Concerted electron transfer

This conception of an 8, 2 reaction as an electron-shift process is obviously equivalent to its conception as an inner sphere electron transfer, i.e. a single electron transfer concerted with the breaking of the R—X bond and the formation of the R—Nu bond. Faced with an experimental system, however, the first question—ET or 8 2 —still remains, whatever intimate description of the 8, 2 reaction one may consider most appropriate. If this is thought of in terms of inner sphere electron transfer, the question thus raised is part of the more general problem of distinguishing outer sphere from inner sphere electron-transfer processes (Lexa et ai, 1981), an actively investigated question in other areas of chemistry, particularly that of coordination complex chemistry (Taube, 1970 Espenson, 1986). 

A first sign of the formation of an organic layer on metallic surfaces by electrochemical reduction of aryl diazonium salts is the disappearance of the expected characteristic broad and irreversible cathodic wave observed by cyclic voltammetry upon an initial scan towards negative potentials. Such blocking effect of the electrochemical response was firstly pointed out by Parker and coworkers.The different steps that take place along the electrografting process were established later on 17,371 been demonstrated that the electron transfer concerted with the loss of... 

Ceric ions react rapidly with 1,2-diols. There is evidence for chelation of cerium and these complexes are likely intermediates in radical generation10 106 The overall chemistry may be understood in terms of an intermediate alkoxy radical which undergoes p-scission to give a carbonyl compound and a hydroxyalkyl radical (Scheme 3.59). However, it is also possible that there is concerted electron transfer and bond-cleavage. There is little direct data on the chemical nature of the radical in termediates. 

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

The authors formulate the mechanism in two steps, first an electron transfer from phenoxide ion to diazonium ion forming a radical pair, followed by attack of the diazenyl radical at the 4-position of the phenoxy radical and a concerted proton release, i. e., without involving the o-complex. Admittedly, there is no experimental evidence against such a concerted process, but also none for it It seems that those authors wanted only to demonstrate the occurrence of radical intermediates, but did not consider the question of the mechanism of the proton release. 

Concanavalin, 2, 773 Concanavalin A, 6,572 manganese, 6,587 Concentration mineral processing flotation, 6,780 Concerted electron transfer oxidases, 6,683... 

Laccase, 6,699 copper, 6,654 cytochrome oxidases concerted electron transfer, 6,683 fungal... 

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... 

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

Leconte and Basset [161-166] proposed two other possible mechanisms (Scheme 39) the first one implies a 1,2 carbon-carbon activation which invokes the de-insertion of a methylidene fragment from a surface metal-alkyl species, and the second implies a 1,3 carbon-carbon bond activation in which the key steps are the formation of a dimetallacyle by y-H activation from a metal-alkyl followed by carbon-carbon bond cleavage via a concerted electron transfer. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Anti stereospecificity is associated with a concerted reductive elimination, whereas single-electron transfer fragmentation leads to loss of stereospecificity and formation of the more stable A-stereoisomer. 

A-Methylphthalimide (288) undergoes photoaddition in acetonitrile to ds-but-2-ene (289) to give d.s-l,6,7-trimethyl-3,4-benzo-6,7-dihydroazepine-2,5-dione (290).238 Evidence supports a concerted [ 2 + J2] pathway to the intermediate 291. Similar additions to other alkenes have been reported.239 Electron transfer quenching has been shown to compete with cycloaddition... 

As depicted in Scheme 1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. How the dynamics of concerted electron transfer/bond breaking reactions (heretofore called dissociative electron transfers) may be modeled, and particularly what the contribution is of bond breaking to the activation barrier, is the first question we will discuss (Section 2). In this area, the most numerous studies have concerned thermal heterogeneous (electrochemical) and homogeneous reactions. 

Although the most numerous investigations of dissociative electron transfer have concerned thermal reactions, photoinduced dissociative electron transfer has also attracted a great deal of recent theoretical and experimental attention. As discussed in Section 6, one of the key questions in the field is whether photoinduced dissociative electron transfers are necessarily endowed with a unity quantum yield as one would predict on purely intuitive grounds. Quantum yield expressions for the concerted and stepwise cases are established and experimental examples are discussed. 

Both thermodynamic and kinetic factors are involved in the competition between concerted and stepwise mechanisms. The passage from the stepwise to the concerted situation is expected to arise when the ion radical cleavage becomes faster and faster. Under these conditions, the rate-determining step of the stepwise process tends to become the initial electron transfer. Then thermodynamics will favor one or the other mechanism according to equation (18). AG )eav is also the standard free energy of cleavage of the ion radical. 

Note, however, that the concerted/stepwise dichotomy discussed here concerns cases in which the intermediate is unstable toward cleavage that occurs by means of intramolecular dissociative electron transfer. As discussed in the foregoing sections, for primary radicals that cleave homolytically, criteria based on the life-time of the intermediate may be pertinent. 

Electronic factors related to orbital overlap also appear to interfere significantly in the dynamics of concerted electron transfer/bond breaking reactions in donor-spacer-cleaving acceptor systems.94... 

There is thus an apparent continuity between the kinetics of an electron transfer leading to a stable product and a dissociative electron transfer. The reason for this continuity is the use of a Morse curve to model the stretching of a bond in a stable product in the first case and the use of a Morse curve also to model a weak charge-dipole interaction in the second case. We will come back later to the distinction between stepwise and concerted mechanisms in the framework of this continuity of kinetic behavior. 

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