Chiral mesoporous organosilica nanospheres: Effect of pore structure on the performance in asymmetric catalysis

Article abstract of DOI:10.1002/chem.201000931

(R)-(+)-1,1′-Bi-2-naphthol ((R)-(+)-Binol)-functionalized (Binol=2,2′-dihydroxy-1,1′-binaphthyl) chiral mesoporous organosilica nanospheres with uniform particle size (100 to 300 nm) have been synthesized by co-condensation of tetraethoxysilane and (R)-2,2′- di(methoxymethyl)oxy-6,6′-di(1-propyl trimethoxysilyl)-1,1′- binaphthyl in a basic medium with cetyltrimethylammonium bromide as the template. Nanospheres with a radiative 2D hexagonal channel arrangement exhibit higher enantioselectivity and turnover frequency than those with a penetrating 2D hexagonal channel arrangement (94 versus 88% and 43 versus 15 h^-1, respectively) in the asymmetric addition of diethylzinc to aldehydes. In addition, under similar conditions, the enantioselectivity of the nanospheres can be greatly improved as the structural order of the framework increases. These results clearly show that the structural order of nanospheres affects enantioselective reactions. The enantioselectivity of the nanospheres synthesized by the co-condensation method is higher than that of nanospheres prepared by a grafting method and even higher than that of their homogeneous counterpart. These results indicate that the bite angle of (R)-(+)-Binol bridging in a more rigid porous network is in a more favorable position for achieving higher enantioselectivity. The efficiency of a co-condensation method for the synthesis of high-performance heterogeneous asymmetric catalysts is also reported. Spheres of influence: (R)-(+)-Binol-functionalized chiral periodic mesoporous organosilicas (PMOs) with different pore structures and morphology are efficient catalysts for the asymmetric addition of diethylzinc to aldehydes. Nanospheres with a radiative 2D hexagonal channel arrangement show the highest activity and ee value of the chiral PMOs investigated (see picture; MOM: methoxymethyl ether, CTAB: cetyltrimethylammonium bromide).

Full text of DOI:10.1002/chem.201000931

FULL PAPER
DOI: 10.1002/chem.201000931
Chiral Mesoporous Organosilica Nanospheres: Effect of Pore Structure on
the Performance in Asymmetric Catalysis
[
a, b]
[a]
[a]
[a]
[a]
[a]
Xiao Liu,
Peiyuan Wang, Lei Zhang, Jie Yang, Can Li,* and Qihua Yang*
Abstract:
(R)-(+)-1,1’-Bi-2-naphthol
trating 2D hexagonal channel arrange-
ment (94 versus 88% and 43 versus
enantioselective reactions. The enantio-
selectivity of the nanospheres synthe-
sized by the co-condensation method is
higher than that of nanospheres pre-
pared by a grafting method and even
higher than that of their homogeneous
counterpart. These results indicate that
the bite angle of (R)-(+)-Binol bridg-
ing in a more rigid porous network is
in a more favorable position for achiev-
ing higher enantioselectivity. The effi-
ciency of a co-condensation method for
the synthesis of high-performance het-
erogeneous asymmetric catalysts is also
reported.
(
(R)-(+)-Binol)-functionalized (Binol=
ꢀ
1
2,2’-dihydroxy-1,1’-binaphthyl)
chiral
15 h , respectively) in the asymmetric
addition of diethylzinc to aldehydes. In
addition, under similar conditions, the
enantioselectivity of the nanospheres
can be greatly improved as the struc-
tural order of the framework increases.
These results clearly show that the
structural order of nanospheres affects
mesoporous organosilica nanospheres
with uniform particle size (100 to
3
00 nm) have been synthesized by co-
condensation of tetraethoxysilane and
R)-2,2’-di(methoxymethyl)oxy-6,6’-
di(1-propyl trimethoxysilyl)-1,1’-bi-
(
naphthyl in a basic medium with cetyl-
trimethylammonium bromide as the
template. Nanospheres with a radiative
Keywords: asymmetric catalysis
·
2D hexagonal channel arrangement ex-
binol derivatives · mesoporous ma-
terials · nanoparticles · organosilica
hibit higher enantioselectivity and turn-
over frequency than those with a pene-
[
4]
[5]
[6]
[1,7]
Introduction
films, hollow spheres, hollow tubes, rods,
liths,
mono-
[
8,9]
[10]
[11]
helical structures, and vesicles, have been suc-
Since the first report of mesoporous silicas by Mobil scien-
tists in the 1990s, much research effort has been devoted to
the controllable synthesis of mesoporous silicas with differ-
ent framework component, mesoporous structure, and exter-
nal morphology, which play important roles in the versatile
cessfully synthesized. For many applications, increasing the
diffusion rate of the guest molecule and the exposed
number of active sites can greatly improve the performance
of the mesoporous silicas. Generally, several approaches are
employed to meet this demand, such as generating a hier-
[1,2]
[9,12]
application of the mesoporous silicas.
To date, mesopo-
archical porous structure in mesoporous silicas,
decreas-
[3]
rous silicas with various morphologies, such as spheres,
ing the particle size of mesoporous silicas to the nano-
[13]
scale, and so on.
As an important branch of mesoporous materials, the pe-
riodic mesoporous organosilicas (PMOs) synthesized from
+
+
[
a] Dr. X. Liu, Dr. P. Wang, Dr. L. Zhang, Dr. J. Yang, Prof. Dr. C. Li,
Prof. Dr. Q. Yang
bridged silane precursors, (R’O) Si-R-Si ACHTUNGTERNN(UGN OR’) , have attract-
3
3
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
57 Zhongshan Road, Dalian 116023 (China)
Fax : (+86)411-84694447
ed much research attention because of their unique proper-
ties (such as high mechanical and hydrothermal stability,
and tunable surface hydrophilicity/hydrophobicity) derived
from the uniformly distributed organic moieties (ethylene,
thiophene, biphenylene, ferrocene, etc.) in the mesoporous
4
E-mail: canli@dicp.ac.cn
yangqh@dicp.ac.cn
+
[14]
framework.
Driven by the ever-increasing demand for
[
b] Dr. X. Liu
nonracemic chiral chemicals, the synthesis of chiral PMOs
with a chiral ligand bridged in the framework is very inter-
esting because the development of chiral porous materials is
one of the key subjects in asymmetric catalysis. The PMOs
Graduate School of the Chinese Academy of Sciences
Beijing 100049 (China)
+
[
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000931.
with chiral ligands in the framework, such as VO ACHTUNGTRENNUNG( salen)
Chem. Eur. J. 2010, 16, 12727 – 12735
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12727

[
15]
(
salen=2,2’-ethylenebis(nitrilomethylidene)diphenol),
Binol bridging in the framework by using a mixture of (R)-
2,2’-di(methoxymethyl)oxy-6,6’-di(1-propyl-trimethoxysilyl)-
1,1’-binaphthyl (BSBinol) and 1,2-bis(trimethoxysilyl)ethane
or tetramethoxysilane as precursor and P123 as surfactant
under acid conditions. In this work, we synthesized chiral
mesoporous organosilicas under different basic conditions to
investigate the effects of pore structure on the catalytic per-
formance. A NaOH aqueous solution (0.2m) was first em-
ployed for the synthesis of a chiral mesoporous organosilica
sample denoted Bulk. The TEM image shows that sample
Bulk has an irregularly shaped morphology with a particle
size larger than 5 mm (Figure 1). The XRD pattern of this
[16]
[17]
trans-diaminocycloxene, and l-tartardiamide, have been
successfully synthesized and used for asymmetric catalysis.
However, they generally show lower activity and enantiose-
lectivity than their homogeneous counterparts, which is also
one of the major obstacles for their use in heterogeneous
asymmetric catalysis. Engineering the mesostructural order-
ing and morphology of the chiral PMOs may be very impor-
tant for achieving high enantioselectivity and catalytic activi-
ty. For example, previous studies demonstrated that the
pore wall rigidity has a big influence on the chiral inducibili-
ty of the chiral PMOs. It has also been shown that the cata-
lytic activity of the mesoporous silicas can be greatly im-
proved by decreasing the particle size to the nanoscale, be-
cause of the short diffusion lengths as a result of the nano-
[19]
[18]
particle size. However, control of the mesostructural or-
dering and morphology of the chiral PMOs is very difficult,
and the synthesis of chiral PMOs with nanospherical mor-
phology has been seldom reported.
Herein, we report the synthesis of chiral mesoporous or-
ganosilica nanospheres with (R)-(+)-1,1’-bi-2-naphthol ((R)-
(
+)-Binol) bridged in the framework (Scheme1). The effects
Figure 1. SEM images, TEM images, and XRD patterns of the (R)-(+)-
Binol-functionalized mesoporous organosilicas Bulk and NS(a), which
were synthesized in NaOH aqueous solution with low H
in ammonia solution, respectively.
2
O/Si ratio and
sample displays a broad diffraction peak at 2q of 1.68, char-
acteristic of the mesoporous materials with wormhole-like
[20]
mesoporous structure (Figure 1). The TEM image of this
sample clearly shows the existence of wormhole-like pores
throughout the sample, further confirming the XRD result
Scheme 1. Synthesis of (R)-(+)-Binol-functionalized chiral PMO nano-
spheres with different pore structures. MOM: methoxymethyl ether;
CTAB: cetyltrimethylammonium bromide; TEOS: tetraethoxysilane; NS,
NS’, and NS(a): different types of nanospheres.
(
Figure 1).
Previous studies showed that the nanospheres can be
formed under basic conditions with high H O/Si molar
2
[21]
of the synthesis medium (pH value and base source), meso-
structural ordering, and morphology on the catalytic perfor-
mance of the resultant materials were investigated. The
asymmetric addition of diethylzinc to aldehydes was chosen
as a model reaction to test the catalytic properties of the
ratio. Therefore, the NS and NS’ samples (Scheme1) were
synthesized in the presence of a large amount of water. The
SEM images of NS and NS’ show that the materials are
both composed of nanospheres with particle size 100 to
150 nm, which suggests that the hydrothermal treatment
process has no obvious influence on the morphology
chiral nanospheres (coordinated with Ti ACHTUNGRETNN(UNG OiPr) ).
4
(
Figure 2). NS’ shows one broad diffraction peak together
with a shoulder in the XRD pattern. From further TEM
characterization, NS’ has a wormhole-like porous structure
similar to that of Bulk but with a higher degree of ordering
(Figure 2). Three well-defined XRD peaks, which can be as-
signed to the (100), (110), and (200) diffractions of a highly
ordered P6mm mesophase, appear in the XRD pattern of
Results and Discussion
Structure and morphology characterization of the chiral
mesoporous organosilicas: Recently, our group reported the
synthesis of chiral mesoporous organosilicas with (R)-(+)-
12728
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Chem. Eur. J. 2010, 16, 12727 – 12735

Chiral Mesoporous Organosilica Nanospheres
FULL PAPER
phology of the mesoporous materials were greatly affected
by the pH of the synthesis medium and the organic addi-
tives. Therefore, mesoporous organosilicas with different
morphology and mesostructure could be obtained by adjust-
ing the pH value of the synthesis medium and by the addi-
tion of organic additives.
The nitrogen sorption isotherms of Bulk, NS’, NS, and
NS(a) are of type IV with a sharp capillary condensation
step in the relative pressure P/P of 0.2 to 0.4, which shows
0
that all the materials have mesopores (Figure 3). The physi-
Figure 2. SEM images, TEM images, and XRD patterns of the (R)-(+)-
Binol-functionalized mesoporous organosilicas NS’ and NS, which were
synthesized in NaOH aqueous solution with high H
and with aging, respectively.
2
O/Si ratio without
[22]
NS (Figure 2).
Straight, parallel channels together with
honeycomb mesopores penetrate throughout the TEM
images of the nanospheres (Figure 2). The TEM and XRD
characterizations show that NS has a more ordered meso-
structure than NS’. The above results suggest that the hydro-
thermal treatment favors the formation of a highly ordered
mesoporous structure.
Through increasing the H O/Si ratio, mesoporous nano-
2
spheres can be easily formed. Rather than NaOH, ammonia
is generally used for the preparation of silica nanoparticles
[23]
by Mouꢁs group.
Therefore, ammonia solution was also
used for the synthesis of chiral mesoporous organosilica
nanospheres. NS(a) is composed of nanospheres with parti-
cle size in the range of 250 to 300 nm (Figure 1). Compared
with NS samples, the particle size of NS(a) is larger. The
structural order of NS(a) was further characterized by XRD
(
Figure 1). Three well-resolved diffraction peaks, indexed to
the (100), (110), and (200) peaks of a highly ordered P6mm
mesophase, were clearly observed in the XRD pattern of
NS(a), thus showing that this material has a 2D hexagonal
mesostructure. The TEM image of NS(a) exhibits striations
radiating from the center of the nanospheres (Figure 1). To
our knowledge, this is the first report of the synthesis of
chiral mesoporous organosilica nanospheres with radiative
2D hexagonal channels.
From the above characterizations, it can be concluded
Figure 3. Nitrogen sorption isotherms of (R)-(+)-Binol-functionalized
that nanospheres with a wormhole-like or 2D hexagonal
mesostructure can be controllably synthesized in basic
medium without and with hydrothermal treatment, respec-
tively. Though NS(a) and NS have a similar structural order,
the channel orientation of these two materials is different.
NS(a) synthesized in ammonia solution has channels radiat-
ing from the center of the nanosphere and NS synthesized
in NaOH solution has straight channels penetrating from
one pole to the other. As we know, the structure and mor-
mesoporous organosilicas synthesized under different conditions.
cal parameters of the materials are summarized in Table 1.
NS(a) has the largest Brunauer–Emmett–Teller (BET) sur-
face area among the nanospheres. NS’ has a higher BET sur-
face area and pore volume than NS, probably due to frame-
work contraction during the hydrothermal treatment pro-
cess. All the nanospheres have a similar pore diameter (2.4–
Chem. Eur. J. 2010, 16, 12727 – 12735
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12729

Q. Yang et al.
ꢀ
1
Table 1. Physicochemical properties of (R)-(+)-Binol-functionalized mes-
oporous organosilicas.
and 0.57 mmolg , respectively. Bulk synthesized in the
strongest basic medium had the lowest organic content
among the four samples, which suggests that the strong basic
conditions are not favorable for the incorporation of an or-
Sample d spacing BET surface Pore
Pore diam- Wall thick-
eter [nm] ness [nm]
2
ꢀ1
[a]
[b]
[nm]
area [m g
]
volume
3
ꢀ1
[cm g ]
[24]
ganic functionality into the mesoporous materials.
NS’
Bulk
NS’
NS
NS(a) 3.9
G-NS 4.0
5.5
4.1
4.0
460
639
444
722
796
0.39
0.64
0.44
0.65
0.52
2.5
2.6
2.4
2.4
2.1
–
–
2.2
2.1
2.5
had a much higher organic content than NS; this is probably
due to the removal of loosely cross-linked organic function-
ality during the hydrothermal treatment process. G-NS pre-
pared by a grafting method had an (R)-(+)-Binol content of
ꢀ
1
[
a] Calculated using the Barrett–Joyner–Halenda (BJH) model on the ad-
0.56 mmolg , which is comparable to that of NS(a).
sorption branch of the isotherm. [b] Calculated by a
0
ꢀpore diameter,
The FTIR spectra of the Bulk, NS’, NS, and NS(a) materi-
p
where a
0
=2d100
/
3
.
als show the characteristic bands of CH for aliphatic C–H
2
ꢀ
1
stretching vibrations at 3000–2900 cm (see the Supporting
ꢀ
1
Information). The bands at 1466, 1400, and 1385 cm can
be ascribed to the vibrations of the aromatic ring of the
Binol group. The FTIR characterization shows the success-
ful incorporation of (R)-(+)-Binol onto the mesoporous
nanospheres.
2.6 nm) and pore wall thickness (2.1–2.2 nm). Bulk shows
similar BET surface area, pore volume, and pore diameter
to those of NS.
For comparison, sample G-NS was also prepared by graft-
ing of BSBinol onto mesoporous silica nanospheres. G-NS
shows a similar XRD pattern and TEM image to those of
NS(a), which suggests that G-NS also has a 2D hexagonal
mesostructure (see the Supporting Information). The nitro-
gen sorption isotherm of the G-NS material is of type IV
To further confirm the incorporation of (R)-(+)-Binol
13
moieties in PMOs, the materials were characterized by
C
cross-polarization magic-angle spinning (CP/MAS) NMR
2
9
13
and Si MAS NMR spectroscopy (Figure 4). In the C CP/
MAS NMR spectrum of the characteristic NS(a), the signals
in the range of 160–110 ppm were ascribed to aryl carbon
atoms of (R)-(+)-Binol. The signals at 46.8, 26.3, and
3
ꢀ1
with a BET surface area of 796 cm g and pore diameter of
.1 nm (Table 1 and the Supporting Information).
2
3
2
1
1
6.1 ppm could be assigned to C , C , and C carbon species
Composition characterization of the chiral mesoporous or-
ganosilica nanospheres: The content of the chiral moiety in
the mesoporous materials was characterized by C, H, and N
elemental analysis (Table 2). The (R)-(+)-Binol content in
G-NS, NS’, NS, Bulk, and NS(a) was 0.56, 0.94, 0.69, 0.36,
Table 2. Ti-promoted asymmetric addition of diethylzinc to benzalde-
[
a]
hyde on chiral mesoporous organosilicas.
[
b]
[b]
[c]
Catalyst Time Conversion
ee
TOF
Binol con-
Binol con-
ꢀ
1
[d]
[d]
[h]
[%]
[%] [h
]
tent
tent
ꢀ
1
ꢀ2
[mmolg
]
[mmolm
]
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3
3
3
3
3
3
G-NS
NS’
NS
6
6
6
6
2
4
95
86
99
36
99
99
76
61
88
33
94
91
6
5
15
1
43
22
0.56
0.94
0.69
0.36
0.57
0.29
0.5ꢂ10
1.4ꢂ10
1.6ꢂ10
0.8ꢂ10
0.8ꢂ10
0.5ꢂ10
Bulk
NS(a)
PBT-
[
e]
1
0
(
(
R)-
+)-
1
99
87
69
–
–
Binol
a] All the reactions were carried out in CH
1.5 mmol), Et
[
(
2
Cl
2
with Ti ACHTUNREGTNNUNG( OiPr)
4
2
Zn (3.0 mmol), and benzaldehyde (1.0 mmol) at 08C. The
molar ratio of Ti to ligand was 13. [b] Conversions and ee values were de-
termined by GC on an HP-Chiral 19091G-B213 capillary column.
[
c] TOF was calculated according to the following equation: TOF=
mmolconverted benzaldehyde/(mmolBinol ꢂh). [d] Determined by elemental analy-
sis based on the C contents. [e] PBT-10 is the material synthesized by co-
condensation of BSBinol and tertramethoxysilane in acidic medium using
2
9
13
Figure 4. Si MAS NMR and C CP/MAS NMR spectra of (R)-(+)-
Binol-functionalized organosilica nanospheres NS(a), which were synthe-
sized in ammonia solution.
[
19]
P123 as surfactant.
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Chiral Mesoporous Organosilica Nanospheres
FULL PAPER
1
2
3
of Si-C H C H C H , respectively. The absence of signals at
5
ment. However, only a few papers have reported the use of
mesoporous silica nanospheres as a solid support for asym-
metric catalysis.
Table 2 summarizes the surface coverage of (R)-(+)-Binol
and the catalytic performance of the materials in the Ti-pro-
moted asymmetric addition of diethylzinc to benzaldehyde;
the reaction profiles are displayed in Figure 5. G-NS synthe-
2
2
2
5.9 and 95.0 ppm shows that the protecting groups (MOM)
can be removed during the process of surfactant extraction.
The signals centered at 58.3 and 16.1 ppm (overlapped with
[26]
1
the signal of C ) may originate from the ethoxy groups
formed during the surfactant-extraction process. The weak
signals at 64.5, 54.0, 29.6, and 9.7 ppm (overlapped with the
signal of ethoxy groups during the surfactant-extraction pro-
1
3
cess) may originate from the residual surfactant. The
C
CP/MAS NMR characterization confirms the incorporation
of organic groups in the mesoporous materials.
29
The Si MAS NMR spectrum of NS(a) shows both the T
and Q silicon sites. The broad signal in the range of ꢀ50 to
ꢀ
80 ppm is for silicon species connected with the organic
group. The signals at ꢀ113.0 and ꢀ103.2 ppm arise from the
4
3
Si species Q (Si(OSi) ) and Q (Si(OH)(OSi) ), respectively.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
4
3
The results of NMR spectroscopy further prove that the or-
ganic group was incorporated in the mesoporous network,
which is consistent with the results of FTIR spectroscopy.
The direct determination of the enantiomeric purity of
(
R)-(+)-Binol in the mesoporous organosilicas is very diffi-
cult. Hence, we used NaOH (1m) to dissolve the mesopo-
rous organosilica and characterized the resultant solution by
circular dichroism (CD). The CD curve of (R)-(+)-Binol
(
see the Supporting Information) showed a negative Cotton
Figure 5. Reaction time profiles of Ti-promoted asymmetric addition of
diethylzinc to benzaldehyde on different kinds of catalysts.
effect centered in the range of 220–270 nm. The solution of
Bulk, NS’, NS, and NS(a) also showed the same negative
Cotton effect centered in the range of 220–270 nm. The re-
sults implied that there was almost no change in the chirality
of the (R)-(+)-Binol groups during the synthesis.
sized by a grafting method can efficiently catalyze the Ti-
promoted asymmetric addition of diethylzinc to benzalde-
hyde. Within 6 h, the conversion of benzaldehyde can reach
95%. However, it takes only 1 h for the homogeneous coun-
terpart to fully convert the benzaldehyde. An enantiomeric
excess (ee) value of 76% was obtained on G-NS, which is
slightly lower than that with homogeneous (R)-(+)-Binol
(87% ee) under identical reaction conditions. The decreas-
ing of the ee value was also observed for solid catalysts pre-
pared by grafting (R)-(+)-Binol onto unmodified porous
To identify the location of (R)-(+)-Binol groups in PMOs
clearly, we used Fe ACHTUNGTRENNUNG( OAc) to coordinate with (R)-(+)-Binol
2
in NS(a) and G-NS. After coordination, the samples were
analyzed by scanning transmission electron microscopy
(
STEM) and energy-dispersive X-ray spectrometry (EDX)
with high-resolution transmission electron microscopy
HRTEM; see the Supporting Information). Figure S-4 in
(
the Supporting Information displays the STEM image and
Fe mapping of the chosen area by using EDX. The white
dots represent the signal of Fe, which has a uniform distribu-
tion in NS(a). On the contrary, the signal of Fe in G-NS has
an irregular distribution and is spread mainly around the
rim of the nanosphere. The results of Fe mapping suggest
that the co-condensation method can lead to PMOs with a
more uniform distribution of (R)-(+)-Binol groups than the
grafting method.
[25]
silica. The turnover frequency (TOF) of G-NS and (R)-
ꢀ1
(+)-Binol is 6 and 69 h , respectively. The much lower TOF
of G-NS is probably due to the irregularly distributed (R)-
(+)-Binol on the mesoporous silica, which makes the access
of the substrate to the active site difficult. Compared with
the grafting method, the co-condensation method favors the
formation of mesoporous silicas with uniformly distributed
organic groups, which may provide a chance to increase the
ee value and catalytic activity of the solid catalysts by in-
creasing the homogeneity of (R)-(+)-Binol.
Catalytic performance of the nanospheres: (R)-(+)-Binol
can efficiently catalyze the asymmetric addition of dialkyl
zinc to aldehydes, which is one of the most important asym-
metric CꢀC bond formation reactions for the production of
NS’ and NS synthesized by the co-condensation method
were tested in the same reaction. Under similar reaction
conditions, it is surprising to find that NS’ and NS with simi-
lar morphology show quite different catalytic performance.
Conversions of 86% with 61% ee and of 99% with 88% ee
were obtained on NS’ and NS, respectively, within 6 h. Cal-
culated from the reaction profile, the TOF of NS is three
times that of NS’. The above characterizations show that
NS’ has a much higher BET surface area and pore volume
pharmaceutically useful chiral secondary alcohols. Because
of the advantages of heterogeneous asymmetric catalysis,
such as easy separation and recycling of the catalyst, the
chiral Binol has been immobilized onto many kinds of solid
[25]
supports. Mesoporous silicas are promising solid supports
because of their high surface area and ordered pore arrange-
Chem. Eur. J. 2010, 16, 12727 – 12735
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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12731

Q. Yang et al.
but less structural order than NS. Also, the surface coverage
of (R)-(+)-Binol on NS’ is comparable to that on NS. The
much lower ee value and TOF of NS’ than of NS can only
be assigned to their different structural order and pore wall
rigidity. It is reported that the bite angle of (R)-(+)-Binol
bite angle of (R)-(+)-Binol may also be affected by the ri-
gidity of the PMOs. Therefore, it could be proposed that a
more ordered mesostructure and rigid pore wall favors the
bite angle of (R)-(+)-Binol in the right position for reaching
higher ee values for asymmetric catalysis. The fact that
NS(a) is more enantioselective than the homogeneous cata-
lyst clearly suggests the positive effect of the ordered pore
wall structure on the enantioselectivity of (R)-(+)-Binol
bridging in the network. (R)-(+)-Binol on G-NS is more
flexible than that on NS(a) and NS. The lower ee value of
G-NS compared with that of NS(a) and NS further confirms
the above conclusion that the more ordered and rigid pore
wall structure favors higher enantioselectivity of the chiral
PMOs.
[27]
determines the enantioselectivity of asymmetric reactions.
It is also generally accepted that the hydrothermal treat-
ment process increases the cross-linking degree of the net-
work. NS has more rigid pore walls than NS’. Thus, the cata-
lytic performance of the PMOs may be influenced by the
structural order of the materials. In this case, it seems that
the ee value and TOF can be greatly accelerated by increas-
ing the structural order from a wormhole-like to 2D-ordered
mesostructure.
To further elucidate the role of the structural order in the
catalytic performance of the chiral PMOs, Bulk and NS(a)
prepared with hydrothermal treatment were also tested in
the Ti-promoted asymmetric addition of diethylzinc to benz-
Choosing NS(a) as a model material, the influence of Ti/
Binol on the catalytic performance was investigated in the
Ti-promoted asymmetric addition of diethylzinc to benzal-
dehyde (Table 3). With the molar ratio of Ti/Binol in the
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
aldehyde (Table 2). Bulk has a less ordered wormhole-like
structure than NS’ from the above characterizations. Within
h, the conversion can only reach 36% with 33% ee. The
Table 3. Influence of Ti/Binol molar ratio on the catalytic properties of
6
[a]
NS(a).
ꢀ
1
TOF of Bulk is only 1 h . The catalytic activity and ee value
of Bulk are much lower than those of NS’. The low catalytic
activity of Bulk can be explained by the large particle size
and low BET surface area, which do not favor the fast diffu-
sion of guest molecules throughout the mesoporous matrix
during the catalytic process. The reason for the low enantio-
selectivity is probably the less ordered pore structure.
ꢀ1
Entry
Ti/Binol [mmolmm
]
Conversion [%]
ee [%]
1
2
3
4
6.5
9.5
13
54
74
99
99
90
92
94
94
NS(a) with an ordered and radiative mesoporous struc-
ꢀ1
ture shows a TOF of 43 h with 94% ee, which is the high-
est among all the PMOs investigated and even higher than
that of the material which we synthesized under acidic con-
20
[
2 2 2
a] All reactions were carried out in CH Cl with Et Zn (3.0 mmol) and
benzaldehyde (1.0 mmol) for 2 h at 08C. Conversions and ee values were
determined by GC on an HP-Chiral 19091G-B213 capillary column.
[19]
ditions (Table 2, PBT-10). The ee value of NS(a) is even
higher than that of the homogeneous catalyst under similar
reaction conditions. The higher TOF observed for NS(a) is
partly due to the higher surface area and pore volume and
partly due to the radiative arrangement of the pore chan-
nels. Furthermore, NS(a) with radiating channels has a
higher activity and enantioselectivity than NS with penetrat-
ing channels. This is probably due to the combined effect of
the bite angle of the Binol in the PMOs and the rigidity of
the mesoporous framework.
range of 6.5 to 20, the ee value varies from 90 to 94%, thus
showing that the Ti/Binol ratio has only a slight influence on
the enantioselectivity. A sharp increase of activity is ob-
served upon increasing the Ti/Binol molar ratio. When the
Ti/Binol molar ratio exceeds 13, further increasing of this
ratio does not influence the performance of the material.
Different kinds of aromatic aldehydes could be converted
into the corresponding secondary alcohols on NS(a) with ex-
cellent conversions (72–99%) and enantioselectivity (71–
94%); all reactions were carried out in CH Cl at 08C for
2 h (Table 4). Among all the aromatic aldehydes investigat-
ed, benzaldehyde shows the highest ee value of 94%. The
aldehyde with a bulky isopropyl group resulted in a higher
ee value than that with a smaller CH group (entries 2 and
3, Table 4). The substrate with electron-donating CH shows
a higher ee value than those with electron-withdrawing Cl
and F (entries 6–8, Table 4), which suggests that the elec-
tronic properties of the substituent have a large influence on
the ee value.
The reusability of NS(a) was investigated in the Ti-pro-
moted asymmetric addition of diethylzinc to benzaldehyde.
Within 2 h, only 31% conversion with 53% ee was obtained
Based on the above results, it could be summarized that
the chiral PMOs with ordered mesostructure (NS and
NS(a)) show higher enantioselectivity than that of the disor-
dered wormlike mesostructure (NS’ and Bulk). Also, the
chiral PMOs synthesized by the co-condensation method
are more enantioselective than the catalyst made by the
grafting method. The enantioselectivity of the chiral PMOs
derives from the (R)-(+)-Binol bridging in the mesoporous
network. It has been previously reported that the bite angle
of the binaphthalene chiral building block determines the
2
2
3
3
[27]
enantioselectivity. By use of a bridged silane precursor for
the synthesis of PMOs, the organic groups should be dis-
persed both in the network and on the surface. Therefore,
the structural order may influence the orientation of the or-
ganic group in the network or on the surface. Moreover, the
12732
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 12727 – 12735

Chiral Mesoporous Organosilica Nanospheres
FULL PAPER
Table 4. Ti-promoted asymmetric addition of diethylzinc to different
kinds of aromatic aldehydes catalyzed by NS(a).
Experimental Section
[
a]
Chemicals and reagents: Diethylzinc (Et
chased from Sigma–Aldrich Company (USA). Ti
2
Zn, 1m in hexane) was pur-
(OiPr) was purchased
A
H
U
G
R
N
U
G
4
from Alfa–Aesar (USA) and was distilled before use. Cetyltrimethylam-
monium bromide (CTAB), benzaldehyde, and other aldehydes were pur-
chased from Acros Company (USA). (R)-(+)-Binol was obtained from
Lianyungang Chiral Chemicals Company (China). 3-Chloropropyltrime-
thoxysilane was obtained from Wuhan Tianmu Science & Technology
Development Co. Ltd. (China). Other reagents were purchased from
Shanghai Chemical Reagent Incorporation of the Chinese Medicine
Group. All solvents were of analytical quality and dried by standard
methods. All the reactions were carried out under an argon atmosphere
by using standard Schlenk techniques. (R)-2,2’-Di(methoxymethyl)oxy-
Entry
R-
Conversion [%]
ee [%]
1
2
3
4
5
6
7
8
H-
99
72
93
94
90
93
99
88
94
82
92
91
92
91
81
71
4-CH
4-CH
4-CH
3-CH
2-CH
2-F-
3
3
3
3
3
-
CH
O-
O-
-
3
ACHTUNGTRENNUNG( CH )-
6
,6’-di(1-propyltrimethoxysilyl)-1,1’-binaphthyl (BSBinol) was synthe-
[19]
sized according to a previous report.
2-Cl-
Synthesis of irregularly shaped bulk chiral mesoporous organosilica with
R)-(+)-Binol in the framework (Bulk): A mixture (8.5 mmol of Si) of
TEOS (1.42 g, 6.8 mmol) and BSBinol (0.596 g, 0.85 mmol) in acetone
2.0 g) was added to an aqueous solution containing CTAB (0.36 g),
NaOH (0.08 g), and deionized water (9 g) at 308C (pH 13.3). After stir-
ring for 12 h at 308C, the reaction mixture was transferred into a Teflon-
lined autoclave and aged at 1108C under static conditions for 72 h. After
filtration, the powder product was washed thoroughly with deionized
water and dried at 808C overnight. The molar composition of the re-
[
Et
a] All reactions were carried out in CH
2
Zn (3.0 mmol), and aldehyde (1.0 mmol) for 2 h at 08C. The molar
2
Cl
2
with Ti
A
H
U
G
R
N
U
G
4
(1.5 mmol),
(
ratio of Ti to ligand was 13. Conversions and ee values were determined
by GC on an HP-Chiral 19091G-B213 capillary column.
(
on the reused catalyst, which is much lower than that ob-
tained on the fresh catalyst (99% conversion with 94% ee).
A previous report shows that the addition of diethylzinc to
benzaldehyde is a stoichiometric reaction for benzaldehyde
and Ti
catalyst are probably due to the decreased Ti/Binol molar
ratio (no Ti(OiPr) was added for the recycling reaction)
2
agents was CTAB/NaOH/Si/H O=1.0:1.9:8.5:500. The surfactant was ex-
tracted by stirring as-synthesized material (1 g) in ethanol (200 mL) con-
taining 12m HCl (2.0 g) at 808C for 24 h. The protecting group was re-
moved at the same time. After filtration, the powder was dried at 808C
overnight. The material is denoted as Bulk, which means the material
with bulk morphology.
[28]
ACHTUNGTRENNUNG( OiPr) . The low activity and ee value of the reused
4
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Synthesis of chiral mesoporous organosilica nanospheres in NaOH solu-
tion (NS’ and NS): In a typical synthesis, CTAB (0.20 g) was dissolved in
deionized water (96 g) under stirring at room temperature. Then NaOH
4
and loss of mesostructural order of the catalyst (see the Sup-
porting Information).
(
2m, 0.70 mL) was added to the solution (pH 12). The temperature of the
solution was raised to 808C. To this clear solution, a mixture of TEOS
1.0 g, 4.8 mmol) and BSBinol (0.4 g, 0.57 mmol) in acetone (2.0 g) was
(
Conclusion
added sequentially and rapidly by injection. After stirring for an addi-
tional 2 h, the reaction mixture was filtered directly or transferred into a
Teflon-lined autoclave and aged at 1108C under static conditions for
(
R)-(+)-Binol-functionalized chiral mesoporous organosilica
7
2 h. After filtration, the powder product was washed thoroughly with
deionized water and dried at 808C overnight. The molar composition of
the reagents was CTAB/NaOH/Si/H O=1:2.6:10.8:9700. The removal of
spheres with different pore structures have been successfully
synthesized in basic medium. Nanospheres with 2D hexago-
nal mesostructure and bulk particles with wormhole-like
structure were obtained by using low H O/Si and high H O/
2
surfactant and protecting group was the same as that for Bulk material.
The materials are denoted as NS’ and NS, respectively, where NS’ refers
to the nanosphere without hydrothermal treatment and NS refers to the
nanosphere with hydrothermal treatment.
2
2
Si ratios in NaOH solution, respectively. Hydrothermal
treatment favors the formation of highly ordered 2D hexag-
onal channels penetrating throughout the nanospheres.
Nanospheres with radiative 2D hexagonal channels can be
obtained in ammonia solution. The catalysis results show
that the nanospheres with highly ordered 2D hexagonal
mesostructure are more enantioselective and catalytically
active than those with a wormhole-like mesostructure, which
suggests that the ordered mesostructure of the chiral PMOs
is one of the important factors for achieving high catalytic
performance in asymmetric catalysis. Our results also indi-
cate that the co-condensation method is more efficient than
the grafting method in view of the uniform coverage of or-
ganic moiety, catalytic activity, and enantioselectivity.
Through finely controlling the morphology and the meso-
structure, efficient chiral PMO catalysts for asymmetric cat-
alysis could be synthesized.
Synthesis of chiral mesoporous organosilica nanospheres in ammonia so-
lution (NS(a)): In a typical synthesis, CTAB (0.44 g) was dissolved in de-
ionized water (16.5 g), NH ·H O (25 wt%, 5.4 g), and EtOH (2.0 g) at
3
2
room temperature (pH 12.7). A mixture of TEOS (1.66 g, 8 mmol) and
BSBinol (0.70 g, 1 mmol) in ethanol (2.6 g) and acetone (2.0 g) was
added to the above solution. After stirring at room temperature for 24 h,
the reaction mixture was transferred into a Teflon-lined autoclave and
aged at 808C under static conditions for 48 h. After filtration, the powder
product was washed thoroughly with deionized water and dried at 808C
overnight. The molar composition of the reagents was CTAB/NH
3 2
·H O/
Si/H O/EtOH=1:66.7:8.3:950:83.3. The removal of surfactant and pro-
2
tecting group was the same as that for Bulk material. The materials are
denoted as NS(a), where “a” refers to the nanosphere synthesized in am-
monia.
Grafting BSBinol onto mesoporous silica nanospheres (G-NS): The syn-
thesis of mesoporous silica nanospheres as a support for grafting was the
same as that of NS(a) except that TEOS was used as the only silica
source. The silica nanospheres (1.0 g) were suspended in DMF (20 mL)
containing BSBinol (2.0 mmol) and pyridine (2.0 mL). The mixture was
stirred for 24 h at 1008C under an argon atmosphere. The solid material
was recovered by filtration, and washed with DMF and acetone sequen-
Chem. Eur. J. 2010, 16, 12727 – 12735
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12733

Q. Yang et al.
tially. For removal of the protecting groups, the material (0.5 g) was
stirred in ethanol (100 mL) containing 12m HCl (1.0 g) at 808C for 24 h.
After filtration, the product was dried at 408C under vacuum overnight.
The material was denoted as G-NS.
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2
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pressure range P/P
from the amount adsorbed at the P/P
0
of 0.05 to 0.25. The total pore volume was estimated
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0
was determined from the adsorption branch by the BJH method. Trans-
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EDX were performed with HRTEM (Philips Tecnai G220 microscope
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1
3
an acceleration voltage of 20–30 kV. Solid-state C (100.5 MHz) CP/
2
9
MAS NMR and solid-state Si (79.4 MHz) MAS NMR experiments
were recorded on a Varian infinity-plus 400 spectrometer equipped with
a magic-angle spin probe in a 4 mm ZrO
eters for C CP/MAS NMR experiments were 10 kHz spin rate, 2 s pulse
2
rotor. The experimental param-
1
3
2
9
delay, 6 min contact time, and 1000–2000 scans, and for Si MAS NMR
experiments were 8 kHz spin rate, 4 s pulse delay, 10 min contact time,
and 3000–5000 scans. Infrared spectra were recorded on a Thermo Nico-
let Nexus 470 FTIR spectrometer. C, H, and N elemental analysis was
performed on a varioEL III apparatus. CD spectra were recorded on a
dual-beam DSM 1000 CD spectrophotometer (Olis, Bogart, GA) with a
1
0 mm quartz cell. Each measurement was the average of five repeated
scans recorded from 220 to 350 nm at room temperature (about 258C).
ꢀ
1
The scanning rate (nmmin ) was automatically selected by the Olis soft-
ware as a function of the signal intensity to optimize data collection.
Catalytic reaction: The catalytic properties of (R)-(+)-Binol-functional-
ized mesoporous materials were investigated in the Ti-promoted asym-
metric addition of diethylzinc to aldehydes. After drying under vacuum
at 1208C for 3 h, (R)-(+)-Binol-functionalized mesoporous material was
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stirred in CH
2
Cl
2
(4 mL) containing Ti
A
H
U
G
R
N
U
G
4
(1.5 mmol) at room tem-
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2
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the reaction. The solid was isolated by filtration and washed with CH
The organic layer was separated from the filtrate and dried over anhy-
drous Na SO . The yield and ee value of the product were measured on
an Agilent 6890 gas chromatograph equipped with a flame ionization de-
tector and an HP-Chiral 19091G-B213 capillary column (30 m
.32 mm ꢂ 0.25 mm).
Recycling of the material: After the catalytic reaction, the catalyst was
separated by centrifugation and washed with CH Cl several times. Then
the reactants, benzaldehyde and diethylzinc, and the solvent CH Cl were
added to the reused catalyst similar to the process described above with-
out addition of Ti(OiPr)
4
Cl solution (1.0 mL) to stop
Cl
2
.
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[
Acknowledgements
This work was financially supported by the National Basic Research Pro-
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Chem. Eur. J. 2010, 16, 12727 – 12735

Chiral Mesoporous Organosilica Nanospheres
FULL PAPER
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Chem. Eur. J. 2010, 16, 12727 – 12735
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12735