Radical-substrate coupling mechanism

Parker VD (1998) Radical reactivity of radical ions in solution. Radical-radical and radical-substrate coupling mechanisms. Acta Chem Scand 52 154-159 Poskrebyshev GA, Neta P, H uie RE (2002) Temperature dependence of the acid dissociation constant of the hydroxyl radical. J PhysChem A 106 11488-11491 Pou S, Hassett DJ, Britigan BE, Cohen MS, Rosen GM (1989) Problems associated with spin trapping oxygen-centered free radicals in biological systems. Anal Biochem 177 1-6 Raghavan NV, Steenken S (1980) Electrophilic reaction of the OH radical with phenol. Determination of the distribution of isomeric dihydroxycyclohexadienyl radicals. J Am Chem Soc 102 3495-3499... 

The situation where the radical-substrate coupling is a preequilibrium to the homogeneous electron transfer step, termed the rsdDISP2 mechanism, prevails when ).-d 3> /d)b. Then equation (6.56) becomes... 

Electrochemical methods were used to obtain kinetic information concerning the cation-radical dimerization of anisole (and related compounds). Two mechanisms were consistent with data A radical-radical coupling (RRC) mechanism and a radical-substrate coupling (RSC) mechanism (Fig. 42) [198]. 

For reactions of MC carried out in the presence of Me4N+, it was shown that the cation must be taken into account in the rate law. Reactions (126)-(128) were proposed to describe the radical anion-substrate coupling mechan-... 

A similar experiment in [C2mim][N(Tf)2] was conducted on a family of arenes, such as pyrene (vii), perylene (viii), chrysene (ix), 1,2-benzanthracene (x), and 2,3-benzanthracene (xi) [9]. For all these compounds, a radical-substrate coupling (RSC) mechanism (Eqs. 15.6-15.9) was observed, instead the radical-radical coupling (RRC) mechanism observed for anthracenes. Uniquely, viii was excellently modelled by a simple E reaction mechanism [9]. 

The radical-substrate dimerization (RSD) mechanism is as depicted in Scheme 2.8, involving, as a first follow-up reaction, coupling of the electron transfer intermediate with the substrate. There are, in fact, several versions of the RSD mechanism according to the nature of the electron transfer step,... 

Cation (anion) radicals of aromatic hydrocarbons should in principle be strong 7r-acceptors (ir-donors) and it is interesting to speculate that CT complexes between substrate and radical ion might play a role in coupling reactions, such as the biaryl coupling mechanism shown in eqn (57). A CT complex of the [ArH—ArH] + type would certainly be more difficult to oxidize than ArH. Thus ArH might be protected from oxidation, and coupling within the complex would take place instead. 

Applying this technique to a series of four substrates, the kinetic data of Table 15 were obtained. Also included are data from homogeneous solution studies on the radical anion of 1,1-diphenyl-ethylene. For the two esters, radical anion coupling seems to be the preferred reaction mode under these conditions, whereas both mechanisms operate for the two nitriles and 1,1-diphenylethylene, k2 being 10-100 times larger than k 2 m these cases. 

Figure 42 The radical-radical coupling mechanism and the radical-substrate mechanisms. (From Ref. 198.)...

Another important set of reactions are the dimerizations, which are usually discussed within the frames of the two mechanisms shown in Scheme 3. If the formation of C results from the coupling of two radicals or radical ions (ii), the reaction is referred to as a radical-radical process (RR). Another route to C, the radical-substrate process (RS), includes the coupling between A and B (iii) resulting in the formation of an intermediate I that is further reduced to C by reaction with B (iv). The direct reduction of I to C at the electrode is usually without importance. Finally, the intermediate C reacts with a reagent X to the product D. These dimerization mechanisms belong to a more general scheme to be discussed in some detail later (Sec. II.C.5). 

Signal-time behavior of the ESR response following a current pulse has been calculated by Goldberg and Bard [367] for a number of mechanisms, including first-order decomposition, radical ion dimerization, and radical ion-substrate coupling, and working curves from which rate constants can be calculated were presented. Application of this approach, which is very similar to that taken, for example, in transmission spectroelec-trochemistry, was demonstrated for the reductive dimerization of a series of activated olefins (Fig. 58), a reaction that has been studied by a number of different electrochemical... 

As one may expect from the peroxidase reaction mechanism described in Eqs. (2-4), the reactivity of the enzyme intermediates towards a particular substrate may be estimated a priori on the basis of the thermodynamic driving force of these two electron-transfer reactions, which is directly related with the difference between the oxidation/reduction potentials of both the enzyme active intermediates (i.e.. Col and Coll) and the substrate radicals. Thus, the thermodynamic driving force for the reaction of Col (or Coll) with the reducing substrates is the difference between the mid-point potentials of the CoI/CoII (or CoII/Felll) and the substrate radical/substrate (R, Hr/RH) redox couples ... 

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