Aniline, nitro-substituted, oxidation

Aromatic azo compounds can be obtained by the oxidation of primary arylamines. The reagent most widely used for this purpose is activated manganese dioxide. This converts aniline and substituted anilines into the corresponding azo compounds in moderate to good yield. At room temperature the products are the n -azobenzenes, which are isomerized to the trans compounds on heating. It is probable that hydrazobenzenes are intermediates in the reaction, but these are more easily oxidized dim the starting anilines (Scheme 7). These oxidations are inhibited by electron-withdrawing substituents and some nitro-substituted anilines fail to react. 

However, the substituents shown in the lefthand column hinder the oxidation of the toluene derivatives completely. Phenols, thiophenols, anilines, o-phenoxy-, and iodine-substituted arenes cannot be oxidized to the aromatic acids, for these substituents act as strong radical scavengers and thus as catalyst and autoxidation poisons. In contrast to m- and p-nitrotoluene, which can be smoothly oxidized to the nitro-substituted benzoic acids, o-nitrotoluene and its derivatives turned out be almost inert. This is due to the high reactivity of the benzylic radical, which ob-... 

Numerous individual substances were detected only in Ru04 extracts e.g. di- to pentachlorinated benzenes, 4-chlorobenzoic acid and 2,4-dichlorobenzoic acid 24, hexachlorocyclohexanes (a-,P-,y- and 5-HCH) 22, a technical mixture obtained during the synthesis of lindane, and the plasticizers alkylsulfonic acid phenylesters 23. These plasticizers were recently identified in riverine sediments (Franke et al. 1998). Furthermore, nitro-substituted benzoic acid and alkylated phenols 24 were observed. The occurrence of aromatic nitro compounds as a result of the oxidation of anilines can be excluded due to the contemporary appearance of amino compounds, e.g. 4-aminobenzoic acid or N-ethylaniline. However, the origin as well as the emission pathway of these compounds is still unknown. 

Amination of aromatic nitro compounds is a very important process in both industry and laboratory. A simple synthesis of 4-aminodiphenyl amine (4-ADPA) has been achieved by utilizing a nucleophilic aromatic substitution. 4-ADPA is a key intermediate in the rubber chemical family of antioxidants. By means of a nucleophibc attack of the anilide anion on a nitrobenzene, a o-complex is formed first, which is then converted into 4-nitrosodiphenylamine and 4-nitrodiphenylamine by intra- and intermolecular oxidation. Catalytic hydrogenation finally affords 4-ADPA. Azobenzene, which is formed as a by-product, can be hydrogenated to aniline and thus recycled into the process. Switching this new atom-economy route allows for a dramatic reduction of chemical waste (Scheme 9.9).73 The United States Environmental Protection Agency gave the Green Chemistry Award for this process in 1998.74... 

Oxidations. Anilines substituted by halo, nitro, cyano, and alkylalkoxy groups arc oxidized by 1 in HO.Ac to the conesponding nitro compounds in 75-90% yield. Over-oxidation is usual when this oxidation is applied to anilines substituted by electron-donating groups. In successful reactions, yields are comparable to those obtained with CF,COjH (1, 821-822). 

Further use of this oxidant to produce a number of poly nitro aromatics was reported by Nielsen and co-workers [71] in their remarkable paper. The authors also reviewed work on the other oxidants used to pass from HN to NO Caro acid, peracetic, permaleic, m-chloroperbenzoic and perbenzoic acids. They pointed out that the power of the oxidant is proportional to the acid strength of deoxy peracid. Peracetic and m-chloroperbenzoic acids are suitable for the oxidation of aliphatic primary amines, whereas peracetic, peroxytrifluoroacetic and peroxymaleic acids are best for the oxidation of ring substituted anilines. Potassium persulphate in sulphuric acid was also used successfully [71 ]. 

ANILINE, A -PHENYL (122-39-4) Comhustible solid. Dust forms explosive mixture with air. Violent reaction when added to hexachloromelamine, trichloromelamine. Often shipped in liquid form (flash point, liquid 307°F/153°C oc). An organic base. Incompatible with strong acids (forms salts), aldehydes, organic anhydrides, isocyanates, oxidizers. Reacts with nitrogen oxides to form Al-nitrosodiphenylamine and mono- and poly-nitro products. Incompatible with organic anhydrides, acrylates, alcohols, aldehydes, alkylene oxides, substituted allyls, cellulose nitrate, cresols, caprolactam solution, epichlorohydrin, ethylene dichloride, isocyanates, ketones, glycols, nitrates, phenols, vinyl acetate. Exothermic decomposition with maleic anhydride. Increases the explosive sensitivity of nitromethane. Attacks light metals in the presence of moisture. 

The advantage of a fluoride leaving group was clear in reaction of a hydroxy-carbamate anion with (fluorobenzene)FeCp complex, where the yield was 72% for F and 41% for Cl [79]. The (l,2-difluorobenzene)FeCp complex underwent smooth double substitution with phenethyl amine, whereas a dichloro complex led to monosubstitution. Replacement of a nitro group in q -nitrobenzene-FeCp" is possible with O, S, and N-nucleophiles [70,80,81]. The nitroarene complexes, which are not known in the Cr(CO)3 series, are prepared by oxidation of the corresponding aniline complexes. 

Vanadium complexes and in particular [VO(acac)2] are the most active catalysts for the oxidation of substituted anilines to nitro compounds. The effect of substituents upon reaction rate corresponds to a reaction involving an electron deficient transition state in that electron withdrawing groups decrease the rate and vice versa. The relative order of reactivity p-Me > m-Me > aniline > p-Cl > p-Br > m-Cl > m-Br is the same as observed in electrophilic aromatic substitution. Straight line correlations between the log of the relative rates and Hammett a or Brown constants were obtained with p values of— 1.42 and — 1.97, respectively, indicating an electron deficient transition state in the rate determining step. 

Recently, a different catalytic system has been reported for the synthesis of urethanes, based on orthometallated ruthenium complexes of the kind [RuL(CO)2C1]2 (best ligand LH = 2-phenylpyridine) in the presence of a base (best NaOMe) [186], The initial rate is first order in catalyst concentration and second order in CO pressure, but is independent on nitrobenzene concentration. If aniline was used as a substrate in place of nitrobenzene, no carbonylation was observed and this was said to represent an evidence that aniline is not an intermediate in the catalytic cycle. However this experiment is inconclusive, since carbonylation of aniline may require the previous oxidation of the starting complex by nitrobenzene, analogously to what found for the Ru(C0)3(DPPE) and Ru3(CO)i2 based systems (vide supra). No experiment was attempted using a nitro compound in the presence of a fferently substituted aniline, analogous to the ones describes for the aforementioned catalytic systems. 

TABLE 2 presents a series of polar chemicals which we found to be less than strictly additive with phenol and octanol. In general, these chemicals are acidic and cause symptoms different from narcosis syndrome. As rules of thumb, phenols or anilines with two or more nitro substituents, or four or more ring substituted halogens do not cause narcosis (II) syndrome. These are more toxic than is estimated from narcosis QSAR models and are likely oxidative phosphorylase uncouplers. This mechanism and associated QSAR will be discussed elsewhere. 

The easiest way to introduce a nitrogen substituent into an arene is by nitration. Yet many desired substituted benzenes have amino functions. Moreover, the nitro group is a meta director, poorly suited for preparing ortho, para-substituted systems. A solution to these problems is provided by the availability of simple reagents that reversibly convert -NO2, a meta director, into -NH2, an ortho, para director. Thus, the nitro group can be reduced to the amino function by either catalytic hydrogenation or exposure to acid in the presence of active metals such as iron or zinc amalgam. The reverse, oxidation of aniline to nitrobenzene, employs trifluoroperacetic acid. 

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