Free-Radical and Electron-Transfer Processes

Perhaps the most important preparative use of aryl free radicals is in the synthesis of biaryls. If an aryl free radical is generated in the presence of a second aromatic compound, substitution leading to the biaryl is observed. Yields in this 

SECTION 8.3. FREE-RADICAL AND ELECTRON-TRANSFER PROCESS 

Ruchardt and B. Freudenberg, Tetrahedron Lett., 3623 (1964) C. Ruchardt and E. Merz, Tetrahedron Lett., 2431 (1964). 

Recent mechanistic investigations have defined a group of nucleophilic aromatic substitutions which proceed by a pathway described as SrnI. The key is an electron transfer from the nucleophile to an aryl halide. The process is a chain 

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The next two chapters demonstrate the power of anionic, free-radical and electron-transfer processes in the systematic construction of highly functionalized five- and six-membered carbocycles. 

In 1972, Ingles reported his studies of Fenton s reagent using redox titration. He found evidence in support of Kremer s complex mechanism theory and concluded that, when suitable complexes are formed, substrates are not oxidized by free radical rather, electron transfer processes might be... 

The redox reactions of carbon free radicals and copper(II) compounds have been portrayed as ligand-transfer and electron-transfer processes 178). The electron-transfer oxidation of alkyl radicals by copper(II) complexes, which are efficient radical interceptors, is considered to proceed via a metastable alkylcopper species which is consumed primarily by oxidative elimination [Eq. (129)] and oxidative solvolysis [Eq. (130)] 143b). The anionic counterion exerts a dominant effect in the selection... 

The conversion of free radicals by electron transfer into carbocations and subsequent initiation of the cationic polymerization has recently been reviewed The electron transfer process... 

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

In redox initiation, the free radicals that initiate the polymerization are generated as transient intermediates in the course of redox reaction. Essentially this involves an electron transfer process followed by scission to give free radicals. A wide variety of redox reactions, involving both organic and inorganic components either wholly... 

A number of metal chelates containing transition metals in their higher oxidation states are known to decompose by one electron transfer process to generate free radical species, which may initiate graft copolymerization reactions. Different transition metals, such as Zn, Fe, V, Co, Cr, Al, etc., have been used in the preparation of metal acetyl acetonates and other diketonates. Several studies demonstrated earlier that metal acetyl acetonates can be used as initiators for vinyl polymeriza-... 

The ESR spectrum of the furan radical anion indicates that the Cem-0 bond is ruptured in the electron transfer process whereby the oxygen atom acquires the negative charge and the C-2 end of the open ring possesses a free radical character ... 

Force-field methods, calculation of molecular structure and energy by, 13,1 Free radical chain processes in aliphatic systems involving an electron-transfer reaction, 23, 271 Free radicals, and their reactions at low temperature using a rotating cryostat, study of, 8. I Free radicals, identification by electron spin resonance, 1, 284... 

Nitroblue tetrazolium (NBT, 3,3 -(3,3,-dimethoxy-l,l,-biphenyl-4,4 -diyl)bis-2-(4-nitrophe-nyl)-5-phenyl-2H-tetrazolium dichloride) is reduced by superoxide to formazan as a final product, which can be measured spectrophotometrically [73]. Although the rate constant for NBT reduction by superoxide is moderately high 5.88+0.12x 104 1 mol 1 s 1 [74], the formation of formazan is not a simple one-electron transfer process, and the final product is formed as a result of disproportionation of intermediate free radicals. Similar to cytochrome c, NBT is easily reduced by the other reductants that confines its application for superoxide detection. Moreover, similar to epinephrine, NBT free radical is apparently... 

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

An increase in the cA-stilbene concentration favors the chain propagation and decreases the probability of termination when the DCNA anion-radicals react with the stilbene cation-radicals. A decrease in the irradiation intensity has a similar effect The chain propagation is the first-order process, whereas termination of the chains is the second-order process. A temperature rise accelerates the accumulation of the stilbene cation-radicals. In this system, the free energy of electron transfer is -53- —44 kJ moD (the cation-radical generation is in fact an endothermal process). If a polar solvent is substituted for a nonpolar one, the conversion of the cii-stilbene cation-radical into the trani-stilbene cation-radical deepens. Polar solvents break ion pairs, releasing free ion-radicals. The cA-stilbene cation-radicals isomerize more easily on being released. The stilbene cation-radical not shielded with a counterion has a more positive charge, and therefore, becomes stabilized in the... 

Electrons are transferred singly to any species in solution and not in pairs. Organic electrochemical reactions therefore involve radical intermediates. Electron transfer between the electrode and a n-system, leads to the formation of a radical-ion. Arenes, for example are oxidised to a radical-cation and reduced to a radical-anion and in both of these intermediates the free electron is delocalised along the 7t system. Under some conditions, where the intermediate has sufficient lifetime, these electron transfer steps are reversible and a standard electrode potential for the process can be measured. The final products from an electrochemical reaction result from a cascade of chemical and electron transfer steps. 

The direct reaction of oxygen with the carbanion from dihydroanthracene does not seem likely. Russell (5) has indicated a preference for a one-electron transfer process to convert the carbanion to a free radical, which then reacts with oxygen to form an oxygenated species. Therefore, we considered a mechanism involving one-electron transfer to form a free radical from the carbanion, which would lead to the formation of anthraquinone and anthracene without having either the hydroperoxide or anthrone as an intermediate. 

Most successful selective fluorination reactions are carried out under conditions which limit any free radical processes and encourage nucleophilic attack on fluorine either by a one- or two-electron transfer process (see Sect. 3.1.1.3). 

The heme iron in the peroxidase is oxidized by the peroxide from III+ to V4- in compound I. The compound I is reduced by two sequential one-electron transfer processes giving rise to the original enzyme. A substrate-free radical is in turn generated. This may have toxicological implications. Thus the myeloperoxidase in the bone marrow may catalyze the metabolic activation of phenol or other metabolites of benzene. This may underlie the toxicity of benzene to the bone marrow, which causes aplastic anemia (see below and chap. 6). The myeloperoxidase found in neutrophils and monocytes may be involved in the metabolism and activation of a number of drugs such as isoniazid, clozapine, procainamide, and hydralazine (see below). In in vitro systems, the products of the activation were found to be cytotoxic in vitro. 

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