Hydrogenation of aromatic nitro compound

Aminoaromatics produced by the selective catalytic hydrogenation of the corresponding nitro precursors are relevant intermediates for agrochemicals, pharma- 

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Hydrogenation of aromatic nitro compounds is very fast, and the rate is limited often by the rate of hydrogen transfer to the catalyst. It is accordingly easy to use inadvertently more catalyst than is actually necessary. Aliphatic nitro compounds are reduced much more slowly than are aromatic, and higher catalyst loadings (6,11) or relatively lengthy reduction times may be... 

We now illustrate the opposite case where the intermediate is in fact a highly undesirable substance, as it presents a health, or even explosion, hazard. The hydrogenation of aromatic nitro compounds, such as the one shown in Fig. 2.6, is industrially important for the production of dyes, whiteners, agrochemicals and pharmaceuticals. The reaction occurs in the presence of a platinum catalyst and proceeds via intermediates, among which the hydroxylamine (-NHOH) species is particularly hazardous, as it is both carcinogenic and explosive. Unfortunately, standard platinum catalysts give rise to high levels of this undesired intermediate. 

Hydrogenation of aromatic nitro compounds [8,18,29] and hydrogenation of benzene derivatives [2,9,21] have been generally accepted as model reactions to check the heterogeneous nature of catalyst, because homogeneous species are not believed to be active. But at least two well-studied examples show that molecular catalysts can hydrogenate benzene [36,37]. 

Hydrogenations of aromatic nitro compounds are important in industry. A nitro group can be easily introduced into a benzene ring and then hydrogenated to the amine. During the hydrogenation, a number of coupling and alkylation reactions are possible as depicted in Fig. 2.31. 

Tyrlik u. M. Kwiecinski, Przem. Chem. 61, 434-437 (1982) . .Hydrogenation of Aromatic Nitro Compounds by Complex Compounds as a Method for Synthesizing Organic Intermediates". 

Hydrogenation catalyst. C,ePd catalyzes hydrogenation of aromatic nitro compounds to anilines and of nitroalkanes to amines (both quantitative). It is also effective for hydrogenation of alkynes to (Z)-alkenes in 90-99% yield, particularly when cthylcnediamine is also present. In fact it is superior to palladium on carbon in the presence of EDA, even though the rate is somewhat slower. 

Stoessel, F. (1993) Experimental study of thermal hazards during the hydrogenation of aromatic nitro compounds. Journal of Loss Prevention in the Process Industries, 6 (2), 79-85. 

Blout and Silverman (57) have reported a good example of selective reduction with Raney nickel. It was found that Raney nickel effectively catalyzes the hydrogenation of aromatic nitro compounds in... 

The nitro group attached to an aromatic ring is one of the most reactive functions toward both chemical and catalytic reductions and in most cases preferentially hydrogenated over other reducible functions. Hydrogenation of aromatic nitro compounds... 

Palladium catalysts, in particular, in the form supported on carbon have frequently been employed for the hydrogenation of aromatic nitro compounds and are often preferred to platinum oxide because of their high activity as well as less tendency toward hydrogenation of the aromatic ring.85,86 3,4-Dimethoxynitrobenzene was hydrogenated to 4-aminoveratrole in a 97% yield over 5% Pd-C in ethanol at room temperature and 0.2 MPa H2 (eq. 9.40). 

The catalytic hydrogenation of aromatic nitro compounds is an industrially important process for the preparation of aromatic amines that are used as intermediates for the manufacture of polyurethanes, rubber chemicals, agricultural products, dyestuffs, photographic chemicals, and drugs, as well as various other chemicals. Stratz has com-... 

Hydrogenation of aromatic nitro compounds with heterogeneous catalysts is often the method of choice for the production of the corresponding anilines. As... 

There now exists evidence for the extension of two-phase catalysis into the new area of Ci-chemistry. Thus, Leitner an co-workers [206] described the biphase hydrogenation of CO2 to formic acid (cf. Section 3.3.4). Two-phase hydrogenations of aromatic nitro compounds with Pd or Rh catalysts are examined by Tafesh and co-workers [207] and others [212 f, 218 d, 226]. 

The validity of suggested relations is self-evident, provided that the assumptions used in deriving them have been fulfilled. Cerveny et al. (102) report the measurements of values of the parameter Sp used in the derived equations, for the hydrogenation of the double bond in aliphatic systems and for the hydrogenation of aromatic nitro compounds to aromatic amines. 2,3-Dimethyl-2-butene and nitrobenzene were the standard substrates used the hydrogenations were performed in seven solvents using 5% Pt on silica gel as catalyst. 

Hydrogenation of aromatic nitro compounds to aromatic amines Hydrogenation of aldehydes and ketortes to alcohols Hydrogenation of atomic olehnic groups Hydrogenation of unsaturatcd nitriles to unsaturated amines Hydrogenation of diolefins artd alkynes to monoolehns Hydration of ethylene oxide alkylation of aromatics with olehns oxidation of alcohols to aldehydes Oxidation of ethylene to ethylene oxide Synthesis of HCN from NH, and CH4 Oxidation of CH3OH to HCHO... 

Selective catalytic hydrogenation of aromatic nitro compounds finds many applications in fine and specialty chemical industries (1). This class of hydrogenation reactions has been studied extensively using various solvents, catalysts and under various reaction conditions (1). The hydrogenation reaction has been found to follow mainly a mechanism that was delineated by Haber in 1898 from his study of electrochemical reduction of nitrobenzene (2). The mechanism, consisting of two types of reaction pathways, is schematically described in Fig. 1. The first pathway is a monomeric one that proceeds in three consecutive steps (a) hydrogenolysis of one of the N-O bonds in the nitro group to produce the nitroso intermediate ... 

The catalytic hydrogenation of aromatic nitro compounds is a potentially hazardous reaction. The safety of a specific operation depends both on the nature of the nitro arene and on the operating conditions [14],... 

Catalytic hydrogenations of aromatic nitro compounds with a stable hydroxylamine intermediate often have two different kinetic phases hydrogen uptake is rapid up to ca 60 %, then distinctly slower in the second phase. This means that reduction of the hydroxylamine to the aniline, formally a hydrogenolysis, is difficult in these cases. In the presence of the promoters discussed in Section 8.5.4.3, the second phase is less pronounced or disappears. This suggests a mechanism which could be called catalytic by-pass (see Figure 4). Experiments in the absence of hydrogen indicated that the vanadium promoters catalyze the disproportionation to give aniline and the nitroso intermediates that re-enter the catalytic cycle. As a consequence, the hydroxylamine does not accumulate and aniline formation is accelerated. 

A silk catalyst of especial efficiency has been developed for hydrogenation of aromatic nitro compounds.51 Boiling silk fibroin fibres with an aqueous solution of potassium tetrachloroplatinate affords a brown, hair-like silk-platinum chelate from which the actual black silk catalyst is obtained by hydrogenation at 9 atm in the presence of 1 % acetic acid. Its activity, calculated on platinum content, for hydrogenation of nitrobenzene to aniline, is 250 times that of platinum dioxide. 

Transfer hydrogenation of aromatic nitro compounds. Aromatic nitro compounds are reduced to anilines when refluxed in excess cyclohexene in the presence of ordinary commerical 10% Pd/C catalyst. The method is very useful for selective reduction of polynitrobenzenes. Halogen, if present, is eliminated. Cyclohexene is superior to cyclohexa-1,3-diene as hydrogen donor. The reaction is usually successful, but slower, with sulfur-containing substrates. 4-Methoxy-2,5-dinitroanisole is reduced to 2,5-dimethoxy-4-nitroaniline in 10 min. in the steam bath under these conditions. 

Rhodium-Phosphine Complexes as Homogeneous Catalysts. Hydrogenation of Aromatic Nitro Compounds ... 

Addition of triethyl amine to Rh(PPhs)3Cl or to complexes formed from [Rh(l,5-hexadiene)Cl]2 and phosphines under hydrogen yields very active catalysts for the hydrogenation of aromatic nitro compounds to amines. The dark brown homogeneous catalyst solutions show highest activity at molar ratios of Rh/PRs/EtsN = 1 1.2 3. Turnovers above 1 mol H2/mol Rh min are achieved. 

Loose, black, nonpyrophoric powder relatively stable in air, provided the paraffin is completely removed and well wetted by water. Shows an activity level similar to that of Raney Ni in hydrogenation of aromatic nitro compounds in aqueous solutions. ... 

Gowda, D. C. Mahesh, B. Catalytic transfer hydrogenation of aromatic nitro compounds by employing ammonium formate and 5% platinum on carbon. Synth. Commun. 2000, 30, 3639-3644. 

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