Radical-substrate coupling

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... 

The reduction of carbon dioxide (Section 2.5.4) raises the question of possible competition between a radical-radical coupling and a radical-substrate coupling according to Scheme 6.3, in which the competition shown in the upper part of Scheme 2.34 is represented symbolically. 

The data shown in Figure 2.36 were gathered at constant current with a value of the current density that brought the electrode potential at the foot of the current-potential characteristic of the system. The concentration of substrate may thus be considered as constant. As discussed in Section 2.5, we consider only the case where the second electron transfer in the radical-substrate coupling pathway occurs at the electrode (ECE). The following equations and conditions apply. 

Radical cations can dimerize in a radical-radical coupling reaction (Scheme la) to afford dimer dications. An alternative pathway to form the dimer dication is a radical-substrate coupling in an electrophilic addition of the radical cation to the nucleophilic substrate. The dimer dication can lose two protons to form a bis-dehydro dimer or react with two nucleophiles to yield a disubstituted dimer. 

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... 

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]. 

At neutral pH, quinoline derivatives may be dimerized in aqueous methanolic solution [264]. It is suggested that a radical-radical reaction occurs at low substrate concentration and a radical-substrate coupling at higher concentrations. 

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,... 

Scheme 3.1. Schematic energy level diagram comparing (a) the radical-anion substrate and (b) the radical-anion radical-anion coupling routes for the clcctrodimerization process. Wavy lines indicate an electron transfer step.

The electro-synthetic reactions of activated alkenes involve carbon-carbon bond formation, which, after much controversy, is now believed generally to involve radical-anion/radical anion coupling rather than the alternative radical-anion/substrate reaction. The history of this mechanistic debate is well documented168. 

The scope was also extended to vinyl halides [117]. These substrates couple with retention of the double bond geometry, indicating that no SET or radicals are involved during oxidative addition. The coupling proceeds as above, giving (3,y-unsaturated esters or ketones in 44—88% yield. For these substrates, no involvement of radicals was proposed, but based on the similarity of the process to those... 

When the thienyl radical 212 couples with PhS ions, the radical anion 210 formed has three competitive reactions bond fragmentation to give the starting materials, ET to the substrate to give product 210, or a C—S bond fragmentation to give the anion 213 and Ph radical (equation 148). 

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.)...

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