Electron-transfer-chain mechanism

Kim and Bunnett in 1970 made unexpected observations of reactions occurring by an electron transfer chain mechanism in aromatic systems. " The selective formation of 60 showed that benzyne intermediates were not formed, and the mechanism of equations (90) and (91) analogous to that found in aliphatic systems (equations 88, 89) was proposed. This process differs in being a chain reaction and not a process in which the product forming step is not a radical-radical combination. 

In connection with the oxirane ring opening, the isomerization of oxirane 6 to ketone 9 in an MeCN—LiClO —(Pt) and a CH Clj—Et NClO —(Pt) system has been reported and was explained on the basis of an electron-transfer chain mechanism via 7 and 8 However, 6 is actually converted to 9 under the preelectrolysis conditions so that the transformation may be explained in terms of an EGA catalyzed reaction... 

Irradiation of NADH model compounds in the presence of benzyl bromide or p-cyanobenzyl bromide in acetonitrile brings about reduction of the benzyl halides to the corresponding toluene compounds114. Like the S l substitution reaction, this photoreduction also occurs via an electron-transfer chain mechanism. Unlike in that case, though, here an electron transfer from the excited state of the NADH compound is solely responsible for the initiation step. In the propagation, the benzyl radical produced by C—Br bond cleavage in the radical anion abstracts hydrogen from the NADH compound. This yields a radical intermediate, from which electron transfer to benzyl bromide occurs readily (equations 39-42). 

Protiodenitration. Simple nitroalkanes are not reduced by 1, but a-nitro nitriles, esters, and ketones are selectively reduced by 1 under irradiation, with selective replacement of NO by H. The reduction occurs via an electron-transfer chain mechanism. Example ... 

A photoinduced oxidative addition of Bu"I and Pr I towards [Rh2(dicp)4] (dicp = 1,3-diisocyanopropane) using low-energy irradiation (553 nm) has been described, and proceeds under a nitrogen atmosphere with quantum yields of 25.2 and 22.6, respectively. Based on the kinetics measured in the presence and absence of quenchers, an electron-transfer chain mechanism has been proposed in which alkyl radicals are involved. " The same authors have also described a new methodology for the experimental determination of redox potentials and have applied this to measure the one-electron oxidation potentials of l-benzyl-l,4-dihydronicotinamide and of [Rh2(dicp)4p. ... 

At a potential of 1.1 to 1.3 V (vs. Ag/Ag+ 0.1 M), (A-benzylaziridine is opened to form a cyclic tetramer, which may be explained by an electron transfer chain mechanism [31]. The same reaction may also be performed by redox catalysis using tris(4-bromophe-nyOamine as mediator at a potential of about 0.75 V (vs. Ag/Ag+). The possibility of catalysis by electrogenerated acid was excluded. 

Scheme 11.18 Electron-transfer-chain mechanism for the synthesis of the 96-Ru dendrimer-cluster complex.

In the initial work, it was found that a catalytic amount of cathodic current at - 1.7 V vs SCE decomposes [FeCp(C6H6)] PF6 in 95 % ethanol on Hg cathode in basic media. Cyclopentadiene, benzene and Fe(OH)2 were characterized and the electron-transfer chain mechanism was proposed to account for the benzene exchange for solvent molecules. The driving force of the propagation cycle is the exergonic cross ET step since the three EtOH ligands do not allow back bonding whereas benzene does. ... 

Once an enzyme-catalysed reaction has occurred the product is released and its engagement with the next enzyme in the sequence is a somewhat random event. Only rarely is the product from one reaction passed directly onto the next enzyme in the sequence. In such cases, enzymes which catalyse consecutive reactions, are physically associated or aggregated with each other to form what is called a multi enzyme complex (MEC). An example of this arrangement is evident in the biosynthesis of saturated fatty acids (described in Section 6.30). Another example of an organized arrangement is one in which the individual enzyme proteins are bound to membrane, as for example with the ATP-generating mitochondrial electron transfer chain (ETC) mechanism. Intermediate substrates (or electrons in the case of the ETC) are passed directly from one immobilized protein to the next in sequence. 

The outer-sphere electron-transfer initiation mechanism cannot account for the observed kinetics, the half-reaction time being more than 100 times greater than that observed. The chain process considerably enhances the global rate of the reaction (without a chain process, the half-reaction time would be three centuries). 

All these mechanisms are non-redox in nature, as is easily found by consideration of the oxidation states of the carbon centers involved. They are furthermore chain reactions, initiated by electron transfer, and the term electron-transfer chain catalysis (ETC catalysis) is therefore appropriate (Alder, 1980). Another term is the DAISET (Double Activation Induced by Single Electron Transfer) concept, recently introduced by Chanon and Tobe (1981). The DAISET (ETC) concept is applicable to inorganic mechanisms too. 

Crofts, A. R., Meinhardt, S. W., Jones, K. R., and Snozzi, M., 1983, The role of the quinone pool in the cyclic electron transfer chain of Rhodopseudomonas sphaeroides. A modified Q-cycle mechanism. Biochim. Biophys. Acta, 723 2029218. 

A novel electron transfer free radical mechanism has been elucidated for sodium naphthalenide induced demercuration. A new reductant for the cleavage of the C—Hg bond, A -benzyl-1,4-dihydro-nicotinamide (BNAH), has also been proposed it reduces alkylmercury(II) acetates via an electron transfer chain substitution mechanism. ... 

Several observations made in whole membranes or in the isolated complexes are in line with these concepts the shifts induced by antimycin A [110,137] and myxothiazol on the absorption spectra of cytochromes b and the alterations of the ESR spectrum of the FeS protein by UHDBT or DBMIB [131]. Moreover, the oxidant-induced reduction of cytochromes b, the key observation for accepting these electron transfer schemes, has been demonstrated in all h/c, complexes isolated so far from mitochondria [134], chloroplasts [111], cyanobacteria [112] and photosynthetic bacteria [110]. In the chloroplast b /f complex this reaction has been demonstrated also in the absence of any exogenously added quinol, indicating that possibly a structurally bound quinone (quinone is always present in the isolated complexes with a stoicheiometry of about 0.5-0.7 mol/mol of cyt. c, [110,111]) is sufficient to drive the reduction of cytochromes [138]. Since a detailed treatment of the genera] mechanism, as well as of the more specific problems of the mitochondrial respiratory chain, are reported in Chapter 3 of this volume, the following discussion will deal only with the specific features of the electron transfer chains in photosynthetic membranes. 

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