ENANTIOSELECTIVE HYDROGENATION OF ETHYL PYRUVATE
267
Separation and identification of the modifier after EP ifier (secondary amine) is formed by reductive alkylation
hydrogenation supplied evidence for the formation of a of 3 with the reactant (EP). This reaction offers the op-
secondary amine 5 by reductive alkylation of the primary portunity of changing the modifier structure by apply-
amine 3 with EP. Formation of 5 on the prereduced Pt ing other carbonyl compounds instead of EP. In the low-
is assumed to precede the hydrogenation of EP, at least pressure range (10 bar) the ee reached with this new mod-
in acetic acid. Modifier 5 and various other derivatives of ifier (82%) is better than those reported with cinchona al-
3 have been synthesized and tested in EP hydrogenation kaloids and their derivatives. For use at high pressure the
(Table 1). From the structural analysis we can conclude that resistance of the anchoring group (naphthalene) against hy-
the presence of an N–C–C–O structural unit, characteristic drogenationneedstobe improved. The studies alsoindicate
of cinchona alkaloids and their derivatives, is not a neces- that the earlier proposal concerning the importance of the
sary requirement for enantiodifferentiation in the hydro- N–C–C–O structural unit (7) does not hold for the (R)-
genation of ꢀ-ketoesters. The distance between the O and 1-(1-naphthyl)ethylamine derived modifiers presented in
N atoms can be shorter or even much longer without los- this work.
ing enantioselection. Moreover, the ee can be substantial
even in the absence of an oxygen function in the modifier
(see, e.g., Table 1). These observations provide strong ev-
ACKNOWLEDGMENT
idence against a possible modifier–reactant interaction, in
which a nucleophilic attraction between the O-atom in the
modifier and the ester carbon atom in EP plays a crucial
role (30).
FinancialsupportofthisworkbytheSwissNationalScienceFoundation
(Program Chiral 2) is gratefully acknowledged.
REFERENCES
The results are in good agreement with our former pro-
posal concerning the nature of enantiodifferentiation in
the hydrogenation of EP in acetic acid (12, 31). The in-
teraction of the protonated N-base modifier with the car-
bonyl O-atom of EP via H-bonding and the sufficiently
strong (“fixed”) adsorption of the complex on the Pt-
surface provide the steric requirements of enantiodiffer-
entiation. Molecular modeling studies of methyl pyruvate
hydrogenation over Pt catalysts modified by cinchonidine
1a or (R)-2-(1-pyrrolidinyl)-1-(1-naphthyl)ethanol 2 can be
found elsewhere (12, 14).
The strong adsorption of the flat aromatic ring (“anchor-
ing” moiety) on a flat Pt surface is a basic requirement in
our model (there is substantial evidence for the adsorption
of naphthalene parallel to Pt(111) (32–34)). Any deviation
from this ideal structure is expected to diminish the enan-
1. Orito, Y., Imai, S., and Niwa, S., J. Chem. Soc. Jpn., 670 (1980).
2. Blaser, H. U., Tetrahedron: Asymmetry 2, 843 (1991).
3. Blaser, H. U., Chem. Rev. 92, 935 (1992).
4. Webb, G., and Wells, P. B., Catal. Today 12, 319 (1992).
5. Baiker, A., DGMK Tagungsber., No. 9305, p. 119, 1993.
6. Blaser, H. U., Jalett, H. P., and Wiehl, J., J. Mol. Catal. 68, 215
(1991).
7. Blaser, H. U., Jalett, H. P., Monti, D. M., Baiker, A., and Wehrli, J. T.,
Stud. Surf. Sci. Catal. 67, 147 (1991).
8. Tungler, A., Tarnai, T., Ma`the´, T., and Petro`, J., J. Mol. Catal. 70, L5
(1991).
9. Griffiths, S. P., Johnston, P., Vermeer, W. A. H., and Wells, P. B., J.
Chem. Soc., Chem. Comm., 2431 (1994).
10. Saus, A., Zimmermann, K., and Gu¨rtler, O., Chem.-Ztg. 115, 252
(1991).
11. Baiker, A., Mallat, T., Minder, B., Schwalm, O., Simons, K. E., and
Weber, J., in “Chiral Reactions in Heterogeneous Catalysis” (G.
Jannes and V. Dubois. Eds.), p. 95. Plenum, New York, 1995.
tioselectivity. The partial hydrogenation of the naphthalene 12. Schwalm, O., Minder, B., Weber, J., and Baiker, A., Catal. Lett. 23,
271 (1994).
ring at pressures higher than 10 bar, resulting in the for-
mation of (5,6,7,8)-tetrahydronaphthalene derivatives (15),
distorts the planar structure of the anchoring moiety and
lowers the ee. For future improvement of our modifiers, it
13. Wang, G., Heinz, T., Pfaltz, A., Minder, B., Mallat, T., and Baiker, A.,
J. Chem. Soc., Chem. Commun., 2047 (1994).
14. Simons, K. E., Wang, G., Heinz, T., Giger, T., Mallat, T., Pfaltz, A., and
Baiker, A., Tetrahedron: Asymmetry 6, 505 (1995).
will be crucial to improve the resistance of the anchoring 15. Minder, B., Mallat, T., Baiker, A., Wang, G., Heinz, T., and Pfaltz, A.,
J. Catal. 154, 371 (1995).
16. Minder, B., Schu¨rch, M., Mallat, T., and Baiker, A., Catal. Lett. 31, 143
aromatic ring against hydrogenation. In this way it should
be possible to achieve high enantioselectivity also at higher
pressure, as with cinchona alkaloids and their most effective
derivatives.
(1995).
17. Mattson, R. J., Pham, K. M., Leuck, D. J., and Cowen, K. A., J. Org.
Chem. 55, 2552 (1990).
18. Benson, S. C., Cai, P., Colon, M., Haiza, M. A., Tokles, M., and Snyder,
J. K., J. Org. Chem. 53, 5335 (1988).
CONCLUSION
19. Juaristi, E., and Madrigal, D., Tetrahedron 45, 629 (1989).
20. Wehrli, J. T., Baiker, A., Monti, D. M., and Blaser, H. U., J. Mol. Catal.
61, 207 (1990).
21. Garland, M., and Blaser, H. U., J. Am. Chem. Soc. 112, 7048 (1990).
22. Freifelder, M., “Practical Catalytic Hydrogenation.” Wiley–Inter-
science, New York, 1971.
(R)-1-(1-naphthyl)ethylamine 3 is a versatile and com-
mercially available precursor for the in situ preparation
of effective modifiers for the platinum-catalyzed enantio-
selective hydrogenation of ethyl pyruvate. In contrast to
the previously used systems, in this case the actual mod-
23. Wehrli, J. T., Baiker, A., Monti, D. M., Blaser, H. U., and Jalett, H. P.,
J. Mol. Catal. 57, 245 (1989).