Oxidation of selenides

Caution Most selenium compounds are toxic consequently care should be exercised in handling them. The hydrogen peroxide oxidation of selenides is highly exothermic, acid-catalyzed, and auto-catalytic. The procedure given for adding the hydrogen peroxide solution should be carefully followed. 

One-electron oxidation of organoselenium and organotellurium compounds results in initial formation of a radical cation (equations (19) and (20)). The eventual fate of the radical cation depends on several variables, but is typically a Se(lV) or Te(lV) compound. The scope of this section will be the one-electron oxidation of selenides and tellurides that are not contained in a heteroaromatic compound, and ones in which the Se and Te are bonded to two carbons, rather than to other heteroatoms. Tellurium- and selenium-containing electron donor molecules have been reviewed. 

More experimental work involving the oxidation of selenides in the presence of substituted olefins is planned to elucidate this point further. 

Selenones.1 Oxidation of selenides to selenones is hampered by the ready decomposition of the intermediate selenoxides. The most useful oxidants are per-acids (m-ClC6H4C03H, CF3C03H) and KMn04 (CH2C12-H20, 20°). 

The oxidation of selenide 55 with subsequent thermolysis of the intermediate, unstable selenoxide yielded 4-methylene-l,3,2-dioxathiolane A-oxide 56 (28%), which is a synthetic equivalent of allene oxides (Scheme 17) <1999T10845>. 

The oxidation of selenides to selenium oxides (Figure 17.35) is faster than the oxidation of sulfides to sulfoxides. The former reaction also succeeds in the presence of C=C double bonds, and even a sulfide group that might be present in the substrate will not react. Tertiary amines yield amine IV-oxides in a similar fashion (Figure 17.36). 

The oxidation of selenides to selenium oxides (Figure 14.29) is faster than the oxidation of sulfides to sulfoxides. The former reaction also succeeds in the presence of... 

Asymmetric [2,3]sigmatropic rearrangements can proceed via optically active selenoxides. It has been shown that the Davis oxidant 158 can be used for the oxidation of selenides such as 172. The reaction product, after oxidation and rearrangement, is the allylic alcohol 173 formed with 35% ee (Scheme 50).279,282 Also Sharpless conditions (Ti(/ -PrO)4, (+)-DIPT, /-BuOOH) have been applied to this reaction and the product has been obtained in 69% ee. When, however, the phenyl selenide moiety in 172 is replaced with an or/ < -nitrophenyl selenide, the selectivity is increased to 92% ee in the allylic alcohol 173 using Sharpless conditions.296 Other selenides such as 2 -pyridyl or ferrocenyl selenides gave much lower selectivities. 

RSeCJIi — ROCHi. Oxidation of an alkyl phenyl selenide with m-chloroperben-zoic acid (2-5 equiv.) in methanol affords the corresponding alkyl methyl ethers in high yield. Oxidation of selenides with a vicinal phenyl group is accompanied by rearrangement of the phenyl group. vtc-Methoxy selenides derived from cycloalkenes are oxidized under these conditions to dimethyl acetals of ring-contracted aldehydes. 

The oxidation of selenoxides to selenones is slow requiring drastic conditions to be used. Diaryl, aryl methyl and dimethyl selenones are prepared by the oxidation of the ctxresponding selenoxides with prolonged exposure to KMn04 or ozone. The direct oxidation of selenides to selenones by FWO with ruthenium(II) complex catalyst, and C u(Mn04)2 has also been described. Aiyl trifluo-... 

Until quite recently the isolation of optically active seienoxides has been limited to those contained in steroids (isolated as diastereoisomeis). < The difficulty in obtaining these compounds was attributed to the racemization through the achiral hydrated intermediates. Simple optically active sel enoxides (S-11% ee) were first prepared by kinetic resolution. Direct oxidation of selenides to seienoxides was first reported using optically active oxaziridine derivatives under anhydrous conditions, but the extent of the asymmetric induction was somewhat unsatisfactory with methyl phenyl selenide as substrate (8-9% Recently much improved enantiomeric excesses (45-73%) were achieved with new oxaziridine reagents such as (70). An attempt at the asymmetric oxidation of more bulky selenides was independently carried out using Bu OCl in the presence of (-)-2-octanol (equation 55), but resulted in unsatisfactory enantioselectivities (ee 1%). Much better results were obtained by the oxidation of p-oxyalkyl aryl selenides (ee 18-40% equation 56) 27 gjyj selenides (ee 1-28%) using... 

Selenoxides (10) are easily prepared by the oxidation of selenides with numerous types of oxidants, including hydrogen... 

There are two methods for obtaining the chiral selenoxides by direct oxidation of the corresponding selenides. One is the enantioselective oxidation of pro-chiral selenides, and the other is the diastereoselective oxidation of selenides bearing a chiral moiety (Eq. 4) ... 

Oxidation of selenides. lenides are converted into selenoxides in high yield by oxidation with singlet oxygen (Rose Bengal sensitizer, CH3OH). In the case of -hydroxy selenides, elimination of RSeOH occurs to some extent to form allylic alcohols. ... 

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