Selenoxides reductions

Allylsilanes in which the silyl group is at the more substituted end of the allyl system have been prepared by a reaction sequence involving the conjugate addition of silylcuprates to a, jS-unsat-urated esters followed by reduction and dehydration via selenoxide elimination38. 

Once the thiol is introduced to the coordination sphere of the selenoxide or telluroxide, a second slower reaction occurs. This step is associated with reduction of the chalcogen(IV) oxidation state to the chalcogen(II) oxidation state, which was demonstrated with dihydroxy telluranes 52 and 53. In the tellurium(IV) oxidation state of 52 and 53, the 5p orbital of tellurium is involved in the three-center, four-electron bond and cannot interact with the carbon 7r-framework. Long-wavelength absorption maxima for 52 and 53 are found at 510 and 500 nm, respectively in water. Reductive elimination generates a tellurium(II) atom, whose 5p orbital can now... 

Several other oxidation reactions of selenoxides and telluroxides are summarized in Fig. 23. Ley, Barton, and co-workers discovered that di-4-methoxyphenyltellur-oxide (54) could be used catalytically as an oxidant in the presence of 1,2-dibromotetrachloroethane. After reduction of the telluroxide to the telluride, the di-4-methoxyphenyltelluride (24) debrominated the 1,2-dibromotetrachloroethane to give the tellurium(IV) dibromide, which was hydrolyzed in situ to give the telluroxide 54. This process was used to oxidize phosphines to phosphine oxides and... 

Deoxygenation of sulfoxides, selenoxides, oximes, and nitroalkanes.2 The reduction of sulfoxides and selenoxides is possible with PI3 or P2I4 in CH2C.l2 even at temperatures of —78°. Yields are 70-95%. 

Oxidation of alkyl iodides, bearing electron-withdrawing groups such as car-bomethoxy and sulfonyl at the a-carbon, with m-chloroperbenzoic acid results in clean elimination to give olefins [Eq. (27)]. This reaction involves reductive / -elimination of the intermediate iodosylalkanes, as observed in thermal peri-cyclic -elimination of sulfoxides and selenoxides. Exclusive syn stereochemistry in the reductive /1-elimination was established by deuterium labeling... 

Assmann, A., Briviba, K., and Sies, H. 1998. Reduction of methionine selenoxide to selenomethionine by glutathione. Arch. Biochem. Biophys. 349, 201-203. 

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... 

A very similar cyclization has been described using elemental selenium and LiHBEt3, as reducing agent in 95% yield. Although the oxidation step was exactly the same, a study of the reduction of selenoxide 110 to selenide 109 has been detailed <2003BCJ381>. 

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... 

Michael reaction of selenophenols. Selenophenols (and other selenides) undergo enantioselective 1,4-addition to cyclohexenone in the presence of catalytic amounts of cinchona alkaloids. Chemical yields are high optical yields are 10-43%. Usually the optical yield can be enhanced by crystallization. In one case the addition product was converted into an optically active allylic alcohol by hydride reduction followed by selenoxide fragmentation. ... 

Several of the procedures discussed in the sulfoxide section describe the successful extension of the method to the reduction of selenoxides, - - and there is little doubt that many of the other procedures cited earlier could be used likewise. Sakaki and Oae used triphenylphosphine selenide and similar se-lenides to reduce selenoxides to selenides in 79-93% yield (equation 19). Using a chiral phosphine selenide, these workers showed that the phosphine oxide formed had suffered predominant inversion, with a stereospecificity of over 80%. Detty has reported the application of the silane PhSeSiMes (12) to the reduction of selenoxides and telluroxides. The reactions are rapid and proceed essentially quantitatively, even in the presence of a hydroxy or carbonyl group. 

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. 

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

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