Process for the preparation of substituted aromatic amino...

Details Process for the preparation of substituted aromatic amino... Process for the preparation of substituted aromatic amino...

1999-03-10

2001-07-10

Barts, Samuel (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Amino nitrogen containing

C564S165000, C564S416000, C564S417000, C564S418000, C564S423000, C562S433000, C562S452000, C562S456000, C560S047000, C560S019000, C560S020000

Reexamination Certificate

active

06258982

ABSTRACT:

This appliction is a 371 of PCT/EP 97/05/157 The present invention relates to a hydrogenation process for the preparation of aromatic amino compounds containing directly on the aryl ring or in a side chain one or more entities that may also undergo hydrogenation, such as carbon multiple bonds, nitrile groups, imino groups or carbonyl groups. The preparation is carried out by catalytic hydrogen cation of the corresponding aromatic nitro compounds in the presence of a phosphorus-modified noble metal catalyst The invention relates also to the use of modified noble metal catalysts for the hydrogenation of aromatic nitro compounds containing carbon multiple bonds and/or substituted by nitrile, imino or carbonyl groups.
It is known that aromatic nitro compounds can be reduced in the presence of noble metal catalysts and hydrogen to aromatic amines with very good Yields. When further hydrogen-ateable groups, for example carbon multiple bonds, nitrile, imino or carbonyl groups, are present at the same time, special measures are required in order to prevent the formation of undesirable secondary products, which otherwise can be separated from the desired product often only by resource-intensive means or, in especially unfavourable cases, not at all. Selective reduction is especially difficult when several hydrogenateable groups are present in a compound.
The prior art contains a number of proposals for the selective reduction of aromatic nitro compounds substituted by hydrogenateable groups. For example, WO 95/32941 and WO 95/32952 describe a process for the hydrogenation of aromatic nitro compounds that are substituted by at least one group having a carbon multiple bond. Noble metal catalysts modified with lead, mercury, bismuth, germanium, cadmium, arsenic, antimony, silver or gold are proposed as catalysts. Although those catalysts are very suitable in principle and can also be used on a production scale, occasional disadvantages emerge in respect of their activity and selectivity.
U.S. Pat. No. 4,020,107 proposes phosphorous acid, hypophosphorous acid or derivatives thereof as additives when aromatic nitro compounds halo-substituted on the aromatic moiety are hydrogenated using Pt or Pd/active carbon and hydrogen.
Although those systems are selective in respect of the halogen substituents present in the molecule, they frequently have a limited reactivity. In many cases, therefore, the formation of considerable amounts of aryl hydroxylamine is observed (J. R. Kosak, in Catalysis of Organic Reactions, Vol 18, (1988), 135-147; idem, in Catalysis in Organic Synthesis, 1980, 107-117).
Catalytic hydrogenations of aromatic nitro compounds to the corresponding aromatic amines proceed via several intermediates. The corresponding nitroso compounds and especially the hydroxylamine intermediate are of importance therein, as described, for example, by M. Freifelder in Handbook of Practical Catalytic Hydrogenation, published by Wiley-lnterscience, New York, 1971.
That hydroxylamine intermediate represents a particular problem in practice because it can, under certain conditions, accumulate in large amounts in the reaction solution. That is true especially of aromatic nitro compounds, the hydrogenation of which produces relatively stable aryl hydroxylamines, and is especially critical when the hydrogenation is carried out in a slurry batch reactor. In an extreme case, several tonnes of aryl hydroxylamine can be formed in that way.
The accumulation of aryl hydroxyiamines is undesirable in many respects. For example, it is known that such compounds are often thermally unstable and when heated, with or without H
2
, can undergo disproportionation with marked emission of heat. The heat being released can trigger further decomposition reactions which may then result in incidents involving serious explosions. W. R. Tong et aL., AICHE Loss Prev. 1977, (11), 71-75 describe such an incident in the reduction of 3,4-dichloro nitrobenzene to 3,4-dichloroaniline.
That instability makes a detailed and resource-intensive thermal study of hydrogenation mixtures indispensable before commencing production. The thermal characteristics of the potential hydroxylamine intermediates especially must be thoroughly studied. F. Stoessel, J. Loss Prev. Process Ind., 1993, Vol 6, No. 2, 79-85 describes that procedure using the example of the hydrogenation of nitrobenzene to aniline.
Furthermore, aryl hydroxylamines are known to be powerful carcinogens and therefore represent a major potential hazard in the event of the hydrogenation being discontinued or being incomplete (J. A. Miller, Cancer Res. 3 (1970), 559).
A third problem area is the preparation of a pure amine. If there are significant amounts of aryl hydroxylamine present during the hydrogenation or at the end of the reaction, that may result in fusions, with undesirable, coloured azo or azoxy products being formed. Since the amount of aryl hydroxylamine may differ from batch to batch, a product quality is obtained that varies in purity and appearance.
The problems mentioned hereinbefore are made even more acute by the fact that neither the concentrations being formed nor even the maximum possible concentrations of that hydroxylamine intermediate can be predicted, even for well known and well studied processes. The presence of impurities in the trace range may trigger the spontaneous accumulation of hydroxylamine intermediates in an unpredictable manner. For example, J. R. Kosak in Catalysis of Organic Reactions, Vol 18, (1988), 135, describes how, in the hydrogenation of 3,4-dichloronitrobenzene, the addition of merely 1% NaNO
3
increases the accumulation from an initial <5% to about 30%.
It has now been found, surprisingly, that the catalytic hydrogenation of aromatic nitro compounds containing directly on the aryl ring or in a side chain one or more entities that may also undergo hydrogenation, such as carbon multiple bonds, nitrile groups, imino groups or carbonyl groups, can be carried out selectively when rhodium, ruthenium, iridium, platinum or palladium catalysts modified with an inorganic or organic phosphorus compound having an oxidation state of less than 5 are used.
Contrary to expectations, it has been shown that, using those catalyst systems, aromatic nitro compounds can be reduced selectively to the corresponding amino compounds, without the unsaturated carbon, —CN or —CO bonds of the substituents on the aromatic nitro compound being hydrogenated at the same time.
Surprisingly, in many cases only low concentrations of hydroxylamine are formed. In cases where relatively large amounts of hydroxylamine are to be expected, however, that hydroxylamine formation can be virtually completely suppressed by adding catalytic amounts of a co-catalyst, for example a vanadium compound. Usually, hydroxylamine concentrations of less than 1% are observed. As a result, it is now possible to use relatively high concentrations of aromatic nitro compounds, which contributes to the process being carried out in an extremely economic manner.
The activity and selectivity of the catalyst systems is high, especially in the case of very sensitive compounds, such as, for example, propargyi nitrobenzoate.
The catalyst systems can be readily prepared from well known and commercially available standard noble metal catalysts, such as standard Pt or Pd catalysts, so that a constant catalyst quality is assured, which is important for large-scale production.
Since no further heavy metal compounds are required for the modification, there is also no possibility of the end products being contaminated with heavy metals.
It is often possible to use a low pressure (about 5 bar) and a relatively low temperature (about 100° C.) in the hydrogenation.
A further advantage of the process over known reduction methods, such as, for example, the Bechamp or sulfide reduction, lies in the fact that no Fe sludges and no acidic or sulfur-containing waste waters, which require disposal, are formed. The product is obtained with a high degree of purity, since virtually no azo or azoxy compounds are form

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