Aryloxyl radicals

In order to explain formation of a furan derivative when a perfluoromethylcyclopropenyl ketone is heated with bromine, a complicated scheme involving intramolecular addition of an enoloxy radical (Cy5/Cy4 case) has been proposed.  

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In a similar system, the reaction of the ferric(edta) complex with peroxycarboxylic acids was probed by adding 2,4,6-tri-fe/r-butyl phenol, ArOH.2 This experiment gave rise to the aryloxyl radical, ArO, which persisted for hours and was detected by its characteristic spectrum. It was indeed formed in the reaction mentioned, at a rate that was independent of [ArOH], It was proposed that ArO results from a reactive oxo-iron intermediate, tentatively (edta)FevO. 

Ioki, Y., Aryloxyl Radicals by Photorearrangement of Nitrocompounds, J. Chem. Soc., Perkin Trans. II, 1240-f242 (f977). 

The oxidation of phenols forming aryloxyl radicals was first recognized by Pummerer and Frankfurter in 1914," who reacted 2,2 -dihydroxy-l,l -binaphthyl with ferricyanide, and isolated dimers and trimers of the radical 20 (equation 8), which was later observed by ESR." Many further examples were studied by Goldschmidt, Muller, and others," and an X-ray crystal structure confirmed dimer formation with C—O bonding at the 4-position for 3-bromo-2,4,6-triphenylphenoxyl. ... 

Substitution patterns on the B-rings appear to be the most important contributors. The 3, 4 -orthodihydroxy structure in the B rings confers the highest ability to the aryloxyl radical formed and participates in electron delocalization. A hydroxyl group at C-3 position is also beneficial to the... 

The kinetics and mechanism of inhibition (inh) of free radical oxidation has been the subject of several earlier reviews . The main reactions for inhibited oxidation by phenols are outlined below. When a phenolic antioxidant is present, peroxyl radicals are trapped by H-atom abstraction from a phenolic hydroxyl group, followed by rapid recombination of peroxyl and resulting aryloxyl radicals (equations 10 and 11). 

The aryloxyl radicals formed in the initial antioxidant reaction of phenols (equation 1) may undergo several different kinds of secondary reactions, including Type (1), rapid combination (termination) with the initiating oxygen-centered radicals (equation 11) Type (2), self-reactions Type (3), initiation of new oxidation chains by H-atom abstraction from the substrate, the so-called prooxidant effect and Type (4), reduction or regeneration by other H-atom donors resulting in synergistic inhibition. The relative importance of these secondary reactions will be considered briefly here, since they may affect the overall efficiency of the antioxidant, which includes the antioxidant activity, as measured by the rate constant, (equation 10), and the number of radicals trapped, n. 

These researchers present a number of arguments and evidence, including large deuterium kinetic isotope effects, in support of a mechanism involving proton-tunneling in a charge transfer complex (equation 29), as the rate-determining step for the reaction of the hindered aryloxyl radical, ArO , with phenolic antioxidants and they propose that the mechanism applies equally well to attack by peroxyl radicals, R—O—O , on phenols. 

The hindered aryloxyl radicals may not model exactly the antioxidant mechanism of phenols with phenoxyl radicals. 

In certain media , even normally nmeactive aryloxyl radicals participate in these so-called pro-oxidant reactions dne to local high concentrations of ArO or certain physical restrictions which prevent their termination by radical-radical reactions. 

It is now generally accepted that the first step involves the formation of aryloxy-lead (IV) triacetates (1). These can decompose homolytically, leading to dimeric products via the aryloxyl radicals (2). They can also decompose heterolytically to cationic aryloxy species, which are then trapped by external nucleophiles. This has been exemplified by the reaction of phenols with lead tetraacetate in the presence of acetic acid or methanol. In the latter case, ortho-mcthoxy derivatives were formed. However, the preferential formation of 6-acetoxy-2,4-dienone derivatives is more likely explained by an intramolecular ligand coupling reaction (3) rather than by occurrence of a cationic... 

This aryloxyl radical may terminate a second chain as follows... 

Matsui, E., Y. Naruta, F. Tani, and Y. Shimazaki (2003). An active-site model of prostaglandin H synthase An iron twin-coronet porphyrin with an aryloxyl radical overhang and its catalytic oxygenation of 1,4-diene. Angew. Chem. Int. Edit. 42, 2744-2747. 

Photolysis of 2-allylphenols (63) leads to a mixture of 2-methylcoumaran (64) and chroman (65) " in a Cy5/Cy6 ratio of 90 10, " a result nearly independent of the substituent on the ring (Scheme 34). " As formation of aryloxyl radicals by phenol photolysis was known, intramolecular aryloxyl... 

R = p-r-butyl), under conditions where the formation of an aryloxyl radical can reasonably be assumed does not lead to the cyclic products of Scheme... 

Thus it seems now established that the cyclized products obtained by photolysis of o-allylphenols do not result from intramolecular addition of aryloxyl radical. Furthermore, the results obtained on metallic salt oxidation lead to the conclusion that intramolecular addition of an aryloxyl radical, even in what is probably the best situation (Cy5/Cy6), is not a favored process. More work is needed before a definitive conclusion can be reached. Nonetheless, this possibility has been retained to account for the cyclized products (48.5% yield) obtained by treating the chalcone (66 Ar =/ -HOC6H4) with aqueous alkali - K3Fe(CN)g (Scheme 35).This must be considered a very favorable case, however, since the (Cy 5) radical is benzylic. 

Fig. 1.38. ESR spectmm of the hindered aryloxyl radical of the antioxidant BHT. After Becconsall et al [780]. From J.K. Becconsall et al, Trans. Faraday Soc. 56, 459-472 (1960). Reproduced by permission of The Royal Society of Chemistry.

Reaction (6) is possible when the A- radical is very reactive. This is what happens with some radicals from sterically hindered phenols or secondary aromatic amines, which constitute most of the important antioxidants [40]. Stabilization of aryloxyl radical increases with the size of ortho- and para-... 

When the aryloxyl radical A- takes part in termination reactions in one of its resonance forms, its antioxidizing effect increases ... 

ROO") produced by lipid oxidation, or with alkoxyl radicals (RO ) arising by decomposition of lipid hydroperoxides. They provide these radicals with hydrogen, thereby interrupting the radical chain autoxidation reaction. The resulting products are phenoxyl (aryloxyl) radicals of antioxidants ... 

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