Preparation of n-aryl- and n-hetarylhydroxylamines

Details Preparation of n-aryl- and n-hetarylhydroxylamines Preparation of n-aryl- and n-hetarylhydroxylamines

1997-07-07

1998-11-03

Lambkin, Deborah C.

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

Organic compounds

Nitrogen attached directly or indirectly to the purine ring...

546310, 546312, 546159, 546183, 548255, 5483661, 564300, 564256, C07D23402, C07D21172, C07D24904, C07D23900

Patent

active

058310932

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for preparing aromatic or heteroaromatic hydroxylamines of the general formula I ##STR3## where R.sup.1 is an unsubstituted or substituted aryl radical or an unsubstituted or substituted hetaryl radical from the pyridine or quinoline groups, by hydrogenation of nitro compounds of the general formula II platinum catalyst on an activated carbon support or in the presence of a palladium catalyst doped with sulfur or selenium on an activated carbon support, which comprises carrying out the reaction in the presence of a nitrogen-substituted morpholine compound of the general formula III ##STR4## R.sup.2 being alkyl radicals having 1 to 5 carbon atoms and R.sup.3 to R.sup.10 being hydrogen atoms or alkyl radicals having 1 to 5 carbon atoms.
The catalytic hydrogenation of aromatic nitro compounds to N-phenylhydroxylamines has been known for a long time (Houben-Weyl, Methoden der Org. Chemie (Methods of Org. Chemistry), Vol. 10/1, pp. 1155-1157; Vol. E 16a, Part 1, pp. 49-53). In comparison to relatively expensive electrochemical reduction and to reduction with metals, such as, for example, with zinc dust, amalgams, inter alia, which have an unfavorable waste material balance, catalytic hydrogenation is the most favorable method from an economic point of view. A problem in this type of reaction is the further reaction to give the aromatic amine, the stable final substance, and the disproportionation of the N-phenylhydroxylamine formed to the corresponding nitroso compounds and anilines. As a result of subsequent reactions, higher molecular weight by-products, such as azoxybenzenes, azobenzenes and hydrazobenzenes, can be formed from these undesirable intermediates and secondary products, which can arise, for example, by condensation of nitrosobenzenes and N-phenylhydroxylamines and further reaction of the azoxybenzenes formed, and under certain circumstances cause substantial losses of yield.
As a rule, Pd or Pt catalysts are recommended for these hydrogenation reactions. In order to obtain yields or selectivities of >50%, according to the present state of knowledge catalyst additives in the form of dimethyl sulfoxide, divalent sulfur compounds or various organic phosphorus compounds are necessary (EP 85890, EP 86363, EP 147879, U.S. Pat. No. 3,694,509, EP 212375).
Using these additives, the improvement in the selectivity is achieved by reducing the reaction rate, which in turn leads to long reaction times. Furthermore, the partial poisoning or inactivation of the catalyst by the additives has the result that the catalyst has usually already lost its activity after one cycle and has to be renewed.
A further method for preparing phenylhydroxylamines is the catalytic hydrogenation of nitroaromatics in the presence of organic nitrogen bases, such as piperidine, pyrrolidine, pyridine, inter alia, which, based on the starting material used, have to be employed in an excess (DE-OS 2 455 238, DE-OS 2 455 887, DE-OS 2 357 370, DE-OS 2 327 412). The yields achievable by this process are, after appropriate working up and purification, 80-85%. It is disadvantageous that using this variant only relatively simple alkyl- and chloronitrobenzenes can be hydrogenated to the corresponding phenylhydroxylamines. Apart from a compound having a 1,3,4-oxadiazole substituent, the hydrogenation of complicated systems by this method is not described.
It is an object of the present invention to make available a process for preparing aromatic and heteroaromatic hydroxylamines which can be carried out simply, is applicable to complicated systems and has a high selectivity and yield.
We have now found that this object is achieved by the process described at the beginning.
It is surprising and was unforeseeable that the partial hydrogenation of aromatic and heteroaromatic nitro compounds to the corresponding hydroxylamines leads to optimum yields and selectivities only in N-alkylmorpholines (N-subst. tetrahydro-1,4-oxazines) as solvents, while according to DE-OS 24 55 238 org. nitrogen bases, such a

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