Chiral Mn(III) salen complexes covalently bonded on modified MCM-41 and SBA-15 as efficient catalysts for enantioselective epoxidation of nonfunctionalized alkenes

Article abstract of DOI:10.1016/j.jcat.2005.11.042

Chiral Mn(III) salen complex supported onto modified mesoporous supports (MCM-41 and SBA-15) were prepared using 3-aminopropyltriethoxysilane as a reactive surface modifier by a covalent grafting method. The supported catalysts showed higher chiral induct

Full text of DOI:10.1016/j.jcat.2005.11.042

Journal of Catalysis 238 (2006) 134–141
www.elsevier.com/locate/jcat
Chiral Mn(III) salen complexes covalently bonded on modified MCM-41
and SBA-15 as efficient catalysts for enantioselective epoxidation
of nonfunctionalized alkenes
Rukhsana I. Kureshy ∗, Irshad Ahmad, Noor-ul H. Khan, Sayed H.R. Abdi, Kavita Pathak,
Raksh V. Jasra
Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar, 364 002, Gujarat, India
Received 29 September 2005; revised 24 November 2005; accepted 28 November 2005
Available online 9 January 2006
Abstract
Chiral Mn(III) salen complex supported onto modified mesoporous supports (MCM-41 and SBA-15) were prepared using 3-aminopropyltri-
ethoxysilane as a reactive surface modifier by a covalent grafting method. The supported catalysts showed higher chiral induction (ee, 71%) for
enantioselective epoxidation of styrene and 4-chlorostyrene in presence of pyridine N-oxide (PyNO) as axial base using aqueous NaOCl (12%)
as an oxidant than seen in its homogeneous counterpart 1 (ee, 48%). SBA-15-based catalyst 3, with a larger pore diameter, was found to be more
active than MCM-41-supported catalyst 2. In addition, bulkier alkenes like indene, 1,2-dihydronaphthalene, and 2,2-dimethylchromene were
efficiently epoxidized with these supported catalysts (ee up to 96%), and the results were comparable to those for the homogeneous system. The
performance of catalysts 2 and 3 was retained for four reuse experiments.
© 2005 Elsevier Inc. All rights reserved.
Keywords: Heterogeneous catalysis; Supported chiral Mn(III) salen; Nonfunctionalized alkenes; Epoxidation; MCM-41 and SBA-15
1. Introduction
tion of chiral salen transition metal complexes onto solid sup-
ports, including organic polymers [13–17] and inorganic solids
of varied porosity [4,18–26] where the metal complexes were
supported via axial or apical coordination or through covalent
bonding of the ligand to the support with moderate to excellent
results for enantioselective epoxidation of nonfunctionalized
alkenes. Furthermore, such heterogenization of chiral Mn(III)
salen complex revealed that the local environment inside the
mesopores and pore size of the support does affect the enan-
tioselectivity of the epoxidation reaction [18,19,21,22]. There-
fore, it is prudent to graft the active metal complex onto solid
support with varied pore sizes to determine the optimum pore
size for efficient diffusion of reactant and product molecules.
Ordered mesoporous solids like MCM-41 and SBA-15, with
their well-defined uniform mesopores and facile surface mod-
ification, are potential materials [27–29] for heterogenization
of valuable chiral homogeneous catalysts. Herein, we describe
the covalent bonding of a homogeneous system (complex 1)
on modified MCM-41 and SBA-15 using 3-aminopropyltri-
ethoxysilane (APTES) as a reactive surface modifier to give
Chiral salen transition metal complexes have become a mat-
ter of current interest [1–7] because of their wide applications
as catalysts in asymmetric epoxidation [1–4], cyclopropanation
[5], aziridination [6], Knoevenagel condensation [7], and selec-
tive hydrogenation [8–10] reactions under homogeneous condi-
tions. Separation and recycling of the homogeneous catalyst is
problematic, however, making the entire process economically
nonviable for industrial applications, more so for expensive cat-
alysts with low turnover numbers. Therefore, heterogenization
of homogeneous catalysts has become an important strategy for
obtaining supported catalysts that retain the active catalytic sites
of a homogeneous analogue while at the same time providing
advantages of easy separation and recycling of the catalyst [11,
12]. Several papers have recently appeared on the heterogeniza-
*
Corresponding author. Fax: +91 0278 2566970.
E-mail address: rukhsana93@yahoo.co.in (R.I. Kureshy).
0021-9517/$ – see front matter © 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2005.11.042

R.I. Kureshy et al. / Journal of Catalysis 238 (2006) 134–141
135
supported complexes 2 and 3. To allow the maximum confor-
mational mobility in the complex needed to obtain a high level
of asymmetric induction, the grafting was done through one of
the fifth positions of the Mn(III) salen complex. The supported
catalysts 2 and 3 effectively catalyzed epoxidation of styrene
and 4-chlorostyrene (ee, 68–71%) with aqueous NaOCl. These
results are significantly better than those achieved with the cata-
lyst 1 under a homogeneous system (ee, 45–48%) and other re-
ported Mn(III) salen complexes [19,30] anchored on MCM-41.
Remarkably, catalysts 2 and 3 worked well for several cycles
even with relatively bulkier alkenes such as indene, 1,2-dihy-
dronaphthalene, and 2,2-dimethylchromene (ee, 67–96%), and
the results are comparable with those of the corresponding ho-
mogeneous system.
Elmer 2400 CHNS analyzer. An inductively coupled plasma
(ICP) spectrometer (Perkin–Elmer, Optima 2000 DV) was used
for Mn estimation. Poweder X-ray diffraction (XRD) patterns
of the samples were recorded on a Philips X’Pert MPD dif-
fractometer using Cu-Kα (λ = 1.5405 Å) radiation with a step
size of 0.02◦ 2θ and a step time of 5 s of curved Cu-Kα mono-
chromator under identical conditions. BET surface area was
determined using N2 sorption data measured at 77 K using
a volumetric adsorption setup (Porous Materials; PMI system
DET sorptometer). The pore diameter of the samples were
determined from the desorption branch of the N2 adsorption
isotherm using the BJH method. Transmission electron mi-
croscopy (TEM) analysis was done with a Philips Tecnai 20.
Scanning electron microscopy (SEM) analysis of the sample
was done with a LEO 1430 VP.
2. Experimental
Cetyltrimethylammonium bromide (CTAB), manganese(II)
acetate (s.d. Fine Chem. Ltd. India), sodium silicate solu-
tion, tetraethyl orthosilicate (TEOS), triblock organic copoly-
mer (EO20–PO70–EO20) Pluronic P123 (Aldrich), HCl (Ran-
baxy, India), APTES (Fluka), aqueous NaOCl (12%) (National
Chemicals, India), and PyNO (Fluka) were used as received.
Indene, styrene, 4-chlorostyrene, and 1,2-dihydronaphthalene
were passed through a pad of neutral alumina before use. 2,2-
Dimethylchromene was synthesized as described previously
[31]. A highly ordered hexagonal siliceous MCM-41 was syn-
thesized as described previously [23]. All of the solvents used
in the present study were purified by known methods [32].
The purity of the solvents and alkenes and analysis of
the product epoxide were determined by gas chromatography
(GC) using a Shimadzu GC 14B instrument equipped with a
stainless-steel column (2 m long, 3 mm i.d., 4 mm o.d.) packed
with 5% SE 30 (mesh size, 60–80) and having a flame ioniza-
tion detector (FID). Ultrapure N2 was used as a carrier gas (rate
30 mL/min) with the injection port temperature set at 200 ◦C.
The column temperature for styrene, 4-chlorostyrene, and in-
dene was in the range of 70–150 ◦C, whereas for chromene and
1,2-dihydronaphthalene, the isothermal temperature was kept
at 150 ◦C. The racemic epoxides were prepared by the epox-
idation of respective alkenes with 3-chloroperbenzoic acid in
CH2Cl2 and were used to determine conversions by compar-
ing the height and area of the GC peaks. The ee’s of styrene,
4-chlorostyrene, and 1,2-dihydronaphthalene oxides were de-
termined by GC on a chiral capillary column (Chiraldex GTA
and BDA). For 2,2-dimethylchromene and indene epoxides, the
ee’s were determined by 1H NMR using the chiral shift reagent
Eu(hfc)3 and by HPLC (Shimadzu SCL-10AVP) using a Chi-
ralcel column (AD/OB/OD/OJ).
2.1. Synthesis of SBA-15
Highly ordered mesoporous SBA-15 was synthesized us-
ing a modified procedure reported by Zhao et al. [33] un-
der hydrothermal conditions using a triblock organic copoly-
mer as a template. In a typical synthesis, 12 g of triblock,
poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene ox-
ide) (EO20–PO70–EO20) (Pluronic P123, mw 5800) was dis-
persed in 90 g of double-distilled water to which 360 g of 2 M
aqueous HCl was added under stirring at ambient temperature
(25–30 ◦C) for 1 h. Finally, 27 g of silica source TEOS was
added to the homogeneous solution under stirring to form a gel
at 313 K for 24 h, and this was allowed to stand for crystalliza-
tion under static hydrothermal conditions at 373 K for 48 h in
a Teflon Parr reactor. The crystallized product was filtered off,
washed with warm distilled water, dried at 383 K, and finally
calcined at 813 K in air for 6 h to remove the template. The
calcined SBA-15 was characterized by powder XRD.
2.2. Preparation of 3-aminopropylsilyl-functionalized
MCM-41 (E) and SBA-15 (F)
A suspension of APTES (4.56 g, 20.63 mmol) and 10 g of
calcined MCM-41/SBA-15 in 90 mL of toluene was heated to
reflux with stirring under inert atmosphere for 24 h. The result-
ing masses were cooled to 25–30 ◦C and filtered. The solids
were filtered and washed successively with dry toluene and di-
ethyl ether, then dried under vacuum at ambient temperature.
The dried material was subjected to Soxhlet extraction with
dry dichloromethane for 24 h. Finally; the solids (E and F)
were dried at 50–55 ◦C under vacuum for 8 h. The charac-
terization was accomplished by microanalysis (Table 1), IR,
diffuse reflectance, UV–vis spectroscopy, XRD, and nitrogen
sorption studies. IR (KBr) for E: 463, 794, 1079, 1633, 2937,
3437 cm−1. Diffuse reflectance UV–vis: 245, 340 nm; IR (KBr)
for F: 460, 798, 1076, 1636, 2937, 3437 cm−1. Diffuse re-
flectance UV–vis: 270, 335, 375 nm.
1H and 13C{1H} NMR spectra were recorded on a 200-
and 50-MHz spectrometer (Bruker, F113V), respectively. The
IR spectra were recorded on a Perkin–Elmer Spectrum GX
spectrophotometer in KBr/nujol mull. Electronic spectra were
recorded in dichloromethane on a Varian UV–vis-NIR CARY
500 SCAN spectrophotometer. Diffuse reflectance spectra were
obtained from UV–vis-NIR scanning spectrophotometer UV-
3101 PC. Microanalysis of the complex was done on a Perkin–

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R.I. Kureshy et al. / Journal of Catalysis 238 (2006) 134–141
Table 1
Physico-chemical characterization data of MCM-41, SBA-15, aminopropyl modified MCM-41 (E) and SBA-15 (F) and supported catalysts 2 and 3
Compound
Mn loading
(mg/100 mg)
BJH pore diameter
(Å)
Total pore volume
BET surface area
Micro analysis, found %
3
2
(cm /g)
(m /g)
C
H
N
C/N
1
–
–
–
22
–
–
–
–
63.02
–
7.86
10.92
–
7.12
–
1.75
1.42
–
4.40
–
1.89
1.04
–
14.13
–
4.16
10.5
–
MCM-41
E
2
SBA-15
F
3
37
28
21
68
57
49
0.727
0.412
0.290
1.120
0.719
0.562
786
464
325
662
364
235
–
23
7.65
14.15
1.66
1.91
1.84
1.38
4.16
10.25
Scheme 1. Synthesis of the complexes 1–3.
2.3. Synthesis of unsymmetrical Mn(III) salen complex 1
found: C, 73.48; H, 8.73; N, 5.17; IR (KBr) 3411, 2957,
2863, 1629, 1597, 1470, 1441, 1390, 1361, 1272, 1251, 1204,
1
1172, 1095, 1044, 878, 827, 712, 644, 590 cm−1; H NMR
The synthesis sequence for non-C2-symmetric salen-based
Mn(III) salen complex 1 is shown in Scheme 1. This involves
the initial preparation of a chiral half unit N-(2-hydroxy-3,5-
di-tert-butylbenzaldehyde)-1-amino-2-cyclohexaneimine (B)
from 3,5-di-tert-butyl salicylaldehyde (A) and 1S,2S-(+)-cy-
clohexanediamine as described previously [34], followed by
condensation of half unit (B) with 5-chloromethyl 3-tert-
butylsalicylaldehyde (C) [35] in dry methanol to form a un-
symmetrical chiral salen-based ligand (D) (Yield, 80–82%),
Anal. Calcd. for C33H47ClN2O2: C, 73.54; H, 8.73; N, 5.20;
(CDCl3, 200 MHz): δ ppm 1.23 (s, 18H), 1.42 (s, 9H), 1.47–
2.10 (m, 8H), 3.30–3.48 (m, 2H), 4.56 (s, 2H), 6.85 (d, 1H
J = 1.9 Hz), 7.05 (d, 2.0 Hz), 7.43 (d, 1H, J = 2.2 Hz),
7.52 (d, 1H J = 2.2 Hz), 8.32 (s, 1H), 8.44 (s, 1H), 13.50
(bs, 1H), 14.48 (bs, 1H); 13C{1H}, δ ppm 24.2, 24.8, 26.2,
29.3, 29.6, 29.8, 33.9, 34.1, 35.2, 45.8, 72.3, 78.1, 116.8, 118.2,
122.4, 126.4, 127.4, 127.6, 136.2, 139.8, 157.9, 161.5, 164.8,
165.7. On complexation with Mn(CH3COO)2·4H2O and its
aerial oxidation in the presence of LiCl, this gave complex 1.

R.I. Kureshy et al. / Journal of Catalysis 238 (2006) 134–141
137
Yield 90%; IR (KBr): 3434, 2955, 2866, 1613, 1536, 1436,
1389, 1252, 1202, 1029, 836, 568 cm−1: UV–vis. (CH2Cl2)
of manganese ion is not involved in the anchoring of the com-
plex 1 to the support, and thus the activity of these supported
catalysts 2 and 3 is akin to that of homogeneous complex 1.
The loading of 1 in supported catalysts was found to be 22–
23 mg/100 mg as determined by ICP and spectrophotometry
(Table 1). The powder XRD patterns of MCM-41 and SBA-15
show a very intense peak assigned to reflection at (100) and two
additional peaks with low intensities at (110) and (200) reflec-
tions, which can be indexed to a hexagonal lattice (Figs. 1A
and 1B, M-1 and S-1). It is observed that on functionalization
with 3-aminopropyltriethoxysilane, the intensities of all of the
peaks of (E and F) decrease marginally with a little shift toward
lower 2θ values (M-2 and S-2), demonstrating the occurrence
of silylation inside the mesopores of MCM-41/SBA-15. After
heterogenization of chiral Mn(III) salen complex 1, intensities
of the peaks at the (110) and (200) reflections were decreased
(M-3 and S-3), indicating that the mesoporous structure of the
supports remained intact under the conditions used for hetero-
genization. SEM micrographs (Fig. 2) revealed that MCM-41
(A) and SBA-15 (B) samples consist of small agglomerates
whose morphology does not change in the supported catalysts 2
(C) and 3 (D).
TEM micrographs of purely siliceous MCM-41 and SBA-
15 revealed hexagonally arranged pore structures when viewed
along the pore direction, along with parallel lattice fringes
on a side view analysis (Figs. 3A and 3D). Whereas SBA-15
prepared in the acidic medium exhibited mesopores of a one-
dimensional channel system, confirming that SBA-15 has a 2D
p6mm hexagonal structure. The presence of equidistant paral-
lel fringes demonstrates the nature of separation between layers
and the unique well-packed arrangement of such monolayers
(Figs. 3C and 3F). The ordered mesoporous structure of the
support was unaffected by anchoring of chiral Mn(III) salen
complex 1 (Figs. 3B and 3E).
FTIR spectra (Fig. 4) of supported catalysts 2 and 3 repre-
sented as (M-3) and (S-3) show bands at 2960 and 2958 cm−1
due to ν(CH2) of the propyl arm belonging to the silylating
agent, These peaks were absent in the IR spectra of calcined
MCM-41 (M-1) and SBA-15 (S-1), confirming the grafting of
chiral salen complex 1 onto MCM-41/SBA-15. To further con-
firm the grafting of the salen unit (structure 1, Scheme 1) onto
modified MCM-41 and SBA-15, the catalysts 2 and 3 were dis-
solved in HF solution, and the resulting mass was extracted with
CH2Cl2. After the solvent was completely removed, the result-
ing brown mass was analyzed by UV–vis and IR spectroscopy,
which showed the presence of the Mn(III) salen complex. These
observations are in accordance with those reported for the het-
erogenization of salen on siliceous material [36].
30
284, 416, 422, 399, 320, 284 nm; [α]D = +663 (c = 0.04 g,
0.064 mmol/100 mL, CH2Cl2).
2.4. Heterogenization of unsymmetrical Mn(III) salen-based
complex 1 on aminopropylsilyl-functionalized MCM-41 and
SBA-15
The surface-modified MCM-41/SBA-15 (E/F) (1 g) was
added to a solution of the unsymmetrical Mn(III) salen com-
plex (1) (352.4 mg, 0.562 mmol) in dry toluene (10 mL), and
the resulting suspension was refluxed for 48 h under inert at-
mosphere. The supported catalyst 2/3 was filtered, washed thor-
oughly with dry toluene and diethyl ether, and extracted repeat-
edly with methanol and dichloromethane on a Soxhlet extractor
until the washings become colorless. All of the washings were
combined, the solvent was evaporated, and the residue was dis-
solved in toluene (10 mL). The difference between the initial
and final concentrations as measured by UV–vis spectroscopy
gave the Mn(III) salen loadings on modified MCM-41 (E) and
SBA-15 (F). The characterization of chiral Mn(III) salen cat-
alysts 2 and 3 was done by microanalysis (Table 1), UV–vis
reflectance spectroscopy (DRS), IR, XRD, ICP, SEM, TEM,
and nitrogen sorption studies (Table 1).
Catalyst 2: Yield, 90%; IR (KBr): 3453, 2960, 1633, 1079,
802, 457 cm−1; diffuse reflectance 275, 330, 430, 530 nm.
Catalyst 3: Yield, 89%; IR (KBr) 3425, 2958, 1629, 1081,
801, 461 cm−1; diffuse reflectance 280, 350, 427, 540 nm.
2.5. Enantioselective epoxidation of nonfunctionalized alkenes
Enantioselective epoxidation reactions were carried out us-
ing catalysts 1, 2, and 3 (0.05 mmol) with styrene, 4-Cl-styrene,
indene, 1,2-dihydronaphthalene, and 2,2-dimethylchromene
(1 mmol) as substrates in 1 mL of dichloromethane (for cat-
alyst 1) and 4 mL of dichloromethane (for catalysts 2 and 3)
under reaction conditions in the presence of PyNO (0.13 mmol)
as an axial base with aqueous buffered 2.75 mmol NaOCl (12%,
pH = 11.3) as an oxidant. The NaOCl was added in five equal
portions at 0 ◦C, and the reaction mass was stirred using a
magnetic stirrer (homogeneous) and a mechanical stirrer (het-
erogeneous) at 800 20 rpm. The epoxidation reaction was
monitored by GC with n-tridecane (0.1 mmol) as a GLC in-
ternal standard for product quantification. After completion of
the reaction, the supported catalysts 2 and 3 were separated by
centrifugation, washed thoroughly with dichloromethane, and
dried for reuse.
Data on BET surface area, pore diameter, and pore volume
are presented in Table 1. A large decrease in BET surface area
was observed on functionalization of modified MCM-41 and
SBA-15, with a reduction in the mesopore diameter and pore
volume (Table 1), suggesting that the complex 1 was present
inside the channels of support material. The solid reflectance
UV–vis spectra of the supported catalysts 2 and 3 showed char-
acteristic bands in complex 1, indicating the presence of chiral
Mn(III) salen in modified MCM-41 and SBA-15 (Fig. 5) [19].
3. Results and discussion
The anchoring of unsymmetrical chiral Mn(III) salen com-
plex 1 was made through the fifth position of the salen com-
plex onto surface-modified MCM-41 and SBA-15, as depicted
in Scheme 1. This strategy has considerable advantages over
others described in the literature [19] from the standpoint of
economy of synthetic steps. Further, the coordination sphere

138
R.I. Kureshy et al. / Journal of Catalysis 238 (2006) 134–141
(A)
(B)
Fig. 1. (A) Powder XRD pattern of calcined MCM-41 (M-1), aminopropyl modified MCM-41 (M-2), supported catalyst 2 (M-3). (B) Powder XRD pattern of
calcined SBA-15 (S-1), aminopropyl modified SBA-15 (S-2), supported catalyst 3 (S-3).
Fig. 2. SEM micrographs of (A) calcined MCM-41, (B) calcined SBA-15, (C) catalyst 2, (D) catalyst 3.
The enantioselective catalytic activities of the chiral Mn(III)
salen complex 1 (homogeneous) supported catalysts onto mod-
ified MCM-41 and SBA-15 (2 and 3) were examined for
epoxidation of styrene, 4-chlorostyrene, indene, 1,2-dihydro-
naphthalene, and 2,2-dimethylchromene at 0 ◦C using aqueous
NaOCl as an oxidant in the presence of PyNO as an axial
base. The data reported in Table 2 show that the supported
catalysts 2 and 3 efficiently promoted the enantioselective re-
action of styrene and 4-chlorostyrene (entries 2, 3, 7, and 8),
producing higher chiral induction (ee, 68–71%) than seen in
its homogeneous version 1 (ee, 45–48%). The increase in chiral
recognition could arise from unique spatial environment created
by both the chiral salen complex and the surface of the supports
used. Similar behavior has been reported earlier for the epoxi-
dation of styrenes [18,22,23,37] with chiral Mn(III) and Cr(III)
salen complexes. In absence of PyNO, the catalyst 1 epoxidized

R.I. Kureshy et al. / Journal of Catalysis 238 (2006) 134–141
139
Fig. 3. TEM micrographs of (A) calcined MCM-41, (B) catalyst 2, (C) pore wall of MCM-41, (D) calcined SBA-15, (E) catalyst 3, (F) pore wall of SBA-15.
(A)
(B)
Fig. 4. (A) IR Spectra of calcined MCM-41 (M-1), aminopropyl modified MCM-41 (M-2), supported catalyst 2 (M-3), unsymmetric salen complex 1. (B) IR Spectra
of calcined SBA-15 (S-1), aminopropyl modified SBA-15 (S-2), supported catalyst 3 (S-3), unsymmetric salen complex 1.
4-chlorostyrene (entry 9; con. 55%, ee 30%), suggesting that
the presence of PyNO is essential for catalyst stability and enan-
tioselectivity. The role of PyNO as an axial base was established
earlier by other authors [38] and also by us [24,35]. MCM-41
and SBA-15 alone showed negligible catalytic activity toward
epoxidation of styrene taken as a representative substrate (2%;
entries 4 and 5).
Furthermore, catalysts 2 and 3 were also found to be ef-
ficient in the epoxidation of indene, 1,2-dihydronaphthalene,
and 2,2-dimethylchromene (entries 11, 12, 14, 15, 17, and 18),
with results comparable to those from the homogeneous reac-
tion carried out with complex 1 under identical reaction condi-
tions. However, the catalytic reactions were found to be slower
(8–12 h) in the supported catalysts 2 and 3 (entries 2, 3, 7, 8, 11,
12, 14, 15, 17, and 18). This behavior is attributed to the diffu-
sional constrains [18] usually present when a catalyst is sup-
ported inside the porous materials. Given this effect, materials
with larger pore sizes would be expected to face less diffusional
resistance. Thus, the higher TOF values obtained with catalyst 3
supported on SBA-15 compared with catalyst 2 supported on
MCM-41 2 are to be expected. The overall performance of cat-
alysts 2 and 3 with varying pore diameters (37–68 Å) is far
better in terms of reactivity and enantioselectivity than that of
other supported Mn(III) salen systems with 3-mercaptopropyl-
triethoxysilane as a reactive surface modifier [20,30].
For reusability experiments, catalysts 2 and 3 were recov-
ered from the reaction mixture of the epoxidation of styrene
by simple centrifugation. The recovered catalyst was washed
thoroughly with CH2Cl2 before reuse. The enantioselectivity
of the recovered catalysts 2 and 3 did not change much dur-
ing four reuse experiments; however, the reaction did slow
down (Table 3), indicating that although the reactive catalyst

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R.I. Kureshy et al. / Journal of Catalysis 238 (2006) 134–141
Table 2
a
Results on the catalytic asymmetric epoxidation of nonfunctionalized alkenes
with chiral Mn(III) salen complex 1 and 2, 3 supported on MCM-41 and
SBA-15
c
d
Entry Catalyst
Substrate
Styrene
Time Conversion
ee
TOF
b
−4
(h)
(%)
(selectivity)
(%)
×10
e
1
2
3
4
5
1
2
3
8
12
9
100 (100)
88 (>99)
94 (>99)
45
6.94
4.16
5.86
–
e
68
e
70
MCM-41
SBA-15
24
24
2
2
–
–
–
e
6
7
8
9
1
2
3
4-Cl-styrene
Indene
7
12
8
100 (100)
96 (>99)
98 (>99)
53 (>99)
48
7.93
4.53
6.87
4.36
e
68
Fig. 5. Diffuse reflectance UV–vis spectra of calcined SBA-15 (S-1), amino-
propyl modified SBA-15 (S-2), covalently bonded Mn(III) salen on modified
SBA-15 (S-3) and complex 1 (1).
e
71
f
e
1
7
30
g
10
11
12
1
2
3
8
12
10
96 (100)
83 (>99)
90 (99)
89
6.73
3.93
5.16
g
remained intact, diffusional resistance increased due to block-
age of some pores with organic matter from previous runs. To
check the leaching of the metal complex, fresh reactant was
added to the solution obtained after centrifugation of the cata-
lyst that showed no further formation of the epoxide. This same
solution did not exhibit any color or the presence of Mn by
ICP. This means that the Mn(III) salen complex 1 supported
onto modified MCM-41 and SBA-15 was intact and stable in
the pore system during the epoxidation reaction.
80
g
81
g
13
14
15
1
2
3
1,2-dihydro-
naphthalene
8
12
9
92 (100)
85 (>99)
94 (>99)
68
6.45
4.07
5.92
g
67
g
72
h
16
17
18
a
1
2
3
2,2-dimethyl-
chromene
8
12
10
99 (>99)
94 (>99)
96 (>99)
99
6.87
4.39
5.44
h
95
h
96
Reactions were performed in CH Cl 1 mL (catalyst 1), 4 mL (catalysts 2
2
2
and 3) with catalyst 0.05 mmol, substrate 1.00 mmol, PyNO 0.13 mmol, aque-
ous NaOCl 2.75 mmol at 0 C.
Characterization of the recycled catalyst (after one cycle) by
FTIR spectra, powder XRD, and CHN analysis demonstrated
entrapment of some of the reactant within the mesopores, which
caused a gradual slowdown of the epoxidation reaction in suc-
cessive recycle experiments. These observations are in accor-
dance with earlier reports based on a supported Mn(III) salen
system [19]. Further, in terms of the transfer of chirality from
catalyst to product epoxide, the configuration of the product
epoxide was the same as that of the catalyst.
◦
b
Based on GC.
c
1
By H NMR using chiral shift reagent (+)Eu(hfc) /chiral capillary column
3
GTA type/chiral HPLC column AD/OB/OD/OJ.
d
Turnover frequency is calculated by the expression [product]/[catalyst] ×
−1
time (s ).
e
Epoxide configuration S.
Reaction conducted in absence of PyNO.
Epoxide configuration 1S,2R.
Epoxide configuration 3S,4S.
f
g
h
4. Conclusion
Chiral Mn(III) salen catalysts 2 and 3 were prepared by het-
erogenizing 1 onto modified mesoporous materials MCM-41
and SBA-15 using 3-aminopropyltriethoxysilane as a reactive
surface modifier by a covalent bonding method. The supported
catalysts demonstrated higher chiral induction (ee, 71%) than
its homogeneous counterpart 1 (ee, 48%) for enantioselective
epoxidation of styrene and 4-chlorostyrene using NaOCl as an
oxidant. The larger-pore size SBA-15 turned out to be a bet-
ter support, because 3 was found to be more active than 2. The
overall performance of the catalysts 2 and 3 is far better in terms
of reactivity and enantioselectivity than that of other supported
Mn(III) salen systems with 3-mercaptopropyltriethoxysilane as
a reactive surface modifier. To further strengthen our observa-
Table 3
a
Recycling data for enantioselective epoxidation of styrene as representative
substrate using supported catalysts 2 and 3
c
b
−4
Run
Catalyst
ee
Conversion
(%)
TOF ×10
−1
(s
)
1
2
3
4
2 (3)
2 (3)
2 (3)
2 (3)
90 (95)
87 (93)
82 (89)
80 (86)
68 (70)
68 (70)
68 (70)
67 (70)
4.16 (5.86)
4.02 (5.74)
3.79 (5.49)
3.70 (5.30)
a
Reactions were performed in CH Cl (4 mL) with the recovered catalysts 2,
2
2
3 substrate 1.00 mmol, PyNO 0.13 mmol, NaOCl 2.75 mmol for 12 h (cata-
lyst 2), 9 h (catalyst 3) at 0 C.
◦
b
Based on GC.
By chiral capillary column GTA type.
c

R.I. Kureshy et al. / Journal of Catalysis 238 (2006) 134–141
141
tions, we are in the process of synthesizing siliceous materials
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In addition, bulkier alkenes, including indene, 1,2-dihydro-
naphthalene, and 2,2-dimethylchromene, were efficiently epox-
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results were comparable with those for the homogeneous sys-
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four reuse experiments.
Acknowledgments
Financial support was provided by the DST, CSIR Net-
work Project on Catalysis, and CSIR (SRF). The authors thank
P.K. Ghosh, Director of CSMCRI, for providing access to the
instrumentation facility.
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