Aromatic cation radical

Electron-transfer activation. Time-resolved spectroscopy shows that the activation of the [ArH, PyNO ] complex by the specific irradiation of the CT absorption band results in the formation of transient aromatic cation radical... 

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...

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... 

The facile reversion of the aromatic cation radical by back electron [Jthct in... 

Temporal evolution of aromatic cation radicals to the Wheland intermediate and the nucleophilic adduct in charge-transfer nitration... 

Most importantly, the direct reaction of anisole cation radical with N02 (64) is established by the pseudo-first-order kinetics observed for the disappearance of AN+- in the presence of added N02 (see Fig. 16A). The magnitude of the second-order rate constant of k2 = 1.5x109m i s evaluated in this way is thus consistent with the spectral decay of the freely diffusing aromatic cation radical (AN+<) in Fig. 15. 

Table 3 Second-order rate constants for the spectral decay of aromatic cation radicals following the charge-transfer excitation of [ArH, MeOPyNOj] complexes."...

Table 4 Second-order reactions of aromatic cation radicals with nitrogen dioxide, pyridine or 2,6-lutidine."...

In acetonitrile at 23°C. Aromatic cation radical (ArH + -) generated by charge-transfer excitation of the aromatic EDA complex from MeOPyN02 with added NO at 355-nm or TNM with added pyridine or "2,6-Iutidine at 532 nm. 

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)... 

The simultaneous annihilation of the aromatic cation radical by its triad partners N02 and pyridine, as presented in Scheme 12, is most apparent in the nature of the aromatic product (i.e., stoichiometry) from mesitylene, among all the other aromatic (benzenoid) donors. Thus, the direct incorporation of N02 and pyridine into the aromatic nucleus is shown by the concomitant ring nitration and pyridination of mesitylene (Kim et al., 1993),... 

The ambiphilic reactivity of aromatic cation radicals, as described in Schemes 12 and 13, is particularly subtle in the charge-transfer nitration of toluene and anisole, which afford uniformly high (>95%) yields of only isomeric nitrotoluenes and nitroanisoles, respectively, without the admixture of other types of aromatic byproducts. Accordingly, let us consider how the variations in the isomeric (ortho meta para) product distributions with... 

Fig. 17 Variation of the rate constants for the homolytic (k2) and nucleophilic (k2) annihilation of various aromatic cation radicals with N02 and pyridine, respectively, as a function of the oxidation potential E x (to gauge ArH+ stability).

Since the latter conditions pertain to aromatic nitration solely via the homolytic annihilation of the cation radical in Scheme 16, it follows from the isomeric distributions in (81) that the electrophilic nitrations of the less reactive aromatic donors (toluene, mesitylene, anisole, etc.) also proceed via Scheme 19. If so, why do the electrophilic and charge-transfer pathways diverge when the less reactive aromatic donors are treated with other /V-nitropyridinium reagents, particularly those derived from the electron-rich MeOPy and MePy The conundrum is cleanly resolved in Fig. 17, which shows the rate of homolytic annihilation of aromatic cation radicals by NO, (k2) to be singularly insensitive to cation-radical stability, as evaluated by x. By contrast, the rate of nucleophilic annihilation of ArH+- by pyridine (k2) shows a distinctive downward trend decreasing monotonically from toluene cation radical to anthracene cation radical. Indeed, the... 

Triad formation in Scheme 10 is a two-step process (88) involving the metastable A-nitropyridinyl radical, whereas the adiabatic electron transfer in Scheme 19 is likely to occur irreversibly with the simultaneous cleavage of the N-N02 bond, as in (89). As a result, the nascent pair (Py and N02) in (88) can suffer greater diffusive separation from ArH+- compared with that in (89). If so, the complexation of the aromatic cation radical by pyridine (90), as recently delineated by Reitstoen and Parker (1991) is (kinetically)... 

The electron-transfer mechanism for electrophilic aromatic nitration as presented in Scheme 19 is consistent with the CIDNP observation in related systems, in which the life-time of the radical pair [cf. (87)] is of particular concern (Kaptein, 1975 Clemens et al., 1984, 1985 Keumi et al., 1988 Morkovnik, 1988 Olah et al., 1989 Johnston et al., 1991 Ridd, 1991 Rudakov and Lobachev, 1991). As such, other types of experimental evidence for aromatic cation radicals as intermediates in electrophilic aromatic nitration are to be found only when there is significant competition from rate processes on the timescale of r<10 los. For example, the characteristic C-C bond scission of labile cation radicals is observed only during the electrophilic nitration of aromatic donors such as the dianthracenes and bicumene analogues which produce ArH+- with fragmentation rates of kf> 1010s-1 (Kim et al., 1992a,b). 

The triethyloxonium cation is an effective oxidant for the production of aromatic cation-radicals, but only as the hexachloroantimonate salt. The analogous EtjO BE or Bu4N SbClg cannot be used to prepare any cation-radical. This is consistent with the previously described function of SbClg as an oxidant. The slow release of SbClj forms the basis for the efficacy of EtjO SbClg. Kochi s group describes this slow release as follows (Rathore et al. 1998a) ... 

As observed, aromatic hydrocarbons gave products of protonation on dissolution in hydrofluoric acid. Oxidation into aromatic cation-radicals did not take place (Kon and Blois 1958). Trifluoro-acetic acid is able to transform aromatics into cation-radicals. This acid is considered a middle-powered one-electron oxidant (Eberson and Radnor 1991). Its oxidative ability can be enhanced in the presence of lead tetraacetate. This mixture, however, should be used carefully to avoid oxidation deeper than the one-electron removal. Thus, oxidation of 1,2-phenylenediamine by the system Pb(OCOCH3)4 -I- CE3COOH -P CH2CI2 leads to the formation of either primary or secondary cation-radicals. The primary product is the cation radical of initial phenylenediamine, whereas the secondary product is the cation radical of dihydrophenazine (Omelka et al. 2001). Sulfuric acid is also used as an one-electron oxidant, especially for aromatic hydrocarbons. In this case, generation of cation radicals proceeds simultaneously with the hydrocarbon protonation and sulfonation (Weissmann et al. 1957). 

There are many testimonies for the cation-radical formation during electrophilic aromatic nitration. Positional selectivities are in line with spin-density distributions. In principle, the attack of N02 radical is probably at the position of the aromatic cation-radical, which bears the maximal spin density. 

Nitrous acid catalysis also takes place in the nitration of such compounds (naphthalene) that are unable to undergo nitrosation on the given conditions or whose nitrosation proceeds slower than nitration. As accepted, the nitrosonium ion is formed from HNOj in acid media. The nitrosonium ion oxidizes an aromatic substrate into a cation-radical and transforms into nitric oxide. The latter reduces nitronium cation to nitrogen dioxide that gives a a-complex with the aromatic cation-radical ... 

Aromatic cation-radicals can also react with NOj", giving nitro compounds. Such reactions proceed either with a preliminary prepared cation-radical or starting from nncharged componnd if iodine and silver nitrite are added. As for mechanisms, two of them seem feasible—first, single electron transfer from the nitrite ion to a cation-radical and second, nitration of ArH with the NOj radical. This radical is quantitatively formed when iodine oxidizes silver nitrite in carbon tetrachloride (Neelmeyer 1904). 

In l,l,l,3,3,3-hexafluoropropan-2-ol, the reaction of l,4-dimethoxy-2,3-dimethylbenzene with a deficit of nitrogen dioxide gives a high concentration of the aromatic cation-radical, which lives long enough and can be detected spectroscopically. In the presence of excessive amounts of N02, this cation-radical decays rapidly giving the 5-nitro derivative of the starting compound (Eberson etal. 1996). 

Gas-phase nitration is important from theoretical and practical points of view. In solution, the solvation of the small nitronium ion should exceed that of the large aromatic cation-radical, and hence electron transfer should be less probable. In the gas-phase process, the solvation is absent and only inner reorganization energy remains significant. 

The ion-radical mechanism is characteristic in cases of substrates, which are ready for one-electron oxidation and capable to give stable cation-radicals in appropriate solvents. As the cited examples show, such a mechanism can really be revealed. However, very rapid transformations of aromatic cation-radicals can mask the ion-radical nature of many other reactions and create an illusion of their nonradical character. At the same time, the ion-radical mechanism demands its own approaches for farther optimization of commercially important cases of nitration. This mechanism deserves onr continned attention. 

Each cation-radical has a positive charge and an unpaired electron. In the simultaneous presence of an anion (A ) and a free radical (R ), both of them can attack a cation-radical in a competitive manner. These two possibilities are shown for the example of an aromatic cation-radical (ArH+ ) ... 

The driving force of the reaction between a radical and an aromatic cation-radical is based on their mutual affinity because both species possess unpaired electrons. There is no coulombic attraction and no coulombic repulsion in this case. The reaction between an anion and an aromatic cation-radical involves electrostatic attraction as the driving force. It necessarily proceeds through a contact (intimate) ion pair Ar+, A. It is obvious that disintegration of the mentioned ion pair has to retard or even prevent ArA formation. 

A few examples in which aromatic cation radicals have been isolated as crystalline salts, actually consist of mixed valence units. For example, crystal structure analysis showed that the naphthalene radical cation (NAP) + forms a mixed valence dimer (NAPy4 in which the two components are arranged face to face in n-stacks with an interplanar separation significantly closer than van der Waals contacts. Such an intermolecular organization arises from the sponta-... 

Heteromolecular -complexes of aromatic cation radicals have been designed from the complexation of aromatic cation radical with a different neutral aromatic donor but such ionic K-heterocomplexes were observed as metastable species in the gas phase or in freon matrices57 58. 

Aromatic hydrocarbons gave products of protonation on dissolving in hydrofluoric acid. Oxidation in aromatic cation radicals did not take place (Kon Blois 1958). Triflu-oroacetic acid is an effective one-electron oxidant (Eberson Radner 1991). Meanwhile, sulfuric acid caused not only dissolution and protonation, but also one-electron oxidation of aromatic hydrocarbons. Sulfonation, naturally, proceeded too (Weissmann et al. 1957). 

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