Aromatic compounds radical attack

In Volume 13 reactions of aromatic compounds, excluding homolytic processes due to attack of atoms and radicals (treated in a later volume), are covered. The first chapter on electrophilic substitution (nitration, sulphonation, halogenation, hydrogen exchange, etc.) constitutes the bulk of the text, and in the other two chapters nucleophilic substitution and rearrangement reactions are considered. 

Nucleophilic substitution is the widely accepted reaction route for the photosubstitution of aromatic nitro compounds. There are three possible mechanisms11,12, namely (i) direct displacement (S/v2Ar ) (equation 9), (ii) electron transfer from the nucleophile to the excited aromatic substrate (SR wlAr ) (equation 10) and (iii) electron transfer from the excited aromatic compound to an appropriate electron acceptor, followed by attack of the nucleophile on the resultant aromatic radical cation (SRi w 1 Ar ) (equation 11). Substituent effects are important criteria for probing the reaction mechanisms. While the SR wlAr mechanism, which requires no substituent activation, is insensitive to substituent effects, both the S/v2Ar and the Sr+n lAr mechanisms show strong and opposite substituent effects. 

Radical cations resulting from oxidation of olefins, aromatic compounds, amino groups, and so on, can react by electrophilic addition to a nucleophilic center as shown, for example, in Scheme 1 [2, 3]. The double bond activated by an electron-donating substituent is first oxidized leading to a radical cation that attacks the nucleophilic center. The global reaction is a two-electron process corresponding to an ECEC mechanism. 

Recently, Behiman and coworkers discussed the mechanism of the Elbs oxidation reaction and explained why the para product predominates over the ortho product in this oxidation. According to the authors, semiempirical calculations show that the intermediate formed by the reaction between peroxydisulfate anion and the phenolate ion is the species resulting from reaction of the tautomeric carbanion of the latter rather than by the one resulting from the attack by the oxyanion. This is confirmed by the synthesis of the latter intermediate by the reaction between Caro s acid dianion and some nitro-substituted fluorobenzenes. An example of oxidative functionalization of an aromatic compound is the conversion of alkylated aromatic compound 17 to benzyl alcohols 20. The initial step in the mechanism of this reaction is the formation of a radical cation 18, which subsequently undergoes deprotonation. The fate of the resulting benzylic radical 19 depends on the conditions and additives. In aqueous solution, for example, further oxidation and trapping of the cationic intermediate by water lead to the formation of the benzyl alcohols 20 (equation 13) . ... 

Mechanistic information from these reactions points to the initial formation of a radical anion of the aromatic compound, followed by loss of halide ion (3.15) subsequent attack by a second enolateanion and electron transfer to a second molecule of aryl halide provides the substitution product, and the reaction is propagated. The operation of a chain mechanism is indicated by the observation that quantum... 

Radical substitution plays a part in the thermal chemistry of aromatic compounds, but not in the photochemistry, except in so far as many radicals that attack aromatic compounds are generated by photochemical methods from other addends. The reason for this is that reactive radicals exist only in low concentrations, and electronically excited states similarly are formed only in low concentrations the rate of bimolecular reaction between two such reactive species is generally much lower than the rates of alternative processes such as attack of the radical on ground-state aromatic compounds. 

We consider as dihydro derivatives those rings which contain either one or two 5p3-hybridized carbon atoms. According to this definition, all reactions of the aromatic compounds with electrophiles, nucleophiles or free radicals involve dihydro intermediates. Such reactions with electrophiles afford Wheland intermediates which usually easily lose H+ to re-aromatize. However, nucleophilic substitution (in the absence of a leaving group such as halogen) gives an intermediate which must lose H and such intermediates often possess considerable stability. Radical attack at ring carbon affords another radical which usually reacts further rapidly. In this section we consider the reactions of isolable dihydro compounds it is obvious that much of the discussion on the aromatic heterocycles is concerned with dihydro derivatives as intermediates. 

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). 

Rate Constants for Hydroxyl Radical Attack on Aromatic Compounds... 

Using the developed model, the k values for 2-CP, 3-CP, and 4-CP are 1.12 x 107, 1.004 x 109, and 1.005 x 108 (1/s), respectively therefore, the dechlorination constants for monochlorophenols follow a decreasing order 3-CP > 4-CP > 2-CP. Because chloride ion can be released only after the rupture of the aromatic ring, the faster the hydroxylation of the parent compounds, the faster the dechlorination process should be. Therefore, the above order can be understood in terms of the effect of the substituents on the reactivity of their parent compounds. It is known that both OH and Cl are ortho and para directors. Under the influence of these directors, the following preference of hydroxyl radical attack is expected ... 

Aruoma Ol (1994) Deoxyribose assay for detecting hydroxyl radicals. Methods Enzymol 233 57-66 Ashton L, Buxton GV, Stuart CR (1995) Temperature dependence of the rate of reaction of OFI with some aromatic compounds in aqueous solution. J Chem Soc Faraday Trans 91 1631-1633 Asmus K-D, Mockel H, Flenglein A (1973) Pulse radiolytic study of the site of OFI radical attack on aliphatic alcohols in aqueous solution. J Phys Chem 77 1218-1221... 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Nucleophilic attack of amines to the radical cations of aromatic compounds [Eq. (6)] is much more favorable than the direct attack to the aromatic rings bearing electron-donating substituents or unsubstituted aromatic hydrocarbons [Eq. 

To demonstrate the intervention of acetoxy radicals, an aromatic substrate (anisole or naphthalene) was added to the electrolyte and the reaction run under constant current conditions. The isolation of aryl acetates from this reaction was considered as evidence for the intermediacy of acetoxy radicals, the aryl acetate being formed via a homolytic attack of the acetoxy radical on the aromatic compound 4S"47). 

Reaction of monocyclic aromatics with O3 and NO3 radicals is generally very slow and unimportant. Atmospheric degradation of aromatics is initiated by OH radical attack. The kinetics of the reaction of OH radicals with aromatic compounds are well established [65]. As seen from Table 2 the fife-time of aromatics with respect to reaction with OH is typically a few days or less. Reaction proceeds by addition to the ring and H-atom abstraction from either the substituent groups or possibly from the ring C - H sites. In all cases the addition channel is dominant. For benzene, toluene, 0, m, p-xylene, and ethyl benzene the H-atom abstraction pathway accounts for 5-10% of the overall reaction [15], the remaining 90-95% proceeds via addition. For toluene ka/(ka + h,) = 0.07 [15,65]. 

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). 

Photoamination of a variety of aromatic compounds with ammonia and alkyl-amines under photosensitized electron transfer mediated reaction conditions has been extensively investigated by Yasuda and coworkers (Scheme 70) [320-324]. The photoamination reactions of stilbene derivatives with ammonia have been utilized for the synthesis of a variety of isoquinoline derivatives [324]. The photoamination is initiated by photochemical electron transfer from the arenes to the electron acceptor followed by nucleophilic attack of ammonia or primary amines on the aromatic radical cations (Scheme 70). 

Electrophilic aromatic substitution is the mechanism suggested in both cases the initial step being attack of the cation radical on the aromatic compound (ArH) (reaction 101), followed by deprotonation of the intermediate (reaction 102) and further oxidation of the radical to the resulting arsonium or stibonium ion (reaction 103). 

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