Selenoxides oxidation

Fig. 17.35. A one-pot combination of a selenide selenoxide oxidation and a selenoxide "pyrolysis" (see Figures 4.10-4.12 for the mechanism).

Several different functional groups can be introduced by selenenylation. The benzeneselenenyl ion tolerates a wide range of anionic groups. However, with differing anionic moieties the reactivity of the benzeneselenenyl ion varies greatly. All adducts can be subsequently converted to either allylic or vinylic moieties by the syn elimination of the corresponding selenoxide (oxidized selenide). 

In natural product synthesis it is sometimes important to eliminate selenoxide oxidatively to form the corresponding olefin. Alumina in combination with tertiary-butyl hydroperoxide in THF offers much improved yields in comparison to conventional methods [77]. Lebar et al. report the application of this reagent in the synthesis of primary allylic alcohols (e.g. equation 4.13) [78]. 

Diphenyl selenoxide Oxidations with selenoxides Oxidative dimerization... 

The 7 a-bromo steroid (9) can also be treated with sodium phenyl selenolate (41). The resultant 7 P-phenyl selenide (13) can be oxidized and the corresponding phenyl selenoxide elirninated to form the 7-dehydtocholesteryl ester (11). 

No oxidizing agent is required for the sulfuric acid promoted cyclization of iV,iV-diphenyl-hydroxylamine to carbazole (13CB3304). The parallel conversion of diphenyl sulfoxide and diphenyl selenoxide to dibenzothiophene (23CB2275) and dibenzoselenophene (39CR(199)53l) is effected by treatment with sodamide. 

Selenosulphonylation 191, 194, 1107-1109 Selenoxides, as oxidizing agents 253 Self-termination, of radicals 1084, 1085,... 

Selenides (R2Se) can be oxidized to selenoxides and selenones. It is possible to oxidize a thioether to a sulfoxide in the presence of an alcohol moiety using MnOa/HCl. ... 

A 30% solution of hydrogen peroxide in water was purchased from Mallinckrodt Chemical Works. The reaction requires 2 molar equivalents of hydrogen peroxide, the first to oxidize the selenide to the selenoxide and the second to oxidize the elimination product, benzeneselenenic acid, to benzeneseleninic acid. The submitters recommend that the hydrogen peroxide solution be taken from a recently opened bottle, or titrated to verify its concentration. 

This ch ter contains reactions which prepare the oxides of nitrogen, sulfur and selenium. Included are W-oxides, nitroso and nitro compounds, nitrile oxides, sulfoxides, selenoxides and sulfones. Oximes are considered to be amines and appear in those sections. Preparation of sulfonic acid derivatives are found in Chapter Two and the preparation of sulfonates in Chapter Ten. 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

Rearrangement of Allylic Sulfoxides, Selenoxides, and Amine Oxides... 

An analogous reaction occurs when allylic selenoxides are generated in situ by oxidation of allylic selenyl ethers.277... 

Selenoxides are even more reactive than sulfoxides toward (3-elimination. In fact, many selenoxides react spontaneously when generated at room temperature. Synthetic procedures based on selenoxide eliminations usually involve synthesis of the corresponding selenide followed by oxidation and in situ elimination. We have already discussed examples of these procedures in Section 4.3.2, where the conversion of ketones and esters to their a, (3-unsaturated derivatives is considered. Selenides can... 

The selenides prepared by any of these methods can be converted to selenoxides by such oxidants as hydrogen peroxide, sodium metaperiodate, peroxycarboxylic acids, 1-butyl hydroperoxide, or ozone. 

Like amine oxide elimination, selenoxide eliminations normally favor formation of the E-isomer in acyclic structures. In cyclic systems the stereochemical requirements of the cyclic TS govern the product composition. Section B of Scheme 6.21 gives some examples of selenoxide eliminations. 

This reaction depends upon the facile solvolysis of (J-haloselenides and the facile oxidative elimination of a selenoxide, which was discussed in Section 6.6.3. An alternative method, which is experimentally simpler, involves reaction of alkenes with a mixture of diphenyl diselenide and phenylseleninic acid.189 The two selenium reagents generate an electrophilic selenium species, phenylselenenic acid, PhSeOH. 

The elimination is promoted by oxidation of the addition product to the selenoxide by f-butyl hydroperoxide. The regioselectivity in this reaction is such that the hydroxy group becomes bound at the more-substituted end of the carbon-carbon double bond. The regioselectivity of the addition step follows Markovnikov s rule with PhSe+ acting as the electrophile. The elimination step specifically proceeds away from the oxygen functionality. 

Other phosphorous compounds, as shown in Table 8, were resolved by the same procedure. The three isomeric phosphinates 27b-d containing a methyl group attached to the aryl substituent could also be resolved, irrespective of the methyl position. From the related phosphine oxides 28a d, however, only those with R=H (28a) and R=m-CH (28c) could be well resolved no satisfactory resolution could be obtained for the other isomers of 28. The efficiency of the optical resolution of alkylaryl-substituted sulfoxides and selenoxides was found to depend similarly on the type of substitution on the aryl ring. 

The organic substrates in Chart 8 can be divided into two main categories in which (i) the oxidation of olefins, sulfides, and selenides involves oxygen atom transfer to yield epoxides, sulfoxides, and selenoxides, respectively, whereas (ii) the oxidation of hydroquinones and quinone dioximes formally involves loss of two electrons and two protons to yield quinones and dinitrosobenzenes, respectively. In order to provide a unifying mechanistic theme for the seemingly disparate transformations in Chart 8, we note that nitrogen dioxide exists in equilibrium with its dimeric forms, namely, the predominant N—N bonded dimer 02N—N02 and the minor N—O bonded isomer ONO—N02 (equation 88). 

The conversion of the polystyrene-supported selenyl bromide 289 into the corresponding acid 290 allowed dicyclohexylcarbodiimide (DCC)-mediated coupling with an amidoxime to give the 1,2,4-oxadiazolyl-substituted selenium resin 291 (Scheme 48). Reaction with lithium diisopropylamide (LDA) and allylation gave the a-sub-stituted selenium resin 292, which was then used as an alkene substrate for 1,3-dipolar cycloaddition with nitrile oxides. Cleavage of heterocycles 293 from the resin was executed in an elegant manner via selenoxide syn-elimination from the resin <2005JC0726>. 

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