Facile synthesis of a mesoporous silica-supported catalyst for Ru-catalyzed transfer hydrogenation of ketones

Article abstract of DOI:10.1039/b714575f

A convenient method for preparation of a mesoporous silica-supported chiral catalyst by postgrafting a homogeneous catalyst on SBA-15 was developed and its application in the asymmetric transfer hydrogenation of aromatic ketones was investigated. The Roya

Full text of DOI:10.1039/b714575f

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Facile synthesis of a mesoporous silica-supported catalyst for Ru-
catalyzed transfer hydrogenation of ketones{
Guohua Liu,* Mei Yao, Fang Zhang, Yan Gao and Hexing Li*
Received (in Cambridge, UK) 21st September 2007, Accepted 18th October 2007
First published as an Advance Article on the web 29th October 2007
DOI: 10.1039/b714575f
A convenient method for preparation of a mesoporous silica-
supported chiral catalyst by postgrafting a homogeneous
catalyst on SBA-15 was developed and its application in the
asymmetric transfer hydrogenation of aromatic ketones was
investigated.
develop a convenient and rapid method for the preparation of
mesoporous silica-supported chiral catalyst 6 by the postgrafting
method based on mesoporous materials (SBA-15), and apply it in
the Ru-catalyzed asymmetric transfer hydrogenation of aromatic
ketones. Our attention focuses on the construction of highly
ordered mesoporous silica-supported chiral catalysts and investi-
gation of different stereocontrol performances through compar-
ison of catalyst 6 with catalyst 5, prepared by the postmodification
method based on mesoporous materials (Ru-SBA-15).
The immobilization of homogeneous chiral catalysts for asym-
metric reactions is a powerful method for the synthesis of optically
active molecules and has attracted a great deal of interest due to the
potential features of easy separation and efficient recycling, as well
The mesoporous silica-supported chiral catalyst, abbreviated as
Ru-SBA-15/(R,R)-DPEN (6), was prepared by the postgrafting
method. As shown in Scheme 1, catalyst 6 was obtained from the
reaction of the commercially available (trisethoxysilyl)ethyldiphe-
nylphosphine of (R,R)-DPEN in dry CH Cl for 5 h at room
1
as minimal product contamination from metal leaching. Recently,
2
many practical approaches have been introduced to immobilize
homogeneous chiral catalysts onto supports through covalent
methods. Especially, the covalent immobilization on mesoporous
3
materials has exhibited some salient features. These mesoporous
2
2
temperature, followed by anchoring onto SBA-15 through
refluxing in toluene for 24 h. IR (KBr): 3450, 2960, 2920, 2870,
21 29
silica-supported chiral catalysts have regular and adjustable pores
that do not allow the aggregation of active catalysts and can keep
excellent stereocontrol performance. Furthermore, they are easy
and reliable to reuse via simple nanofiltration. Besides, they also
have a facile preparation, remarkable thermal and mechanical
stability, and high density of catalytically active units because of the
high surface area. Due to these advantages, the immobilization of
homogeneous catalysts on mesoporous materials represents a
rapidly growing field that is on the verge of being applied in
industry. Recently, some of mesoporous materials, such as MCM-
1
650, 1440, 1050, 959, 806, 692, 545, 468 cm
;
Si MAS-NMR
13
(79.5 MHz): 2116, 2106, 297 ppm; C CP-MAS (100.6 MHz):
31
129, 72, 63, 22 ppm; P CP-MAS (169.3 MHz): 71 ppm; elemental
analysis (%): C 3.39, H 2.55, N 0.19. For comparison, catalyst 5
was also synthesized by the postmodification of (R,R)-DPEN onto
9a–b
Ru-SBA-15
with a similar strategy. The inductively coupled
plasma (ICP) optical emission spectrometer analyses showed that
the Ru-loading amounts in catalysts 5 and 6 were 6.80 and 6.26 mg
per gram of catalyst. The pore sizes of catalysts 5 and 6 were
measured to be 4.8 and 6.3 nm, as shown in Table 1 in Fig. 1.
The powder XRD patterns revealed that catalysts 5 and 6
showed one similar intense peak and two weak peaks indicative of
4
5
4
1 (ordered hexagonal), MCM-48 (ordered cubic) and SBA-15
6
ordered hexagonal), have been used successfully as the supports
(
to immobilize homogeneous chiral catalysts.
Optically active 1,2-diphenylethylenediamine [(R,R)-DPEN] is a
highly effective chiral ligand for the transfer hydrogenation of
(
100), (110), and (200) reflections in Fig. 1, suggesting that the
hexagonal arrayed pore structure (p6mm) could be preserved after
10
modification and postgrafting. The decrease in the peak intensity
7
ketones. The immobilization of its derivatives [(R,R)-TsDPEN]
8
onto mesoporous materials has also been explored. However,
implied that the modification or postgrafting might disturb the
ordered mesoporous structure to a certain degree. The TEM
morphologies further confirmed that both catalysts displayed a
two-dimensional hexagonal arrangement of one-dimensional
channels with uniform size (Fig. 2). N2 adsorption–desorption
isotherms in Fig. 3 revealed that both catalysts exhibited the
typical IV type isotherms with a steep increase in adsorption at
P/P = 0.40–0.70. On the basis of the N adsorption–desorption
apart from the tedious synthetic process, these MCM-41 or SBA-
15-supported chiral catalysts still afforded worse recycling than
that of those immobilized on silica gel. Moreover, detailed
discussion of their differences, especially the structures of MCM-
4
1 or SBA-15-supported chiral catalysts, is unavailable.
9
Recently, we reported a series of mesoporous catalysts,
9a–b
especially Ru-SBA-15,
and their application in catalytic
0
2
processes. As an extension of our previous study, we herein
isotherms, some structural parameters were calculated and listed in
Table 1, which is inserted in Fig. 1. It is found that the catalysts by
the modification or postgrafting method resulted in a decrease in
the nanopore size, surface area, and pore volume (sample 5 versus
3, and sample 6 versus SBA-15). This could be attributed to the
coverage by the ruthenium complexes on the channel surfaces of
the Ru-SBA-15 or SBA-15, resulting in an increase in the wall
Department of Chemistry, College of Life and Environmental Science,
Shanghai Normal University, Shanghai, China.
E-mail: ghliu@shnu.edu.cn; Fax: + 86 21 64322511;
Tel: + 86 21 64321819
{
Electronic supplementary information (ESI) available: Experimental
procedures for the syntheses and characterizations of 5 and 6, and
analytical data for obtained chiral aromatic alcohols. See DOI: 10.1039/
b714575f
9a–b
thickness.
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Chem. Commun., 2008, 347–349 | 347

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Scheme 1 Syntheses of immobilized catalysts 5 and 6.
The asymmetric transfer hydrogenation of ketones was carried
7
out according to the reported method. Generally, catalyst 6 was
found to be highly effective in the asymmetric transfer hydro-
genation of aromatic ketones. The preliminary results are
summarized in Table 2. Taking the acetophenones as an example,
the asymmetric transfer hydrogenation provided (R)-1-phenyl-1-
ethanol in more than 99% yield and 99.5% ee (entry 1), which is
higher than that obtained with the parent, molecular catalyst,
7f–7g
[
RuCl (PPh
2
3
)
2
(R,R)-DPEN)].
Besides, catalyst 6 also gave
values of more than 98% ee using 1-acetonaphthone and
substituted acetophenones as substrates (entries 2–5). Of particular
note is that the reaction can be run at a much higher S : C ratio
without obviously affecting the ee value, as exemplified by the
hydrogenation of 8 at S/C = 500 (entry 6).
Fig. 1 The powder XRD patterns of the mesoporous silica-supported
chiral catalysts 5 and 6.
a
Table 2 Asymmetric transfer hydrogenation of aromatic ketones
b
b
Entry
Substrate
Catalyst
Run
Conv. (%)
Ee (%)
Fig. 2 The TEM images of the catalysts 5 and 6 viewed along [100]
directions.
1
2
3
4
5
6
7
8
9
7
8
9
10
11
8
7
7
8
8
6
6
6
6
6
6
1
1
1
1
1
1
1
1
2
3
4
5
6
7
8
.99
.99
.99
.99
.99
.99
81.9
17.9
.99
.99
.99
.99
.99
.99
.99
99.5
99.9
98.4
99.1
99.8
c
97.2
5
2 + 3
59.1
79.3
d
6
6
6
6
6
6
6
99.8
99.6
d
1
1
1
1
1
1
a
0
1
2
3
4
5
d
8
8
8
8
99.2
99.5
99.4
98.4
d
d
d
d
8
93.6
2 2
Reactions were carried out in CH Cl . Reaction conditions:
catalyst (5.03 mmol of Ru), i-PrOH (0.013 mol), i-PrOK (0.1 mol),
ketone (0.5 mmol), reaction temperature (50 uC), reaction time (24–
b
8 h), argon atmosphere. Determined from chiral GC analysis. The
4
absolute configuration of the product is R. Data were obtained at
c
Fig. 3 Nitrogen adsorption–desorption isotherms of the mesoporous
silica-supported chiral catalysts 5 and 6.
d
S/C = 500. Recovered catalysts were used.
3
48 | Chem. Commun., 2008, 347–349
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In order to further compare the catalytic performances, two
maintained high enantioselectivity, which is beneficial for possible
industrial applications.
control experiments were also carried out using 5 and the 1 : 1
(
mole ratio) mixture of Ru-SBA-15 and (R,R)-DPEN as catalysts
We are grateful to the China National Natural Science
Foundation (No.20673072), Shanghai Sciences and Technologies
Development Fund (No.05JC14074) and Shanghai Leading
Academic Discipline Project (No.T0402) for financial support.
under similar reaction conditions. It was found that 5 afforded the
corresponding alcohol in 81.9% conversion and 59.1% ee, while the
1
: 1 (mole ratio) mixture of Ru-SBA-15 and (R,R)-DPEN gave
the corresponding alcohol in only 17.9% conversion and 79.3% ee
entries 7 and 8). As compared with catalyst 6, the low
(
Notes and references
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In conclusion, we report the facile preparation of a mesoporous
silica-supported chiral catalyst 6 by postgrafting, in which 6 was
obtained by anchoring 4 onto SBA-15 through refluxing in toluene
for 24 h. This kind of heterogeneous catalyst showed high catalytic
activities (more than 99% yields for all tested ketones) and excellent
enantioselectivities (more than 98% ee) for the asymmetric transfer
hydrogenation of various aromatic ketones. Besides, catalyst 6
could be readily recovered and reused in multiple consecutive
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Chem. Commun., 2008, 347–349 | 349