Other anion radicals

Among other anion radicals studied are those of halouracils (Bansal et al., 1972 Bhatia and Schuler, 1973b). In these cases the electron adducts can either protonate on carbon or eliminate a halide ion. It has been found that the 5-fluorouracil anion radical proto-nates on carbon, the 5-bromo derivative eliminates bromide, whereas the 5-chloro radical undergoes both processes (83) with equal rates at pH 5-2 (Bhatia and Schuler, 1973b). The radical formed upon... 

Other anion-radicals containing the thiophene heterocycle in an otherwise hydrocarbon molecular environment and whose ESR spectra have been analyzed are 110 (for which hyperfine splittings are given in gauss), the anionic counterpart of 94, and the anion-radical of 963 2,343. thieno[2,3-ZjJthiophene (111) failed to give a persistent radical by alkali metal reduction. 

Paramagnetic molecules can also be generated inside the zeolite cavities by using ionizing y- and X-ray radiation. The most studied molecules are Oj and CI2, but other anion radicals such as SO2 and COj can also be generated inside zeolites. The super-oxide ion O2 is obtained by interaction of O2 with transition metal ions and their complexes in zeolites. While NO is a one electron system, Oy is a one hole system. Thus both molecules have similar ESR spectra with g<2.0023 for NO andg>2.0023 for O2 (see Tables 7 and 8). If the O2 is coordinated to transition metal ions such as Co +, typical hyperfine patterns are seen if the transition metal ion has a nuclear spin 1 0 [97]. 

Sodium naphthalene [25398-08-7J and other aromatic radical anions react with monomers such as styrene by reversible electron transfer to form the corresponding monomer radical anions. Although the equihbtium (eq. 10)... 

Both CSs and CSs were also successfully generated by the fragmentation of ionized 4,5-dioxo-2-thioxo-l,3-dithione (65) and 2-thioxo-l,3-dithiole (66) (90JA3750). Tire three sulfur atoms in the anion and cation radicals were chemically equivalent, suggesting that they take the D h (or C2u) form (67 or 68). On the other hand, under similar conditions, 3-thioxo-1,2-dithiole (69) yielded two isomeric cation radicals the (or 2 ) form and the carbon disulfide 5-sulfide form (70). Ab initio calculations on three electronic states of CS3 at the 6-31G -l-ZPVE level indicated that the C21, form (68) was more stable than the carbon disulfide 5-sulfide form (70) in the neutral (both singlet and triplet states) and the anion radical states, but 68 was less stable than 70 in the radical cation state. 

The reactions of polymeric anions with appropriate azo-compounds or peroxides to form polymeric initiators provide other examples of anion-radical transformation (e.g. Scheme 7. 6). ""7i However, the polymeric azo and peroxy compounds have limited utility in block copolymer synthesis because of the poor efficiency of radical generation from the polymeric initiators (7.5.1). 

A large number of other sensitizers has been investigated for use in photolytic de-diazoniation. The excited states of these compounds (S ) react either by direct electron transfer (Scheme 10-97), as for pyrene, or by reaction with an electron donor with formation of a sensitizer anion radical which then attacks the diazonium ion (Scheme 10-98). An example of the second mechanism is the sensitization of arenedi-azonium ions by semiquinone, formed photolytically from 1,4-benzoquinone (Jir-kovsky et al., 1981). 

The structural requirements of sulphones to react cathodically and to possess specific electrochemical properties are summarized in Scheme 1. In other words, condition (a) means that aromatic sulphones and a unsaturated sulphones are electroactive, i.e., electron transfer to the LUMO leads to the anion radical, but a cleavage reaction (see b) is mainly observed when R S02 " is a fairly good leaving group. Consequently, the two main classes of electroactive sulphones may react differently with aromatic sulphones, ArS02—R, cleavage is strongly favoured, while with unsaturated sulphones ... 

Here, the relative stability of the anion radical confers to the cleavage process a special character. Thus, at a mercury cathode and in organic solvents in the presence of tetraalkylammonium salts, the mechanism is expected16 to be an ECE one in protic media or in the presence of an efficient proton donor, but of EEC type in aprotic solvents. In such a case, simple electron-transfer reactions 9 and 10 have to be associated chemical reactions and other electron transfers (at the level of the first step). Those reactions are shown below in detail ... 

Other similar cyclic structures may present quite unexpected behaviour. Let us give the example of 46, where X is Cl or Br. Such structures are very easily reduced44 (polished platinum microelectrodes are preferred owing to the reaction of mercury with C—X linkages), and the presence of an anion radical of some stability can be demonstrated in the... 

More recently it has become apparent that proton equilibria and hence pH can be equally important in aprotic and other non-aqueous solvents. For example, the addition of a proton donor, such as phenol or water, to dimethylformamide has a marked effect on the i-E curve for the reduction of a polynuclear aromatic hydrocarbon (Peover, 1967). In the absence of a proton donor the curve shows two one-electron reduction waves. The first electron addition is reversible and leads to the formation of the anion radical while the second wave is irreversible owing to rapid abstraction of protons from the solvent by the dicarbanion. 

Ionic polysulfides dissolve in DMF, DMSO, and HMPA to give air-sensitive colored solutions. Chivers and Drummond [88] were the first to identify the blue 83 radical anion as the species responsible for the characteristic absorption at 620 nm of solutions of alkali polysulfides in HMPA and similar systems while numerous previous authors had proposed other anions or even neutral sulfur molecules (for a survey of these publications, see [88]). The blue radical anion is evidently formed by reactions according to Eqs. (5)-(8) since the composition of the dissolved sodium polysulfide could be varied between Na2S3 and NaaS with little impact on the visible absorption spectrum. On cooling the color of these solutions changes via green to yellow due to dimerization of the radicals which have been detected by magnetic measurements, ESR, UV-Vis, infrared and resonance Raman spectra [84, 86, 88, 89] see later. 

On the other hand, in the anion radical of fulvalene and the cation radical of heptafulvalene, the energy gaps between the ground and lowest excited state (which is in both cases doubly degenerate in the Hiickel approximation (Fig. 4)) are predicted to be reasonably large (1.4 and 1.7 eV, respectively), so that these radicals would not suffer a symmetry reduction. 

The calculated bond lengths for the 2 structure of the anion radical of heptafulvalene shown in Fig. 7 indicate that in one of the ring there exists a significant bond fixation to the same extent as that in the neutral heptafulvalene, while in the other ring bond lengths are nearly equalized. The calculated spin densities, presented in Table 3, indicate that the unpaired spin is localized essentially on the latter ring. 

In the anion and cation radicals of fulvalene (XXI) the situation turns out to be quite reversed. Removal of an electron from the neutral molecule to produce the cation radical results in a symmetry reduction (Dj -> C2 ), the stabilization energy being calculated to be 17.8 kcal mole . On the other hand, addition of an electron to form the anion radical leaves the molecular symmetry unaffected. 

Inspection of Fig. 7 reveals that in the cation radical of XXI a marked bond fixation exists in one of the rings, while bond lengths are nearly equalized in the other ring, on which the unpaired spin is delocalized (Table 3). On the other hand, in the anion radical of XXI, there is a moderate bond fixation in both rings, and the unpaired spin is delocalized over the entire molecule. 

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

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. 

The negative ClogP term shows that highly hydrophilic molecules for this data set would present better inhibitory activities against topo I. Two compounds (Ri = R3 = R4 = R5 = H, R2 = NO2 and Ri = R3 = R4 = R5 = H, R2 = F) in Table 4 for the development of QSAR Eq. 6 were deemed to be outliers on the basis of their deviation (>2s). The outlier (Ri = R3 = R4 = R5 = H, R2 = NO2) is much more active than expected, by three times the standard deviation. This may be due to the formation of nitro anion radicals that interact with DNA [48]. The other derivative (Ri = R3 = R4 = R5 = H, R2 = F) is... 

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