Reduction, of nitro compounds

FIGURE 22.16 Synthesis of arylamines by reduction of nitro groups. 

FIGURE 22.17 Reduction of aliphatic nitro compounds to amines. 

Hydrogen telluride and sodium telluride in a protic solvent easily reduce nitroarenes to anilines. In contrast, sodium telluride in an aprotic solvent exhibits a milder reducing capacity and reduces the nitro compounds only to azo compounds.  

The reducing power of sodium hydrogen telluride towards nitro compounds is dependent on their structure. Unhindered nitrobenzenes are reduced to azoxybenzenes whereas sterically hindered nitrobenzenes are reduced to anilines by simple stirring with 5 mol equiv of the reagent at room temperature for 1 h. 

Under alkahne conditions, sodium hydrogen telluride reduces nitrobenzene to A -phenyl-hydroxylamine.  

A more practical procedure for the reduction of nitroarenes to A-arylhydroxyl amines is the use of NaBH4 in the presence of a catalytic amount of tellurium. 

The reduction of nitro- and dinitroalkanes with sodium hydrogen telluride gives, respectively, diazenes (dimers of nitrosoalkenes) and olefins.-  

Preparation Sign in at www.cengage.com/login to read the MSDSs for the chemi-cais used or produced in this procedure. 

Combine 1 g of the nitro compound and 2 g of granulated tin in a small round-bottom flask containing a stirbar. Attach a reflux condenser, and add 20 ml of 3 fW hydrochloric acid in small portions to the stirred mixture through the condenser. 

If the nitro compound is insoluble in the aqueous medium, add 5 mL of 95% ethanol to increase its solubility. 

After the addition is complete, warm the mixture with stirring for 10 min at 70-80 °C. Decant the warm, homogeneous solution into 10 mL of water, and add sufficient 12 Msodium hydroxide solution to dissolve the tin hydroxide completely. Extract the basic solution with several 10-mL portions of diethyl ether. Dry the ethereal solution over potassium hydroxide pellets, decant the liquid, and remove the ether by one of the techniques described in Section 2.29. 

The residue contains the primary amine. Convert it to one of the derivatives described in Section 25.14. 

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Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

Arylamines are generally prepared by the reduction of nitro compounds. When only small quantities are to be reduced and the time element is important and cost is a secondary consideration, tin and hydrochloric acid may be employed, for example ... 

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

Reduction of Nitro Compounds. The mechanism for catalytic hydrogenation of nitro compounds has been the subject of many iavestigations and there is much evidence that this reaction proceeds through several iatermediate species. The most widely accepted mechanism for the hydrogenation of nitro compounds was proposed by Haber ia 1898 (2) (see Fig. 1). 

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). 

Neta.1 Ama.lga.ms. Alkali metal amalgams function in a manner similar to a mercury cathode in an electrochemical reaction (63). However, it is more difficult to control the reducing power of an amalgam. In the reduction of nitro compounds with an NH4(Hg) amalgam, a variety of products are possible. Aliphatic nitro compounds are reduced to the hydroxylamines, whereas aromatic nitro compounds can give amino, hydra2o, a2o, or a2oxy compounds. 

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... 

The validity of the Hammett relationship log K/Ko = pa- has been extensively investigated for five-membered heteroaromatic compounds and their benzo analogues. The ratio Pheterocycie/Pbenzene is closest to Unity for thiophene. Judged from work on the polarographic reduction of nitro compounds, the ability to transmit electronic effects is HC=CH = S < O < NH. 

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. ... 

Platinum may be more useful than palladium in reduction of nitro compounds containing functions easily reduced by palladium. Hydrogenation of I over 5% Pd-on-C was nonselective with hydrogenolysis of the benzyl ethers competing with nitro hydrog ation, but over PtO in ethanol 2 was obtained in 96% yield (4). 

Catalytic cleavage of the nitrogen-oxygen bond occurs very frequently as in reduction of nitro compounds, oximes, and various heterocyclics these reactions are discussed in separate chapters. Considered here are N-oxides, hydroxylamincs, and N- and C-nitroso compounds. 

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

Isocyanates and isothiocyanates are reduced to methylamines on treatment with LiAlH4. Lithium aluminium hydride does not usually reduce azo compounds (indeed these are the products from LiAlH4 reduction of nitro compounds, 19-59), but these can be reduced to hydrazo compounds by catalytic hydrogenation or with... 

Reduction of nitro compounds is a classic application of WGSR systems (Equation (17)). 

Reductive carbonylation of nitro compounds (in particular aromatic dinitro compounds) is an important target in industry for making diisocyanates, one of the starting materials for polycarbamates. At present diisocyanates are made from diamines and phosgene. Direct synthesis of isocyanates from nitro compounds would avoid the reduction of nitro compounds to anilines, the... 

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