Selenium transfer reactions

A new radical chain group transfer reaction which does not involve tin reagents has been reported. The reaction proceeds by a photosensitized electron transfer reductive activation of PhSeSiR.3 using 1,5-dimethoxynaphthalene as the sensitizer [95ACIE2669]. In contrast to the tellurium transfer described above, the selenium transfer reaction gave higher diastereoselectivity (4 1 vs 2 1). 

A small number of reports on the chemistry of selenophenes and tellurophenes appeared during the past year. A selenium transfer reaction between the bis-anion derived from phenylenediacetonitrile and selenium oxychloride (SeOCl2) gave the corresponding... 

Many molybdenum and tungsten complexes have been synthesized to date, mimicking oxygen, sulfur and selenium transfer reactions. Studying their kinetic characteristics can be undertaken via several alternative routes and procedures. These methodologies and their respective results are presented in the following parts of this chapter. 

Halogen, Sulfur, and Selenium Group Transfer Reactions... 

Another important group of methyl transfer reactions are those from methyl corrinoids to mercury, tin, arsenic, selenium, and tellurium. For example, Eq. 16-44 describes the methylation of Hg2+. These reactions are of special interest because of the generation of toxic methyl and dimethyl mercury and dimethylarsine. 

In common with related heterocycles, 2-benzoselenophene is unstable and cannot be isolated but can be stored in solution and easily undergoes the Diels-Alder reaction [146], In contrast, the 1,3-disubstituted 2-benzoselenophenes are stable compounds and synthesis of 1,3-dicyano 2-benzoselenophene (97) starting from o-xylene dicyanide (96) using SeOCL, as selenium transfer reagent was reported (Scheme 27) [147],... 

Selenium-containing molecules have also been used as precursors for radical seleno group transfer reactions. This is a very powerful method for radical additions to alkenes and alkynes it is especially interesting from an atom economy point of view since all atoms remain in the product molecule. The free-radical addition of selenosulfonates 146 can be initiated either photochemically or thermally using AIBN. The addition of 146 not only to alkynes 147,255-257 km also to alkenes258-261 or allenes,261 has been reported and the products such as 148 are versatile building blocks for subsequent reactions (Scheme 39). For example, vinyl selenides 148 can be easily transformed into allenes. 

A photosensitized activation of carbon-selenium bonds was also used for performing phenylseleno group transfer reactions. This process involves a photosensitized electron transfer (PET) as the initial step in the reaction sequence. Fragmentation affords a radical and phenylselenolate, which is oxidized to diphenyl diselenide in the presence of oxygen. The cyclized radical is then trapped by diphenyl diselenide to afford the final product. This process is quite general for intramolecular radical reactions.70,266... 

Due to their singlet character, carbenes with sulfur-, selenium-, silicon-, or tin-substituents add to olefins with high stereospecificity, thus guaranteeing mechanism-based stereoselectivity. While high simple diastereoselectivity has been observed in certain examples, the diastereofacial selectivity of these carbenes is almost unknown, and examples of enantioselective carbene transfer reactions are completely lacking. 

Other atoms and groups apart from hydrogen are susceptible to abstraction by free radicals. The most important from a synthetic point of view are bromine, iodine, sulfur, and selenium substituents. Group transfer reactions can occur inter- or intramolecularly. Indeed, we have already encountered one example in the addition of polyhalogenated methanes to alkenes. The chain is propagated by a bromine atom transfer. 

Recently, a variety of sulfijr- and selenium-bearing compounds were identified through the use of GC/MS techniques [51-53], element specific technique of gas chromatography with atomic emission detection (GC-AED) [54], and by proton-transfer-reaction mass spectrometry (PTR-MS)[55] in the breath of persons after the ingestion of garlic. 

SECTION 12.5. HALOGEN, SULFUR, AND SELENIUM GROUP TRANSFER REACTIONS... 

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