Magnetically recoverable chiral catalysts immobilized on magnetite nanoparticles for asymmetric hydrogenation of aromatic ketones

Article abstract of DOI:10.1021/ja053881o

Novel heterogenized asymmetric catalysts were synthesized by immobilizing preformed Ru catalysts on magnetite nanoparticles via the phosphonate functionality and were characterized by a variety of techniques, including TEM, magnetization, and XRD. These nanoparticle-supported chiral catalysts were used for enantioselective heterogeneous asymmetric hydrogenation of aromatic ketones with very high enantiomeric excess values of up to 98.0%. The immobilized catalysts were easily recycled by magnetic decantation and reused for up to 14 times without loss of activity and enantioselectivity. Orthogonal nature of the present catalyst immobilization approach should allow the design of other superparamagnetic nanoparticle-supported asymmetric catalysts for a wide range of organic transformations. Copyright

Full text of DOI:10.1021/ja053881o

Published on Web 08/17/2005
Magnetically Recoverable Chiral Catalysts Immobilized on Magnetite
Nanoparticles for Asymmetric Hydrogenation of Aromatic Ketones
Aiguo Hu,† Gordon T. Yee,‡ and Wenbin Lin*,†
Department of Chemistry, CB #3290, UniVersity of North Carolina, Chapel Hill, North Carolina 27599, and
Department of Chemistry, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061-0212
Received June 13, 2005; E-mail: wlin@unc.edu
Scheme 1. Immobilization of Chiral Ru Catalyst on Magnetite
Nanoparticles
Asymmetric catalysis provides a powerful method for the
synthesis of optically active molecules that serve as precursors to
pharmaceutically important compounds.1 Although numerous highly
selective chiral catalysts have been developed in the past three
decades, their practical applications in industrial processes are
hindered by their high costs as well as difficulties in removing trace
amounts of toxic metals from the organic products. To overcome
these problems, many different approaches have been used to
generate heterogenized asymmetric catalysts.2 The heterogenized
catalysts are, however, typically less effective than their homoge-
neous counterparts. There thus exists a need to develop new,
innovative approaches toward the design of recoverable and reusable
asymmetric catalysts.
Crystalline nanoparticles with appropriate surface coatings are
readily dispersible in organic solvents. Owing to their small sizes,
nanoparticles tend to have very high surface areas. Spherical
nanoparticles of 10 nm diameter, for example, have a calculated
surface area of 600 m2/cm3, which is comparable to that of many
porous supports for catalyst immobilization. Nanoparticles can thus
be used as novel supports to prepare “heterogenized” asymmetric
catalysts that are more accessible to the reactants. Aerogel-derived
nanocrystalline MgO was recently used to prepare a nanohetero-
geneous catalyst for Claisen-Schmidt condensation of benzalde-
hydes with acetophenones to yield chalcones, which were converted
to chiral epoxy ketones by subsequent asymmetric epoxidation
reactions.3 BINAP-capped Pd nanoparticles were recently shown
to catalyze the asymmetric hydrosilylation of styrene to afford chiral
alcohols.4 The difficulty in recovering nanoparticle-supported
asymmetric catalysts by settling or filtration, however, precludes
their widespread applications.5
DMF at elevated temperatures. Immobilization of 1 on MNPs was
carried out by ultrasonicating a mixture of 1 and MNPs in anhydrous
methanol at room temperature for 1 h.10 The MNPs coated with 1
(designated as 1-MNP1 and 1-MNP2) were isolated by magnetic
decantation and washed with methanol three times. The loading of
1 on MNPs was estimated by quantifying the amount of 1 remaining
in the solution using 31P NMR spectroscopy.
Transmission electron microscopy (TEM) images showed slight
aggregation of MNPs after immobilization of 1 (Figure 1),
presumably because the [Ru(BINAP-PO3)(DPEN)Cl2] moieties on
the MNP surfaces are less effective in preventing the aggregation
of the MNPs (upon solvent evaporation) than oleic acid. Powder
X-ray diffraction (XRD) patterns confirmed that both MNPs were
magnetite. The average sizes of MNPs were estimated using
Scherrer’s equation, and they did not change upon the immobiliza-
tion of 1. The average diameter is 8.9 nm for 1-MNP1, while the
average diameter is 6.6 nm for 1-MNP2. Magnetization curves
measured at 300 K showed that the MNPs modified with the [Ru-
Superparamagnetic materials are intrinsically nonmagnetic but
can be readily magnetized in the presence of an external magnetic
field. As a result of this unique property, superparamagnetic
materials have been widely used in biomedical applications, such
as protein purification, cell sorting, MRI contrast enhancement, and
drug targeting.6 Recent advances in the synthesis of superparamag-
netic nanoparticles facilitate their exploitation in many technological
and biomedical applications.7 Herein, we wish to report the design
of novel magnetically recoverable heterogenized chiral catalysts
and their applications in highly enantioselective asymmetric
hydrogenation of aromatic ketones.8
Magnetite nanoparticles (MNPs) used for this work were
synthesized by the thermal decomposition (MNP1)9 or by copre-
cipitation method (MNP2).10 Ruthenium(II) complex with phos-
phonic acid-substituted BINAP [Ru(BINAP-PO3H2)(DPEN)Cl2]
(1) was synthesized by treating [Ru(benzene)Cl2]2 with (R)-2,2′-
bis(diphenylphosphino)-1,1′-binaphthyl-4-phosphonic acid (BINAP-
PO3H2) followed by (R,R)-1,2-diphenylethylenediamine (DPEN) in
Figure 1. (a) TEM image of as-synthesized MNP1. (b) TEM image of
1-MNP1. (c) Magnetization curves for MNP1 and 1-MNP1 measured at 300
K. (d) Magnetization curves for MNP2 and 1-MNP2 measured at 300 K.
† University of North Carolina.
‡ Virginia Polytechnic Institute and State University.
9
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J. AM. CHEM. SOC. 2005, 127, 12486-12487
10.1021/ja053881o CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Enantio Excess Values (% ee) for the Heterogeneous
runs, but began to drop for the fifth run. The 1-MNP2 system could
be used for asymmetric hydrogenation for an impressive number
of 14 times (with no deterioration of conversion and enantiomeric
excess).15
Hydrogenation of Aromatic Ketonesa
In summary, we have designed novel magnetite nanoparticle-
supported chiral Ru complexes that catalyze heterogeneous asym-
metric hydrogenation of aromatic ketones with remarkably high
activity and enantioselectivity. The heterogenized catalysts can be
readily recycled by magnetic decantation and used for asymmetric
hydrogenation for up to 14 times without loss of activity and
enantioselectivity. Orthogonal nature of the present catalyst im-
mobilization approach should allow the design of other superpara-
magnetic nanoparticle-supported asymmetric catalysts for a wide
range of organic transformations.
substrate
BINAP
1
1-MNP1
1-MNP2
Ar ) Ph, R ) Me
83.0
96.9
75.2
94.0
83.1
60.0
80.7
85.4
87.0
98.0
84.9
94.1
85.6
75.9
82.5
89.0
87.6
98.0
87.6
95.1
87.9
76.6
87.6
88.9
81.7
97.6
82.0
91.1
80.5
70.6
77.7
86.3
Ar ) 1-naphthyl, R ) Me
Ar ) 2-naphthyl, R ) Me
Ar ) 4-tBu-Ph, R ) Me
Ar ) 4-Me-Ph, R ) Me
Ar ) 4-Cl-Ph, R ) Me
Ar ) 4-MeO-Ph, R ) Me
Ar ) Ph, R ) Et
a All of the reactions were carried out at room temperature with 0.1 mol
% of catalyst and 1 mol % of KOtBu under 700 psi of hydrogen pressure
in 20 h. The ee values were determined by GC on a Supelco â-Dex 120
column. All of the conversions were >99% as judged by the integrations
of GC peaks.
Acknowledgment. W.L. thanks the NSF (CHE-0512495) for
financial support, and G.T.Y. thanks the NSF (CHE-023488) for
the SQUID magnetometer. W.L. is an A.P. Sloan Fellow, a
Beckman Young Investigator, a Cottrell Scholar of Research Corp,
and a Camille Dreyfus Teacher-Scholar. We thank Dr. Bin Cheng
for help with TEM images, and Dr. Guangbin Wang for magnetic
measurements.
Table 2. Results of Catalyst Reuse Experiments
1-MNP1
conversion
1-MNP2
conversion
run
% ee
% ee
1
4
5
100
100
92
98.0
98.0
97.5
96.8
100
100
100
100
100
99
97.0
97.6
97.7
97.8
97.7
96.7
95.1
Supporting Information Available: Experimental procedures and
five figures. This material is available free of charge via the Internet at
http://pubs.acs.org.
6
35
10
14
15
References
35
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Cl2] and comparable to those of the homogeneous counterpart 1.13
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[Ru(BINAP-PO3)(DPEN)Cl2].
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We have also successfully reused both 1-MNP1 and 1-MNP2 for
asymmetric hydrogenation of 1-acetonaphthone without the dete-
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system was used for six cycles of hydrogenation without loss of
enantioselectivity. The activity did not decrease for the first four
JA053881O
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J. AM. CHEM. SOC. VOL. 127, NO. 36, 2005 12487