Reduction of nitro compounds and oximes to hydroxylamines

Under suitable conditions it is possible to isolate the A-substituted hydroxylamines that are formed as intermediates in the reduction of nitro compounds. For this purpose it is essential in the reduction of aromatic nitro compounds to work with neutral or nearly neutral solutions suitable reducing agents are hydrogen and platinum oxide catalysts in glacial acetic acid,82,83 zinc dust in ammonium chloride solution,84 aluminum amalgam,85 and ammonium sulfide.86 Aliphatic nitro compounds may be reduced as their alkali salts (nitronates) by diborane in tetrahydrofuran, then giving A-alkylhydroxyl-amines 87 for instance, A-cyclohexylhydroxylamine is thus obtained from nitrocyclohexane in 53% yield. However, aliphatic nitro compounds are converted into A-alkylhydroxylamines more simply by catalytic hydrogenation in the presence of palladium-barium sulfate unlike aromatic nitro compounds, aliphatic nitro compounds require an acid medium for reduction to hydroxylamines an oxalic acid medium has proved the most suitable. 

Alteration of nitrogen groups in carbon-nitrogen compounds 

When a mixed catalyst containing silver is used, the hydrogenation of nitroalkanes can be conducted so that reduction stops at the oxime stage this has been studied in detail for nitrocyclohexane.74 

Oximes of aliphatic aldehydes and ketones are hydrogenated to TV-alkyl-hydroxylamines in the presence of platinum black in aqueous-alcoholic hydrochloric acid 90 also a mixture of the ketone and hydroxylamine hydrochloride may be hydrogenated.91 However, with arylalkyl and aryl ketones the latter method yields amines. 

A reducing agent that converts all types of oximes equally into iV-mono-substituted hydroxylamines is diborane 92 for example, diborane reduces cyclohexanone oxime to iV-cyclohexylhydroxylamine in 82% yield and similarly affords A-heptyl- (81%), A-(/ -nitrobenzyl-(68 %), and iV(-a methylbenzyl)-hydroxylamine (55%). 

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