Angewandte
Chemie
DOI: 10.1002/anie.200905626
Heterogeneous Catalysis
A Practical Heterogeneous Catalyst for the Suzuki, Sonogashira, and
Stille Coupling Reactions of Unreactive Aryl Chlorides**
Myung-Jong Jin* and Dong-Hwan Lee
Transition-metal-catalyzed coupling reactions have contrib-
uted greatly to the straightforward and facile construction of
carbon–carbon bonds.[1] Significant progress in this area has
been achieved with a variety of homogeneous palladium
catalysts.[2,3] However, homogeneous catalysis suffers from
the problematic separation of the expensive catalyst from the
product for re-use.[4] Moreover, the homogeneous palladium
catalysts tend to lose their catalytic activity because of
palladium metal aggregation and precipitation.[5] These
problems are of particular environmental and economic
concern in large-scale syntheses. Heterogenization of the
existing homogeneous palladium catalysts could be an
attractive solution to this problem.[6] There has been consid-
erable interest in the development of heterogeneous catalytic
systems that can be efficiently re-used whilst keeping the
inherent activity of the catalytic center.
Aryl iodides and bromides have been widely employed as
substrates in heterogeneous coupling reactions.[7–9] From a
practical point of view, the use of aryl chlorides is highly
desirable because they are readily available and inexpensive.
However, they are much more difficult to activate than aryl
iodides and bromides.[2a] The activation of aryl halides is
particularly challenging for heterogeneous catalysts, and
although there have been many reports
beneficial properties, such as invariant catalytic activity and
stability.[12]
We have previously reported the use of a (b-oxoimin-
ato)(phosphanyl)palladium complex as an efficient catalyst in
coupling reactions.[3f] In this context, we have prepared
triethoxysilyl-functionalized palladium complex 2, which can
be anchored easily onto the surface of the silica. Commer-
cially available Fe3O4 nanoparticles, with an average diameter
of 20 nm, were coated with a thin layer of silica using a sol-gel
process to give silica-coated Fe3O4 (3; SiO2@Fe3O4).[13] The
silica shell has plenty of hydroxyl groups for potential
derivatization with different functional groups, and also
protects the magnetite core from abrasion under harsh
shaking conditions.
The silylated palladium complex 2 was successfully
immobilized on the surface of robust SiO2@Fe3O4 (3)
(Scheme 1). Schiff-base condensation of 2,4-pentanedione
with (3-aminopropyl)triethoxysilane under microwave heat-
ing afforded 1 in only 3 min in quantitative yield. Deproto-
nation of 1 with EtOTl in tetrahydrofuran, followed by
treatment with [Pd2(m-Cl)2Me2(PPh3)2],[14] led to the forma-
tion of 2. Magnetic-nanoparticle-supported (b-oxoiminato)-
(phosphanyl)palladium complex 4 was obtained by reaction
of heterogeneous reactions in the litera-
ture, successful examples using deacti-
vated aryl chlorides are quite rare.[10]
Therefore, the development of high-per-
formance catalysts for practical catalytic
coupling reactions is of ongoing interest.
Magnetite Fe3O4 nanoparticles have
recently emerged as promising supports
for immobilization because Fe3O4-sup-
ported catalysts can be separated from
the reaction medium by an external
permanent magnet.[11] This circumvents
time-consuming and laborious separation
steps, and allows for practical continuous
catalysis. In particular, Fe3O4 nanoparti-
cles coated with a thin layer of silica have
Scheme 1. Synthesis of magnetic nanoparticle-supported (b-oxoiminato)(phosphanyl) palla-
dium complex 4. a) NH2(CH2)3Si(OEt)3, microwave heating; b) EtOTl, [Pd2(m-Cl)2Me2(PPh3)2],
tetrahydrofuran, RT; c) tetraethyl orthosilicate; d) 2, toluene, 100 8C, 12 h.
of 2 with 3 in refluxing toluene (see the Supporting
[*] Prof. M.-J. Jin, Dr. D.-H. Lee
À
Information). Palladium catalyst Pd SiO2@Fe3O4 (4), with a
Department of Chemical Science and Engineering, Inha University
Incheon 402-751 (South Korea)
Fax: (+82)32-872-0959
loading of 0.21 mmol of palladium per gram, was prepared for
this study, and the palladium content was confirmed by
inductively coupled plasma atomic emission spectrometry
(ICP-AES). TEM images of 4 show the core–shell structure of
the particles, and the silica coating, which has a uniform
thickness of 7 nm (Figure 1). Herein, we report the use of a
highly active and magnetically recyclable catalyst 4 in the
E-mail: mjjin@inha.ac.kr
[**] This work was supported by National Research Foundation of Korea
Grant funded by the Korean Government (2009-0072013).
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2010, 49, 1119 –1122
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1119