Radical attack on the aromatic ring

Neither the relative number of benzylic hydrogens nor the base strength accounts for the slow oxidation rate of the methylnaphthalenes. Formation of radicals in the presence of aromatic hydrocarbons can lead to radical attack on the aromatic ring. Addition of phenyl or methyl radical to the ring gives a cyclohexadienyl radical that may disproportionate or dimerize, or undergo hydrogen abstraction by another radical (3, 9,13). 

Because hydroxyl radicals have indiscriminate reactivity, they can react with almost all types of organic and inorganic compounds. Most aromatic compounds undergo radical attack on the aromatic ring in a manner similar to that of benzene systems. The products and the rate constants for hydroxyl radical attack on aromatic compounds are listed in Table 5.11. The data were obtained from the pulse radiolysis studies (Buxton et al., 1988). 

The decomposition of the unsymmetrical peroxides (Table 82) does not present anything particularly unusual in rates. The enthalpy of activation for the decomposition of benzoyl phenylacetyl peroxide and phenylacetyl peroxide are similar. An analysis of the products from the decomposition of acetyl benzoyl peroxide in conjunction with kinetic datahaveled Walling and Cekovic to suggest an induced decomposition which is initiated by methyl radical attack on the aromatic ring, viz. 

The most studied commercial polymer in the chemical modification, Udel polyethersulfones are based on bis-phenol A, in which the electron-donating isopropylidene moiety increases the electronic density of the whole phenoxy sites and, in particular, increases the ether basicity. This may favour chain breaking during the sulfona-tion step [29,32] the former may favour free-radical attacks on the aromatic rings. 

The same radical cation is believed to be the intermediate when oxidation of aromatics is carried out with peroxydisulfate739,745 or peroxydiphosphate.746 In the presence of suitable oxidants such as Cu(II), phenolic products are formed with high selectivity. This supports the suggestion that initial attack on the aromatic ring gives rise to 96, which, in turn, reacts rapidly with water to yield the hydroxy-cyclohexadienyl radical ... 

The most vexed subject in this field is the site of radical attack on substituted aromatic rings. Some react cleanly where we should expect them to. Phenyl radicals add to naphthalene 7.36, to anthracene 7.37 and to thiophene 7.38, with the regioselectivity shown on the diagrams. In all three cases, the frontier orbitals are clearly in favour of this order of reactivity (because of the symmetry in these systems, both HOMO and LUMO have the same absolute values for the coefficients). 

With alkoxyalkynes and aromatic ketones, in addition to the expected acrylate esters (equation 85) alkylidenecycloheptatrienes are also formed in the photo-reaction , probably by a reaction from the biradical intermediate involving radical attack on an aromatic ring (equation 86). 

The most vexed subject in this field is the site of radical attack on substituted aromatic rings. Some react cleanly where we should expect them to. Phenyl radicals add to naphthalene (399), to anthracene (400)323 and to thiophene... 

This novel anodic methoxyiation may proceed via the fluorosulfonium ion B in a Pummerer-type mechanism as shown in Scheme 6.11. In this mechanism, the cation radical A of the sulfide is trapped by a fluoride ion, and this step should suppress side reactions from the cation radical A (such as dimerization and nucleophilic attack on an aromatic ring) even when deprotonation of A is slow due to the weak electron-withdrawing Rf groups or electron-donating substituents on the benzene ring. Since fluoride ions are much weaker nucleophiles compared to methoxide, it is reasonable that the methoxyiation predominates in methanol. 

Photocycloadditions of higher order than 2 + 2) are sometimes encountered, but they are not so general as the (2 +2) reactions. Often they arise in reactions that occur by way of radical cations 2.83), when electrophilic attack on an aromatic ring may divert the reaction from cyclobutane formation, or in those that are promoted... 

In some cases, degradation products may be produced as a result of attack on both the aromatic ring and aliphatic components of the compound. This is the case for diuron where hydroxylated photoproducts 1 and 2 (Scheme 4) are produced by reaction of HO with the aromatic ring (Process A) to form an adduct similar to that described above with subsequent loss of a chlorine atom [15]. The more important degradation pathway in this instance is via hydroxyl radical attack on the methyl of the dimethylurea group (Process B). 

Based on the data in Table 14.4, OH radical attack appears to occur at either the ortho or the ipso positions on the aromatic rings. The predicted position of attack appears to vary based on the method used to calculate the energies (i.e. MP2 versus B3LYP), as is clearly evident in the o-xylene/OH and p-xylene/OH systems. In all cases, the MP2 method tends to favor the ipso adduct much more than DFT. 

When considering the rate constants for deprotonation reported in Table 2 one should, however, take into account that in methylbenzene radical cations with < 3 methyl groups, nucleophilic attack of water on the aromatic ring can compete efficiently with side-chain deprotonation. An elegant explanation which accounts for this competition has been provided for the toluene radical cation on the basis of the three-electron three-orbital three-configuration approach [135, 136]. Three electrons are involved in the deprotonation reaction—the unpaired electron delocalized over... 

The reaction is most probably initiated by radical attack of a reduced carbonyl function on the aromatic ring in the adjacent system. The product is formed as the radical anion but reoxidized by air during work-up. In the presence of proton donors, or in alcoholic solvents, reduction of 98 gives a mixture of acyclic and partly hydrogenated cyclic products [288]. Substituted 98, such as the 4,4, 5,5 -tetracarboxylic acid, gives coupling in basic alcoholic medium but not in DMF [289]. 

The heterocycle undergoes photochemical ring rearrangement to (21) (Equation (4)) by cleavage of the weakest bond (Se—N) in the molecule and attack of the resultant selenium radical on the aromatic ring <86BCJ2179>. 

Trisylbenzene [tris(trimethylsilyl)methylbenzene] when photolyzed is highly susceptible to homolysis (ESR spectroscopy demonstrated the presence of radicals) the radicals recombined by attack of the silyl radical on the aromatic ring as the major pathway16 (equation 5). 

Formation of a new carbon-oxygen bond in aromatic photocyclization is seen in the reaction of phenanthrene-4-carboxylic acid (175) in the presence of t-butyl hypoiodite, which involves radical attack on the ring system. Attack by an oxy-radical is also evident in the formation of a heterocyclic oxygen compound in the... 

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