Fenton-type chemistry

As expected based on our knowledge of gas-phase chemistry, in addition to the Fenton type chemistry involving iron, photolysis of Os, H202, HONO, and HNO-, are all potential OH sources in clouds and fogs. In addition, the photolysis of nitrite, nitrate, and HOJ in aqueous solutions can also form OH. In short, there are many potential sources of OH in clouds and fogs. 

All the complexes in Table I have the ability to serve as immediate or indirect precursors to high valent species. The hydroperoxides react with one-electron reductants in Fenton-type chemistry to generate... 

Since H202 is easier to handle than 02, we will focus on the use of the former. Many metals can be used for this transformation [50]. Among them, iron compounds are of interest as mimics of naturally occurring non-heme catalysts such as methane monooxygenase (MMO) [51a] or the non-heme anti-tumor drug bleomycin [51b]. Epoxidation catalysts should meet several requirements in order to be suitable for this transformation [50]. Most importantly they must activate the oxidant without formation of radicals as this would lead to Fenton-type chemistry and catalyst decomposition. Instead, heterolytic cleavage of the 0—0 bond is desired. In some cases, alkene oxidation furnishes not only epoxides but also diols. The latter transformation will be the topic of the following section. 

Figure 2.12 Possible formation offerryl ion during Fenton-type chemistry.

Early work with transition metals, following Fenton-type chemistry see Fenton Chemistry), employed the transition metal as a catalyst for the decomposition of a primary oxidant to yield radicals such as RO- and -OH that readily abstract an H atom from alkanes, RH, to give R-. These radicals... 

Fe(III) salts are known to oxidise electron-rich centres to foster the formation of radical species. They are particularly efficient in the oxidation of aromatic systems or a carbanion to the corresponding carbon-centred radical which undergoes C-C bond formation to yield the coupled products. For a successful synthesis, it is important to work in the absence of reactive synthetic molecules other than those which form the combination of radicals. Barton et al. used a simple water-soluble diselenide derivative that shows radical scavenger properties towards alkyl and hydroxyl radicals in Fenton-type chemistry (Fe2+-H202)4 The reaction rate between the produced alkyl radical and the diselenide overwhelms self-termination and halogen transfer reactions. The ability of diselenide to scavenge alkyl and hydroxyl radicals [ 3(0 °C) = 6.1 x 108 M-1 s-1] could be exploited as a new tool in both synthetic and mechanistic work conducted in aqueous media (Scheme 8.5).4... 

Iron salts or iron exchanged MMT deactivate H2O2 with the generation of O2, and show low alkane oxidation activity (< 0.5%). Complexation of iron by organic ligands has very specific effects on the spin state (II, III) and its electrophilicity and allows non-Fenton-type chemistry [13,14]. Mononuclear ferrous N,N -bis(2-pyridinecarboxamide)-l,2/3-R complexes in Na-Y zeolite (with R= alkane or benzene) are oxo-type catalysts [13]. Here a different reactivity pattern is expected for the dinuclear ferric complexes [14]. 

The reactions of superoxide radicals with metal complexes have received much attention as O2" can play a role in cycling oxidation states in metal complexes (I). This feature has been implicated in catalytic processes such as the Fenton-type chemistry (M + + H2O2 + HO + OH" + O2"... 

The Fenton-type chemistry between Fe q and several rhodium hydro-peroxides yields Rh(IV) species believed to have the general formula L(H20)Rh0, Eq. (11) (72 although this was not established experimentally for these short-lived transients. Just like the other LMO species generated by this route (58 the compounds L(H20) RhO (L = (NH3)4, L, and L ) react rapidly with the Fe + present in solution. The addition of substrates for L(H20)Rh0 resulted in a competition, as shown for methanol in Eqs. (12) and (13). 

While radical species can hardly abstract H atom from benzene rings, some of them, in particular HO radical, are easily trapped by aromatic molecules. Radical hydroxylation most often follows Fenton-type chemistry that involves generation of HO in the course of homolytic decomposition of HjOj induced by an iron(II) salt and addition of HO to an aromatic nuclear to give hydroxycyclo-hexadienyl radical I. This radical may dimerize, be oxidized to phenols (Cu(II) is one of the most effective oxidants for this), or undergo an acid-catalyzed collapse to radical cation [15, 28]. Scheme 14.3 shows the classical mechanism suggested by Walling [28]. 

Most of the studies on these sulfide-derived three-electron bonded species have been carried out in aqueous solutions and by using hydroxyl radicals, generated by radiolysis or Fenton-type chemistry, as oxidants [66-68,70]. The OH... 

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