Aryl nitro compounds

The sulfonation of aromatic nitro compounds is not of great synthetic importance because it is generally easier to nitrate an aromatic sulfonic acid. However, in some cases, this may cause nitrodesulfonation and also sulfonation, unlike nitration, often gives mainly one product rather than a mixture of isomers, consequently sulfonation of aromatic nitro compounds is sometimes a useful procedure. 

In the aryl nitro compounds, the powerful electron-withdrawing (—1, -M) effect of the nitro group results in the sulfonation of nitrobenzene being more difficult than that of benzene. Early work claimed that heating nitrobenzene with excess chlorosulfonic acid (two equivalents) afforded m-nitrobenzenesulfonyl chloride, but under drastic conditions (150 °C, 20 hours) the only product was reported to be tetrachloro-/ -benzoquinone (chloranil). 

In more recent work, nitrobenzene 89, by heating with chlorosulfonic acid (four equivalents) at 100-120 °C (8 hours), afforded the w-sulfonyl chloride. The 

Nitrobenzene (89, 184.5 g) by reaction with a mixture of iodine (1.1 g) and 98% sulfuric acid (57 g) in chlorosulfonic acid at 30 °C afforded pentachloroni-trobenzene 90 (87% yield) (Equation 28). The product is useful as a soil fungicide and the s)mthesis provides another example of the use of a mixture of chlorosulfonic acid and iodine as a chlorinating reagent (see Section 3, p 50). The rate of chlorination of nitrobenzene 89 to pentachloronitrobenzene 90 by the chlorosulfonic acid-iodine mixture increased with the iodine concentration. However, the rate was less than that observed in the presence of iodine chlorides. The most effective catalyst for chlorination was found to be a mixture of 0.75% iodine chlorides in chlorosulfonic acid which was stable for 3 days.  

Chlorosulfonic acid in the presence of iodine catalyst at 85 °C converted nitrobenzene into a mixture of w-, o- and / -chloronitrobenzenes in a ratio of 80.6 13.9 5.5% with combined yield of 7% and in this reaction the intermediate was sulfuryl chloride (S02Cl2).  

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Reduction of aryl nitro compounds (Sec tion 22 9) The standard method for the preparation of an arylamine is by nitra tion of an aromatic ring followed by reduction of the nitro group Typical re ducing agents include iron or tin in hydro chloric acid or catalytic hydro genation... 

Mercuration of aromatic compounds can be accomplished with mercuric salts, most often Hg(OAc)2 ° to give ArHgOAc. This is ordinary electrophilic aromatic substitution and takes place by the arenium ion mechanism (p. 675). ° Aromatic compounds can also be converted to arylthallium bis(trifluoroacetates), ArTl(OOCCF3)2, by treatment with thallium(III) trifluoroacetate in trifluoroace-tic acid. ° These arylthallium compounds can be converted to phenols, aryl iodides or fluorides (12-28), aryl cyanides (12-31), aryl nitro compounds, or aryl esters (12-30). The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. 

Reaction of halo sulfonic acid esters with boranes 10-125 Alcoholysis of sulfonic acid derivatives 13-15 Vicarious substitution of aryl nitro compounds... 

Scheme 4. The compounds and intermediates on the rear plane of the bicubic system (farthest from the reader) are protonated on the pyridine nitrogen atom those on the front plane (nearest the reader) are not. Laviron s work has shown that the reduction of 14 and its corresponding N-oxide34, and indeed probably most aryl nitro compounds, proceeds by an ECEC sequence leading to the neutral N,N-dihydroxy [ArN(OH)2] intermediate at all proton concentrations from Ho = —6 to pH 9.6. This substance then loses water to form the nitroso compound, which then undergoes a second sequence leading to the arylhydroxylamine.

Reduction of aryl nitro compounds with less-powerful reducing agents, especially in alkaline media, gives what may appear to be a mysterious conglomerate of bimolecular reduction products. For example, with nitrobenzene,... 

With deuterium-labeled dimethylaniline as the excited electron donor, primary electron transfer to aryl halides is followed by proton transfer yielding reduced aryl nitro compounds and functionalized dimethylanilines (193). 

ArN02 ArNH2.1 Aryl nitro compounds are reduced to arylamines by Ni2B at 40° in 3N HC1 or 15N NH4OH in 80-96% yield without effect on alkene, keto, nitrile, amide, carboxyl, or ester functional groups. Nitroso-, azoxy-, azo-, and hydrazobenzene are reduced to amines under the same conditions. 

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