Dinitrobenzenes, 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... 

Raney copper is another Raney type catalyst that is prepared from a copper-aluminum alloy. This catalyst has been used infrequently but does show some reaction selectivity not possible with other catalysts. Of particular interest is its use for the selective hydrogenation of substituted dinitrobenzenes (Eqn. 11.6).2 This catalyst, as well as Raney cobalt, generally promotes fewer side reactions than does Raney nickel. 25... 

Fig. 14.1. Steric effects in the selective hydrogenation of 1 -substituted-2,4-dinitrobenzenes.

The selective hydrogenation of one nitro group of a dinitrobenzene represents yet another type of reaction selectivity observed with aromatic nitro groups. This reaction, which follows a combination of Type 11 and Type 111 selectivity (Chapter 5) (Eqn. 19.7) is influenced by a number of factors. It was found that in the hydrogenation of 2,6-dinitrotoluene to 2-amino-6-nitrotoluene (Eqn. 19.8), platinum and rhodium were more selective than palladium. With Pt/Al203... 

The traditional treatment of nitrobenzene (1) with iron and acid, called Bechamp reduction, was employed almost exclusively in the production of aniline (2) and many aromatic amines until the 1960s1,2 (Scheme 1). The reduction is straightforward, and can also be achieved by catalytic hydrogenation, sodium sulfide reduction and zinc reduction with caustic soda. Nitrotoluenes and nitroxylenes are hydrogenated under pressure over a nickel catalyst supported on kieselguhr. The sulfide reduction is useful in selective reduction, such as of m-dinitrobenzene to m-nitroaniline. 

The studies in Table 9.3 show the use of a number of different conditions, especially with regard to the substrate, solvent and the support for Pd and Pt. The conditions often result from a limited survey by the authors to find the solvent and catalyst that give optimum rate and selectivity for their chosen substrate. For example, Khilnani and Chandalia found that hydrogenation of m-dinitrobenzene was much faster in acetone than in ethanol, and studied the kinetics in acetone. Despite these differences, one might hope to find some conunon kinetic features in these studies, at least for those using the same metal and similar solvents. 

The deposition of PdCl2 on interlamellar silylamine afforded a catalyst that catalyzed the selective hydrogenation of only one nitro group of dinitrobenzene 170 to nitroaniline 171 (Scheme Exclusive hydrodebromination was observed with p-bromonitrobenzene to give nitrobenzene, but only nitro reduction was observed with dichloronitrobenzene 176. The formyl group of 174 was stable under the reaction conditions. The catalyst was only... 

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