Redox catalysis

Many redox reactions require the hypothesis that intermediate oxidation states are needed to meet kinetic laws, which is made possible by the presence of the catalyst. 

Hence the reduction reaction [13.R6] of ferric ion by tin (II), which is extremely slow without a catalyst, is strongly catalyzed by chloride ions that are assumed to permit the presence of oxidation state III for tin  

With a high iron (III) over tin (II) concentration ratio, the rate law is  

In this expression, the constant k depends on the chloride ion concentration. 

The following mechanism was first proposed by Weiss. It occurs according to the steps [13.R6a] and [13.R6b]  

Redox reactions can be catalyzed by reducible cations substituted into the framework of zeolitic systems as well as polymorphic AIPO4 systems or by cations not located in the framework but in the micropores. In Chapter 8 we will discuss more extensively catalysis by Tia,Si(i 2,)02 systems using peroxides. Here we will initiate the discussion on redox catalysis with Coa Al(i 2.)P04 oxidation catalysts where reducible ions such as Co + substitute for AP+. Catalytic oxidation carried out with oxygen provides an opportunity to discuss radical-type chemistry. A second system that we will discuss is photochemical oxidation induced by the strong electrostatic field of ion-exchanged cations. We will subsequently discuss catalysis by Fe and Fe ion exchanged zeolites with comparisons to Zn + systems and the important role of the corresponding oxycation. 

For the iron system we first discuss N2O decomposition and then describe the selective oxidation with N2O to produce benzene from phenol. The N2O decomposition reaction will be an example that illustrates additional complexity of catalytic systems, with selforganizing features. 

Although in catalytic reactions, in particular on the surface of solid catalysts, it is not formally correct to distinguish between acid-base and redox catalysis, because usually they are both involved, this distinction is often common. The two main classes of reactions are selective hydrogenation and selective oxidation. 

Catalytic hydrogenation is a common operation in both industrial and laboratory catalytic syntheses. As an example, in the synthesis of vitamins 10-20% of all reaction steps are catalytic hydrogenations. 

Many different functions can be hydrogenated with a few catalytically active metals. However, when dealing with multifunctional molecules, catalytic activity for the desired transformation is not enough, because the catalyst should also be chemo-selective. In other words, it should not affect other reducible functional groups. 

Knowledge on selective reduction is growing and today several commercial catalysts can be selected from a catalogue. In pharmaceutical syntheses the time to develop the industrial production of a new product or to scale-up laboratory preparations are the critical factors. Therefore, the availability of various catalysts and knowledge would reduce considerably the time, even if the synthesis is not optimized (which could be a less serious problem in the production of high value products). This 

Aromatic nitro groups ArNOj ArNHj Ni, Pd, Pt Various 

Metals can accelerate the decomposition of hydroperoxides that are formed e.g., by Eq. 19.2. 

The second reaction in Eq. 20.7 regenerates the original metal ion M . In the combined reaction, the hydroxyl anion and the hydrogen cation recombine to water. This reaction is used purposely in hydroperoxide curing of unsaturated poly(ester) resins. 

One important mechanism in order to delay degradation reactions is the addition of scavengers bearing S-H moieties. These groups react with the highly reactive radicals as they convert them into quasi inert radicals S.  

Cyclic mechanisms have been described with stable radicals in hindered amine stabilizers. 

For example, in Eq. 20.9, the N-0- unit is regenerated, as simultaneously peroxides are formed. 

---

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

Indirect cathodic reduction of sulphones. Redox catalysis.1014... 

In this example24 redox catalysis kinetics is governed partly by chemical reaction, i.e., the scission of C6H5S02CH3. For given concentrations of pyrene and sulphone at sweep rate v one can find values of klk/k2 from published graphs23 in the case of EC processes. 

Redox catalysis appears also to be an elegant way to conduct alkylations. In this case the RX compound has to produce a free radical R which is sufficiently reactive toward the electron carrier A or its reduced form. Generally the mechanism, developed mainly for the case of alkyl halides, may be summarized as in reactions 25, 29-36. 

A general theory based on the quantitative treatment of the reaction layer profile exists for pure redox catalysis where the crucial function of the redox mediator is solely electron transfer and where the catalytic activity largely depends only on the redox potential and not on the structure of the catalyst This theory is consistent... 

Fig. 3. Steady state concentration profiles of catalyst and substrate species in the film and diffusion layer for for various cases of redox catalysis at polymer-modified electrodes. Explanation of layers see bottom case (S + E) f film d diffusion layer b bulk solution i, limiting current at the rotating disk electrode other symbols have the same meaning as in Fig. 2 (from ref.

Certainly, the same arguments apply for chemical redox catalysis , but as discussed above, thinner films may be effective in this case. Hence, it will be reasonable to work with modified electrodes having a large effective area instead of thick films, i.e. three-dimensional, porous or fibrous electrodes. The notorious problem with current/potential distribution in such electrodes may be overcome by the potential bias given by selective redox catalysts. Some approaches in this direction are described in the next section. 

An irreversible reaction of the intermediate of a redox reaction will greatly facilitate redox catalysis by thermodynamic control. A good example is the reduction of the carbon halogen bond where the irreversible reaction is the cleavage of the carbon halogen bond associated, or concerted, with the first electron transfer -pEe... 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

In the theoretical treatment of ion exchange polymers the roles of charge propagation and of migration of ions were further studied by digital simulation. Another example of proven 3-dimensional redox catalysis of the oxidation of Ks[Fe(CN)5] at a ruthenium modified polyvinylpyridine coated electrode was reported... 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

Moreover, redox catalysis may provide valuable information concerning thermodynamics and kinetics. Thus, let us recall that the value of °Arso r/AfSO k the kinetic constant... 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

The catalysis of hydrogen peroxide decomposition by iron ions occupies a special place in redox catalysis. This was precisely the reaction for which the concept of redox cyclic reactions as the basis for this type of catalysis was formulated [10-13]. The detailed study of the steps of this process provided a series of valuable data on the mechanism of redox catalysis [14-17]. The catalytic decomposition of H202 is an important reaction in the system of processes that occur in the organism [18-22]. 

In the case of cobalt ions, the inverse reaction of Co111 reduction with hydroperoxide occurs also rather rapidly (see Table 10.3). The efficiency of redox catalysis is especially pronounced if we compare the rates of thermal homolysis of hydroperoxide with the rates of its decomposition in the presence of ions, for example, cobalt decomposes 1,1-dimethylethyl hydroperoxide in a chlorobenzene solution with the rate constant kd = 3.6 x 1012exp(—138.0/ RT) = 9.0 x 10—13 s—1 (293 K). The catalytic decay of hydroperoxide with the concentration [Co2+] = 10 4M occurs with the effective rate constant Vff=VA[Co2+] = 2.2 x 10 6 s— thus, the specific decomposition rates differ by six orders of magnitude, and this difference can be increased by increasing the catalyst concentration. The kinetic difference between the homolysis of the O—O bond and redox decomposition of ROOH is reasoned by the... 

Two-Step (Push-Pull, Ping-Pong) Mechanisms Two-step mechanisms are typical of chemical catalytic processes, as opposed to redox catalysis processes, that are discussed and exemplified in Section 6.2. The first step following the generation at the electrode of the active form of the catalyst, Q, is the formation of an adduct, C, with the substrate A (Scheme 2.11). C requires an additional electron transfer to regenerate the initial catalyst, P. There are then two main possibilities. One is when C is easier to reduce (or oxidize in oxidative processes) than P. The main route is then a homogeneous electron... 

APPLICATION OF REDOX CATALYSIS TO FAST FOLLOW-UP REACTIONS... 

---