Oxygen radical intermediates

The control of anomeric stereochemistry continues to fuel the investigation into the synthetic utility of (x-oxygenated radical intermediates. Moreover, it has proven to be a valuable tool in organic synthesis, especially in the stereoselective synthesis of various substituted tetrahydropyrans, y>>n-l,3-dioxanes, and carbohydrate derivatives. The recent discovery of non-equilibrium radical reactions and conformation-induced self-regeneration of stereocenters should provide new opportunities in the ever-expanding field of a-oxygenated radical chemistry. 

No clear picture of the primary radical intermediate(s) in the HO2 photooxidation of water has appeared. The nature of the observed radical species depends on the origin and pretreatment of the HO2 sample, on the conditions and extent of its reduction, on the extent of surface hydroxylation, and on the presence of adventitious electron acceptors such as molecular oxygen (41). The hole is trapped on the terminal OH group (54). 

The principal components of atmospheric chemical processes are hydrocarbons, oxides of nitrogen, oxides of sulfur, oxygenated hydrocarbons, ozone, and free radical intermediates. Solar radiation plays a crucial role in the generation of free radicals, whereas water vapor and temperature can influence particular chemical pathways. Table 12-4 lists a few of the components of each of these classes. Although more extensive tabulations may be found in "Atmospheric Chemical Compounds" (8), those listed in... 

The presence of oxygen can modify the course of a fiee-radical chain reaction if a radical intermediate is diverted by reaction with molecular oxygen. The oxygen molecule, with its two unpaired electrons, is extremely reactive toward most free-radical intermediates. The product which is formed is a reactive peroxyl radical, which can propagate a chain reaction leading to oxygen-containing products. 

A possible mechanism for the formation of the furanones 6 and 7 is illustrated in Scheme 2. The initial alkoxy radical generated from the alcohol 5 and lead tetraacetate (LTA) undergoes /3-scission to produce the acyl radical intermediate 9. Subsequent cyclization to 10 proceeds through attack of the radical at the carbonyl oxygen. The resulting Pb(IV) intermediate 11 finally collapses via the reductive... 

Various side reactions may complicate RAFT polymerization. Transfer to solvents, monomer and initiator occur as in conventional radical polymerization. Other potential side reactions involve the intermediate radicals 165 and 167. These radicals may couple with another radical (Q ) to form 271 or disproportionate with Q to form 270. They may also react with oxygen. The intermediate radicals 165 and 167 are not known to add monomer. 

The reaction with nitrite proceeds smoothly and with relatively high yields of the corresponding nitroarene (see Sec. 10.6). Obviously a major part of the driving force of this reaction is the formation of a stable, i. e., an energetically favorable, radical, nitrogen dioxide. With the hydroxide ion — a much stronger nucleophile than the nitrite ion — the reaction is expected to produce very unstable radicals, the hydroxy radical OH and the oxygen radical anion O, from the diazohydroxide (Ar - N2 — OH) and the diazoate (Ar-N20 ) respectively. Consequently, dediazoniation in alkaline aqueous solution does not follow the simple Scheme 8-41 with Yn = OH, but instead involves diazoanhydrides (Ar — N2 —O —N2 —Ar) as intermediates (see Sec. 8.8). 

It is noteworthy that metallic copper or cuprous bromide used under nitrogen atmosphere shows only a very short induction time. This last result points out the inhibitor role of the oxygen of the air atmosphere and most likely the important role taken either by reduced species or by radical intermediates in the catalytic cycle. 

There are examples in which base radicals undergo reaction with adjacent base residues. The 5-(2 -deoxyuridinyl)methyl radical (63, Scheme 8.30) can forge an intrastrand cross-link with adjacent purine residues. Cross-link formation is favored with a guanine residue on the 5 -side of the pyrimidine radical and occurs under low-oxygen conditions. A mechanism was not proposed for this process, but presumably the reaction involves addition of the nucleobase alkyl radical to the C8-position of the adjacent purine residue. Molecular oxygen likely inhibits crosslink formation by trapping the radical 63, as shown in Scheme 8.24. The radical intermediate 89 must undergo oxidation to yield the final cross-linked product 90,... 

Under low oxygen conditions, C5 -sugar radicals can react with the base residue on the same nucleotide. In purine nucleotides, the carbon-centered radical 91 can add to the C8-position of the nucleobase (Scheme 8.31). Oxidation of the intermediate nucleobase radical 92 yields the 8,5 -cyclo-2 -deoxypurine lesion 93197,224,225,230-233 Similarly, in pyrimidine nucleotides, the C5 -radical can add to the C6-position of nucleobase. Reduction of the resulting radical intermediate yields the 5, 6-cyclo-5,6-dihydro-2 -deoxypyrimidine lesion 94,234-236... 

The trapping of the radical intermediate in demercuration by oxygen can be exploited as a method for introduction of a hydroxy substituent (see p. 295). The example below and Entries 3 and 4 in Scheme 4.6 illustrate this reaction. 

The reactions of oxygen with metal-alkyl derivatives typically involve radical intermediates (73, 74), and a likely mechanism for the reactions of [TpBut]MgR with 02 is illustrated in Scheme 6. Specific... 

By complexation of MnNaY with 1,4,7-trimethyltriazacyclononane, a new heterogeneous catalyst was obtained for olefin epoxidation with H202. Excellent epoxide selectivities were obtained, with limited epoxide solvolysis. The oxygenation appears to go through a radical intermediate. The manganese trizacyclononane epoxidation catalyst was also heterogenized via surface gly-cidylation.103... 

Ionomer membranes are used in fuel cells in order to separate the anode and cathode compartment and to allow the transport of protons from the anode to the cathode. The typical membrane is Nation , which consists of a perfluorinated backbone and side chains terminated by sulfonic groups. In the oxidizing environment of fuel cells, Nation , as well as other membranes, is attacked by reactive oxygen radicals, which reduce the membrane stability. Direct ESR was used recently in our laboratory to detect and identify oxygen radicals as well as radical intermediates formed in perfluorinated membranes upon exposure to oxygen radicals [73,74]. The three methods used to produce oxygen radicals in the laboratory and the corresponding main reactions are shown below. 

The effectors of the mammalian host immune attack against filaria include reactive oxygen intermediates. Filarial nematodes express glutathione peroxidase, thioredoxin peroxidase and superoxide dismutase at their surface - enzymes believed to protect the nematode from this attack (Selkirk et al., 1998). A bacterial catalase gene has been identified that most probably derives from the endosymbiont genome (Henkle-Duhrsen et al., 1998) this enzyme may contribute with other enzymes to the protection of both Wolbachia and its nematode host from oxygen radicals. 

Note that under these conditions the thermal reaction in equation (90) is too slow to compete.) Finally, the stoichiometry for the oxygen-atom transfer from NO to the donor cation radical in equation (93) is independently established by the reaction of isolated cation radical intermediates with NO. 251,252... 

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