Reduction of aromatic nitro compounds with hydrazine hydrate in the presence of an iron oxide/hydroxide catalyst. III. The selective reduction of nitro groups in aromatic azo compounds

The study on the range of applications of a 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 monosubstituted 3- and 4-nitrophenylazobenzenes were reduced selectively by hydrazine hydrate in the presence of the iron oxide/hydroxide catalyst. The selectivity for the nitro group reduction vis-a-vis that of the azo bridge in substituted nitrophenylazobenzenes is higher for compounds with the nitro group in para position to the azo bridge than for the corresponding meta counterparts. This selectivity was enhanced with stronger electron-attracting properties of the substituent. Moreover, the selectivity for the reduction of the nitro group vis-a-vis that of the azo function can be enhanced by increasing the reaction temperature as the Arrhenius activation energy for the nitro reduction is about twice that for the azo cleavage. (C) 1999 Elsevier Science B.V. All rights reserved.