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4: Selenides & Tellurides

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    24017

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    Carbohydrates containing selenium–carbon bonds react with tin and silicon hydrides to gen­er­ate carbon-centered radi­cals. Phenyl selenides are the most com­mon type of selenium-containing carbohydrate used in radical formation. As radical precursors, phenyl selen­ides rival the reactivity of bromides and iodides. (Absolute rate con­stants for reaction of simple organic iodides,1 phenyl sel­enides,2 and bromides2,3 with (CH3Si)3Si· are 4.0 x 109, 9.6 x 107, and 2.0 x 107 M-1s-1, re­spect­ively.)

    Selenophenyl glycosides have a distinct advantage over the cor­res­ponding iodides and bro­mides when it comes to radical formation because anomeric phenyl selenides are therm­ally more stable than the corresponding anomeric halides. Anomeric iodides are, in fact, too unstable to have a significant role in gener­ating pyranos-1-yl or fur­an­os-1-yl radicals. Anomeric bromides are accept­able radical precursors in many instances, but when they are too unstable, phenyl selenides become attractive alternatives. Phenyl selenide advan­tage is apparent in the gen­er­ation of fur­an­os-1-yl radicals where glycosyl bromides typically are unable to survive the heating at reflux in benzene or toluene that normally is used in such reac­tions. Selenophenyl glycosides are stable enough under these conditions to avoid nonradical, thermal decomposition and, therefore, they are able to form the desired radicals.4,5

    Organotellurium compounds represent another source of carbohydrate radicals. Although selenides are used much more frequently as starting mater­ials for radical formation, tellurides undergo many of the same types of reac­tion. A problem with many tellurium-containing com­pounds is that they decom­pose so readily ­that they can be difficult to purify and store.


    This page titled 4: Selenides & Tellurides is shared under a All Rights Reserved (used with permission) license and was authored, remixed, and/or curated by Roger W. Binkley and Edith R. Binkley.

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