Oxygen-centered radical mediated

Other Oxygen-Centered Radical-Mediated Polymerizations... 

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). 

Figure 3. OH radical-induced degradation of thymine. Oxidation involves either OH addition to the C5-06 double bond or OH-mediated hydrogen abstraction from the methyl group. Molecular oxygen then adds to the resulting carbon-centered radicals. This reaction yields hydroperoxide that further evolves into stable products, such as thymidine glycol (ThdGly), 5-hydroxymethyl-uracil (HMdUrd) and 5 formyl-uracil (5-FordUrd). N- represents the bond to the DNA backbone.

Reactions herein significantly differ from most standard nBu SnH-mediated radical transformations that employ a variety of well-known precursors for carbon-centered radicals, including halides, thioacyl moieties, olefins, selenides and sulfides. Most of these potentially useful precursors are sacrificed and lost during the radical reaction and are not available for subsequent manipulations [7]. However, O-stannyl ketyl intermediates conserve the carbonyl oxygen for further functionalization. 

Other methods, among which thermolysis or photolysis of tetrazene [59], photolysis of nitrosoamines in acidic solution [60], photolysis of nitrosoamides in neutral medium [61], anodic oxidation of lithium amides [62], tributylstannane-mediated homolysis of O-benzoyl hydroxamic derivatives [63, 64], and spontaneous homolysis of a transient hydroxamic acid sulfinate ester [65] could have specific advantages. The redox reaction of hydroxylamine with titanium trichloride in aqueous acidic solution results in the formation of the simplest protonated aminyl radical [66] similarly, oxaziridines react with various metals, notably iron and copper, to generate a nitrogen-centered radical/oxygen-centered anion pair [67, 68]. The development of thiocarbazone derivatives by Zard [5, 69] has provided complementary useful method able to sustain, under favorable conditions, a chain reaction where stannyl radicals act simply as initiators and allow transfer of a sulfur-containing... 

Scheme 10.27 Catalytic cycle of HppE. Dashed arrows indicate electron transport. In this scheme HPP binds to iron1". After a one-electron reduction, dioxygen binds and reoxidizes the iron center. The peroxide radical is capable of stereospecifically abstracting the (pro-R) hydrogen. Another one-electron reduction is required to reduce one peroxide oxygen to water. Epoxide formation is mediated by the resulting ironlv-oxo species.

---