Electron transfer reactions cation radical peroxidation

Electron-transfer reactions between cytochrome c and cytochrome c peroxidase have been studied extensively because of the well-characterized structures and biophysical properties of the reactants [146-150]. It is well known that the resting ferric form of cytochrome c peroxidase is oxidized by hydrogen peroxide to compound I, which contains an oxyferryl heme moiety in which the iron atom has a formal oxidation state of 4-1- [146-150]. The other is a porphyrin n radical cation or organic radical (R +) localized on an amino acid residue of Trp-191 [151-154] this is formed by transfer of the oxidized equivalent to the amino acid side chain [150]. The site of electron transfer in the reduction of compound I has been controversial and two forms of compound II have been identified, (P)Fe =0 containing the oxyferryl heme Fe(IV) [155-158] and [(P)Fe ] + containing Fe(III) and the porphyrin % radical cation which oxidizes the amino acid side-chain to produce an organic radical [(P)Fe +, R" ] [159 165] as shown in Scheme 10. 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

However, in certain cases, the rate of electron uptake by a particular species just happens to be slow. For example, electron transfer between the methyl viologen radical cation (MV ) and hydrogen peroxide has a rate constant of 2.0 (mol dm ) s , while the reaction between MV and just about any other chemical oxidant known is so fast as to be dijfusion-controlled. The reason for this is simply not known at the present time. 

In 2004, a report appeared that 9,10-dimethylanthracene-9,10-endoperoxide (48) arose from an electron-transfer photooxidation of 9,10-dimethylanthracene (38) with the use of the sensitizer Acr+-Mes in 02-saturated CH3CN at 0 °C [32]. Subsequent coupling of the anthracene radical cation and the superoxide ion generated endo-peroxide 48. Endoperoxide 48 was detected during the initial stage of the photooxidation, but was not isolated over time, the reaction yielded 10-hydroxanthrone, anthraquinone, and H202. 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

Photolysis of FL at a silica gel/air interface leads to the generation of 9-fluorenone (FLO) as the only isolable product. No dark reactions were observed and singlet molecular oxygen is not involved in the reaction. Transient spectroscopy shows that both the triplet state and the radical cation of FL are formed, thus indicating that an electron transfer mechanism is involved. Loss of a proton from the radical cation and subsequent reaction with molecular oxygen yields FL peroxide radicals, leading to the formation of 9-hydroxyfluorene. The latter readily photolyzes on the silica surface to produce FLO. 

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