Radicals reactivity with aromatic substrates

The formation of benzoic acid, benzaldehyde, biphenyl and phenol may be explained by an addition of C02 to benzene, reaction (R26). Disproportionation of the adduct, C02 -CHD, yields benzene with elimination of formate, and benzoic acid, reaction (R27). Benzaldehyde would then be formed by the subsequent reduction of benzoic acid (see above). Alternatively, the C02 -CHD adduct may eliminate formate and add water, hence producing a hydroxycyclohexadienyl radical (HO-CHD), reaction (R28), in agreement with the observed reactivity of sulfate and phosphate radical adducts of aromatic substrates in aqueous solution and as expected for the relatively good leaving group HC02 . HO-CHD can either disproportionate to yield phenol and benzene with elimination of water, reaction (R29), or recombine yielding biphenyl as shown in reaction (R30). 

The very reactive phenyl radical reacts with the aromatic substrate 2, present in the reaction mixture. Subsequent loss of a hydrogen radical, which then combines with 7 to give 4, yields a biaryl coupling product e.g. the unsymmetrical biphenyl derivative 3 ... 

From the decomposition mechanism and the products formed it can be deduced that DCP primarily generates cumyloxy radicals, which further decompose into highly reactive methyl radicals and acetophenone, having a typical sweet smell. Similarly, tert-butyl cumyl peroxide (TBCP) forms large quantities of acetophenone, as this compound still half-resembles DCP. From the decomposition products of l-(2-6 rt-butylperoxyisopropyl)-3-isopropenyl benzene ( ), it can be deduced that the amount of aromatic alcohol and aromatic ketone are below the detection limit (<0.01 mol/mol decomposed peroxide) furthermore no traces of other decomposition products could be identified. This implies that most likely the initially formed aromatic decomposition products reacted with the substrate by the formation of adducts. In addition, unlike DCP, there is no possibility of TBIB (because of its chemical structure) forming acetophenone. As DTBT contains the same basic tert-butyl peroxide unit as TBIB, it may be anticipated that their primary decomposition products will be similar. This also explains why the decomposition products obtained from the multifunctional peroxides do not provide an unpleasant smell, unlike DCP [37, 38]. 

In general, from among the protic solvents, only liquid ammonia (the first used)1 is particularly useful, and is still used more than any other solvent despite the low temperature at which reactions have to be carried out (b.p. -33 °C) and the fact that solubilities of some aromatic substrates and salts (M+Nu-) are poor. Ammonia has the added advantage of being easily purified by distillation, being an ideal system for production of solvated electrons, and has very low reactivity with basic nucleophiles and radical anions, and aryl radicals. Also, poor solubilities can sometimes be ameliorated by use of cosolvents such as THF. In addition it can be used as a solvent for the in situ reductive generation of nucleophiles such as ArSe- and ArTe- ions, e.g. the formation of PhTe- from diphenyl ditelluride (equation 16).54 55... 

The excited states of substrate species are very reactive. They react with available free-radical oxidants with subsequent generation of partially oxidized intermediates. Although organic compounds are assumed to react predominantly with HO and H02 /02 y degradation of certain compounds can also take place directly by activation caused by UV, which improves the ability of the organics to be oxidized by H2Oz or hydroxyl radicals. Certain aromatics and olefins can also react with H202 directly. Phenol is an extensively studied example. These reactions may further enhance the oxidation of aromatics and olefins. The reaction intermediates (Int) are unstable and are further oxidized to mineral species (C02, HzO, and HC1) ... 

Peroxidase, also known as hydrogen peroxide oxidoreductase (EC 1.11.1.7), was the first enzyme that was investigated for its ability to accomplish the treatment of aqueous aromatic compounds [7], The system of reactions involved in the catalytic cycle of several peroxidases has been studied extensively and is summarized in Fig. 2. The catalytic cycle involves a progression of the enzyme through three oxidation states, i.e., native state (HRP), Compound I (HRP ) and Compound II (HRP i), following its reaction with hydrogen peroxide and its subsequent reactions with an aromatic substrate (AH2). The products of this cycle are water and highly reactive free radicals (AH ). It is these free radicals that spontaneously combine to produce insoluble precipitates. 

The main feature of carbanions derived from nitriles lies in the dependence on the aromatic substrate involved thus, two different outcomes of the substitution reaction are possible formation of the substitution compound by ET to the substrate from the radical anion intermediate 7, formed by coupling of phenyl radicals and acetonitrile anion, or formation of products from elimination of the cyano group as is the case with phenyl halides [31,32] (Sch. 3). The same reactivity pattern is found with halothiophenes [33]. 

The substitutions with nucleophilic radicals become particularly interesting only with electron-deficient aromatic substrates, as the ionic nucleophilic substitutions. Carbon free radicals are the most common nucleophilic radicals and they are obviously among the most important organic free radicals. Heteroaromatic bases on the other hand are electron-deficient aromatic substrates which readily react with nucleophilic species. The protonation of heteroaromatic bases strongly increases their electron-deficient nature and therefore the reactivity towards nucleophilic reagents, while the reactivity towards electrophilic species is strongly... 

The previous sections leave no doubts that aromatic compounds, react with positively charged electrophiles to form a-complexes-arenium ions. But are they the primary intermediates It is not by accident that the problem of preliminary formation of radical cations has arisen. Its statement is an attempts to explain the orientational peculiarities of electrophilic aromatic substitution of hydrogen. The widespread view that the orientation in the reactions of aromatic compounds with electrophiles is dictated by the relative stabilities of the cr-complexes explains but a part of the accumulated material. In the first place this refers to the meta- and para-orienting effects of electron-releasing substituents in benzene in terms of the QCT -approach and to that of the relative reactivity of various aromatic substrates... 

ArPb(OzC-CF3)2 Ar+ + Pb(02C CF3)2]. The aryl cations have been trapped with aromatic compounds to give biaryls [with certain substrates, notably poly-methylbenzenes, high yields (up to 88 %) are obtained], but with reactive aromatic substrates aryl cations are not the precursors to the biaryls and in these cases it is proposed that reaction proceeds via preliminary complex formation between the substrate and a species which contains an aryl-lead bond. Oxidative coupling of methyl-substituted benzenes by the reagent Pb(0Ac)4-CFs C02H to give biaryls and diarylmethane is also considered to involve formation of a radical cation in the primary step. A study has also been made of the plumbylation of monohalogeno-benzenes with Pb(OAc)4-CF3 COsH. ... 

Products and reaction intermediates observed in the present study are rationalized by a dual reactivity of the C02 radical anion with substituted benzenes, i.e. one-electron reduction of and radical addition to the aromatic ring. For substrates with electron-withdrawing substituents, i.e., with positive u values such as X = NO2, COOH and COH, the observed products can be explained by an electron transfer from CO2 to the substrates as the primary reaction channel. Benzene and chlorobenzene react by both electron transfer and radical addition. For toluene, with an electron-donating substituent, products of radical addition are found. The dual reactivity of COf radical anion has recently been reported for thymine," in agreement with our observations. 

A novel intramolecular alkylation using a spiroketal (14) with BFj.OEt, in THF at reflux forms the benzene-fused 8-oxabicyclo[3.2.1]octane ring system (15) in satisfactory yield. IV-Tosylpipecolinic acid (16) in the presence of sulfuric acid in benzene forms the unexpected aromatized derivative (17) in 18% yield and the mechanism is suggested to involve the reaction of intermediate (18) with benzene to form (19). Cumyl and r-butyl hydroperoxides have been used for the electrophilic alkylation of activated aromatic substrates, mainly phenols and phenol ethers.The hydroperoxides behave differently as far as catalysis and regioselectivity are concerned. The latter is believed to be explicable by steric and reactivity/selectivity considerations. Electrophilic r-butylation may be followed by radical reactions due to the r-butyl... 

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