obtained for the first three runs. When the catalyst was run for the
sixth time, the reaction could still reach 99% conversion. A longer
reaction time was needed to complete the reaction, partly due to
the loss of solid catalysts during the course of recovery. The
activity is relatively lower than that of the homogeneous catalyst
because of the diffusion limitation. Moreover, Ru-TsDPEN/SBA-
16 can catalyze different kinds of ketones to corresponding chiral
alcohols with high activity and enantioselectivity.
Table 3 Asymmetric transfer hydrogenation of ketones on Ru-
TsDPEN and Ru-TsDPEN/SBA-16-2Ph (C)a
Entry
R
Catalysts
S/Cb
Conv. % (h)c
Ee %
In conclusion, the chiral metal complex catalysts can be trapped
in the cage of mesoporous materials like SBA-16 by modifying the
entrance pore size of the cage using silylation. The entrapped chiral
catalyst can be easily recycled without significant loss of catalytic
performance and can show catalytic performance comparable to
that in a homogeneous catalysis process. This strategy could be
generally applicable for various chemical transformations and
nano-reactor design.
1
2
3
4
5
6
7
8
9
a
H
H
H
H
Ru-TsDPEN
100
100
.99 (1.5)
.99 (5)
.99 (6)
92 (6)
99 (19)
.99 (8)
.99 (8)
.99 (8)
94 (7.5)
93 (S)
93 (S)
92 (S)
92 (S)
92 (S)
87 (S)
88 (S)
88 (S)
90 (S)
C (1st)d
C (2nd)
C (3rd)
C (6th)
C
H
p-F
p-Cl
o-Cl
o-MeO
200
200
200
200
C
C
C
The reaction was carried out with 0.2 mmol of ketones, 0.016 mmol
of Et4N+Br2, 1.5 mmol of HCOONa in 0.3 g of water at 313 K,
catalyst C is Ru-TsDPEN/SBA-16-2Ph (the pore entrance blocked
with diphenyldichlorosilane). Conversion and ee value were obtained
The authors would like to thank the National Natural Science
Foundation of China (Grant No. 20321303 and 20673113),
National Basic Research Program of China (2003CB615803) and
Knowledge Innovation Program of the Chinese Academy of
Science (DICP K2006B2).
b
from GC analysis (HP-Chiral19091G-B213 capillary column). The
molar ratio of substrate/Ru. The data in parentheses are the
c
d
reaction time (hour). The catalyst is run for the first time.
Notes and references
loss of solid catalysts during the course of recovery (for the 12th
run, the catalyst weight is ca. 65% of the original one). The filtrate
does not show any activity for the ring-opening reaction. This
confirms that the conversion is contributed by the catalyst trapped
in the cage of the mesoporous materials. Catalyst Co(Salen)/SBA-
16-C8 was also used to catalyze the asymmetric ring-opening of
propylene oxide (entries 9–12, Table 2). No obvious loss of activity
and ee was observed even when Co(Salen)/SBA-16-C8 was run for
the thirteenth time (entry 12, Table 2). After the thirteen cycles of
reaction, the Co content in Co(Salen)/SBA-16-C8 was 87% of that
in the fresh one.
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TsDPEN is among the most efficient catalysts for the
asymmetric transfer hydrogenation of prochiral ketones.18,19 The
heterogeneous catalyst Ru-TsDPEN/SBA-16-2Ph was prepared by
the encapsulation of Ru-TsDPEN in the mesoporous cage of
SBA-16 using diphenyldichlorosilane as silylation agent. The UV–
vis absorption measurements show that the pore entrance size of
SBA-16 modified by diphenyldichlorosilane can effectively confine
Ru-TsDPEN within the mesoporous cage (Fig. S6 in ESI). Table 3
summarizes the results of the asymmetric transfer hydrogenation
of different ketones in the HCOONa–H2O system. The hetero-
geneous catalyst Ru-TsDPEN/SBA-16-2Ph exhibits comparable
enantioselectivity with the homogeneous counterpart. The recycle
reaction test shows that similar conversion and ee value are
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1088 | Chem. Commun., 2007, 1086–1088
This journal is ß The Royal Society of Chemistry 2007