P-Xylene radical cations

Conversion increases from toluene to p-xylene to p-methoxytoluene due to the electron-donating effects of the substituents. The increased number of electron-donating substituents creates a positive charge on the reaction center and activates the ring for increased oxidation rates. This effect is in agreement with the radical cation oxidation mechanism proposed by Som-rani et al. (1995). Electron density on the reaction center was shown to play an important role in determining oxidation rates of toluenes. 

Photochemical electron transfer reactions of electron donor-acceptor pairs in polar solvents provide a convenient and effective method for the generation of radical cations which can be trapped by complex metal hydrides. One of the most effective systems is based on irradiation of a solution of substrate, sodium borohydride and 1,4- or 1,3-dicyanobenzene. A range of bi- and poly-cyclic aromatic hydrocarbons has been converted into the dihydro derivatives in this way. An especially important aspect of this route to dihydroaromatic compounds is that it may give access to products which are regioisomeric with the standard Birch reduction products. Thus, o-xylene is converted into the 1,4-dihydro product (229) rather than the normal 3,6-dihydro isomer (228). The m- and p-xylenes are similarly reduced to (230) and (231), respectively. ... 

The importance of stereoelectronic effects has been assessed quantitatively by comparing the CH3/CH2CH3 and CH3/CH2tBu reactivity ratios (statistically corrected) in the radical cations of p-ethyltoluene (ETT), p-neopentyltoluene (NPT), 5,6-dimethylindane (DMI), and 2,2,5,6-tetramethylindane (TMI) (Scheme 35) [148], In the two indane derivatives the methylene groups in positions 1 and 3 bear a substituent which resembles an ethyl and a t-butyl group, respectively. In the p-xylene series, on going from ETT + to NPT"" a dramatic 1400-fold decrease in reactivity is observed for deprotonation from the methylene position, behavior which is readily explained on the basis of a stereoelectronic effect (see above and Scheme 34). 

With monoalkylbenzenes and m-xylene no evidence has been obtained for formation of an intermediate radical cation, the complex decomposing to a benzyl radical (path i) either by a direct HAT or via a concerted electron-proton transfer mechanism. The latter seems more likely on the basis of the observation of the order of reactivity ethylbenzene > isopropylbenzene > toluene (statistically corrected) for reaction i, in line with the stereoelectronic effects observed for the deprotonation of alkylbenzene radical cations [153]. With more readily oxidizable substrates such as p-xylene and methylbenzenes with >3 methyl groups, an analogous complex is formed this, however, is characterized by a lifetime of < 20 ns and decomposes into the intermediate radical cation (experimentally observed) from which the benzyl radical is formed by deprotonation (paths ii and Hi). 

The naphthalimide (227) imdergoes a SET process with arylalkenes such as p-xylene. The initial process yields a radical cation/radical anion pair within... 

Among ArH + which coupled readily with N02 in the gas-phase, were cation radicals of benzene, toluene, p-xylene, mesitylene, and fluoroben-zene, all of which couple at about the same rate (1.2-3.7 cm3 molecule-1 s-1). The cation radicals of m- and p-difluorobenzene coupled with N02 about 10 times more slowly. Naphthalene, which was anticipated to couple easily, did so only very slowly. These results may be related to the relative stabilities of the cation radicals and their corresponding a-complexes in the gas phase. 

Metal-ion catalysed air oxidations of aromatic compounds are industrially important. Oxidation of a methyl to a carboxylic acid group, such as in the first stage of the manufacture of terephthalic acid from p-xylene, is believed to involve peroxidation of benzylic carbon [formed in eqn (29)] (Andrulis et al., 1966). The reactions leading to benzyl acetates [eqns (29)-(31) for example] must therefore be carried out in the absence of air. Oxidations of alkylaromatics in the presence of oxygen and involving cation radical intermediates have been reported (Onopchenko et al., 1972 Holtz, 1972 Scott and Chester, 1972). 

In a related example, the photolysis of cerium(IV) ammonium nitrate generates the NO radical (Eq. 10) which has recently been demonstrated to react with alkylbenzenes via an electron transfer reaction. This particular oxidant is able to generate radical cations of substituted benzenes with ionization potentials less than that of p-xylene (8.44 eV). 

For industrial applications of ceric oxidation a regeneration of the oxidant is necessary. Electrolytic conversion of Ce(III) into Ce(IV) is described in some publications or patent literature. Thus, p-xylene was converted into 4-methyl-benzaldehyde by catalytic amounts of ceric sulfate in sulfuric acid with an electrolytic regeneration of Ce(IV) (70). Benzylic oxidation appears to start by the formation of a cation radical which then gives a benzyl radical. 

The PET reactions of isobutylene(2-methylpropene) (21) in the absence of methanol have yielded a novel photochemical nucleophile-alkene combination aromatic substitution reaction. The solvent, acetonitrile, was found to act as a nucleophile and add to the alkene radical cation to give a distonic radical cation that subsequently adds to the 1,4-dicyanobenzene radical anion. Cyclization to the ortho position of the phenyl group leads to product formation. Two-colour, two-laser techniques have been used to study the photolysis of a, a -dichloro-o- or -p-xylene to yield xylylenes. ... 

The interaction of Fe + in Fe,H-ZSM-5 with NH3 and pyridine led to a complete disappearance of the low-field Unes at g2 = 5.65 and g3 = 6.25, and interaction with H2O to their considerable decrease. In any event, the intensity at gi = 4.27 was markedly enhanced. This was especially pronoimced with NHj and pyridine indicating an increase of the crystal field symmetry upon adsorption of these powerfiil Ugands. Interaction with O2 resulted in a considerable but reversible broadening of the Fe + ESR lines caused by dipole-dipole interaction of Fe + with O2. With NHj, the samples of Fe,H-ZSM-5 were reduced at higher temperatures (823 K) as indicated by the disappearance of the signals of Fe + and formation of Fe clusters. Reoxidation did not fiilly restore the original spec-triun. Interaction with p-xylene yielded an ESR spectrum characteristic of p-xylene cation radicals. 

Calculated Spin-Density Distribution (p,) in Xylene Cation-Radicals and Determined Partial Factors ((p,) of Ring-Attack Rates for Nitrations of Neutral Isomeric Xylenes with Nitric Acid in Acetic Anhydride... 

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