Selective reduction of nitro groups in aromatic azo compounds

This work deals with the selective reduction of aromatic nitro compounds to the corresponding aromatic amines with hydrazine hydrate in the presence of catalytic amounts of a modified iron oxide hydroxide compound. The dependence of the rate of reduction on the nature and the position of additional substituents other than the nitro group was determined. The rate is enhanced by electron-withdrawing substituents and decreased by electron-donating groups. Moreover, our study on the range of application of this cheap iron oxide hydroxide modification as a H-transfer catalyst opened up a promising new route for the selective reduction of nitro groups in aromatic azo compounds. A series of mono-substituted 4-nitroazobenzenes were selectively reduced by hydrazine hydrate in the presence of the iron oxide hydroxide catalyst. The selectivity for the reduction of the nitro group vis-a-vis that of the azo bridge was increased with stronger electron-withdrawing properties of the substituent R. For 4-nitroazobenzenes with electron-donating substituents the rate constants of the reductive cleavage of the azo bridge and of the nitro group reduction are of the same order of magnitude. For the reduction of the nitro group, (1) in the unsubstituted 4-nitroazobenzene and for the reductive cleavage of the azo function (3) in the corresponding 4-aminoazobenzene (Scheme 1), the Arrhenius activation energies were determined to be E-a(1) = 81.5 kJ/mole and E-a(3) = 46.6 kJ/mole, respectively. Thus, the selectivity for the reduction of the nitro group vis-a-vis that of the azo function can be enhanced by higher reaction temperatures.