Palladium catalyzed the Suzuki cross-coupling reaction using a fluorous NHC ligand

Article abstract of DOI:10.1016/j.jfluchem.2012.04.011

The utilization of N-heterocyclic carbene palladium complex immobilized on fluorous silica gel through fluorous-fluorous interactions in the Suzuki cross-coupling reaction was described. The reactions were carried out under aerobic and phosphine-free conditions with moderate to excellent product yields. The catalyst was easily recovered and reused three times without significant loss of activity.

Full text of DOI:10.1016/j.jfluchem.2012.04.011

Journal of Fluorine Chemistry 140 (2012) 107–111
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Short communication
Palladium catalyzed the Suzuki cross-coupling reaction using a
fluorous NHC ligand
a
a
a,b,
Li Wan , Hong Yu , Chun Cai
*
a
Chemical Engineering College, Nanjing University of Science and Technology, 200 Xiaolingwei, Nanjing 210094, PR China
b
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
The utilization of N-heterocyclic carbene palladium complex immobilized on fluorous silica gel through
fluorous–fluorous interactions in the Suzuki cross-coupling reaction was described. The reactions were
carried out under aerobic and phosphine-free conditions with moderate to excellent product yields. The
catalyst was easily recovered and reused three times without significant loss of activity.
ß 2012 Elsevier B.V. All rights reserved.
Received 25 January 2012
Received in revised form 17 April 2012
Accepted 19 April 2012
Available online 1 May 2012
Keywords:
Suzuki coupling
Fluorous
N-heterocyclic carbene
Palladium
1
. Introduction
The Suzuki reaction catalyzed by palladium is one of the most
stability and have higher dissociation energy. Therefore, NHCs
can enhance the reactivity and stability of palladium catalysts
compared with phosphines [18–22]. Recently, several supported
NHC–Pd catalysts by immobilization of the corresponding
homogeneous complexes on Wang resin [23], Merrifield resin
[24], polystyrene [25,26] have been developed to combine the
advantages of both homogeneous and heterogeneous catalysts in a
number of cross-coupling reactions. However, the polymer back-
bones in these catalysts are very expensive and most of them suffer
from the drawback that many of their catalytic active sites are in
the interior of the support. Therefore, it is often necessary to use a
high loading of these palladium-based catalysts in a typical
reaction.
Recently, Bannwarth and co-workers [27,28] have reported
some immobilizing perfluoro-tagged palladium catalysts which
were prepared via adsorption of palladium(II) complexes contain-
ing perfluorinated phosphine ligands on fluorous silica gel (FSG).
These heterogeneous catalysts showed the advantages of their
utilization (separation and recovery of perfluoro-tagged palladi-
um) in the Suzuki cross-coupling reaction in comparison with
fluorous biphasic catalysis approaches using expensive and
environmentally persistent perfluorinated solvents. Vallribera
and co-workers [29–31] have developed some new immobilized
phosphine-free palladium systems in the alkynylation of aryl
halides and the Heck reaction. Wang and co-workers reported the
immobilized palladium nanoparticles on fluorous silica gel to
catalyze the Suzuki reaction in water [32]. Although these systems
are efficient in catalyzing these two C–C bond-forming reactions,
the palladium catalysts were prepared in multiple steps, using
2
2
powerful routes for the formation of C(sp )–C(sp ) bonds. This
reaction has been used to make biaryl derivatives that are
important intermediates in polymers, liquid crystals, pharmaceu-
ticals, and herbicides [1–6]. From an industrial point of view, one of
significant issues in the Suzuki reaction has focused on heteroge-
neous catalyst. Homogeneous catalysts have been of somewhat
limited use, mainly because of the difficulty in separation from the
reaction products that in turn may lead to economical/environ-
mental problems especially in the case of expensive or toxic metal
catalysts. Therefore, a heterogeneous catalyst for the Suzuki
reaction is still needed for industrial applications.
Since Arduengo and co-workers isolated the first stable N-
heterocyclic carbene, N-heterocyclic carbenes (NHCs) are widely
used as ligands in inorganic and organometallic chemistry [7–14].
NHCs were first considered as simple phosphine mimics in
organometallic chemistry [15]. However, increasing experimental
data clearly showed that NHC–metal catalysts can surpass their
phosphine-based counterparts in both activity and scope [16,17].
NHCs behave like typical
chemistry. Moreover, NHCs have excellent air and moisture
s-donor ligands in metal coordination
*
Corresponding author at: Chemical Engineering College, Nanjing University of
Science and Technology, 200 Xiaolingwei, Nanjing 210094, PR China.
Tel.: +86 25 84315514; fax: +86 25 84315030.
E-mail address: c.cai@mail.njust.edu.cn (C. Cai).
0022-1139/$ – see front matter ß 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2012.04.011

1
08
L. Wan et al. / Journal of Fluorine Chemistry 140 (2012) 107–111
Scheme 1. Preparation of the fluorous NHC–Pd/FSG.
expensive starting materials and high palladium contamination in
the products still limit their industrial applications. To address
these concerns, the use of simple, low-cost, phosphine-free,
recoverable and recyclable heterogeneous catalyst would provide
obvious advantages in many synthetic applications.
the corresponding products in good to excellent yields. Further-
more, the heterogeneous catalyst was recovered from the reaction
mixture by a simple filtration and reused three times without
significant loss of its activity.
We herein report an efficient fluorous heterogeneous NHC–Pd
catalyst for the Suzuki cross-coupling reaction. In the presence of
2. Results and discussion
0
.1 mol% palladium catalyst, the reactions of aryl halides with
N-heterocyclics carbenes are easily accessible from imidazo-
lium salts, which can in turn be prepared by alkylation of
arylboronic acids in DMF/water underwent efficiently to generate
Fig. 1. (a) TEM image and particle size distribution histogram of the fluorous NHC–Pd/FSG. (b) TEM image and particle size distribution histogram of the catalyst after 3 runs.

L. Wan et al. / Journal of Fluorine Chemistry 140 (2012) 107–111
109
Table 1
a
Optimization study for the Suzuki reaction of 4-bromoanisole with phenylboronic acid.
base, Cat.
CH O
Br + (HO) B
CH O
3
2
3
solvent
.
b
Entry
Solvent
TBAB (mol%)
Base
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
H
H
H
H
H
2
2
2
2
2
O
O
O
O
O
0
50
50
100
50
0
K
K
K
K
K
K
K
K
K
2
2
3
3
3
2
2
2
2
CO
CO
3
3
2
39
48
70
62
78
12
78
50
39
81
99
74
99
89
2
PO
PO
PO
4
4
4
2
2
6
EtOH/H
2
O (v/v = 1:0.5)
CO
CO
CO
CO
3
3
3
3
2
DMF/H
DMF/H
DMF/H
DMF/H
DMF/H
DMF
2
O (v/v = 1:1)
O (v/v = 1:0.5)
O (v/v = 0.5:1)
O (v/v = 1:1)
O (v/v = 1:1)
0
2
2
2
2
2
0
2
0
2
1
1
1
1
0
1
2
0
Cs
2
CO
3
2
0
K
K
K
K
3
PO
3
PO
3
PO
3
PO
4
4
4
4
2
0
2
3
c
DMF/H
DMF/H
2
O (v/v = 1:1)
O (v/v = 1:1)
0
0.5
0.5
1
4
2
0
a
b
c
Reaction conditions: 4-bromoanisole (1.0 mmol), phenylboronic acid (1.5 mmol), base (2.0 mmol), catalyst (0.1 mol%) in 2 mL solvent at 100 8C in air.
Isolated yield.
0.05 mol% catalyst loading.
0
imidazoles [33]. The N,N -difluoroalkyl-substituted imidazolium
salt 1 was prepared by alkylation of imidazole with 1H,1H,2H,2H-
perfluorodecyl iodide in toluene in one pot (Scheme 1). The
product was obtained by filtration and washed with solvents
without further purification in moderate yield (43%).
catalyst as gray solid. The TEM image was shown in Fig. 1a. It was
obvious that the palladium nanoparticles were successfully
dispersed in the fluorous silica gel with an average size of 3–5 nm
[34,35]. Due to the low palladium loading on the fluorous silica gel,
there are no obvious peaks in XRD analysis.
N-heterocyclic carbene metal complex can be prepared by ligand
substitution reactions using free carbenes, which are accessible via
deprotonation of the corresponding imidazolium salts in THF with
KO-t-Bu at room temperature. The fluorous NHC–Pd complex was
isolated as gray solid. The immobilization of the catalyst was
according to the usual procedure, which heating the fluorous NHC–
Pd complex in perfluorooctane at 100 8C for 12 h, then added FSG
In order to evaluate the catalytic activity of the immobilized
fluorous NHC–Pd catalyst in Suzuki coupling reactions, we first
investigated the reaction between 4-bromoanisole and phenyl-
boronic acid in the presence of 0.1 mol% Pd catalyst. To begin our
study, we examined various parameters to optimize reaction
conditions. Water is cheap, readily available, nontoxic, and
nonflammable, which offers practical advantages over organic
solvents. Using water as solvent the yield was given in 39%
(Table 1, entry 1). It has been reported that additives such as
(C ; 35–70
8
mm) and stirred the mixture at the same temperature for
2
h. The fluorous solvent was vaporized to obtain the immobilized
Table 2
a
Suzuki cross-coupling reactions of aryl halides with arylboronic acids.
K PO , Cat.
3
4
X + (HO)2B
DMF/H O
R1
R2
2
R1
R2
.
b
Entry
X
R
1
R
2
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
Br
Br
Br
Br
Br
Br
Br
Br
Br
I
H
H
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
98
98
94
99
95
99
99
96
97
99
99
99
98
96
98
97
95
92
35
43
18
4-CH
2-CH
3
O
O
H
3
H
4-NO
2-NO
2
H
2
H
4-CF
3-CF
3
H
3
H
4-CH
3
CO
H
4-CHO
H
H
1
1
1
1
1
1
1
1
1
1
2
2
H
I
4-NO
2-NO
2
2
H
I
H
I
4-CH
2-CH
4-CH
4-CH
4-CH
4-CH
H
3
H
I
3
H
Br
Br
Br
Br
Cl
Cl
Cl
3
3
3
3
O
O
O
O
4-Cl
4-CF
3
4-CH
4-CH
H
3
O
3
4-NO
4-CH
2
H
1
3
H
1
a
Reaction conditions: aryl halide (1.0 mmol), arylboronic acid (1.5 mmol), K
Isolated yield.
3 4 2
PO (2 mmol), catalyst (0.1 mol% Pd) in 2 mL DMF/H O (v/v = 1:1) at 100 8C in air.
b

1
10
L. Wan et al. / Journal of Fluorine Chemistry 140 (2012) 107–111
100
3. Conclusions
90
80
70
60
50
40
30
20
10
0
In summary, we have prepared and characterized a novel
immobilized fluorous NHC–Pd catalyst and successfully applied it
for the Suzuki reaction. The catalyst showed high catalytic activity
9
9
91
85
for the reaction of aryl halides and arylboronic acid in DMF/H
2
O
(
v:v = 1:1). And the corresponding coupling products were
obtained in moderate to excellent yields. Moreover, the catalyst
could be easily recovered by filtration and reused three times
without significant loss of activity. The simple procedure for
catalyst preparation, easy recovery and reusability of the catalyst is
expected to contribute to its utilization for the development of
benign chemical process.
1
2
3
Run
4. Experimental
Fig. 2. Recycling experiments of the catalyst.
4.1. General
tetrabutylammoniumbromide(TBAB)canenhancethereaction rate
in the coupling reactions. However, only a little increase in yield
All the reagents were commercially available and used without
any further purification. The solvents were dried before use. IR
spectra were recorded in KBr disks with a Bomem MB154S FT-IR
when using 50 mol% TBAB (Table 1, entry 2). Then, we chose K
as a base in the Suzuki reaction and a good yield was obtained (Table
, entry 3). Increase in the amount of TBAB and prolongation of the
3 4
PO
1
13
spectrometer. H NMR and C NMR were recorded on Bruker DRX
500 and tetramethylsilane (TMS) was used as a reference.
Palladium content was measured by inductively coupled plas-
ma-atomic emission spectroscopy (ICP-AES) on a PE5300DV
instrument. The XRD spectra were recorded on a Bruker D8
ADVANCE X-ray diffraction spectrometer at 40 kV, 40 mA.
Transmission electron microscope (TEM) images were collected
on a JEOL-2100 transmission electron microscopy at 200 kV and
the images were recorded digitally with a Gatan 794 charge-
coupled device (CCD) camera. The TEM measurements were made
by sonication of the nanoparticulate material in perfluorodecalin
for several minutes, then one drop of the finely divided suspension
was placed on a specially produced structureless carbon support
film having a thickness of 4–6 nm and dried before observation.
1
reaction time did not result in the satisfactory level of the
corresponding product yield (Table 1, entries 4 and 5). It was
known that organic/aqueous co-solvent has the merit of good
solubility of the organic reactants and the inorganic base. Compared
with EtOH/H
entries 6 and 8). Next, we examined the effect of the volume ratio of
DMF/H O and the base on the product yield (Table 1, entries 7–11).
Asshown inTable1, thebestvolumeratioofDMF/H O is1:1, and the
highest yield was obtained when using K PO as base in 0.5 h. When
the catalyst loading was decreased to 0.05 mol%, a slightly low yield
2 2
O, a higher yield was obtained in DMF/H O (Table 1,
2
2
3
4
(89%) was observed (Table 1, entry 14).
Under the optimized conditions, we then evaluated the
efficiency of the immobilized fluorous NHC–Pd catalyst with
different substrates. As shown in Table 2, aryl bromides bearing
either electron-donating or electron-withdrawing substitutents
in the ortho and para positions, afforded the corresponding
biphenyls in excellent yields. The trifluoromethyl substituted aryl
bromides were more activated than bromobenzene in terms of
yields (Table 2, entries 6 and 7). Next, we examined aryl iodides for
the Suzuki reaction. And excellent yields of the corresponding
products were obtained under the optimized conditions. In
addition, the coupling reaction of 4-bromoanisole could be
efficiently carried out using various substituted arylboronic acids
4.2. Preparation of 1,3-bis(1H,1H,2H,2H-perfluorodecyl)imidazolium
iodide
A sealed tube was charged with imidazole (0.35 g, 5 mmol),
1H,1H,2H,2H-perfluorodecyl iodide (8.61 g, 15 mmol) and 10 mL
toluene. The mixture was heated at 110 8C overnight. After being
cooled to room temperature, the imidazolium salt was filtered,
washed with 5 mL EtOAc, 5 mL H
2
O, and dried at 50 8C to give 2.3 g
product (43%, white solid). H NMR (acetone-d ): 3.20 (m, 4H),
4.96 (t, 4H, J = 7.0 Hz), 8.10 (s, 2H), 9.77 (s, 1H). C NMR (acetone-
6
d ): d 32.8, 44.1, 108–122 (m), 125.1, 139.6. MS (ESI), m/z (%): 961
1
6
d
1
3
(
Table 2, entries 15–18). We also examined whether aryl chlorides
ꢀ
+
were active for the Suzuki reaction in our system. In this case, the
corresponding products were obtained in poor yields except for a
moderate yield obtained with 4-nitrochlorobenzene after 1 h
reaction (Table 2, entry 20).
[MꢀI ] (100), IR (KBr)
n
3414, 3142, 3057, 1580, 1334, 1203, 1149,
ꢀ
1
659 cm . Anal. Calc. for C23 34I (1088.20): C, 25.39; H, 1.02;
11 2
H N F
N, 2.57. Found: C, 25.72; H, 1.13; N, 2.49.
Catalyst recovery and reuse are important issues that deter-
mine the applicability of a heterogeneous catalyst. The immobi-
lized fluorous NHC–Pd catalyst can be recovered by simple
filtration and reused several times without significant loss of
activity (Fig. 2). The slight decrease of the yield may be due to the
size of palladium nanoparticles increased to about 5–8 nm and the
aggregation of palladium nanoparticles (Fig. 1b). The coupling
crude products obtained by extraction were detected with low
palladium contents (<3 ppm). This Pd level was a little higher than
the specifications required by the pharmaceutical industry
regarding the final purity of the products (Pd < 2 ppm). This
might be the fluorous–fluorous interactions were not strong
enough to highly stabilized the NHC–Pd complex on the fluorous
silica gel, so that it caused the Pd leaching in the reaction and
contaminated the Suzuki products.
4.3. Preparation of bis[1,3-bis(1H,1H,2H,2H-
perfluorodecyl)imidazol-2-ylidene]di-iodopalladium(II)
In an oven-dried Schlenk flask, the imidazolium salt 1 (0.272 g,
0.25 mmol), PdCl
2
(0.022 g, 0.125 mmol), KO-t-Bu (0.028 g,
0.25 mmol) and THF (5 mL) were added. The resulting suspension
was stirred at room temperature for 24 h under an inert
atmosphere. Then the solution was filtered and the solid was
2
washed with 5 mL EtOAc, 5 mL H O respectively, and dried under
vacuum at 50 8C for 6 h to obtain the fluorous NHC–Pd complex as
1
gray powder (0.269 g, 94%). H NMR (acetone-d
4.95 (t, 8H, J = 7.0 Hz), 8.09 (s, 4H). IR (KBr) 3468, 3103, 1657,
1565, 1534, 1498, 1471, 1412, 1242, 1205, 1149 cm . Anal. Calc.
for C46 Pd (2282.81): C, 24.20; H, 0.97; N, 2.45. Found: C,
24.64; H, 1.05; N, 2.38.
6
): d 3.19 (m, 8H),
n
ꢀ
1
22 4 68 2
H N F I

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