Electron abstraction, initiation

Purified ligninase H8 produced by P. chrysosporium in stationary cultures oxidized pyrene to pyrene-1,6- and pyrene-l,8-quinones in high yield, and experiments with showed that both quinone oxygen atoms originated in water (Figure 8.25). It was suggested that initial one-electron abstraction produced cation radicals at the 1 and 6 or 8-positions (Hammel et al. 1986), whereas in... 

Bieber reported that the reaction of bromoacetates is greatly enhanced by catalytic amounts of benzoyl peroxide or peracids and gives satisfactory yields with aromatic aldehydes. A radical chain mechanism, initiated by electron abstraction from the organometallic Reformatsky reagent, is proposed (Scheme 8.27).233 However, an alternative process of reacting aldehydes with 2,3-dichloro-l-propene and indium in water followed by ozonolysis provided the Reformatsky product in practical yields.234 An electrochemical Reformatsky reaction in an aqueous medium and in the absence of metal mediator has also been reported.235... 

LiP catalyze several oxidations in the side chains of lignin and related compounds [26] by one-electron abstraction to form reactive radicals [27]. Also the cleavage of aromatic ring structures has been reported [28]. The role of LiP in ligninolysis could be the further transformation of lignin fragments, which are initially released by MnP. 

However, Bawn et al., take the view that when polymerization of an alkyl vinyl ether is initiated by a stable ion, such as tropylium, the initiation involves electron abstraction from the monomer with formation of a radical cation and a tropyl radical [52] ... 

In order to assess reactivity in this context we need first to recognise that the chemical initiation of cationic polymerizations can occur in different ways. Disregarding the rarer initiations involving electron abstraction or hydride abstraction from a monomer, the most common are... 

We found that when phenols (or lignins) are oxidized in alkaline media the initial step is a one-electron abstraction from the phenolic hydroxyl group forming a phenoxyl radical. We discovered an electrochemical method which allows us to determine roughly the free enthalpy of this particular reaction (phenol —> phenoxyl radical). This method has been... 

The phosphorescence decay kinetics of the triplet excited states of CuP molecules (Fig. 14) is adequately described by Eq. (16). Using this equation one can obtain the values of the parameter p = (Tra /2) In2 veT from the initial non-exponential part of the phosphorescence decay curves and the values of t = l/ k, i.e. the characteristic time of phosphorescence decay, from the final exponential part. Then the data on the dependence of the quantum yield of CuP phosphorescence on the concentration of C(N02)4 have been used to estimate the effective radii of electron tunneling from triplet excited copper porphyrins to C(N02)4 within the time x R, = (ac/2) In vet (Table 3). In doing so, the quenching of CuP luminescence by electron abstraction was assumed to be the only process leading to a decrease in the quantum yield of CuP phosphorescence in the presence of C(N02)4. From Table 3 an electron is seen to tunnel, within the lifetime of triplet excited states x at 10-4s, from CuP particles to C(N02)4 molecules over the distance R, 11 A. Further, the parameter vc and ae for different porphyrins were estimated from the values of (3, Rt, and x. These values are also cited in Table 3. 

Oxidation of organic substrates (flavones, phenols, indoles, thiols, ascorbic acid, etc.) using Co(salen)/02 systems is often interpreted in terms of initial production of radicals by H-atom (or electron) abstraction by the Co(II)02 or Co(III)02Co(III) moieties (122-127) (cf. Scheme 2), Reaction 38. In some of the conditions used, for example in... 

The photoreaction investigated to test the above model is the well-known electron-transfer-initiated intermolecular hydrogen abstraction reaction of carbonyl compounds [277]. Under the conditions employed, one of the guest mole-... 

The chemistry of [ j -CpCr(CO)3]2 and [ -Cp Cr(CO)3]2 has been studied in detaiP " . The dimers are in equilibrium with 17-electron monomers reactions with alkyl halides and R3SnCl are consistent with a halogen abstraction initiated by the monomeric Cr complex . If no S-hydrogen is present in R, the reaction proceeds as follows ... 

In an attempt to eliminate the steric effect of substituents, Kieboom and van Bekkum (75) hydrogenated substituted 2-aryl-3-methyl-2-butenes and 3,4-dihydro-l,2-dimethylnaphthalenes. They found that the polar effect of substituents on the rate of hydrogenation on a palladium catalyst was virtually negligible, and hence, that the character of the activated complex of the rate determining step was similar to the initial state. The adsorptivity of substrates was reduced by electron abstracting substituents, in agreement with the view that the substrate is adsorbed as a surface n complex. 

Based on the known chemistry of flavin photolysis reactions, it appears unlikely that thymine dimer cleavage occurs via a direct energy transfer mechanism (160). One proposal suggests that in the model reaction with 1-deazariboflavin, the thymine dimer radical anion is formed via electron donation from the excited sensitizer (164). Alternatively, electron abstraction by the excited flavin could occur, resulting in the thymine dimer radical cation (159, 160), although it is unlikely that reduced flavin would act as an electron acceptor. A schematic for this mechanism is illustrated in Scheme 33, where the initial formation of a sensitizer-dimer complex is consistent with the observed saturation kinetics. The complex is activated by excitation of the ionized sensitizer (pH > 7), and electron donation to the dimer forms the dimer radical anion and the zwitterionic, neutral 1-deazariboflavin radical (162). Thymine dimer radical would spontane-... 

Figure 6.10. The cytochrome P450-catalyzed oxidation of quadricyclane involving an initial electron abstraction step.

The ability of P450 enzymes to oxidize nitrogen atoms to radical cations via an initial electron abstraction is supported by a mmiber of experimental results. The finding that the 4-alkyl group of 3,5-(6/s)carbethoxy-2,6-dimethyl-4-alkyl-l,... 

Results of these kinds, then, lead to the aforementioned concensus that a cation radical is formed at a Lewis-acid site. There is no knowledge, however, about the fate of the abstracted electron. It has been suggested that the electron deposited initially at each site is spread over the surface or within the crystal lattice of the catalyst so that the esr signal becomes broadened and undetectable (Stamires and Turkevich, 1964). There is also a lot of evidence that molecular oxygen may be the true electron acceptor, and this possibility, in turn, is part of the much-debated question of the role of oxygen in cation radical formation. 

The Mn =0 entity is a very powerful oxidant able to mediate two different types of oxidative reactions transfer of an oxygen atom on a substrate (mimic of cytochrome P450, Fig. 21A) or electron abstraction (mimic of peroxidase. Fig. 21B). It is formally written Mn =0 since it corresponds to a two-electron oxidation with respect to the initial Mn complex. However,... 

The effect of the addition of citric acid on the photocatalytic reduction of hazardous Cr(VI) to less hazardous Cr(III)) with titania catalysts has been studied by means of in situ EPR of chromium species by Meichtry et al. [52] using titania P25 as photocatalyst. Reduction experiments of Cr(VI) solution were performed under near-UV (366 nm) irradiation under acidic conditions (pH 2) with bubbling air. It is found that the addition of citric acid fadhtates Cr(VI) reduction with a stepwise reduction of the chromate CrO/ (V)) via formation of Cr(V) and Cr( IV) and finally Cr(III) spedes observed. In the absence of dtric add, a cycHng between the different valence states of chromium occurs because of reduction and reoxidation processes by OH radicals. The maximum rate (fivefold increase) of Cr(VI) reduction is achieved at an initial citric add/Cr(VI) molar ratio of 1.25. Citric acid is oxidized to its anionic radical by electron abstraction of surface-trapped holes Cit + h+ j, -> Cit . 

One important characteristic of cobalt porphyrins is their ability to bind or react with small molecules, such as NO [27, 67, 70, 91, 93, 100], CO [36, 114, 115], O2 [314-320], or CO2 [321], and several studies have focused on the chemical and/or electrochemical reactivity of (P)Co toward these small molecules. The interaction of cobalt porphyrins with NO and the electrochemical properties of the resulting cobalt-nitrosyl porphyrins have been investigated by several research groups [7]. (TPP)Co(NO) exhibits two oxidations and three reductions at a microelectrode in CH2CI2 [90]. The NO group remains coordinated after electrooxidation and the initial electron abstraction from (TPP)Co(NO) was proposed to involve the porphyrin jr-ring system. Other electrode reactions were accompanied by a dissociation of NO from the compound and the site of electron transfer could not be determined. 

ABSTRACT. The electrochemical oxidation of chromium carbonyl thiolates RSCr(CO)5" has been investigated. Preparative scale oxidation yields the disulfide complex (RSSR)Cr(CO)5 as the major product the mechanism of its formation could be established by cyclic voltammetry experiments the short lived 17 electron radical initially formed undergoes a firagmentation reaction giving the unsaturated pentacarbonyl chromium and the alkylthiyi radical which readily dimerizes. Recombination of these two species gives the product. A slower reaction was shown to take place in the difiiision layer it results in the formation of the binuciear thiolate complex RSCr2(CO)io the redox behaviour of which is presented. 

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