Unstable radical anions

The combination of electrochemical and EPR studies can provide valuable information about unstable S-N radical species. A classic early experiment involved the electrochemical reduction of S4N4 to the anion radical [S4N4] , which was characterized by a nine-line EPR spectrum. The decay of the radical anion was shown by a combination of EPR and... 

The reaction with nitrite proceeds smoothly and with relatively high yields of the corresponding nitroarene (see Sec. 10.6). Obviously a major part of the driving force of this reaction is the formation of a stable, i. e., an energetically favorable, radical, nitrogen dioxide. With the hydroxide ion — a much stronger nucleophile than the nitrite ion — the reaction is expected to produce very unstable radicals, the hydroxy radical OH and the oxygen radical anion O, from the diazohydroxide (Ar - N2 — OH) and the diazoate (Ar-N20 ) respectively. Consequently, dediazoniation in alkaline aqueous solution does not follow the simple Scheme 8-41 with Yn = OH, but instead involves diazoanhydrides (Ar — N2 —O —N2 —Ar) as intermediates (see Sec. 8.8). 

The radical anions of polysilanes have been studied as unique a-conjugated radical ion species.2,4,56,57 Although many radical anions of polysilanes have been reported, most of them are unstable species that can only be observed at low temperatures. Quite recently, we found that the radical anions of longer ladder polysilanes are persistent at room temperature.58... 

Nitrobenzyl chlorides are also reduced by microsomes through one-electron reduction mechanism. Moreno et al. [47] suggested that p- and o-nitrobenzyl chlorides are reduced by rat hepatic microsomes to unstable radical anions, which are decomposed to form benzyl radicals under anaerobic conditions. However, in the presence of dioxygen the radical anions of these compounds participate in futile redox cycling yielding superoxide (Figure 24.2). In contrast to p- and o-nitrobenzyl chlorides, m-nitrobenzyl chloride was reduced by microsomes to a relatively stable m-nitrobenzyl radical anion. 

The redox chemistry of [4]radialenes shows similarities as well as differences with respect to [3]radialenes (see elsewhere1 for a more detailed comparison). The simplest [4]radialene for which a redox chemistry in solution is known appears to be octa-methyl[4]radialene (94). It has been converted into the radical anion 94 (with potassium, [2.2.2]cryptand, THF, 200 K) and into the radical cation 94 + (with AICI3/CH2CI2, 180 K)82. Both species are kinetically unstable, but the radical cation is less stable than the radical anion and disappears even at 180 K within 2 hours, probably by polymerization. For the success of the oxidation of 94 with the one-electron transfer system... 

The redox chemistry of stannyl metalcarbonyl clusters has been studied in one case177. CV of the cluster PhSnCo3(CO)i2 shows an irreversible reduction peak —0.74 V vs Ag wire, at 100 mV s 1. Faster scans were not reported. It is concluded that the radical anion [PhSnCo3(CO)i2r is unstable, in contrast to the silicon cluster PhSiCo3(CO)n which gives a stable radical anion (reported lifetime of 2.3 s) at —0.26 V (200 mV s 1). 

A further group of nonbenzenoid aromatics is the series of odd-membered cations and anions such as cycloprope-nium (14) and tropylium cations (15) as well as cyclopentadienyl (16) and cyclononatetracenyl anions (17). Regarding the arguments for the properties of Hiickel-like 4 + 2 jr-systems, all these molecules should be energetically stabilized. Obviously, this is not fulfilled in all cases. The tropylium cation (15) can be reduced in a one-electron step to the tropyl radical even at A = +0.06 V vs. SCE [85, 86]. The radical is unstable and rapidly dimerizes to bitropyl. The hep-taphenyl tropylium radical is stable on the voltammetric timescale, but decays... 

When X = CN, a stable anion-radical is obtained as evident from the ESR spectrum. When X = NO2, the ESR signal cannot be observed. This anion-radical is unstable and gives rise to the dimer (isolated Scheme 3.5). 

As stated above, most radical anions encountered in the literature result from the addition of an electron to a formal n orbital. This is generally true because n orbitals are generally more accessible (energetically) than a orbitals, which are more localized and more strongly perturbed by the addition of an electron. However, from an electronic viewpoint there is no reason, a priori, to expect that all a bonds will be unstable to electron attachment. From simple perturbation MO theory, the interaction of a radical... 

However, its isoelectronic unsymmetrical counterpart, 7r-cyclopen-tadienyl-7r-cycloheptatrienylchromium(0) can be reduced to a stable anion radical (41). The orbital occupied by the unpaired electron has dominant ligand tt character. It appears to consist of a combination of the antibonding e2 MO s of the cyclopentadienyl (Cp) and cyclohepta-trienyl (Tr) 7r-systems with about one-third Cp and two-thirds Tr, respectively (41). Reduction of 7r-cyclopentadienyl-7r-methylcyclohepta-trienylchromium leads to an unstable radical anion that decomposes by loss of the seven-membered ligand (41). 

The reactions most commonly involved in flavin redox chemistry are shown in Equations 1.15-1.17. One-electron reduction of the flavin (Eq. 1.15) produces a relatively stable radical anion. Protonation of the radical anion produces an unstable neutral radical (Eq. 1.16), which will be rapidly reduced by another electron (Eq. 1.17) to give the flavohydroquinone anion. 

Before getting into the subject, classifying in accordance with their process and mechanism the electrolytic initiation reactions which have appeared in the literature will afford the reader a better understanding. The reactions are classified firstly into two types cathodic and anodic. For the cathodic reaction, generation of free-radical and radical-anion, and formation of unstable monomer and active catalyst are visualized from the corresponding references. 

Generation of free-radicals by Kolbe s reaction is well-known [Eq. (10)]. Formation of a radical-cation of monomer [Eq. (11)] has never been been proved and is only a possible conjecture from the right reverse consideration of the radical-anion formation at the cathode [Eq. (6)], although the perchlorate anion has actually been found to yield an unstable perchlorate free-radical by discharge at the anode. Nor is it certain that the monomer radical-cation is formed by direct discharge from the anode [Eq. (12)]. The ring-opening polymerization of oxides, caprolactam and isocyanides is also initiated on the electrode. A few examples of condensation polymerization have developed recently, like Eq. (7) and (12). Details of this work are described in the appropriate section. 

Radical anions derived from 2,5-diformylthieno[3,2-6]thiophene (39) as well as (40) were studied in connection with the conformational analysis of heteroaromatic carbonyl compounds. Different ESR signals were given by distinct rotational isomers. Information on interconversion of the rotamers could not be obtained since the radicals were unstable at the temperatures necessary for interconversion. On the other hand, ketyl radicals derived from ketones (41) and (42) are relatively more stable at the temperatures needed for the study of the conformational mobility in these systems. The ESR spectra of the bis-thienothienyl ketyls from (41) and (42) at room temperature show that the unpaired electron is coupled to three pairs of equivalent protons. Both spectra exhibited a certain amount of asymmetry, which was enhanced by lowering the temperature. At -10°C the highfield part of the spectra split into new lines arising from two species which have similar hyperfine splitting, but different g factors. These have been identified as the rotational isomers of the radicals. The two preferred conformations are cis-trans and trans-trans. An examination... 

Other examples of the use of electron acceptors whose ion radicals are unstable with respect to fragmentation to an anion and a radical capable of initiation of polymerization were provided by Eaton (63,101,102). It was shown that -nitrobenzyl halides could be used in dye-sensitized compositions of semiconductor pigments such as Ti02 and CdS to induce polymerization of vinyl monomers using visible light. The sequence of events is outlined in eqs. 46-49 and Scheme 6 ... 

The structure and energy of a series of ions generated from penta-cyclo[3.3.1.13,7.01 3.05 7]decane (7) has been explored by using HF, MP2 and DFT methods to estimate enthalpy changes of isodesmic disproportionation reactions and by considering the reorganization of frontier orbitals as a consequence of addition or removal of electrons from the neutral molecule.8 The dication (72+), which is considered to be Three-dimensionally homoaromatic , is stable relative to a localized structure with similar features but is highly unstable compared to the radical cation (7+i)- hi contrast, the dianion (72 ) is unstable relative to the radical anion (T) and shows no evidence of electron delocalization. 

Photooxidation and y-radiolysis agree in their products reasonably well as long as the base is damaged. The reason for this may be that in the photooxidation a dCyd radical cation / menadione radical anion pair is the first intermediate. The reaction of the dCyd radical cation with water could give rise to OH-adducts (as in radiolysis). The most important product, however, is dUrd, that is, deaminated dCyd (Table 10.16) which is not, and cannot be, formed upon radiolysis. The mechanism of its formation is not discussed in the original paper, but it is not unlikely that it results from an unstable intermediate (adduct ) formed in the reaction of the dCyd radical cation / menadione radical anion pair in the solvent cage. 

To utilize the strong reducing power of the 3(da po) excited states of the platinum and iridium dimers, the nonproductive back electron transfer reactions need to be inhibited. An effective way to accomplish this is to use acceptors that are thermally unstable after the initial electron transfer. Reduction of alkyl halides has been shown to lead to short-lived radical anions RXT, which rapidly decompose to give R- and X (k... 

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