Fenton-like reaction intermediates

The reaction is postulated to proceed through a hydroperoxide intermediate, [Cu2(L-H)(OOH)]. Experimental studies into Fenton and Fenton-like reactions point out that the detailed mechanism involves the formation of a low-valent cation.However, while [(H20)5Cr " 02H ] decomposes directly to [Cr(H20)6] " and OH in the presence of >0.1 M EtOH, 2-PrOH, and 2-BuOH, [(H20), Cu 02H ] reacts directly with the alcohols. In the absence of organic substrate, the latter complex does yield [Cu(H20) ] and OH. 

Fenton chemistry continues to attract the interest of mechanistic chemists. An account entitled Comments on the mechanism of the Fenton-like reaction has appeared. In particular, the question of whether hydroxyl and alkoxy radicals are intermediates was discussed. ... 

The reaction of hydrogen peroxide with copper(I) salts produces a Fenton-like hydroxylating system involving reactive hydroxyl radical intermediates (equation 265).486,491 Hydroxylation of benzene to phenol can be achieved by air in the presence of copper(I) salts in an acidic aqueous solution.592 593 This reaction is not catalytic (phenol yields are ca. 8% based on copper(I) salts) and stops when all copper(I) has been oxidized to copper(II). A catalytic transformation of benzene to phenol can occur when copper(II) is electrolytically reduced to copper(I) (equation 266).594,595... 

The reaction of hydrogen peroxide with copper(I) salts produces a Fenton-like hydroxylating system involving reactive hydroxyl radical intermediates (equation Hydroxylation of... 

Under this approaeh, one of the biggest question is if the reaction (19) is not to slow to compete with the reaetion (18). On the other hand, Fe appears to be a weaker oxidant than OH radieal Koppenol and Liebman 1984 Rahhal and Richter 1988). Experimental evidences (using the electron paramagnetic resonance spin-trapping method) have shown the existence of several intermediated oxidant in the Fenton proeess, like OH bound OH and high-valence iron speeies Yamazaki and Piette 1991 De Laat and Gallard 1999 Gallard and De Laat 2001). 

Although several studies indicate that HO is formed in Fenton systems according to Eq. 37 and it is responsible for the efficiency of degradative reactions, it is presently believed that other Fe(IV) or Fe(V) species like Fe03+ and ferryl complexes, are also active agents in the processes [53— 55,58, 112]. For example, Kremer [112] identified a mixed valence binuclear species, FeOFe 5+, and proposed a new mechanism for the Fenton reaction, in which Fe02+ acts as the key intermediate. 

These footprinting analyses, based on enzymic and chemical digestion, are now widely used to define DNA (and RNA) and their complexes with various ligands. Recently active radical probes have been used as footprinting agents in protection assays in a variety of systems (e.g., Tullius and Dombroski, 1986 Chalepakis and Beato, 1989 Hayes and Tullius, 1989 Schickor et al., 1990). Such probes rely on active radical intermediates, most likely hydroxyl radicals, released by Fe(II) in the presence of an electron donor, probably via a Fenton reaction. In addition, hydroxyl radicals also appear to react with DNA in a conformation-specific manner which may allow some prediction of DNA secondary structure (see Burkhoft and Tullius, 1987 Zorbas et al., 1989 Lu et al., 1990). 

In turn, the very same catalyst is also likely to exhibit catalase activity, i.e., the reversal of 02 activation, thereby depleting the free 0) equivalents necessarily formed as intermediates Reaction 8). Moreover, metal-catalyzed decomposition of peroxides according to Reaction 8b often involves radical pathways e.g., Fenton chemistry), thereby increasing the risk of the occurrence of non-selective pathways. Accordingly, systems for selective mediation of reactions with 02 need to display a delicate balance of rate constants if, for example. Reactions 7a and 8 a have to be fast compared with all possible competing chaimels. 

Another classification of C-H activation methods is as inner-sphere and outer-sphere mechanisms. Inner-sphere mechanisms can be defined as those that involve the formation of a carbon-metal bond from a C-H bond, while outer-sphere mechanisms involve the cleavage of a C-H bond by a metal-containing species to generate a reactive intermediate, but without a metal-carbon bond. A disadvantage of this classification is that it assumes that the mechanism is known The reactions discussed in this chapter would be considered inner-sphere. Reactions such as the Fenton reaction would be considered outer-sphere. A grey area is likely to exist between the two mechanisms. Another disadvantage of this classification is that the term inner-sphere mechanism tells us nothing about the mechanism beyond the formation of a metal-carbon bond ... 

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