Electron transfer catalytic chain

Fig. 17. Electron-transfer catalytic cycle, where the chain-carrying species is an alkyl radical. 

Fig. 19. Electron-transfer catalytic cycles. The possible chain-carrying species are Ni XL, allylNi XL, or RNi,uX2L.

Several processes that are catalytic (a photon is not a substance) in photons and involve a catalytic quantity of one compound have been reported. Different labels were associated with such an overall situation electron transfer induced chain reactions [7], photoinduced catalytic reactions [8], or photogenerated catalysis [9]. The main experimental observations which characterize such processes are ... 

NDO can be classified as class III dioxygenase the electron transfer chain involves a Rieske-type ferredoxin. Electrons enter NDO through the Rieske-type cluster of the dioxygenase. Kauppi et al. (11) have suggested that the binding site of NDO for the ferredoxin involves the 6 strands 10 and 12 of the Rieske domain as well as residues from the catalytic domain that form a depression in the protein surface close to Cys 101, which is a ligand of the Rieske cluster. In Rieske proteins from be complexes, access to this side of the cluster is blocked by an acidic surface residue (Asp 152 in the ISF, Glu 120 in RFS). 

Moreover, an electron transfer chain could be reconstituted in vitro that is able to oxidize aldehydes to carboxylic acids with concomitant reduction of protons and net production of dihydrogen (213, 243). The first enzyme in this chain is an aldehyde oxidoreductase (AOR), a homodimer (100 kDa) containing one Mo cofactor (MOD) and two [2Fe—2S] centers per subunit (199). The enzyme catalytic cycle can be regenerated by transferring electrons to flavodoxin, an FMN-con-taining protein of 16 kDa (and afterwards to a multiheme cytochrome and then to hydrogenase) ... 

Cu(II) EPR signal in nitriles as solvent as well as by polarographic measurements 144>. Similarly, the EPR signal disappeared when Cu(OTf)2 was used for catalytic cyclo-propanation of olefins with diazoesters 64). In these cases, no evidence for radical-chain reactions has been reported, however. The Cu(acac)2- or Cu(hfacac)2-eatalyzed decomposition of N2CHCOOEt, N2C(COOEt)2, MeCOC(N2)COOEt and N2CHCOCOOEt in the presence of cyclopropyl-substituted ethylenes did not furnish any products derived from a cyclopropylcarbinyl - butenyl rearrangement128. These results rule out the possible participation of electron-transfer processes and radical intermediates which would arise from interaction between the olefin and a radical species derived from the diazocarbonyl compound. 

The formation of the monocationic intermediate (ArH)2Fe+ attendant upon the charge-transfer excitation of either the ferrocene or methylan-thracene EDA complex (7a and 7b) is responsible for the photo-induced de-ligation of bis(arene)iron(II), as described in (6). Thus, transient electrochemical studies (Karpinski and Kochi, 1992a,b) show that the catalytic de-ligation of (ArH)2Fe+ proceeds rapidly via a (two-step) electron-transfer chain or ETC process (8). 

The electron-transfer chain (ETC) catalytic process (or, electrocatalysis) is the catalysis of a reaction triggered by electrons (through a minimal quantity of an oxidizing or reducing agent) without the occurrence of an overall change in the oxidation state of the reagent. 

The rate of an electron transfer from the reduced catalyst to the substrate is also important. If the rate is excessively high, the electron exchange will occur within the preelectrode space and the catalytic effect will not be achieved. If the rate is excessively low, a very high concentration of the catalyst will be needed. However, at high concentration, the anion-radicals of the catalyst will reduce the phenyl radicals. Naturally, this will be unfavorable for the chain process of the substitution. As catalysts, substances that can be reduced at potentials by 50 mV less negative than those of the substrates should be chosen. The optimal concentration of the catalyst must be an order lower than that of a substrate (Swartz and Stenzel 1984). 

Electroreductive one-electron initiation of cyclization was described for the series of E,E-, 1-dibenzoyl-l,6-heptadiene and its derivatives (Roh et al. 2002, Felton and Bauld 2004). In this case, the catalytic effect was also observed (the actual consumption of electricity was substantially less than theoretical). The same bis(enones) can also be cyclized on the action of the sodium salt of chrysene anion-radical in THF, but with no catalytic effect. Optimum yields were obtained only when 70-120 mol% of the initiator was used, relative to a substrate (Yang et al. 2004). The authors suggest that tight ion pairing of the sodium cation with the product anion-radical in THF (which is a somewhat nonpolar solvent) slows down the intermolecular electron transfer to the bis(enone) molecules. Such an electron transfer would be required for chain propagation. 

In the catalyst reoxidation step, contrary to the electron-transfer step, the polymer ligand should shrink because of the formation of the Cu(II) complex. Therefore, the polymer chain may partially repeat are expansion and contraction occurring during the catalytic cycle. When one has a view of the polymer-Cu catalyst as a whole, each part of the polymer catalyst domain, which is drifted in solution, is seen to be fluctuating during the catalytic process [Fig. 32(b)]. The fluctuating shape of biopolymers in enzymic reactions has been pointed out, and the dynamically conformational change of a flexible polymer chain is considered to be one of the effects of the polymer catalyst. 

The process implies a first electrochemical step with a very fast conversion of the ferroceno/ferrocinium couple, due to the short length of the alkyl chain, and a second chemical step with a simple electron transfer between the iron complex in solution and that of the monolayer. Moreover, the thiols block the gold surface in such a way that the Fe(CN)g- oxidation will take place due solely to the ferrocene mediation at the monolayer, and with a very high efficiency (i.e., the catalytic way is observed at potentials 500 mV lower than those corresponding to a gold electrode with a C6SH monolayer). 

---