Magnetic nanoparticle supported polyoxometalates (POMs) via non-covalent interaction: Reusable acid catalysts and catalyst supports for chiral amines

Article abstract of DOI:10.1039/c1cc14178c

Magnetic polyoxometalates (POMs) are obtained by a simple sonication between functionalized magnetic nanoparticles and polyoxometalates. This material can be used not only as a highly active acid catalyst, but also as a catalyst support for chiral amines.

Full text of DOI:10.1039/c1cc14178c

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COMMUNICATION
Magnetic nanoparticle supported polyoxometalates (POMs) via
non-covalent interaction: reusable acid catalysts and catalyst supports
for chiral aminesw
Xiaoxi Zheng, Long Zhang, Jiuyuan Li, Sanzhong Luo* and Jin-Pei Cheng
Received 12th July 2011, Accepted 30th September 2011
DOI: 10.1039/c1cc14178c
Magnetic polyoxometalates (POMs) are obtained by a simple
sonication between functionalized magnetic nanoparticles and
polyoxometalates. This material can be used not only as a highly
active acid catalyst, but also as a catalyst support for chiral
amines.
area (1–5 m2 gÀ1),8 their immobilization on large surface area
materials are important to achieve better performance in many
heterogeneous catalytic applications.5 Various solid supports
such as silica, active carbon and mesoporous molecular sieve,
have been used to immobilize POMs. However, to the best of
our knowledge, MNPs have not been used as POM supports.
Herein, we report the non-covalent immobilization of POMs
on MNPs as efficient acid catalysts for the Friedel–Crafts
reaction of indole and chalcones. Furthermore, the MNP-
POMs can also serve as a support for chiral amine catalysts.
The MNP-supported POM catalysts were prepared via the
well-applied ‘‘acid–base’’ strategy. Since MNPs are acid sensitive,
they were first protected with a 1–2 nm thickness silica layer.9
The obtained SiO2-MNP was then treated with the commercial
(3-aminopropyl)triethoxysilane in refluxing toluene to afford
MNP-1.3b–d The loading of amino groups on MNP-1 was shown
to be 1.46 mmol gÀ1 as determined by elemental analysis.
Following the same procedure, MNP-2 was also prepared and
the loading of imidazolium group was determined to be
0.60 mmol gÀ1 (Scheme 1).3b The immobilization of POM on
MNP-1 or MNP-2 was achieved by sonication of a mixture of
MNPs (MNP-1 or MNP-2) and POM (H3PW12O40, 1.1 equiv.)
in dry THF for 1 h. The MNP supported POMs (MNP-1-PW
and MNP-2-PW) were isolated by magnetic decantation and
washed twice with THF. The molar ratio of functional group
(amino or imidazolium group) and POM was determined to be
nearly 1 : 1 in the immobilized catalysts. The magnetic core was
analyzed by XRD and the observed diffraction pattern is consistent
with the JCPDS database for magnetite (Fig. 1a). The room-
temperature magnetization curves (Fig. 1b) proved that the
magnetic nanoparticles are superparamagnetic.
Magnetic nanoparticles (MNPs), which are available from
cheap materials via simple synthesis and easily tunable by
structural surface modifications, have drawn much attention
in the past few decades due to their vast applications including
drug delivery, biosensors, enzyme immobilization, environ-
mental remediation and so on.1 Recently, their application as
catalyst supports has become a hot topic due to their unique
properties such as high surface area, good dispersion, super-
paramagnetic behavior as well as low toxicity.2 The most
attractive feature of MNP-supported catalysts is that they
can be recycled by simple magnetically driven separation,
thereby eliminating the requirement of catalyst filtration and
centrifugation. Furthermore, improved activity is usually
achieved in the nanometre-sized supported catalyst due to
the high surface area and good dispersion properties. In the
past few years, several transition-metal catalysts,2a enzyme
catalysts2a as well as organocatalysts3 have been successfully
immobilized on MNPs with good catalytic activity and reusability.
Up to now, most of these MNP-supported catalysts employed
covalent immobilization strategies, requiring additional synthetic
manipulation in most cases. Non-covalent immobilization,
endowed with the merits such as easy modification and combi-
natorial flexibility, has been much less explored with MNPs.4
Due to their strong acidity and favorable redox properties,
polyoxometalates (POMs) and their derivatives have been
widely used in organic synthesis and catalytic reactions as acid
and oxidation catalysts.5 Meanwhile, their applications as
catalyst supports have also been explored and good reusability
could be realized by taking advantage of their large frame-
work.6,7 However, since bulk POMs have low specific surface
With the immobilized POM catalysts in hand, the Friedel–Crafts
reaction of indole and chalcone was chosen as a model reaction
to examine the acidic catalytic activity and reusability. To our
Beijing National Laboratory for Molecular Sciences (BNLMS),
CAS Key Laboratory of Molecular Recognition and Function,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
P. R. China. E-mail: luosz@iccas.ac.cn; Fax: (+86)10-62554449
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc14178c
Scheme 1 Preparation of MNP-1-PW and MNP-2-PW.
Chem. Commun., 2011, 47, 12325–12327 12325
c
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Fig. 1 (a) XRD of the magnetic core (Fe3O4); (b) magnetic curve of
MNP-1 (solid line) and MNP-1-PW (dashed line).
Fig. 2 Reusability of MNP-1-PW (black columns) and MNP-2-PW
(gray columns). Reaction conditions: indole (0.25 mmol), chalcone
(0.20 mmol), THF (0.2 mL), catalyst (5 mol%), 12 h.
delight, the reaction proceeds very well in many solvents and
the best results were obtained when using THF as solvent
(Table S2, ESIw). With the optimized conditions, the substrate
scopes were next explored. As shown in Table 1, the reactions
between different chalcones and indoles proceeded smoothly
to give the desired products with good yields.
The recyclabilities of MNP-1-PW and MNP-2-PW were
then evaluated. After each run, CH2Cl2 was added to dilute the
reaction mixture and the organic layer was simply decanted with
the assistance of a small magnet. The catalyst was quickly
concentrated to the side wall of the reaction vial once a magnet
is placed nearby. After a simple wash by CH2Cl2 and subsequent
vacuum removal of residual solvent, the catalyst was reused for
the next run. Both magnetic POMs showed excellent activity and
reusability for 12 recycles (Fig. 2).10
Scheme 2 Preparation of magnetic POM supported chiral amine
catalyst MNP-1-PW-A.
shown in Scheme 2) in the hybrids was confirmed by IR (see
Fig. S3, ESIw). In spite of the multidentate nature of chiral
amines and POM, the hybrid magnetic solid remains as
nanometer sized particles (Fig. S4, ESIw) and is well dispersible
in organic solvents such as DCM (Fig. 3c). The loading of the
catalyst was 0.55 mmol gÀ1 as determined by elemental
analysis.
In our previous studies, it was found that the POMs were
efficient catalyst supports for chiral amine catalysts via non-
covalent interaction.7 However, the POM-supported chiral
amines generally showed limited reusability due to the loss
of catalyst upon biphasic recycle and reuse. Bearing in mind
the excellent reusability of MNP supported catalysts, we have
tested magnetic POMs as non-covalent supports for chiral
amines. As shown in Scheme 2, chiral amine hybrids with
magnetic POMs can be easily prepared by mixing MNP-PW and
chiral amine via sonication in dry THF. The resulting hybrid
solid was washed with THF and dried under vacuum at room
temperature. The presence of chiral amine (compound A as
The MNP-PW-supported chiral amine catalysts were next
evaluated in typical enamine-based asymmetric direct aldol
reactions. With the identified optimal catalyst MNP-1-PW-A
(see ESIw for the screening results),11 the reaction scope was
then examined with a series of aldehyde acceptors and aldol
donors using the optimized catalyst. In the presence of 5 mol%
of MNP-1-PW-A, acetone reacted with various aromatic
aldehydes to afford the desired products with high yields and
enantioselectivities (Table 2). Other aldol donors such as
Table 1 Substrate scope of Friedel–Crafts reactionsa
MNP-1-PW
MNP-2-PW
R1
R2
t/h
Yieldb (%)
t/h
Yieldb (%)
Entry
1
2
3
4
5
6
7
8
H
H
H
H
H
H
4-Cl
4-Me
5-MeO
5-Me
5-Br
5-Cl
5-I
6-Cl
H
H
20
20
7
7
7
7
12
12
93
96
98
97
99
98
90
88
20
20
6
6
6
6
12
12
94
96
97
97
98
98
92
90
Fig. 3 Catalyst recycling: (a) Reaction conditions: 4-nitrophenylaldehyde
(0.25 mmol), acetone (0.20 mL), MNP-1-PW-A (5 mol%), 13 h.
(b) TEM image of catalyst MNP-1-PW-A after 10 cycles; (c) MNP-1-
PW-A dispersed in CH2Cl2; (d) catalyst separation by a small magnet.
a
Reaction conditions: Catalyst (5 mol%), indole derivative (0.25 mmol),
b
chalcone derivative (0.20 mmol), THF (0.2 mL). Isolated yield.
c
12326 Chem. Commun., 2011, 47, 12325–12327
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Table 2 MNP-1-PW-A catalyzed aldol reaction of acetonea
realized via non-covalent assembly with POMs as a non-
covalent anchor. The resulted non-covalently assembled
catalyst could be reused up to 11 times with essentially no
loss of activity and enantioselectivity.
We gratefully acknowledge the Natural Science Foundation of
China (NSFC 20972163 and 21025208), MOST (2011CB808600)
and CAS for financial support.
Entry
R
t/h
Yieldb (%)
ee (%)c
1
2
3
4
5
6
2-NO2C6H4
3-NO2C6H4
4-CF3C6H4
4-CNC6H4
4-ClC6H4
12
12
30
30
48
48
83
87
81
80
72
77
89
89
90
87
87
88
Notes and references
1 For reviews, see: (a) A.-H. Lu, E. L. Salabas and F. Schuth, Angew.
Chem., Int. Ed., 2007, 46, 1222; (b) J. Fan and Y. Gao, J. Exp.
Nanosci., 2006, 1, 457; (c) Y.-W. Jun, J.-S. Choi and J. Cheon,
Chem. Commun., 2007, 1203; (d) R. Banerjee, Y. Katsenovich,
L. Lagos, M. Mclintosh, X. Zhang and C.-Z. Li, Curr. Med.
Chem., 2010, 17, 3120.
2-BrC6H4
a
Reaction conditions: Catalyst (5 mol%), acetone (0.20 mL), aldehyde
b
c
(0.25 mmol). Isolated yield. Determined by chiral HPLC.
cyclohexanone and cyclopentanone also worked very well in
this catalytic system (Table 3). For comparison, the results
obtained from the same reactions using POM-chiral amine
hybrid PW-A are also listed in Table 3 (entries 1 vs. 2). The
magnetic POM supported catalyst MNP-1-PW-A showed
slightly higher stereoselectivity and enantioselectivity albeit
with a little loss of activity.
2 For recent reviews, see: (a) V. Polshettiwar, R. Luque, A. Fihri,
H. Zhu, M. Bouhrara and J.-M. Basset, Chem. Rev., 2011,
111, 3036; (b) K. V. S. Ranganath and F. Glorius, Catal. Sci.
Technol., 2011, 1, 13; (c) S. Shylesh, V. Schunemann and
¨
W. R. Thiel, Angew. Chem., Int. Ed., 2010, 49, 3428;
(d) A. Schatz, O. Reiser and W. J. Stark, Chem.–Eur. J., 2010,
16, 8950; (e) Y. Zhu, L. P. Stubbs, F. Ho, R. Liu, C. P. Ship,
J. A. Maguire and N. S. Hosmane, ChemCatChem, 2010, 2, 365;
(f) C. W. Lim and I. S. Lee, Nano Today, 2010, 5, 412.
The recyclability of MNP-1-PW-A was examined using the
model reaction between acetone and 4-nitrobenzylaldehyde.
The process of the catalyst recycling is the same as the MNP-1-
PW in the Friedel–Crafts reaction. To our delight, the non-
covalently assembled magnetic chiral amine catalyst showed
excellent reusabilities. The yield and enantioselectivity was
maintained at similar level even after 11 cycles (Fig. 3a), which
is significantly improved comparing with POM supported
chiral amines. The TEM image of the catalyst after 10 recycles
indicated that the catalyst is quite robust and no significant
aggregation was found (Fig. 3b).
3 For recent examples, see: (a) B. Karimi and E. Farhangi, Chem.–Eur.
J., 2011, 17, 6056; (b) P. Riente, C. Mendoza and M. A. Pericas,
´
J. Mater. Chem., 2011, 21, 7350; (c) X. X. Zheng, S. Z. Luo, L. Zhang
and J.-P. Cheng, Green Chem., 2009, 11, 455; (d) B. G. Wang,
B. C. Ma, Q. Wang and W. Wang, Adv. Synth. Catal., 2010,
352, 2923; (e) S. Z. Luo, X. X. Zheng and J.-P. Cheng, Chem.
Commun., 2008, 5719; (f) S. Z. Luo, X. X. Zheng, H. Xu, X. L. Mi,
L. Zhang and J.-P. Cheng, Adv. Synth. Catal., 2007, 349, 2431.
4 For
a recent non-covalent immobilization on MNPs, see:
S. Wittmann, A. Shatz, R. N. Grass, W. J. Stark and O. Reiser,
¨
Angew. Chem., 2010, 122, 1911 (Angew. Chem., Int. Ed., 2010,
49, 1867).
5 For reviews, see: (a) I. V. Kozhevnikov, Catalysis by Polyoxo-
metalates, Wiley, Chichester, UK, 2002; (b) R. Neumann, In
In summary, we have successfully developed, for the first
time, a novel type of non-covalently immobilized POM catalysts
using magnetic nanoparticles as supports. These immobilized
acid catalysts showed excellent catalytic capabilities in Friedel–Crafts
reactions of indoles and could be reused for at least 11 times.
In addition, the immobilization of chiral amines on MNPs was
Modern Oxidation Methods, ed. J. E. Backvall, Wiley-VCH, Wein-
¨
heim, 2004, pp. 223–251; (c) C. L. Hill, In Comprehensive Coordi-
nation Chemistry II, ed. A. G. Wedd, Elsevier, Oxford, 2004, vol. 4,
pp. 679–759; (d) I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171;
(e) N. Mizuno and M. Misono, Chem. Rev., 1998, 98, 199;
(f) M. Sadakane and E. Steckhan, Chem. Rev., 1998, 98, 219;
(g) T. Ueda and H. Kotsuki, Heterocycles, 2008, 76, 73.
6 For recent applications of POMs supported catalysis, see:
(a) C. Boglio, G. Lemiere, B. Hasenknopf, S. Thorimbert,
E. Lacote and M. Malacria, Angew. Chem., 2006, 118,
3402–3405 (Angew. Chem., Int. Ed., 2006, 45, 3324);
(b) R. Augustine, S. Tanielyan, S. Anderson and H. Yang, Chem.
Commun., 1999, 1257; (c) R. L. Augustine, P. G. N. Mahata,
C. Reyes and S. K. Tanielyan, J. Mol. Catal. A: Chem., 2004,
216, 189.
Table 3 MNP-1-PW-A catalyzed aldol reaction of various aldol
donorsa
7 For a recent review of non-covalent immobilization of organo-
catalysts, see: (a) L. Zhang, S. Z. Luo and J.-P. Cheng, Catal. Sci.
Technol., 2011, 1, 507. For non-covalently supported asymmetric
organocatalysis with POMs, see: (b) S. Z. Luo, J. Y. Li, H. Xu,
L. Zhang and J.-P. Cheng, Org. Lett., 2007, 9, 3675; (c) J. Y. Li,
S. S. Hu, S. Z. Luo and J.-P. Cheng, Eur. J. Org. Chem., 2009, 132;
(d) J. Y. Li, S. Z. Luo and J.-P. Cheng, J. Org. Chem., 2009,
74, 1747; (e) J. Y. Li, X. Li, P. X. Zhou, L. Zhang, S. Z. Luo and
J.-P. Cheng, Eur. J. Org. Chem., 2009, 4486.
8 Y. Izumi, K. Urabe and M. Onaka, Zeolite, Clay and Heteropoly
Acid In Organic Reactions, Kodansha/VCH, Tokyo, 1992, p. 99.
9 M. Shokouhimehr, Y. Piao, J. Kim, Y. Jang and T. Hyeon, Angew.
Chem., Int. Ed., 2007, 46, 7039.
10 The use of 3 mol% of catalyst (MNP-1-PW) has also been
examined in the Friedel–Crafts reaction, showing relatively lower
yield in 15 h but still good recyclability: 1st run, 78% yield; 2nd
run, 73% yield; 3rd run, 75% yield.
11 MNP-2-PW-A demonstrated similar performance in the aldol
reaction and the results are listed in Tables S4 and S5 (ESIw).
Entry
n
R
t/h Yieldb (%) syn/antic
eed (%)
1
1
1
1
1
1
1
2
2
2
2
2
4-NO2C6H4
4-NO2C6H4
2-NO2C6H4
3-NO2C6H4
4-CF3C6H4
4-ClC6H4
2-NO2C6H4
3-NO2C6H4
4-NO2C6H4
4-CF3C6H4
4-ClC6H4
5
6
5
97
86
97
96
88
86
98
97
97
93
92
6 : 94
23 : 77
24 : 76
16 : 84
17 : 83
20 : 80
13 : 87
14 : 86
14 : 86
13 : 87
17 : 83
97
95
98
98
97
96
99
98
97
98
98
2e
3
4
5
6
7
8
9
10
11
5
12
48
8
8
6
11
48
a
Reaction conditions: Catalyst (5 mol%), ketone (0.20 mL), aldehyde
b
(0.25 mmol). Isolated yield. Determined by chiral HPLC. Deter-
e
mined by chiral HPLC. PW-A (1 mol%) was employed.
c
d
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 12325–12327 12327