Aromatic nitro compounds rearrangement

Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

When aromatic nitro compounds are treated with cyanide ion, the nitro group is displaced and a carboxyl group enters with cine substitution (p. 854), always ortho to the displaced group, never meta or para. The scope of this reaction, called the von Richter rearrangement, is variable. As with other nucleophilic aromatic substitutions, the reaction gives best results when electron-withdrawing groups are in ortho and para positions, but yields are low, usually < 20% and never > 50%. 

Rearrangement of aromatic nitro compounds upon treatment with... 

Further complications of the reduction of aromatic nitro compounds are the possibility of complete reduction to aromatic amines (which may condense with nitroso compounds to give the desired azo compounds), reduction of azo compounds to the corresponding hydrazo compounds, followed by a benzidine (or semidine) rearrangement. It is clear, therefore, that the level of reducing agent used and other reaction conditions are quite critical. 

Aromatic Nitro Compounds The molecular ion peak of aromatic nitro compounds (odd number for one N atom) is strong. Prominent peaks result from elimination of an N02 radical (M - 46, the base peak in nitrobenzene), and of a neutral NO molecule with rearrangement to form the phenoxy cation (M - 30) ... 

When 2,4,6-tri- or 2,3,4,6-tetraarylated pyrylium tetrafluoroborate is treated with nitromethane under different conditions rearrangement is also observed. No rearrangement occurs when the two components are heated together with an excess of potassium t-butylate in t-butanol (route a). Under strong basic conditions the intermediate 11 probably loses the acidic proton at position 1 to give the aromatic nitro compound 12. 

Reductions of aromatic nitro compounds often proceed to generate mixtures of nitroso and hydroxyl-amine products which then condense to form azoxy and, eventually, azo compounds. This bimolecular reduction is practical only for the generation of symmetrically substituted azo compounds. The situation can be further complicated if the reduction continues such that aromatic amines are formed the amines may then condense with the intermediate nitroso compounds to generate hydrazo compounds which can then undergo a benzidine rearrangement. 

When strong sulfuric acid is used with aromatic nitro compounds containing a free para position, rearrangement of the intermediate phenylhy-droxylamine to a p-aminophenol takes place. A smooth platinum cathode... 

BuUen, J. V. Ridd, J. H. The rearrangement of aromatic nitro compounds. Part 2 the rearrangement of substituted nitrophenols in trifluoromethanesulfonic acid. J. Chem. Soc., Perkin Trans. 2 1990, 1675-1679. 

Aromatic nitro compounds can be identified by characteristic (M — NO)", as well as (M — N02), peaks in their mass spectra. The large enthalpic advantage of 2 eV for the rearrangement loss of NO vs. simple cleavage loss of NO2 arises from the much-higher resonance stabilization of ArO" than of Ar (Section 8.2). This NO elimination can be enhanced by an ortho-situated functional group (Yinon 1982,1992). Loss of NO may also occur from aromatic EE ions, such as protonated nitrobenzene (Crombie and Harrison 1988 Equation 8.124). 

Aromatic aminosulfonic acids, which play a major role in connection with pigment lakes, are produced by sulfonating the corresponding nitro compound and then reducing it to an aminosulfonic acid. An alternative technique, known as baking process, involves exposing an amine/dihydrosulfate to a temperature of 200 to 300°C in order to effect rearrangement to p-aminosulfonic acid. Ortho-sulfonation prevails if the para position is occupied. In contrast to sulfonation techniques with sulfuric acid, this method avoids wastewater contamination with sulfuric acid. 

The mechanism of the coupling reaction has been very exhaustively studied. Summarising first what has already been mentioned, it must be noted that the reaction is not confined to the aromatic series, for diazo-compounds condense also with enols and with the very closely related aliphatic aci-nitro-compounds. The final products of these reactions are not azo-compounds, but the isomeric hydrazones formed from them by rearrangement. 

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... 

Nitro compounds, in particular aromatic and heterocyclic derivatives, absorb strongly in the near UV. They have properties similar to ketones in their excited state. These compounds are characterized by an unpaired electron in the n0 orbital and thus by a radical character. A typical example of this radical character is the easy intramolecular hydrogen abstraction in nifedipine and related vasodilators (Sch. 7) (18). Another manifestation of the radical character of the nitro group is the rearrangement often observed with nitrated five-membered heterocycles, as in the case of metronidazole (Sch. 8) (19). 

Aromatic and aliphatic primary amines can be oxidized to the corresponding nitro compounds by peroxy acids and by a number of other reagents. The peroxy acid oxidations probably go by way of intermediate hydroxylamines and nitroso compounds (Scheme 2). Various side reactions can therefore take place, the nature of which depends upon the structure of the starting amine and the reaction conditions. For example, aromatic amines can give azoxy compounds by reaction of nitroso compounds with hy-droxylamine intermediates aliphatic amines can give nitroso dimers or oximes formed by acid-catalyz rearrangement of the intermediate nitrosoalkanes (Scheme 3). 

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