Negative redox catalysis

6 NEGATIVE CATALYSIS IN ALCOHOL OXIDATION 2.6.1 Negative redox catalysis 

Hydroxyperoxy radicals can induce both oxidation and reduction. If the inhibitor is present in two states, oxidized and reduced, and each state reacts with hydroxyperoxy radicals only, terminating the chains, then negative catalysis will take place, each inhibitor molecule terminating chains an infinite number of times. This is the case on addition of CuS04 to cyclohexanol [83]. Cupric ions in a concentration of 10-smolel I virtually stop the initiated oxidation of cyclohexanol. The mechanism of the retarding action of cupric ions is 

The first stage is suggested to be the rate-limiting one. Dependence of the oxidation rate on [Cu2+] is expressed as 

Rate coefficients for the reactions of hydroxyperoxy radicals with compounds of transition metals in alcohol 

Similar results were obtained when transition metal stearates were added to cyclohexanol (Table 5). The dioxymine complexes of Co, Cu and Fe retard oxidation of 2-propanol [274] by termination of chains. The rate of termination obeys the equation 

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As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

The second alternative to bypass a difficult RX reduction consists of using redox catalysis [29], Thus, the reduction of RX can be performed at the much less negative one-electron reversible reduction potential (Equation 12.18) of an adequate redox mediator M, which delivers the electron to RX through an homogeneous electron transfer when RX does exist (Equation 12.19), or for a concerted bondbreaking RX reduction (Equation 12.20) ... 

In all of the examples considered, Ei/2 of the acceptor was much more negative than that of the donor. However, in liquid phase one-electron transfer from a donor to an acceptor can proceed even with an unfavorable difference in the potentials if the system contains a third component, the so-called mediator. The mediator is a substance capable of accepting an electron from a donor and sending it instantly to an acceptor. Julliard and Chanon (1983), Chanon, Rajzmann, and Chanon (1990), and Saveant (1980, 1993) developed redox catalysis largely for use in electrochemistry. As an example, the reaction of ter-achloromethane with /V,/V,/V ,Af-tetramethyl-p-phenylenediamine (TMPDA) can be discussed. The presence of p-benzoquinone (Q) in the system provokes electron transfer (Sosonkin et al. 1983). Because benzoquinone itself and tetrametyl-p-phenylenediamine interact faintly, the effect is evidently a result of redox catalysis. The following schemes reflect this kind of catalysis ... 

Using spin markers it could be shown that redox catalysis occurs in which the solvent itself plays the role of an electron carrier. Thus indirect reduction of aromatic halides having more negative potentials than benzonitrile has been achieved at the reduction potential of benzonitrile when it was used as a solvent211. 

Benzoyl-CoA reductase has three cysteine-ligated [Fe4S4] clusters with very negative redox potential, two of which are involved in the transfer of electrons to the aromatic ring of benzoyl-CoA. This reaction is coupled to ATP hydrolysis. The redox centres were characterized by EPR and Mossbauer spectroscopies. A single-turnover, rapid-freeze EPR study was presented for this enzyme. From the analysis of the different EPR-active species during catalysis it was concluded that a free-radical species is involved in catalysis. ... 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

As seen in Figure 2.1, the substitution of the aluminum ions of the octahedral layer with iron(II) ions leads to the formation of the negative layer charge of montmorillonite. In addition, as a cation, iron ions can be present in the interlayer space also. In both places, the oxidation state of iron can be Fe(II) and Fe(III). Reactions involving the back-and-forth transition between the two oxidation states play a significant role in the redox processes of rocks and soils as well as in the catalysis of redox reactions (Stucki et al. 2002 Stucki 2008). 

Important consequences result from the increase of the redox potential of metal clusters with their nuclearity. Indeed, independently of the metal, the smaller clusters are more sensitive to oxidation and can undergo corrosion even by mild oxidizing agents. Moreover, size-dependent redox properties explain the catalytic efficiency of colloidal particles during electron transfer processes. Their redox potentials control their role as electron relays the required potential being intermediate between the thresholds of the potentials of the donor (more negative) and of the acceptor (more positive). Catalytic properties of the nanoparticles are thus size-dependent. Haruta and co-workers reported that gold nanoparticles smaller than 5 nm have potential applications in catalysis as they are very active in... 

The only reversible redox process observed under rapid scanning of potential in the entire scheme occurs between tetrahydropterin and the quinonoid tautomer of dihydropterin in step (a) [Scheme 2.3). All other redox processes in the scheme lead to unstable dihydropterin forms that rapidly rearrange to the most stable tautomer, 7,8-dihydro tautomer in step (b). While 7,8-dihy-dropterin can be oxidized to pterin in step (c), it is a much less favorable oxidation process requiring potentials -500 mV more positive than that for the reversible tetrahydro/quinonoid oxidation. Likewise 7,8-dihydropterin is reducible to tetrahydropterin but at potentials over 1 V more negative than reversible quinonoid/tetrahydro reduction. Such a low reduction potential accounts for unlikely participation of simple 7,8-H2pterins in any redox step of Moco catalysis. Reduction of fully oxidized pterin also generates an unstable 5,8-dihydropterin tautomer, which rearranges to the 7,8-dihydro tautomer step (d) before further reduction to tetrahydropterin can occur. 

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