Free radical generation, electrochemical

The techniques described here for monitoring flash-photolyzed solutions are clearly applicable to other situations in which the solution s composition is modified suddenly. Henglein and co-workers have used them in some interesting electrochemical studies of free radicals generated by pulse radiolysis [77,78]. They have been able to resolve events on a 10"5 time scale. 

We present here a brief account of the specific dimerization, and other related, reactions undergone by a variety of purine and pyrimidine derivatives, and a number of related compound s, during the course of their electrochemical reduction at the surface of a mercury electrode. A characteristic feature of these reactions is the transfer of an electron to the compound, accompanied, or preceded, by its protonation. The resultant free radicals, generated by a one-electron reduction process, rapidly dimerize to products in which each of the monomeric components possesses an additional electron and an additional proton, relative to the parent monomer1 7). (See Scheme 1)... 

In general, the thermodynamics of free radicals has been, up to now, almost unknown due to their extremely high reactivity and electroactivity. For clarification, several methods were proposed [225,226] from anodic half-wave potentials of carbanions and organo-lithium compounds. Redox catalysis may strongly help to estimate redox potential of a certain number of radicals generated electrochemically. Thus, as seen previously in Scheme 4 and in Scheme 6, in the absence of other side reactions, there are two main different ways... 

Olea-Azar, C., Rigol, C., Mendizabal, F., MoreUo, A., Maya, J.D., Moncada, C., Cabrera, E., Di Maio, R., Gonzalez, M., and Cerecetto, H. (2003). ESR spin trapping studies of free radicals generated from nitrofuran derivative analogues of nifurtimox by electrochemical and Trypanosoma cruzi reduction. Free Radic Res 37, 993-1001. 

In addition to heat and light, generation of free radicals can be accomplished by using 7-rays, X-rays or through electrochemical means. In general, however, these methods do not tend to be so widely used as those involving benzoyl peroxide or AIBN as initiators. 

The previous chapter covered radical cation cyclization reactions that were a consequence of single-electron oxidation. In the following section, radical anion cyclization reactions arising from single-electron reduction will be discussed. In contrast to the well documented cyclization reactions via carbon-centered free radicals [3, 4], the use of radical anions has received limited attention. There are only a few examples in the literature of intramolecular reductive cyclization reactions via radical anions other than ketyl. Photochemi-cally, electrochemically or chemically generated ketyl radical anions tethered to a multiple bond at a suitable distance, have been recognized as a promising entry for the formation of carbon-carbon bonds. 

Because of the difficulties described earlier, electroanalytical studies are usually performed separately from radical generation studies. But a flat cell has been designed [26] (Fig. 29.19) to permit simultaneous monitoring of the electrochemical and EPR response of a free-radical system (SEEPR). The auxiliary electrode extends along the edges of the working electrode, which diminishes the problems of iR drops and provides better uniformity of current density than is possible with conventional electrode placement. This cell is used primarily for short-term (on the order of seconds) electrochemical experiments, such as... 

The cell shown schematically in Figure 29.20b permits external generation, followed by EPR detection. The solution can either be recirculated to the electrolysis cell or discarded after observation. Umemoto [36] used a similar apparatus to generate moderately stable radicals coulometrically, followed by stopped-flow measurements of the decay kinetics. Forno [37] used a more elaborate recirculating system with two electrochemical cells in series. The unstable product of the first electrolysis was pumped to the second electrolysis cell, where it was converted to a free radical and thence to the cavity for observation (Sec. VI.A). 

Previous work in our laboratory (3) and in others (4) has established that the primary photoprocess in a variety of excited carbanions involves electron ejection. This photooxidation will generate a reactive free radical if recapture of the electron is inhibited. Parallel generation of these same carbon radicals by electrochemical oxidation reveals an irreversible anodic wave, consistent with rapid chemical reaction by the oxidized organic species (5). Little chemical characterization of the products has been attempted, however (6). 

Although less common, ketyl anions can also be generated by removal of an a-hydrogen from an alkoxide (Figure 1, reaction 3). An interesting example where a ketyl anion is formed as an intermediate in this manner is provided by the electrochemically-initiated reduction of an aryl halide by an alkoxide anion via the free radical chain process illustrated in Scheme l6. 

Electrochemical reduction of aryl halides in the presence of olefins (94), (equation 54) leads to the formation of arylated products (95). Electroreduction of several aralkyl halides at potentials ranging from -1.24 V to -1.54 V (see) gives products which involve dimerization, cyclization, and reduction to the arylalkanes. Carbanions and/or free radicals were again postulated as intermediates79. Aryl radicals generated from the electrochemical reduction of aryl halides have been added to carbon-carbon double bonds80,81. Electrochemical reduction of aryl halides in the presence of olefins leads to the formation of arylated products78. Preparative scale electrolyses were carried out in solvents such as acetonitrile, DMF and DMSO at constant potential or in liquid ammonia at constant current. The reaction is proposed to involve an S l mechanism. 

Pyridinyl radical studies have stimultated the development of useful equipment, such as those for thin film spectroscopy, for variable i th measurements under oxygen-free conditions (the VV-cell ) and a spectr< lectrochemical cell. In addition, a new technique for measuring the photodissodation spectra of unstable species which can be generated electrochemically was introduced. 

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