Enantioselective hydrogenation catalyst
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 3, March, 2009
501
during modification is 71%, whereas with addition of NaBr
(from 1 to 5 g), it is somewhat lower and does not exceed
65% under the same hydrogenation and modification conꢀ
ditions. The degree of conversion is not virtually changed
and remains high. Under other hydrogenation conditions
and on variation of amount of NaBr, a decrease in both
the degree of conversion of compound 1b and the optical
yield is observed.
For Ni—Co catalysts (see Table 3) in the absence of
NaBr, the optical yield is 48%, whereas with addition
of the salt, both the degree of conversion of compound 1b
and the optical yield decrease with the value of the latter
falling to 34%. When the temperature of hydrogenation of
compound 1b, the pressure of hydrogen, and the temꢀ
perature of modification are elevated, the degree of conꢀ
version increases from 23 to 60%, but the ee value still
does not exceed 34%.
ently, decreases upon treatment of the catalysts with NaBr
with the concentration of racemic centers remaining fairly
high. For these reasons, the degree of conversion remains
the same. Our data appear to favor the suggestion on
effect of the salt additives for Ni catalysts on the formaꢀ
tion of stereodifferentiating centers. For the Ni—Co conꢀ
tacts, the picture is even more complicated, since their
surface is enriched with cobalt.13
In conclusion, in agreement with the published data,5
the addition of NaBr not always has a positive effect on
the efficiency of Ni catalysts in asymmetric hydrogenaꢀ
tion. This effect has a complicated mechanism, that still
is not clear.
References
1. E. Klabunovskii, G. V. Smith, A. Zsigmond, Heterogeneous
Enantioselective Hydrogenation. Theory and Practice, Springer,
Dordrecht, 2006, p.118.
Thus, the asymmetryꢀinducing properties of powdered
Ni and Ni—Co catalysts can not be improved by addition
of NaBr into the modifying solution of TA.
2. E. I. Klabunovskii, A. A. Vedenyapin, Asimmetricheskii
kataliz. Gidrogenizatsiya na metallakh [Asymmetric Catalysis.
Hydrogenation on Metals], Nauka, Moscow, 1980, 94.
3. T. Harada, M. Yamamoto, S. Onaka, M. Imaida, H. Ozaki,
A. Tai, Y. Izumi, Bull. Chem. Soc. Jpn, 1981, 54, 2323.
4. L. J. Bostelar, W. M. H. Sachtler, J. Mol. Catal., 1984, 27, 387.
5. T. Osawa, Sh. Mita, A. Iwai, T. Miyazaki, O. Takayasu,
T. Harada, I. Matsuura, Chem. Lett., 1997, 1131.
6. N. D. Zubareva, E. I. Klabunovskii, Izv. Akad. Nauk SSSR,
Ser. Khim., 1988, 1172 [Bull. Acad. Sci. USSR, Div. Chem.
Sci. (Engl. Transl.), 1988, 37, 1031].
7. N. D. Zubareva, I. A. Ryndakova, E. I. Klabunovskii,
Kinetika i Katal., 1988, 29, 1485 [Kinet. Catal. (Engl. Transl.),
1988, 29].
8. T. Ninomiya, Bull. Chem. Soc. Jpn, 1972, 45, 2548.
9. T. Ninomiya, Bull. Chem. Soc. Jpn, 1972, 45, 2555.
10. E. I. Klabunovskii, V. I. Neupokoev, Yu. I. Petrov, A. E.
Fasman, G. Kh. Areshidze, Izv. Akad. Nauk SSSR, Ser.
Khim., 1980, 121 [Bull. Acad. Sci. USSR, Div. Chem. Sci.
(Engl. Transl.), 1980, 29, 106].
There are different opinions on the effect of NaBr on
the catalytic activity and efficiency of Ni catalysts in asymꢀ
metric hydrogenation. Some authors3 believe that NaBr
blocks racemic centers, since the efficiency in asymmetꢀ
ric hydrogenation of the catalysts increases followed treatꢀ
ment with NaBr (the optical yield in the hydrogenation of
methyl acetoacetate 1a increases). In this case, however,
the degree of conversion decreases to lower level than in
hydrogenation without addition of NaBr. At the same
time, no considerable decrease in the degree of converꢀ
sion was found during study of supported Ni contacts4. It
was suggested4 that NaBr influences the formation of
stereodiscriminating centers. For the Raney catalysts, the
additional removal of aluminum metal, remaining after
the leaching of the Ni—Al alloys is observed, The appearꢀ
ance of nickel ions necessary for the formation of stereoꢀ
discriminating centers on the surface is also described.
These centers contain the metal in the reduced and
nonreduced states, along with a substrate and a modiꢀ
ficator.1,2
11. T. Tanabe, Y. Izumi, Bull. Chem. Soc. Jpn, 1973, 46, 1550.
12. H. Ozaki, A. Tai, Y. Izumi, Chem. Lett., 1974, 935.
13. N. D. Zubareva, O. P. Tkachenko, N. S. Telegina, G. V.
Dorokhin, E. I. Klabunovskii, A. Yu. Stakheev, L. M. Kustov,
Izv. Akad. Nauk, Ser. Khim., 2007, 2265 [Russ. Chem. Bull.,
Int. Ed., 2007, 56, 2349].
The absence of a positive effect of NaBr on Ni and
Ni—Co catalysts, possibly, is due to the presence of a
sufficient amount of nickel and cobalt ions on their surꢀ
face, which were detected13 during the study of the cataꢀ
lyst surface composition by XPS method. At the same
time, the amount of stereodiscriminating centers, apparꢀ
Received July 2, 2008;
in revised form February 10, 2009