Cations toluene radical

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... 

In acetonitrile (AN), the toluene cation-radical has high thermodynamic acidity, its pK is between -9 and -13 (Nicholas and Arnold 1982). In the same solvent (AN), neutral toluene has... 

To elevate p-selectivity in nitration of toluene is another important task. Commercial production of p-nitrotoluene up to now leads with twofold amount to the unwanted o-isomer. This stems from the statistical percentage of o m p nitration (63 3 34). Delaude et al. (1993) enumerate such a relative distribution of the unpaired electron densities in the toluene cation-radical—ipso 1/3, ortho 1/12, meta 1/12, and para 1/3. As seen, the para position is the one favored for nitration by the attack of NO (or NO2 ) radical. A procednre was described (Delande et al. 1993) that used montmorillonite clay supported copper (cupric) nitrate (claycop) in the presence of acetic anhydride (to remove excess humidity) and with carbon tetrachloride as a medinm, at room temperature. Nitrotoluene was isolated almost quantitatively with 23 1 76 ratio of ortho/meta/para mononitrotoluene. 

Formation of radicals having a lower energy than that of the starting cation radicals is obviously favorable for their deprotonation. The cation radicals of toluene and other alkylbenzenes are illustrative examples. As shown (Sehested Holcman 1978), these cation radicals lose protons even in very acidic aqueous solutions. The deprotonation rate does not, in general, depend on the medium acidity. In acetonitrile (AN), the toluene cation radical has high thermodynamic acidity, its pKa is between 9 and -13 (Nicholas ... 

Medium acidity is not essential for the deprotonation of the toluene cation radical, but medium basicity provokes the abstraction of the cation radical proton. Although this effect is obvious, special experiments were undertaken to define it more accurately (Bernstein 1992 and references therein). The studies show that at least three water molecules are re-... 

This reaction scheme is applicable to C H bond activation by strong electron acceptors. For example, the formation of benzyl acetate from toluene using Co111 as the oxidant has been shown to occur via the toluene cation radical [41] ... 

Aryl and aliphatic amines are perhaps the best known D molecules primarily due to the availability of the nonbonded electrons on nitrogen for PET processes. The simplest bond cleavage reaction for amine cation-radicals is heterolytic C—H cleavage or deprotonation. It is well known that the acidity of protons on a species after loss of an electron, i.e., after one-electron oxidation, is greatly enhanced [7]. For example, the pKa of the benzylic protons in the toluene cation-radical have been estimated to be —10 [7] compared to pKa value of 35 before oxidation. The process of proton loss can be extremely fast, on the time scale of picoseconds [8]. 

Ledwith and Russell (1974b) have found that chlorination of benzene, toluene and other aromatic molecules is easily achieved in aqueous acetonitrile containing sodium peroxydisulfate and copper(II) chloride. Toluene, for example, gives no benzyl chloride but a mixture of chlorotoluenes (58% o-, 4% m-, and 38% p-) consistent with the spin distribution in the toluene cation radical. The amount of copper chloride used can be catalytic provided another source of chloride ion (LiCl) is added. Reaction is attributed to the very fast transfer of chlorine atom from copper(II) chloride to the cation radical (132) the metal halide is thus regarded as a trap for the aromatic cation radical. In the absence of copper(II) chloride, reactions of toluene with peroxydisulfate ion and chloride ion give... 

Scheme VIILI2. Transformations of the toluene cation radical.

The cation—radical intermediate loses a proton to become, in this case, a benzyl radical. The relative rate of attack (via electron transfer) on an aromatic aldehyde with respect to a corresponding methylarene is a function of the ionization potentials (8.8 eV for toluene, 9.5 eV for benzaldehyde) it is much... 

Cyclizations of dihydroxystilbene 256 using 4 mol % of chiral ruthenium complexes under photolytic conditions were investigated by Katsuki et al. (Scheme 65) [167]. Coordination of alcohols/phenols to Ru(IV) species generates a cation radical with concomitant reduction of metal to Ru(III). Cycli-zation of this oxygen radical followed by another cyclization provides the product 257. Catalyst 259 provided 81% ee of the product in chlorobenzene solvent. Optimization of the solvent polarity led to a mixture of toluene and f-butanol in 2 3 ratio as the ideal solvent. Substituents on the phenyl rings led to a decrease in selectivity. Low yields were due to the by-product 258. 

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

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... 

Shono et al. (1979) recommend the use of thioanisole as a catalyst that allows lowering the electrode potential in the oxidation of the secondary alcohols into ketones. The cation-radical of thioanisole is generated at a potential of up to +1.5 V in acetonitrile containing pyridine (Py) and a secondary alcohol. (The background electrolyte was tetraethylammonium p-toluene sulfonate.) Thioanisole is recovered and, therefore, a ratio of RXR )CHOH PhSMe = 1 0.2 is sufficient. The yield of ketones depends on the nature of the alcohol and varies from 70 to 100%. 

Turning from the intramolecular process to the intermolecular ones, we now extend our comparison of the thermal and cation-radical cyclizations. It is also interesting to take sonication into account as a route to initiate cyclizations. The reaction between 2-butenal A,A-dimethylhydrazone (a diene) and 5-hydroxy-l,4-naphthoquinone (a dienophile) gives such an opportunity. In toluene, at 20°C, the reaction follows as depicted in Scheme 7.28 (Nebois et al. 1996). 

One-electron oxidation of toluene results in the formation of a cation radical in which the donor effect of the methyl group stabilizes the unit positive charge. Furthermore, the proton abstraction from this stabilized cation radical leads to the conjugate base, namely, the benzyl radical. This radical also belongs to the it type. Hence, there is resonance stabilization in the benzyl radical. This stabilization is greater in the benzyl radical than in the tt cation radical of toluene. As a result, the proton expulsion appears to be a favorable reaction, and the acid-base equilibrium is shifted to the right. This is the main cause of the acidylation effects that the one-electron oxidation brings. 

Sometimes, a transition metal salt is deliberately added to a mixture of a substrate and a persulfate salt (Dobson et al. 1986). The free or metal-coordinated sulfate anion radical reacts with an organic substrate, giving rise to a substrate cation radical (Minisci et al. 1983 Itahara et al. 1988 Telo Vieira 1997). One typical example is the reaction between toluene and SOJ in Scheme 1-96 ... 

If the sulfate anion radical is bound to the surface of a catalyst (sulfated zirconia), it is capable of generating the cation radicals of benzene and toluene (Timoshok et al. 

The reversible reaction of chloroiron(III) octaethylisobac-teriochlorin (or the Br, NCS, or PhS analog) with r-acid ligands such as carbon monoxide, t-butyl isocyanide or PF3 in toluene solution produces EPR spectra at 77 K that are indicative of a r-cation radical (equation 16) ... 

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