Process for the hydrogenation of imines

Details Process for the hydrogenation of imines Process for the hydrogenation of imines

1998-01-26

1999-01-12

Burn, Brian M.

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

Organic compounds

Amino nitrogen containing

549 68, 564415, C07C20952

Patent

active

058593004

DESCRIPTION:

BRIEF SUMMARY
This application is a 35 USC 371 of PCT/EP96/03143, filed Jul. 13, 1996.
The present invention relates to a process for the hydrogenation of imines with hydrogen and with iridium-diphosphine complexes under elevated pressure in the presence of solid acids.
U.S. Pat. No. 4,994,615 describes a process for the asymmetric hydrogenation of prochiral N-arylketimines wherein iridium catalysts having chiral diphosphine ligands are used. U.S. Pat. No. 5,011,995 describes a process for the asymmetric hydrogenation of prochiral N-alkylketimines using the same catalysts. U.S. Pat. No. 5,112,999 discloses polynuclear iridium compounds and a complex salt of iridium, which contain diphosphine ligands, as catalysts for the hydrogenation of imines. U.S. Pat. No. 5,371,256 and EP-A-0 612 758 describe iridium complexes with chiral ferrocenyldiphosphine ligands for the homogeneous enantioselective hydrogenation of imines.
Those homogeneous catalysis processes have proved valuable, although it is evident, especially in the case of relatively large batches or on an industrial scale, that the catalysts frequently tend to become deactivated to a greater or lesser extent depending on the catalyst precursor, the substrate and the diphosphine ligands that are used. In many cases, especially at elevated temperatures--for example at temperatures >25.degree. C., which are necessary for a short reaction time--it is not possible to achieve complete conversion. For industrial applications of the hydrogenation processes, therefore, the catalyst productivity is too low to be economically viable. Those disadvantages are also not eliminated by the addition of metal halides described in U.S. Pat. No. 4,994,615, U.S. Pat. No. 5,011,995 and U.S. Pat. No. 5,112,999.
It has now been found, surprisingly, that the catalyst activity can be considerably increased if the reaction mixture contains solid acids. It has also unexpectedly been found that at the same time the deactivation of the catalysts can be considerably reduced or completely eliminated. It has also been found, surprisingly, that when asymmetric catalysts are used the enantioselectivity is high, and high optical yields of up to 80% can be achieved, even at reaction temperatures of more than 50.degree. C. In addition, it is of particular advantage to the working-up of the reaction mixture that the solid acids can be removed from the reaction mixture simply by filtration. It has also unexpectedly been found that it is even possible to obtain higher conversion rates and optical yields using a smaller amount of catalyst, which offers very considerable economic advantages since the molar ratio of imine to iridium catalyst can be greatly increased.
The invention relates to a process for the hydrogenation of imines with hydrogen under elevated pressure in the presence of iridium catalysts containing diphosphine ligands, with or without an inert solvent, the reaction mixture containing a soluble ammonium chloride, bromide or iodide or a metal chloride, bromide or iodide, wherein the reaction mixture additionally contains at least one solid acid with the exception of ion exchangers.
Suitable imines are especially those which contain at least one >C.dbd.N--group. If the groups are substituted asymmetrically and are thus compounds having a prochiral ketimine group, it is possible in the process according to the invention for mixtures of optical isomers or pure optical isomers to be formed if enantioselective or diastereo-selective iridium catalysts are used. The imines may contain further chiral carbon atoms. The free bonds in the above formulae may be saturated with hydrogen or organic radicals having from 1 to 22 carbon atoms or organic hetero radicals having from 1 to 20 carbon atoms and at least one hetero atom from the group O, S, N and P. The nitrogen atom of the group >C.dbd.N--may also be saturated with NH.sub.2 or a primary amino group having from 1 to 22 carbon atoms or a secondary amino group having from 2 to 40 carbon atoms. The organic radicals may be substituted, for example,

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