Oxidation of radicals

A large number of carbon-centered free radicals, which are formed by photolysis or thermolysis of commercially available free radical initiators, can be oxidized by onium ions by reaction according to Eq. (10-4)  

Carbocations generated in this way can add directly to appropriate monomers (e.g., tetrahydrofuran, cyclohexene oxide, n-butyl vinyl ether) or can form Bronsted acids by abstracting hydrogen from surrounding molecules. This method, which is commonly referred to as free-radical-promoted cationic polymerization, is quite versatile, because the user may rely on a large variety of radical sources. Some of them are compiled in Table 10.9. 

A sufficiently high negative value of the free energy, AG, is required for the occurrence of reaction according to Eq. (10-4). AG, in units of kj mol , can be estimated with the aid of Eq. (10-5), the modified Rehm-Weller equation. 

1/2 half-wave potentials, in units of V, of oxidation 

Photoinitiator Electron-donating free radical Generation of radical 

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The low yield in this reaction might be caused by a number of reasons. First, the overall reaction is only rapid for readily enolizable compounds. 1,3-Dicarbonyl compounds will therefore be a better choice as compared to acetic acid. Second, to prevent oxidation of radical 54, it is advantageous to work with excess diene and therefore speed up trapping of 54 through diene addition. Finally, lactone 55 can, as an enolizable compound itself, also be oxidized by manganese(III) acetate and form various oxidation products. Shorter reaction time and the use of understoichiometric amounts of oxidant might therefore benefit the overall result. All these factors have been taken into account in the synthesis of bicyclic /-lactone 56, which has been obtained from cyanoacetic acid and 1,3-cyclohexadiene in 78% yield within 15 min reaction time (equation 25)60,88. 

A constant extra amount of oxygen is, therefore, taken up over and above that required to form AO,. This may reasonably be attributed to the oxidation of radicals formed in the solvent reaction, where it is known that different products are formed in presence of Oa. 

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

Copper(I) can also be regenerated by electron transfer oxidation of radicals produced by hydrogen transfer from solvent by alkoxy radicals.101 Thus, reactions carried out in hydrocarbon solvents will produce alkyl radicals that are oxidized by Cu(II) at rates approaching diffusion control.95-100... 

Copper(II) is known61 63b>65 to be more effective than other metal oxidants for the electron transfer oxidation of radicals to the corresponding cations. 

When oxygen is admitted into the sample containing the polymer particles with macroradicals, the oxidation of radicals takes place almost immediately, and they are transformed into peroxide radicals with their characteristic asymmetrical spectrum (Fig. 3). After the oxidation process is over, at temperatures below — 50° C the amount of peroxide... 

The oxidation of radicals in our experiments was carried out at 760 mm Hg oxygen pressure and at low temperatures. The macroradicals were generated by mechanical degradation of polymers in vacuo. The polymeric particles, made during the degration procedure, fell into a... 

The first stage of this unusual reaction involves debenzylation of the substrate to form 2-aminobenzimidazole polyaniones. The formation of 2,2 -azobenzimida-zolone is the result of autooxidation of 2-aminobenzimidazole di- and trianiones, when 2-nitrobenzimidazole is formed on oxidizing of radical anions [660]. 

Under what conditions can one-electron oxidation of radicals compete with other radical reactions ... 

The reaction follows the latter pathway when oxidation of radical 234 is not so easy. In those cases, an alternate course involving iodine atom transfer to the substrate is followed. This allows the formation of iodoalditols [196,197], difluoroalditols (241, 75%) [198], vinyl azides [199], or vinyl sulfones (although in that particular case, the mechanism may be more sophisticated due to the possible 6-elimination of the sulfur group) [200]. 

The reactions shown in Scheme 2.140 involve the oxidation of radical species, for example 419a, as the termination step in a radical addition. An alternative and highly useful protocol was elaborated with a system composed of a radical initiator and a hydrogen donor capable of reducing a similar radical intermediate. Utilization of azobisisobutyronitrile (AIBN) plus Bu3SnH was shown to be especially efficient for this purpose. In this system, BusSnH fulfills a dual... 

The mechanism is considered to involve generation and oxidation of radicals ... 

As methyl-ethyl ether can be prepared in both these ways the view that it and other ethers are oxides of radicals in which the two alkyl groups are joined directly to oxygen, appears to have a satisfactory experimental basis. 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo their usual decarboxylation. This electron transfer can be effected by strong one-electron oxidants. Such reactions have been observed with Mn(III), Ag(II), Ce(IV), and Pb(IV). The metal ion is also capable of oxidizing the radical intermediate, so the products are of the same type as those observed in reactions involving oxidation of radicals by Cu(II), although there are competing reactions. In terms of preparative usefulness, the oxidative decarboxylation by Pb(IV) in the presence of halide salts probably is of the most value. For example, oxidation of... 

According to Berzelius the acid anhydrides are oxides of radicals and the acids are formed from these by addition of water e.g. acetic acid is C H 0 + H 0. Laurent said that no acid can be represented as the hydrocarbon from which it is derived plus oxygen. This hydrocarbon must always lose part... 

The resistivity of polyaniline (pH 1.5) is dependent upon applied potential. The resistivity exhibits a minimum whose magnitude and position with respect to potential depends upon pH (Figure 2) as was first observed by Paul et al." The electrochemistry of polyaniline has been studied in some detail and provides for an interpretation of the resistivity data." The first anodic wave, which corresponds to the oxidation of diamine units to radical cations, is essentially pH-independent. However, the second wave, which is the result of oxidation of radical cations to quinone diimines, is pH-dependent as the result of immediate deprotonation of the quinonediimine. Thus, the second wave moves cathodically (closer to the first wave) as pH increases, and the region of minimum resistivity (Figure 2) in turn narrows while the magnitude of minimum resistivity increases. The resistivity is therefore intimately dependent on two parameters, pH and potential. 

The efficiency of cationic photoinitiators can be enhanced by the use of free-radical sources such as benzoin alkyl ethers and alkoxyacetophenones. In the presence of THF, photo-active radical sources and diaryliodonium salts would be expected to yield cations as outlined in Scheme 1. The neral method of producing cations by the oxidation of radicals produced from any source has been demonstrated for the polymerization of vinyl ethers and THF. ... 

The oxidation of alcohols in iron(n)-hydrogen peroxide systems has been re-examined, and the effects of copper(ii) ions described. A new kinetic analysis has been developed, and the nature of the interactions of the radicals produced on reaction of the peroxide with the metal ion indicates that, in the reaction with isopropyl alcohol, )S-hydroxyalkyl radicals are not oxidized by iron(m) but dimerize, whereas in the presence of copper(n) they are oxidized to glycols. It is suggested that oxidation of radicals by iron(m) involves an electron-transfer process, which may in some cases be reversible,... 

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