Formation of Radical Cations

In the direct effect of ionizing radiation on DNA, radical cations are the primary products (Chap. 12). For this reason, their reactions are of considerable interest. Obviously, photoionization (e.g., at 193 nm) and laser multi-photon excitation leads to such species (e.g., Candeias and Steenken 1992b Malone et al. 1995 Chap. 2.2). Base radical cation electron pairs have been proposed to be the first observable intermediates with a lifetime of 10 ps for Ade and four times longer for the other nucleobases (Reuther et al. 2000). Radical cations are also assumed to be intermediates in the reactions of photosensitization reactions with qui-nones, benzophenone, phthalocyanine and riboflavin (Cadet et al. 1983a Decar-roz et al. 1987 Krishna et al. 1987 Ravanat et al. 1991, 1992 Buchko et al. 1993 Douki and Cadet 1999 Ma et al. 2000). Nucleobase radical cations may be produced by electrochemical oxidation (Nishimoto et al. 1992 Hatta et al. 2001) or with strongly oxidizing radicals (for a compilation of their reduction potentials see Chap. 5.3). Rate constants are compiled in Table 10.3. 

Short-lived adducts may be formed as intermediates in the reactions of the oxidizing inorganic radicals with the nucleobases, and it is therefore not always fully excluded that processes observed at very short times and attributed to the reactions of radical cations are in fact due to such intermediates. It may be mentioned that, for example, a long-lived S04 -adduct is observed in the reaction of S04, with maleic acid (Norman et al. 1970). It has been suggested that S04, in its reactions with the pyrimidines forms only an adduct and does not give rise to radical cations (Lomoth et al. 1999). The observation of heteroatom-centered radicals by EPR from the nucleobases Ura, Thy and Cyt (Catterall et al. 1992) as well as dCyd (Hildenbrand et al. 1989) (see below) has been taken as evidence that in the reaction of S04 with pyrimidines radical cations are likely, albeit 

Br2 can also be used to oxidize good electron donors, but at least with the pyrimidines its rate of reaction is too slow to be of any importance. Instead, degradation may occur by Br2, the product of the disproportionation of Br2, as has been shown for Thd (Cadet et al. 1983b). 

Usually, radical cations have much lower pfCa values than their parent compounds. A typical examples is phenol, whose pfCa value is at 10 while that of its radical cation is at -2 (Dixon and Murphy 1976). Thus in this case, ionization causes an increase in acidity by 12 orders of magnitude. It is hence expected that also the nucleobase radical cations should be much stronger acids than their parents. This has indeed been found in all systems where equilibrium conditions are established, and the consequences for DNA base pairs have been discussed (Steenken 1992). 

Pyrimidines. Photoexcited anthraquinone-2,6-disulfonate undergoes ET with Thy and its methyl derivatives as indicated by Fourier transform EPR (Geimer et al. 1997). These pyrimidine radical cations deprotonate at N( 1) thereby giving rise to the corresponding N-centered radicals [reaction (6)]. 

In this context, in the narrow sense the term nucleobase radical cation signifies a species identical to that which is produced upon one-electron oxidation of the base. Its deprotonation leads to a radical whose unpaired spin is largely heteroatom-sited. Reprotonation of this radical could in principle give rise to radical cations that are not identical but tautomeric to the original radical cation. 

The observation of heteroatom-centered radicals by EPR from the nucleobases uracil, thymine [cf. reactions (94) and (95)], cytosine [23, 99] as well as 2 -deoxycytidine [21, 99] is evidence that a radical cation is indeed a likely intermediate in the reaction of the sulfate radical with pyrimidines. 

Behaviour similar to that of SOA is expected of the phosphate radical anion. In the latter case, however, an adduct radical is observed by EPR. The fact that this is the C(6)-adduct not the C(5) one was explained through the assumption that a rapid 5,6-shift follows the initial addition at C(5), which leads to the thermodynamically more stable C(6)-phosphate adduct [102]. At pH 6.5 the 

6-shift is sufficiently slow to allow the interception the C(5)-phosphate adduct by a spin trap [103]. 

Pulse radiolysis shows that the pyrimidine radical cations are fairly strong acids and rapidly deprotonate at a heteroatom [reaction (98)]. As protonation/deprotonation reactions at heteroatoms are easily reversible, the radical cations are regenerated upon reprotonation. Deprotonation at carbon or reaction with water yields the final free-radical products [reactions (99) - (101)]. It is noted that in thymidine [23] and 5 -thymidylic acid [104] the allylic thymine radical is observed by EPR and there is very little question that its precursor is the thymine radical cation. The identification of the C(6)-OH-5-yl radical by EPR supports the view [100] that reaction with water competes with the deprotonation at methyl. Due to the ready oxidation of the (reducing) C(5)-OH-6-yl by peroxodisulfate, this type of radical is only observed at low peroxodisulfate concentrations in these systems, i.e. the (oxidizing) C(6)-OH-5-yl radicals are correspondingly enriched under conditions favourable to a chain reaction [22]. In the case of 1,3-dimethyluracil the interesting characteristics of 

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This sequence of formation of radical cation which is followed by a C—S bond scission into alkyl radical and alkyl sulfonyl cation was previously suggested by the same authors for the radiolysis of polyfolefin sulfonefs in the solid state72 and was confirmed by scavenger studies73. Scavengers are ineffective in crystalline solids such as dialkyl sulfones and hence could not be used in this study. 

Loss of electrons from the polymer chain with the formation of radical cations (polarons) and dications (bipolarons). 

Unlike the 4H- imidazoles (219), (223), (224) electrochemical oxidation of the nitrone group in 4-R-3-imidazoline-3-oxides (228), (230-232), as in a-PBN and DMPO is of irreversible nature. Therefore, the formation of radical cations... 

Explanation of the observed de-oxygenation with the formation of the corresponding imines in the photochemical reaction of a-aryl-A-methylaldonitrones, confirms the intermediate formation of radical cations resulting from PET (441, 467). 

Figure 6. Electrophilic polymerization of thiophene. Initial 71-bonding, formation of radical cation, electrophilic addition.

Much work conducted in low-temperature matrices has shown that the primary chemical process induced by y-irradiation is formation of electrons (e ) and positive holes (h+), the latter eventually leading to the formation of radical cations of the component(s) with the lowest ionization potential (Symons, 1997). This means that an added spin trap may be transformed into its radical cation by y-irradiation and thus create conditions for inverted spin trapping, as already described for PBN and DMPO above in experiments designed to study this aspect. 

Similar to the intramolecular addition of neutral carbon-centered radicals to alkenes, the formation of radical cations starting from alkenes with subsequent cyclization offers a convenient method for constructing carbocyclic ring systems. In contrast to the regioselective 1,5-ring closure (5-cxo-trig cyclization) of the... 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

Some of the materials highlighted in this review offer novel redox-active cavities, which are candidates for studies on chemistry within cavities, especially processes which involve molecular recognition by donor-acceptor ii-Jt interactions, or by electron transfer mechanisms, e.g. coordination of a lone pair to a metal center, or formation of radical cation/radical anion pairs by charge transfer. The attachment of redox-active dendrimers to electrode surfaces (by chemical bonding, physical deposition, or screen printing) to form modified electrodes should provide interesting novel electron relay systems. 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

Our radiolysis studies also indicate that phosphonates react quite slowly with the superoxide anion radical. Although our studies do not support the formation of radical cations as an initial oxidation step, we cannot rule out the possibility that radical cations are not involved in the oxidation of the C—P bond, as previously proposed [44], It is also possible that more electron-rich organphosphorus compounds or organophosphorus compounds in the adsorbed state may exhibit different redox and hydroxyl radical chemistries than what is observed under pulse radiolysis employing homogeneous conditions. 

Although some radicals and cation radicals are postulated for chemical and electrochemical transformations of 2-benzopyrylium cations (Sections III,F,1 and IV,B)> attempts to record their electron spin resonance (ESR) spectra failed, obviously because of a low stability of these radicals. However, the structural combination of hydroxy aryl and 2-benzopyrylium fragments favors the formation of radical cations 301-303, and their ESR spectra were recorded on oxidation of the corresponding 2-benzopyrylium salts with lead tetraacetate (87RRC417). 

Thus, the degradative oxidation of chlorophenols proceeds by a hydroxy-lated species (Equation 6.99 and Equation 6.100), followed by ring opening to yield aldehydes and ultimate degradation of C02 and CP. It was suggested that the first step is the formation of radical cation by acid-catalyzed dehydration of radicals formed due to the interaction of OH with chlorophenols. 

Water Elimination, Heterolytic P-Fragmentation and Formation of Radical Cations 118... 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46],... 

Treatment of arenes or heteroarenes with oxidants can lead to the formation of radical cations by SET. These radical cations can dimerize, oligomerize, or react with other radicals present in the reaction mixture deprotonation of the resulting intermediates yields the final products (Scheme 3.18). 

Lewis acids such as A1C13, SbCl5, or PFS have been used successfully to generate a variety of radical cations. Antimony pentachloride was first used with hydrocarbons such as benzene or anthracene [22, 23]. Salts obtained from aromatic amines with this reagent were found to be paramagnetic [24] eventually, well resolved ESR spectra identified the formation of radical cations [25,26]. Although an electron transfer mechanism must be involved, the fate of the complementary radical anions and details of their decay are poorly understood. Once again, it appears doubtful that Lewis acids are suitable oxidants for the study of the sometimes delicate substrates discussed in this review. 

Oxidizing/Reducing Entities. Other reactions depend on oxidation/reduction processes. Among them is polymerization of various aromatic molecules. Such polymerizations are preceeded by formation of radical cations of the aromatic hydrocarbon. Cu-montmorillonites are capable of catalyzing such reactions (136). 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

There exists still some controversy as to the nature of the electron-acceptor sites on oxide surfaces that lead to the formation of radical cations. Various... 

In contrast to oxidations with Mn(III) acetate, the oxidation of alkylbenzenes by the stronger oxidant, Co(III) acetate, appears to involve only electron transfer. No competition from classical free radical pathways is apparent. Waters and co-workers,239,240 studied the oxidation of a series of alkylbenzenes by Co(III) perchlorate in aqueous acetonitrile. They observed a correlation between the reactivity of the arene and the ionization potential of the hydrocarbon which was compatible with the formation of radical cations in an electron transfer process. 

Surprisingly, alkanes containing tertiary C—H bonds showed poor reactivity in these reactions.2943 b 29Sa d Thus, isobutane was less reactive than n-butane, and methylcyclohexane less reactive than cyclohexane (cf., lower reactivity of cumene to toluene). In the series of normal alkanes, n-butane reacted faster than n-pentane. n-Undecane was unreactive. These results are inconsistent with a normal free radical autoxidation. The authors used the analogy with arene oxidations to postulate that formation of radical cations by electron transfer from the alkane to Co(III) was a critical factor ... 

Both the styrene monomer and the neutral dimer can trap a migrating positive hole or positive charge from solvent radical-cations (solventt) or related cationic species, which leads to the formation of radical cations, dimer cations, and bonded dimer cations. 

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