Heterogeneous organocatalysis at work: Functionalization of hollow periodic mesoporous organosilica spheres with MacMillan catalyst

Article abstract of DOI:10.1002/chem.201100072

We report a new method for the synthesis of hollow-structured phenylene-bridged periodic mesoporous organosilica (PMO) spheres with a uniform particle size of 100-200 nm using α-Fe₂O₃ as a hard template. Based on this method, the hollow-structured phenylene PMO could be easily functionalized with MacMillan catalyst (H-PhPMO-Mac) by a co-condensation process and a "click chemistry" post-modification. The synthesized H-PhPMO-Mac catalyst has been found to exhibit high catalytic activity (98 % yield, 81 % enantiomeric excess (ee) for endo and 81 % ee for exo) in asymmetric Diels-Alder reactions with water as solvent. The catalyst could be reused for at least seven runs without a significant loss of catalytic activity. Our results have also indicated that hollow-structured PMO spheres exhibit higher catalytic efficiency than solid (non-hollow) PMO spheres, and that catalysts prepared by the co-condensation process and "click chemistry" post-modification exhibit higher catalytic efficiency than those prepared by a grafting method.

Full text of DOI:10.1002/chem.201100072

DOI: 10.1002/chem.201100072
Heterogeneous Organocatalysis at Work: Functionalization of Hollow
Periodic Mesoporous Organosilica Spheres with MacMillan Catalyst
Jiao Yi Shi, Chang An Wang, Zhi Jun Li, Qiong Wang, Yuan Zhang, and Wei Wang*[a]
Abstract: We report a new method for
the synthesis of hollow-structured
phenylene-bridged periodic mesopo-
rous organosilica (PMO) spheres with
a uniform particle size of 100–200 nm
H-PhPMO-Mac catalyst has been
found to exhibit high catalytic activity
(98% yield, 81% enantiomeric excess
(ee) for endo and 81% ee for exo) in
asymmetric Diels–Alder reactions with
water as solvent. The catalyst could be
reused for at least seven runs without a
significant loss of catalytic activity. Our
results have also indicated that hollow-
structured PMO spheres exhibit higher
catalytic efficiency than solid (non-
hollow) PMO spheres, and that cata-
lysts prepared by the co-condensation
process and “click chemistry” post-
modification exhibit higher catalytic ef-
ficiency than those prepared by a graft-
ing method.
using a-Fe2O3 as
a hard template.
Based on this method, the hollow-
structured phenylene PMO could be
easily functionalized with MacMillan
catalyst (H-PhPMO-Mac) by a co-con-
densation process and a “click chemis-
try” post-modification. The synthesized
Keywords: asymmetric catalysis
heterogeneous catalysis
·
·
hollow structures · organocatalysis ·
mesoporous materials
Introduction
ed work has focused on the construction of hollow-struc-
tured catalysts for metal catalysis. For example, Kim et al.[7]
reported that hollow Pd spheres exhibit good catalytic activ-
ity in Suzuki cross-coupling reactions. Liang et al.[8] synthe-
sized hollow Pt nanospheres, which showed good electroca-
talytic activity in the oxidation of methanol. Ikeda and co-
workers prepared Pt[9a] and Rh[9b] nanoparticles encapsulat-
ed in hollow mesoporous carbon shells, which served as ex-
cellent heterogeneous hydrogenation catalysts. Song and co-
workers[10] also prepared Pd nanoparticles inside hollow
mesoporous silica spheres, which exhibited superior activity
in Suzuki coupling reactions. To immobilize organocatalysts,
Yang and co-workers[11a] pioneered the research by prepar-
ing ethane–silica hollow nanospheres with microwindows in
the shells through a “polymeric micelles” method. Through
a post-modification process, these authors further offered
the first two examples of hollow nanospheres functionalized
with chiral diamine[11b] or prolinamide[11c] organocatalysts,
which showed promise as highly efficient, water-tolerant cat-
alysts. An important aspect yet to be developed is enlarge-
ment of the micropores in the shells of such hollow struc-
tures to mesopores, which will extend catalytic applications
to large reactants or products.
Complementing metal- and biocatalysis, organocatalysis has
become one of the most rapidly growing research areas in
synthetic organic chemistry during the past decade.[1] Many
types of asymmetric organocatalysts have been developed
for applications in a wide range of organic methodologies,
most prominently those providing efficient and environmen-
tally benign access to chiral compounds such as drugs and
bioactive natural products. For example, imidazolidinone,
the so-called MacMillan catalyst[2] (Scheme 1), is one of the
most versatile organocatalysts, and has been utilized in a va-
riety of highly enantioselective reactions, such as the Diels–
Alder reaction,[2a,e] 1,3-dipolar cycloaddition,[2b] Friedel–
Crafts alkylation,[2c] and so on. However, the practical appli-
cation of asymmetric organocatalysis is generally hindered
due to the requirement for high catalyst loadings and the
difficulty in separating the catalyst from the product. Ac-
cordingly, immobilization of organocatalysts on solid sup-
ports, such as polymers,[3] porous materials,[4] or magnetic
nanoparticles,[5] has been further explored to achieve asym-
metric catalysis under heterogeneous conditions.
It is being increasingly recognized that hollow structures
with a large fraction of void space and high surface area are
advantageous for catalytic applications.[6] Most of the report-
Periodic mesoporous organosilicas (PMOs) are synthe-
sized from organo-bridged precursors (R’O)3Si-R-SiACHTUNGTRENNUNG(OR’)3
in the presence of a surfactant.[12] Due to their combined ad-
vantages of ordered mesoporous structure, unique pore-wall
functionality, and tunable surface properties,[13] PMO mate-
rials are of potential importance in catalytic applications.[13e]
Accordingly, many efforts have been directed towards the
designed synthesis and catalytic applications of chiral
PMOs.[14] Mesoporous materials in nanoparticle size, espe-
cially with a hollow nanostructure, may decrease diffusion
[a] J. Y. Shi, C. A. Wang, Z. J. Li, Q. Wang, Y. Zhang, Prof. Dr. W. Wang
State Key Laboratory of Applied Organic Chemistry
College of Chemistry and Chemical Engineering
Lanzhou University, Lanzhou 730000 (P.R. China)
Fax : (+86)9318915557
E-mail: wang_wei@lzu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100072.
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Scheme 1. Schematic representation of the synthesis of hollow-structured phenylene PMO (H-PhPMO, path A) and further functionalization with Mac-
Millan catalyst (H-PhPMO-Mac, path B) through a co-condensation process and click chemistry post-modification.
lengths, increase accessibility to active sites, and thus en-
hance catalytic activity.[6b,15] The first example of the prepa-
ration of PMO materials with hollow nanospheres was re-
ported by Luꢁs group, in which a dual-templating approach
was explored.[16] Ha and co-workers[17] also prepared meso-
porous ethane–silica hollow nanospheres through a hard-
templating route using polystyrene latex spheres. Frçba and
co-workers[18] reported the synthesis of core–shell PMO ma-
terials with a solid silica core and a phenylene PMO shell.
However, no catalytic application of these hollow PMO ma-
terials was reported.
sized for the first time by using hematite (a-Fe2O3) nanopar-
ticles as a hard template. The a-Fe2O3 nanoparticles were
prepared according to a procedure described in the litera-
ture.[19] As determined from scanning electron microscope
(SEM) images, the obtained a-Fe2O3 nanoparticles had a di-
ameter of (100Æ20) nm (Figure S1 in the Supporting Infor-
mation). These a-Fe2O3 nanoparticles were dispersed in an
aqueous solution containing cetyltrimethylammonium bro-
mide (CTAB) and NaOH, to which 1,4-bis(triethoxysilyl)-
benzene (BTEB) was then added. The hydrolyzed BTEB
molecules self-assembled with CTAB micelles around the a-
Fe2O3 nanoparticles, resulting in the formation of ordered
PMO shells. After extraction of the surfactant and acid etch-
ing, the hollow-structured phenylene-bridged PMO (H-
PhPMO) was obtained.
Herein, we report a hard-templating route for the prepa-
ration of hollow phenylene-bridged PMO (denoted as H-
PhPMO) by using hematite nanoparticles as the hard tem-
plate. Based on this new method, the hollow-structured
phenylene PMO could be easily functionalized with Mac-
Millan catalyst (denoted as H-PhPMO-Mac) through a co-
condensation process and “click chemistry” post-modifica-
tion. The hydrophobic surface
For comparison, the powder X-ray diffraction (XRD) pat-
terns of PMO samples synthesized without (Figure 1a) and
with (Figure 1b) a-Fe2O3 as the core template are shown in
properties of PMO[13c,e] and ac-
cessible mesopores between the
inner and outer surfaces of the
hollow structures[6] accelerate
the mass-transport process,
which renders the H-PhPMO-
Mac catalyst highly active in
asymmetric Diels–Alder reac-
tions.
Results and Discussion
Synthesis of hollow-structured
phenylene PMO (H-PhPMO):
As depicted in Scheme 1
(path A),
hollow-structured
Figure 1. Powder XRD patterns of a) PhPMO and b) H-PhPMO, TEM images of c) PhPMO and d) H-
phenylene PMO was synthe- PhPMO, and SEM images of e) PhPMO and f) H-PhPMO.
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W. Wang et al.
Figure 1. In each case, the single reflection peak seen in the
low-angle region is typical of mesoporous materials. A trans-
mission electron microscope (TEM) image of solid (non-
hollow) phenylene PMO (denoted as PhPMO) indicates a
spherical morphology with most diameters in the range 100–
200 nm (Figure 1c). As revealed in Figure 1d, the hollow-
structured phenylene PMO (H-PhPMO) was successfully
synthesized. The particle size of the product was mostly in
the range 100–200 nm, with a hollow core diameter of about
90 nm. These hollow-structured PMO particles were largely
monodispersed, with some exceptions of two or more cores
due to the unsatisfactory dispersion of a-Fe2O3 nanoparti-
cles. SEM images (Figure 1e, f) also showed that both
PhPMO and H-PhPMO had spherical morphologies, with
particle sizes in the range 100–200 nm. The nitrogen adsorp-
tion–desorption isotherms (Figure 2) indicate that both
PhPMO and H-PhPMO show type IV behavior, which is
also characteristic of mesoporous solids. A hysteresis loop
for H-PhPMO was observed in the range 0.4–1.0 P/P0,
which may be attributed to the delayed evaporation of ni-
trogen from the hollow voids blocked by the surrounding
mesopores.[20] The mesostructural properties of PhPMO and
H-PhPMO are listed in Table 1, which indicate that H-
PhPMO has higher surface area, and larger pore size and
pore volume. Solid-state 13C cross-polarization magic-angle
spinning (CP/MAS) NMR and 29Si MAS NMR measure-
ments (Figures S2 and S3 in the Supporting Information)
confirmed that the pore walls of H-PhPMO were composed
of covalently-bonded O1.5Si-C6H4-SiO1.5 units. As revealed
by thermogravimetric analysis (Figure S4 in the Supporting
Information), the benzene groups remained intact in the
PMO network at up to 5508C in air.
Figure 2. N2 adsorption–desorption isotherms of PhPMO and H-PhPMO.
Functionalization of hollow-structured PMO materials with
MacMillan catalyst (H-PhPMO-Mac): On account of its
large fraction of void space, high surface area, and good
thermal stability, the hollow-structured H-PhPMO may act
as an excellent support for the immobilization of organoca-
talysts. Accordingly, we set out to prepare hollow PMO
spheres functionalized with a typical symmetric organocata-
lyst, namely the MacMillan catalyst. As depicted in
Scheme 1 (path B), by a co-condensation procedure employ-
ing a-Fe2O3 as a hard template, we first synthesized hollow
phenylene PMO material functionalized with organic azide
(denoted as H-PhPMO-N3). On the basis of the click
chemistry approach (1,3-dipolar cycloaddition of organic
Table 1. Description and mesostructural properties of phenylene PMO samples.
Sample name
Sample description
Pore
size[a]
[nm]
BET surface
area [m2 gÀ1
Pore volume
[cm3 gÀ1
N
Mac
]
G
]
content[b]
[%]
content[c]
ACHTUNGTRENNUNG ]
[mmolgÀ1
PhPMO
H-PhPMO
H-PhPMO-N3
Phenylene PMO
2.9 (3.1[d]
)
1017 (1197[d]
)
0.66 (0.81[d]
)
– (–[d]
)
– (–[d]
)
Hollow-structured phenylene PMO
Azide-functionalized hollow-structured PhPMO,
prepared by co-condensation process
MacMillan-catalyst-functionalized hollow-structured
PhPMO, prepared by click chemistry
Azide-functionalized PhPMO, prepared by
co-condensation process
MacMillan-catalyst-functionalized PhPMO,
prepared by click chemistry
MacMillan-catalyst-functionalized PhPMO,
prepared by a grafting method
3.8 (3.3[e]
2.9
)
1219 (1224[e]
1212
)
0.75 (0.88[e]
0.81
)
– (–[e]
3.38
)
– (–[e]
–
)
H-PhPMO-Mac
PhPMO-N3
3.1
2.8
4.7
4.4
6.9
3.5
644
1188
480
265
253
400
0.47
0.70
0.38
0.26
0.36
0.26
4.09
3.26
3.89
4.17
3.71
2.07
0.50
–
PhPMO-Mac
0.47
0.59
0.53
0.27
PhPMO-Mac-G
H-PhPMO-Mac-G
H-PhPMO-Mac-R7
MacMillan-catalyst-functionalized hollow-structured
PhPMO, prepared by a grafting method
H-PhPMO-Mac catalyst recycled after seven runs
of Diels–Alder reaction
[a] Calculated from the adsorption branch of the isotherms. [b] Nitrogen content in the sample, determined by elemental analysis. [c] Loading amount of
MacMillan catalyst, determined by elemental analysis or semi-quantitative IR estimation. [d] Another batch of PhPMO sample, used for further grafting
of MacMillan catalyst to prepare PhPMO-Mac-G. [e] Another batch of H-PhPMO sample, used for further grafting of MacMillan catalyst to prepare H-
PhPMO-Mac-G.
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Hollow Periodic Mesoporous Organosilica Spheres
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tern. Hysteresis loops in the
range 0.4–1.0 P/P0 indicate that
both had a hollow structure
with a mesoporous shell. After
the click reaction, the surface
area
was
reduced
from
1212 m2 gÀ1 (for H-PhPMO-N3)
to 644 m2 gÀ1 (for H-PhPMO-
Mac) and the pore volume was
reduced
from
0.81
to
0.47 cm3 gÀ1 (Table 1). All of
these data indicate the success-
ful incorporation of MacMillan
catalyst into the hollow-struc-
tured PMO material.
The incorporation of func-
tional groups into the hollow-
structured PMO was further
confirmed by solid-state 13C
Figure 3. Powder XRD patterns of a) H-PhPMO-N3 and b) H-PhPMO-Mac, TEM images of c) H-PhPMO-N3
and d) H-PhPMO-Mac, and SEM images of e) H-PhPMO-N3 and f) H-PhPMO-Mac.
azides to alkynes),[21] the hollow-structured phenylene PMO
could be easily functionalized with the MacMillan catalyst.
The product obtained is denoted as H-PhPMO-Mac
(Scheme 1). The XRD patterns of H-PhPMO-N3 (Figure 3a)
and H-PhPMO-Mac (Figure 3b) indicate that each PMO
material has a mesoporous structure. TEM images of H-
PhPMO-N3 (Figure 3c) and H-PhPMO-Mac (Figure 3d)
confirmed that monodispersed and hollow-structured PMO
materials were successfully obtained after the co-condensa-
tion procedure (for H-PhPMO-N3) and the click reaction
(for H-PhPMO-Mac). SEM images (Figure 3e and f) also
showed both H-PhPMO-N3 and H-PhPMO-Mac to be com-
posed of monodispersed particles. Nitrogen adsorption–de-
sorption isotherms (Figure 4) of H-PhPMO-N3 and H-
PhPMO-Mac revealed that each PMO material had a well-
defined mesostructure giving rise to a type IV isotherm pat-
CP/MAS NMR spectroscopy (Figure 5). In the 13C CP/MAS
NMR spectrum of azide-functionalized hollow PMO (H-
PhPMO-N3, Figure 5a), the peak at d=133 ppm relates to
the carbon atoms of the benzene ring. The three signals at
d=53, 22, and 9 ppm correspond to different methylene
groups of propyl azide. In the 13C CP/MAS NMR spectrum
of MacMillan-catalyst-functionalized hollow PMO (H-
PhPMO-Mac, Figure 5b), the signals at d=174, 158, 114, 76,
Figure 5. 13C CP/MAS NMR spectra of a) H-PhPMO-N3, b) H-PhPMO-
Mac, and c) H-PhPMO-Mac-R7 (H-PhPMO-Mac catalyst recycled after
seven runs of Diels–Alder reaction). Asterisks denote spinning sidebands.
The assignments of 13C chemical shifts are indicated in the chemical
structures.
Figure 4. N2 adsorption–desorption isotherms of H-PhPMO-N3, H-
PhPMO-Mac, and H-PhPMO-Mac-R7 (H-PhPMO-Mac catalyst recycled
after seven runs of Diels–Alder reaction).
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W. Wang et al.
59, 40, 30, 26, 20, and 13 ppm can be assigned to the skele-
ton carbons of the MacMillan catalyst. The signal at d=
144 ppm due to the carbon atoms of the newly formed tria-
zole ring also indicates the successful functionalization
through the click reaction. The peak at d=50 ppm is as-
To compare the catalytic activity with that of H-PhPMO-
Mac, solid (non-hollow) spheres of MacMillan-catalyst-func-
tionalized phenylene PMO (denoted as PhPMO-Mac) were
also prepared. In the absence of a-Fe2O3, solid spheres of
azide-functionalized phenylene PMO (denoted as PhPMO-
N3) were synthesized through the co-condensation proce-
dure, and PhPMO-Mac was obtained after the click reac-
tion. The XRD patterns (Figure S6 in the Supporting Infor-
mation) and nitrogen adsorption–desorption isotherms (Fig-
ure S7 in the Supporting Information, results summarized in
Table 1) confirmed that both PhPMO-N3 and PhPMO-Mac
had mesoporous structures. SEM images (Figure S8 in the
Supporting Information) show that both had largely mono-
dispersed spherical morphologies with the majority of the
particles in the size range 100–200 nm. IR and 13C CP/MAS
NMR (Figures S9 and S10a in the Supporting Information)
confirmed that the MacMillan catalyst had been successfully
incorporated into PhPMO. According to elemental analysis
and semi-quantitative IR estimation, about 77% of the
azide groups in PhPMO-N3 were converted during the click
reaction, and the loading amount of MacMillan catalyst in
PhPMO-Mac was 0.47 mmolgÀ1 (Table S1 in the Supporting
Information).
À
cribed to surface methoxy groups (Si OCH3) formed in the
process of Soxhlet extraction using methanol. Therefore, the
13C CP/MAS NMR spectra recorded for H-PhPMO-N3 and
H-PhPMO-Mac provide convincing evidence for the incor-
poration of organic azide and the MacMillan catalyst, re-
spectively, into the framework of the hollow PMOs. More-
over, the IR spectrum of H-PhPMO-N3 (Figure 6a) displays
For comparison, we also synthesized hollow-structured
MacMillan-catalyst-functionalized PhPMO (denoted as H-
PhPMO-Mac-G) and solid spheres (denoted as PhPMO-
Mac-G) by a grafting method. The synthetic procedure is
outlined in Scheme 2. XRD patterns (Figures S11 and S12 in
the Supporting Information) and nitrogen adsorption–de-
sorption isotherms (Figures S13 and S14 in the Supporting
Information, results summarized in Table 1) confirmed that
both H-PhPMO-Mac-G and PhPMO-Mac-G had mesopo-
rous structures. SEM images (Figures S15 and S16 in the
Supporting Information) showed that they were composed
of monodispersed particles. IR (Figure S17 in the Supporting
Information) and 13C CP/MAS NMR spectra (Figure S10 b
and c in the Supporting Information) confirmed that the
Figure 6. IR spectra of a) H-PhPMO-N3, b) H-PhPMO-Mac, and c) H-
PhPMO-Mac-R7 (H-PhPMO-Mac catalyst recycled after seven runs of
Diels–Alder reaction).
a strong absorbance at 2104 cmÀ1, which is characteristic of
the stretching vibration of azide groups. Based on elemental
analysis, the loading amount of azide groups in H-PhPMO-
N3 was 0.80 mmolgÀ1. After the click reaction, the IR ab-
sorption intensity of the azide
group band at 2104 cmÀ1 de-
creased dramatically and a car-
bonyl stretching band at
1675 cmÀ1 appeared (Figure 6b),
indicating covalent attachment
of the MacMillan catalyst to
the hollow-structured PMO.
Applying
a semi-quantitative
estimation developed by Malvi
et al.,[22] we determined that
around 79% of the azide
groups in H-PhPMO-N3 were
converted during the click reac-
tion. Accordingly, the loading
amount of MacMillan catalyst
in
H-PhPMO-Mac
was
0.50 mmolgÀ1 (Figure S5 and
Table S1 in the Supporting In-
formation).
Scheme 2. Schematic representation of the immobilization of MacMillan catalyst on solid PhPMO spheres (de-
noted as PhPMO-Mac-G) and on hollow spheres (denoted as H-PhPMO-Mac-G) by a grafting method.
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Hollow Periodic Mesoporous Organosilica Spheres
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MacMillan catalyst was immobilized on the surface of the
PhPMO materials. According to elemental analysis, the
loading amounts of MacMillan catalyst were 0.53 mmolgÀ1
on H-PhPMO-Mac-G and 0.59 mmolgÀ1 on PhPMO-Mac-
G, respectively.
(entries 9 and 10). The MacMillan catalyst immobilized by
the grafting method may have been inhomogeneously dis-
tributed on the PhPMO support, possibly with most such
moieties being located on the surface or near the pore
mouths of the PMO material,[23] and this would have hin-
dered the mass-transport process, thereby rendering the cat-
alyst poorly active and giving only low enantioselectivity.[15b]
The recyclability of the H-PhPMO-Mac catalyst in the
asymmetric Diels–Alder reaction was further investigated
and the results are listed in Table 3. The H-PhPMO-Mac
catalyst could be reused for at least seven runs without a no-
ticeable decrease in enantioselectivity (from 81% ee to
78% ee), although with some
Catalytic performance of hollow-structured PhPMO materi-
als functionalized with MacMillan catalyst (H-PhPMO-
Mac): To evaluate the catalytic activity of H-PhPMO-Mac
catalyst, the Diels–Alder cycloaddition of 1,3-cyclopenta-
diene with trans-cinnamaldehyde was selected as a model
reaction. Table 2 summarizes the catalytic performances of
loss of the reaction yield (from
Table 2. Comparison of catalytic performances of the MacMillan catalyst on different PhPMO supports.
98% to 86%). These results
suggest that H-PhPMO-Mac
catalyst may be advantageously
applied as a remarkably water-
tolerant catalyst for asymmetric
Diels–Alder reactions. After
the H-PhPMO-Mac catalyst
had been recycled seven times,
the loading amount of the Mac-
Millan moiety in the recycled
catalyst had decreased from
0.50 to 0.27 mmolgÀ1 (Table S1
in the Supporting Information).
An XRD pattern (Figure S18 in
the Supporting Information) in-
dicated that the recycled cata-
Entry
Catalyst
t [h]
Solvent
Yield
[%][b]
endo/exo[c]
endo
exo
ee[d] [%]
ee[d] [%]
1
2
3
4
5
6
7
8
9
1
24
24
24
24
24
24
12
12
12
12
CH3CN/H2O[a]
CH3CN/H2O[a]
CH3CN/H2O[a]
CH3CN/H2O[a]
CH3CN/H2O[a]
H2O
H2O
H2O
H2O
H2O
97
58
42
46
38
80
98
84
86
80
1:1.1
1:1.1
1:1.1
1:1.0
1:1.0
1:1.4
1:1.1
1:1.1
1:1.2
1:1.1
1
95
89
87
72
70
93
81
79
63
59
92
86
85
69
68
91
81
78
62
62
H-PhPMO-Mac
PhPMO-Mac
H-PhPMO-Mac-G
PhPMO-Mac-G
1
H-PhPMO-Mac
PhPMO-Mac
H-PhPMO-Mac-G
PhPMO-Mac-G
10
[a] CH3CN/H2O, 95:5 (v/v). [b] Yield of the isolated product. [c] Determined by H NMR spectroscopy. [d] De-
termined by HPLC. TFA=trifluoroacetic acid.
lyst retained
a
mesoporous
the MacMillan catalyst on different supports. We also chose
Table 3. Recyclability of H-PhPMO-Mac catalyst.
a homogeneous catalyst (1) as a reference. Catalyst 1 gave a
97% yield (with 95% enantiomeric excess (ee) for endo,
92% ee for exo) in CH3CN/H2O (Table 2, entry 1) and an
80% yield (with 93% ee for endo, 91% ee for exo) in H2O
(Table 2, entry 6). When CH3CN/H2O was used as solvent,
both PhPMO-Mac and H-PhPMO-Mac gave good ee values,
but with poor yields (Table 2, entries 2 and 3). To our de-
light, when H2O was used as solvent, a high yield of 98%
and high enantioselectivities of 81% ee (endo) and 81% ee
(exo) were obtained for H-PhPMO-Mac catalyst (Table 2,
entry 7). Under all conditions tested, the hollow-structured
PhPMO catalysts (H-PhPMO-Mac, Table 2, entries 2 and 7)
exhibited higher catalytic activity than their solid (non-
hollow) counterparts (PhPMO-Mac, entries 3 and 8). The
accessible mesopores within the walls of the hollow spheres
may provide plentiful channels between the inner and outer
surfaces, which facilitate the diffusion of solvents, reactants,
and products, and thus accelerate chemical reactions.[6]
Entry Recycle Time [h] Yield[a][%] endo/exo[b] endo
ee[c] [%] ee[c] [%]
exo
1
2
3
4
5
6
7
8
Fresh
1st
2nd
3rd
4th
5th
6th
7th
12
12
12
12
12
21
21
21
98
96
90
92
76
94
86
86
1:1.1
1:1.1
1:1.1
1:1.2
1:1.2
1:1.1
1:1.2
1:1.1
81
83
83
81
84
80
79
78
81
81
82
82
83
81
79
78
1
[a] Yield of the isolated product. [b] Determined by H NMR spectrosco-
py. [c] Determined by HPLC.
structure, but with decreased order in the mesostructure. N2
adsorption–desorption isotherms (Figure 4, bottom) indicat-
ed that its surface area had been reduced to 400 m2 gÀ1 and
that the pore volume had been reduced to 0.26 cm3 gÀ1
(Table 1). The 13C CP/MAS NMR spectrum showed addi-
tional signals at d=44 and 26 ppm (Figure 5c), while the IR
spectrum remained almost unchanged (Figure 6c). A TEM
image further revealed that the hollow structure of H-
For comparison, H-PhPMO-Mac-G (hollow spheres) and
PhPMO-Mac-G (solid spheres) prepared by the grafting
method were also tested in the Diels–Alder cycloaddition of
1,3-cyclopentadiene and trans-cinnamaldehyde. Both sam-
ples exhibited low catalytic activity with low enantioselectiv-
ity in CH3CN/H2O (Table 2, entries 4 and 5) and in H2O
Chem. Eur. J. 2011, 17, 6206 – 6213
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6211

W. Wang et al.
HCl (6 mL) in ethanol (200 mL) at 608C for 6 h, a surfactant-free sample
was obtained.
PhPMO-Mac had been destroyed (Figure S19 in the Sup-
porting Information). The loss of catalytic active sites and
damage of the hollow structure may be the main reasons for
the loss of catalytic activity after seven runs.
Synthesis of hollow-structured azide-functionalized PMO (H-PhPMO-
N3): Hematite nanoparticles (1.2 g) were dispersed by ultrasound in a so-
lution containing CTAB (0.5 g), deionized water (240 mL), and 0.5m
NaOH (7.0 mL). The system was then heated to a constant temperature
of 808C, whereupon BTEB (2.16 mL) was slowly added under vigorous
stirring. After 15 min, 3-azidopropyltrimethoxysilane (0.27 g) was slowly
added. After stirring at 808C for 105 min, the white solid product was
collected by filtration, washed thoroughly with water and ethanol, and
dried at room temperature. After twofold extraction with a solution of
36 wt% HCl (6 mL) in ethanol (200 mL) at 608C for 6 h, and etching in
a solution of 2m HCl at 808C for 2 h, a surfactant-free and hematite-free
sample was obtained.
Conclusion
We have developed a new method for the synthesis of
hollow-structured PMO spheres using a-Fe2O3 as a hard
template. Based on this method, hollow-structured phenyl-
ene PMO (H-PhPMO) could be easily functionalized with
the MacMillan catalyst (H-PhPMO-Mac) through a co-con-
densation process and a click chemistry post-modification.
Results have indicated that these hollow-structured MacMil-
lan-catalyst-functionalized PhPMO spheres, that is, H-
PhPMO-Mac catalyst, exhibit higher catalytic activity than
solid (non-hollow) PhPMO-Mac catalyst and may be advan-
tageously applied as a water-tolerant and highly efficient
catalyst for asymmetric Diels–Alder reactions. Moreover,
the H-PhPMO-Mac catalyst can be reused for at least seven
runs without significant loss of catalytic activity. Our results
have also indicated that the catalysts (PhPMO-Mac and H-
PhPMO-Mac) prepared by the co-condensation process are
superior in terms of catalytic activity to immobilized cata-
lysts (H-PhPMO-Mac-G and PhPMO-Mac-G) prepared by
a grafting method.
Synthesis of MacMillan-catalyst-functionalized PMOs (PhPMO-Mac and
H-PhPMO-Mac) by click chemistry: An azide-functionalized sample
(PhPMO-N3 or H-PhPMO-N3) (1.8 g) and CuI (0.11 g, 0.54 mmol) were
suspended in THF (10 mL). A solution of 1 (1.70 g, 5.4 mmol) in THF
(25 mL) was added under Ar, and then N,N’-diisopropylethylamine
(DIPEA) (1.9 mL, 11 mmol) was added dropwise. The suspension was
stirred at 508C for 3 days. The solid was then collected by filtration,
washed thoroughly several times with acetonitrile and methanol, subject-
ed to by Soxhlet extraction with methanol for 48 h, and then dried under
vacuum at 608C.
Synthesis of MacMillan-catalyst-immobilized PMOs (PhPMO-Mac-G
and H-PhPMO-Mac-G) by a grafting method: The MacMillan catalyst
was anchored onto the surface of phenylene PMO samples according to
a
strategy described in the literature (Scheme 2).[26] CuI (3.2 mg,
0.017 mmol) and DIPEA (0.35 mL, 2 mmol) were added to a solution of
(0.33 g, 1.05 mmol) and 3-azidopropyltrimethoxysilane (0.203 g,
1
1.0 mmol) in THF (10 mL) protected by Ar. The resulting mixture was
stirred at room temperature for 24 h, the solvent was removed under re-
duced pressure to quantitatively afford 2 as a light yellow oil, and this
was used directly in the next step without further purification. The result-
ing crude yellow product was mixed with a PMO sample (1.0 g) and tolu-
ene (30 mL), and the suspension was heated at 908C for 2 days under Ar.
The solid was collected by filtration, washed thoroughly several times
with acetonitrile and methanol, subjected to Soxhlet extraction with
methanol for 48 h, and then dried under vacuum at 608C.
Experimental Section
Chemicals and reagents: 1,4-Bis(triethoxysilyl)benzene (BTEB),[24] 3-azi-
dopropyl-trimethoxysilane,[25] and hematite nanoparticles[19] were pre-
pared according to the respective literature procedures.
General procedure for asymmetric Diels–Alder reactions catalyzed by
MacMillan catalysts on different supports: Trifluoroacetic acid
(0.05 mmol) was added to a stirred solution of catalyst (0.05 mmol) in
H2O (1 mL) and the mixture was stirred for 5 min at room temperature.
(E)-Cinnamaldehyde (31.8 mL, 0.25 mmol), used soon after purification,
was added, followed by cyclopentadiene (104 mL, 1.25 mmol). The mix-
ture was stirred at room temperature for a specified time. After the addi-
tion of CH3CN, the catalyst was separated by centrifugation and the
liquid layer was removed by decantation. The catalyst was washed with
four portions of Et2O, the washings were pooled with the aforementioned
liquid layer, and then the combined liquid phase was extracted four times
with Et2O. After evaporation of the solvent under vacuum, the residue
was purified by column chromatography on silica gel (hexane/ethyl ace-
tate, 12:1, v/v) to give the product as a colorless oil. The endo to exo
ratio was determined by 1H NMR (400 MHz). The product was convert-
ed into the corresponding alcohol with NaBH4 and the enantiomers were
separated by HPLC using a Daicel chiral OJ-H column (hexane/iPrOH,
70:30; flow rate 1.0 mLminÀ1). The recovered H-PhPMO-Mac catalyst
was employed for the next run after evacuation under vacuum.
Characterization: Solution 1H and 13C NMR spectra were recorded in
CDCl3 on a Bruker Avance III 400 MHz NMR spectrometer using tetra-
methylsilane (TMS) as an internal standard. Powder X-ray diffraction
(XRD) measurements were made with a PANalytical XꢁPert Pro with
XꢁCelerator detector (step size: 0.0088, step time: 38.34 s). The nitrogen
adsorption and desorption isotherms were measured at 77 K using a Mi-
cromeritics ASAP 2020M system. The samples were degassed at 1208C
for 8 h before the measurements. Surface areas were calculated from the
adsorption data using the Brunauer–Emmett–Teller (BET) method. The
pore size distribution curves were obtained from the adsorption branches
Synthesis of phenylene PMO (PhPMO): In a typical synthesis, CTAB
(0.1 g) was dissolved in deionized water (48 mL) and 0.5m NaOH
(1.4 mL). The system was then kept at a constant temperature of 808C,
and BTEB (0.48 mL) was slowly added under vigorous stirring. After
stirring for 2 h at 808C, the white solid product was collected by filtra-
tion, washed thoroughly with water and ethanol, and dried at room tem-
perature. After twofold extraction with a solution of 36 wt% HCl (2 mL)
in ethanol (60 mL) at 608C for 6 h, a surfactant-free sample was ob-
tained.
Synthesis of hollow-structured phenylene PMO (H-PhPMO): Hematite
nanoparticles (0.20 g) were dispersed by ultrasound in a solution contain-
ing CTAB (0.1 g), deionized water (48 mL), and 0.5m NaOH (1.4 mL).
The system was then heated to a constant temperature of 808C, where-
upon BTEB (0.48 mL) was slowly added under vigorous stirring. After
stirring for 2 h at 808C, the white solid product was collected by filtra-
tion, washed thoroughly with water and ethanol, and dried at room tem-
perature. After twofold extraction with a solution of 36 wt% HCl (2 mL)
in ethanol (60 mL) at 608C for 6 h, and etching in a solution of 2m HCl
at 808C for 2 h, a surfactant-free and hematite-free sample was obtained.
Synthesis of azide-functionalized phenylene PMO (PhPMO-N3): CTAB
(0.5 g) was dissolved in deionized water (240 mL) and 0.5m NaOH
(7.0 mL). The system was then heated to a constant temperature of 808C,
whereupon BTEB (2.16 mL) was slowly added under vigorous stirring.
After 15 min, 3-azidopropyltrimethoxysilane (0.27 g) was slowly added.
After stirring at 808C for 105 min, the white solid product was collected
by filtration, washed thoroughly with water and ethanol, and dried at
room temperature. After twofold extraction with a solution of 36 wt%
6212
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 6206 – 6213

Hollow Periodic Mesoporous Organosilica Spheres
FULL PAPER
using the Barrett–Joyner–Halenda (BJH) method. The morphology and
size of the obtained samples were characterized with a JEOL-6701F
field-emission scanning electron microscope (SEM) operated at 10 kV.
Transmission electron microscope (TEM) images were obtained with a
JEOL JEM-2010 instrument operating at 200 kV. Solid-state NMR meas-
urements were performed on a Bruker WB Avance II 400 MHz spec-
trometer. 13C CP/MAS NMR spectra were recorded with a 4 mm double-
resonance MAS probe at a sample spinning rate of 10.0 kHz; a contact
time of 2 ms (ramp 100) and a pulse delay of 3 s were applied. 29Si high-
power proton-decoupling (HPDEC) MAS NMR spectra were recorded
with a 4 mm double-resonance MAS probe and a sample spinning rate of
10.0 kHz; a p/4 single-pulse excitation of 2.0 ms and a pulse delay of 30 s
were applied. FTIR spectra were recorded on a Nicolet NEXUS 670 in-
strument. Elemental analysis was carried out on an Elementar Analysen-
systeme GmbH VarioEL V3.00 elemental analyzer. Thermogravimetric
analysis (TGA) was performed on an STA 449C Jupiter instrument over
the temperature range from ambient to 9008C under air atmosphere
(heating rate 108CminÀ1). Enantiomeric excesses were determined by
HPLC on a Waters 1525 Delta system with a Daicel chiral OJ-H column
(eluents: iPrOH and n-hexane).
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Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (Nos.: 20972064 and 20933009), the 111 Project, and
the Key Grant Project of the Chinese Ministry of Education (No.:
309028).
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Received: January 8, 2011
Published online: April 19, 2011
Chem. Eur. J. 2011, 17, 6206 – 6213
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6213