Nitro compounds to amines

The Zinin reduction is also usehil for the reduction of aromatic nitro compounds to amines in the laboratory. It requires no special equipment, as is the case with catalytic hydrogenations, and is milder than reductions with iron and acid. Usually ammonium or alkah sulfides, hydrosulftdes or polysulftdes are used as the reactant with methanol or ethanol as the solvent. 

Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

In the reduction of nitro compounds to amines, several of the iatermediate species are stable and under the right conditions, it is possible to stop the reduction at these iatermediate stages and isolate the products (see Figure 1, where R = CgH ). Nitrosoben2ene [586-96-9] C H NO, can be obtained by electrochemical reduction of nitrobenzene [98-95-3]. Phenylhydroxylamine, C H NHOH, is obtained when nitrobenzene reacts with ziac dust and calcium chloride ia an alcohoHc solution. When a similar reaction is carried out with iron or ziac ia an acidic solution, aniline is the reduction product. Hydrazobenzene [122-66-7] formed when nitrobenzene reacts with ziac dust ia an alkaline solution. Azoxybenzene [495-48-7], C22H2QN2O, is... 

Nickel compounds Hydrogenations (e.g. Raney nickel) Conversion of synthesis gas to methane Reduction of organo nitro compounds to amines Carcinogenic (nickel subsulphide). Skin sensitization... 

Analogous to DPNH (144-146), 1,4-dihydropyridines (147) act as reducing agents. For instance, the conversion of aromatic nitro compounds to amines (148) and reduction of enones to ketones (749) has been achieved. 

Hydrogen gas can be replaced by ammonium formate for the reduction of nitro compounds to amines. The ammonium formate method is efficient, and the rapid workup procedure by simple filtratkin makes it widely used for converting the NO to the NH. ° For example, ct-nitro esters are reduced to ct-amino esters in excellent yields on treatment v/ith HCO NH and PdAZ in methanol. ... 

The hydrogenation in the presence of Pd/C is also effective for the conversion of nitro compounds to amines.94 The Michael addition of nitromethane to 2-alkenoic esters followed by catalytic hydrogenation using 10% Pd/C in acetic acid and hydrolysis is a convenient method for the preparation of 3-alkyl-4-aminobutanoic acids, which are important y-amino acids for biological study (Eq. 6.48).94b The reduction can be carried out at room temperature and atmospheric pressure. 

Bechamp A process for reducing organic nitro-compounds to amines, using iron, ferrous salts, and acetic acid. Invented by A. J. Bechamp in 1854 and still used for making certain aromatic amines. 

Iron(II) sulphate is a by-product in many industrial processes, such as the manufacture of titanium dioxide, the pickling of steel sheet before galvanising and the reduction of aromatic nitro compounds to amines using iron catalysts. Conversion of waste iron (II) salts to usable iron oxide pigments, where the quality requirements are not too stringent, is therefore a useful proposition, since it uses up chemicals that would otherwise be regarded as waste products. 

Electrocatalytic hydrogenation has the advantage of milder reaction conditions compared to catalytic hydrogenation. The development of various electrode materials (e.g., massive electrodes, powder cathodes, polymer film electrodes) and the optimization of reaction conditions have led to highly selective electrocatalytic hydrogenations. These are very suitable for the conversion of aliphatic and aromatic nitro compounds to amines and a, fi-unsaturated ketones to saturated ketones. The field is reviewed with 173 references in [158]. While the reduction of conjugated enones does not always proceed chemoselectively at a Hg cathode, the use of a carbon felt electrode coated with polyviologen/Pd particles provided saturated ketones exclusively (Fig. 34) [159]. 

Once a favorite reducing agent for the conversion of aromatic nitro compounds to amines, tin is used nowadays only exceptionally. The reason is partly unavailability, the necessity for the use of strongly acidic media, and... 

The most popular reducing agent for conversion of aromatic nitro compounds to amines is iron [166]. It is cheap and gives good to excellent yields [165, 582]. The reductions are usually carried out in aqueous or aqueous alcoholic media and require only catalytic amounts of acids (acetic, hydrochloric) or salts such as sodium chloride, ferrous sulfate or, better still, ferric chloride [165]. Thus the reductions are run essentially in neutral media. The rates of the reductions and sometimes even the yields can be increased by using iron in the form of small particles [165]. Iron is also suitable for reduction of complex nitro derivatives since it does not attack many functional groups [555]. 

All of these substances can be reduced to benzenamine with tin and hydrochloric acid. As a result, each could be, but not necessarily is, an intermediate in the reduction of nitro compounds to amines. Formation of the bimolecular reduction products is the result of base-induced reactions between nitroso compounds and azanols or amines and possibly further reduction of the initially produced substances (see Exercise 24-18). 

The microwave-assisted reduction of aromatic nitro compounds to amines has also been achieved with SnCl2-2H20/Et0H as the reducing agent (Scheme 4.33). Alkenes and esters are not affected by this reagent1. 

The reducing ability of iron(0) complexes has been exploited for functional group interconversion, for example reduction of aromatic nitro compounds to amines by dodecacarbonyltriiron [17]. 

Ready conversion of nitro compounds to amines further extends the applicability of the deamination process. For example, the nitro group may be introduced into an aromatic nucleus for the purpose of closing a position to other groups in subsequent steps of a synthetic sequence, and, finally, the nitro group is replaced by hydrogen. This blocking effect is utilized in the preparation of 2-chlororesorcinol, which cannot be obtained by direct chlorination of resorcinol.187... 

Reduction of NO. This Ti(ll) reagent reduces aromatic and aliphatic nitro compounds to amines in THF/r-butyl alcohol at 0° in yields of 85-95%. Halo, cyano, and ester groups are not reduced. 

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