Selenium-centered radicals

Most polymerizations in this section can be categorized as stable (Tree) radical-mediated polymerizations (sometimes abbreviated as SFRMP). In the following discussion systems have been classed according to the type of stable radical involved, which usually correlates with the type of bond homolyzed in the activation process. Those described include systems where the stable radical is a sulfur-ccntered radical (Section 9.3.2), a selenium-centered radical (Section 9.3.3), a carbon-centered radical (Sections 9.3.4 and 9.3.5), an oxygen-centered radical (Sections 9.3.6, 9.3.7), or a nitrogcn-ccntcrcd radical (Section 9.3.8). Wc also consider polymerization mediated by cobalt complexes (Section 9.3.9) and certain monomers (Section 9.3.5). 

Reactivity and Selectivity of Oxygen-, Sulfur- and Selenium-Centered Radicals Towards Allenes... 

Taking advantage of the formation of selenium-centered radicals, a cyclization reaction was attempted (Eq. 28). The reduction of the selenium-methylene bond appears, however, to be the preferred pathway, and the desired pyrrolidine compound forms in only 11% yield. 

The wide variety of methods available for the synthesis of orga-noselenides,36 and the observation that the carbon-selenium bond can be easily cleaved homolytically to give a carbon-centered radical creates interesting possibilities in organic synthesis. For example, Burke and coworkers have shown that phenylselenolactone 86 (see Scheme 16), produced by phenylselenolactonization of y,S-unsaturated acid 85, can be converted to free radical intermediate 87 with triphenyltin hydride. In the presence of excess methyl acrylate, 87 is trapped stereoselectively, affording compound 88 in 70% yield 37 it is noteworthy that the intramolecular carbon-carbon bond forming event takes place on the less hindered convex face of bicyclic radical 87. 

Various other heteroatom-centered radicals have been generated as initiating species. These include silicon-, sulfur-, selenium- (see 3.4.3.1). nitrogen- and phosphorus-centered species (see 3.4.3.2). Kinetic data for reactions of these radicals with monomers is summarized in Table 3.10. 

The vast majority of radical cascades initiated by addition of higher main group (VI)-centered radicals to alkynes focus predominantly on S-centered radicals, mainly thiyl radicals, whereas considerably fewer intermolecular addition reactions involving radicals with the unpaired electron located on selenium or even tellurium are known. 

Simultaneous introduchon of both sulfur and selenium functions into carbon-carbon unsaturated compounds via a radical mechanism is also demonstrated by selenosulfonahori [149] and selenothiocarboxylatiorl [150] (Scheme 15.70). In these addihon reactions, attack of sulfur-centered radicals at the terminal position of alkenes and the subsequent Sh2 reaction on the selenium lead to the formahon of anti-Markovnikov adducts regioselechvely. The selenosulfonahon can be apphed to a variety of unsaturated compounds, for example alkynes, allenes, and vinylcyclopropanes, and combination with the selenoxide syn-elimination procedure... 

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