Reduction of nitro compounds

The reduction of nitroaromatics is conducted on an industrial scale.[1] Many methods exist, such as:

• Catalytic hydrogenation using: Raney nickel[2] or palladium-on-carbon,[3][4][5] platinum(IV) oxide, or Urushibara nickel.[6]
• Iron in acidic media.[7][8][9]
• Sodium hydrosulfite[10]
• Sodium sulfide (or hydrogen sulfide and base). Illustrated by the selective reduction of dinitrophenol to the nitroaminophenol.[11]
• Tin(II) chloride[12]
• Titanium(III) chloride
• Samarium[13]
• Hydroiodic acid[14]

Metal hydrides are typically not used to reduce aryl nitro compounds to anilines because they tend to produce azo compounds. (See below)

Several methods have been described for the production of aryl hydroxylamines from aryl nitro compounds:

• Raney nickel and hydrazine at 0-10 °C[15]
• Electrolytic reduction[16]
• Zinc metal in aqueous ammonium chloride[17]
• Catalytic Rhodium on carbon with excess hydrazine monohydrate at room temperature [18]

Treatment of nitroarenes with excess zinc metal results in the formation of N,N'-diarylhydrazine.[19]

Treatment of aromatic nitro compounds with metal hydrides gives good yields of azo compounds. For example, one could use:

• Lithium aluminium hydride[20]
• Zinc metal with sodium hydroxide.[19] (Excess zinc will reduce the azo group to a hydrazino compound.)

Hydrodenitration (replacement of a nitro group with hydrogen) is difficult to achieve but can be effected by catalytic hydrogenation over platinum on silica gel at high temperatures.[21] The reaction can also be effected through radical reaction with tributyltin hydride and a radical initiator, AIBN as an example.[22]

Aliphatic nitro compounds can be reduced to aliphatic amines by several reagents:

• Catalytic hydrogenation using platinum(IV) oxide (PtO₂)[23] or Raney nickel[24]
• Iron metal in refluxing acetic acid[25]
• Samarium diiodide[26]
• Raney nickel, platinum on carbon, or zinc dust and formic acid or ammonium formate[6]

α,β-Unsaturated nitro compounds can be reduced to saturated amines by:

• Catalytic hydrogenation over palladium-on-carbon
• Iron metal
• Lithium aluminium hydride[27] (Note: Hydroxylamines and oximes are typical impurities.)
• Lithium borohydride or sodium borohydride and trimethylsilyl chloride[28]
• Red-Al[29]

Aliphatic nitro compounds can be reduced to aliphatic hydroxylamines using diborane.[30]

The reaction can also be carried out with zinc dust and ammonium chloride:[31][32][33]

R-NO₂ + 4 NH₄Cl + 2 Zn → R-NH-OH + 2 ZnCl₂ + 4 NH₃ + H₂O

Nitro compounds are typically reduced to oximes using metal salts, such as tin(II) chloride[34] or chromium(II) chloride.[35] Additionally, catalytic hydrogenation using a controlled amount of hydrogen can generate oximes.[36]