Polymer-supported N-heterocyclic carbene-palladium complex for heterogeneous suzuki cross-coupling reaction

Article abstract of DOI:10.1021/jo050721m

Poly(1-methylimidazoliummethyl styrene)-surface grafted-poly(styrene) resin was prepared for the first time as a polymer-supported N-heterocyclic carbene (NHC) precursor for palladium complex by suspension polymerization. To prepare this polymer-supported NHC precursor, 1-methyl-3-(4-vinylbenzyl)imidazolium hexafluorophosphate, [MVBIM] [PF₆^-], was synthesized as a monomer and copolymerized with styrene and DVB in water. This polymer-supported NHC precursor with imidazolium as a ligand, which exists solely on the surface of the resin, was well characterized by FE-SEM, CLSM, and IR spectroscopy. The precursor containing imidazolium readily formed a stable complex with Pd(OAc)₂, and this polymer-supported N-heterocyclic carbene-palladium complex exhibited excellent catalytic activity for Suzuki cross-coupling reaction in an aqueous medium. The catalyst was recovered quantitatively from the reaction mixture by simple filtration and was able to be reused for a number of recycles with consistent activity in all of the coupling reactions.

Full text of DOI:10.1021/jo050721m

Polymer-Supported N-Heterocyclic Carbene-Palladium Complex
for Heterogeneous Suzuki Cross-Coupling Reaction
Jong-Ho Kim, Jung-Woo Kim, Mohammadreza Shokouhimehr, and Yoon-Sik Lee*
School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea
yslee@snu.ac.kr
Received April 12, 2005
Poly(1-methylimidazoliummethyl styrene)-surface grafted-poly(styrene) resin was prepared for the
first time as a polymer-supported N-heterocyclic carbene (NHC) precursor for palladium complex
by suspension polymerization. To prepare this polymer-supported NHC precursor, 1-methyl-3-(4-
-
vinylbenzyl)imidazolium hexafluorophosphate, [MVBIM][PF
6
], was synthesized as a monomer and
copolymerized with styrene and DVB in water. This polymer-supported NHC precursor with
imidazolium as a ligand, which exists solely on the surface of the resin, was well characterized by
FE-SEM, CLSM, and IR spectroscopy. The precursor containing imidazolium readily formed a stable
complex with Pd(OAc) , and this polymer-supported N-heterocyclic carbene-palladium complex
2
exhibited excellent catalytic activity for Suzuki cross-coupling reaction in an aqueous medium.
The catalyst was recovered quantitatively from the reaction mixture by simple filtration and was
able to be reused for a number of recycles with consistent activity in all of the coupling reactions.
Introduction
There have been many successful demonstrations of
homogeneous catalysis using NHCs as ligands for transi-
N-Heterocyclic carbenes (NHCs) have garnered a great
deal of interest as transition metal ligands in the area
of organic and inorganic chemistry since they were first
discovered by O¨ fele and Wanzlick in the late 1960s.
These NHCs have the same σ-donor and exhibit a low
π-acceptor ability as phosphine in terms of their metal
coordination chemistry. Recently, the NHC ligands have
turned out to have an excellent air and moisture stability
and dissociation energies quantified by theoretical cal-
4
5
6
tion metals such as palladium, ruthenium, rhodium,
7
and nickel. However, these homogeneous catalyst sys-
tems have some basic problems in terms of the separation
and recycling of the catalysts. Additionally, they induce
contamination of the ligand residue in the products.
1
2
(
4) (a) Zhang, C.; Huang, J.; Trudell, M. L.; Nolan, S. P. J. Org.
Chem. 1999, 64, 3804. (b) Xu, L.; Chen, W.; Xiao, J. Organometallics
000, 19, 1123. (c) Andrus, M. B.; Song, C. Org. Lett. 2001, 3, 3761.
2
3
culations for different metals higher than those of other
(d) Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R. H. Chem. Commun.
2
001, 210. (e) Yang, C.; Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3,
ligands. Therefore, they have been applied to many
organic reactions, utilizing the transition metal catalysts
as efficient ligands.
1
511. (f) Grasa, G. A.; Viciu, M. S.; Huang, J.; Zhang, C.; Trudell, M.
L.; Nolan, S. P. Organometallics 2002, 21, 2866. (g) Gst o¨ ttmayr, C.
W. K.; B o¨ hm, V. P. W.; Herdtweck, E.; Grosche, M.; Herrmann, W. A.
Angew. Chem., Int. Ed. 2002, 41, 1363.
(
5) (a) Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H.
*
Corresponding author. Phone: +82-2-880-7080. Fax: +82-2-876-
Tetrahedron Lett. 1999, 40, 2247. (b) Ackermann, L.; Furstner, A.;
Weskamp, T.; Kohl, F. J.; Herrmann, W. A. Tetrahedron Lett. 1999,
40, 4787. (c) Hung, J.; Schanz, H. Z.; Stevens, E. D.; Nolan, S. P.
Organometallics 1999, 18, 5375. (d) Jazzar, R. F. R.; Macgregor, S. A.;
Mahon, M. F.; Richards, S. P.; Whittlesey, M. K. J. Am. Chem. Soc.
2002, 124, 4944. (e) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.;
Wilhelm, T. E.; Scholl, M.; Choi, T.-L.; Ding, S.; Day, M. W.; Grubbs,
R. H. J. Am. Chem. Soc. 2003, 125, 2546. (f) Ung, T.; Hejl, A.; Grubbs,
R. H.; Schrodi, Y. Organometallics 2004, 23, 5399. (g) Lebel, H.; Janes,
M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5046.
(6) (a) K o¨ cher, C.; Herrmann, W. A. J. Organomet. Chem. 1997, 532,
261. (b) Chen, A. C.; Ren, L.; Decken, A.; Crudden, C. M. Organome-
tallics 2000, 19, 3459.
9
625.
(
1) (a) O¨ fele, K. J. Organomet. Chem. 1968, 12, 11. (b) Wanzlick, H.
J.; Sch o¨ nherr, H. J. Angew. Chem. 1968, 80, 154. (c) For a recent
review, see: Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(2) (a) O¨ fele, K.; Herrmann, W. A.; Mihalios, D.; Elison, M.;
Herdtweck, E.; Scherer, W. J. Organomet. Chem. 1993, 459, 177. (b)
Herrmann, W. A.; O¨ fele, K.; Elison, M.; K u¨ hn, F. E.; Roesky, P. W. J.
Organomet. Chem. 1994, 480, C7.
(3) (a) Weskamp, T.; Kohl, F. J.; Hieringer, W.; Gleich, D.; Her-
rmann, W. A. Angew. Chem., Int. Ed. 1999, 38, 2416. (b) Schwarz, J.;
B o¨ hm, V. P. W.; Gardiner, M. G.; Grosche, M.; Herrmann, W. A.;
Hieringer, W.; Raudaschl-Sieber, G. Chem.-Eur. J. 2000, 6, 1773.
10.1021/jo050721m CCC: $30.25 © 2005 American Chemical Society
6714
J. Org. Chem. 2005, 70, 6714-6720
Published on Web 07/26/2005

NHC-Pd Complex for Suzuki Cross-Coupling Reaction
Therefore, the development of polymer-supported and
insoluble transition metal catalysts has attracted a great
deal of attention in organic chemistry. This heteroge-
neous catalysis system has several advantages, such as
fast recovery and the simple recycling of the catalysts
by filtration, which in turn prevents the contamination
of the ligand and decreases the environmental pollution
caused by residual metals in the waste. Several kinds of
supported NHC-transition metal complexes have been
designed, so as to combine the advantages of both
homogeneous and heterogeneous catalysts. For example,
Herrmann’s group synthesized an NHC-palladium com-
plex in solution and then immobilized it on Wang resin,
SCHEME 1. Synthesis of the Polymerizable Ionic
Liquid Monomer (1), [MVBIM][PF -]
6
Results and Discussion
Preparation of Poly(1-methylimidazoliummethyl
styrene)-sg-PS Resin. (a) Synthesis of the Polymer-
izable Ionic Liquid Monomer (1). As shown in Scheme
1, to prepare poly(1-methylimidazoliummethyl styrene)-
sg-PS resin by suspension polymerization, where sg
8
which was used in Heck coupling reaction. Blechert’s
group reported a polymer-supported olefin metathesis
9
catalyst. In this case, they built up the polymer-anchored
ligand precursor and then treated it with a metal
compound. The monolithic system, which was function-
alized by living NHC-ruthenium termini, was prepared
denotes surface grafted, 1-methyl-3-(4-vinylbenzyl)imid-
-
azolium hexafluorophosphate (1), [MVBIM][PF
6
], was
first synthesized as a monomer, which plays the role of
a carbene ligand. To accomplish this, 1-methylimidazole
was dissolved in CHCl and reacted with 4-chloromethyl
3
by Buchmeister’s group for ring-closing and ring-opening
metathesis reactions.10 Very recently, we also reported
that the NHC-Pd catalyst, which was immobilized onto
styrene. After the reaction was completed, the result-
ing 1-methyl-3-(4-vinylbenzyl)imidazolium chloride,
Merrifield resin, exhibited excellent catalytic activity in
11
Suzuki reaction. However, all of the previously reported
polymer-supported catalysts have their catalytic active
sites in the entire regions of the supports, which makes
it difficult for the reagents to diffuse into the interior of
the supports and eventually decreases the reaction rate
and catalytic activity. If the catalytic active sites were
located at the surface of the resin, all of the chemical
-
-
[
MVBIM][Cl ], was obtained in 98% yield. [MVBIM][Cl ]
was dissolved in acetone, and NaPF was added to
6
-
-
-
6 6
replace Cl with PF . The final [MVBIM][PF ] mono-
mer (1) was synthesized in 97% yield. This ionic liquid
monomer was not only insoluble in the aqueous phase,
but also in the styrene phase, and consequently, it was
located at the interface between the aqueous phase and
the organic phase in the suspension polymerization
system.
12
reactions would be more effective.
In this article, we describe the development of a
polymer-supported NHC precursor prepared by suspen-
sion polymerization, the development of its palladium
complex, which has the catalytic active sites located only
on the surface of the polymer, and the application of the
palladium complex to heterogeneous Suzuki cross-
coupling reaction in an aqueous phase. When the het-
erogeneous catalytic system is applied to Suzuki reaction,
the reaction proceeds very rapidly and affords excellent
yields owing to the existence of the catalyst on the surface
of the resin beads. This polymer-supported NHC-pal-
ladium catalyst shows outstanding reusability without
the loss of its catalytic activity.
(b) Suspension Polymerization. Poly(1-methylimi-
dazoliummethyl styrene)-sg-PS resin was prepared by
utilizing a conventional suspension polymerization sys-
tem with a reactor and an overhead stirrer, as shown in
Scheme 2. The synthetic ionic liquid monomer was
suspended in an aqueous phase. The organic phase
consisted of styrene, DVB, and benzoyl peroxide as an
initiator, and this was added to the aqueous phase with
a constant stirring (250 rpm) under a nitrogen atmo-
sphere. After the completion of the polymerization,
spherical particles with a white color were obtained.
The polymer-supported NHC precursor, poly(1-meth-
ylimidazoliummethyl styrene)-sg-PS resin, was analyzed
by an elemental analyzer to quantify the amount of
imidazolium groups on the beads by measuring the
nitrogen content. The loading level of the imidazolium
groups was 0.23 mmol/g when 3 g of the ionic liquid
monomer and 7 mL of styrene were used. This loading
level could be adjusted by changing the amount of ionic
liquid monomer.
(7) (a) B o¨ hm, V. P. W.; Weskamp, T.; Gst o¨ ttmayr, C. W. K.;
Herrmann, W. A. Angew. Chem., Int. Ed. 2000, 39, 1602. (b) Sato, Y.;
Sawaki, R.; Mori, M. Organometallics 2001, 20, 5510. (c) Douthwaite,
R. E.; Green, M. L. H.; Silcock, P. J.; Gomes, P. T. Organometallics
2
001, 20, 2611. (d) B o¨ hm, V. P. W.; Gst o¨ ttmayr, C. W. K.; Weskamp,
T.; Herrmann, W. A. Angew. Chem., Int. Ed. 2001, 40, 3387. (e) Dorta,
R.; Stevens, E. D.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2003,
1
25, 10490. (f) Wang, X.; Liu, S.; Jin, G.-X. Organometallics 2004, 23,
002.
6
(8) Schwarz, J.; B o¨ hm, V. P. W.; Gardiner, M. G.; Grosche, M.;
Characterization of Poly(1-methylimidazolium-
methyl Styrene)-sg-PS Resin with Scanning Elec-
tron Microscopy (SEM) and Confocal Laser Scan-
ning Microscopy (CLSM). As shown in Figure 1, all of
the resins were the bead types with a special shape and
a diameter of ∼38-150 µm. The fluorescent dye, 5(6)-
carboxytetramethyl rhodamine, was adsorbed by the
beads. These fluorescent dye-adsorbed beads were visual-
ized by CLSM to confirm the location of imidazolium,
which plays the role of the carbene precursor on the
beads. As shown in the CLSM image, all of the red
Herrmann, W. A.; Hieringer, W.; Raudaschl-Sieber, G. Chem.-Eur.
J. 2000, 6, 1773.
(9) Sch u¨ rer, S. C.; Gessler, S.; Buschmann, N.; Blechert, S. Angew.
Chem., Int. Ed. 2000, 39, 3898.
(
10) Mayr, M.; Mayr, B.; Buchmeiser, M, R. Angew. Chem., Int. Ed.
2
001, 40, 3839.
(
(
11) Byun, J. W.; Lee, Y. S. Tetrahedron Lett. 2004, 45, 1837.
12) (a) Cho, J. K.; Park, B. D.; Lee, Y. S. Tetrahedron Lett. 2000,
4
1, 7481. (b) Cho, J. K.; Kim, D. W.; Namgung, J. Y.; Lee, Y. S.
Tetrahedron Lett. 2001, 42, 7443. (c) Cho, J. K.; Park, B. D.; Park, K.
B.; Lee, Y. S. Macromol. Chem. Phys. 2002, 203, 2211. (d) Kim, H. Y.;
Cho, J. K.; Chung, W. J.; Lee, Y. S. Org. Lett. 2004, 6, 3273. (e) Kim,
J. H.; Jun, B. H.; Byun, J. W.; Lee, Y. S. Tetrahedron Lett. 2004, 45,
5
827.
J. Org. Chem, Vol. 70, No. 17, 2005 6715

Kim et al.
FIGURE 1. (a) SEM image of the poly(1-methylimidazoliummethyl styrene)-sg-PS resin. (b) CLSM image of the 5(6)-
carboxytetramethyl rhodamine-adsorbed poly(1-methylimidazoliummethyl styrene)-sg-PS resin.
SCHEME 2. Suspension Polymerization for the Preparation of the Poly(1-methylimidazoliummethyl
styrene)-sg-PS Resin
SCHEME 3. Preparation of the
Polymer-Supported NHC-Palladium Complex (2)
TABLE 1. Loading Levels of the Immobilized Pd on the
Resina
amount of Pd(OAc)2 23 (1 equiv) 46 (2 equiv) 92 (4 equiv)
(
µmol)
loading level of Pd
0.017
0.03
0.11
b
(
mmol/g)
a
The initial loading capacity of imidazolium on the beads was
b
0
.23 mmol/g. The loading levels of palladium on the beads were
calculated by ICP-AES.
Characterization of Polymer-Supported NHC-
Palladium Complex (2). (a) Inductively Coupled
Plasma-Atomic Emission Spectrometry (ICP-AES).
The polymer-supported NHC-palladium complex (2)
with a dark green color was analyzed by ICP-AES to
quantify the amount of palladium immobilized on the
beads. The loading level of the palladium on the beads
was 0.11 mmol/g when a 4-fold excess amount of Pd-
colored imidazolium groups, which resemble islands,
were located on the surface of the beads (Figure 1b).
Preparation of Polymer-Supported NHC-Pal-
ladium Complex (2). As shown in Scheme 3, a mixture
of the polymer-supported NHC precursor and Pd(OAc)
2
was shaken in DMSO at 50 °C for 4 h, and then the
temperature was increased to 100 °C.13 The reaction was
then allowed to proceed for a further 30 min at 100 °C,
leading to the production of beads with a dark green
color. Various amounts of Pd(OAc) were used to deter-
2
mine the optimal loading of immobilized palladium, as
shown in Table 1.
2
(OAc) was used, as shown in Table 1. We concluded that
almost all of the imidazolium groups (0.23 mmol/g) on
the surface of the beads participated in the formation of
the Pd-NHC complex.
(
b) Energy Dispersive X-ray (EDX) and Infrared
Spectroscopy. After immobilizing the palladium, the
polymer-supported NHC-palladium complex (2) was
(
13) Herrmann, W. A.; Schwarz, J.; Gardiner, M. G. Organometallics
1
999, 18, 4082.
6716 J. Org. Chem., Vol. 70, No. 17, 2005

NHC-Pd Complex for Suzuki Cross-Coupling Reaction
SCHEME 4. Heterogeneous Suzuki
Cross-Coupling Reaction of Ph-I and Ph-B(OH)
2
SCHEME 5. Reusability of the Polymer-Supported
NHC-Pd Complex in Suzuki Cross-Coupling
Reaction
TABLE 3. Reusability of the Polymer-Supported
a
NHC-Pd Complex in Suzuki Cross-Coupling Reaction
recycle
1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th
yield (%)b 95 95 94 93 95 94 95 94 94
92
a
Iodobenzene (0.5 mmol), phenylboronic acid (0.6 mmol), NHC-
Pd (1 mol %), Na2CO3 (2.5 mmol), and DMF/H2O (1:1), 50 °C and
h. Isolated by column chromatography.
b
1
FIGURE 2. (a) EDX spectrum for the PS region in the
polymer-supported NHC-palladium complex (2). (b) EDX
spectrum for the imidazolium region in the polymer-supported
NHC-palladium complex (2).
Heterogeneous Suzuki Cross-Coupling Reaction.
(a) Catalytic Activity of Polymer-Supported NHC-
Palladium Complex (2). The catalytic activity of the
polymer-supported NHC-palladium catalyst (2) with a
palladium loading of 0.11 mmol/g was investigated in the
palladium-catalyzed Suzuki reaction, as shown in Table
2. First, several solvents were examined by using the
polymer-supported NHC-Pd catalyst (1 mol %) in the
reaction of iodobenzene and phenylboronic acid as a
model reaction. Biphenyl was obtained with an excellent
yield in both DMF/water (1:1) and dioxane/water (1:1)
TABLE 2. Heterogeneous Suzuki Cross-Coupling
Reaction of Ph-I and Ph-B(OH)2a
b
entry
solvent
base
T (°C) t (h) yield (%)
1
2
3
4
5
6
7
8
9
DMF/water (1:1)
Na2CO3
50
50
50
50
50
50
50
50
50
1
1
95
94
72
94
94
20
36
45
-
dioxane/water (1:1) Na2CO3
THF/water (1:1)
DMF/water (1:3)
DMF/water (1:7)
water
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Cs2CO3
1
2
3
3
water
12
12
12
(
95 and 94%, respectively; Table 2, entries 1 and 2). In
water
t
addition, the reaction rate was so fast that most of the
iodobenzene was converted into biphenyl within l h at
50 °C, which was quite different from the previous
results.15 Comparing to the results of other polystyrene-
water
KO Bu
a
Iodobenzene (0.5 mmol), phenylboronic acid (0.6 mmol), NHC-
b
Pd (1 mol %), and bases (2.5 mmol). Isolated by column chro-
matography.
1
5c
based polymer bead derived from Merrifield resin,
which has imidazolium group in the whole region of the
bead, this high reaction rate might be due to the
distinctive feature of the catalyst, in that all of the
catalytically active sites were located on the surface of
the beads, which means that the reagents do not need to
diffuse into the interior of the beads. Moreover, the local
concentration of the reagents was higher in the catalyti-
cally active species on the surface of the beads, and this
may have promoted the rapid reaction.
Recently, water has received much attention as a
solvent in organic reactions,16 because it is readily
available and environmentally safe. Hence, we decreased
the amount of organic solvents in heterogeneous Suzuki
reaction. Even the reaction in mixtures of DMF and
water with ratios of 1:3 and 1:7 gave biphenyl in excellent
analyzed by EDX to ascertain whether palladium binds
specifically to the ligand or not. According to the results
of the EDX spectra shown in Figure 2, palladium was
only present in the imidazolium region (Figure 2b), and
no palladium was detected in the PS region (Figure 2a).
We concluded that the palladium was immobilized specif-
ically to the carbene ligand on the surface of the beads.
Considering the results of the ICP-AES and EDX obser-
vations, we were able to indirectly determine the struc-
ture of the polymer-supported NHC-palladium complex
(
2). Additionally, we also confirmed that palladium was
immobilized specifically at the C2 position of the imida-
zolium on the bead with IR spectroscopy. Before im-
mobilization of palladium, the strong band of quaternary
-
1
imidazolium on the bead appeared at 1157 cm in the
1
4
IR spectrum. However, it disappeared, and the band of
(
15) (a) Uozumi, Y.; Nakai, Y. Org. Lett. 2002, 4, 2997. (b) Yamada,
-
1
alkene at 1665 cm became stronger after immobiliza-
tion of palladium.
Y. M. A.; Takeda, K.; Takahashi, H.; Ikegami, S. J. Org. Chem. 2003,
68, 7733. (c) Byun, J. W.; Lee, Y. S. Tetrahedron Lett. 2004, 45, 1837.
(
16) (a) Li, C. J. Chem. Rev. 1993, 93, 2023. (b) Uozumi, Y.;
Shibatomi, K. J. Am. Chem. Soc. 2001, 123, 2919. (c) Uozumi, Y.;
Danjo, H.; Hayashi, T. J. Org. Chem. 1999, 64, 3384. (d) Uozumi, Y.;
Kimura, T. Synlett 2002, 12, 2045.
(
14) Petrak, K.; Degen, I.; Beynon, P. J. Polymer. Science 1982, 20,
7
83.
J. Org. Chem, Vol. 70, No. 17, 2005 6717

Kim et al.
TABLE 4. Heterogeneous Suzuki Cross-Coupling
Reaction of Aryl Iodides and Bromides with Phenyl
SCHEME 6. Heterogeneous Suzuki
Cross-Coupling Reaction of Aryl Iodides and
Bromides with Phenyl Boronic Acid
a
Boronic Acid
TABLE 5. Heterogeneous Suzuki Cross-Coupling
a
Reaction of Arylboronic Acids with 4-Iodoanisole
a
4
-Iodoanisole (0.5 mmol), arylboronic acid (0.6 mmol), NHC-
Pd (1 mol %), Na2CO3 (2.5 mmol), and DMF/H2O (1:1), 50 °C.
Isolated by column chromatography.
b
level of reusability are very important factors. In this
experiment, the catalyst was reused 10 times in the
reaction of iodobenzene and phenylboronic acid under
DMF/water (1:1) for 1 h at 50 °C (Scheme 5). It was
notable that the polymer-supported NHC-Pd catalyst
still showed consistent catalytic activity after being
reused so many times. The reaction at the 9th and 10th
recycled runs gave biphenyl in 94 and 92% yields,
respectively, as shown in Table 3. In addition, the filtrate
resulting from the reaction mixture had no catalytic
activity under identical conditions. We concluded that
there was no leaching of the catalytically active species
from the beads during the reaction.
(
c) Suzuki Cross-Coupling Reaction of Aryl Io-
a
dides and Bromides with Phenylboronic Acid. To
extend the scope of the heterogeneous reaction, Suzuki
cross-coupling reaction of various aryl iodides and bro-
mides with phenylboronic acid was examined in very mild
conditions, as shown in Table 4.17 All of the aryl iodides
were converted to the corresponding biaryls with excel-
lent yields within 1 h at 50 °C, regardless of the
substituent, as shown in Table 4 and Scheme 6. The
reaction of the electron-deficient aryl bromides with
phenylboronic acid proceeded smoothly to afford the
biaryls with excellent yields within 6 h at 50 °C (Table
Aryl halides (0.5 mmol), phenylboronic acid (0.6 mmol), NHC-
Pd (1 mol %), Na2CO3 (2.5 mmol), and DMF/H2O (1:1), 50 °C.
Isolated by column chromatography.
b
yields (Table 2, entries 4 and 5), which is appropriate
for an industrial application. However, biphenyl was
obtained in low yield in water only, because of the poor
solubility of iodobenzene in water. Using Na CO as a
2 3
base gave a good yield in Suzuki reaction. Scheme 4
shows the heterogeneous Suzuki cross-coupling reaction
of Ph-I and Ph-B(OH)
(
ladium Complex. For the industrial application of
heterogeneous Suzuki cross-coupling reaction, the life-
time of the polymer-supported NHC-Pd catalyst and its
2
.
4
, entries 9-12 and 17). The electron-rich aryl bromides
including 4-bromotoluene and 4-bromoanisole were also
b) Reusability of Polymer-Supported NHC-Pal-
(
17) The coupling of these substrates was reviewed in Miyaura, N.;
Suzuki, A. Chem. Rev. 1995, 95, 2457.
6718 J. Org. Chem., Vol. 70, No. 17, 2005

NHC-Pd Complex for Suzuki Cross-Coupling Reaction
SCHEME 7. Heterogeneous Suzuki
Cross-Coupling Reaction of Arylboronic Acids
with 4-Iodoanisole
DVB (0.7 mL, 50% in ethylvinylbenzene), and benzoyl peroxide
(0.22 g) as an initiator, was added to the aqueous phase with
constant stirring (250 rpm) under a nitrogen atmosphere. The
mixture was then polymerized for 24 h at 80 °C. The resulting
polymer resins were sieved and washed with distilled water,
MeOH, and MC. They were dried in vacuo overnight.
(c) CLSM Analysis. The fluorescent dye, 5(6)-carboxy
tetramethyl rhodamine (34.5 µmol, 3 equiv), was dissolved in
DMF (3 mL) and added to the resin (0.23 mmol/g, 50 mg). After
shaking the mixture for 4 h at room temperature, we washed
the resin extensively with DMF, MC, and MeOH. The resin
beads were examined by means of a CLSM.
easily converted to the corresponding biaryls with yields
of 93 and 95%, respectively, for 6 h at 50 °C (Table 4,
entries 7, and 14-16). As mentioned above, this polymer-
supported NHC-Pd catalyst was so effective that all of
the aryl iodides and bromides with the electron-deficient
and -rich substituents were converted very rapidly to the
corresponding products under mild conditions.
Preparation of Polymer-Supported NHC-Palladium
Complex (2). (a) Immobilization of Palladium on the
Resin. The poly(1-methylimidazoliummethyl styrene)-sg-PS
2
resin (100 mg, 0.23 mmol/g) and Pd(OAc) (1, 2, and 4 equiv)
were mixed and stirred in DMSO (10 mL) at 50 °C for 4 h.
After increasing the temperature to 100 °C, the stirring was
continued for a further 30 min. After the completion of the
reaction, the resin was filtered and washed vigorously with
DMSO (10 mL × 5), distilled water (10 mL × 5), and MeOH
(10 mL × 5). The resin was then dried under reduced pressure
to give the complex (2).
(
d) Suzuki Cross-Coupling Reaction of Arylbo-
ronic Acids with 4-Iodoanisole. We further examined
the reactions of various arylboronic acids with 4-iodoani-
sole in DMF/water (1:1), as shown in Table 5.17 The
reaction with the electron-rich arylboronic acids pro-
ceeded readily to afford the respective biaryls with a high
yield within 1 h at 50 °C (Scheme 7). Generally, in a
Suzuki reaction, the hydrolysis of arylboronic acids with
the electron-withdrawing groups occurs and causes the
yield to be decreased.18 However, the reaction of the
electron-deficient arylboronic acids with 4-iodoanisole
using the polymer-supported NHC-Pd catalyst (1 mol
(b) ICP-AES Analysis. The resin beads (50 mg) were
treated with a mixture (5 mL) of hydrochloric acid and nitric
acid (3:1, v/v) at 100 °C for 4 h. Following this, the resulting
orange colored solution was filtered, and the recovered resin
beads were washed with distilled water (2.5 mL × 6). The
filtrate was diluted to 50 mL with distilled water and analyzed
by ICP-AES.
General Procedure for Heterogeneous Suzuki Cross-
Coupling Reaction. The polymer-supported NHC-Pd com-
plex (2) (50 mg, 1 mol % Pd, 0.11 mmol-Pd/g) was suspended
in DMF (2 mL). After adding a mixture of arylhalide (0.5
%) was performed successfully in DMF/water (1:1).
Conclusion
2 3
mmol), arylboronic acid (0.6 mmol, 1.2 equiv), and Na CO (2.5
mmol, 5.0 equiv) in distilled water (2 mL), we agitated the
reaction mixture in a shaking incubator at 50 °C for a specific
period of time. The coupling reactions were performed under
an air atmosphere. The resulting reaction mixture was filtered
and washed with distilled water (4 mL × 5) and diethyl ether
In summary, a poly(1-methylimidazoliummethyl sty-
rene)-sg-poly(styrene) resin was developed as a polymer-
supported NHC precursor, with the ligand present only
on the surface of the beads, by simple suspension
polymerization. This precursor readily formed a stable
complex with palladium, and this polymer-supported
NHC-Pd catalyst showed excellent catalytic activity in
Suzuki cross-coupling reactions of various aryl iodides
and bromides with arylboronic acids in a DMF/aqueous
phase. Of particular note is that this polymer-supported
catalyst showed outstanding reusability without any loss
of catalytic activity in Suzuki cross-coupling reaction.
(
4 mL × 5). The organic phase was separated and dried over
MgSO , and diethyl ether was evaporated under reduced
4
pressure. The final biaryl product was isolated by column
chromatography and identified by gas chromatography/mass
spectroscopy (GC-MS) and nuclear magnetic resonance spec-
troscopy (NMR). The isolation yield was calculated from the
mass value of the product.
1
Biphenyl (3). H NMR (DMSO-d
6
): δ (ppm) 7.33 (q, 2H),
7
.44 (q, 4H), 7.64 (d, 4H). GC-MS: m/z 154.
1
4
-Hydroxybiphenyl (4). H NMR (CDCl
3
): δ (ppm) 4.82
Experimental Section
(s, 1H), 6.89 (d, 2H), 7.29 (q, 1H), 7.42 (m, 4H), 7.55 (d, 2H).
GC-MS: m/z 170.
Preparation of Poly(1-methylimidazoliummehyl sty-
rene)-sg-PS Resin. (a) Synthesis of the Polymerizable
Ionic Liquid Monomer (1). 1-Methylimidazole (5 g, 61
1
4
-Acetyl-4′-methoxybiphenyl (18). H NMR (CDCl
3
): δ
(
(
ppm) 2.63 (s, 3H), 3.87 (s, 3H), 6.99 (d, 2H), 7.52 (d, 2H), 7.66
d, 2H), 8.02 (d, 2H). GC-MS: m/z 266.
3
mmol) was dissolved in CHCl (50 mL), and 4-chloromethyl
Experimental Procedure for Reusability Test of Poly-
styrene (12 g, 79.3 mmol) was added. After stirring the mixture
for 8 h at 50 °C, we washed the resulting 1-methyl-3-(4-
mer-Supported NHC-Pd Complex. The polymer-supported
NHC-Pd complex (2) (50 mg, 1 mol % Pd, 0.11 mmol-Pd/g)
was suspended in DMF (2 mL). After adding a mixture of
iodobenzene (102 mg, 0.5 mmol), phenylboronic acid (75 mg,
-
vinylbenzyl)imidazolium chloride, [MVBIM][Cl ], with ethyl
acetate several times. After evaporating the solvent, we
-
dissolved [MVBIM][Cl ] (14 g, 59.8 mmol) in acetone (300 mL)
0
.6 mmol), and Na
2 3
CO (265 mg, 2.5 mmol) in distilled water
6
and added NaPF (12 g, 71.8 mmol) with constant stirring.
(
2 mL), we agitated the reaction mixture in a shaking
The reaction was then allowed to proceed for 2 days at room
incubator at 50 °C for 1 h. The resulting reaction mixture was
filtered and washed with distilled water and diethyl ether. The
filtered catalyst was reused 10 times for the same reaction.
The yield was calculated from the mass value of the product
after isolation with column chromatography.
temperature. After evaporating the solvent, the final ionic
-
liquid monomer (1), [MVBIM][PF
6
], was extracted from water
with MC and then dried in vacuo overnight.
b) Suspension Polymerization. The synthetic ionic
liquid monomer (3.0 g), [MVBIM][PF
aqueous phase, which consisted of water (150 mL) and PVA
0.75 g). An organic phase, which consisted of styrene (7.0 mL),
(
-
6
], was suspended in an
(
Acknowledgment. This work was supported by the
Nano-Systems Institute-National Core Research Center
(NSI-NCRC) program of KOSEF, Korea, and the Brain
(18) Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett 1992, 2, 207.
J. Org. Chem, Vol. 70, No. 17, 2005 6719

Kim et al.
1
Korea 21 Program supported by the Ministry of Educa-
tion & Human Resources Development.
polymerizable ionic liquid monomer, and GC-MS and H NMR
data for all of the products obtained from Suzuki cross-coupling
reactions. This material is available free of charge via the
Internet at http://pubs.acs.org.
Supporting Information Available: Solid-state 13C NMR
data and IR spectra for polymer support and palladium
1
31
complex, ESI-MS, H NMR, and P NMR data for the
JO050721M
6720 J. Org. Chem., Vol. 70, No. 17, 2005