Aromatic Radical Ions

Generally, highly reactive aromatic ion-radicals can undergo a variety of other uni- and bi-molecular processes that compete effectively with back ET and lead to an overall transformation, some examples of which are illustrated in Chart 6 (with Ar representing an aryl group) [59]. 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

Productive bimolecular reactions of the ion radicals in the contact ion pair can effectively compete with the back electron transfer if either the cation radical or the anion radical undergoes a rapid reaction with an additive that is present during electron-transfer activation. For example, the [D, A] complex of an arene donor with nitrosonium cation exists in the equilibrium with a low steady-state concentration of the radical pair, which persists indefinitely. However, the introduction of oxygen rapidly oxidizes even small amounts of nitric oxide to compete with back electron transfer and thus successfully effects aromatic nitration80 (Scheme 16). 

The identical stoichiometries and the color changes that are observed in thermal and photochemical aromatic osmylations point to the ion-radical pair Ar+, OsO T as the seminal intermediate in both activation processes. It is similarly possible that the osmylation of olefinic donors may proceed via the same types of reactive intermediates as delineated for the aromatic osmylation. 

The explanation for the dichotomy between aromatic nitration versus de-alkylative oxidation in equation (84) lies in the dynamics of ion-radical triad (which is predictably modulated by solvent polarity and added inert salt). For example, the nonpolar dichloromethane favors aromatic nitration via a radical-pair collapse of ArH+, NO, 239 i.e.,... 

The formation of the Wheland intermediate from the ion-radical pair as the critical reactive intermediate is common in both nitration and nitrosation processes. However, the contrasting reactivity trend in various nitrosation reactions with NO + (as well as the observation of substantial kinetic deuterium isotope effects) is ascribed to a rate-limiting deprotonation of the reversibly formed Wheland intermediate. In the case of aromatic nitration with NO, deprotonation is fast and occurs with no kinetic (deuterium) isotope effect. However, the nitrosoarenes (unlike their nitro counterparts) are excellent electron donors as judged by their low oxidation potentials as compared to parent arene.246 As a result, nitrosoarenes are also much better Bronsted bases249 than the corresponding nitro derivatives, and this marked distinction readily accounts for the large differentiation in the deprotonation rates of their respective conjugate acids (i.e., Wheland intermediates). 

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

A general theory of the aromatic hydrocarbon radical cation and anion annihilation reactions has been forwarded by G. J. Hoytink 210> which in particular deals with a resonance or a non-resonance electron transfer mechanism leading to excited singlet or triplet states. The radical ion chemiluminescence reactions of naphthalene, anthracene, and tetracene are used as examples. 

Metal ketyls are ion-radicals analogous to semiquinone ion radicals and may be considered either oxygen or carbon free radicals. They are readily prepared by treating aromatic ketones with alkali metals in dry ether, benzene, or liquid ammonia under an inert atmosphere.124 125 Benzophenone potassium has been shown to be paramagnetic in the solid state.126... 

A reaction analogous to the formation of metal ketyls is the formation of negative ion-radicals not only from aromatic nitro compounds but also from aromatic hydrocarbons like naphthalene. These substances are highly colored and exhibit paramagnetic resonance absorption.128... 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

Fig. 5 (A) Typical time-resolved picosecond absorption spectrum following the charge-transfer excitation of tropylium EDA complexes with arenes (anthracene-9-carbaldehyde) showing the bleaching (negative absorbance) of the charge-transfer absorption band and the growth of the aromatic cation radical. (B) Temporal evolution of ArH+- monitored at Amax. The inset shows the first-order plot of the ion...

The recent time-resolved spectroscopic studies described above (Sections 2 and 3) identify the charge-transfer excitation (/n cr) of aromatic EDA complexes with various types of acceptors (A) to their ion-radical pairs [ArH+-,A ] (Mataga, 1984 Hilinski et al., 1984 Jones, 1988). Such electronic transitions in weak EDA complexes, like those of the halogen acceptors, are mainly associated with the excited states, such as in (32), since the variations in the ground state are minor owing to formation constants K that are not strongly dependent on the arene donor (Briegleb, 1961, pp. 106 ff.). 

Charge-transfer activation of aromatic EDA complexes with N-nitropyridi-nium ion for the spontaneous formation of the aromatic cation radical in the reactive triad... 

AN+- (Reitstoen and Parker, 1991). In other words, the triad of reactive fragments produced in (63) in the charge-transfer excitation of the EDA complex with A-nitropyridinium ion is susceptible to mutual (pairwise) annihilations leading to the Wheland intermediate W and the nucleophilic adduct N (Scheme 12), so that the observed second-order rate constant ku for the spectral decay of ArH+- in Table 3 actually represents a composite of k2 and k2. A similar competition between the homolytic and nucleophilic reactivity of aromatic cation radicals is observed in the reaction triad (55)... 

Intra-complex TMS+ abstraction by F yields the trimethylenemethane radical anion 33. Similarly, a number of other (mostly aromatic) distonic radical anions have been generated. Using the same approach, several other highly unsaturated distonic negative ions, such as the benzyne radical anions, were also studied164. 

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