Electron-transfer oxidation formulation

Transition metals (iron, copper, nickel and cobalt) catalyse oxidation by shortening the induction period, and by promoting free radical formation [60]. Hong et al. [61] reported on the oxidation of a substimted a-hydroxyamine in an intravenous formulation. The kinetic investigations showed that the molecule underwent a one-electron transfer oxidative mechanism, which was catalysed by transition metals. This yielded two oxidative degradants 4-hydroxybenzalde-hyde and 4-hydroxy-4-phenylpiperidine. It has been previously shown that a-hydroxyamines are good metal ion chelators [62], and that this can induce oxidative attack on the a-hydroxy functionality. 

Mechanistic Formulation of Electron Transfer. The Importance of the Work Term. Accordingly, the electron transfer mechanism can be considered in the light of the standard potentials E° for each redox couple, i.e., E x for the oxidation of the donor (D D+ + e ) and E ed for the reduction of the acceptor (A + e" A"). Thus the general reaction scheme for an irreversible process is represented by (20) ... 

In addition to simple electron transfers in which no chemical bond is either broken or formed, numerous organic reactions, previously formulated by movements of electron pairs, are now understood as processes in which an initial electron transfer from a nucleophile (reductant) to an electrophile (oxidant) produces a radical ion pair, which leads to the final products via the follow-up steps involving cleavage and formation of chemical bonds [11-23], The follow-up steps are usually sufficiendy rapid to render the initial electron transfer the rate-determining step in an overall irreversible transformation [24], In such a case, the overall reactivity is determined by the initial electron-transfer step, which can also be well designed based on the redox potentials and the reorganization energies of a nucleophile (reductant) and an electrophile (oxidant). 

As shown in Fig. 4, a FenNO + species has been proposed to be formed following coordination of NO into Fe(III). This reaction appears to be related to the one in Eq. (3), associated with NP formation. However, the electron-transfer step must proceed in an inner-sphere way in the Fe(III) porphyrin system, subsequently to the water-release. As anticipated above, the evidence on the detailed electronic structure in the ferri-hemes is still ambiguous (24,25). Whether the species formed in the reaction of the oxidized form of heme cd1 nitrite reductase (NiR) with NO is best formulated as FenNO + or FeinNO has been qualified as a matter of semantics... 

Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton.

As demonstrated in this review, photoinduced electron transfer reactions are accelerated by appropriate third components acting as catalysts when the products of electron transfer form complexes with the catalysts. Such catalysis on electron transfer processes is particularly important to control the redox reactions in which the photoinduced electron transfer processes are involved as the rate-determining steps followed by facile follow-up steps involving cleavage and formation of chemical bonds. Once the thermodynamic properties of the complexation of adds and metal ions are obtained, we can predict the kinetic formulation on the catalytic activity. We have recently found that various metal ions, in particular rare-earth metal ions, act as very effident catalysts in electron transfer reactions of carbonyl compounds [216]. When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give 9 spedes at different oxidation and protonation states, as shown in Scheme 29. Each species can... 

An additional consideration in formulating redox reactions is the possibility of catalysis by substances that mediate the transfer of electrons between the bulk reductant (or oxidant) and the substrate being transformed. Such considerations arise frequently in many areas of chemistry, especially electrochemistry and biochemistry (e.g., 97). In environmental applications, the most common model for mediated electron transfer involves a rapid and reversible redox couple that shuttles electrons from a bulk electron donor to a contaminant that is transformed by reduction. 

The Butler-Volmer formulation of electrode kinetics [16,17] is the oldest and least complicated model constructed to describe heterogeneous electron transfer. However, this is a macroscopic model which does not explicitly consider the individual steps described above. Consider the following reaction in which an oxidized species, Ox, e.g. a ferricenium center bound to an alkanethiol tether, [Fe(Cp)2]+, is converted to the reduced form, Red, e.g. [Fe(Cp)2], by adding a single electron ... 

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