Mechanism comproportionation

Israeli, A., Patt, M., Oron, M., Samuni, A., Kohen, R., and Goldstein, S. 2005. Kinetics and mechanism of the comproportionation reaction between oxoammonium cation and hydroxy-lamine derived from cyclic nitrones. Free Radical Biology and Medicine 38 317-324. 

It should be recalled that in the case of the EE mechanism, in solution one can have the comproportionation reaction ... 

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

Inasmuch as flavins can accommodate two electrons but possess a relatively stable one-electron intermediate, an obvious question which can be asked of any flavin-mediated two electron redox reaction is whether or not the mechanism includes the radical species on a direct line between reactants and products. The mere observation of semiquinones in a reaction mixture is not sufficient evidence for their intermediacy, due to the existence of side reactions such as comproportionation (F -I- FH2 2 FH-) which can generate radicals rapidly. Bruice has discussed this question from a physical-organic point of view and concluded that there must exist a competition between one-electron and two-electron processes and that the actual mechanism should be determined mainly by the free energy of formation of substrate radical and the nucleophilicity of the substrate. Bruice has analyzed a variety of systems which he feels should proceed via one-electron mechanisms among these are quinone and carbonyl group reduction by FH2... 

In this section, the electrochemical behavior of an EE mechanism with two reversible electron transfer reactions will be studied. It will also be shown that for this electrode process (given in reaction scheme (3.II)) in both cases, i.e., normal ordering and potential inversion, the disproportionation/comproportionation reaction (3) can take place in the diffusion layer. 

Moreover, the current-potential curves are affected by the disproportionation reaction therefore, other variables (the rate constant for the disproportionation reaction) must be taken into account. Since experimental results for many interesting systems show clear evidence of slow kinetics, ad hoc simulation procedures have typically been used for the analysis of the resulting current-potential curves [31, 38, 41, 48]. As an example, in reference [38], it is reported that a clear compropor-tionation influence is observed for an EE mechanism with normal ordering of potentials and an irreversible second charge transfer step. In this case, the second wave is clearly asymmetric, showing a sharp rise near its base. This result was observed experimentally for the reduction of 7,7,8,8-tetracyanoquinodimethane in acetonitrile at platinum electrodes (see Fig. 3.20). In order to fit the experimental results, a comproportionation rate constant comp = 108 M-1 s-1 should be introduced. 

References [40,41] report the chronoamperometric analysis of the response of an EE mechanism with non-reversible charge transfer processes including the consideration of a fast comproportionation step [40], indicating that strong differences in the diffusion coefficients of the different species are needed to cause a clear influence of the comproportionation process in the electrochemical response. 

In this section, the current-potential curves of multi-electron transfer electrode reactions (with special emphasis on the case of a two-electron transfer process or EE mechanism) are analyzed for CSCV and CV. As in the case of single and double pulse potential techniques (discussed in Sects. 3.3 and 4.4, respectively), the equidiffusivity of all electro-active species is assumed, which avoids the consideration of the influence of comproportionation/disproportionation kinetics on the current corresponding to reversible electron transfers. A general treatment is presented and particular situations corresponding to planar and nonplanar diffusion and microelectrodes are discussed later. 

A frequently used indirect method involves cyclizable (cf. (7)) or other mechanistic probes which should provide evidence for free radical intermediates and thus for SET [19,37,59]. However, Newcomb and Curran have pointed out the pitfalls of such an approach especially if iodide precursors are used [17]. The supposedly radical-indicative reaction may come about albeit slower by a different, nonradical mechanism or the radical formation may occur via a secondary process which is not directly related to the first reaction step. A similar side-route can be made responsible for the appearance of stable radical compounds which may arise via a comproportionation reaction between non-reduced starting material and the doubly reduced species which can be formed from a hydro form (the normal product, Eq. (5)) and the usually strongly basic organometallic or hydridic reagents (Eq. (9)) [58]. The ability of strong bases to produce reduced radical species via complicated electron/proton transfer processes has been known for some time in the chemistry of quinones and quaternary salts [60,61]. 

Recently Holm and co-workers (131) have described a new OAT reaction system that cycles between (L-NS)2Mo 02 (Fig. 10b) and (L-NS)2Mo 0 (Fig. 12). The steric hindrance of the two bulky p-tert-butylphenyl groups prevents comproportionation [Eq. (15)] to form di-nuclear [Mog Os] centers (37). This reaction system is thermodynamically competent to oxidize or reduce all enzymatic substrates except those requiring the [Mo OS] center as oxidant. The system is stable in the presence of strong oxo donors, such as MesNO and 104. The kinetics of substrate oxidation are second order and sensitive to substrate, indicating a different mechanism than the previously studied system based upon (L-NS2)Mo02 (130). 

In the case of multi-E mechanisms, kinetic fronts associated with comproportionation reactions can take place in solution instead of next to the electrode surface. In this particular case, very dense grids or automatically adaptive grids are employed for accurate description of the concentration profiles (see Chapter 6) [1, 2]. 

As an example, Figure 6.2 shows the voltammograms of the EE mechanism where both electron transfers are reversible, diffusion is the only active mechanism and no side reactions take place but where the diffusion coefficient of species B is significantly larger than that of species A and C Db = 4 Da/c- Under such conditions we can observe that the voltammetry is significantly affected by the kinetics of the comproportionation reaction. 

For the sake of simplicity we will tackle the case of a one-electron transfer (7.1), although the same methodology here described has been successfully applied to the study of a munber of systems with different mechanisms, including comproportionation reactions [3] and amalgamation processes [5], and using different electrochemical techniques chronoamperometry [4], cyclic voltammetry [6] and multipulse voltammetries [7]. 

This cell has been applied successfully for in situ electrochemical EPR, deducing both kinetics and reaction mechanisms, for radicals with lifetimes typically in the range 10 to 100 ms Sect. 3.2.4 will examine its application to ECE [58], DISPl [74], FC [68], and comproportionation reaction mechanisms [75-77]. Other applications include the study of an ECEEE mechanism... 

Fig. 4.9 Comparison of concentration profile schematics for an EE mechanism where the comproportionation reaction is fast (top) or inactive (bottom).

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