Cation-radicals, heteroaromatic

The main direction of decomposition of the cation radical formed at the first stage is a deprotonation leading to a neutral free-radical particle which later oxidizes into a heteroaromatized cation (Scheme 3.90, pathway a). The opposite pathway is rarely observed, i.e., oxidation of the cation radical into a dication foregoing the deprotonation stages (Scheme 3.90, pathway b) [250]. When single-electron transfer occurs along with a cation-radical particle, aromatiza-tion is also observed (Scheme 3.90, pathway c) [292] at the expense of the elimination of a hydrogen atom in the solvent cell , i.e., where repeated collisions of two particles take place. A fourth variation is possible and involves the decomposition of the cation radical and the formation of molecular... 

For the purpose of this review the term heteroaromatic is applied to 7r-radical species when they may be regarded as arising from aromatic heterocyclic molecules or ions by addition or removal of an odd number (usually one) of electrons. Thus, entities such as the anion- and cation-radicals of pyridine (1 and 2) are clearly heteroaromatic. The dilemma whether or not to regard thianthrene (3) as aromatic (its central ring possessing eight electrons) does not arise for the thianthrene cation radical (4) it is heteroaromatic on the grounds that it may formally arise by one-electron reduction of the aromatic thianthrenium dication (5). 

Such polycyclic aromatic hydrocarbons as anthracene or heteroaromatics as acridine, phenazine and 2,4,5-triphenyl oxazole act as Jt-donors for the Jt-acceptors AN and alkyl methacrylates [50-53]. Again, the interaction of the donor excited states with vinyl monomers leads to exciplex formation. But, the rate constants (k ) of these quenching processess are low compared to other quenching reactions (see Table 1). The assumed electron transfer character is supported by the influence of the donor reduction potential on the k value (see Table 1), and the detection of the monomer cation radicals with the anthracene-MMA system. Then, the ion radicals initiate the polymerization, the detailed mechanism of which is unsolved,... 

The unsubstituted TTE 16 is nonaromatic, in the Hiickel sense. Oxidation to the cation radical and dication occurs sequentially and reversibly at relatively low potentials ( 1/2 = 0.37 V and 1/2 = 0.67 V vs. SCE in 147t-electron system. In contrast to the neutral TTF, both the cation radical and dication are aromatic as a result of the brt-electron heteroaromaticity of the 1,3-dithiolium cation. The radical cation and dication can be isolated as stable crystalline compounds due to the effective resonance stabilization of the aromatic dithiolium and, to a minor extent, the polarizable sulfur atoms <1996SR1, 1997SL1211, 1999PS99, 2001AGE1372>. 

Russell and co-workers have obtained cation-radicals in the 1,2-dithiete system. Such radicals are cyclic, conjugated within the heterocycle, and possess (4 -P 1) electrons, with = 1 they are consequently heteroaromatic witliin the definition given in the introduction to Part I. 3,4-Dimethyl-l,2-dithiete cation radical 89 (R = R = Me) was obtained by treatment of acetoin in sulfuric acid with sulfide ion. The radical is persistent at ambient temperature and unaffected by oxygen. This evident stability, and its formation from open-chain precursors to the exclusion of acyclic possibilities such as 90, whose oxygen equivalent exists, implies that aromatic stabilization of 89 and similar radicals is a matter of fact and not merely definition. 

Thianthrene cation-radical 178 is one of the most studied of heteroaromatic radicals. Its individual chemistry has proved very rich additionally, it is a persistent, readily recognized radical which has often been used as a well-behaved radical in systems where the prime interest has been the physical chemistry rather than the substrate itself. This chapter cannot deal fully with the chemistry of 178 however, aspects have been reviewed on several and recent occasions. " ... 

The most easily oxidized molecules are those containing 7T-eIectrons and heteroatoms with unshared pairs. Consequently, most of our treatment concerns aromatic and heteroaromatic cation radicals. Cation radicals of saturated molecules are, of course, easily but fleetingly made in the mass spectrometer, and even the anodic oxidation of alkanes is achievable and describable in terms of cation radical chemistry (Clark et al., 1973). 

Cation radicals exhibit characteristic esr spectra. Many of the aromatic and heteroaromatic cation radicals are colored and therefore have characteristic electronic absorption spectra in the visible region. Detailed descriptions of these properties are outside the scope of this account. Changes in esr and absorption spectra are used to follow other physical and chemical changes of the cation radicals. Among the physical phenomena of interest to us are reversible dimerizations and disproportionation. 

It is understandable that electron-rich molecules, i.e., aromatics, heteroaromatics and alkenes, are among the most well explored in present cation radical chemistry, since such molecules are relatively easily oxidized. Some aromatics and heteroaromatics, furthermore, give cation radicals which are so... 

Direct evidence for the involvement of solution species in these redox reactions was reported at about this same time by our group. We found that under certain solution conditions, the molecular radical cations (M ) of some divalent metal porphyrins (e.g., Ni octaethylporphyrin (NiOEP), ZnOEP, and VOOEP) formed by this electrochemical process could be observed in positive-ion ES mass spectra.Certain other easy-to-oxidize species like polyaromatic hydrocarbons (PAHs), aromatic amines, and heteroaromatics were also oxidized at the emitter electrode and observed as cationic radicals. Molecular ions formed by loss of an electron had not been observed in ES mass spectra prior to those reports. Our work served to illustrate that analyte species, under the appropriate operational conditions, could be directly involved in the redox reactions in the metal spray capillary and that the products of their reactions could be observed in the gas phase. 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

One-electron oxidation of organoselenium and organotellurium compounds results in initial formation of a radical cation (equations (19) and (20)). The eventual fate of the radical cation depends on several variables, but is typically a Se(lV) or Te(lV) compound. The scope of this section will be the one-electron oxidation of selenides and tellurides that are not contained in a heteroaromatic compound, and ones in which the Se and Te are bonded to two carbons, rather than to other heteroatoms. Tellurium- and selenium-containing electron donor molecules have been reviewed. 

Although difluoroenoxysilanes are typical nucleophiles, they can also react with other nucleophiles via their oxidation into radical cations. The oxidative homocoupling and the cross-coupling with heteroaromatics and alcohols proceeds very well in the presence of Cu triflate as the oxidizing reagent (Figure 2.24). " ... 

Using Saveanfs terminology, such a process is called redox catalysis in its proper meaning, while Shono formed the expression homomediatory system . This type of mechanism was already schematically presented in the case of an oxidation in Eqs. (2) to (4). To this category of redox catalysts belong, for example, the radical anions and cations of aromatic and heteroaromatic compounds and some reactions of triaryl amine radical cations. 

Heteroatoms also influence the charge carried by heteroaromatic radicals, just as they influence that of diamagnetic aromatic species. Thus, addition of an electron to the aromatic cations 15 and 16 yields radicals which are, respectively, neutral and cationic, albeit isoelectron ic with benzene anion-radicals. 

Similar problems arise with quinolinium salts as both C2 and C4 are susceptible to radical addition. Where either of these positions is blocked by a substituent, reactions often proceed in high yield. For example, Minisci et al. have described a useful method of formylating heteroaromatic bases which begins with an iron(II) promoted reduction of /-butyl hydroperoxide <86JOC536>. The /-butyloxy radical thus produced abstracts a hydrogen atom from 1,3,5-trioxane 34 giving intermediate 33. Union of 33 and the quinolinium salt 35 next produces radical cation 36, which is oxidised to 32 by iron(III) (Scheme 14). 

Tiecco et al. found that 4-acylp5Tidines were prone to ipso-substitution reactions with 3°— alkyl radical intermediates such as 1-adamantyl and bicyclo[2,2,2]octyl radical <76CC329>. They reasoned that the reaction began with an electron transfer from S04 to the heteroaromatic base leading to radical cation 87. Addition of Ad to C4 then gave acetate 88,... 

Illustrative examples of the ionization potential of heteroaromatics and of the structure of the corresponding radical cations are reported in Table 1 [5, 16 24]. 

Table 1. Ionization potential and structure of the radical cations of some representative heteroaromatic compounds.

---