Selective Reduction of the Nitro-group Using Co2(CO)8-H2O

Abstract

Recently, we reported a new reducing agent prepared from Co2(CO)8 and H2O for the selective 1,4-reduction of α ,βunsaturated carbonyl compounds 1 as well as a tandem reductive Pauson-Khand reaction. 2 We became interested in the reduction power especially the reactivity and the selectivity of this new reducing agent in reducing other functional groups. Reduction of aromatic nitro compounds to amines is a very useful synthetic transformation for which a vast array of reagents has been developed. 3 Though the most general methodology for this conversion is catalytic hydrogenation 4 as it is an economical and effective method, particularly in large scale reactions, the reaction has a limited utility in the presence of other reducible functional groups. 5 The selective reduction of nitro group in presence of other reducible functional groups were also achieved using metal based reduction systems. 6 However, the selective reduction of the nitro group in presence of carbonyl group could not easily be attainable under these conditions. Therefore we became interested in a possibility of the selective reduction of nitro groups in presence of other reducible functional groups including carbonyl groups and halides. Herein, we report that various nitro compounds are selectively and readily reduced to their corresponding amino derivatives in presence of other functional groups. The reaction was tested on p-chloronitrobenzene with the reaction conditions used in the previous report of unsaturated carbonyl compound reduction. 1 Since the nitro group requires more reducing agent than unsaturated carbonyl groups, 2 equivalent of Co 2(CO)8 was used for the reaction with the corresponding amount of water. Dimethoxyethane (DME) was used as the solvent since DME was the optimal solvent for the reduction of the unsaturated carbonyl compounds. To our delight, the reaction completed in 30 minutes and the reaction mixture contained only the desired aniline compound without any trace of other byproducts. When a less amount than 2 equivalent of Co2(CO)8 was used, the reduction was not complete. Then, the reaction was applied to aromatic nitro