Hydrogenation aromatic nitro compounds

Figure 11.14 Formation of instable intermediates during the catalytic hydrogenation of nitro-aromatic compounds. The decomposition of phenyl hydroxyl amine (I) is shown in the thermogram at 70% of hydrogen uptake. The decomposition of the nitro aromatic compound (D) decreases as the hydrogenation progresses.

Nitro-polycyclic aromatic hydrocarbons, referred to as nitro-aromatic compounds hereafter, constitute one of the most troubling classes of environmental pollutants. They are derivatives of polycyclic aromatic hydrocarbons (PAHs) that contain two or more fused aromatic rings made of carbon and hydrogen atoms and at least one nitro group (Fig. 10.1). Concern about these compounds arises partly from their ubiquity nitro-aromatic compounds are released to the environment directly from a variety of incomplete combustion processes [1] and are also formed in situ by atmospheric reactions of PAHs [2]. Nitro-aromatic compounds have been found in grilled food in diesel, gasoline, and wood-smoke emissions and are commonly found in atmospheric particulate matter, natural waters, and sediment [3-8],... 

Ziegler catalysts prepared from nickel or cobalt 2-ethylhexanoate and triethylaluminum (3 or 4 1 ratio) are very efficient for the hydrogenation of aromatic compounds.o-Xylene (51) gives a mixture of cis- (52) and trans-1,2-dimethylcyclohexane (53) in 65 35 ratio. Phenol (54), dimethyl phthalate and dimethyl terephthalate are also reduced in high yield, while nitro-substituted benzene and phenols cannot be reduced. Catalytic activity toward benzene hydrogenation decreases in the order Ni > Co > Fe > Cr > Cu. ... 

The selectivity of the hydrogenation halo-nitro aromatic compounds can be influenced by cyclodextrins, as additives, or by using cyclodextrin-derived catalysts [232] (see Ch. 10). 

By an analogous method Pt-silk and Rh-silk catalysts were prepared and possessed activities for hydrogenation of nitro-aromatic compounds, ketones, and alkenes. 

The asymmetric-transfer hydrogenation of aromatic compounds to enantiopure cyclic nitro compounds with two contiguous stereocentres in benzene is catalysed by H8-BfNOL-derived phosphoric acid with Hantzsch ester as the hydrogen source. The 1,4-// addition intermediate, formed in the initial step of the reaction, was isolated. The product is obtained with 97% yield and 99% ee. ... 

Cyclohexanones may serve as precursors to aromatic amines in a reductive alkylation, the source of hydrogen being aromatization of the cyclohexanone (66). In a variation, an aromatic nitro compound acts as both an amine precursor and a hydrogen acceptor (64). 

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... 

When aromatic compounds are reacted with hydrogen, the catalyst used determines which part of the molecule reacts. Thus, with the right catalyst, a nitro group can be converted to an amine without adding hydrogen to the ring. In this case the simplest aromatic amine (aniline) is produced. 

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. 

Competitive consecutive reactions are combinations of parallel and series reactions that include processes such as multiple halogenation and nitration reactions. For example, when a nitrating mixture of HN03 and H2S04 acts on an aromatic compound like benzene, N02 groups substitute for hydrogen atoms in the ring to form mono-, di-, and tri-substituted nitro compounds. 

In recent years, the importance of aliphatic nitro compounds has greatly increased, due to the discovery of new selective transformations. These topics are discussed in the following chapters Stereoselective Henry reaction (chapter 3.3), Asymmetric Micheal additions (chapter 4.4), use of nitroalkenes as heterodienes in tandem [4+2]/[3+2] cycloadditions (chapter 8) and radical denitration (chapter 7.2). These reactions discovered in recent years constitute important tools in organic synthesis. They are discussed in more detail than the conventional reactions such as the Nef reaction, reduction to amines, synthesis of nitro sugars, alkylation and acylation (chapter 5). Concerning aromatic nitro chemistry, the preparation of substituted aromatic compounds via the SNAr reaction and nucleophilic aromatic substitution of hydrogen (VNS) are discussed (chapter 9). Preparation of heterocycles such as indoles, are covered (chapter 10). 

If an aromatic compound contains saturated aliphatic side chains nitration carried out under the above conditions takes place always in the benzene nucleus and not in the side chain. Since the carbon atoms of benzene are each united directly to only one hydrogen atom, the nitro-derivatives obtained are tertiary and therefore incapable of forming salts, nitrolic acids, or pseudonitroles, as do the primary and secondary nitro-compounds. 

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. 

Ruthenium is commonly used with other platinum metals as a catalyst for oxidations, hydrogenations, isomerizations, and reforming reactions. The synergetic effect of mixing ruthenium with catalysts of platinum, palladium, and rhodium lias been found for the hydrogenations of aromatic and aliphatic nitro compounds, ketones, pyndine, and nitriles. 

SELECTIVE HYDROGENATION OF AROMATIC AND ALIPHATIC NITRO COMPOUNDS BY HYOROGEN TRANSFER OVER MgO... 

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

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