Observations on the reduction of aryl nitro groups with palladium‐sodium borohydride

The reduction of aryl nitro groups by the palladium‐sodium borohydride system can be controlled in some instances to give products differing from those of catalytic hydrogenation. With six equivalents of borohydride added to the catalyst 2,2′‐dinitrobiphenyl formed only 2,2′‐diaminobiphenyl. With added sodium hydroxide and varying amounts of borohydride and catalyst, the reduction can be controlled to give benzo[ c ]cinnoline, benzo[ c ]cinnoline 5‐oxide or benzo[ c ]cinnoline 5,6‐dioxide. In a closed system, the reagents reduce both the nitro and olefin functional groups in 6‐nitro‐5,8‐dimethoxy‐1,4‐dihydro‐1,4‐ethano‐naphthalene. In an open flask flushed with argon, the reduction is confirmed to the nitro group. Reduction of 2‐chloro and 4‐chloronitrobenzene with palladium‐borohydride gives substantial yields of the appropriate chloroanilines. In contrast, hydrogen and palladium give aniline as the major product from each of these. These results suggest that the addition of sodium borohydride to palladium on carbon produces a reductive entity differing from that of catalytic hydrogenation.