Heterogeneous chiral Mn(III) salen catalysts for the epoxidation of unfunctionalized olefins immobilized on mesoporous materials with different pore sizes

Article abstract of DOI:10.1016/j.tet.2008.10.051

Two chiral Mn(III) salen complexes were immobilized onto a series of mesoporous MCM-41 and MCM-48 materials with different pore sizes and the as-synthesized catalysts were active and enantioselective for the asymmetric epoxidation of styrene and indene. T

Full text of DOI:10.1016/j.tet.2008.10.051

Tetrahedron 65 (2009) 305–311
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Heterogeneous chiral Mn(III) salen catalysts for the epoxidation of
unfunctionalized olefins immobilized on mesoporous materials
with different pore sizes
*
Kai Yu, Zhicheng Gu, Runan Ji, Lan-Lan Lou, Shuangxi Liu
Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 June 2008
Received in revised form 13 October 2008
Accepted 17 October 2008
Available online 1 November 2008
Two chiral Mn(III) salen complexes were immobilized onto a series of mesoporous MCM-41 and MCM-48
materials with different pore sizes and the as-synthesized catalysts were active and enantioselective for
the asymmetric epoxidation of styrene and indene. The results of XRD, FTIR, DR UV–vis, and N2 sorption
showed that the chiral Mn(III) salen complexes were anchored in the channels of mesoporous materials.
The influence of organic silicane dosage on the catalytic performance was studied and the optimum
dosage of organic silicane for preparing heterogeneous catalysts was determined. Furthermore, the effect
of the fine-tuning of pore size on the performance of heterogeneous catalysts was discussed. In general,
larger pore size of the supports could lead to higher conversions and the compatible pore size with
substrate may be responsible for the improved enantiomeric excess (ee) values.
Keywords:
Chiral Mn(III) salen complex
Mesoporous materials
Fine-tuning of pore size
Immobilization
Ó 2008 Elsevier Ltd. All rights reserved.
Asymmetric epoxidation
1. Introduction
on the catalytic performance of mesoporous material-supported
chiral Mn(III) salen complex has been reported. Zhang et al.3,15
Chiral Mn(III) salen complexes have proven activity and enan-
tioselectivity for the asymmetric epoxidation of unfunctionalized
olefins,1,2 which are highly significant in the pharmaceutical and
agrochemical fields to synthesize chiral building blocks that could
be transformed into other useful chiral compounds through
regioselective ring-opening reactions.3 However, the catalyst al-
ways suffers from separation and recycling problems in homoge-
neous system. Consequently, the heterogenization of chiral Mn(III)
salen complexes within inorganic matrixes has received great at-
tention4–13 over the last decade. The heterogeneous catalysts could
be easily separated from the reaction system and simply recycled
several times, moreover, the immobilization of chiral Mn(III) salen
complexes could effectively decrease the formation of inactive di-
immobilized the chiral Mn(III) salen complexes into the nano-
pores of mesoporous supports MCM-41(1.6 and 2.7 nm), SBA-15
(7.6 nm), and activated silica (9.7 nm) with different pore sizes
and the influence of pore size on the catalytic performance was
studied. Kureshy et al.16,17 supported the chiral Mn(III) salen
complex onto the mesoporous materials MCM-41 and SBA-15,
and found that the SBA-15-based catalyst, with a larger pore
diameter, was more active than MCM-41-supported catalyst.
Similarly, Thomas et al.24 and Raja et al.25 also studied the in-
fluence of pore size on catalytic performance for other hetero-
geneous asymmetric reactions.
It is generally believed that the pore size of parent supports
could influence the activity and enantioselectivity of the hetero-
geneous chiral Mn(III) salen catalysts. However, the effect of the
fine-tuning of pore size on the catalytic performance of meso-
porous material-supported chiral Mn(III) salen complex has seldom
been systematically investigated for the heterogeneous asymmetric
epoxidation. In this work, the chiral Mn(III) salen complexes were
immobilized onto a series of mesoporous MCM-41 and MCM-48
materials, which possess the pore sizes varied on the angstrom
length-scale. The as-synthesized catalysts were evaluated in the
epoxidation of styrene and indene. The effects of organic silicane
dosage and the fine-tuning of pore size on the performance of
heterogeneous catalysts were studied.
meric m
-oxo Mn(IV) species.14
The mesoporous siliceous materials have large surface areas,
controllable pore sizes of 2–50 nm, and abundant silanol groups
on the surface, allowing ready material diffusion and easily or-
ganically functionalized and grafted to catalytic active species,
and thus were widely applied in the immobilization of chiral
Mn(III) salen complexes.3,15–23 Recently, the influence of pore size
* Corresponding author. Tel./fax: þ86 22 23509005.
E-mail address: sxliu@nankai.edu.cn (S. Liu).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.10.051

306
K. Yu et al. / Tetrahedron 65 (2009) 305–311
2. Experimental
resulting gel was allowed to crystallize at 383 K for 52 h in a Teflon-
lined autoclave. The solid product was filtered, washed with
deionized water, dried at room temperature, and calcined at 823 K
in air for 6 h to remove the template. According to the difference in
the alkyl chain length of the template, the as-synthesized materials
were marked as nMCM-41 (n¼12, 14, 16, 18).
2.1. Materials
3-Mercaptopropyltrimethoxysilane, cis/trans-1,2-diaminocyclo-
hexane, and 2-tert-butylphenol were supplied by Alfa Aesar.
m-Chloroperbenzoic acid (m-CPBA) was purchased from Acros
Organics. N-Methylmorpholine-N-oxide (NMO) was provided by
Aldrich. Indene was purchased from Fluka. Alkyltrimethyl-
ammonium bromides CnH2nþ1(CH3)3–NBr (CnTAB for brevity, n¼12,
14, 16, 18; AR) were obtained from Jintan Huadong Chemical Re-
search Institute in Jiangsu. (1R,2R)-(þ)-1,2-Diphenylethylenedi-
amine (>99% ee) was provided by Likai Chiral Tech. Co. Ltd. in
Chengdu. (1R,2R)-(ꢀ)-1,2-Diaminocyclohexane was resolved from
the technical grade isomers mixture in >99% ee by the reported
procedure.26 Tetraethyl orthosilicate (TEOS), 2,20-azobis(2-meth-
ylpropionitrile) (AIBN), and styrene were provided by Jiangtian Co.
Ltd. in Tianjin. Racemic epoxides were synthesized by the epoxi-
dation of corresponding olefins with m-CPBA in CHCl3 at 273 K27
and detected by gas chromatography (GC). All of the solvents used
in the present study were purified before use.
2.3.2. Synthesis of MCM-48 with different pore sizes
The mesoporous materials MCM-48 with different pore sizes
were synthesized according to the procedures as follows. When
CnTAB (n¼14,16) was used astemplate, CnTAB wasdissolved inwarm
deionized water, and to this solution the required quantity of TEOS
and the aqueous solution of NaOH were added in sequence under
vigorous stirring. After stirring for 2 h at room temperature, a gel
with a molar composition of 1TEOS/0.46CnTAB/0.41NaOH/52.95H2O
was obtained. The resulting gel was allowed to crystallize at 383 K
for 72 h in a Teflon-lined autoclave. When CnTAB (n¼12) was used as
template, the molar composition of the gel was 1TEOS/1.10CnTAB/
0.46NaOH/112H2O, and the gel was crystallized at 393 K for 168 h.
Then, the solid product was filtered, washed with deionized water,
and dried in air at room temperature overnight. The template was
removed by calcination at 823 K in air for 6 h. According to the dif-
ference in the alkyl chain length of the template, the as-synthesized
materials were marked as nMCM-48 (n¼12, 14, 16).
2.2. Characterization
Powder X-ray diffraction (XRD) patterns were performed on an
R/max-2500 diffractometer with Cu K
a
radiation at 40 kV and
2.4. Synthesis of the homogeneous chiral Mn(III) salen
complex
100 mA in a scan range of 1.5ꢁ<2 <10ꢁ. FTIR spectra were carried
q
out on KBr pellets in a BRUKER VECTOR 22 spectrometer. Diffuse-
reflectance UV–vis (DR UV–vis) spectra were obtained on
a Shimadzu UV-2550 UV–vis spectrophotometer in the range of
220–800 nm. N2 adsorption–desorption analysis was carried out at
77 K on a Micromeritics TriStar 3000 apparatus. The analytical data
were processed by the BET equation for surface areas and by the
BJH model for pore size distributions. Elemental analysis was per-
formed on a Perkin–Elmer 240C analyzer. 1H and 13C{1H} NMR
spectra were recorded at 300 and 75 MHz, respectively, using
a Varian Mercury Vx-300 spectrometer. The content of Mn in the
heterogeneous catalysts was determined by an ICP-9000 (NþM)
spectrometer (TJA Co.), after calcination and solubilization of the
samples with hydrogen fluoride and aqua regia. The products of
The chiral Mn(III) salen complexes 1 and 2 were obtained
through the synthesis sequence given in Scheme 1 according to the
literature.31
2.4.1. Synthesis of 3-tert-butyl-2-hydroxy-5-vinylbenzaldehyde (D)
3-tert-Butyl-2-hydroxybenzaldehyde (A) was synthesized from
2-tert-butylphenol as described previously.32 Compound A (27.0 g,
0.15 mol), paraformaldehyde (10.0 g, 0.33 mol), and tetrabutyl-
ammonium bromide (4.7 g, 14.6 mmol) were stirred in concen-
trated hydrochloric acid (110 mL) at 313 K for 3 days.33 The reaction
mixture was extracted with diethyl ether (3ꢂ150 mL), and the or-
ganic phase was washed with 5% NaHCO3 (2ꢂ100 mL), brine
(2ꢂ100 mL), and dried over MgSO4. Evaporation of the solvent
under vacuum afforded 3-tert-butyl-5-chloromethyl-2-hydroxy-
benzaldehyde (B) as a yellow crystalline solid (34.0 g, 99% yield). 1H
epoxidation reaction were determined by GC with a chiral
clodextrin capillary column (RESTEK RT-BetaDEXse, 30 mꢂ0.25 -
mmꢂ0.25 m), using a Rock GC7800 gas chromatograph equipped
b-cy-
m
with a flame ionization detector. Ultrapure nitrogen was used as the
carrier gas and the column temperature was programmed in
the range of 333–423 K. For each of the epoxidation reactions, the
conversion and ee value were determined by averaging the values
from two parallel measurements, of which the results should not
differ by more than 0.5%, otherwise additional measurements on
the same sample need to be done.
NMR (CDCl3, 300 MHz): d (ppm) 1.43 (s, 9H), 4.59 (s, 2H), 7.44 (d,
J¼1.8 Hz, 1H), 7.53 (d, J¼1.8 Hz, 1H), 9.87 (s, 1H), 11.87 (s, 1H).
A mixture of compound B (15.9 g, 0.070 mol) and triphenylphos-
phine (18.3 g, 0.070 mol) was refluxed in cyclohexane (150 mL) for
1 h. After cooling the solution to room temperature, the product was
filtered, washed with diethyl ether, and dried under vacuum. Then, 3-
tert-butyl-5-formyl-4-hydroxybenzyl(triphenylphosphonium) chlo-
ride (C) (30.9 g, 90% yield) was obtained as a white powder. 1H NMR
2.3. Synthesis of siliceous mesoporous materials
(CDCl3, 300 MHz):
J¼1.8 Hz, 2H), 7.62–7.86 (m, 15H), 9.70 (s, 1H), 11.82 (s, 1H).
d
(ppm) 1.13 (s, 9H), 5.66 (d, J¼13.8 Hz, 2H), 7.32 (d,
A series of highly ordered siliceous mesoporous materials MCM-
41 and MCM-48 with different pore sizes were synthesized
according to the literature method,28–30 using the alkylammonium
salts CnTAB (n¼12, 14, 16, 18) with different alkyl chain lengths as
templates and TEOS as silica source.
Compound C (24.6 g, 0.050 mol) was stirred in 170 mL of 40%
aqueous formaldehyde, and an aqueous solution of 12.5 N NaOH
(55 mL) was added, keeping the reaction temperature below 313 K.
After stirring at room temperature for 2 h, the mixture was cooled
with ice and neutralized with 6 N HCl. At pHz7, the aqueous phase
was extracted with cyclohexane. Evaporation of the solvent afforded
a semisolid that was purified by flash chromatography (dichloro-
methane). After distilled under vacuum (380 K, 0.1 mmHg), 3-tert-
butyl-2-hydroxy-5-vinylbenzaldehyde (D) (5.87 g, 57% yield) was
obtained as light yellow crystals. 1H NMR (CDCl3, 300 MHz):
2.3.1. Synthesis of MCM-41 with different pore sizes
The highly ordered hexagonal siliceous MCM-41 materials with
different pore sizes were synthesized using a gel (molar) compo-
sition of 1TEOS/0.12CnTAB/8NH4OH/114H2O. In a typical synthesis,
the template of CnTAB was dissolved in warm deionized water. To
this solution, ammonia and TEOS were added orderly under vig-
orous stirring. After stirring at room temperature for 0.5 h, the
d
(ppm) 1.43 (s, 9H), 5.21 (d, J¼11.1 Hz, 1H), 5.66 (d, J¼17.7 Hz, 1H),
6.68 (dd, J¼17.4, 10.8 Hz, 1H), 7.41 (d, J¼2.1 Hz, 1H), 7.61 (d,
J¼2.1 Hz, 1H), 9.89 (s, 1H), 11.79 (s, 1H).

K. Yu et al. / Tetrahedron 65 (2009) 305–311
307
CHO
OH
CHO
OH
t-Bu
CHO
OH
t-Bu
CHO
OH
t-Bu
CH2O
NaOH
(CH2O)n
HCl
Ph3P
-Cl+Ph3PH2C
ClH2C
t-Bu
D
B
A
C
EtOH
R1
R2
2
R1
N
H
H2N
H
NH2
R2
R1
R2
H
H
H
H
N
N
O
N
Mn
O
Mn(OAC)2 4H2O
LiCl
OH HO
t-Bu t-Bu
Cl
t-Bu
t-Bu
L1: R1=R2= -(CH2)4-
L2: R1=R2= Ph
1: R1=R2= -(CH2)4-
2: R1=R2= Ph
Scheme 1. The synthesis process of chiral Mn(III) salen complexes 1 and 2.
2.4.2. Synthesis of the chiral salen ligand L1
Compound
2.5. Heterogenization of chiral Mn(III) salen complexes 1
and 2
D
(1.23 g, 6.00 mmol) and (1R,2R)-(ꢀ)-1,2-di-
aminocyclohexane (0.34 g, 3.00 mmol) were dissolved in 15 mL of
hot ethanol, and the resulting solution was refluxed for 1 h. After
partial removal of the solvent, the solution was kept at 273–
277 K for 12 h, and the product L1 was collected by filtration as
deep yellow crystals (1.33 g, 91%). 1H NMR (CDCl3, 300 MHz):
The synthesis process is shown in Scheme 2. The calcined
siliceous mesoporous support (1.0 g) and the required quantity of
3-mercaptopropyltrimethoxysilane were stirred in 80 mL of an-
hydrous toluene under refluxing for 8 h. The resulting solid was
filtered, washed with ethanol, and dried at 323 K under vacuum for
12 h. The dried mercaptopropylsilyl-functionalized mesoporous
material (1.0 g), the chiral Mn(III) salen complex 1 or 2 (0.20 mmol),
and AIBN (0.015 g, 0.10 mmol) were refluxed under nitrogen in
30 mL of oxygen-free chloroform for 12 h. The samples were col-
lected by filtration, washed with ethanol, and Soxhlet-extracted
with dichloromethane for 24 h. According to the difference of the
supports and the chiral Mn(III) salen complexes, the heterogeneous
catalysts were marked as nM41-1(or nM41-2) (n¼12, 14, 16, 18) and
nM48-1 (or nM48-2) (n¼12, 14, 16).
d
(ppm) 1.41 (s, 18H), 1.45–1.97 (m, 8H), 3.31–3.34 (m, 2H), 5.04
(d, J¼10.8 Hz, 2H), 5.49 (d, J¼17.4 Hz, 2H), 6.55 (dd, J¼17.4,
11.1 Hz, 2H), 7.02 (d, J¼2.1 Hz, 2H), 7.31 (d, J¼2.1 Hz, 2H), 8.28 (s,
2H), 13.96 (s, 2H). 13C{1H} NMR (75 MHz in CDCl3):
d (ppm) 24.2,
29.3, 32.9, 34.8, 72.3, 110.8, 118.4, 127.2, 127.5, 136.3, 137.1, 159.8,
165.3.
2.4.3. Synthesis of the chiral salen ligand L2
Compound D (613 mg, 3.00 mmol) and 319 mg (1.50 mmol) of
(1R,2R)-(þ)-1,2-diphenylethylenediamine were refluxed in 10 mL
of ethanol. Following the same procedure described above for the
preparation of ligand L1, 842 mg of ligand L2 (96% yield) was
obtained as deep yellow crystals. 1H NMR (CDCl3, 300 MHz):
2.6. Asymmetric epoxidation of unfunctionalized olefins
d
(ppm) 1.42 (s, 18H), 4.72 (s, 2H), 5.05 (d, J¼10.8 Hz, 2H), 5.49 (d,
The epoxidation reactions were carried out using the catalysts
(0.015 mmol) with styrene and indene as substrates (1 mmol) and
J¼17.4 Hz, 2H), 6.55 (dd, J¼17.7, 10.8 Hz, 2H), 7.02 (d, J¼2.1 Hz,
2H), 7.18–7.26 (m, 10H), 7.33 (d, J¼2.1 Hz, 2H), 8.35 (s, 2H), 13.84
(s, 2H). 13C{1H} NMR (75 MHz in CDCl3):
d (ppm) 29.3, 34.8, 80.0,
R1
SH
R2
110.9, 118.3, 127.5, 127.6, 127.8, 128.0, 136.3, 137.3, 139.3, 160.2,
166.9.
H
H
N
O
N
O
+
Si
O
Mn
Cl
2.4.4. Synthesis of the chiral Mn(III) salen complexes 1 and 2
The chiral ligand L1 (or L2) (2.0 mmol) and Mn(OAc)2$4H2O
(1.24 g, 5.0 mmol) were refluxed in ethanol (30 mL) for 1 h. Then,
LiCl (0.6 g, 15 mmol) was added, and the mixture was refluxed for
an additional 1 h under exposure to the air. The solvent was com-
pletely removed under vacuum and the residue was dissolved in
dichloromethane (100 mL). The organic phase was washed with
deionized water (3ꢂ25 mL), brine (2ꢂ20 mL), and then dried over
MgSO4. After evaporation of the solvent, the residue was recrys-
tallized from dichloromethane/hexane to give 1 (or 2) as a brown
solid. Mn(III) salen complex 1: FTIR (KBr): 3077, 2956, 2862, 1611,
1538, 1456, 1436, 1386, 1340, 1310, 1265, 1233, 1202, 1165, 1097,
1029, 987, 898, 825, 793, 772, 557 cmꢀ1; DR UV–vis: 284, 320, 366,
448, 532 nm. Mn(III) salen complex 2: FTIR (KBr): 3077, 2956, 2862,
1606, 1538, 1456, 1436, 1386, 1340, 1310, 1270,1239, 1207, 1165, 987,
882, 851, 814, 783, 725, 699, 568 cmꢀ1; DR UV–vis: 284, 328, 366,
456, 550 nm.
O
O
support
t-Bu
t-Bu
ME-nMCM-41
ME-nMCM-48
1: R1=R2= -(CH2)4-
2: R1=R2= Ph
CHCl3 reflux
AIBN 12h
R1
R2
H
H
N
O
N
O
O
O
O
Mn
Cl
Support=
nMCM-41
nMCM-48
Si
S
t-Bu
t-Bu
nM41-1(or 2), nM48-1(or 2)
Scheme 2. Preparation of the heterogeneous chiral Mn(III) salen catalysts.

308
K. Yu et al. / Tetrahedron 65 (2009) 305–311
Table 1
The structure parameters of mesoporous materials nMCM-41 and nMCM-48
1436
1538
2862
2956
(a)
(a)
Sample
SBET (m2 gꢀ1
)
Pore volume
Pore diameter (Å)
(cm3 gꢀ1
)
(b)
(b)
18MCM-41
16MCM-41
14MCM-41
12MCM-41
16MCM-48
14MCM-48
12MCM-48
1112
941
841
1.11
31.7
27.9
21.9
18.9
26.0
23.2
18.6
0.82
0.71
0.59
1.11
0.80
0.82
873
(c)
1200
1201
1257
(c)
(d)
m-CPBA (2 mmol) as an oxidant in 10 mL of dichloromethane
containing toluene (40 L) as an internal standard and NMO
1456
(d)
1541
1386
1340
1310
m
2854
2923
(5 mmol) as an axial base at 273 K. Once the reaction was com-
pleted, the supported catalysts were separated by filtration, and the
filtrate was washed with 1 N NaOH (10 mL) and brine (10 mL), then
dried over MgSO4. The conversion and ee values were determined
by GC, with toluene as an internal standard. For the heterogeneous
catalysis system, the filtrate was detected by ICP-AES, and no ob-
vious Mn leaching (<0.5%) was found.
3200 3000 2800
1600
Wavenumber (cm-1)
1500
1400
1300
Figure 2. FTIR spectra of (a) Mn(III) salen complex 1, (b) the heterogeneous catalyst
16M48-1, (c) Mn(III) salen complex 2, and (d) the heterogeneous catalyst 16M48-2.
3. Results and discussion
Figure 2. The bands near 3000 cmꢀ1 were due to C–H stretching
vibrations of alkyl groups, and the IR bands near 1540 cmꢀ1 were
attributed to the stretching vibrations of azomethine groups (H–
C]N). The deformation vibrations of C–H bond in alkyl groups
appeared in the range of 1340–1456 cmꢀ1. The characteristic IR
bands of the chiral Mn(III) salen complexes could be also observed
in the FTIR spectra of heterogeneous catalysts. The results of FTIR
characterization confirmed the successful immobilization of chiral
Mn(III) salen complex.
The diffuse-reflectance UV–vis spectra of chiral Mn(III) salen
complexes (1 and 2) and the heterogeneous catalysts (16M48-1 and
16M48-2) are shown in Figure 3. In the spectrum of chiral Mn(III)
salen complexes, the bands at 284, 320, 328, and 366 nm were due
to the charge transfer transition of salen ligand, the bands at 448
and 456 nm could be attributed to ligand-to-metal charge transfer
transition, and the bands at 532 and 550 nm may be assigned to the
d–d transition of Mn(III) salen complexes. The spectra of hetero-
geneous catalysts 16M48-1 and 16M48-2 showed features similar
to those of homogeneous catalysts 1 and 2. However, after immo-
bilization of Mn(III) salen complex on mesoporous materials, the
bands at 284 and 448 nm shifted to 268 and 434 nm for 16M48-1,
and the bands at 284 and 456 nm shifted to 271 and 440 nm for
16M48-2. The blue shift indicated the presence of interaction
between Mn(III) salen complex and the support, and further con-
firmed the immobilization of chiral Mn(III) salen complexes on the
supports.
3.1. Characterization of the supports
The mesoporous materials nMCM-41 and nMCM-48 were
characterized by N2 sorption. The BET surface area, pore volume,
and pore diameter of all the parent supports are given in Table 1.
The obtained mesoporous materials nMCM-41 and nMCM-48 have
large BET surface area and pore volume. Moreover, with the de-
crease in alkyl chain length of the templates, the pore diameter of
the mesoporous materials regularly decreased on the angstrom
length-scale.
3.2. Characterization of the heterogeneous catalysts
The powder XRD patterns of the calcined supports (16MCM-41
and 16MCM-48), the mercaptopropylsilyl-functionalized supports
(ME-16MCM-41 and ME-16MCM-48), and the heterogeneous cat-
alysts (16M41-1, 16M41-2, 16M48-1 and 16M48-2) are shown in
Figure 1. After modification by 3-mercaptopropyltrimethoxysilane
and immobilization of Mn(III) salen complexes, the intensities of all
peaks decreased. However, the well-resolved reflections (110) and
(200) for 16MCM-41-supported catalysts and reflection (220) for
16MCM-48-supported catalysts indicated that the mesoporous
structure of heterogeneous catalysts still kept good periodicity.
The FTIR spectra of chiral Mn(III) salen complexes (1 and 2) and
the heterogeneous catalysts (16M48-1 and 16M48-2) are shown in
A
B
(100)
(211)
(220)
(110)
(200)
(a)
(a)
(b)
(c)
(d)
(b)
(c)
(d)
2
4
6
2Theta (deg.)
8
10
2
4
6
2Theta (deg.)
8
10
Figure 1. (A) Powder XRD patterns of (a) calcined 16MCM-41, (b) ME-16MCM-41, (c) 16M41-1, and (d) 16M41-2. (B) Powder XRD patterns of (a) calcined 16MCM-48, (b) ME-
16MCM-48, (c) 16M48-1, and (d) 16M48-2.

K. Yu et al. / Tetrahedron 65 (2009) 305–311
309
surface area, pore volume, and pore diameter was observed on
immobilization of chiral Mn(III) salen complexes onto mesoporous
materials, which indicated that the active centers were located
mainly on the inner surfaces of the supports.
284
448
532
268
284
434
320
328
3.3. The heterogeneous asymmetric epoxidation
(a)
(b)
The heterogeneous catalysts were evaluated in the epoxidation
of styrene and indene in CH2Cl2 at 273 K for 8 h using m-CPBA/NMO
as the oxidant. For comparison, the chiral Mn(III) salen catalysts 1
and 2 were also investigated under the same reaction conditions,
but shortening the reaction time to 2 h. The effects of 3-mercap-
topropyltrimethoxysilane dosage and the fine-tuning of pore size
on the catalytic performance for the epoxidation of unfunctional-
ized olefins were studied.
456
271
366
(c)
(d)
440
550
300
400
500
Wavelength (nm)
600
700
800
3.3.1. Effect of 3-mercaptopropyltrimethoxysilane dosage on the
catalytic performance
Figure 3. Diffuse-reflectance UV–vis spectra of (a) Mn(III) salen complex 1, (b) the
heterogeneous catalyst 16M48-1, (c) Mn(III) salen complex 2, and (d) the heteroge-
neous catalyst 16M48-2.
The chiral Mn(III) salen complexes were covalently anchored
onto the surface of mesoporous materials through 3-mercapto-
propyltrimethoxysilane. To investigate the effect of 3-mercapto-
propyltrimethoxysilane dosage on the catalytic performance of
heterogeneous catalysts, different amounts of 3-mercaptopropyl-
trimethoxysilane were immobilized on 16MCM-41 and 16MCM-48.
The as-synthesized catalysts (e.g., 16M41-1 (x mL/g), where x
denoted the amount of 3-mercaptopropyltrimethoxysilane used in
the system containing 1.0 g of parent support) were evaluated in
the asymmetric epoxidation of styrene. To effectively study the
influence of 3-mercaptopropyltrimethoxysilane dosage, an excess
of chiral Mn(III) salen complexes was used in the process of im-
mobilization and the same amount of heterogeneous catalysts
The calcined mesoporous supports and the heterogeneous
catalysts were characterized by N2 sorption. Figure 4 shows the
low-temperature N2 adsorption–desorption isotherms and the
pore size distributions of the heterogeneous catalysts 16M41-1,
16M48-1, 16M41-2, and 16M48-2. It can be seen that the immo-
bilized catalysts maintained characteristic type IV isotherms34 and
uniform pore sizes in the mesopore ranges. The textural parameters
of all the heterogeneous catalysts are presented in Table 2. Com-
pared with parent supports (Table 1), a large decrease in BET
20
20
A
B
adsorption
desorption
adsorption
desorption
16M41-1
16
16
16M48-1
12
12
8
0.08
0.06
0.04
0.02
0.00
0.06
0.04
8
0.02
0.00
0
50
100
150
0
50
100
150
4
4
Pore Diameter (Å)
Pore Diameter (Å)
0.0
0.2
0.4
0.6
Relative Pressure (P/P0)
0.8
1.0
0.0
0.2
0.4
0.6
Relative Pressure (P/P0)
0.8
1.0
20
16
12
8
D
C
adsorption
desorption
adsorption
desorption
25
16M41-2
20
15
10
5
16M48-2
0.10
0.08
0.06
0.04
0.02
0.00
0.08
0.06
0.04
0.02
0.00
0
50
100
150
05
01
00
150
Pore Diameter (Å)
4
Pore Diameter (Å)
0.0
0.2
0.4
0.6
Relative Pressure (P/P0)
0.8
1.0
0.0
0.2
0.4
0.6
Relative Pressure (P/P0)
0.8
1.0
Figure 4. N2 adsorption–desorption isotherms and pore diameter distribution profiles (inset) of (A) 16M41-1, (B) 16M48-1, (C) 16M41-2, and (D) 16M48-2.

310
K. Yu et al. / Tetrahedron 65 (2009) 305–311
Table 2
Table 4
The structure parameters of heterogeneous catalysts
Asymmetric epoxidation of styrene and indene catalyzed by the catalysts 1, nM41-1,
and nM48-1a
Sample
SBET (m2 gꢀ1
)
Pore volume
Pore diameter
(Å)
(cm3 gꢀ1
)
Entry
Catalyst
Substrate
Styrene
Time (h)
Conv. (%)
ee (%) c
b
18M41-1
16M41-1
14M41-1
12M41-1
16M48-1
14M48-1
12M48-1
18M41-2
16M41-2
14M41-2
12M41-2
16M48-2
14M48-2
12M48-2
755
744
779
0.62
0.58
0.51
0.50
0.62
0.46
0.58
0.60
0.62
0.48
0.51
0.85
0.48
0.57
27.0
24.4
19.1
17.3
19.0
18.9
17.3
27.1
24.2
19.0
17.5
21.1
17.4
17.0
1
1
2
8
8
8
8
8
8
8
2
8
8
8
8
8
8
8
99.8
71.8
62.4
60.1
49.4
69.3
45.5
45.3
99.1
94.4
80.8
76.5
54.1
63.0
53.2
35.2
47.0 (R)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
18M41-1
16M41-1
14M41-1
12M41-1
16M48-1
14M48-1
12M48-1
1
18M41-1
16M41-1
14M41-1
12M41-1
16M48-1
14M48-1
12M48-1
24.4 (R)
20.9 (R)
18.5 (R)
8.0 (R)
29.6 (R)
11.9 (R)
16.6 (R)
88.4 (1R,2S)
58.4 (1R,2S)
47.0 (1R,2S)
50.5 (1R,2S)
30.8 (1R,2S)
44.1 (1R,2S)
37.9 (1R,2S)
45.1 (1R,2S)
769
1142
1083
1001
753
770
775
Indene
788
1154
1099
1055
(0.18 g, ca. 0.15 mol %) was employed in reaction system. Table 3
gives the results of styrene epoxidation catalyzed by the hetero-
geneous Mn(III) salen complexes that were anchored through dif-
ferent amounts of 3-mercaptopropyltrimethoxysilane onto the
surfaces of 16MCM-41 and 16MCM-48. According to the results of
elemental analysis, ca. 60% of 3-mercaptopropyltrimethoxysilane
were linked on the surfaces of mesoporous supports.
a
Reactions were performed in CH2Cl2 (10 mL) with catalyst (0.015 mmol), sub-
strate (1 mmol), NMO (5 mmol), and m-CPBA (2 mmol) at 273 K.
b
Conversion % determined by GC with chiral column using toluene as internal
standard.
c
ee % determined by GC with RESTEK RT-BetaDEXse chiral column.
3.3.2. Effect of the fine-tuning of pore size on the catalytic
performance
As shown in Table 3, the dosage of 3-mercaptopropyl-
trimethoxysilane was related to the catalytic performance of het-
erogeneous catalysts. For the catalysts 16M41-1 and 16M48-1,
0.4 mL of 3-mercaptopropyltrimethoxysilane was the most favor-
able to 1.0 g of parent supports. However, for the catalysts 16M41-2
and 16M48-2, 0.2 mL of 3-mercaptopropyltrimethoxysilane to 1.0 g
of parent supports was the best dosage. The high content of
3-mercaptopropyltrimethoxysilane would increase the diffusional
resistance of chiral Mn(III) salen complexes into the channels and
reactants tothe active sites located on the inner surfaces of supports.
On the other hand, low content of 3-mercaptopropyltrimethoxy-
silane would decrease the loading of chiral Mn(III) salen complexes
and increase the leaching of active sites during the process of
Soxhlet-extraction with dichloromethane. It could be seen from
Table 3, the catalysts 16M41-2 and 16M48-2 obtained better cata-
lytic performance at low dosage of 3-mercaptopropyltrimethoxy-
silane than the catalysts 16M41-1 and 16M48-1. This is attributed
mainly to the larger molecular size of complex 2, which led to the
higher diffusional resistance at high organic silane loading.
The heterogeneous catalysts supported by a series of meso-
porous MCM-41 and MCM-48 materials with different pore sizes
were evaluated in the epoxidation of styrene and indene, and the
effect of the fine-tuning of pore size on their catalytic performance
was studied. On the immobilization of chiral Mn(III) salen com-
plexes 1 and 2, the optimum 3-mercaptopropyltrimethoxysilane
dosages were applied. The loading of complexes 1 and 2 in the
heterogeneous catalysts was in the range of 0.07–0.08 mmol/g
based on Mn element analysis by ICP-AES.
Table 4 gives the results of asymmetric epoxidation of styrene and
indene catalyzed by the catalysts 1, nM41-1, and nM48-1. In general,
for the catalysts nM41-1 and nM48-1, the catalytic activity increased
with increasing the pore size. For instance, the highest conversions of
styrene and indene were attained by the catalysts 18M41-1 and
16M48-1, which possess the largest pore size. However, the enan-
tioselectivity didn’t run parallel to catalytic activity. The catalysts
18M41-1 and 16M48-1 exhibited the highest enantioselectivity
Table 3
Table 5
Asymmetric epoxidation of styrene catalyzed by the immobilized chiral Mn(III) salen
complexesanchoredthroughdifferent amounts of3-mercaptopropyltrimethoxysilanea
Asymmetric epoxidation of styrene and indene catalyzed by the catalysts 2, nM41-2,
and nM48-2a
b
c
Entry
Catalyst
Conv. (%)b
ee (%)c
Entry
Catalyst
Substrate
Styrene
Time (h)
Conv. (%)
ee (%)
1
2
3
4
5
6
7
8
16M41-1 (0.2 mL/g)
16M41-1 (0.4 mL/g)
16M41-1 (0.6 mL/g)
16M41-1 (1.0 mL/g)
16M48-1 (0.2 mL/g)
16M48-1 (0.4 mL/g)
16M48-1 (0.6 mL/g)
16M48-1 (1.0 mL/g)
16M41-2 (0.1 mL/g)
16M41-2 (0.2 mL/g)
16M41-2 (0.4 mL/g)
16M41-2 (0.6 mL/g)
16M48-2 (0.1 mL/g)
16M48-2 (0.2 mL/g)
16M48-2 (0.4 mL/g)
16M48-2 (0.6 mL/g)
51.7
62.4
59.4
57.6
62.1
69.3
58.1
44.0
58.4
64.7
61.3
58.9
72.3
76.9
73.2
58.1
17.3 (R)
20.9 (R)
19.0 (R)
16.6 (R)
26.1 (R)
29.6 (R)
18.0 (R)
15.4 (R)
25.7 (R)
25.7 (R)
23.7 (R)
22.2 (R)
40.1 (R)
41.2 (R)
39.0 (R)
22.7 (R)
1
2
3
4
5
6
7
8
2
2
8
8
8
8
8
8
8
2
8
8
8
8
8
8
8
99.5
64.7
64.5
59.9
54.3
76.9
53.9
47.4
98.7
86.2
81.8
79.6
68.0
95.2
64.8
55.8
66.0 (R)
30.1 (R)
25.7 (R)
24.1 (R)
14.6 (R)
41.2 (R)
16.4 (R)
11.4 (R)
87.0 (1R,2S)
48.2 (1R,2S)
43.8 (1R,2S)
43.9 (1R,2S)
28.4 (1R,2S)
58.8 (1R,2S)
30.7 (1R,2S)
25.6 (1R,2S)
18M41-2
16M41-2
14M41-2
12M41-2
16M48-2
14M48-2
12M48-2
2
18M41-2
16M41-2
14M41-2
12M41-2
16M48-2
14M48-2
12M48-2
9
9
Indene
10
11
12
13
14
15
16
10
11
12
13
14
15
16
a
a
Reactions were performed in CH2Cl2 (10 mL) with catalyst (0.18 g, ca. 1.5 mol %),
Reactions were performed in CH2Cl2 (10 mL) with catalyst (0.015 mmol), sub-
styrene (1 mmol), NMO (5 mmol), and m-CPBA (2 mmol) at 273 K for 8 h.
strate (1 mmol), NMO (5 mmol), and m-CPBA (2 mmol) at 273 K.
b
b
Conversion % determined by GC with chiral column using toluene as internal
Conversion % determined by GC with chiral column using toluene as internal
standard.
standard.
c
c
ee % determined by GC with RESTEK RT-BetaDEXse chiral column.
ee % determined by GC with RESTEK RT-BetaDEXse chiral column.

K. Yu et al. / Tetrahedron 65 (2009) 305–311
311
among the heterogeneous catalysts, but the catalyst 12M48-1
References and notes
obtained higher ee values than 14M48-1 for the epoxidation of sty-
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This work was supported by the National Natural Science
Foundation of China (grant 20773069) and the National Key Tech-
nologies R&D Program of China (grant 2006BAC02A12).