For reduction of nitro compounds

Lithium aluminum hydride is a convenient reagent for reduction of nitro compounds, nitriles, amides, azides, and oximes to primary amines. Catalytic hydrogenation works also. Aromatic nitro compounds are reduced best by reaction of a metal and aqueous acid or with ammonium or sodium polysulfides (see Section 23-12B). Reduction of /V-substituted amides leads to secondary amines. 

General procedure for reduction of nitro compounds with Raney nickel and hydrazine 58 The nitro compound (1 g) is dissolved in ethanol (10 ml) and treated with a two- to three-fold molar amount of 100% hydrazine hydrate. The mixture is warmed on a water-bath, and a small amount of Raney nickel is added. Reaction is complete in 5-60 min, as can be recognized from the facts that gas evolution stops and the previously yellow solution becomes colorless. Then fresh catalyst is added and the mixture is boiled to destroy the excess of hydrazine. The catalyst is filtered off and the filtrate is boiled with charcoal, filtered again, and, after cooling, poured into cold water. 

Since the nitroso group is the first stage in reduction of a nitro group, the conditions under which the former is reduced to an amino group differ, as might be expected, little from those used for reduction of nitro compounds. 

Aluminum amalgam (Al/Hg) is also a useful reagent that is most commonly employed for hydrogenolysis of sulfur compounds.533 Reduction of sulfoxides is an important application,534 as is reduction of sulfones. Aluminum amalgam reduces the a-phenylsulfonyl moiety in 554 to give lactone 555 in 68% yield.535 Aluminum amalgam has also been used for reduction of nitro compounds to the amine,536 and reduction of azides to the amine.532... 

Co -", Ni "", Cu -", and Zn ) [73], GR-Pd-CdS [76], and hierarchical CdS-ZnO-GR hybrids [108]. It is found that their photoactivities for reduction of nitro compounds to amino compounds with ammonium formate (HCOONH4) for hole scavenger in N2 atmosphere (Scheme 8.11) under visible light irradiation are all remarkably enhanced as compared to the blank semiconductors [73,100-102,106, 107]. It has been concluded that the selective reduction efficiency can be driven by appropriate introduction of GR into the matrix of pure semiconductor, which can boost the transfer and prolong the lifetime of the electrons photoexcited from the semiconductor due to the tighter connection between GR and the semiconductor, as well as the optimization of the atomic charge carrier transfer pathway across the interface between GR and the semiconductor. 

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). 

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. ... 

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... 

Spin trapping has been widely used for superoxide detection in various in vitro systems [16] this method was applied for the study of microsomal reduction of nitro compounds [17], microsomal lipid peroxidation [18], xanthine-xanthine oxidase system [19], etc. As DMPO-OOH adduct quickly decomposes yielding DMPO-OH, the latter is frequently used for the measurement of superoxide formation. (Discrimination between spin trapping of superoxide and hydroxyl radicals by DMPO can be performed by the application of hydroxyl radical scavengers, see below.) For example, Mansbach et al. [20] showed that the incubation of cultured enterocytes with menadione or nitrazepam in the presence of DMPO resulted in the formation of DMPO OH signal, which supposedly originated from the reduction of DMPO OOH adduct by glutathione peroxidase. 

Imanaka—heterogenization of Rh complexes. In 1991, Imanaka and coworkers124 reported the heterogenization of Rh complexes by binding them to aminated polymers. As discussed previously, these findings led to fruitful research by Ford, Pardey, and others. The isolated polymer-bound Rh carbonyl anion complex was found to be reusable for reactions such as water-gas shift and reduction of nitro compounds. The polymer-bound Rh complexes were characterized by infrared spectroscopy. Water-gas shift activity (80 mol H2 per mol Rh6(CO)i6 in 24 hours) was recorded using the Rh complexes at 100 °C with 0.92 atm of CO, 2.16 ml H20, 0.05 mmol Rh6(CO)16, aminated polystyrene, 5.0 mmol of N, 5.56 ml ethoxyethanol and reduction of nitro-compounds (e.g., aliphatic nitro compounds to nitriles, oximes to nitriles, hydroxylamines to nitriles, and N-oxides to amines) occurred at 40 °C. 

Two basic methods are used for the manufacture of sulphur dyes. In one, the starting materials are baked either with sulphur alone or with sulphur and sodium sulphide at a temperature between 180 and 350 °C. Alternatively the intermediates are heated under reflux in aqueous or alcoholic sodium polysulphide this process may also be carried out under pressure at temperatures up to about 130 °C. Following sulphurisation the dye is precipitated by means of air or chemical oxidation, acidification or a combination of these methods. The sulphurisation process results in the evolution of hydrogen sulphide, which is usually absorbed in aqueous sodium hydroxide for use elsewhere - in the reduction of nitro compounds, for example. 

For the reduction of nitro-compounds on a small scale, tin or stannous chloride and concentrated hydrochloric acid are the most suitable reagents. Solids are often difficult to reduce in the absence of a solvent and call for the addition of alcohol or glacial acetic acid. When the addition of water to a sample of the reaction mixture produces a clear solution it is known that the reduction is complete. The base will, of course, be present as hydrochloride, and hydrochlorides are, almost without exception, soluble in water. It should be noted, however, that sparingly soluble double salts with stannous chloride are often formed, but these double salts are usually soluble in boiling water. 

For the reduction of nitro-compounds containing a group which may be attacked by nascent hydrogen, as, for example, an aldehyde group, an unsaturated side chain, and so on, special methods must he applied. In such cases ferrous hydroxide or iron powder (cf. Chap. VII. 5, arsanilic acid) are often used. The reduction is carried out thus a weighed amount of ferrous sulphate is caused to act, in the presence of alkali (potassium or sodium hydroxide, baryta), on the substance to he reduced. In this way it is possible to reduce, for example, o-nitro-benzaldehyde to aminobenzaldehyde, and o-nitrocinnamic acid to amino-cinnamic acid. 

Titanium ions can also he used as redox catalysts for the indirect cathodic reduction of nitro compounds (417). The electroreduction is carried out in an H20-H2S04/Ti(S04)2-(Pb/Cu) system at 45 80°C under 5 20Am . Nitrobenzene, dinitrobenzene, nitrotoluene, 2,4-dinitrotoluene, 2-nitro-m-xylene, nitro-phenol, 2,4-dinitrophenol, nitrophenetole, o-nitroanisole, 4-nitrochlorotoluene, ni-trobenzenesulfonic acid, and 4,4 -dinitro-stilbene-2,2 -disulfonic acid can all be reduced by this procedure to the corresponding amino compounds (418) in good yields (Scheme 146) [513-516]. Tin... 

Stannous chloride is used most frequently for the reduction of nitro compounds [177, 178, 179] and of quinones [180, 181], It is also suitable for conversion of imidoyl chlorides [182] and of nitriles [183] to aldehydes, for transformations of diazonium salts to hydrazines [184], for reduction of oximes [f[Pg.30]

Titanous chloride (titanium trichloride) is applied in aqueous solutions, sometimes in the presence of solvents increasing the miscibility of organic compounds with the aqueous phase [199, 200]. Its applications are reduction of nitro compounds [201] and cleavage of nitrogen-nitrogen bonds [202] but it is also an excellent reagent for deoxygenation of sulfoxides [203] and amine oxides [199] (Procedure 38, p. 214). 

With the exception of the nitroso stage, all the intermediate stages of the reduction of nitro compounds can be obtained by controlled catalytic hydrogenation [572], and all reduction intermediates were prepared by reduction with appropriate hydrides or complex hydrides. However, the outcome of many hydride reductions is difficult to predict. Therefore more specific reagents are preferred for partial reductions of nitro compounds. 

In this paper, the selectivity of the ECH method for the reduction of nitro compounds to the corresponding amines on RCu electrodes will be compared with that of reduction by RCu alloy powder in alkaline aqueous ethanol. In the latter method (termed chemical catalytic hydrogenation (CCH)), chemisorbed hydrogen is generated in situ but by reduction of water by aluminium (by leaching of the alloy) (equation [12]). The reductions by in situ leaching must be carried out in a basic medium in order to ensure the conversion of insoluble Al(OH)3 into soluble aluminate (equation [12]). The selectivity and efficiency of the electrochemical reduction of 5-nitro-indoles, -benzofurane, and -benzothiophene at RCu electrodes in neutral and alkaline aqueous ethanol will also be compared with that of the classical reduction with zinc in acidic medium. 

It is natural to presuppose that the reduction of nitro compounds should lead to the nitroso compounds, at least as an intermediate stage. Until quite recently, no reductive processes for the formation of nitrosoalkanes were known [3], More recently, some indirect evidence is said to show that, on electrolytic reduction of tertiary aliphatic nitro compounds, the final t-alkyl-hydroxylamines are produced by the intermediate formation of nitroso compounds which were not isolated [99]. 

The reduction of aromatic nitro compounds is believed to proceed to an intermediate mixture of nitroso compounds and substituted hydroxylamines which are not isolated but condense to form an azoxy compound which, in turn, is reduced to an azo compound. Contributing evidence to substantiate this mechanism is that the reduction of a mixture of two aromatic nitro compounds leads to a mixture of azo compounds consistent with that predicted if each of the nitro compounds were reduced to a nitroso compound and a hydroxylamine and these, in turn, reacted with each other in all possible combinations. This observation also implies that the bimolecular reduction of nitro compounds is practical only from the preparative standpoint for the production of symmetrically substituted azo compounds. Spectrophotometric studies of the reaction kinetics of the reduction of variously substituted nitro compounds may, however, uncover reasonable procedures for the synthesis of unsymmetrical azo compounds. 

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