Heteroatom oxidations selenium

Since dioxiranes are electrophilic oxidants, heteroatom functionalities with lone pair electrons are among the most reactive substrates towards oxidation. Among such nucleophilic heteroatom-type substrates, those that contain a nitrogen, sulfur or phosphorus atom, or a C=X functionality (where X is N or S), have been most extensively employed, mainly in view of the usefulness of the resulting oxidation products. Some less studied heteroatoms include oxygen, selenium, halogen and the metal centers in organometallic compounds. These transformations are summarized in Scheme 10. We shall present the substrate classes separately, since the heteroatom oxidation is quite substrate-dependent. 

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. 

The use of hypervalent iodine reagents for heteroatom-heteroatom bond forming reactions is well established in the context of classical oxidation chemistry [1-11]. For example, oxidations of anilines to azobenzenes, thiols to disulfides, and sulfides to sulfoxides with aryl-A3-iodanes were documented decades ago [1-5]. During the last ten years, particular attention has also been given to oxidative transformations of compounds derived from heavier elements, including the interception of reaction intermediates or initially formed products with external nucleophiles. A second important development is the utilization of sulfonyliminoiodanes, ArI = NS02R, for heteroatom-nitrogen bond formation, especially for imidations of sulfur, selenium, phosphorus and arsenic com-... 

The syntheses from [4 + 1] atom fragments, in which the Group VIA heteroatom is introduced between two nitrogen atoms, are the most widely applicable and versatile methods available for construction of the 1,2,5-thiadiazole and 1,2,5-selenadiazole ring systems. These methods have been applied to the synthesis of monocyclic and polycyclic aromatic forms of these ring systems in addition to the direct formation of 1-oxides and 1,1-dioxides, 2-oxides, quaternary salts and reduced forms. The earliest use of the [4 + 1] synthesis dates back to 1889 when Hinsburg prepared 2,1,3-benzothiadiazole (1) and 2,1,3-benzoselenadiazole (2) from o-phenylenediamine and sodium bisulfite, or selenium dioxide, respectively. 

The preparative electrochemical oxidation of a-heteroatom-substituted tetraorganosilanes gives rise to cleavage of the C-Si bond and the introduction of a nucleophile such as methanol on the carbon [Eq. (27)]. Various reactions of this type have been reported for compounds containing oxygen [117-119], nitrogen [121], sulfur [116,119,120,122,123], and selenium [123]. 

Selenoxide elimination occurs under relatively mild conditions in comparison to the elimination reactions described above. Selenoxides undergo spontaneous yn-elimi-nation at room temperature or below and thus have been used for the preparation of a variety of unsaturated compounds. The selenide precursors can be obtained by displacement of halides or sulfonate esters with PhSeNa. Oxidation of the selenides with hydrogen peroxide or tert-huiyX hydroperoxide, sodium periodate, or peroxycar-boxylic acids furnishes the corresponding selenoxides. Their eliminations usually favor formation of the less substituted olefin in the absence of heteroatom substituents or delocalizing groups. Since selenium compounds are toxic, they should be handled with care. 

However, the 4-hydroperoxyflavin, acting in these cases as an electrophile, is also able to oxidize other nucleophilic substrates and in particular heteroatoms such as sulfur[91, selenium[91> 92 nitrogen[731 and phosphorous[91. Indeed, CHMO oxygen-... 

Ozonolysis of Carbon-Heteroatom Bonds and Heterocyclie Compounds. The ozonolysis of nitrogen, phosphorus, sulfur, and selenium to form iV-oxides, phosphine oxides and phospho-nates, sulfoxides, and selenoxides that can be used to functionalize various substrates has been weU documented. A useful application of the power of ozonolysis is to cleave carbon heteroatom bonds for conversion into carbonyl compounds. A useful... 

There are many [2,3]-sigmatropic rearrangements involving a variety of heteroatoms as well as carbon. The mechanism is common with elements that are prepared to change their oxidation state by two so that an arrow can both start and finish on that atom. The examples in this section involve sulfur and selenium, which can both form stable compounds at three oxidation states S or Se(II), S or Se(IV), and S or Se(VI). 

Sufficient information on selenium-stabilized carbanions is now available both to require and to allow classification of the field. The carbanions may be of several types, depending on the oxidation level (selenide or selenoxide) of the heteroatom and on the status of the appendages. 

The neighboring group effect becomes particularly apparent within the SaS series relative to the oxidation of a simple sulfide. The generally lower oxidation potentials for the selenium- and phosphorus-containing compounds not only prove participation of these heteroatoms but also reflect their lower ionization potentials compared with sulfur. As discussed before, the best orbital overlap for formation of the 2(r/l(r three-electron bond is obtained for the five-membered ring (n = 3), evidenced by the lowest oxidation potentials and the most blue-shifted absorptions. (Remember that absolute optical transition energies may... 

The addition of nucleophiles to the cr-center of the selenium linker has been described by several groups among them Nicolaou et aL who investigated the formation of several spiro compounds 624 upon cleavage from solid supports [327] (Scheme 91). Crucial for these transformations are heteroatom-nucleophiles which can attack the released compoimds after sel-eniiun linker oxidation and elimination. Following the above-mentioned and similar routes, Nicolaou et aL synthesized diverse orthoesters and aUylic orthoesters [328]. 

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