Aromatic anion radicals homogeneous electron transfer from

The rate constant of the heterogeneous electron transfer from a platinum electrode to aromatic disulfides in DMF is low, whereas the homogeneous electron transfer from suitable radical anions is much faster these can be generated at potentials more negative than eP of the disulfides but less negative than their peak potentials [223]. 

It was found that the cleavage of the primarily formed radical anion was the ratedetermining step in an indirect reduction of tosylates in DMF when R was aliphatic, whereas with R aromatic the cleavage was so fast that the homogeneous electron transfer from the reduced catalyst became rate-determining [249],... 

One approach to this problem [40, 41] generates the alkyl radicals by homogeneous electron transfer to an alkyl halide from an aromatic radical anion. These... 

A bsolute rate constants for electron transfer reactions of some aromatic molecules in solution have been reported in our earlier work (2) using the pulse radiolysis method. The transfer of an electron from various radical anions to a second aromatic compound in solution was observed directly. Of the rate constants for nine donor-acceptor pairs investigated, two were found to be lower than the diffusion controlled values, and a correlation with such parameters as the reduction potential difference of the pair was considered. These measurements have been extended to additional transfer pairs for which the reduction potential difference is small. The objective of this work, in addition to furnishing new data for electron transfer rates, is to provide an adequate test of theories of the rate of homogeneous electron transfer in polar liquids (10, 11,12,13, 14, 15,16,17). 

Figure 4.4. Homogeneous electrocatalytic redticiion of 2-bromopyridine in ditnethylfomiamide. Forward electron transfer rate constant vs. standard potential of the aromatic radical-anion donor. Data from ref, [29],...

Figure 4.6. Homogeneous dissociative electron transfer reaction between aromatic radical-anions and (a) di-(4-cyanophenyl) disulphide, (b) diphenyl disulphide in dimethyl formamide. Data from ref [31J.

The indirect reduction of many organic substrates, in particular alkyl and aryl halides, by means of radical anions of aromatic and heteroaromatic compounds has been the subject of numerous papers over the last 25 years [98-121]. Many issues have been addressed, ranging from the exploration of synthetic aspects to quantitative descriptions of the kinetics involved. Saveant et al. coined the expression redox catalysis for an indirect reduction, in which the homogeneous reaction is a pure electron-transfer reaction with no chemical modification of the mediator (i.e., no ligand transfer, hydrogen abstraction, or hydride shift reactions). In the following we will consider such reactions and derive the relevant kinetic equations to show the kind of kinetic information that can be extracted. 

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