Oxidation-reduction reactions Intramolecular electron transfer

Chromium(II) is a very effective and important reducing agent that has played a significant and historical role in the development of redox mechanisms (Chap. 5). It has a facile ability to take part in inner-sphere redox reactions (Prob. 9). The coordinated water of Cr(II) is easily replaced by the potential bridging group of the oxidant, and after intramolecular electron transfer, the Cr(III) carries the bridging group away with it and as it is an inert product, it can be easily identified. There have been many studies of the interaction of Cr(II) with Co(III) complexes (Tables 2.6 and 5.7) and with Cr(III) complexes (Table 5.8). Only a few reductions by Cr(II) are outer-sphere (Table 5.7). By contrast, Cr(edta) Ref. 69 and Cr(bpy)3 are very effective outer-sphere reductants (Table 5.7). 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Two separate but somewhat interwoven themes have emerged from the study of inner-sphere reactions. The first is the use of product and rate studies to establish the existence of inner-sphere pathways and then the exploitation of appropriate systems to demonstrate such special features as remote attack . In the second theme the goal has been to assemble the reactants through a chemical bridge and then to study intramolecular electron transfer directly following oxidation or reduction of the resulting dimer (note equation 7). It is convenient to turn first to chemically prepared, intramolecular systems since many of the theoretical ideas and experimental results for outer-sphere reactions can be carried over directly as an initial basis for understanding the experimental observations. 

Therefore, the role that the ferrocenium group plays in the in vitro cytotoxicity appears to be that of an intramolecular electron acceptor. The inertness of the non-phenolic compound 7 to pyridine in this model system shows that a phenolic group is necessary for the reaction to take place. Likewise, for the unconjugated 20a-c, chemical reduction of the Fe(III) atoms was not observed suggesting that the electron transfer process occurs through a coupling in the molecular Ti-system. Thus, as soon as an adequate base is available, a substantially fast intramolecular electron transfer may occur, thereby leading to the oxidation of the phenolic moiety made easier because of its displacement by the reaction of the phenoxy cation with the pyridine base [153-155]. 

Although a previous chapter in this volume provides a broader perspective on the reactivity of radical cations, in this section we will examine intramolecular electron-transfer reactions coupled with or followed by cleavage of a bond in odd electron species such as radical cations, radical zwitterions and radical anions. In particular, this paragraph will be divided in oxidative and reductive bond-cleavage processes. Because this field is however too large to be covered extensively here, the discussion will be limited to selected examples—for oxidative cleavages, side-chain fragmentation reactions of alkylaromatic radical cations and decarboxylation reactions of radical zwitterions derived from benzoic and arylalkanoic acids, and for reductive... 

C (66). If electron transfer from type 1 to type 3 copper couples the two halves of the enzyme cycle, as proposed for laccase, then this intramolecular redox reaction must be extremely rapid to account for the effects of trace dioxygen on the reduction of the type 1 copper. Consequently, despite the fact that an ambiguous assignment of a type 1 to type 3 transfer is not possible in this example, facile intramolecular electron transfer processes probably ensure a rapid distribution of electrons among the type 1 and type 3 copper centers, at least in some of the enzyme molecules. The equilibrium distribution, and quite conceivably the relative rates of approach to this state, should be influenced by the oxidation-reduction potentials, which, as described earlier in this chapter (Figure 5), favor electron occupancy of the type 3 copper pairs at 10.0°C. 

Such a sacrificial mechanism, although fully successful, is less appealing than the intramolecular one because it leads to the formation of waste products. However, instead of using a sacrificial reductant, that is, an electron donor molecule that undergoes a fast decomposition reaction after electron transfer has taken place, a reversible reductant, giving rise to a stable oxidized form, may be successfully employed, provided that the back electron transfer process can be slowed down by a wise choice of the partners. 

Likewise, differences in reaction driving forces do not explain such a difference in rates. Interestingly, it was found that the rate of intramolecular electron transfer in Pa-NiR was accelerated four orders of magnitude by cyanide coordination to the oxidized heme-dj (104) it was suggested that, in the absence of cyanide, a coordination change takes place upon reduction of this heme. 

Figure 21. Reaction scheme describing the stepwise reduction of P. aeruginosa and P. stutzeri NiR. Symbols Circle Heme-c Square Heme-di. Empty symbols represent an oxidized site, filled ones a reduced site. Each protein subunit includes one heme-c and the heme-di below it. A single arrow represents intermolecular reduction of heme-c by the external reductant. These reactions are all assumed to be irreversible and to occur with the same, near diffusion controlled, rate constant. The double arrows represent reversible intramolecular electron transfer within a subunit. It is assumed that heme-di is not reduced by in/crmolecular ET, and that there is no intramolecular electron transfer between the two subunits. The asterisk indicates that there ate two fmms of the species. They differ by...

In peptides and proteins, oxidation of tryptophan is followed by tryptophanyl radical reduction by tyrosine, leading to tyrosinyl radical. This reaction was shown first by Prutz and co-workers (120). Azide radicals are very convenient for this study. This process is easily visualized by pulse radiolysis since both free radicals absorb at different wavelengths (table 3) and the time scale for this reaction goes to microsecond for small peptides to millisecond for proteins. This reaction may occur by intramolecular step and thus it constitutes an excellent model to investigate long range intramolecular electron transfer. These results will be discussed further (see 5.1). 

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