Concerted electron

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. 

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... 

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. 

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. 

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... 

It is also worth emphasizing that recent theoretical work on photoinduced stepwise and concerted electron transfer/bond-breaking reactions opens the route to a more systematic combination than before of the electrochemical and photochemical approaches to the same problems. 

It follows that the value of the electrochemical transfer coefficient may allow the distinction between stepwise and concerted electron-transfer-bond-breaking reactions when a chemical bond of normal strength is involved (Andrieux and Saveant, 1986b Andrieux et al., 1990b). If the reduction wave possesses the characteristics of a process controlled by slow electron transfer rather than controlled by a follow-up reaction, and if a is significantly larger than 0.5, then one can conclude that the reaction proceeds in a stepwise manner. The same is true when the wave exhibits the characteristics of a process controlled by a follow-up reaction, electron transfer remaining at equilibrium. 

Fig. 6 Stepwise and concerted electron transfer and bond breaking. Schematic representation of the potential energy surface, (a) Stepwise process, a > 0.5. (b) Concerted process, a < 0.5. (Adapted from Andrieux et ai, 1985.)...

In Other classes of organic halides, for example perfluoroalkyl and vinyl halides, the distinction between stepwise and concerted electron-transfer-bond-breaking upon reduction by outer sphere heterogeneous and/or homogeneous electron donors is less unambiguous than in the case of aryl and alkyl halides. As discussed in Section 3, they also present the interest of being active substrates in Sg l reactions. 

COUPLING OF ARYL RADICALS WITH NUCLEOPHILES AS A CONCERTED ELECTRON-TRANSFER-BOND-FORMING REACTION... 

The addition of the nucleophile to the aryl radical is the reverse of the cleavage of substituted aromatic anion radicals that we have discussed in Section 2 in terms of an intramolecular concerted electron-transfer-bondbreaking process and illustrated with the example of aryl halides. The present reaction may thus be viewed conversely as an intramolecular concerted electron-transfer-bond-forming process. The driving force of the reaction can be divided into three terms as in (131). The first of these, the... 

There are no large differences between the reactivities of PhS , (EtO)2PO and CHjCOCHj" with the same aryl radical, but CN appears to be significantly less reactive. It is not easy to evaluate the respective role of the bond dissociation free energy and of the Nu-/Nu" standard potential in equation (13) in this connection because of the paucity of available data concerning these two quantities. An explanation of the low reactivity of CN" should thus await the availability of such data as well as that of a precise expression of the intrinsic barrier in a model of these intramolecular concerted electron-transfer-bond-breaking (or forming) reactions. 

Mechanistically, analogous to what already is suggested for 5-azido- or S-diazomethyl-1,2,3-triazoles (Section II,C), the participation of open-chain intermediates 287 or 288 is proposed (Scheme 46) the subsequent hetrocyclization engaging the pivotal sulfur and the azido or diazo-methyl side-chain develops into the rearranged 285 or 290, respectively. On the other hand, a concerted electronic reorganization involving the side-chain... 

ULTRAFAST EFFICIENT SWITCHING OF CONCERTED ELECTRON-NUCLEAR DYNAMICS IN MOLECULES... 

A different type of concerted reaction involves the bacterial cytochrome c peroxide, where two hemes are coupled together, so that hydrogen peroxide undergoes a two-electron reduction to water without the formation of radical species. In a number of dioxygenases, dioxygen is reduced to peroxide by concerted electron transfer from [2Fe-2S] and non-heme Fe11 centres. 

A concerted electron transfer mechanism, with formation of an alkyl radical and quinone radical anion, has been proposed to account for the products of reaction of benzophenone with alkyllithium or Grignard reagents 92 the ratio of addition to reduction products is dependent on the alkyl group and not on the metal. 

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