Palladium-loaded renewable polymer as a green heterogeneous catalyst for cross-coupling reactions under microwave irradiation

Article abstract of DOI:10.3184/174751914X14175181969937

A new palladium catalyst was prepared by immobilising ligand 2, 2'-dipyridylamine on the backbone of an acidic rosin polymer from gum rosin, on to which palladium(II) was bound via coordination. The catalyst at a low loading of 0.2 mol% was found to be highly effective for Suzuki-Miyaura coupling reactions of aryl halides and arylboronic acids under microwave irradiation in the presence of 1 equiv. of Na₂CO₃, affording excellent yields of the corresponding biaryls. Moreover, the catalyst exhibited very good recyclability over three cycles.

Full text of DOI:10.3184/174751914X14175181969937

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JOURNAL OF CHEMICAL RESEARCH 2014
VOL. 38 DECEMBER, 715–718
RESEARCH PAPER 715
Palladium-loaded renewable polymer as a green heterogeneous catalyst for
cross-coupling reactions under microwave irradiation
Dan Pan, Aiqun Wu, Pengfei Li, Haitang Xu, Fuhou Lei and Liqun Shen*
College of Chemistry and Chemical Engineering, Guangxi University for Nationalities, Key Laboratory of Development and Application of Forest
Chemicals of Guangxi, Nanning 530006, P.R. China
A new palladium catalyst was prepared by immobilising ligand 2, 2′‑dipyridylamine on the backbone of an acidic rosin polymer
from gum rosin, on to which palladium(II) was bound via coordination. The catalyst at a low loading of 0.2 mol% was found to
be highly effective for Suzuki–Miyaura coupling reactions of aryl halides and arylboronic acids under microwave irradiation in
the presence of 1 equiv. of Na2CO3, affording excellent yields of the corresponding biaryls. Moreover, the catalyst exhibited very
good recyclability over three cycles.
Keywords: renewable polymer, palladium, heterogeneous catalysts, Suzuki–Miyaura reaction, microwave irradiation
In the past 30 years, palladium‑mediated cross‑coupling
reactions have become popular as convenient techniques for the
formation of carbon–carbon bonds.1–6 There have been several
reports of highly‑active catalysts that can be used in various
homogeneous C–C cross‑coupling reactions (see refs 7–10).
Despite the considerable progress in homogeneous catalysis
that has been made, problems relating to the separation and
recycling of the catalyst persist. A promising approach for
achieving this goal is the development of heterogeneous
catalytic systems giving scope for efficient recovery and
repeated use in green processes.11–15 In this context several
studies have reported the synthesis of heterogeneous palladium
catalysts, e.g., based on palladium‑loaded polymers,16
palladium‑loaded inorganic solids,17,18 and dendrimers.19,20 In
addition, renewable polymers have become a rapidly growing
area, as these materials could potentially replace or partially
replace environmentally and energy unfavourable petroleum‑
derived polymers.21–23
Results and discussion
Synthesis of the Pd‑loaded polymer (Scheme 1)
2,2′-Dipyridylamine
1 was prepared by base‑catalysed
nucleophilic substitution of the chloro group of 2‑chloropyridine
by 2‑pyridylamine. The renewable resin polymer was treated
with thionyl chloride at 80 °C to convert the carboxylic acid
residues to acid chlorides to form derivatised polymer 2. Then
2 was treated with excess 2,2′-dipyridylamine and pyridine
at 100 °C for 3 h. (Ligand 2,2′-dipyridylamine is known to
possess a remarkable binding ability to transition metal ions.27)
The product of this reaction was a 2,2′-dipyridylamidated
polymer 3. Finally, treatment of 3 with Pd(OAc)2 yielded the
palladium(II)‑loaded polymer 4.
Characterisation of the amidated polymer 3 and Pd‑loaded
catalyst 4
The chemical conversion from renewable polymer to immobiled
2,2′-dipyridylamine renewable resin polymer 3 was verified
by FTIR analysis as shown in Fig. 1. In the spectrum of the
renewable polymer (a), the peaks attributed to the O–H bond
of the carboxyl group in the polymer display around 3495 cm–1
and the C=O bond shows an absorption at 1743 cm–1. The curve
(b) of immobilised 2,2′-dipyridylamine on renewable resin
polymer 3 shows the absence of the hydroxy band at 3495 cm–1
We recently developed a new class of renewable polymers
using gum rosin,24–26 a natural resource abundantly available.
We now report the use of a palladium‑on‑rosin catalyst in the
Suzuki–Miyaura cross‑coupling reaction of aryl halides and
phenylboronic acids under microwave irradiation.
toluene
C4H9OK
+
N
N
H
N
N
NH2
N
NH2
1
O
O
O
SOCl2
N
N
1
N
OH
cycohexane
Cl
pyridine
3
2
O
N
O
N
Pd(OAc)2
Pd2+
OH
N
acidic rosin polymer from gum rosin
4
Scheme 1
* Correspondent. E‑mail: shen.760@osu.edu
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716 JOURNAL OF CHEMICAL RESEARCH 2014
(b)
(a)
(b)
(a)
3500
3000
2500
2000
1500
1000
500
0
20
40
60
80
100
Wavenumbers(cm-1)
( )
2θ °
Fig. 2 Wide angle powder XRD image of (a) polymer 3 and (b) Pd‑loaded
catalyst 4.
Fig. 1 IR spectra of (a) the renewable polymer and (b) the amidated
polymer 3.
and the appearance of bands at 1662, 1604, 1563 cm–1 (amide,
pyridine) which revealed that the amidation reaction had been
successful.
The resulting Pd‑loaded catalyst 4 was characterised by
its XRD pattern (Fig. 2). The Bragg reflections at 2θ values
of 39.9°, 46.5°, 68.3°, 82.3° and 86.6° correspond to the index
face‑centred cubic crystalline structure of the palladium
(111), (200), (220), (311) and (222) lattice planes respectively,
suggesting the presence of crystalline palladium.
100
Compound
Compound
3
4
80
60
40
20
0
The amount of immobilised Pd on the renewable rosin
polymer 3 was quantified by ICP‑AES. The loading level of Pd
on the resin was found to be 0.093 mmol g–1.
Since Suzuki–Miyaura reactions are usually carried out
at high temperature, the thermal stability and recyclability
of a catalyst is important. We therefore carried out a
thermogravimetric (TG) analysis. The TG curves in Fig. 3
of polymer 3 and Pd‑loaded catalyst 4 show that the catalyst
systems were stable up to 225 °C under a nitrogen atmosphere.
The first mass decrease was due to the release of assimilated
water. This was followed by weight loss caused by oxidative
decomposition of the rosin polymer.
0
100
200
300
400
500
600
700
Temperature(℃)
Fig. 3 Thermogravimetric curves of amidated polymer 3 and Pd‑loaded
catalyst 4.
Moreover the reactions would not proceed without base (entry
6). From the solvent screening, (1:1) EtOH/H2O was found to
be optimal (entries 7–12). If just water was used, only moderate
results were obtained (entry 12). We found that the solvent
system is an important factor for the Suzuki–Miyaura in the
heterogeneous catalytic system, as such a reaction requires
both the compatibility of the solid support and the solubility of
the organic and inorganic reagents.
To evaluate the effectiveness of the microwave‑assisted
reaction, we carried out a comparison using the optimal
conditions, between the catalysed coupling reaction of 4‑bromo
acetophenone with 4‑methoxyphenylboronic acid, heated by
conventional methods and microwave radiation. The results
are listed in Table 2. It is clear that the microwave‑assisted
Suzuki–Miyaura reactions superior to conventional heating
being faster and giving a greater yield.
Catalysis of the Suzuki–Miyaura reactions
Compared to traditional heating, microwave‑heating can
enhance the rate of reaction, reduce side reactions and increase
product yields,28–30 so we chose to employ this method to
accelerate our reaction. Initially, the reaction conditions of
the Suzuki–Miyaura reaction using the Pd‑loaded catalyst
4 were optimised as regards the solvent and base using as a
model the reaction between 4‑bromoacetophenone (5 X=Br,
R1=4‑MeCO) and 4‑methoxyphenylboronic acid (6 R2=MeO)
in ethanol/water (1:1) as solvent (Scheme 2). Na2CO3 gave
the highest yield of 4-methoxy-4′-acetylbiphenyl among the
several bases tested, as Table 1 shows. Other inorganic bases,
such as K3PO4, KOH and K2CO3 are slightly less efficient,
while Et3N gave the product in a much lower yield (entry 5).
Pd catalyst
4 (0.2 %)
R2
B(OH)2
R2
+
X
120 oC, MW
R1
R1
5 min, Na2CO3,
EtOH/H2O
7
5
6
Scheme 2
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JOURNAL OF CHEMICAL RESEARCH 2014 717
Table 1 The effect of varying the base and solvent in the Suzuki–Miyaura
cross‑coupling reaction of 4‑bromoacetophenone (5 X=Br, R1=4‑
MeCO) with 4‑methoxyphenylboronic acid (6 R2=MeO) under microwave
irradiation (Scheme 2)a
Table 3 Suzuki–Miyaura couplings of various aryl halides (5 X=Cl, Br
or I; R1=various) with phenyl‑(6 R2=H) or 4‑methoxyphenyl‑boronic acid
(6 R2=MeO) under the optimised conditions (Scheme 2)a
Entry
X
R1
6
Product
Yield/%b
Entry
Base
Solvent (1:1)
Yield/%b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
I
H
H
H
H
H
H
H
H
H
H
H
H
H
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
7m
7n
7o
7p
7q
7r
98
98
97
97
92
97
95
96
94
95
87
71
99
98
97
98
96
97
95
96
95
98
80
66
1
2
3
4
5
6
7
8
9
Na2CO3
K3PO4
KOH
K2CO3
Et3N
(without base)
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
DMF/H2O
DMAC/H2O
MeCN/H2O
MeCOMe/H2O
H2O
99.7
98.5
92.4
94.7
76.9
0
99.7
99.0
98.0
57.8
95.7
77.2
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
I
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
4‑MeCO
4‑CHO
3‑CHO
2‑CHO
4‑OMe
4‑CH3
4‑OH
4‑Cl
H
H
4‑NO2
H
4‑MeCO
4‑CHO
3‑CHO
2‑CHO
4‑OMe
4‑CH3
4‑OH
4‑Cl
10
11
12
aReaction conditions:4‑bromoacetophenone (5 X=Br, R1=4‑MeCO)
(0.5 mmol), 4‑methoxy phenylboronic acid (6 R2=MeO) (0.65 mmol),
base (1 mmol), solvent (2 mL) and 0.2 mol% Pd catalyst 4 were subjected
to microwave irradiation (60 W) for 5 min at 120 °C. DMAC, N,N‑
Dimethylacetamide.
bDetermined by HPLC analysis.
7s
7t
7u
7v
7w
7x
Table 2 The effect of varying the heating mode for Suzuki–Miyaura cross‑
coupling reaction of 4‑bromoacetophenone (5 X=Br, R1=4‑MeCO) with
4‑methoxyphenylboronic acid (6 R2=MeO) (Scheme 2)a
H
H
4‑NO2
Entry
Heating mode
Time/min
Yield/%b
aReaction conditions: aryl halide (0.5 mmol), phenyl‑(6 R2=H) or
4‑methoxyphenylboronic acid (6 R2=MeO) (0.65 mmol), Na2CO3 (1 mmol),
EtOH/H2O (1:1) (2 mL) and 0.2 mol% Pd‑loaded catalyst 4 were subjected
to microwave irradiation (60 W) for 5 min at 120 °C.
1
2
3
4
Microwave irradiation
Conventional reflux
Conventional reflux
Conventional reflux
5
30
60
99.7
38.7
69.5
93.1
120
bIsolated yield after purification by column chromatography.
aReaction conditions: 4‑bromoacetophenone (0.5 mmol), 4‑methoxy
phenylboronic acid (0.65 mmol), Na2CO3 (1 mmol), EtOH/H2O (1:1)
(2 mL) and 0.2 mol% Pd catalyst 4 were subjected to heating at various
temperatures or to microwave irradiation (60 W) for 5 min at 120 °C.
bDetermined by HPLC analysis.
Table 4 Recycling efficiency of the Suzuki–Miyaura cross‑coupling of
iodobenzene (5 X=I; R1=H) with 4‑methoxyphenylboronic acid (6 R2=4‑
MeO) under the optimised conditions over six cycles (Scheme 2)a
Cycles/n
1
2
3
4
5
6
Conversion/%b
99.9
97
92.7
82.2
75.5
53
The catalytic system was applicable to a wide range of aryl
halides and two different types of boronic acid (Scheme 2)
(Table 3). The results show that the couplings of aryl bromides
with arylboronic acids whether bearing an electron‑donating
or an electron‑withdrawing group proceeded in good yield
(entries 2–10). Similar results were obtained in the reaction of
iodobenzene with arylboronic acids (entry 1). We also explored
the scope of this reaction using aryl chlorides, and found that the
conversion of aryl chloride was very good (87%, entry 11). The
study of the coupling reaction using 4‑methoxyphenylboronic
was also carried out, and the corresponding products were
produced in excellent yields (entries 13–24).
aReaction conditions: iodobenzene (5 X=I; R1=H) (0.5 mmol),
4‑methoxyphenylboronic acid (6 R2=MeO) (0.65 mmol), Na2CO3 (1 mmol),
(EtOH/H2O (1:1) (2 mL) and recycled 0.2 mol% Pd catalyst 4 were
subjected to microwave irradiation (60 W) for 5 min at 120 °C.
bDetermined by HPLC analysis.
the Suzuki–Miyaura cross‑coupling reactions of aryl halides
and phenylboronic acids under microwave irradiation.
Experimental
Unless otherwise noted, reagents and materials were commercially
available and were used as received without further purification.
Solvents were purified according to standard methods. The acidic
rosin polymer (acid value: 4.464 mmol g–1) was prepared according
to established procedures.26 Melting points were measured using a
WRS‑1B apparatus. The 1H NMR spectra were recorded on a Bruker
Avance 600 MHz spectrometer in CDCl3 using TMS as internal
standard. Chemical shifts (δ) are given in ppm and coupling constants
(J) in Hz. The IR spectra were performed in the range 400–4000 cm–1
using KBr pellets on a Nicolet IS10 FTIR spectrometer. HPLC
detection was carried out on a C18 reversed‑phase column (5 µ,
200×4.6 mm) using MeOH:water (80:20)(v/v) as the mobile phase at
a flow rate of 0.8 mL min–1. Microwave reactions were carried out in
a Biotage Initiator 60. X‑ray powder diffraction (XRD) images were
determined on a XD‑3 and TGA analyses on a STA 449 F3. ICP‑AES
spectra data were recorded on an iCAP‑6000 SERIES.
Catalyst recycling
To evaluate the reusability of the Pd‑loaded catalyst 4, a
recycling test was carried out using the recovered catalysts from
the Suzuki–Miyaura cross‑coupling reaction of iodobenzene
with 4‑methoxyphenylboronic acid (Table 4). The reaction
was carried out with Na2CO3 in a mixture of (1:1) EtOH/H2O
under microwave irradiation for 5 min. Upon completion of the
reaction, products were extracted with dichloromethane and
the yields were determined by HPLC. The resulting Pd catalyst
4 was reused with the same substrates for the next cycle. As can
be seen from Table 4, the Pd‑loaded catalyst 4 could be used
three times with no significant decrease in the yields. But on
the sixth cycle, the yield had dropped to 53%.
Conclusion
Synthesis of 2,2′-dipyridylamine 1
In conclusion, we have prepared a palladium catalyst loaded
onto a renewable rosin polymer and applied it successfully to
Potassium t‑butoxide (15.50 g, 138.1 mmol) was added to a solution
of 2‑aminopyridine (10.00 g, 106.2 mmol) and 2‑chloropyridine
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718 JOURNAL OF CHEMICAL RESEARCH 2014
(13.27 g, 116.8 mmol) in toluene (100 mL), and the mixture was stirred
at 120 °C for 36 h under a nitrogen atmosphere. After cooling to room
temperature and removing solvent under vacuum, the crude product
was extracted with ether and the combined ethers then washed with
water three times. The organic layer was dried with magnesium sulfate,
filtered and the solvent was removed under reduced pressure. The
residue was purified by chromatography on silica gel to yield a white
solid,31 13.73 g (75.5%). m.p. 90.3–91.6 °C (lit.32 94–95 °C), 1H NMR: δ
8.28–8.27 (m, 2H), 8.19 (brs, 1H), 7.61–7.55 (m, 4H), 6.85–6.83 (m, 2H).
This work was supported by the National Natural Science
Foundation of China (No. 21062002), Natural Science
Foundation of Guangxi Zhuang Autonomous Region (No.
2014GXNSFAA118032) and colleges and universities in
Guangxi science and technology research projects (ZD2014046,
2013YB079).
Received 27 October 2014; accepted 5 November 2014
Paper 1402976 doi: 10.3184/174751914X14175181969937
Published online: 19 December 2014
Synthesis of the amidated polymer 3
After the rosin polymer (2.00 g, acid value: 4.46 mmol g–1) was
swelled in dry cyclohexane (20 mL) at room temperature for 4 h,
thionyl chloride (3.19 g, 26.78 mmol) was added with cooling.
Then the mixture was stirred at 80 °C for 3 h. After cooling to room
temperature and removing the excess thionyl chloride, the residue
was washed twice with cyclohexane to afford a white solid compound
2. Then, pyridine (1.06 g, 13.4 mmol) was added to a mixture of
compound 2 and 2,2′-dipyridylamine 1 (1.83 g, 10.7 mmol) in dry
1,4‑dioxane (20 mL), and the reaction mixture was refluxed at 100 °C
for 3 h. After removing solvent under reduced pressure, the residue
was washed three times with water, and then dried in vacuum. A white
solid compound 3 was obtained, yield 2.21 g; IR (KBr): 2949, 1726,
1662, 1604, 1563, 1438, 1247, 1150, 1018, 771 cm–1.
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Suzuki–Miyaura coupling reaction; general procedure
Into a 10 mL glass vial were placed aryl halide (0.5 mmol), phenyl‑ or
4‑methoxy phenylboronic acid (0.65 mmol), base (1 mmol), Pd‑loaded
catalyst 4 (0.2 mol% Pd) and EtOH/water (1:1) (2 mL), and a magnetic
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Catalyst stability and recycling
Iodobenzene (0.5 mmol), 4‑methoxy phenylboronic acid (0.65 mmol),
Na2CO3 (1 mmol), EtOH/H2O (1:1) (2 mL), Pd‑loaded catalyst 4
(0.2 mol% Pd) and a magnetic stirrer was added into a 10 mL glass
vial. The vial was sealed and placed into the microwave cavity.
The reaction was carried out at 120 °C for 5 min under microwave
irradiation. After cooling to room temperature, the reaction contents
were extracted with dichloromethane (5 mL×2), and the solid polymer
catalyst was filtered off and placed into another glass vial for the next
reaction cycle. The filtered resins were reused five times for the same
reaction. The yields were also determined by HPLC.
39 Y.M.A. Yamada, S.M. Sarkar and Y. Uzoumi, J. Am. Chem. Soc., 2012,
134, 3190.
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