Nitroanilines, rearrangement

Diaminoazobenzene was reported by Nietzki to have been prepared by diazotizing -nitroaniline and coupling the product with aniline. The resulting 4-nitrodiazoaminobenzene is rearranged and the nitro group reduced. The submitters tried several times to carry out this procedure but were unsuccessful. 4,4 -Diaminoazobenzene has been prepared by the oxidation of -nitroaniline with potassium persulfate followed by the reduction of the nitro groups. ... 

Fig. 2. Variation of log k with H0 for the rearrangement of N-nitroaniline in perchloric Acid...

Azide 367 is prepared from 4-r -butyl-2-nitroaniline in 76% yield by its diazotization followed by treatment with sodium azide. In a 1,3-dipolar cycloaddition with cyanoacetamide, azide 367 is converted to triazole 368 that without separation is directly subjected to Dimroth rearrangement to give derivative 369 in 46% yield. Reduction of the nitro group provides ortfc-phenylenediamine 371 in 91% yield <2000EJM715>. Cyclocondensation of diamine 371 with phosgene furnishes benzimidazol-2-one 370 in 39% yield, whereas its reaction with sodium nitrite in 18% HC1 leads to benzotriazole derivative 372, which is isolated in 66% yield (Scheme 59). Products 370 and 372 exhibit potassium channel activating ability <2001FA841>. 

The most synthetically useful methods for benzofuroxans are (1) oxidation of o-quinone dioximes (2) decomposition of o-nitroaryl azides and (3) oxidation of o-nitroanilines. Benzofuroxans can also be formed as a result of Boulton-Katritzky rearrangement (see Section 5.05.5.2.1). 

The acid-catalysed rearrangement of A-nitroaniline derivatives continues to provide convenient synthetic routes to some nitro compounds which are difficult to obtain by other methods. A recent example68 is given in Scheme 13, where the introduction of the third nitro group into the aromatic ring is brought about by rearrangement of the... 

Ridd and coworkers71 have now shown unequivocally that radical pairs are indeed involved in this rearrangement by observation of strong enhancement of 15N NMR signals in both the reactant and product, when reaction was carried out with 15N-labelled nitro groups in both A-methyl-A-nitroaniline and also in A-methyl-A-nitro-2,5-dichloro (and dibromo) aniline. 

Substituted 2-nitroanilines also rearrange in concentrated sulphuric acid at 110°C to give both products of rearrangement 72 and 73, where again 1,3-nitro group migration occurs81. 

Confirmation of the radical mechanism was provided by Ridd and Sandall35, who detected radical cations by nitrogen-15 NMR of the reaction mixture for the nitramine rearrangement of 2.6-dibromo-jV-nitroaniline and of A -methyl/V-nitroaniline labelled with nitrogen-15 in the nitro group. The results did not allow the authors to indicate whether the products were formed within the solvent cage or from separated radicals. 

Ridd and Yoshida explored the At-nitration of aromatic nitroanilines with nitronium hex-afluorophosphate in nitromethane with the aim of studying the Bamberger rearrangement (Chapter 4, Section 4.5). In was found that amines of low basicity essentially undergo complete A -substitution if the amine is in excess 2,4-dinitroaniline and 2,3-dinitroaniline form A, 2,4-trinitroaniline and At,2,3-trinitroaniline, respectively, as the sole products. More basic nitroanilines give mixtures of C-substituted, At-substituted and poly-substimted materials. The use of nitronium salts for At-nitration has been extensively reviewed. ... 

The principal methods for forming the heterocyclic ring of benzofuroxans involve oxidation of o-quinone dioximes, thermolysis of o-nitroaryl azides, and oxidation of o-nitroanilines (Scheme 25). Ring chain tautomerism (Section 4.05.5.2.1) for the A -oxides of asymmetrically substituted benzofuroxans is more facile than for monocyclic analogues and mixtures of isomers may result. Benzofuroxans are also formed by Boulton-Katritzky rearrangement of 7-nitro-2,l-benzisoxazoles and 4(7)-nitrobenzofuroxans (Section 4.05.5.2.5). 

The para-nitro ester 71d generated only 4-nitroaniline (70%) and 4,4 -dinitroazoxybenzene (10%) when it underwent decomposition (Scheme 29). These products could have been derived from either a triplet nitrene or a triplet nitrenium ion precursor. Homolysis of the N—O bond to generate radical intermediates was ruled out because of the nearly quantitative yield of pivalic acid derived from 71d. The pivaloxy radical would have undergone rapid decarboxylation to generate CO2 and the rert-butyl radical under these conditions. Since no rearrangement product was observed, it was tentatively concluded that this ester underwent direct decomposition to 4-nitrophenyl-nitrene without the intermediacy of a nitrenium ion. ... 

The results in Table 2.4 are also relevant to preparative scale reactions. Adapting the definition of selectivities (Equation 2.10) for the reaction shown in Scheme 2.21 by rearranging Equation 2.14 gives a definition of S for competing methanolysis and aminolysis (Equation 2.17). Although the molar concentration of methanol in almost pure solvent is high (24.7 M), the major product is amide even when the concentration of m-nitroaniline is only 10-2 M (Table 2.4), and S is calculated from Equation 2.17 to be over 8000 very high yields are predicted for reactions in more concentrated solutions of m-nitroaniline. In contrast, under the same conditions, the less basic amine o-nitroaniline has an S value of only 6 [44] ... 

Above 170 °C the amidrazone ylide (36) decomposes with loss of triethylamine and concurrent cyclization to give an 85% yield of 2-phenylbenzimidazole (Scheme 19) (B-75M140800). Poorer yields ( 40%) are obtained when N-benzyl-o-nitroaniline is pyrolyzed in the presence of iron oxalate. No doubt this last reaction is similar in many respects to the reactions shown in Scheme 2. Both 2-phenyl-imidazoles and -benzimidazoles (as well as other 2-substituted analogues) can be obtained as a result of thermal rearrangement of the 1-substituted isomers (Section 4.07.1.2.2), by radical substitution methods (Section 4.07.1.7) or via the 2-lithio derivatives (Sections 4.07.1.6, 4.07.3.7). 

The aromatic nitramine rearrangement has been known already for more than 100 years. This reaction can proceed as an acid-catalyzed, thermal and photochemical reaction (see reviews in References 23,250-252). /V-Nitroanilinc 169, being treated with strong acids, rearranges to give ort/ro-nitroanilinc 170 as main product (50-60%), para-nitroaniline 171 (5-10%) and some side products (equation 66)251. 

The nitration of a number of anilines 172 in ca 70% sulfuric acid was reported as ipso-attack followed by a 1,3-rearrangement of the nitro group. The reaction occurs through the (pso-intermediate 173 to afford orf/zo-nitroaniline 174257 (equation 68). 

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