the authors believe the diselenide dication, not the radical cation, to be the intermediate that reacts with the nucleophile. 

Additions of selenium electrophiles to double bonds have most frequently been used as part of a synthetic sequence, because the addition products are very versatile building blocks in synthesis. They can undergo a variety of subsequent transformations and can, therefore, serve as precursors for the generation of radicals 27 in radical reactions. Using a selenoxide elimination, new double bonds as shown in 28 can be generated. After oxidation of the selenide to the seleneone the selenium moiety can be replaced by a second nucleophile to generate compounds of type 29 (Scheme 2). 

Ley and Barton s observation that di-4-methoxyphenyltelluride could be used catalytically was the first entry into the use of in situ generated selenoxides or telluroxides as catalysts. As shown in Fig. 8, a variety of different nucleophiles can be introduced via the selenoxide or telluroxide followed by reductive elimination to generate oxidized product and reduced selenide or telluride. If the nucleophile is relatively inert to oxidation by hydrogen peroxide, then the reduced selenide or telluride can be reoxidized by hydrogen peroxide and the overall oxidation of the nucleophile becomes catalytic in the selenide or telluride. In the case of thiols, disulfides are the final product and the selenides or tellurides exhibit thiolperox-idase-like activity 60-62 64 82 83 If halide salts (chloride, bromide, iodide) are the nucleophiles, then positive halogen sources are the oxidized products and the selenides and tellurides exhibit haloperoxidase-like activity.84-88 The phenoxypro-pyltelluride 59 has been used as a catalyst for the iodination and bromination of a variety of organic substrates as shown in Fig. 24.87... 

Selenides are also nucleophilic and produce isolable selenonium salts (9) when treated with alkyl halides. They are easily oxidized to selenoxides (10) and further to selenones (11) under more forcing conditions (see Section 4). Reduction of selenides to the corresponding hydrocarbons is most conveniently achieved with nickel boride,or with tri-n-butyl- or triphenyltin hydride under radical conditions. " Other reagents for reductive deselenization include Raney nickel, lithium triethylborohydride, and lithium in ethylamine (Scheme 4). Benzylic selenides undergo radical extrusion reactions under thermal or photolytic conditions to produce... 

Electrophilic addition of PhSeCl to alkene 319 is highly stereo- and regioselective (steric effects) and provides adduct 338 if carried out in the presence of an acetate nucleophile. Methanolysis followed by treatment with formalin and protection of the endo alcohol gives ketone 339. Low-temperature oxidation of the selenide 339 generates a selenoxide, which does not... 

No other derivatives of the tertiary cation, such as the aUylic alcohol in the last part, can r formed this way because the rearrangement is too fast. The reaction with PhSe-SePh and NaBH a trick to get this allylic alcohol. The true reagent is PhSe , formed by reduction of the Se-Se bcr. It is very nucleophilic and will attack even the tertiary epoxide to give a selenide. Oxidation to rh selenoxide leads to a fast concerted cis elimination. The intermediate selenide is unstable as wel m very smelly and must be oxidized immediately before it decomposes. 

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