Recyclable and ligandless suzuki coupling catalyzed by carbon nanotube-supported palladium nanoparticles synthesized in supercritical fluid

Article abstract of DOI:10.1080/00397910600942925

Carbon nanotube-supported palladium nanoparticles prepared by a supercritical fluid deposition method show high activities for catalyzing Suzuki coupling reactions, and the catalysts can be recycled and reused at least six times without losing activity. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600942925

This article was downloaded by: [Illinois State University Milner Library]
On: 27 November 2012, At: 22:46
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office:
Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An International
Journal for Rapid Communication of
Synthetic Organic Chemistry
Publication details, including instructions for authors and subscription
information:
http://www.tandfonline.com/loi/lsyc20
Recyclable and Ligandless Suzuki Coupling
Catalyzed by Carbon Nanotube‐Supported
Palladium Nanoparticles Synthesized in
Supercritical Fluid
Horng‐Bin Pan a , Clive H. Yen a , Byunghoon Yoon a , Masaki Sato a & Chien
M. Wai a
a Department of Chemistry, University of Idaho, Moscow, Idaho, USA
Version of record first published: 24 Nov 2006.
To cite this article: Horng‐Bin Pan, Clive H. Yen, Byunghoon Yoon, Masaki Sato & Chien M. Wai (2006):
Recyclable and Ligandless Suzuki Coupling Catalyzed by Carbon Nanotube‐Supported Palladium Nanoparticles
Synthesized in Supercritical Fluid, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 36:23, 3473-3478
To link to this article: http://dx.doi.org/10.1080/00397910600942925
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial
or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or
distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the
contents will be complete or accurate or up to date. The accuracy of any instructions, formulae,
and drug doses should be independently verified with primary sources. The publisher shall not
be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or
howsoever caused arising directly or indirectly in connection with or arising out of the use of this
material.

Synthetic Communicationsw, 36: 3473–3478, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600942925
Recyclable and Ligandless Suzuki Coupling
Catalyzed by Carbon Nanotube-Supported
Palladium Nanoparticles Synthesized in
Supercritical Fluid
Horng-Bin Pan, Clive H. Yen, Byunghoon Yoon,
Masaki Sato, and Chien M. Wai
Department of Chemistry, University of Idaho, Moscow, Idaho, USA
Abstract: Carbon nanotube–supported palladium nanoparticles prepared by a super-
critical fluid deposition method show high activities for catalyzing Suzuki coupling
reactions, and the catalysts can be recycled and reused at least six times without
losing activity.
Keywords: Carbon nanotube, palladium nanoparticles, supercritical fluid, Suzuki
coupling
INTRODUCTION
The palladium-catalyzed Suzuki coupling reactions between arylboronic acids
and aryl halides are widely used in organic syntheses.[1] Both homogeneous
and heterogeneous catalysts have been used for the coupling reactions. In
general, heterogeneous catalysis is less effective than homogeneous
catalysis but has advantages, including easy separation of catalysts for
recycling. The efficiency of heterogeneous catalysis can be improved by
employing nanoparticle catalysts because of their extremely large surface-
to-volume ratios. An early study using palladium nanoclusters stabilized by
tetraalkylammonium salts or by poly(vinylpyrrolidone) (PVP) as catalysts
for Suzuki coupling reactions was reported by Reetz and coworkers.[2]
Received in the USA April 19, 2006
Address correspondence to Chien M. Wai, Department of Chemistry, University of
Idaho, Moscow, ID 83844, USA. E-mail: cwai@uidaho.edu
3473

3474
H.-B. Pan et al.
Recent reports showed that the shape of catalytic metal nanoparticles stabil-
ized by PVP could affect catalysis, and near-spherical palladium nanoparticles
were very effective for catalyzing the Suzuki coupling reactions.[3] Because
metal nanoparticles are generally unstable, exploration of appropriate
supports for stabilizing catalytic nanoparticles is a key factor for their success-
ful applications in heterogeneous catalysis. Carbon nanotubes provide a new
type of support for stabilizing catalytic nanoparticles. Because of their small
sizes, carbon nanotubes can be uniformly dispersed in a solution by mechan-
ical stirring, thus increasing contacts between the reactants and the catalyst.
Attaching metal nanoparticles to carbon nanotubes in aqueous solutions
usually involves tedious processes with little control of particle size and dis-
tribution. A simple method of depositing metal nanoparticles on surfaces of
multiwalled carbon nanotubes (MWCNT) using supercritical fluid carbon
dioxide as a medium was reported recently.[4] The method involves
chemical reduction of a metal precursor dissolved in supercritical carbon
dioxide in the presence of carbon nanotubes. Condensation and aggregation
of the reduced metal lead to formation of metal nanoparticles on surfaces of
curved carbon substrates. Near-spherical palladium nanoparticles with
narrow size distributions can be formed on carbon nanotube surfaces. The
curvature of the nanotubes is probably responsible for the formation of
metal nanoparticles of a certain size range due to the contact angles allowed
by the substrate surface during condensation.[4] The catalytic activities of
carbon nanotube–supported metal nanoparticles prepared by this method
for organic reactions, including coupling reactions, have not been investi-
gated. We have recently studied a number of Suzuki coupling reactions in
methanol catalyzed by multiwalled carbon nanotube-supported palladium
nanoparticles (Pd/MWCNT) prepared by a supercritical fluid deposition
method. Very efficient Suzuki coupling reactions were observed under
moderate temperatures without expensive ligands, and the Pd/MWCNT
catalyst could be easily recycled for repeated use without losing activity.
This new type of palladium catalyst appears to offer opportunities for a
wide range of applications including organic syntheses and chemical manu-
facturing processes.
EXPERIMENTAL
The Pd/MWCNT catalyst was prepared according to a procedure described
in a previous report.[4] The method involved hydrogen reduction of
palladium (II)–hexafluoroacetylacetone in supercritical fluid CO2 in the
presence of MWCNT at 808C and 150 atm. After preparation, the
samples were washed and sonicated in methanol and then dried for charac-
terization. Transmission electron microscopy (TEM) images showed
spherical particles attached to the surfaces of carbon nanotubes, and XPS
(X-ray photon spectroscopy) showed that the Pd (3d5/3) and Pd (3d3/2
)

Recyclable and Ligandless
3475
peaks at 335.3 eV and 340.6 eV, respectively, were consistent with the lit-
erature data for zero-valent metallic palladium. Energy dispersive X-ray flu-
orescence (EDS) was used for quantitative analysis of the Pd metal loading
in our carbon nanotube catalysts, which was about 3% by weight. A com-
mercial palladium/carbon black (Pd/C) catalyst was obtained from Aldrich.
General Procedures for Suzuki Coupling Reactions
A 100 mL, three-necked, round-bottomed flask with a magnetic stirring bar
was charged with Pd/MWCNT (10 mg, 3% Pd by weight), phenylboronic
acid (1.1 mmol), aryl halide (1.0 mmol), sodium acetate (2.2 mmol), and
methanol (25 mL). The mixture was vigorously stirred under reflux conditions
in the open air. The extent of reaction was monitored by thin-layer chromato-
graphy (TLC) and 300 MHz nuclear magnetic resonance (NMR) spectroscopy
(Bruker AMX 300).
After the reaction completion, the Pd/MWCNT settled down to the
bottom of the flask. The organic layer was carefully removed. The solution
was added with water and extracted with dichloromethane. The combined
organic solution was dried with anhydrous MgSO4 and concentrated in
vacuo. The residue was purified by flash chromatography on silica gel with
ethyl acetate–hexane ¼ 1:10 as the eluent to give the desired products. The
products were identified by comparison with 1H NMR with those of
authentic samples. The Pd/CNT was recovered almost quantitatively. After
washing with methanol, the catalyst was reused for the next coupling
experiment.
RESULTS AND DISCUSSION
The results of Suzuki coupling between phenylboronic acid and 1-iodo-4-
nitrobene catalyzed by the Pd/MWCNT are shown in Table 1. In entry 1,
after 30 min of reaction at 658C, the isolated yield showed at least 94%
conversion to the product (nitro-biphenyl) with a turnover frequency (TOF)
of 709 h21. In comparison with a commercially available Pd/C catalyst, the
TOF of the commercial catalyst for the same reaction under the same con-
ditions was only 30 h21, about 24 times slower. Similarly, in the case of
coupling between phenylboronic acid and iodobenzene, it required about
2 h to reach 95% of conversion. The TOF of MWCNT-supported Pd
catalyst was also higher than that found with the commercial Pd/C catalyst
by about a factor of 7 (Table 1).
The Suzuki cross-coupling reactions between phenylboronic acid and
iodobenzene catalyzed by PVP-Pd colloidal in 40% EtOH aqueous solution
under reflux conditions (temperature of boiling 40% EtOH about 808C)
were reported by Li et al.[5] The reaction required 12 h to reach 95%

3476
H.-B. Pan et al.
Table 1. Comparison of Pd/MWCNT with conventional Pd/C for Suzuki coupling
Rxn
time (h)
Isolated
yield %
Entry
1
Aryl halide
Product
TOF/h
1-Iodo-4-
nitrobenzene
1-Iodo-4-
0.5a
3.5b
R55NO2
93a
94b
709
2
R55NO2
30
nitrobenzene
Iodobenzene
Iodobenzene
3
4
2a
4b
R55H
R55H
95a
95b
177
27
a10 mg Pd/CNT(3%) as described in the experimental section.
b10 mg of conventional palladium catalyst on activated carbon (Pd content 10%,
dry wt.) was used after dried in a oven for 2 h at 1008C.
conversion, and the Pd nanoparticles were agglomerated after one cycle,
resulting in a loss of catalytic activity.
The speed of the coupling reaction depends on the nature of the
substituted aryl halide. Table 2 shows the results of Suzuki coupling of
phenylboronic acid and aryl halides with different substitution groups. With
electron-withdrawing substitution groups (2NO2, 2COCH3), the reaction
rate is faster than nonsubstituted aryl halide. On the contrary, for the
electron-donating groups (2OCH3), the reaction rate is slower than nonsubsti-
tuted aryl halide. Aryl bromide is slower than corresponding aryl iodide for the
coupling reaction with phenylboronic acid, but the reaction still proceeds well
Table 2. Pd/MWCNT catalyzed Suzuki coupling of phenylboronic acid with aryl
halides
Rxn
time (h)
Isolated
yield %
Entry
Aryl halide
Product
TOF/h
1
2
3
4
5
4-Iodoacetophenone
4-Bromoacetophenone
4-Iodoanisole
0.75
1.5
3.5
2.5
5
94
94
92
94
95
473
236
118
142
71
2-Iodothiophene
1,4Diiodobenzene
6
7
1-Bromo-4-nitrobenzene
Bromobenzene
1
92
95
355
101
3.5

Recyclable and Ligandless
3477
Table 3. Suzuki coupling reaction of 4-iodobenzene and
phenylboronic acid using recycled Pd/MWCNT as catalyst
Pd/CNT
Conversion %
Isolated yield %
Fresh
.99
.99
.99
.99
.99
.99
.99
94
96
95
94
95
96
96
1st reused
2nd reused
3rd reused
4th reused
5th reused
6th reused
with good conversion using the Pd/CNT catalyst. In a previous report,
Kabalka et al. used a large amount of palladium black (50 mg) as catalyst
for Suzuki coupling with aryl bromides, and the substrates were found ineffec-
tive.[6] In entry 4, coupling between 2-iodothiophene and phenylboronic acid
was also studied, and the reaction proceeded well, as shown in Table 2.
However, coupling of 1,4-diiodobenzene with 2 molecules of phenylboronic
acid to form triphenyl is the slowest reaction observed in this study,
requiring 5 h to reach 95% conversion.
The activity of the recycled Pd/MWCNT was tested by the reaction of
4-iodobenzene with phenylboronic acid. As shown in Table 3, the activity
of the MWCNT-supported Pd catalyst remained almost unchanged even
after recycling six times. TEM micrographs of the recycled Pd/MWCNT
catalysts showed little change of the particle density on the MWCNT
surfaces, but some larger particles up to 40 nm were observed after six uses.
CONCLUSION
In summary, we have shown that Pd/MWCNT made by a supercritical fluid
deposition method can efficiently catalyze the Suzuki coupling reactions
without expensive ligands and can be reused many times without loss of
catalytic activity. Biaryl structures are present in a variety of common
natural products,[7] conjugated polymers,[8] and liquid crystalline
materials[8,9] that require Suzuki coupling reactions for synthesis. The
Pd/CNT catalyst described in this report should have a wide range of appli-
cations in organic synthesis and design for durable nanoparticle catalysis.
ACKNOWLEDGMENTS
This work was supported by U. S. Air Force Office for Scientific Research
(AFOSR) (F49620-03-1-0361).

3478
H.-B. Pan et al.
REFERENCES
1. (a) Miyaura, N.; Suzuki, A. Palladium-catalyzed cross-coupling reactions of orga-
noboron compounds. Chem. Rev. 1995, 95, 2457–2483; (b) Suzuki, A. Recent
advances in the cross-coupling reactions of organoboron derivatives with organic
electrophiles. J. Organomet. Chem. 1999, 576, 147–168; (c) Kotha, S.;
Lahiri, K.; Kashinath, D. Recent applications of the Suzuki–Miyaura cross-
coupling reactions in organic sythesis. Tetrahedron 2002, 58, 9633–9695;
(d) Moreno-Manas, M.; Pleixats, R. Formation of carbon–carbon bonds under
catalysis by transition-metal nanoparticles. Acc. Chem. Res. 2003, 36, 638–643.
2. Reetz, M. T.; Breinbauer, R.; Wanninger, K. Suzuki and Heck reaction catalyzed
by preformed palladium clusters and palladium/nickel bimetallic clusters. Tetra-
hedron Lett. 1996, 37, 4499–4502.
3. Narayanan, R.; El-Sayed, M. A. Effect of colloidal nanocatalysis on the metallic
nanoparticle shape: The Suzuki reaction. Langmuir 2005, 21, 2027–2033.
4. (a) Ye, X. R.; Lin, Y. H.; Wai, C. M. Decorating catalytic palladium nanoparticles
on carbon nanotubes in supercritical carbon dioxide. Chem. Commun. 2003,
642–643; (b) Ye, X. R.; Lin, Y. H.; Wang, C.; Engelhard, M. H.; Wang, Y.;
Wai, C. M. Supercritical fluid synthesis and characterization of catalytic metal
nanoparticles on carbon nanotubes. J. Mater. Chem. 2004, 14, 908–913.
5. (a) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Suzuki cross-coupling
reactions catalyzed by palladium nanoparticles in aqueous solution. Org. Lett.
2000, 2, 2385–2388; (b) Narayanan, R.; El-Sayed, M. A. Effect of catalysis on
the stability of metallic nanoparticles: Suzuki reaction catalyzed by PVP–
palladium nanoparticles. J. Am. Chem. Soc. 2003, 125, 8340–8347.
6. Kabalka, G. W.; Namboodiri, V.; Wang, L. Suzuki coupling with ligandless
palladium and potassium fluoride. Chem. Commun. 2001, 775.
7. Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Palladium-catalyzed cross-coupling
reactions in total synthesis. Angew. Chem. Int. Ed. 2005, 44, 4442–4489.
8. Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, P. H. J. About supram-
plecular assemblies of p-conjugated systems. Chem. Rev. 2005, 105, 1491–1546.
9. (a) Shen, D.; Pegenau, A.; Diele, S.; Wirth, I.; Tschierske, C. Molecular design of
nonchiral bent-core liquid crystals with antiferroelectric properties. J. Am. Chem.
Soc. 2000, 122, 1593–1601; (b) Kishikawa, K.; Harris, M. C.; Swager, T. M.
Nematic liquid crystals with bent-rod shapes: Mesomorphic thiophenes with
lateral dipole moments. Chem. Mater. 1999, 11, 867–871.