Magnetic nanoparticle-supported Hoveyda-Grubbs catalysts for ring-closing metathesis reactions

Article abstract of DOI:10.1039/b911999j

Magnetically recyclable Hoveyda-Grubbs catalyst can be readily assembled using magnetic nanoparticles as support, and this catalyst combines convenient recyclability and excellent activity on ring-closing metathesis (RCM) reactions.

Full text of DOI:10.1039/b911999j

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Magnetic nanoparticle-supported Hoveyda–Grubbs catalysts
for ring-closing metathesis reactionsw
Chao Che,*a Wenzhao Li,a Shengyue Lin,a Jiwei Chen,a Jie Zheng,a Jiun-chen Wu,a
Qunxiong Zheng,a Guoqing Zhang,a Zhen Yang*ab and Biwang Jiang*a
Received (in Cambridge, UK) 18th June 2009, Accepted 3rd August 2009
First published as an Advance Article on the web 24th August 2009
DOI: 10.1039/b911999j
Magnetically recyclable Hoveyda–Grubbs catalyst can be
readily assembled using magnetic nanoparticles as support,
and this catalyst combines convenient recyclability and excellent
activity on ring-closing metathesis (RCM) reactions.
The Grubbs-type1 and Hoveyda–Grubbs-type2 catalysts are
the most widely used catalysts in ring-closure metathesis
(RCM). They are extraordinarily versatile to tolerate a variety
of functional groups in the alkenes and are compatible with a
wide range of solvents. Despite these advantages, the separation
of the soluble catalyst from the product and any reaction
solvent remains difficult. Current effort3 toward solving the
problem is focused on anchoring catalysts to solid or soluble
matrices via ligand exchange.4 In this context, immobilized
catalysts offer inherent operational and economic advantages
over their homogeneous counterparts.
Fig. 2 (a) TEM image of magnetic nanoparticles; (b) magnetization
curves of magnetic nanoparticles.
reaction mixture with the aid of an external magnet.7 In this
respect, the development of an MNP catalysts based on
ruthenium carbene complexes would find a wide range of
applications both in academia and industry. Herein, we report
our recent effort in developing a magnetic nanoparticle-
supported Hoveyda–Grubbs ruthenium carbene complex for
promoting RCM reactions, which lead to a convenient
synthesis of five-, six- and seven-membered carbocycles or
heterocycles in high yields.
During recent years we have witnessed a significant progress
on nanoparticle catalysis because the nanoparticle-based
catalysts not only show superior catalytic activities to their
corresponding bulk materials,5 but also increase their loading
capacity. However, one issue associated with the application
of nanoparticle-based catalysis is the removal of the catalysts
from the reaction system by conventional methods, such as
filtration, which frequently leads to the blocking of filters and
valves (Fig. 1).
The method utilized for the preparation of magnetic
nanoparticles (iron oxide) was reported by Hyeon,8 and the
procedure allowed us to prepare monodispersed iron oxide
nanoparticles (about 10 nm) in a narrow range of particle-size
distribution. The formed magnetic nanoparticles were
characterized by transmission electron microscopy (TEM),
and the TEM image of the nanoparticles (Fig. 2(a)) shows
that they are relatively uniform with an average size of 10 nm.
The magnetic behavior of the magnetic nanoparticles was
Recently, magnetic nanoparticles (MNPs)6 emerged as new
catalyst supports because of their facile separation from the
investigated using
a Magnetic Properties Measurement
System-5 (MPMS, Quantum Design) magnetometer. The
Fig.
1 Illustration of the separation of magnetic nanoparticle
catalysis from the reaction system in RCM reaction.
a Laboratory of Chemical Genomics, School of Chemical Biology and
Biotechnology, Shenzhen Graduate School of Peking University,
Shenzhen 518055, China. E-mail: zyang@pku.edu.cn
b Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education and Beijing National Laboratory of
Molecular Science (BNLMS), College of Chemistry and the State
Key Laboratory of Natural and Biomimetic Drugs, School of
Pharmaceutical Science, Peking University, Beijing 100871, China
w Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b911999j
Scheme 1 Synthesis of magnetic nanoparticle-supported catalyst 3.
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field-dependent magnetization curve at 300 K reveals that the
catalyst is superparamagnetic with a saturation magnetization
of 36.5 emu gÀ1 (Fig. 2(b)). Surface modification of magnetic
nanoparticles with amino groups was then carried out using
organic silane bifunctional groups (3-aminopropyltriethoxy-
silane, APTS), which served as robust anchors to immobilize
the functional ligands on the iron oxide shells of the magnetic
nanoparticles. The resulting amino-functionalized particles
Table 2 Experiments for recycle of magnetic catalyst 3
had an amino group loading level of 0.28 mmol gÀ1
.
Scheme 1 shows the synthesis of the magnetic nanoparticle
supported Hoveyda–Grubbs catalyst. Ligand 14o was coupled
to the magnetic nanoparticles under standard conditions to
form 2, which was then treated with the first-generation
Grubbs catalyst in the presence of CuCl to provide the desired
magnetic Hoveyda–Grubbs catalyst 3 as a purple powder.
The Ru loading level of the obtained magnetic catalyst as
determined by ICP-AES is 0.025 mmol gÀ1
.
To test the efficiency and reactivity of the new magnetic
catalyst 3 in the RCM reaction, substrates 4–12 were treated
with one single batch of catalyst 3 (2.5 mol%) in consecutive
manner in CH2Cl2 at 40 1C, and the corresponding products
4p–12p were obtained in excellent yields (Table 1).
For evaluation of recyclability, substrates 4 and 10 were
selected, and the RCM reactions were run with recycled
catalyst 3 up to 22 times. The results as illustrated in Table 2
show almost no loss of the catalytic efficiency.
Table 1 Activity of magnetic catalyst 3 in RCM reactionsa
It is worthwhile to mention that after the reaction, the
catalyst was simply collected using a magnetic bar, and
the reaction mixture was then transferred out of the flask.
The isolated catalyst was washed twice with CH2Cl2 and then
employed in the next reaction under nitrogen.
In summary, we have developed a novel type of Hoveyda–
Grubbs catalyst by assembling magnetic nanoparticles with
Grubbs I catalyst. This type of catalyst was found to be
effective in the synthesis of a series of cyclic olefins, and found
to be active with no loss of catalytic efficiency after repeated
use. We expect this type of magnetic nanoparticle-based
catalyst to find broad applications in organic synthesis.
This work is supported by grants of the National Science
Foundation of China (20325208, 20672004 and 20521202).
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5992 | Chem. Commun., 2009, 5990–5992