Saveant studies

The pioneering work of Conant showed that the one-electron reduction of pyridinium ion by low-valent vanadium produces the corresponding carbon radical, which dimerizes to give a homocoupling product. Saveant studied the electrochemical reduction of stable iminium salts, and observed two reduction waves in the polarograms (Scheme 5.27). The first wave corresponds to the one-electron reduction process for which dimerization occurs. This process presumably involves formation of the carbon centered radical. The second wave is concerned with the formation of the amine by two-electron reduction. Wayner performed extensive work on oxidation and reduction potentials of carbon radicals.A modulated... 

ORR catalysis by Fe or Co porphyrins in Nation [Shi and Anson, 1990 Anson et al., 1985 Buttry and Anson, 1984], polyp5rrolidone [Wan et al., 1984], a surfactant [Shi et al., 1995] or lipid films [CoUman and Boulatov, 2002] on electrode surfaces has been studied. The major advantages of diluting a metalloporphyrin in an inert film include the abUity to study the catalytic properties of isolated molecules and the potentially higher surface loading of the catalyst without mass transport Umit-ations. StabUity of catalysts may also improve upon incorporating them into a polymer. However, this setup requires that the catalyst have a reasonable mobUity in the matrix, and/or that a mobile electron carrier be incorporated in the film [Andrieux and Saveant, 1992]. The latter limits the accessible electrochemical potentials to that of the electron carrier. 

Bertran, J., Gallardo, I., Moreno, M. and Saveant, J. M. Dissociative electron transfer. Ab initio study of the carbon-halogen bond reductive cleavage in methyl and perfluoromethyl halides. Role of the solvent, JAm.Chem.Soc., 114 (1992), 9576-9583... 

The reduction of organic halides in the presence of aromatic hydrocarbons, the subject of detailed kinetic studies, provide rate constants for the homogeneous ET [147-150] and the follow-up reaction [151]. The theoretical basis for this kind of experiment ( homogeneous redox catalysis ) was laid by Saveant s group in a series of papers during the years 1978-80 [152-157]. Homogeneous ET also plays an important role in the protonation of anion radicals [158]. 

In order to illustrate the application of LSV in mechanistic analysis we can look at the redox behavior of the formazan-tetrazolium salt system which we studied some years ago [17], 1,3,5-Triphenyl formazane was oxidized at controlled potential in CH3CN-Et4NC104 solution to 2,3,5-triphenyl tetrazolium perchlorate which was then isolated in quantitative yield. Coulometry showed that the overall electrode reaction was a two-electron oxidation. It has been shown that the rate of variation of Ep with log v was 30 mV per decade of sweep rate and that there was no variation of the peak potential with the concentration of 1,3,5-triphenylformazan. According to Saveant s diagnostic criteria (Table 1), four mechanistic schemes were possible e-C-e-p-p, e-C-d-p-p, e-c-P-e-p and e-c-P-d-p. If cyclization is the rate-determining step, then the resulting e-C-e-p-p and e-C-d-p-p mechanisms would not imply variation of Ep with the concentration of base. However, we have observed the 35 mV shift of Ep cathodically in the presence of 4-cyanopyridine as a b e. These observations ruled out the first two mechanisms. The remaining possibilities were then e-c-P-e and e-c-P-d, as shown in Scheme 3. 

Saveant and co-workers have shown that the kinetics for C—C cleavage of radical cations of NADH model compounds fall into this pre-equilibrium regime. Similarly, the slope of log(A obs) vs. AG°ab for a number of C—C containing ion radicals also is about -1/(2.303/ 7 ). The first clear demonstration of the transition from activation to counterdiffusion control was found in a study of the fragmentation of anion radicals of a-aryloxyacetophenones (ArC(0)CH20Ar ). This study used a combination... 

The electroreduction of phosphonium salts, and the way in which ylids are formed, has been studied in some detail Saveant et al. recognised that residual... 

Convolution potential sweep voltammetry (CPSV) refers to the mathematical transformation of LSV current—potential data resulting in curves with shapes like conventional polarograms which are suitable for logarithmic analysis. The method was first proposed for the study of electrode kinetics by Imbeaux and Saveant [74] but is equivalent in all respects to a semi-integral technique reported earlier by Oldham [75— 77]. A very readable description of the method has been presented by Bard and Faulkner [21]. 

The conclusions of these studies were disputed by Saveant and coworkers [90] who claimed that the data are indeed consistent with the simple eCeh mechanism and implied that the previous interpretation was influenced by experimental error. More recently [91], they re-examined the data and found that it was necessary to propose a somewhat more complex mechanism involving reactions of AN- complexed with either one or two molecules of PhOH. 

The definitive study on this topic was reported by Saveant and coworkers117. They note a dichotomy between behavior of reagents believed to be involved in outer-sphere (redox) and inner-sphere (chemical) electron transfer processes. Outer-sphere reagents such as... 

Initiation by electrochemical induction may have the disadvantage of low yields of substitution due to the reduction of the radicals formed near the electrode, mainly in those cases in which the radical anion of the halide compound fragments at a considerably high rate. Redox catalysis, that is activation involving a suitable ET mediator, is an important means to avoid termination steps in electrochemically induced reactions. This approach has been extensively studied by the Saveant group15. A general equation has been proposed in order to predict the yield of ET-initiated S l chain reactions and related mechanisms under preparative electrochemical conditions in the presence of a redox mediator35. 

The mechanism of the electrochemical cleavage of the C-X (X = F, Cl, Br and I) bond has been extensively studied (see for example, Miller and Riekena 1969 Pause et al. 2000 Costentin et al. 2003 Wang et al. 2004 Sanecki and Skital 2007) and theoretical models (Battistuzzi et al. 1993 Kuznetsov et al. 2004 Zhang et al. 2005 Golinske and Voss 2005), mostly based on the Marcus theory (Marcus 1964) for the homogeneous and heterogeneous (electrochemical) electron transfer (ET), have been formulated (Saveant 1987, 2000 Maran et al. 2001 Costentin et al. 2006a, b) and tested with the aim to predict the experimental outcomes. 

E is for an electron transfer at the electrode, indicates a homogeneous electron transfer and C is a homogeneous chemical reaction. During a chrono-amperometric study of a process believed to go by a mechanism such as that above, the response did not fit theoretical data for the ECE mechanism and it was necessary to include the homogeneous electron transfer (70) in the mechanism in order to account for the data (Hawley and Feldberg, 1966). A detailed theoretical study of this problem has shown that the E step predominates over the second E step in situations where it would be possible to distinguish between the two (Amatore and Saveant, 1977, 1978, 1979, 1980). 

In more recent work concerned with the protonation of anthracene radical anion (AN ) by phenol in DMF, no indications of deviations from ECE behaviour were found during DPSC (Amatore and Saveant, 1980) or DCV studies (Ahlberg and Parker, 1981b,c). However, when the reaction was examined in the presence of phenoxide ion it became apparent that the protonation step is reversible and the kinetic law is quite complex (Parker, 1981i). The data reproduced in Table 4 are instructive with regard to the use of some... 

A triactivated olefin, p-methylbenzylidenemalononitrile (MBM) [14], has also been used as a substrate in electrohydrodimerization studies (Nadjo et al., 1975 Nadjo and Saveant, 1976). Because of the expected extensive delocalization of charge and the odd electron in the anion radical MBM , this structure would appear to be rather extreme to be considered as a model... 

The loss of halide ion from the radical anions of aromatic compounds is a facile reaction in solution. This reaction has been studied extensively by electrode techniques (Lawless and Hawley, 1969a,b Bartek et al., 1970, 1972 Nadjo and Saveant, 1971a Houser et al., 1973 Nelson et al., 1973 M Halla et al., 1978, 1980 Saveant and Thiebault, 1978 Gores et al., 1979 Pinson and Saveant, 1974, 1978 Amatore e/a/., 1979 Parker, 1981k,l). The anion radicals of halonitrobenzenes, halobenzonitrilies, haloanthracenes, halo-naphthalenes, halobenzophenones and haloacetophenones have received the most attention. 

Slower reactions, for example those of the halobenzophenone radical anions, had previously been studied in DMSO (M Halla et al., 1978), DMF, CHgCN, 2-cyanopropane and liquid NHj (Nadjo and Saveant, 1971a Saveant and Thiebault, 1978). Typical rate constants for cleavage reaction (148) in DMF at 20°C were reported to be 10 (p-Cl). 740 (w-Br) and 8 x 10 (p-Br). 

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