Redox steps

Azirines (three-membered cyclic imines) are related to aziridines by a single redox step, and these reagents can therefore function as precursors to aziridines by way of addition reactions. The addition of carbon nucleophiles has been known for some time [52], but has recently undergone a renaissance, attracting the interest of several research groups. The cyclization of 2-(0-tosyl)oximino carbonyl compounds - the Neber reaction [53] - is the oldest known azirine synthesis, and asymmetric variants have been reported. Zwanenburg et ah, for example, prepared nonracemic chiral azirines from oximes of 3-ketoesters, using cinchona alkaloids as catalysts (Scheme 4.37) [54]. 

An example of such a catalytic EC process is the oxidation of dopamine in the presence of ascorbic acid (4). The dopamine quinone formed in the redox step is reduced back to dopamine by the ascorbate ion. The peak ratio for such a catalytic reaction is always unity. 

This corresponds with MacDiarmid s observations which show that the second redox step is strongly pH-dependent. MacDiarmid further differentiated his redox model to take account of the fact that pure leucoemaraldine with its amine-N is already protonated at pH values 2, and that the totally oxidized pemigraniline with its less basic imine-N can also be protonated. This gives the following (simplified) reaction scheme ... 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

In this case, the actual redox step is preceded by the formation of an adduct or a complex between the catalyst, the substrate and dioxygen. The order of these reaction steps is irrelevant as long as the rate determining step is Eq. (8). If Eqs. (6) and (7) are rapidly established pre-equilibria the reaction rate depends on the concentrations of all reactants. In some instances, the rate determining step is the formation of the MS complex and the reaction rate is independent of the concentration of dioxygen. 

The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

In the case of the Ru(III)-chelates, the crowded coordination sphere around the metal center prevents the coordination of 02. Thus, the corresponding kinetic model postulates that ascorbic acid is oxidized by Ru(III) in two subsequent redox steps and Ru(II) is reoxidized by 02 ... 

The intense blue color of the reaction mixture was assigned to the paramagnetic [Fe(HDMG)2(MeIM)(DTBSQ )]+ complex which is characterized by a broad spectral band at imax 680 nm and a distinct doublet with g = 2.00425 and cqn = 3.135 G in the visible and ESR spectra, respectively. This iron(II) species is not involved in a direct redox step and acts only as a reservoir for the semi-quinone radical. 

The experimental observations were interpreted by assuming that the redox cycle starts with the formation of a complex between the catalyst and the substrate. This species undergoes intramolecular two-electron transfer and produces vanadium(II) and the quinone form of adrenaline. The organic intermediate rearranges into leucoadrenochrome which is oxidized to the final product also in a two-electron redox step. The +2 oxidation state of vanadium is stabilized by complex formation with the substrate. Subsequent reactions include the autoxidation of the V(II) complex to the product as well as the formation of aVOV4+ intermediate which is reoxidized to V02+ by dioxygen. These reactions also produce H2O2. The model also takes into account the rapidly established equilibria between different vanadium-substrate complexes which react with 02 at different rates. The concentration and pH dependencies of the reaction rate provided evidence for the formation of a V(C-RH)3 complex in which the formal oxidation state of vanadium is +4. 

The three rate constants for Eq. (98) correspond to the acid-catalyzed, the acid-independent and the hydrolytic paths of the dimer-monomer equilibrium, respectively, and were evaluated independently (107). The results clearly demonstrate that the complexity of the kinetic processes is due to the interplay of the hydrolytic and the complex-formation steps and is not a consequence of electron transfer reactions. In fact, the first-order decomposition of the FeS03 complex is the only redox step which contributes to the overall kinetic profiles, because subsequent reactions with the sulfite ion radical and other intermediates are considerably faster. The presence of dioxygen did not affect the kinetic traces when a large excess of the metal ion is present, confirming that either the formation of the SO5 radical (Eq. (91)) is suppressed by reaction (101), or the reactions of Fe(II) with SO and HSO5 are preferred over those of HSO3 as was predicted by Warneck and Ziajka (86). Recently, first-order formation of iron(II) was confirmed in this system (108), which supports the first possibility cited, though the other alternative can also be feasible under certain circumstances. 

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

The first reduction, which is likely centred on the Cr(III)— Cr(II) process, has substantial electrochemical reversibility (A2sp = 70 mV, at 0.2 V s-1), thus suggesting that no significant structural rearrangements accompany such redox step. Unfortunately, no further investigations have been carried out to clarify whether the successive reductions are centred on the metal or on the ligand. 

Another concept has been developed on a refined model based on two-step redox systems typical for organic compounds [94]. This concept treats a polymer chain with the degree of polymerization n as X weakly interacting segments containing k monomeric units, each of which can be charged up to a diionic state in two redox steps with different potentials (1 < k n, X k = n). 

Scheme4 Redox steps during charging/discharging of PANI.

Obviously, for success in this approach, the rate of the redox step producing the intermediate must be at least as fast as the decomposition of the intermediate. This can be sometimes accomplished by increasing the reactant concentrations, since the first step is second order and the second step is first order. 

In the theory of SWV, two different types of surface EE mechanisms have been treated [91,92], O Dea et al. [91] considered a mechanism in which the first redox step was chemically reversible, whereas the second one was a totally irreversible process. In the succeeding study [91], a more general case has been treated consisting of two quasireversible redox transformations, as indicated by (2.129) ... 

Combining (2.137) and (2.138) with kinetic equation (2.135), and (2.138) and (2.139) with (2.136), integral equations are readily obtained as general solutions for each redox step. The numerical solution is represented by the following set of recursive formulas ... 

This analysis corresponds to a comparison of different two-step processes characterized by identical kinetics of the first redox step and different kinetics of the second one. For log(o)i) = -3.5 (curve 1 in Fig. 2.66), 0)2 exhibits no influence on the net peak current, whereas for log(o)i) = -1 (curve 2 in Fig. 2.66) the effect is very week. Over the interval log(o)i) < -3.5 limiting conditions are reached and kinetics of the overall reaction is solely controlled by the first redox step, which is slow and electrochemically irreversible. 

Two experimental systems have been used to illustrate the theory for two-step surface electrode mechanism. O Dea et al. [90] studied the reduction of Dimethyl Yellow (4-(dimethylamino)azobenzene) adsorbed on a mercury electrode using the theory for two-step surface process in which the second redox step is totally irreversible. The thermodynamic and kinetic parameters have been derived from a pool of 11 experimental voltammograms with the aid of COOL algorithm for nonlinear least-squares analysis. In Britton-Robinson buffer at pH 6.0 and for a surface concentration of 1.73 X 10 molcm, the parameters of the two-step reduction of Dimethyl Yellow are iff = —0.397 0.001 V vs. SCE, Oc,i = 0.43 0.02, A sur,i =... 

The reduction potential of the second redox step overlaps with the potential of the first one, resulting in an overall four-electron four-proton irreversible reduction. The features of the voltammetric response are controlled by the competitionbetween reaction pathways of the hydrazo-form, which can be either reoxidized back to the azo-form or irreversibly reduced to the electroinactive amines. 

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