Selenium polymer-supported

Another example of the resin-capture-release technique which should see widespread applications in the future is the selenium-mediated functionalization of organic compounds. Polymer-supported selenium-derived reagents [34] are very versatile because a rich chemistry around the carbon-selenium bond has been established in solution and the difficulties arising from the odor and the toxicity of low-molecular weight selenium compounds can be avoided. Thus, reagent 26 (X = Cl) was first prepared by Michels, Kato and Heitz [35] and was employed in reactions with carbonyl compounds. This treatment yielded polymer-bound a-seleno intermediates, which were set free back into solution as enones from hydrogen peroxide induced elimination. Recently, new selenium-based functionalized polymers 26 (X = Br)-28 were developed, which have been utilized in syntheses according to Scheme 11 (refer also to Scheme 3) [36],... 

The concept of nucleophilic selenium reagents has also been applied to solid-phase synthesis. Polymer-supported selenium reagents have attracted the interest of synthetic chemists because of their facile handling without the formation of toxic and odorous by-products. The polystyrene-bound selenium-containing reagent... 

Polymer-supported reagents have attracted growing interest because they can be applied to combinatorial chemistry and solid-phase synthesis. Some newly developed selenium-containing polymers have been reported recently. These polymers show selective reactivity without forming toxic and odorous byproducts in the form of low-molecular-weight selenium compounds. 

Recent advances in combinatorial chemistry has led to the development of several polymer-supported selenium reagents, exemplified by Scheme 15.39 [103-109]. 

Scheme 18.31 Synthesis of a library of benzopyrans employing the use of a polymer-supported selenium resin.

Polymer-supported selenium regents have been prepared to avoid some of the obnoxious and toxic properties of these reagents as monomers (Michels et al., 1976). The reagents were used to convert 4-methylcyclohexanone into 4-methylcyclohex-2-enone, to hydrobro-minate ethyl 2-bromopropionate, and to glycolate 2-methyl-2-heptene. 

Selenium-based linkers 590 can also be synthesized from polystyrene via lithiation and subsequent addition of dimethyldiselenide to the polymer support. Reaction of bromine gives selenenyl bromide that can be reduced to the lithium selenium resin in the presence of liBH4 that is alkylated subsequently [318]. Other approaches include the addition of former synthesized selenium-containing building blocks to the resin, for example, the hnker systems generated by Elofsson et al. [319] and Nakamura et al. [320,321]. 

Recently, the semi-synthesis of Vancomycin (48) on solid supports was accomplished using an allylic linker (Scheme 3.2) [123, 124]. Polymer-bound chiral electrophilic selenium reagents have been developed and applied to stereoselective se-lenylation reactions of various alkenes (Tab. 3.9) [125]. 

Subsequent epidemiological studies have supported the association between better health and long-term consumption of diets rich in foods of plant origin. " However, whether this is because such diets minimize exposure to deleterious substances (e.g., oxidized cholesterol, pyrolysis mutagens, salt, saturated fat, etc.), or maximize intake of certain beneficial nutrients (e.g., isothiocyanates and other sulfur-containing plant constituents, mono-unsaturated fatty acids, and poly-unsaturated fatty acids, PPT, poly acetylenes, selenium, terpenes, etc.) or some combination as advocated in the Polymeal concept, remains unknown. " An in vitro study indicates that there may be mechanistic basis for true synergy between PPT and isothiocyanates. ... 

With a polymer-bound chiral selenium bromide reagent, the stereoselective activation of olefins has been accomplished (entry 19) 24 Uehlin and Wirth24 found that both mesoporous silica and polystyrene-supported... 

For synthetic purposes, Pd catalyst on a polymeric support was developed and the addition reaction was carried out with excellent product yields of 94-99% and excellent stereoselectivity >99 1 (Scheme 3.64) [113]. After completion of the reaction the catalyst was easily isolated by filtration and a pure product was obtained after solvent evaporation (>98% purity without any additional purification procedures). Unfortunately, this synthetic approach was not applicable to Ph2Se2 addition to alkynes, since, at 140 °C, triphenylphosphine bound to the polymer reacted with Ph2Se2, resulting in selenium atom transfer to the phosphorus (Se=PR3) and formation of Ph2Se as a by-product. The mechanism of this side reaction was addressed in a joint experimental and theoretical study, which revealed the relationship between the C-Z and Z—Z bonds activation by Pd complexes [114]. 

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