Dinitroanilines, hydrogenation selective

Pd-C in both ethanol and DMF, little, if any, selectivity was obtained product ratios were 50 50 in ethanol and 60 40 in DMF. Palladium black in ethanol, which afforded 4-amino-5-nitroveratrole in 70-75% yield from 4,5-dinitroveratrole, provided no selectivity with 2,4-dinitroaniline, as well as with o- and m-dinitrobenzenes both nitro groups were hydrogenated.140 Reduction by sodium borohydride and Pd-C in methanol also does not appear to be selective, as 1,2,4-triaminobenzene is formed.141 It is of interest that over Pt-C in acidic alcohol 2,4-dinitroaniline is selectively hydrogenated at the 4-nitro group to give 4-amino-2-nitroaniline in 70% yield.142... 

Formation of diamines from dinitro compounds, which are unable to interact intramolecularly, presents no problem. Very large volumes of diaminotoluene, a precursor to toluene diisocyanate, are produced by hydrogenation of dinitrotoluene over either nickel or palladium-on-carbon. Selective hydrogenation of one or the other of two nitro groups is much more of a challenge, but a number of outstanding successes have been recorded. A case in point is the hydrogenation of 2,4-dinitroaniline (11) to 4-nitro-l,2-benzenediamine (12) (2) or to 2-nitro-l,4-benzenediamine (10). 

The synthesis of TATB (14) from the reaction of 2,3,4,5,6-pentanitroaniline (31) with ammonia has been reported. " In one route, 2,3,4,5,6-pentanitroaniline (31) is synthesized from the nitration of 3,5-dinitroaniline (30) " the latter is obtained from the selective reduction of TNB ° or via a Schmidt reaction with 3,5-dinitrobenzoic acid. Another route to 2,3,4,5,6-pentanitroaniline (31) involves the selective reduction of TNT (1) with hydrogen sulfide in ammonia followed by nitration of the resulting 4-amino-2,6-dinitrotoluene (46), during which the methyl group is lost by oxidation-decarboxylation. 

Lyle and LaMattina selectively hydrogenated 4-substituted 2,6-dinitroanilines to the corresponding nitrophenylenediamines over 10% Pd-C in 1,2-dimethoxyethane-chloroform as solvent (eq. 9.56).135 With use of ethanol instead of 1,2-di-methoxyethane, 4-trifluoromethyl-2,6-dinitroaniline was completely hydrogenated to give the corresponding triaminobenzene. 

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. 

Selective reduction of polynitro compounds. 2,6-Dinitroanilines can be reduced to nitrophenylenediamines by partial hydrogenation with 20% Pd on... 

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