Highly active new chiral Co(iii) salen catalysts immobilized by electrostatic interaction with sulfonic acid linkages on ordered mesoporous SBA-16 silica

Article abstract of DOI:10.1039/b906350a

New chiral cobalt(iii) salen complexes immobilized via HO ₃S-linkers on ordered SBA-16 by electrostatic interactions showed very high activity in enantioselective ring-opening reactions of racemic epoxides.

Full text of DOI:10.1039/b906350a

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Highly active new chiral Co(III) salen catalysts immobilized by
electrostatic interaction with sulfonic acid linkages on ordered
mesoporous SBA-16 silica
Yong-Suk Kim, Xiao-Feng Guo and Geon-Joong Kim*
Received (in Cambridge, UK) 31st March 2009, Accepted 8th May 2009
First published as an Advance Article on the web 5th June 2009
DOI: 10.1039/b906350a
New chiral cobalt(III) salen complexes immobilized via
HO3S-linkers on ordered SBA-16 by electrostatic interactions
showed very high activity in enantioselective ring-opening
reactions of racemic epoxides.
and organosilane compounds in the presence of surfactants
and H2O2. Alternatively, the addition of mercaptopropyl-
triethoxysilane (MPTES) or 2-(4-chlorosulfonylphenyl)ethyl-
trimethoxysilane (CSPTMS) into the reactant mixtures for
SBA-16 was adopted for introducing sulfonic acid groups
on their pore walls. The active sulfonic group was formed
through the oxidation of propanethiol or chlorosulfonylphenyl
groups during the synthesis of mesoporous SBA-16 materials
by H2O2.
Here, the new application of sulfonic acid (ÀSO3H)
functionalized SBA-16 is introduced as an effective method
for the immobilization of chiral salen complexes on surfaces.
This approach is so simple that it can provide a facile route for
anchoring the active sites in one step as compared to covalent
attachment which requires complicated procedures in the
The MTPES (or CSPTMS) functionalized SBA-16 materials
were prepared using EO106PO70EO106 (F127; Aldrich)
as a structure directing agent. The molar composition of
each substrate for 1 g of copolymer (as a basis) was X
TEOS: (0.0227 À X) MPTS or CSPTMS precursor : 0.0405
BuOH : 0.0201 HCl : 2.74 H2O : Z H2O2; where X = 0.0227
(0%), 0.0204 (10%), 0.0183 (20%) or 0.0159 (30%). The
numbers in parentheses indicate the percentage of silicon as
MTPES (from Aldrich) (or CSPTMS (from Gelest) in the
initial mixture. Some approaches have been demonstrated in
the synthesis of the sulfonic acid-functionalized SBA-15 or
MCM-41 materials by direct one-pot condensation reaction
between TEOS and organosilane compounds CSPTMS
(or MPTES) in the presence of surfactants and H2O2.7–10 To
the best of our knowledge, the synthesis of sulfonic acid-
containing SBA-16 material has not been explored to date.
In direct incorporation of sulfonic group on the surfaces,
too large a content of MPTES or CSPTMS in the starting
synthetic mixtures lowers the regularity of the mesopores, and
thus there is a limitation on the concentration of acid sites on
the mesoporous materials. Table 1 summarizes the preparation
conditions and the physicochemical properties of the sulfonic-
modified SBA-16 materials. The BET surface areas and pore
volumes were decreased as the introduced amount of MTPES
or CSPTMS increased.
synthesis of various functionalized linkers.1,2 Aromatic
(À1)
SO3
anion containing homogeneous chiral Co(III) salen
complexes have been successfully prepared and used as active
catalysts by the Jacobson group.3,4 Those homogeneous
cobalt(III)-salen complexes bearing a sulfonic acid group
(as a charge-balancing anion) exhibited high activity and
enantioselectivity in asymmetric ring opening (ARO) reactions
of racemic epoxides by various nucleophiles. However, the
immobilization of cobalt(III) salen complexes by electrostatic
interactions with alkyl and aromatic sulfonic groups via
HO3S-functionalized linkers on inorganic supports, and
the application of those heterogenized complexes in chiral
catalysis have not been reported. Our main methodology is
the direct oxidation of monomeric cobalt(II) salen by an SO3H
group linked through propyl- or phenethyl-silane via ionic pair
formation5,6 on the support to yield catalytically active chiral
cobalt(III)-SO3 salen species. In this context, mesoporous
materials containing sulfonic acid groups were chosen as
candidates for immobilization of cationic chiral salen
complexes. During the reaction between monomeric cobalt(II)
salen and HO3S-functionalized SBA-16 under air bubbling in
toluene, the cobalt(II) salen became oxidized to cobalt(III), and
the increased +1 charge of cobalt(III) in salen would be
balanced by the SO3(À1) anion linked to the tether on support
surfaces, providing a strong and stable linkage as ionic
pairs. Unreacted cobalt(II) salen ligand remained in the +2
oxidation state in solution without attachment.
For the SBA-16 samples synthesized with CSPTMS/
(CSPTMS + TEOS) molar ratio of 0.2, the XRD result (in
Fig. 1) confirmed that the CSPTMS-functionalized SBA-16
material possess a cubic ordering, as evidenced by the
appearance of the (110), (200) peaks with additional broad
peaks at 2y = 2.01. N2 adsorption/desorption isotherms
of sulfonic acid-functionalized SBA-16 are the type IV
(H2) hysteresis loop that is typical for mesoporous materials
with ordered cubic channels. The TEM image shown in
Fig. 1 also confirms the high mesoscopic order of arenesulfonic
functionalized mesoporous silicas synthesized in this work,
showing that SBA-16 has a regular cubic array of uniform
channels.
Furthermore, for the purpose of anchoring chiral salen
complexes to the inorganic supports, we investigated the
synthetic method how to obtain sulfonic acid-functionalized
SBA-16 materials by direct co-condensation of alkoxysilane
Fine Material Synthesis Lab, Department of Chemical Engineering,
Inha University, 253 Yonghyun-dong, Nam-gu, Incheon, 402-751,
South Korea. E-mail: kimgj@inha.ac.kr; Fax: +82-32-872-0959;
Tel: +82-32-860-7472
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Table 1 Physicochemical and textural properties of extracted SBA-16 samples containing sulfonic acid moieties
Molar composition Textural properties
No
Organosilane
X
Z
Pore sizea/A
SBET/m2 gÀ1
Vp/cm3 gÀ1
Sulfur contentb
Incorporation (%)
1
2
3
4
MTPES
MTPES
CSPTMS
CSPTMS
0.0204
0.0183
0.0204
0.0183
0.0204
0.0183
0.0102
0.0102
38
46
32
43
885
664
826
780
1.41
0.82
1.23
0.96
1.39
2.50
1.34
2.37
83c
75d
80c
71d
a
b
c
d
Mean pore size from adsorption branch applying the BJH model, mmol of sulfur per g-SiO2, 1.67 mmol and 3.34 mmol of sulfur per g-SiO2
(as a theoretical value).
Fig. 1 XRD pattern, N2 isotherm and TEM image of the [110] zone
axes for SBA-16 cubic mesoporous silica containing sulfonic
acid groups (20 mol% CSPTMS containing SBA-16 in the starting
mixture).
To observe the trends in the catalytic properties of new
heterogeneous chiral salens, the ARO of racemic (Æ) epichloro-
hydrin (ECH) by nucleophiles such as water and phenol
derivatives were examined. SBA-16 catalysts bearing sulfonic
acid groups were used as a support to anchor the optically
active salen complexes. The enantioselectivity of chiral cobalt-
salen complexes anchored on the mesoporous materials were
compared to those of homogeneous catalysts with a similar
structure (cata 1 and 2 in Scheme 1). The homogeneous Co(III)
salen catalysts containing MSA, CSA or NBSA anions
(catalysts 1–4) showed a relatively high activity and enantio-
selectivity as shown in Table 2.
The chiral Co(III) salen complexes attached to propylsulfonic
anion groups on SBA-16 exhibited remarkably high activity,
and they could be employed at very low loadings to obtain
the enantio-enriched products without suffering a solubility
problem or severe deactivation. The HKR of (Æ)-ECH (entries
5, 6) and (Æ)-methyl-4-[(oxiran-2-yl)methoxy]benzene acetate
(entries 9, 10) by using chiral salen catalysts led to optically
pure (S)-epoxides (498 ee%, 441% product yield). Significantly
accelerated rates in the HKR or ARO reaction of racemic
epoxides by nucleophiles were observed using a chiral
Co(III)–SO3 salen catalyst linked by arenesulfonic acid on
SBA-16 in this work.
Scheme 1
SBA-16 can avoid the necessity of catalyst separation by
evaporation. Since this is a heterogeneous catalyst, it offers
the practical advantages of facile separation and isolation of
the catalyst from liquid products. The activity of recycled
catalysts diminishes by 5% as compared to that obtained in
the first run at the third reuse in HKR of ECH (same reaction
as entries 5 and 6 in Table 2). This may be due to the
dissociation of attached Co(III) salen from the supports in
the presence of highly polar products such as 1-chrolopropane-
2,3-diol. However, high enantioselectivity (498% ee) was
obtained with prolonged reaction time. Further, the slightly
deactivated catalysts could be recovered again by treatment
As another application on new reactions, the synthesis of
chiral atenolol could be performed efficiently via ring opening
of racemic ECH with methyl-p-hydroxyphenyl acetate as
shown in Table 2. The reactions were completed within 12 h
in good yields with promising enantioselectivities up to
98% ee (entries 14, 15) using our heterogeneous chiral (A),
(B)/SBA-16 salen catalysts to produce optically pure (R)-methyl
2-(4-(3-chloro-2-hydroxypropoxy)phenyl) acetate (MCHPA).
Transformation of chiral MCHPA into the chiral atenolol
can be simply accomplished by an additional two-step treatment
(À1)
after HKR. The use of cobalt(III)–SO3
salen linked to
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Table 2 Enantioselective ring opening of terminal epoxides catalyzed by chiral Co(III) salen complexes
Entry
Nu–H
R
Catalysta
Loadingb (mol%)
t/h
ee% of epoxide (yield, %)c
1
2
3
4
5
6
H2O
H2O
H2O
H2O
H2O
H2O
CH2Cl
CH2Cl
CH2Cl
CH2Cl
CH2Cl
CH2Cl
Cata 1
Cata 2
Cata 3
Cata 4
Cata(A)/SBA-16
Cata(B)/SBA-16
0.5
0.5
0.5
0.5
0.4
0.4
9
9
9
9
7
6
95 (42)
97 (45)
98 (47)
97 (46)
98 (44)
498 (45)
7
8
H2O
H2O
Cata(A)/SBA-16
Cata(B)/SBA-16
0.6
0.5
10
9
498 (43)
98 (41)
9
CH2Cl
CH2Cl
CH2Cl
CH2Cl
CH2Cl
CH2Cl
Cata 1
Cata 2
Cata 3
Cata(A)/SBA-16
Cata(B)/SBA-16
Cata(B)/SBA-16
2.0
2.0
2.0
1.5
1.5
1.0
12
12
12
12
10
12
93 (41)
96 (43)
98 (46)
498 (44)
98 (42)
98 (43)
10
11
12
13
14
Phenol
a
b
The catalysts are shown in Scheme 1; Cata(A)/SBA-16 and Cata(B)/SBA-16 correspond to samples 2 and 4 of Table 1. Catalyst loading is
c
based on the Co unit. ee% of product was analyzed by chiral GC and HPLC. The yield is based on mol% of reactant epoxides.
with additional Co(II) salen under air in toluene via
(À1)
of monomeric chiral cobalt(III) salen to SO3 group through
propyl- or phenethyl-linkage by electrostatic interactions has
been developed to yield catalytically active sites. On the basis
of asymmetric catalysis, it is noteworthy that the chiral (salen)
complexes obtained by the present procedure can be applied as
active and effective recyclable catalysts for the ARO of
terminal epoxides by nucleophiles such as water and phenol.
reattachment of Co(III) salen as Co(III) to SO3
groups. In
other reactions; especially ring opening of racemic ECH with
phenol derivatives (entries 12–14), no significant dissociation
of cobalt-salen complexes into solution was found during the
repeated use of catalysts, with the high catalytic activity of the
fresh catalyst being retained. The catalysts could be reused
several times without further treatment after reaction in THF
solvent without significant loss in activity. This means Co(III)
salens are attached strongly and are stable during the reaction.
In addition, to verify that some of the reaction was not caused
by the contribution of dissolved homogeneous catalyst, the
solid catalyst was removed from an incomplete reaction batch
and the reaction was continued (the heterogeneous catalysts
were filtered out at ca. 20, 55 and 80 ee%, respectively, in
three different batches (for reaction 12). For each reaction,
the reaction stopped and no increase of conversion was
investigated.
Notes and references
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3375.
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1374.
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5 J. M. Fraile, J. I. Garcia, J. Massam and J. A. Mayoral, J. Mol.
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6 H. Zang, Y. Zang and C. Li, Tetrahedron: Asymmetry, 2005, 16,
2417.
In conclusion, we have demonstrated a simple procedure for
the direct synthesis of SBA-16 mesoporous silica functionalized
with alkyl and aromatic sulfonic groups. The formation of
sulfonic groups was investigated with the enhanced mesoscopic
ordering in replacing TEOS with MPTES or CSPTMS
(o20 mol%). Also, the efficient new immobilization method
7 O. Kwon, S. Park and G. Seo, Chem. Commun., 2007, 4113.
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This journal is The Royal Society of Chemistry 2009
4298 | Chem. Commun., 2009, 4296–4298