2.6- dinitrotoluene, nitro group reduction

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]... 

A similar example is 2,6 dinitrotoluene where reduction in rats of a nitro group to a hydroxylamine occurs, which yields a liver carcinogen (see chap. 5). 

The hydroxylamino group may react further. A Gatterman reaction has, however, been observed only in the reduction of 2,3-dinitrotoluene. When a carboxyl group is neighbor to the hydroxylamino group, an isoxazolone may be formed this reaction has been used as an indication of which nitro group is most easily reduced in dinitrobenzoic acids [152]. 

Subsequently reductive desulphonation was tried by methods which do not involve the reduction of nitro groups. A small yield (ctf. 5%) was obtained [lOOj of 2,4-dinitrotoluene on desulphonation of 2.4 dinilro-5-sulplionic acid with sodium borohydride. 

Reduction of nitro groups may occur under either aerobic or anaerobic conditions, and the complete sequence of reduction products is produced from 2,6-dinitrotoluene by Salmonella typhimurium strain TA 98 (Sayama et al. 

Both aminodinitrotoluene isomers are products of the first reduction step in cell suspensions of Desulfovibrio (39). However, most publications on TNT reduction indicate that the nitro group para to the methyl group is reduced first, with 4-amino-2,6-dinitrotolu-ene being the main reduction product (23, 32, 37). Only Naumova et al. (33) observed the reduction of TNT mainly to 2-amino-4,6-dinitrotoluene by 2i Pseudomonas strain. Figure 6 illustrates the pathway of complete TNT reduction and summarizes the characteristics of the sequential reduction steps. 

The initial step in the metabolism of TNT by nearly every organism seems to be reduction (8-13, 18, 19, 27, 32, 51, 52, 59, 63, 64, 74, 87, 96, 97). Most often nitrotoluenes are biotransformed to aromatic amines (31, 40) through a series of reduction steps as described by Yamashima et al. (99). The para nitro group of TNT is usually reduced preferentially to the ortho group. A second series of reductions predominantly leads to the formation of 2,4-diaminonitrotoluene (2,4-DAT). Reduction of the third nitro group usually only occurs under anaerobic conditions (55). Under aerobic conditions, hydroxylamino metabolites are sufficiently long lived to lead to the formation of azoxy compounds or polymers which are products of condensation of hydroxylamino and nitroso dinitrotoluenes (56). 

The mineralization of 2,4-dinitrotoluene by P. chrysosporium also started with the reduction of one nitro group leading to either 2-amino-4-nitrotoluene or 4-amino-2-ni-trotoluene (53), which was analogous to the initial reduction of TNT. 2-Amino-4-nitro-toluene was a substrate for the manganese-dependent peroxidase and was converted to 4-nitro-l,2-benzoquinone. The authors did not investigate the metabolic fate of 4-amino-2-nitrotoluene but assumed that the reactions involved in its degradation are similar. No formylated products were described (53). But it is possible that the transformation of aminonitrotoluenes by the manganese-dependent peroxidase occurs faster than a possible formylation. 

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