Nitro groups, aromatic partial

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

Although the possibility of bioremediation of sites contaminated with nitrotoluene waste is clearly possible, important issues should be clearly appreciated. These include (a) additional carbon sources may be necessary to accomplish partial or complete reduction of nitro groups, (b) under aerobic conditions, dimeric azo compounds may be formed as terminal metabolites, and (c) aromatic amines may be incorporated into humic material by covalent bonding and thereby resist further degradation. 

Nitro Compounds. Under mild conditions, aromatic nitro compounds are hydrogenated easily to amines.518 The reaction may give partially reduced products, according to the circumstances. Palladium, platinum, and nickel are used frequently for this reaction. For example, nitro and benzyl ester functions are reduced on Pd(OH)2/C on THF and on Pd/C in EtOH.519 Aliphatic nitro groups are reduced more slowly. 

The X-ray structure of l-piperidino-2,4-dinitrobenzene was published (95MI801). The piperidine ring exhibits a slightly distorted chair conformation the N-atom is almost in a plane due to the partial C,N double bond, and the aromatic ring shows a slight boat deformation with the o/p-nitro groups twisted out of plane. UV and NMR data indicate that the molecule in solution presents a conformation similar to that in the solid state. In the crystalline anhydrous N-methylpiperidine betaine 86 (cf. Scheme 33) the piperidine ring adopts the usual chair conformation... 

The reaction sequence involved in the hydrogenation of aromatic nitro groups is shown in Scheme 19.1. This can be classed as a complex Type III selectivity. The end product from all paths is the aniline (10), but intermediates such as hydroxylamines (11), azo (12), azoxy (13), and hydrazo (14) compounds are present and can sometimes be isolated under the proper reaction conditions. In general, the dimeric products usually form in alkaline media, the partially reduced monomeric species form in neutral solutions, and anilines are produced in acid. The best yields of partially reduced products are obtained when the reaction is interrupted before it stops spontaneously and when it is carried out in the presence of various modifiers. ... 

Manufacture of many important amino intermediates used for dyes and other purposes is usually by conversion or replacement of a substituent. For example, as already mentioned, in substituted nitro compounds, the nitro groups may be reduced with iron/hydrochloric acid, hydrogen and catalyst, or zinc in aqueous alkali. Partial reductions can be brought about with sodium sulfide. Amino groups may be introduced by replacing halogens in the aromatic ring. Another approach to amination is by attack on a substituted aromatic compound with ammonia or amines. Thus, for example, direct amination of p-chloronitrobenzene (15a) in the presence of a copper catalyst affords p-nitroaniline (15b) in almost quantitative yield l,4-dichloro-2-nitrobenzene (16) is converted in a similar way to 4-chloro-2-nitroaniline (17). Reactions of ammonia with carboxylic acids or anhydrides are carried out on an industrial scale. 

Reviews of the biodegradation and biotransformation of nitroaroamatic compounds have been given (Spain 1995a,b Crawford 1995) to which reference should be made for details, so that only a very brief summary is justified here. Three principal reactions are found among aerobic bacteria (1) oxidative elimination of nitrite, (2) partial or complete reduction of the nitro group, and (3) reduction of the aromatic ring (Chapter 6, Section 6.8.2). 

Selective hydrogenation of one or two nitro groups in an aromatic dinitro compound is the basis for the synthesis of otherwise unattainable molecules. Carefully chosen catalysts, under totally different reaction conditions, have been met with success. Partial reduction of 2,6-dinitroanilines to nitrophenylenediamines occurs in 60-90% yield over 10% palladium-on-carbon at RT [equation (a)], although most heterogeneous catalysts do not afford such selectivity. 

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