Oxidative substitution, aromatic compounds

One 7i-bond of an aromatic ring can be converted to a cyclohexadiene 1,2-diol by reaction with enzymes associated with P. putida A variety of substituted aromatic compounds can be oxidized, including bromobenzene, chlorobenzene, " and toluene. In these latter cases, introduction of the hydroxyl groups generates a chiral molecule that can be used as a template for asymmetric syntheses. " ... 

Selifonov SA, M Grifoll, RW Eaton, PJ Chapman PJ (1996) Oxidation of naphthenoaromatic and methyl-substituted aromatic compounds by naphthalene 1,2- dioxygenase. Appl Environ Microbiol 62 507-514. 

Goumont et al. exploited this kind of reactivity for the nucleophilic substitution of the hydrogen atom in position 5 by carbon nuclophiles <20030BC2192> (Scheme 18). These authors reported that 6,8-dinitrotetrazolo[l,5- ]pyr-idine 11 easily reacts with potassium nitropropenide to yield an adduct similar to those obtained with alcohols 12. This adduct when oxidized by cerium ammonium nitrate yields the nitroalkyl-substituted aromatic compound 64. 

The conversion of aromatic compounds comprises coupling, nuclear and ben-zylic substitution, and in some cases, addition. Homo- and in a more limited scope, heterocoupling is achieved for unsubstituted and substituted aromatic compounds in direct or indirect anodic processes. Chemically, there is a limited variety of expensive oxidation reagents available, but a large scope of transition... 

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

Some reactions of 2,2 -bipyridine /V-oxides have been reported. The l,T-dioxide is nitrated readily to 4,4 -dinitro-2,2 -bipyridine 1,T-dioxide. ° ° °" 2,2 -Bipyridine 1-oxide is also nitrated in the 4 position. The nitro groups in 4,4 -dinitro-2,2 -bipyridine l,T-dioxide are reactive, being replaced by chlorine with concentrated hydrochloric acid," by bromine with acetyl bromide, by hydroxyl with dilute sulfuric acid, and by alkoxy groups with sodium alkoxides. Some of the dialkoxy derivatives are useful catalysts for the oxidation of aromatic compounds. The dinitro dioxide is deoxygenated to 4,4 -dinitro-2,2 -bipyridine with phosphorus trichloride in chloroform, and other substituted l,T-dioxides behave similarly, but with phosphorus trichloride alone, 4,4 -dichloro-2,2 -bipyridine results. The dinitro dioxide is reduced by iron powder in acetic acid to 4,4 -diamino-2,2 -bipyridine, whereas 4,4 -dichloro-2,2 -bipyridine l,T-dioxide is converted to its 4,4 -diamino analogs with amines. Related reactions have been described. ... 

F. Effenberger, Acc. Chem. Res. 22,27-35 (1989) l,3,5-Tris(dialkylamino) benzenes Model Compounds for the Electrophilic Substitution and Oxidation of Aromatic Compounds". 

Reactions of para- or mefa-substituted aromatic compounds hydrolysis, hydration of alkenes, substitution, oxidation, enzyme-catalyzed oxidations some type II photooxidations Hydrolysis and many other reactions of aliphatic organic compounds... 

Oxidation. A primary intermediate, i.e., a para-substituted aromatic compound such as a derivative of 1,4-diaminobenzene or of 4-aminophenol, is oxidized to a quinonediimine or quinonemonoimine, respectively. The imine then reacts with a secondary intermediate (coupler), which is a meta-substituted compound such as as a derivative of 1,3-diaminobenzene, 3-aminophenol, or resorcinol. Another oxidation step yields an indo, phenazine, or oxazine dye which is three times the size of the precursors. The mechanism of this coupling reaction is shown in Scheme 5.1. 

Figure 4.2 Hammett relationship (log 11/2 versus a) in the benzoylation reaction of benzene and substituted aromatic compounds with benzoyl chloride over H-BEA zeolite modified by indium oxides...

There are two general routes to arylsulfonyl chlorides. The first involves the conversion of an already sulfur-substituted aromatic compound to the sulfonyl chloride. Thus arylsulfonic acids or their alkali metal salts yield sulfonyl chlorides by treatment with a variety of chlorinating agents such as phosphorus pentachloride, thionyl chloride, phosgene, and chlorosulfonic acid. Alternatively, substituted thiophenols or aryl disulfides can be oxidized by chlorine-water to the sulfonyl chloride.6... 

Although its oxidizing power is not very high, the superoxide radical is able to degrade substituted aromatic compounds with high absorption in the UV range. 

On the other hand, microbial treatment of simple alkyl-substituted aromatic compounds (e.g., 1-isopropyl-4-methylbenzene328 332) results in oxygenation of the methyl group, rather than the chemically more susceptible isopropyl group333. Further oxidation of the benzylic alcohol to the corresponding carboxylic acid is common and, in most cases, represents the only isolated product. 

One-electron oxidation of aromatic compounds (ArH) leads primarily to corresponding radical cation which exist either in monomeric (ArH +) or dimeric form [(ArH)2 ] the latter usually formulated as r-dimer [70]. However, radical cations are reactive species and can undergo further reaction yielding more persistent radical cations e.g. oxidation of rert-butylbenzene or of toluene or o-xylene yielded radical cation of 4,4 -di-rerf-butyl biphenyl, 4,4 -bitoluene or 3,3, 4,4 -tetramethyl biphenyls, products of further a-coupling, proton loss and further one-electron oxidation [71]. This is a well-known pathway of biaryl dehydrodimerization, explored in anodic and metal-ion oxidation of ArH [72, 73]. Other compounds with high reactivity in (T-coupling are alkoxy and amino substituted ArH [73]. Thus a risk with characterization of radical cations is that hardy survivors and not primary radical... 

Oxidation of Arenes and Alkyl-Substituted Aromatic Compounds... 

Oxidation of Alkyl-Substituted Aromatic Compounds with Air 443... 

The simplicity of the easily surveyed reaction equation is strongly misleading. The reaction mechanism of the autoxidation of alkyl-substituted aromatic compounds consists of several complex steps - free-radical chain reactions triggered by oxidation catalysts. In general, two initiation steps can be distinguished [5, 6, 10] ... 

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