Process for the hydrogenation of imines

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

1997-12-05

1999-03-23

Burn, Brian M.

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

Organic compounds

Amino nitrogen containing

C07C20938

Patent

active

058862250

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for the hydrogenation of imines with hydrogen under elevated pressure in the presence of hydrogen iodide.
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.
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.
It has now been found, surprisingly, that the catalyst activity can be increased by a factor of 10 or more if the reaction mixture contains hydrogen iodide. 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.
The invention relates to a process for the hydrogenation of imines with hydrogen under elevated pressure in the presence of iridium catalysts and with or without an inert solvent, wherein the reaction mixture contains hydrogen iodide.
Suitable imines are especially those which contain at least one >C.dbd.N-- 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, by F, Cl, Br, C.sub.1 -C.sub.4 haloalkyl wherein halogen is preferably F or Cl, --CN, --NO.sub.2, --CO.sub.2 H, --CONH.sub.2, --SO.sub.3 H, --PO.sub.3 H.sub.2, or C.sub.1 -C.sub.12 alkyl esters or amides, or by phenyl esters or benzyl esters of the groups --CO.sub.2 H, --SO.sub.3 H and --PO.sub.3 H.sub.2. Aldimine and ketimine groups are especially reactive, with the result that using the process according to the invention it is possible selectively to hydrogenate >C.dbd.N-- groups in addition to the >C.dbd.C< and/or >C.dbd.O >C.dbd.N--N--
The process according to the invention is suitable especially for the hydrogenation of aldimines, ketimines and hydrazones with the formation of corresponding amines and hydrazines, respectively. The ketimines are preferably N-substituted. It is preferable to use chiral iridium catalysts and to hydrogenate enantiomerically pure, chiral or prochiral ketimines to prepare optical isomers, the optical yields (enantiomeric excess, ee) being, for example, higher than 30%, especially higher than 50%, and yields of more than 90% being achievable. The optical yield indicates the ratio of the two st

REFERENCES:
patent: 4994615 (1991-02-01), Spindler et al.
patent: 5011995 (1991-04-01), Pugin et al.
patent: 5112999 (1992-05-01), Osborn et al.
patent: 5463097 (1995-10-01), Togni et al.
Angewandte Chemie, International Edition, vol. 29, No. 5, pp. 558-559, May 1990.

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