(R)-(+)-binol-functionalized mesoporous organosilica as a highly efficient pre-chiral catalyst for asymmetric catalysis

Article abstract of DOI:10.1002/asia.200900737

(R)-(+)-Binol-functionalized chiral periodic mesoporous organosili-cas (PMOs) with different framework compositions were successfully synthesized by cocondensation of (R)-2,2′-di(methoxymethyl)oxy-6,6'-di(1-propyl- trimethoxysilyl)-1,1'-binaphthyl (BSBi-n

Full text of DOI:10.1002/asia.200900737

FULL PAPERS
DOI: 10.1002/asia.200900737
(
R)-(+)-Binol-Functionalized Mesoporous Organosilica as a Highly Efficient
Pre-Chiral Catalyst for Asymmetric Catalysis
[
a, b]
[a]
[a, b]
[a, b]
[a]
Xiao Liu,
Peiyuan Wang, Yan Yang,
Peng Wang,
and Qihua Yang*
Abstract: (R)-(+)-Binol-functionalized
chiral periodic mesoporous organosili-
cas (PMOs) with different framework
compositions were successfully synthe-
sized by cocondensation of (R)-2,2’-di-
structure in an acidic medium but the
mesoporous materials synthesized
to aldehydes. The chiral PMO with
ethane and (R)-(+)-Binol in the frame-
work exhibits an enantioselectivity as
high as 90% with a turnover frequency
using a mixture of TMOS and BSBinol
can only be obtained in a weak acidic
buffer solution. All the materials are
efficient catalysts (coordinated with Ti)
for asymmetric addition of diethylzinc
ꢀ
1
(TOF) of 104 h , which is even higher
(
methoxymethyl)oxy-6,6’-di(1-propyl-
than the homogeneous (R)-(+)-Binol
ꢀ
1
trimethoxysilyl)-1,1’-binaphthyl (BSBi-
nol) with 1,2-bis(trimethoxysilyl)ethane
catalyst (83% ee with TOF of 96 h )
using toluene as solvent under similar
conditions. This work demonstrates the
positive effect of the rigid pore wall in
increasing the enantioselectivity of the
chiral PMOs.
(
(
BTME)
and
tetramethoxysilane
Keywords: asymmetric catalysis
·
TMOS) using triblock copolymer
chirality · heterogeneous catalysis ·
P123 as a template in acidic solution.
The mixture of BTME and BSBinol
can result in a highly ordered meso-
mesoporous materials
effects
·
solvent
Introduction
solids, which have demonstrated application potential in
[2]
asymmetric catalysis and chiral separation. Several chiral
ligands such as VO(Salen) (Salen=N,N’-bis(3,5-di-tertbutyl-
salicylidene)-1,2-cyclohexanediamine), trans-diaminocyclo-
Asymmetric catalysis is one of the most general and power-
ful strategies for the synthesis of modern pharmaceuticals
and agrochemicals. Heterogeneous asymmetric catalysis has
attracted a great deal of research interest because of the
easy purification of the product and recycling of the chiral
catalysts. However, most heterogeneous catalysts show
lower activity and enantioselectivity than their homogene-
ous counterparts. Therefore, the synthesis of highly efficient
chiral solids is one of the challenges in the field of heteroge-
neous asymmetric catalysis.
ACHTUNGTRENNUNG
[3]
[4]
[5]
[6]
[7]
hexene, l-tartardiamide, benzylic ether, chiral amine,
[8]
and chiral alkane have been incorporated in the frame-
work of mesoporous silicas. However, most chiral PMOs
show lower catalytic activity and enantioselectivity com-
pared with their homogeneous counterparts. The synthesis
of chiral PMOs with high catalytic activity and enantioselec-
tivity still remains a challenge.
[1]
(R)-(+)-1,1’-Bi-2-naphthol [(R)-(+)-Binol] with binaph-
thalene as the chiral building block has been one of the
most useful chiral ligands for both stoichiometric reactions
Chiral periodic mesoporous organosilicas (PMOs) with
chiral ligands in the framework are novel chiral porous
[9]
and catalytic asymmetric catalysis since the 1990s. The bite
angles of binaphthalene building blocks with C symmetrical
2
[
a] Dr. X. Liu, Dr. P. Wang, Dr. Y. Yang, Dr. P. Wang, Prof. Q. Yang
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics
axes may be sensitive to the microenvironment, which can
be tuned to improve the enantioselectivity for the asymmet-
[10]
Chinese Academy of Sciences Institution
ric catalysis. (R)-(+)-Binol has been grafted onto mesopo-
4
57 Zhongshan Road, Dalian 116023 (China)
rous silicas such as SBA-15 and cellular foam using different
linker groups. The author found that (R)-(+)-Binol sup-
Fax : (+86)411-84694447
E-mail: yangqh@dicp.ac.cn
[11]
ported on cellular foam exhibited a higher enantioselectivity
than that on SBA-15. This shows that the pore structure of
the support plays an important role in asymmetric catalysis.
The integration of (R)-(+)-Binol in the mesoporous frame-
work may further reveal the influence of the rigid porous
[
b] Dr. X. Liu, Dr. Y. Yang, Dr. P. Wang
Graduate School of the Chinese Academy of Sciences
Beijing 100049 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900737.
1232
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Chem. Asian J. 2010, 5, 1232 – 1239

wall on the enantioselectivity of the supported chiral cata-
lyst, which is very important for the design and synthesis of
a highly efficient solid catalyst for asymmetric catalysis. Re-
cently, the incorporation of (R)-(+)-Binol into the frame-
tetramethoxysilane (TMOS) were used as a co-silane precur-
sor for the synthesis. The combination of BTME and BSBi-
nol is more favorable for the synthesis of materials with or-
dered mesoporous structures than the combination of
TMOS and BSBinol. The catalytic activity of the chiral
PMOs was tested in Ti-promoted asymmetric addition of di-
ethylzinc to aldehydes. Moreover, the influence of solvents
on the catalytic performance of the chiral PMOs with a ho-
mogeneous chiral catalyst is reported.
[12]
work of PMOs was reported. However, the OH of (R)-
+)-Binol was chosen as the site to be derivatized, which
(
may reduce the application potentials of the chiral PMOs in
asymmetric catalysis. Previously, we tried to synthesize
chiral PMOs with (R)-(+)-Binol in the framework using eth-
[
13]
ylene as the linker group.
Unfortunately, SiꢀC cleavage
was observed during the synthesis of PMOs, which caused a
decrease of enantioselectivity of the resultant chiral solid in
asymmetric catalysis.
Results and Discussion
Synthesis of Chiral PMOs with Different Framework
Compositions
In this work, we report the synthesis of (R)-(+)-Binol-
functionalized mesoporous organosilicas using (R)-2,2’-di-
(
methoxymethyl)oxy-6,6’-di(1-propyltrimethoxysilyl)-1,1’-bi-
The mesoporous organosilicas with bridging ethane and (R)-
(+)-Binol in the framework were synthesized by coconden-
sation of BTME and BSBinol in an acidic medium using the
P123 surfactant as the template. At least one sharp diffrac-
tion peak was observed in the XRD patterns of PBB-n (n=
naphthyl (BSBinol) as a chiral silane precursor (Scheme 1).
To investigate the effect of the framework composition and
the porous structure on the catalytic performance of the
chiral PMOs, 1,2-bis(trimethoxysilyl)ethane (BTME) and
Scheme 1. The synthesis of A) (R)-(+)-2,2’-di(methoxymethyl)oxy-6,6’-di(1-propyltrimethoxysilyl)-1,1’-binaphthyl (BSBinol) and B) (R)-(+)-Binol-func-
tionalized chiral periodic mesoporous organosilicas with different framework compositions.
1
0, 20, 30) materials, showing that all materials have an or-
Abstract in Chinese:
dered mesostructure (Figure 1). The TEM images (Figure 2)
of PBB-n (n=10, 20, 30) materials show a 2D hexagonal ar-
rangement of mesopores throughout the sample, further
confirming that all these materials have an ordered 2D hex-
agonal mesostructure. The N sorption isotherms (Figure 3)
2
of all PBB-n (n=10, 20, 30) materials are of type IV with a
sharp capillary condensation step in the relative pressure P/
Chem. Asian J. 2010, 5, 1232 – 1239
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1233

Q. Yang et al.
FULL PAPERS
Figure 3. N
2
sorption isotherms of (R)-(+)-Binol-functionalized mesopo-
rous organosilicas.
distributed color. This may be due to the unmatchable hy-
drolysis and condensation rate between TMOS and BSBi-
nol. It is well known that the hydrolysis and condensation
rates of silane precursors are directly related to their struc-
ture and are highly sensitive to the pH of the synthesis
medium. Thus, by adjusting the hydrolysis–condensation ki-
netics of different silane precursors, the microphase-separa-
tion behaviors of silane precur-
Figure 1. Powder X-ray diffraction patterns of (R)-(+)-Binol-functional-
ized mesoporous organosilicas.
sors during the assembly pro-
cess can be selectively con-
trolled, which will consequently
result in mesoporous materials
with various pore structures.
Compared with commonly used
strongly acidic or basic synthe-
sis media, the mild acidic buffer
medium is beneficial for the
control of the hydrolysis and
condensation of different kinds
of silane precursors. PBT-n
samples were synthesized in
HOAc–NaOAc (pH 4.4) buffer
solution in the presence of the
P123 surfactant according to
[16]
our previous publication.
Three diffraction peaks could
be clearly observed in the XRD
pattern of the PBT-n samples
Figure 2. TEM images of (R)-(+)-Binol-functionalized mesoporous organosilicas.
(
Figure 1). The TEM images of
P range of 0.60–0.70, which suggests that all materials have
PBT-n clearly show the ordered 2D hexagonal arrangement
of the mesopores, further confirming that PBT samples have
a 2D hexagonal mesostructure (Figure 2). The intensities of
the (100) diffraction peak decrease sharply as the amount of
BSBinol in the initial mixture increases. This is usually ob-
served for the mesoporous organosilicas synthesized using a
0
a large and uniform mesopore. PBB-n samples have a pore
diameter of approximately 6 nm with a wall thickness in the
range of 4.8 to 5.7 nm (Table 1). The thick pore wall of the
PBB-n materials is mainly attributed to the existence of
[15]
ethane moieties in the mesoporous framework.
[17]
Under similar conditions, we also tried to prepare the
mesoporous organosilicas by using a mixture of TMOS and
BSBinol as precursors. However, phase separation was ob-
served during the synthesis and the product had an unevenly
mixture of TMOS and bis-silylated silane as precursor.
All PBT-n samples show a type IV isotherm pattern, similar
to the PBB samples (Figure 3). The BET surface area and
pore diameter decrease as the amount of BSBinol in the ini-
1234
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Chem. Asian J. 2010, 5, 1232 – 1239

Functionalized Mesoporous Organosilica
Table 1. Physicochemical properties of (R)-(+)-Binol-functionalized mes-
oporous organosilicas.
Sample
D
BET
Pore
Pore
diameter
[nm]
Wall
thickness
[nm]
[
a]
[b]
[c]
spacing surface area volume
2
ꢀ1
3
ꢀ1
[nm]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[m g
]
A
H
U
G
E
N
N
]
PBB-10
PBB-20
PBB-30
PBT-10
PBT-20
PBT-30
9.3
9.8
9.2
11.3
11.3
11.2
666
599
444
578
413
239
0.73
0.66
0.51
1.11
0.87
0.55
5.9
5.6
5.6
10.8
7.9
7.0
4.8
5.7
5.0
2.2
5.2
5.9
[
P
a] Total pore volume obtained from the volume of N
2
adsorbed at P/
29
13
Figure 4. Si MAS NMR (left) and C CP-MAS NMR (right) spectra of
R)-(+)-Binol-functionalized mesoporous organosilicas: A) PBB-30 and
0
=0.99. [b] Calculated using the BJH model based on the adsorption
(
branch of the isotherms. [c] Wall thickness=a
a
0
ꢀpore diameter, in which
B) PBT-30.
pffiffi
0
=2d100
/
3
.
tial mixture increases. It is worth noting that the wall thick-
ness of the PBT-n samples increases sharply from 2.2 nm for
PBT-10 to 5.9 nm for PBT-30 (Table 1). This unusually sharp
increase of the wall thickness suggests the aggregation of
Binol groups in the pore wall.
PBB-n materials with an ordered mesoporous structure
can be easily synthesized under conventional acidic condi-
tions. However, PBT-n materials can only be obtained in a
mild buffer solution. When the initial molar ratio of BSBi-
nol is the same, the PBB-n materials generally have a higher
surface area than PBT-n materials. The above results indi-
cate that the combination of BTME and BSBinol is more fa-
vorable for the synthesis of materials with an ordered meso-
porous structure than the combination of TMOS and BSBi-
nol.
age of the SiꢀC bond was observed. Moreover, the absence
0
29
of the T [SiC(OH) ] site in the Si MAS NMR spectra of
3
PBB-30 and PBT-30 also indicates that the chiral moiety is
actually integrated with both ends in the pore walls instead
of dangling in the pore as a pendant group.
1
3
In the C CP-MAS NMR spectra of PBT-30 and PBB-30,
the signals in the range of d=160–110 ppm were ascribed to
aryl carbon atoms of (R)-(+)-Binol. The signals related to
1
2
3
1
2
3
C , C , and C carbon species of SiꢀC H C H C H were ob-
2
2
2
served at d=16.6, 26.8, and 46.4 ppm, respectively. The
sharp signal at d=5.1 ppm of PBB-30 is ascribed to the
ethane moiety bridging in the mesoporous framework. The
absence of signals at d=55.9 and 95.0 ppm of PBB-30 and
PBT-30 shows that the protecting groups can be removed
[
18]
during the process of surfactant extraction.
The signals
centered at d=59.0 and 16.6 ppm (overlapped with the
1
signal of C ) of PBB-30 and PBT-30 may originate from the
Composition Characterization of the PBB and PBT
Materials
ethoxy groups formed during the process of surfactant ex-
traction. The above characterizations confirm the successful
synthesis of chiral PMOs with (R)-(+)-Binol moieties inte-
grated in the framework.
The FTIR spectra of PBB-10 and PBT-10 are presented in
the Supporting Information. The vibrational bands of naph-
thyl units of PBB-10 could be clearly observed at 1621 and
The direct determination of the enantiomeric purity of
(R)-(+)-Binol in the mesoporous organosilicas is very diffi-
cult. Therefore, we used NaOH (1m) to dissolve the meso-
porous organosilica and characterized the resultant solution
by using circular dichroism (CD). The CD curve of (R)-(+)-
Binol (Figure 5A) showed a negative Cotton effect centered
in the range of 220–270 nm. The solution of PBT-30 also
showed the same negative Cotton effect centered in the
range of 220–270 nm (Figure 5B). The results implied that
there was almost no change in the chirality of (R)-(+)-Binol
groups during the synthesis.
ꢀ
1
1
-
411 cm . The CH stretching vibrations of -CH CH - and
2 2
ꢀ
1
CH CH CH - appear at 2965 and 2925 cm . Similarly to
2 2 2
PBB-10, the vibrational bands of naphthyl units of PBT-10
ꢀ
1
could be found at 1627 and 1464 cm . The CꢀH stretching
vibrations of -CH CH CH - were observed at 2978 and
2
2
2
ꢀ
1
2
929 cm .
29
The solid-state Si magic-angle spinning (MAS) NMR
13
and C cross-polarization (CP) MAS NMR spectra of PBB-
0 and PBT-30 are displayed in Figure 4. Both T and Q sili-
con sites were clearly observed in the Si MAS NMR spec-
3
2
9
tra of PBT-30. The signals at ꢀ110.6 and ꢀ103.1 ppm arise
To identify the location of (R)-(+)-Binol groups in PMOs
clearly, we used Fe ACHTUNGERTNNUNG( OAc) to coordinate with (R)-(+)-Binol
2
4
3
from the Si species Q [Si
A
H
U
T
E
N
N
(OSi) ] and Q [Si(OH)
A
H
U
G
R
N
N
(OSi) ], re-
4
3
spectively. The signals at ꢀ65.3 and ꢀ61.2 ppm can be as-
cribed to the organosilicon species T [SiC(OSi) ] and T
in PBT-10. After coordination, the sample was analyzed by
scanning transmission electron microscopy (STEM) and
energy-dispersive X-ray (EDX) spectroscopy on a high-reso-
lution transmission electron microscope (HRTEM;
Figure 6). Figure 6A is the STEM image and Figure 6B is
the Fe mapping of the chosen area using EDX. The red dots
show the signal of Fe and that it has a uniform distribution.
Therefore, (R)-(+)-Binol groups are uniformly distributed
3
2
AHCTUNGTRENNUNG
3
2
9
[
SiC(OH)
trum of PBB-30 shows only T silicon sites at ꢀ59 and
63 ppm, which are assigned to T [SiC(OH)(OSi) ] and T
SiC(OSi) ] of Si species related to the mixture of (R)-(+)-
Binol and ethane, respectively. The absence of a Q site in
ACHTUNGTRENNUNG( OSi) ], respectively. The Si MAS NMR spec-
2
2
3
ꢀ
ACHTUNGTRENNUNG
2
[
ACHTUNGTRENUNG
3
2
9
the Si MAS NMR spectra of PBB-30 means that no cleav-
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1235

Q. Yang et al.
FULL PAPERS
Table 2. Ti-promoted asymmetric addition of diethylzinc to benzalde-
[
a]
hyde on chiral PMOs.
[
b]
[b]
[c]
[d]
[d]
Catalyst
Conv.
%]
ee
TOF
Binol content
Binol content
ꢀ
1
ꢀ1
ꢀ2
[
[%]
A[ h
H
U
G
R
N
N
]
A[ mmolg
H
U
G
R
N
U
G
]
A[ mmolm
H
U
G
R
N
N
]
-
-
-
-
-
-
3
3
3
3
3
3
PBB-10
PBB-20
PBB-30
PBT-10
PBT-20
PBT-30
90
92
99
99
67
78
86
90
92
91
88
90
87
17
21
29
22
4
0.26
0.65
0.73
0.29
0.42
0.62
–
0.4ꢁ10
1.1ꢁ10
1.6ꢁ10
0.5ꢁ10
1.0ꢁ10
2.6ꢁ10
–
3
69
[
e]
(
R)-(+)-Binol 99
[
a] Unless otherwise stated, all the reactions were carried out in CH
with Ti(OiPr) (1.5 mmol), Et Zn (3.0 mmol), and benzaldehyde
1.0 mmol) at 08C for 4 h. The molar ratio of Ti to ligand is 13. [b] Con-
2 2
Cl
A
H
U
G
R
N
U
G
4
2
(
versions and ee values were determined by GC on a HP-Chiral 19091G-
B213 capillary column. [c] The TOF was calculated according to the fol-
lowing equation: TOF=mmolconverted benzaldehyde/(mmolBinol ꢁh). [d] (R)-
(
+)-Binol contents of PBB-n and PBT-n were determined by thermogra-
vimetric analysis and elemental analysis based on the C content, respec-
tively. [e] Data were obtained using the homogeneous catalyst over 1 h.
Figure 5. Circular dichroism (CD) spectra of A) (R)-(+)-Binol and
B) PBT-30 dissolved in 1m NaOH.
Figure 6. A) STEM image and B) chemical mapping of Fe/PBT-10.
in the mesoporous material. In other words, (R)-(+)-Binol
groups were incorporated both inside the pore wall and on
the surface of the pore wall.
Catalytic Properties of the Chiral PMOs
Figure 7. The time profiles of the Ti-promoted asymmetric addition of di-
ethylzinc to benzaldehyde catalyzed by chiral PMOs.
(
R)-(+)-Binol–Ti is an efficient chiral catalyst for the asym-
[9]
metric addition of diethylzinc to benzaldehyde. Therefore,
the catalytic performance of (R)-(+)-Binol-functionalized
chiral PMOs was tested for the same reaction after coordi-
value increasing from 86 to 92%, which is probably due to
the increased concentration of (R)-(+)-Binol in the material
(Table 2).
nation with Ti ACHTUNGTRENNUNG( OiPr) . The catalytic results of PBB-n and
4
PBT-n materials are summarized in Table 2. To compare the
activity of the solid catalysts, the reaction kinetics were also
investigated (Figure 7). In the presence of PBB-n and PBT-
n samples, the reaction between diethylzinc and benzalde-
hyde goes smoothly. The conversion of benzaldehyde in-
creases sharply as the reaction time increases within the first
Contrary to the PBB-n samples, the TOF of the PBT-n
samples decreases from PBT-10 to PBT-30. The TOF of
ꢀ
1
PBT-10 is 22 h (Table 2). However, the TOF of PBT-20
ꢀ
1
and PBT-30 is only in the range of 3 to 4 h . From Table 1,
we can see that the pore wall thickness of PBT-n samples in-
creases sharply from PBT-10 to PBT-20, which indicates that
the chance of (R)-(+)-Binol being buried in the pore wall of
PBT-n (n=20, 30) is increased. The thick pore wall may be
2
00 min and gradually reaches a plateau. From PBB-10 to
PBB-30, the turnover frequency (TOF) of the sample gradu-
ꢀ
1
ally increases from 17 to 29 h accompanied with the ee
1236
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Chem. Asian J. 2010, 5, 1232 – 1239

Functionalized Mesoporous Organosilica
the main reason for the low catalytic activity of PBT-n (n=
The big difference in solvent effect between the chiral
PMOs and homogeneous (R)-(+)-Binol may derive from
the different microenvironment surrounding the chiral cata-
lyst. In chiral PMOs, (R)-(+)-Binol was distributed uniform-
ly in the pore wall. During the catalytic process, the reac-
tants should pass through the nanopore to access (R)-(+)-
Binol. The nanopore may make the interaction between
(R)-(+)-Binol–Ti (the active catalyst formed by coordina-
tion of (R)-(+)-Binol and Ti) and the reactants tighter than
that under homogeneous conditions, which may result in the
different solvent effect for chiral PMOs and homogeneous
(R)-(+)-Binol.
20, 30). The above results show that the accessibility of the
chiral ligands to the substrates is a key factor that influences
the catalytic activity of the chiral PMOs. The ee of the PBT
samples is similar and in the range of 91 to 88%.
ꢀ
1
The TOF and ee of PBT-10 are 22 h and 91%, respec-
tively, which are higher than those of PBB-10. The higher ee
and TOF of PBT-10 are probably due to the higher (R)-(+)-
Binol density and thinner pore wall of PBT-10 than those of
PBB-10 (Tables 1 and 2). Compared with homogeneous cat-
alyst (R)-(+)-Binol (87% ee), the chiral PMOs, for example,
PBB-n (n=20, 30) and PBT-10, show slightly higher enan-
tioselectivity. This may be from the slight change of the bite
By choosing PBB-30 as the model catalyst, the catalytic
scope of the chiral PMOs was investigated using CH Cl as
[10]
angles of (R)-(+)-Binol integrated in the pore wall. New
techniques are needed for further characterization of the
chirality of the chiral solids.
2
2
solvent (Table 4). From the results we find that the alde-
Generally, the solvent has a big influence on the catalytic
Table 4. Catalytic properties of PBB-30 in the Ti-promoted asymmetric
addition of diethylzinc to aromatic aldehydes.
[19]
properties of the catalysts. Therefore, the catalytic perfor-
mance of PBB-30, PBT-10, and (R)-(+)-Binol in different
solvents was investigated (Table 3). The homogeneous (R)-
[a]
Table 3. Effect of solvent in Ti-promoted asymmetric addition of diethyl-
zinc to benzaldehyde.
[
a]
ꢀ1
Entry
R
Conv. [%]
ee [%]
TOF [h
]
1
2
3
4
5
6
7
8
9
H
99
60
61
40
98
88
72
57
50
92
93
88
91
89
88
84
83
68
29
12
13
7
24
14
13
11
13
2-CH
3-CH
4-CH
3-CH
4-CH
3-Cl
3
3
3
3
3
O
O
(
R)-(+)-Binol
PBB-30
ee [%] TOF [h
PBT-10
ee [%] TOF [h ]
ꢀ
1
ꢀ1
ꢀ1
ee [%] TOF [h
]
]
CH
toluene 83
hexane 78
2
Cl
2
87
69
96
87
92
90
79
29
104
103
91
86
81
22
95
101
3-Br
4-CF
3
[
a] The reaction conditions and calculation method for the TOF are the
[
a] The reaction conditions and calculation method for the TOF are the
same as that in Table 2.
same as that in Table 2.
hydes with substituted groups have a lower TOF than ben-
zaldehyde. This could be ascribed to the transmission diffi-
culty for larger reactant molecules. Higher ee values were
obtained for the substrates with electron-donating groups
(2-CH -, 4-CH -, and 4-CH O-) than those with electron-
(
+)-Binol shows lower ee but slightly higher TOF in nonpo-
lar solvents such as toluene and hexane than in a polar sol-
vent, CH Cl . Unlike their homogenous counterparts, the
2
2
3
3
3
TOFs of PBB-30 and PBT-10 are much higher in toluene
and hexane than in CH Cl . For example, the TOF of PBB-
withdrawing groups (3-CH -, 3-CH O-, 3-Cl-, 3-Br-, and 4-
3 3
CF -). Steric hindrance also plays an important role in ee
2
2
3
ꢀ
1
30 can reach as high as 104 h in toluene, which is more
values for the substrates with electron-withdrawing groups.
than three times higher than that in CH Cl . By using tolu-
With the size of substituted groups increasing, the ee values
2
2
ene as the solvent, the TOF of PBB-30 is even higher than
its homogenous counterpart. The ee of PBT-10 with a more
hydrophilic surface decreased from 91 to 86 and to 81% as
the solvent was shifted from CH Cl to toluene and to
decrease gradually from 84% (3-Cl-) to 68% (3-CF -). All
3
the experimental results indicate that the size of reactant
molecules and the electronic and steric properties of the
substituents have a remarkable effect on the catalytic activi-
ty and enantioselectivity of the chiral PMOs.
2
2
hexane. It is interesting that for PBB-30 a 90% ee was
found in toluene, although a decrease of ee was observed in
hexane. When using toluene as solvent, PBB-30 (TOF of
The recyclability of the chiral PMOs was investigated by
choosing PBB-30 as the model catalyst (Table 5). The cata-
lyst was recovered from the reaction mixture by means of
filtration under an argon atmosphere after the reaction and
was washed with CH Cl several times. The recovered cata-
ꢀ
1
1
04 h with 90% ee) is more active and enantioselective
ꢀ1
than its homogeneous counterpart (TOF of 96 h with
3% ee). This is probably due to the fact that the transition
8
2
2
state for the formation of a single enantiomer could be
greatly enhanced by the hydrophobic surface using toluene
as solvent.
lyst was directly used in the next reaction without addition
of extra Ti(OiPr) . An 88% conversion with 89% ee was ob-
tained in the second run. However, only 15% conversion
AHCTUNGTRENNUNG
4
Chem. Asian J. 2010, 5, 1232 – 1239
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1237

Q. Yang et al.
FULL PAPERS
Table 5. Recyclability of PBB-30 in the Ti-promoted asymmetric addition
of diethylzinc to benzaldehyde.
(R)-(+)-Binol in the framework was found to be more enan-
tioselective and active than its homogeneous counterpart.
The primary results indicate that highly efficient chiral
PMOs for asymmetric catalysis could be obtained by tuning
the mesostructure and the rigid microenvironment of the
chiral ligand in the nanopore of the chiral PMOs.
Run
Cycle
t [h]
Conv. [%]
ee [%]
[
a]
1
2
3
4
fresh
1st
2nd
3rd
4
4
6
6
99
88
15
99
92
89
11
79
Experimental Section
[
b]
Synthesis of BSBinol
[
a] The reaction conditions and calculation method for the TOF are the
same as that in Table 2. [b] The recovered catalyst was washed with 10%
HCl in MeOH, H O, and acetone and was used in the reaction under the
The schematic description of the synthesis of BSBinol is outlined in Sche-
me 1A. Typically, n-butyllithium (100 mmol) in hexane was added to a
2
solution
of
(R)-(+)-6,6’-dibromo-2,2’-di(methoxymethyl)oxy-1,1’-bi-
same conditions as that in Table 2.
[14]
naphthyl (13.30 g, 25 mmol) in THF (160 mL) at ꢀ788C under argon.
After stirring at ꢀ788C for 1 h, the solution was warmed to room temper-
ature and stirred for 1 h. Then the mixture was cooled to ꢀ788C and 3-
chloropropyltrimethoxysilane (11.90 g, 60 mmol) was added dropwise.
The mixture was stirred at ꢀ788C for 1 h. After warming to room tem-
perature, the mixture was stirred overnight. Then the solvent was re-
moved in vacuo and the residue was extracted with diethyl ether
with 11% ee was obtained in the third run. The sharp de-
crease of activity and ee is probably due to the destruction
of active sites during the recycling process. The actual cata-
lytic species is supposed to be a dimeric Ti structure that has
one Binol ligand and six isopropoxy ligands.
third run, the recovered catalyst was washed successively
with 10% HCl/MeOH, water, and acetone to remove resid-
(150 mL). A yellow solid was obtained after removing diethyl ether in
[20]
1
After the
vacuo. H NMR (400 MHz, CDCl
3
, 258C, TMS): d=0.90 (4H), 1.82
(
4H), 2.63 (4H), 3.16 (6H), 3.83 (18H), 5.05 (4H), 7.07–7.35 (4H), 7.53–
1
3
7
.70 (2H), 7.85–8.00 (2H), 8.05–8.38 ppm (2H); C NMR (100 MHz,
, 258C, TMS): d=14.2 (-Si-CH -CH -CH -), 26.8 (-Si-CH -CH
-), 47.7 (-Si-CH -CH -CH -), 51.0 (CH -O-Si), 56.0 (-OCH OCH
95.2 (-OCH OCH ), 117.3–153.6 ppm (CBinol).
CDCl
CH
3
2
2
2
2
2
-
[21]
ual Ti species from the solid catalyst. In the fourth run,
the recovered catalyst showed a conversion of 99% with
2
2
2
2
3
2
3
),
2
3
7
9% ee under similar conditions to the first run. The meso-
Synthesis of PBB-n by cocondensation of BSBinol with 1,2-
bis(trimethoxysilyl)ethane
porous structure of PBB-30 after the fourth run was ana-
lyzed by XRD (Figure 1). The recycled PBB-30 has a similar
XRD pattern to the fresh one, showing no destruction of
the mesostructure during the catalytic process. The de-
creased enantioselectivity is probably due to the Ti residue,
which cannot be removed during the washing step. The
above results indicate that the chiral PMOs are relatively
stable during the catalytic process and can be recycled.
P123 (0.55 g) and KCl (3.49 g) were dissolved in 2m HCl (16.50 g) and
(3.75 g) under vigorous stirring. To this solution, mixture
7.00 mmol of Si) of BSBinol and BTME in acetone (2.00 g) was added
H
2
O
a
(
at 358C. The reaction mixture was stirred at 358C for 24 h and then
transferred into a teflon-lined autoclave and aged at 1008C under static
conditions for 24 h. The surfactant was extracted and the protecting
2 3
group (-CH OCH ) was removed simultaneously by stirring the as-syn-
thesized material (1.0 g) in ethanol (200 mL) containing 12m HCl (2.0 g)
at 808C for 24 h. After filtration, the powder was dried at 808C over-
night. The materials are denoted PBB-n, in which n (n=10, 20, 30) is the
mol% of BSBinol/(BSBinol+BTME).
Conclusion
Synthesis of PBT-n by cocondensation of BSBinol with tetramethoxysilane
(
R)-(+)-Binol-functionalized chiral PMOs were successfully
In a typical synthesis, P123 (0.50 g) was dissolved in ethanol (0.80 g) and
acetic acid/sodium acetate buffer solution (14 mL; pH 4.4; HOAc: 0.52m;
NaOAc: 0.27m) at 258C under vigorous stirring. Sodium silicate (1 mL,
synthesized by means of a cocondensation method using
BTME and TMOS as co-silane precursor. The combination
of BTME and BSBinol is more favorable for the synthesis
of materials with an ordered mesoporous structure than the
combination of TMOS and BSBinol. The results of FTIR
and solid-state NMR spectroscopy confirm the integrity of
2 2
20% of SiO , 6% of Na O) was added to the above solution. After stir-
ring at 258C for 10 min, a mixture (4.25 mmol of Si) of BSBinol and
TMOS in acetone (2.00 g) was added dropwise. The reaction mixture was
stirred at 408C for 24 h and was then transferred into a teflon-lined auto-
clave and aged at 1008C under static conditions for 24 h. The surfactant
was extracted and the protection group (-CH OCH ) was removed simul-
(
R)-(+)-Binol in the ordered 2D mesostructure framework.
2
3
The catalytic performance of the chiral PMOs was investi-
gated by means of Ti-promoted asymmetric addition of di-
ethylzinc to aromatic aldehydes. At higher (R)-(+)-Binol
concentration, PBB-n materials show a higher catalytic ac-
tivity than PBT-n materials. This is mainly due to the fact
that most (R)-(+)-Binol species buried in the thick pore
wall of PBT materials are difficult for the reactants to
access during the catalytic process. The chiral PMOs exhibit
higher enantioselectivity but lower catalytic activity than
taneously by stirring the as-synthesized material (1.0 g) in ethanol
200 mL) containing 12m HCl (2.0 g) at 808C for 24 h . After filtration,
the powder was dried at 808C overnight. The materials are denoted PBT-
n, in which n (n=10, 20, 30) is the mol% of 2BSBinol/(2BSBinol+T-
MOS).
(
Characterization
X-ray powder diffraction (XRD) patterns were recorded on a Rigaku
RINT D/Max-2500 powder diffraction system using CuKa radiation. The
nitrogen sorption experiments were performed at ꢀ1968C on an ASAP
2
020 system. The samples were outgassed at 1208C for 5 h before the
their homogeneous counterparts in CH Cl . When toluene
2
2
measurement. The Brunauer–Emmett–Teller (BET) surface area was cal-
culated from the data in the relative pressure range of 0.05 to 0.25. The
was used as the solvent, the chiral PMO with ethylene and
1238
www.chemasianj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2010, 5, 1232 – 1239

Functionalized Mesoporous Organosilica
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Received: December 21, 2009
2
65–288.
Published online: March 16, 2010
Chem. Asian J. 2010, 5, 1232 – 1239
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1239