Carbon-carbon and carbon-nitrogen coupling reactions catalyzed by palladium nanoparticles derived from a palladium substituted Keggin-type polyoxometalate

Article abstract of DOI:10.1021/ol026689p

(matrix presented) Palladium 15-20 nm particles stabilized by a Keggin-type polyoxometalate were prepared by reduction of K₅PPdW₁₁O₃₉ with H₂. The nanoparticles were shown to be effective catalysts for Suzuki-, Heck-, and Stille-type carbon-carbon coupling and carbon-nitrogen coupling reactions of bromoarenes in aqueous media. Chloroarenes were also reactive in reaction media without solvent.

Full text of DOI:10.1021/ol026689p

ORGANIC
LETTERS
2002
Vol. 4, No. 20
3529-3532
Carbon−Carbon and Carbon−Nitrogen
Coupling Reactions Catalyzed by
Palladium Nanoparticles Derived from a
Palladium Substituted Keggin-Type
Polyoxometalate
,§
Vladimir Kogan,† Zeev Aizenshtat,† Ronit Popovitz-Biro,‡ and Ronny Neumann*
Department of Organic Chemistry, The Hebrew UniVersity of Jerusalem,
Jerusalem, Israel 91904, DiVision of Chemical SerVices, Weizmann Institute of Science,
RehoVot, Israel 76100, and Department of Organic Chemistry, Weizmann Institute of
Science, RehoVot, Israel 76100
ronny.neumann@weizmann.ac.il
Received August 7, 2002
ABSTRACT
Palladium 15−20 nm particles stabilized by a Keggin-type polyoxometalate were prepared by reduction of K PPdW O39 with H . The nanoparticles
5
11
2
were shown to be effective catalysts for Suzuki-, Heck-, and Stille-type carbon−carbon coupling and carbon−nitrogen coupling reactions of
bromoarenes in aqueous media. Chloroarenes were also reactive in reaction media without solvent.
Various carbon-carbon cross-coupling reactions such as the
Suzuki, Heck, and Stille reactions and carbon-nitrogen
coupling reactions are generally considered to be homoge-
neous reactions most commonly catalyzed by soluble Pd(II)
species with a wealth of ligands.1 Although stabilizing
phosphane ligands are often used in these reactions, increas-
ingly simple palladium salts such as Pd(OAc)2 in the presence
of quaternary ammonium salts (the Jeffrey system)2 or water3
are being used effectively in such reactions. Reetz and
Westermann4 noted the formation of colloidal Pd nanopar-
ticles stabilized by quaternary ammonium salts in such
systems5 and claimed that a true heterogeneous catalyst was
functioning. More recently, additional palladium nanoparticle
systems such as palladium hollow spheres6 and nanopar-
ticulate palladium species involving stabilization by ionic
liquids,7 dendrimers,8 polyvinylpyridine,9 or fluorous ligands10
have been described as catalysts active for carbon-carbon
coupling reactions. The idea that such nanoparticulate
† The Hebrew University of Jerusalem.
‡ Division of Chemical Services, Weizmann Institute of Science.
§ Department of Organic Chemistry, Weizmann Institute of Science.
(1) (a) Diederich, F.; Stang, P. J. Metal-Catalyzed Cross-Coupling
Reactions; Wiley-VCH: Weinheim, Germany, 1998. (b) Cornils, B.;
Herrmann, W. A. Applied Homogeneous Catalysis with Organometallic
Compounds; VCH: Weinheim, Germany, 1996. (c) Beller, M.; Bolm, C.
Transition Metals for Organic Synthesis; Wiley-VCH: Weinheim, Germany,
1998.
(3) (a) Beletskaya, I. P. Pure Appl. Chem. 1997, 69, 471. (b) Genet, J.
P.; Savignac, M. J. Organomet. Chem. 1999, 576, 305.
(4) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165.
(5) (a) Reetz, M. T.; Breinbauerm R.; Wanniger, K. Tetrhedron Lett.
1996, 37, 4499. (b) Beller, M.; Fischer, H.; Ku¨hlein, K.; Reisinger, C.-P.;
Herrmann, W. A. J. Organomet. Chem. 1996, 520, 257.
(6) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc.
2002, 124, 7642.
(2) (a) Jeffrey, T.; David, A. Tetrahedron Lett. 1998, 39, 5751. (b) Jeffrey,
T. Tetrahedron, 1996, 52, 10113.
10.1021/ol026689p CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/31/2002

palladium systems are truly heterogeneous and do not involve
soluble intermediates has been disputed at least in somewhat
different cases where commercially available Pd/C or
Pd/Al2O3 catalysts are used.11 A dissolution/reprecipitation
process of palladium has been concluded to operate, although
this does not necessarily limit the practical advantages of
such “heterogeneous” systems.11b
for 4 h. After the slurry was cooled, the blue-black solid
was filtered, washed with dichloromethane, and dissolved
at 90 °C in 25 mL of water. Originally, a dark blue solution
was obtained due to the presence of the reduced polyoxo-
metalate moiety. However, after exposure of the solution to
air (2-3 h) in order to reoxidize the polyoxometalate, a
yellow-red solution indicative of formation of Pd clusters is
formed.4 The stabilization of nanoparticles by the polyoxo-
metalate is thought to be due to the high anionic charge of
the polyoxometalate.13 A transmission electron microscope
image, Figure 1, revealed formation of spherical 15-20 nm
The use of polyoxometalates as catalysts has become an
important area of research over the past two decades.12 One
sub-area of this research field involves the use of noble metal
clusters stabilized by polyoxometalates. Thus, Rh0 and Ir0
nanoparticles have be used for the hydrogenation of alkenes.13
Recently, we have shown that palladium nanoparticles
stabilized by a Keggin-type polyoxometalate showed unusual
selectivity in hydrogenation reactions, for example, arene
hydrogenation in the presence of a ketone.14 In this paper,
we describe the use of palladium nanoclusters stabilized by
a polyoxometalate as catalysts for carbon-carbon (Suzuki,
Heck, and Stille) and carbon-nitrogen coupling reactions.
The previously reported palladium-substituted Keggin-type
polyoxometalate14,15 formulated by elemental analysis as
K5[PPd(H2O)W11O39]‚12H2O 16-17 was used as a precursor
for the preparation of palladium nanoparticles. Thus, the
palladium nanoparticles, hereafter referred to as catalyst A,
were prepared as follows. K5[PdPW11O39]‚12H2O (1 g) was
reacted with 5 g of acetophenone at 200 °C under 30 bar H2
(7) (a) Deshmukh, R. R.; Rajagopal, R.; Srinivaan, K. V. Chem. Commun.
2001, 1544. (b) Battistuzzi, G.; Cacchi, S.; Fabrizi, G. Synlett 2002, 439.
(c) Hamill, N. A.; Hardacre, C.; McMath, S. E. J. Green Chem. 2002, 4,
139.
(8) Rahim, E. H.; Kamounah, F. S.; Fredericksen, J.; Christensen, J. B.
Nanoletters 2001, 1, 499.
Figure 1. Pdx - ([PW11O39]7-)y nanoparticles.
(9) Pathak, S.; Greci, M. T.; Kwong, R. C.; Mercado, K.; Prakash, G.
K. S.; Olah, G. A.; Thompson, M. E. Chem. Mater. 2000, 12, 1985.
(10) Rocaboy, C.; Gladysz, J. A. Org. Lett. 2002, 4, 1993.
(11) (a) Biffis, A.; Zecca, M.; Basato, M. Eur. J. Inorg. Chem. 2001,
1131. (b) Ko¨hler, K.; Heidenreich, R. G.; Krauter, J. G. E.; Pietsch, J. Chem.
Eur. J. 2002, 8, 622.
(12) (a) Kozhevnikov, I. V. Chem. ReV. 1998, 98, 171. (b) Okuhara, T.;
Mizuno, N.; Misono, M. AdV. Catal. 1996, 41, 113. (c) Hill, C. L.; Prosser-
McCartha, C. M. Coord. Chem. ReV. 1995, 143, 407. (d) Neumann, R.
Prog. Inorg. Chem. 1998, 47, 317.
(13) (a) Aiken, J. D., III; Lin, Y.; Finke, R. G. J. Mol. Catal. A: Chem.
1996, 114, 29. (b) Lin, Y.; Finke, R. G. J. Am. Chem. Soc. 1994, 116,
8335. (c) Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382.
(d) Lin, Y.; Finke, R. G. Inorg. Chem. 1994, 33, 4891. (e) Aiken, J. D., III;
Finke, R. G. J. Am. Chem. Soc. 1999, 121, 8803. (f) Aiken, J. D., III; Finke,
R. G. J. Mol. Catal. A: Chem. 1999, 145. (g) Weddle, K. S.; Aiken, J. D.,
III; Finke, R. G. J. Am. Chem. Soc. 1998, 120, 5653.
nanoparticles. Electron diffraction measurements were con-
sistent with the formation of Pd(0) clusters with a closed-
packed arrangement of the Pd atoms. 31P NMR measure-
ments, δ ) -13.3 ppm, indicate the presence of [PW11O39]7-
polyoxometalate anions. Catalyst A, therefore, is formulated
7-
as Pdx - ([PW11O39]
)
y.
The results of Suzuki cross-coupling reactions carried out
using various aryl bromide substrates and phenylboronic acid
with diisopropylamine as a base is presented in Table 1. The
results showed nearly a quantitative formation of the biphenyl
product for a range of substrates using an environmentally
preferred aqueous-alcohol reaction medium. No debromi-
nation was observed, although for aryl iodides such deiodo-
nation was encountered (up to 20% depending on the
substrate). 2-Thiophenboronic acid and 3-thiophenboronic
acid yielded similar results. The Suzuki coupling reactions
with phenylboronic acid on 4-bromotoluene and 1-bromo-
4-chlorobenzene as representative substrates were also carried
out using alternative bases instead of diisopropylamine.
Under typical reaction conditions (1 mmol of substrate, 2
mmol of phenylboronic acid, 2.5 of mmol base, 1 mL (∼0.01
mmol Pd) of catalyst A in 8 mL of water and 2 mL of EtOH,
∼80-85 °C, 12 h), results similar to those reported in Table
1 were observed using potassium acetate, cesium carbonate,
potassium fluoride, or triethylamine.
(14) Kogan, V.; Aizenshtat, Z.; Neumann, R. New J. Chem. 2002, 26,
272.
(15) (a) Liu, H.; Sun, W.; Yue, B.; Jin, S.; Deng, J.; Xie, G. Synth.
React. Inorg. Met.-Org. Chem. 1997, 27, 551. (b) Kuznetsova, N. I.;
Detusheva, L. G.; Kuznetsova, L. I.; Fedetov, M. A.; Likholobov, V. A. J.
Mol. Catal. A: Chem. 1996, 114, 131.
(16) K5PPdW11O39‚12H2O was prepared by adding dropwise PdCl2 (1.1
mmol, 200 mg) dissolved in 20 mL of deionized water to a solution of
Na7PW11O39‚xH2O (1.0 mmol, 3.2 g) dissolved in 20 mL of hot deionized
water. After additional heating for 1 h at 90 °C, a saturated solution of KCl
(20 mL) was added and the solution was cooled. The brown precipitate
was collected and recrystallized from water (yield 2.9 g, 90%). ICP
elemental analysis experimental (calcd) K - 5.86 (6.08), P - 1.02 (0.96),
Pd - 3.12 (3.31), W - 63.15 (62.94), H2O - 7.37 (7.29). 31P NMR (85%
H3PO4 as external standard): major peak at -15.17 ppm (95%) attributable
to [PPd(H2O)W11O39]5- and/or [PW11O39Pd - O - PdPW11O39],15b minor
peak at -13.32 ppm (5%) [PW11O39]7- impurity. IR - 1100, 1046, 950,
851, 805 cm-1
.
(17) (a) Kabalka, G. W.; Pagni, R. M.; Hair, C. M. Org. Lett. 1999, 1,
1423. (b) Kabalka, G. W.; Namboodiri, V.; Wang, L. Chem. Commun. 2001,
775.
3530
Org. Lett., Vol. 4, No. 20, 2002

In a specific example to investigate the synthetic utility
of the reaction and the recovery/recycle of the catalyst,
chlorobenzene (0.3 g, 2.55 mmol) was reacted with phenyl-
boronic acid (0.32 g, 2.62 mmol) in the presence of 400 mg
of catalyst B at 130 °C for 18 h. Biphenyl (0.35 g, 95%
yield) was recovered by extraction with hot CH2Cl2 (2 × 10
mL); the combined organic phase was washed with water
and dried with MgSO4. The catalyst was reused under the
same conditions with essentially no loss in activity.
It is interesting to comment also on whether the Pdx -
([PW11O39]7-)y - KF on alumina catalyst is a true hetero-
geneous catalyst. Thus, insoluble poly(4-bromostyrene) (0.1
g) was reacted with 3-formylphenylboronic acid (0.15 g, mp
110 °C) in the presence of catalyst B (200 mg) at 140 °C
for 12 h. The polymer was recovered after the reaction. IR
analysis showed no characteristic aldehyde peak at ∼1700
cm-1, indicating that 3-formylphenylboronic acid had not
reacted with the 4-bromopolystyrene substrate. On the other
hand, 4-bromotoluene reacted quantitatively with 3-formyl-
phenylboronic acid under the same reaction conditions.
Therefore, it may be concluded that the coupling reaction is
initiated by reaction of the haloarene with the Pd catalyst at
the surface of the nanoparticle rather than with dissolved
Pd species. It may be assumed that the oxidative addition to
Pd(0) with the ArX substrate may lead to dissolution of
palladium and its reprecipitation at the nanoparticle surface
at the conclusion of the catalytic cycle. Alternatively, the
ArPd(II)X intermediate could remain at the nanoparticle
surface throughout the catalytic cycle. Since KF is essentially
insoluble in the reaction mixture, we assume that the
completion of the catalytic cycle occurs at the nanoparticle
surface.
Table 1. Suzuki Cross-Coupling Catalyzed by Pdx -
([PW11O39]7-)y Nanoparticles in EtOH/H2Oa
substrate
product
yieldb
1-bromo-4-nitrobenzene
4-bromobenzonitrile
4-bromotoluene
2-bromotoluene
bromobenzene
4-nitro-1,1′-biphenyl
4-cyano-1,1′-biphenyl
4-methyl-1,1′-biphenyl
2-methyl-1,1′-biphenyl
biphenyl
>99
97
89
87
94
91
92
>99
1-bromo-4-chlorobenzene
4-bromoacetophenone
4-bromobenzoic acid
4-chloro-1,1′-biphenyl
4-acetyl-1,1′-biphenyl
4-carboxy-1,1′-biphenyl
a Reaction conditions: 1 mmol of substrate, 2 mmol of phenylboronic
acid, 2.5 mmol of diisopropylamine, 1 mL (∼0.01 mmol Pd) of catalyst A
in 8 mL of water and 2 mL of EtOH, ∼80-85 °C, 12 h. At the completion
of the reaction, the solution was extracted by dichloromethane and analyzed
by GC and GC-MS. b Yield is in mol % as computed by GC with
pentadecane as the external standard. Response factors were determined
by calibration of standards.
In the H2O-EtOH reaction medium, chloroarenes, except
for 1-chloro-4-nitrobenzene, reacted sluggishly. However,
chloroarenes successfully underwent Suzuki cross-coupling
with phenylboronic acid in the absence of solvent using a
supported catalyst system, Table 2. Supported catalyst B was
Table 2. Solvent-Free System for Suzuki Cross-Coupling of
Chloroarenesa
substrate
product
yieldb
1-chloro-4-nitrobenzene
4-chlorobenzonitrile
4-chloroacetophenone
2-chlorotoluene
4-nitro-1,1′-biphenyl
4-cyano-1,1′-biphenyl
4-acetyl-1,1′-biphenyl
2-methyl-1,1′-biphenyl
biphenyl
>99 (98)
>99 (53)
>99 (25)
The Pdx - ([PW11O39]7-)y catalyst was also effective for
other types of coupling reactions. For example, Stille-type
coupling reactions between 4-bromotoluene and 1-chloro-
4-nitrobenzene with tetraphenyltin (1 mmol of substrate, 2
mmol of tetraphenyltin, 2.5 mmol of diisopropylamine, 1
mL (∼0.01 mmol Pd) of catalyst A in 5 mL of water and 5
mL of DMF, 110 °C, 12 h) yielded almost quantitative yields
of 4-methyl-1,1′-biphenyl and 4-nitro-1,1-biphenyl, respec-
tively, Scheme 1.
c
94
chlorobenzene
>99
>99
>99
2-chloropyridine
3-chloropyridine
2-phenylpyridine
3-phenylpyridine
a Reaction conditions: 1 mmol of substrate, 2 mmol of phenylboronic
acid, 280 mg of catalyst B, 130 °C, 16 h. Values in parentheses are results
obtained for reaction under the conditions described in Table 1. At the
completion of the reaction, the solution was extracted by dichloromethane
and analyzed by GC and GC-MS. b Yield is in mol % as computed by GC
with pentadecane as the external standard. Response factors were determined
by calibration of standards. c 2,4-Dichlorotoluene (6%) was formed.
prepared by adding 0.4 g of KF and 5.6 g of γ-Al2O3 (Strem,
surface area 250 m2/g) to 25 mL of catalyst solution A. Water
was removed under vacuum yielding a Pdx - ([PW11O39]7-)y
- KF catalyst impregnated on alumina. The results, Table
2, show that chloroarenes can effectively be coupled with
phenylboronic acid using a variety of chloroarenes substituted
with either electron-withdrawing or electron-donating moi-
eties. In one case, with 2-chlorotoluene as the substrate, some
2,4-dichlorotoluene was formed as a byproduct probably via
acid-catalyzed transfer chlorination (halogen “dance” reac-
tion) presumably due to the acidity of the polyoxometalate
moiety. Previously, Suzuki coupling reactions in a solvent-
free system with heterogeneous Pd catalysts showed good
activity for iodoarenes but no or only marginal activity for
chloroarenes.17 This is the major advantage of this catalytic
system.
Scheme 1. Stille Coupling Catalyzed by Pdx -
([PW11O39]7-)y Nanoparticles
Heck-type coupling with both styrene and methylacrylate
as alkenes and with bromoarenes, e.g., 4-bromotoluene and
activated chloroarenes, e.g., 1-chloro-4-nitrobenzene, Scheme
2, was also demonstrated. Under typical reaction conditions
(1 mmol of substrate, 2.5 of mmol alkene, 3 mmol of
diisopropylamine, 1 mL (∼0.01 mmol Pd) of catalyst A in
8 mL of water and 2 mL of EtOH, ∼80-85 °C, 16 h), nearly
Org. Lett., Vol. 4, No. 20, 2002
3531

Scheme 2. Heck Coupling Catalyzed by Pdx -
([PW11O39]7-)y Nanoparticles
Scheme 3. Carbon-Nitrogen Coupling Catalyzed by Pdx -
([PW11O39]7-)y Nanoparticles
Palladium nanoparticles stabilized by polyoxometalates
were found to be versatile catalysts for carbon-carbon and
carbon-nitrogen coupling reactions of bromoarenes in
aqueous media or chloroarenes in the absence of solvent.
quantitative yields of the respective stilbene and methyl
cinnamate derivatives were obtained in all cases.
Acknowledgment. This research was supported by the
Minerva Foundation and the Kimmel Center for Molecular
Design and in part by the Israel Science Foundation. R.N.
is the Israel and Rebecca Sieff Professor of Organic
Chemistry.
Finally, carbon-nitrogen coupling reactions, for example,
with pyrrolidine and 4-bromotoluene or 1-chloro-4-nitroben-
zene (1 mmol of substrate, 2 mmol of pyrrolidine, 1 mL
(∼0.01 mmol Pd) of catalyst A in 5 mL of water and 5 mL
of DMF, 90 °C, 12 h), were also effectively catalyzed by
the Pdx - ([PW11O39]7-)y catalyst, Scheme 3.
OL026689P
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Org. Lett., Vol. 4, No. 20, 2002