Redox catalysis radicals

Redox catalysis appears also to be an elegant way to conduct alkylations. In this case the RX compound has to produce a free radical R which is sufficiently reactive toward the electron carrier A or its reduced form. Generally the mechanism, developed mainly for the case of alkyl halides, may be summarized as in reactions 25, 29-36. 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

FIGURE 2.27. Redox catalysis of the reductive cleavage of 1-chloronaphthalene by the electrochemically generated anion radical of 4-methoxybenzophenone in DMSO at 0.05 V/s for excess factor of 0.5. Fitting of the experimental data points. 

For the sake of comparison and mutual validation of methods for measuring large follow-up reaction rate constants, it is interesting to apply different methods to the same system. Such a comparison between high-scan-rate ultramicroelectrode cyclic voltammetry, redox catalysis, and laser flash photolysis has been carried out for the system depicted in Scheme 2.25, where methylacridan is oxidized in acetonitrile, generating a cation radical that is deprotonated by a base present in the reaction medium.20... 

As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

The very fact that chemical catalysis involves the formation of an adduct opens up possibilities of selectivity, particularly stereoselectivity, that are absent in redox catalysis. Several examples of homogeneous chemical catalysis are described in the following section, illustrating the improvements that can be achieved when passing from redox to chemical catalysis. It remains true that redox catalysis has several useful applications that have already been discussed, such as kinetic characterization of fast follow-up reactions (Section 2.3) and determination of the redox properties of transient radicals (Section 2.6.4). 

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

Aromatic anion radicals may be used as outer-sphere electron donors, thus giving rise to redox catalysis. The ensuing variations of the electron transfer rate constant with the driving force are shown in Figure 3.2b for... 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

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]. 

Radical-anion complexes Scope of this review 91 Thermodynamic and kinetic methodologies Voltammetric methods 92 Homogeneous redox catalysis 94 Convolution analysis 98 Laser flash photolysis 102 Photoacoustic calorimetry 103 Thermochemical estimates 105 Fleduction of C—O and O O bonds 106 Reduction of ethers 107 Reduction of peroxides and endoperoxides Reduction of S—S and C—S bonds 136 Reduction of disulfides 137 Reduction of sulfides 150 Concluding remarks 157 Fleferences 160... 

A more recent investigation has been carried out on the homogeneous ET to an extended series of diaryl disulfides (X = NH2, OMe, H, F, C02Et, CN. NO2) in DMF. The redox catalysis approach was applied extensively. The mechanism of the homogeneous reaction between electrogenerated radical anion donors D and (ArS)2 takes place according to the sequence (equations 80-83) ... 

Using Saveanfs terminology, such a process is called redox catalysis in its proper meaning, while Shono formed the expression homomediatory system . This type of mechanism was already schematically presented in the case of an oxidation in Eqs. (2) to (4). To this category of redox catalysts belong, for example, the radical anions and cations of aromatic and heteroaromatic compounds and some reactions of triaryl amine radical cations. 

The methods for the theoretical treatment of the homogeneous redox catalysis have been mainly developed by Saveant et al. These methods allow to calculate the lifetimes of short-lived anion radicals and the standard potentials of the substrates from redox-catalytic experiments which are directly not accessible... 

In addition, it has been shown that, for the same telogen and fluoroolefin, a radical telomerisation leads to higher molecular weights than those obtained from redox catalysis [21-24]. 

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