Magnetically recoverable SiO2-coated Fe3O4 nanoparticles: A new platform for asymmetric transfer hydrogenation of aromatic ketones in aqueous medium

Article abstract of DOI:10.1039/c0cc03730c

A magnetically recoverable chiral rhodium catalyst exhibited excellent catalytic activity and enantioselectivity in asymmetric transfer hydrogenation of aromatic ketones in aqueous medium, which could be recovered easily via a small magnet and used repetitively ten times without obviously affecting its enantioselectivity.

Full text of DOI:10.1039/c0cc03730c

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 2583–2585
www.rsc.org/chemcomm
COMMUNICATION
Magnetically recoverable SiO2-coated Fe3O4 nanoparticles: a new
platform for asymmetric transfer hydrogenation of aromatic ketones in
aqueous mediumw
Yunqiang Sun, Guohua Liu,* Hongyuan Gu, Tianzeng Huang, Yuli Zhang and Hexing Li*
Received 8th September 2010, Accepted 29th November 2010
DOI: 10.1039/c0cc03730c
A magnetically recoverable chiral rhodium catalyst exhibited
excellent catalytic activity and enantioselectivity in asymmetric
transfer hydrogenation of aromatic ketones in aqueous medium,
which could be recovered easily via a small magnet and
used repetitively ten times without obviously affecting its
enantioselectivity.
Directly anchoring homogeneous chiral catalysts on
magnetic nanoparticles can provide a more convenient strategy
to tailor the uniform shape of catalysts, making recovery and
reuse much easier via an suitable external magnetic field.6–8
More importantly, immobilization of a homogeneous chiral
catalyst on the outside surface of a rigid nanoparticle can keep
the same chiral microenvironment as a homogeneous catalyst
and can maintain excellent stereocontrol performance.
However, potential magnetic aggregation of bare magnetic
nanoparticles7 and magnetic loss of magnetic mesocellular8
are still challenges for practical application. Herein, we
develop a convenient method for preparing a magnetically
recoverable chiral rhodium catalyst through direct complexation
of [Cp*RhCl2]2 with (S,S)-TsDPEN-modified SiO2-coated
Fe3O4 nanoparticles, and apply it to asymmetric transfer
hydrogenation of aromatic ketones in aqueous medium. The
key feature is that this SiO2-coated magnetic nanoparticle can
not only act as a new platform to anchor different types of
chiral catalysts for asymmetric catalysis but also can greatly
improve dispersibility of active species to decrease aggregation
of active catalysts. In particular, the tailored structure and
surface chemistry of SiO2-coated magnetic nanoparticles are a
benefit to build an analogously homogeneous chiral micro-
environment and to keep an excellent catalytic efficiency.
The chiral rhodium catalyst anchored on the magnetic
nanoparticles, denoted as Cp*-Rh-TsDPEN-MNPs (5), was
prepared according to the procedures illustrated in Scheme 1.
Firstly, the SiO2-coated Fe3O4 magnetic nanoparticles (3) with
ca. 400 nm average diameter were synthesized readily via a
Asymmetric catalysis in environmentally friendly processes
represents an important branch of green chemistry, which
can greatly reduce pollution.1 Learning from enzyme catalysis
in biochemical processes, most studies are focused on
homogeneous chiral catalysts. For example, chiral Z5-Cp*–M
(M = Ru, Rh and Ir) complexes bearing N-sulfonylated 1,2-
diamines have been widely used in asymmetric hydrogenation
of ketones2 and some of them can significantly accelerate
asymmetric transfer hydrogenation of ketones in aqueous
medium.3 However, their practical applications are still
hindered due to difficulties in separation and reuse, leading
to a high cost and even a heavy pollution from metallic ions.
Although immobilized homogeneous catalysts on general
polymer or inorganic materials4 have exhibited obvious
advantages of easy separation and efficient recycling, most
catalysts still suffer from lower catalytic efficiencies than
corresponding homogeneous catalysts owing to uncontrollable
hindrance and changes in the chiral microenvironment of
active sites. Previously, we reported a series of heterogeneous
catalysts anchored on ordered silica supports,5 some of them
had exhibited excellent enantioselectivities in asymmetric
catalysis because of ordered mesoporous channels and good
compatibility of surface functionalities. However, the irregular
shape with long pore channels of silica supports will inevitably
decrease the catalytic efficiencies and also add a problem in
catalyst separation. Thus, based on highly catalytic efficiency,
giving consideration to highly efficient recycling of catalyst has
attracting extensive research interest.
similar reported solvothermal method,9 followed by
a
SiO2-coated procedure.10 (S,S)-TsDPEN-derived ligand 211
was then grafted onto the magnetic nanoparticles. Finally,
The Key Laboratory of Resource Chemistry of Ministry of Education,
Shanghai Normal University, Shanghai, China.
E-mail: ghliu@shnu.edu.cn, HeXing-Li@shnu.edu.cn;
Tel: +86 21 64321819
w Electronic Supplementary Information (ESI) available: [Experimental
procedures and analytical data for obtained chiral alcohols]. See
http://dx.doi.org/10.1039/c0cc03730c/
Scheme 1 Preparations of the catalyst 5.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2583–2585 2583

View Article Online
Table 1 Asymmetric transfer hydrogenation of aromatic ketonesa
Entry Ar
Catalyst
Conv. (%)b ee (%)b
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
5
97.2
98.5c
86.1
97.7
97.3c
84.0
96.1
93.2
92.4
92.4
90.8
90.1
Cp*RhTsDPEN
4 + [Cp*RhCl2]2
3 + Cp*RhTsDPEN 80.3
5
5
5
5
5
Fig. 1 (a) Wide-angle powder XRD patterns of 3 and 5. (b) XPS
spectra of Cp*RhTsDPEN and 5.
4-ClPh
4-FPh
4-MePh
4-OMePh
2-Naphthyl
99.2
97.8
97.6
97.8
99.8
the catalyst 5 was successfully obtained as a brown powder via
directly complexing 4 with [Cp*RhCl2]2 using NEt3 as a basic
reagent (see ESIw). The inductively coupled plasma (ICP)
optical emission spectrometer analysis showed that the Rh
loading amount in the catalyst 5 was 16.92 mg (0.164 mmol)
per gram of catalyst, which was consistent with the 1 : 2 mole
ratio of Rh to N atom confirmed by elemental analysis that
calculated a mass % of N (0.46%) with 0.329 mmol content.
Wide-angle XRD patterns (Fig. 1a) showed that, after being
immobilized with rhodium complex, the typical diffraction
peaks indicative of Fe3O4 were still clearly observed,12
suggesting that the surface modification had no significant
effect on the nature of Fe3O4. An FT-IR spectrum of the
catalyst 5 (Fig. S1w) demonstrated successful incorporation of
them onto the magnetic nanoparticles, in which relatively
weak absorption bands around 3100–2800 cmÀ1 for n(C–H)
and 1594–1445 cmÀ1 for n(CQC) were derived from the
organometallic rhodium complex.13 Scanning electron micro-
scopy (SEM) revealed that the catalyst 5 presented the
uniform size of nanoparticles with average diameter around
400 nm (Fig. 2a). Transmission electron microscopy (TEM)
further confirmed that this core–shell catalyst 5 was encapsulated
by a thin silica layer of B30 nm in thickness (Fig. 2b). The
magnetization curves (Fig. 2c) showed that 3 and catalyst 5
were superparamagnetic and had the magnetization saturation
values of 132.8 and 58.9 emu gÀ1, in which catalyst 5 was
readily dispersible in the reaction system and could be easily
separated with a small magnet near the bottle (Fig. 2d).
Table 1 summarizes catalytic performances in asymmetric
transfer hydrogenation of aromatic ketones. In general, high
conversions and high enantioselectivities were obtained for all
tested aromatic ketones. Taking acetophenone as an example
(entry 1), it was found that 97.2% conversion and 97.7% ee
value of (S)-1-phenyl-1-ethanol were obtained with 1 h
reaction time, which was comparable to that of the parent
catalyst (entry 1 versus entry 2). The high catalytic activity of
a
Reaction conditions: catalysts (4.0 mmol of Rh, based on the ICP
analysis), HCO2Na (0.68 g, 10.0 mmol), Bu4NBr (0.29 g, 0.80 mmol),
ketone (2.0 mmol) and 2.0 mL water, reaction temperature (40 1C),
reaction time (1.0 h). Determined by chiral GC or HPLC analysis
c
(Fig. S3). Data were obtained using the homogeneous catalyst.
b
the catalyst 5 could be attributed to the high dispersion of
rhodium active sites onto the outer surface of the SiO2-coated
Fe3O4 nanoparticles. While, the high enantioselectivity
confirmed that the homogeneous microenvironment could
be preserved after being immobilized. This behaviour was
proved by the XPS spectra (Fig. 1b), in which the same
binding energies of catalyst 5 as Cp*RhTsDPEN3a and
an obvious difference from [Cp*RhCl2]2 were observed
(Fig. S2w).
To gain better insight into the nature of catalyst 5 and to
eliminate the non-covalent adsorption on catalytic process,
two control experiments were carried out using 4 plus
[Cp*RhCl2]2 and 3 plus Cp*RhTsDPEN as chiral catalysts.
It was found that the former afforded the corresponding
alcohol with 86.1% conversion and 84.0% ee value (entry 3),
while the latter gave the corresponding alcohol with 80.3%
conversion and 96.1% ee value (entry 4). The former suggested
that the catalyst synthesized by an in situ postmodification
method could result in a medium catalytic performance.
Lower catalytic activity and enantioselectivity than catalyst 5
may be due to the fact that the small part of [Cp*RhCl2]2 had
not been coordinated on the catalytic process, in which the
part of loss of Rh was detected by ICP analysis in solution.
The latter indicated that the chiral microenvironment had not
been changed from non-covalent adsorption. Low conversion
suggested that part of the catalyst did not participate in the
catalytic reaction due to the jam from physical absorption.
Complete disappearance of activity and enantioselectivity
indicated that the non-covalent adsorption in the catalytic
process could be eliminated when employing the catalyst after
Soxhlet extraction (3 plus Cp*RhTsDPEN). This deduction
was proved by ICP analysis (nearly the same loading amount
of Rh was detected in solution after Soxhlet extraction).
An important feature of the heterogeneous catalyst 5 is that
it is convenient to recover and recycle. As shown in Fig. 2d, the
catalyst 5 was quantitatively recovered via a small magnet.
Meanwhile, in ten consecutive reactions the recycling of
catalyst 5 still afforded 91.9% conversion and 92.5% ee value
using acetophenone as a substrate (Fig. 3).
Fig. 2 (a) SEM image, (b) TEM image and (c) magnetization curves
of 3 and the catalyst 5 measured at 300 K. (d) The separation–
redispersion process of the catalyst 5.
c
2584 Chem. Commun., 2011, 47, 2583–2585
This journal is The Royal Society of Chemistry 2011

View Article Online
J. Bacsa and J. L. Xiao, Chem.–Eur. J., 2008, 14, 7699;
(c) P. N. Liu, J. G. Deng, Y. Q. Tu and S. H. Wang, Chem.
Commun., 2004, 2070; (d) G. Li, X. F. Wu, W. P. Chen,
F. Hancock, F. King and J. L. Xiao, Org. Lett., 2004, 6, 3321;
C. Wang, C. Q. Li, X. F. Wu, A. Pettman and J. L. Xiao, Angew.
Chem., Int. Ed., 2009, 48, 6524; (e) N. A. Cortez, G. Aguirre,
M. Parra-Hake and R. Somanathan, Tetrahedron Lett., 2009, 50,
2228.
4 (a) J. M. Thomas and R. Raja, Acc. Chem. Res., 2008, 41, 708;
(b) M. Heitbaum, F. Glorius and I. Escher, Angew. Chem., Int. Ed.,
2006, 45, 4732; (c) Z. Wang, G. Chen and K. L. Ding, Chem. Rev.,
2009, 109, 322.
Fig. 3 Reusability of 5 using acetophenone as a substrate.
5 (a) G. H. Liu, J. Y. Wang, T. Z. Huang, X. H. Liang, Y. L. Zhang
and H. X. Li, J. Mater. Chem., 2010, 20, 1970; (b) G. H. Liu,
M. Yao, F. Zhang, Y. Gao and H. X. Li, Chem. Commun., 2008,
347; (c) G. H. Liu, M. Yao, J. Y. Wang, X. Q. Lu, M. M. Liu,
F. Zhang and H. X. Li, Adv. Synth. Catal., 2008, 350, 1464;
(d) G. H. Liu, M. M. Liu, Y. Q. Sun, J. Y. Wang, C. S. Sun and
H. X. Li, Tetrahedron: Asymmetry, 2009, 20, 240; (e) J. L. Huang,
F. X. Zhu, W. H. He, F. Zhang, W. Wang and H. X. Li, J. Am.
Chem. Soc., 2010, 132, 1492.
6 (a) S. L. Brock and K. Senevirathne, J. Solid State Chem., 2008,
181, 1552; (b) B. Panella, A. Vargas and A. Baiker, J. Catal., 2009,
261, 88; (c) G. Chouhan, D. Wang and H. Alper, Chem. Commun.,
2007, 4809; (d) M. L. Kantam, J. Yadav, S. Laha, P. Srinivas,
B. Sreedhar and F. Figueras, J. Org. Chem., 2009, 74, 4608.
7 (a) A. Hu, T. Gordon, G. T. Yee and W. Lin, J. Am. Chem. Soc.,
2005, 127, 12486; (b) O. Gleeson, R. Tekoriute, Y. K. Gun’ko and
S. J. Connon, Chem.–Eur. J., 2009, 15, 5669.
8 (a) J. Li, Y. M. Zhang, D. F. Han, Q. Gao and C. Li, J. Mol. Catal.
A: Chem., 2009, 298, 31; (b) D. Lee, J. Lee, H. Lee, S. Jin, T. Hyeon
and B. M. Kim, Adv. Synth. Catal., 2006, 348, 41.
9 X. Q. Xu, C. H. Deng, M. X. Gao, W. J. Yu, P. Y. Yang and
X. M. Zhang, Adv. Mater., 2006, 18, 3289.
10 Y. H. Deng, D. W. Qi, C. H. Deng, X. M. Zhang and D. Y. Zhao,
J. Am. Chem. Soc., 2008, 130, 28.
11 P. N. Liu, P. M. Gu, F. Wang and Y. Q. Tu, Org. Lett., 2004, 6,
169.
In conclusion, we supplied a facile approach to prepare a
magnetically recoverable rhodium catalyst, which exhibited
excellent catalytic activities (up to 99%) and enantioselectivities
(up to 97% ee) in the asymmetric transfer hydrogenation of
aromatic ketones. In particular, the heterogeneous catalyst
could be recovered easily by using a magnet, in which neither
filtration nor extraction was necessary. The recycling catalyst
(reused 10 times) still showed high catalytic efficiency without
obviously affecting its enantioselectivity, showing good
potential for industrial application.
We are grateful to China National Natural Science
Foundation (20673072), Shanghai Sciences and Technologies
Development Fund (10dj1400103 and 10jc1412300) and
Shanghai Municipal Education Commission (08YZ71 and
S30406) for financial support.
Notes and references
1 P. J. Walsh, H. M. Li and C. A. de Parrodi, Chem. Rev., 2007, 107,
2503.
2 (a) T. Ikariya and A. J. Blacker, Acc. Chem. Res., 2007, 40, 1300;
(b) S. E. Clapham, A. Hadzovic and R. H. Morris, Coord. Chem.
Rev., 2004, 248, 2201; (c) J. E. D. Martins, G. J. Clarkson and
M. Wills, Org. Lett., 2009, 11, 847; (d) R. Malacea, R. Poli and
E. Manoury, Coord. Chem. Rev., 2010, 254, 729.
12 D. H. Zhang, G. D. Li, J. X. Li and J. S. Chen, Chem. Commun.,
2008, 3414.
13 (a) A. S. M. Chong and X. S. Zhao, J. Phys. Chem. B, 2003, 107,
12650; (b) Q. Y. Hu, J. E. Hampsey, N. Jiang, C. J. Li and
Y. F. Lu, Chem. Mater., 2005, 17, 1561; (c) G. Chaplais,
J. L. Bideau, D. Leclercq and A. Vioux, Chem. Mater., 2003, 15,
1950.
3 (a) X. F. Wu, X. H. Li, A. Zanotti-Gerosa, A. Pettman, J. Liu,
A. J. Mills and J. L. Xiao, Chem.–Eur. J., 2008, 14, 2209;
(b) X. F. Wu, J. Liu, D. D. Tommaso, J. A. Iggo, C. R. Catlow,
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2583–2585 2585