A novel graphene-supported palladium catalyst for suzuki-miyaura coupling of less reactive heteroaryl halides in water

Article abstract of DOI:10.1002/bkcs.10894

An efficient reduced graphene oxide-supported heterogenized palladium complex with pyrene-tagged ketoiminato ligand has been developed and used for cross-coupling of less reactive heteroaryl halides in water. Desirable catalytic activity was observed with high (up to 98%) conversion of the less reactive reactants under relatively mild conditions. The efficiency and green nature of the catalyst were confirmed without significant loss of activity.

Full text of DOI:10.1002/bkcs.10894

Article
DOI: 10.1002/bkcs.10894
BULLETIN OF THE
A. Taher et al.
KOREAN CHEMICAL SOCIETY
A Novel Graphene-Supported Palladium Catalyst for Suzuki–Miyaura
Coupling of Less Reactive Heteroaryl Halides in Water
‡
‡,
Abu Taher,† Dong-Jin Lee,† Ik-Mo Lee,†, Md. Lutfor Rahman, and Md. Shaheen Sarker
*
*
†Department of Chemistry, Inha University, Incheon 402-751, South Korea. *E-mail: imlee@inha.ac.kr
‡Faculty of Industrial Sciences and Technology, University Malaysia Pahang, 26300 Gambang, Kuantan,
Pahang, Malaysia. *E-mail: shaheen@ump.edu.my
Received June 8, 2016, Accepted July 20, 2016, Published online August 29, 2016
An efficient reduced graphene oxide-supported heterogenized palladium complex with pyrene-tagged
ketoiminato ligand has been developed and used for cross-coupling of less reactive heteroaryl halides in
water. Desirable catalytic activity was observed with high (up to 98%) conversion of the less reactive
reactants under relatively mild conditions. The efficiency and green nature of the catalyst were confirmed
without significant loss of activity.
Keywords: Graphene oxide, Heterogeneous catalyst, Palladium, Cross-coupling, Heteroaryl halides
Introduction
attention due to their convenient molecular attachment in
organic synthesis.14 The widely used covalent immobiliza-
tion strategies often require nontrivial pretreatment of the
solid surface. However, noncovalent methods exploiting
strong π interactions between poly-aromatic hydrocarbons
and nano-graphitized surfaces offer a convenient and practi-
cal approach for the immobilization. Therefore, noncova-
lent methods6 have attracted great interest in the field of
catalysis. Because of the inherent nature of the pyrene moi-
ety to afford noncovalent interactions with nano-graphitic
surfaces,15 complexes with a pyrene tag have been intro-
duced on graphitized surfaces16 and have also been used in
catalysis, proving interesting recyclability properties.17
Despite these impressive recent improvements, the catalytic
protocol still suffers from some disadvantages, such as
metal leaching and low reactivity for several important sub-
strate classes. Therefore, the development of reusable cata-
lysts for the SMC reaction under mild conditions is highly
desirable from the view points of both green chemistry and
current organic synthesis. Here we report a highly efficient
and reusable rGO-supported Pd complex (4) for the cross-
coupling of less reactive and sterically hindered heteroaryl
halides in water under mild reaction conditions (Scheme 1).
Heterobiaryl compounds are a very common structural
motif in various natural products, pharmaceuticals, and bio-
logically active organic molecules.1 The palladium (Pd)-
catalyzed Suzuki–Miyaura coupling (SMC) reaction is one
of the most characteristic and frequently used methods to
construct such carbon–carbon bonds in organic synthesis,2
as it facilitates the coupling of heteroaryl halides with boro-
nic acids. In recent years, SMC reactions have received
much attention for the synthesis of heteroatom-containing
biaryl compounds with great industrial potential.3 Even
though a large number of reports have been published in
this field of research, most heterocyclic coupling partners
have not been widely used4 and only limited progress has
been made in heterobiaryl production. This may be due to
the fact that coupling reactions with heteroaryl substrates
have proven more challenging than the analogous couplings
of simpler substrates. Thus, more efficient catalytic meth-
ods for the corresponding coupling products are required.5
Even though satisfactory results have been obtained in lim-
ited cases, long reaction time at relatively high temperature
and pressure was usually required.3
On the other hand, the incorporation of a homogeneous
catalyst onto a support is one of the most challenging work
in the field of catalysis, because this allows easier reproces-
sing of the used catalyst.6 In addition, recycling of homoge-
neous metal catalysts is considered one of the important
objectives in the field of green chemistry.7 One such an
environmentally benign process involves the development
of solid-supported catalysts exhibiting high catalytic effi-
ciency even in an aqueous medium.8 Thus, efforts have
been made to immobilize homogeneous catalysts on vari-
ous support materials, such as nanoparticles,9 inorganic
solids,10 polymers,11 dendrimers,12 and perfluorinated
tags.13 Graphene surfaces have recently gained much
Experimental
General Information. All reagents were purchased from
commercial sources and used without any prior purification,
unless otherwise stated. 1H and 13C NMR spectra were
recorded at either 300 or 400 MHz. Chemical shifts were
measured in parts per million (δ ppm) referenced to the sol-
vent peak or 0.0 ppm for tetramethylsilane peak. The FT-IR
spectra were obtained on a Nicolet iS10 FT-IR spectrome-
ter. All products were isolated by chromatography using a
silica gel (0.035–0.070 mm) column, unless otherwise
noted. X-ray powder diffraction (XRD) patterns were
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KOREAN CHEMICAL SOCIETY
purified by column chromatography using MeOH as an elu-
ent to afford (pyren-1-yl)methanamine 1 in 90% (0.24 g)
yield.1H NMR (400 MHz, CDCl3) δ 8.31–7.97 (m, 9H,
arom); 4.56 (d, 2H, J = 1.2 Hz); 1.66 (s, 2H); 13C NMR
(100 MHz, CDCl3) δ 136.74–122.67 (16C, arom), 44.33 (s,
1C, alph); IR (cm−1): 3364 (N H stretch), 3038 (C H,
arom), 1570 (C C, arom), 1183 (C N, stretch), 881, 838
and 709 (C H, bend); Anal. Calcd. for C17H13N
(231.29): C, 88.28; H, 5.67; N, 6.06. Found: C, 88.02; H,
5.83; N, 5.91%.
Synthesis of 4-((Pyrene-1-yl)methylamino)pent-3-en-2-
one (2).
A solution of pentane-2,4-dione (0.171 g,
1.72 mmol) in absolute ethanol (6 mL) was added drop-
wise with continuous stirring to a solution of (pyren-1-yl)
methanamine 1 (0.36 g, 1.56 mmol) in ethanol(10 mL).
The resulting solution was mildly refluxed with stirring for
12 h. The color changed from pale green to bright orange.
The reaction mixture was then allowed to cool at RT and
concentrated under reduced pressure. After purification of
the crude product using silica gel column chromatography
(EtOAc/hexane, 1:1), 2 was obtained as a white solid in
Scheme 1. Synthesis of reduced graphene oxide-supported cata-
lyst 4.
recorded on a Rigaku diffractometer using Cu Kα radiation
(λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS) mea-
surements were carried out on a K-Alpha instrument
(Thermo Scientific). For transmission electron microscopy
(TEM) measurements, a drop of the sample suspension was
placed on Formvar and carbon-coated positive-glow-dis-
charge-treated copper grid and subsequently blotted dry by
a filter paper. The prepared samples were examined in a
JEM-2100 F field-emission transmission electron micro-
scope (FE-TEM) with energy dispersive spectroscopy
(EDS) and the images were recorded using a charge-coupled
device camera. Scanning electron microscopy (SEM)
images and EDS elemental mapping were observed with an
SU8010 microscope (Hitachi, Japan). Thermogravimetric
analysis (TGA) was carried out on a Scinco S-1000 system
under a N2 gas flow. The Raman spectra were measured
using an excitation wavelength of 457.9 nm provided by a
Spectra-Physics model 2025 argon ion laser. Elemental
analysis was carried out using a Flash EA1112 analyzer
(Thermo Electron Corporation). Gas chromatography
(GC) or GC–mass spectrometry (GC–MS) was performed
on an Agilent 6890N GC coupled to an Agilent 5975 Net-
work mass-selective detector. The Pd content was measured
using inductively coupled plasma-atomic emission spectros-
copy (ICP-AES) by an OPTIMA 4300 DV instrument
(Perkin-Elmer). UV–vis spectra (diffuse reflectance mode)
were recorded on a UV–vis spectrophotometer (Varian,
Australia). The spectra were recorded in the range
200–800 nm using boric acid as the standard of reflectance.
1
68% (0.329 g) yield. H NMR (400 MHz, CDCl3): δ 11.39
(s, 1H, NH); 8.18–7.89 (m, 9H, C-H, Ar); 5.11 (d, 2H,
J = 6.4 Hz); 5.09 (s, 1H, CH); 2.06 (s, 3H, CH3); 1.99 (s,
3H, CH3) ppm; 13C NMR (100 MHz, CDCl3): δ 195.45,
163.06, 131.21–121.71 (16C, Ar), 96.14, 44.81, 28.88,
18.96 ppm; IR (cm−1): 3283 (N H stretch, amine), 3037
(C H, Ar), 2919 (C H, Al), 1662 (C O), 1581 (C C,
Ar), 1190 (C–N stretch), 837, 748 and 705 (C H “oop,”
Ar); Anal. Calcd. for C22H19NO (313.39): C, 84.31; H,
6.11; N, 4.47. Found: C, 84.36; H, 6.34; N, 5.01%.
Synthesis of Pyrene-tagged Pd complex (3). A solution
of (CH3CN)2PdCl2 (0.27 g, 1.05 mmol) in acetone (9 mL)
was added to a stirred solution of 4-((pyrene-1-yl)methyla-
mino)pent-3-en-2-one
2 (0.33 g, 1.05 mmol, acetone,
7 mL). The resulting solution was gently stirred at 45ꢀC for
2 h during which time, pale orange crystals precipitated
out. The reaction flux was then removed from the heating
bath and allowed to cool at room temperature. The products
were separated by filtration and washed repeatedly with
acetone to give the corresponding pyrene-tagged Pd-
complex 3 in 80% (0.41 g) yield. 1H NMR (400 MHz,
DMSO-d6): δ 11.07 (s, 1H, NH); 8.35–7.95 (m, 9H, C–H,
Ar); 5.23 (d, 2H, J = 5.2 Hz); 5.06 (s, 1H, CH); 2.07 (s,
3H, CH3); 1.85 (s, 3H, CH3) ppm; 13C NMR (100 MHz,
DMSO-d6): δ 192.96, 163.60, 145.28, 131.71–122.84
(15 C, Ar), 95.30, 44.25, 28.44, 18.74 ppm; IR (cm−1):
3281 (N–H stretch, amine), 2862 and 2940 cm−1 (C H),
1668 (C O), 1593 (C C), 1187 (C N stretch), 844 (C H
“oop”); Anal. Calcd. for C22H19NOPdCl2 (490.72): C,
53.85; H, 3.90; N, 2.85. Found: C, 53.98; H,
3.98; N, 2.98%.
Synthesis of (Pyren-1-yl)methanamine (1). A solution of
NaOH (0.054 g, 1.34 mmol) in water (12 mL) was added
dropwise to a stirring solution of 1-pyrenemethylamine
hydrochloride (0.30 g, 1.12 mmol) in methylene chloride
(MC) (12 mL). The resulting solution was gently stirred at
room temperature (RT) for 0.5 h and, at the end of reaction,
the color changed from yellow to white. The reaction mix-
ture was diluted with MC and separated. The organic layer
was dried over magnesium sulfate (MgSO4) and evaporated
under reduced pressure. The crude reaction product was
Synthesis of 4. To a solution of pyrene-tagged Pd complex
3 (10 mg, 0.02 mmol) in CH2Cl2, reduced graphene oxide
(50 mg) was added. The mixture of reagents was sonicated
for 10 min and then stirred for 10 h at room temperature.
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A Highly Efficient and Recyclable Palladium Catalyst
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The solvent was removed by using a glass filter, and the
resulting catalyst was washed with sufficient amount of
CH2Cl2 and dried under vacuum at 80ꢀC to give the desired
rGO-supported Pd complex (4). The loading amount of Pd
content (0.21 mmol/g) onto the graphene was measured by
weight gain and ICP analyses.
General Procedure for the Suzuki–Miyaura Coupling
Reaction. To a 10-mL reaction vial, heteroaryl halide
(1.0 mmol), boronic acid (1.2 mmol), K3PO4 (2.0 mmol),
tetra-butylammonium bromide (TBAB) (0.5 mmol), and
4 (0.1 mol %) in water (3.5 mL) were added. The reaction
mixture was stirred at 85ꢀC and the reaction progress was
monitored by GC–MS analysis. After completion of the
reaction, it was diluted with H2O and CH2Cl2. The organic
layer was separated from mixture, the dried organic layer
over MgSO4, and evaporated under reduced pressure. The
crude reaction product was purified using column chroma-
tography on silica-gel to afford the corresponding product
with isolated yield up to 98%.
Procedure for Recycling the Pd Complex 4. In the recy-
cling experiment of the substrate, the reaction was per-
formed according to the general procedure of the SMC
reaction. After completion and cooling down of the reac-
tion, the Pd complex 4 was separated from the mixture by
centrifugation. The separated Pd complex 4 was washed
with CH2Cl2 and water and directly used for the next reac-
tion without any change in reaction conditions.
corresponding to hydroxyl group22 because water mole-
cules are usually adsorbed on the surface of rGO flakes or
are inserted between the layers during sonication.23 The
UV–vis absorption spectrum of 4, 3, and rGO were ana-
lyzed (Figure 1(a)). No UV signal was found for the pure
rGO sample, whereas UV signals were well overlaid for
3 and 4 due to the presence of pyrene units that absorb at
342 nm, and it proves that pyrene is grafted on the gra-
phitic surface.24 Moreover, EDS spectra showed the exist-
ence of palladium on the rGO surface (Figure 1(b)). Thus,
we confirmed that the complex 3 was successfully anchored
onto the graphitic surface. The morphology of 4 was
observed by FE-TEM and HR-SEM analyses (Figure 1). In
Figure 1(c) and (d), the TEM images of fresh 4 and that
after conjugative recycling of 4 are shown. The elemental
mapping by EDS performed by means of HR-SEM of cata-
lyst 4 showed the homogeneous distribution of Pd in the
hybrid materials (Figure 1(f )).
Raman spectroscopy was used to prove the structural fea-
ture of graphene-based materials. Figure 2(a) illustrates the
featured areas of the Raman spectra for rGO and 4. The I
(D)/I(G) value in fresh rGO is 1.35, but this value in
4 decreased to 1.32, which indicated the incorporation of
3 on the graphitic surface. Moreover, an additional peak at
432 cm−1 was observed that clearly indicated the presence
of palladium on the graphitic surface,25 which also con-
firmed by XPS survey analysis (Figure 2(c)). The thermal
stability of the prepared catalyst 4 was measured using
TGA analysis (Figure 2(b)).
Results and Discussion
The TGA result revealed the excellent thermal stability
of 4. It is assumed that the higher weight loss of 4 was due
to the thermal decomposition of complex 3 that was
anchored to the graphitic surface.
To gain further insights on the surface changes of rGO
associated with the chemical reaction shown in Scheme 1,
we also performed XPS analysis (Figure 3). The N 1s spec-
trum of 4 can be deconvoluted into three components at
The (pyren-1-yl)methanamine 1 was prepared from com-
mercially available 1-pyrenemethylamine hydrochloride
under basic condition at room temperature in CH2Cl2/
H2O. Compound 2 was prepared by the reaction of 1 with
pentane-2,4-dione in refluxing ethanol. The Pd complex
3 was prepared from 2 with (CH3CN)2PdCl2 in acetone at
45ꢀC for 2 h. The Pd complex 3 is a nonplanar and pale
yellow crystalline powder, which is confirmed by density
functional theory optimization and XRD analysis
(Figures S1 and S2, Supporting Information). The Pd com-
plex 4 was obtained by immersing 3 and commercially
available rGO in CH2Cl2 followed by filtration and washing
repeated with an excess amount of CH2Cl2 at room temper-
ature (Scheme 1). The maximum loading of palladium in
4 was measured by ICP analysis (0.21 mmol/g), which
exactly matched with the calculated value. The palladium
complex 3 was anchored onto the rGO layer through non-
covalent interaction,18,19 and the presence of 3 on the rGO
layers was examined by FT-IR, FE-TEM, HR-SEM, XPS,
EDS, TGA, ICP, UV–vis, and Raman spectroscopy ana-
lyses. The FT-IR spectra of both complexes 3 and 4 show
intense bands in the region 1681–1644 cm−1 (Figure S3)
due to the existence of carboxylato and phenyl ring stretch-
ing vibrations.20 The N H stretching peaks at
3300–3350 cm−1 indicate the presence of the NH group.21
We found an intense and broad band at 3400–3600 cm−1
399.17, 400.04, and 400.89 eV, corresponding to C NH,
N H, and NH3 , respectively (Figure 3(a)).26,27 The peak
+
for C N (285.4 eV) in 4 confirmed the presence of amino
groups (Figure 3(b)). Furthermore, the O 1s spectra were
reasonably fitted into a couple of peaks at 531.76 eV for
C O, and 535.57 eV for O C O, indicating the presence
of the carbonyl group in 4 (Figure 3(c)). For the as-
prepared 4, the presence of the Pd 3d3/2 (343.13 eV) and
Pd 3d5/2 (337.60 eV) peaks indicates the successful incor-
poration of Pd(II) on the graphitic surface (Figure 3(d)).28
The SMC reaction is widely used for building units of
biaryl in organic synthesis,29,30 and the use of inexpensive
heteroaryl halides would be extremely desirable for indus-
trial applications. Because of high stability of the catalyst
4, efforts were immediately expanded to use this complex
for the SMC of heteroaryl halides. To obtain the optimal
reaction conditions, the SMC reaction of 2-bromopyridine
with phenylboronic acid was performed in water using cat-
alyst 4, and the results are summarized in Table 1. The
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Figure 1. (a) UV–vis diffuse reflectance spectra of 3, 4, and rGO. (b) EDS analysis of 4 (note that the Cu peaks come from the copper
grid). TEM images of 4 (c) before use and (d) after the fourth run. (e) SEM image of 4. (f ) EDS mapping image of catalyst 4 showing the
homogeneous distribution of palladium.
Figure 2. (a) Raman spectra of rGO and 4. The corresponding I(D)/I(G) values are shown for each spectrum. (b) TGA plots of rGO and
4 with at the heating rate of 20ꢀC/min under air. XPS analysis of catalyst 4: (c) survey spectra and (d) survey spectra after the fourth run.
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Figure 3. XPS analysis of 4: (a) N 1s, (b) C 1s, (c) O 1s, and (d) Pd 3d.
Table 1. Optimization of 4 in the SMC reaction.a
4
+
Br
(HO)2B
Base,TBAB
Water
N
N
Entry
4 (mol%)
TBAB (mmol)
Base
Time (h)
T (ꢀC)
Yieldb (%)
1
2
3
4
5
6
7
8
0.1
0.1
0.1
0.1
0.02
0.01
0.005
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K2CO3
Na2CO3
CsF
KOAc
NaOH
Et3N
(i-Pr)2NEt
Piperazine
Et2NH
(n-Bu)3N
Pyridine
5
2
90
85
85
85
85
85
85
55
85
85
85
85
85
85
85
85
95
95
95
96
97
95
98
95
96
95
96
85
83
76
46
25
81
63
51
0.1
0.3
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
1.6
1
1.3
1.5
2
3
1
1
1
1
1
1
1
1
1
1
1
9
10
11
12
13
14
15
16
17
18
19
a
Trace
Trace
–
Reaction conditions: 2-bromopyridine (1.0 mmol), PhB(OH)2 (1.2 mmol), base (2.0 mmol), and water (3.5 mL).
Isolated yield is given in parenthesis.
b
yield of the products gradually increased with increase of
of 4 and K3PO4 as base in water at 90ꢀC to give the corre-
sponding coupling product in 97% yield within 5 h (entry
1). Then, the phase-transfer additive TBAB was added in
the reaction mixture to enhance the reaction yield. As
expected, the yield of the coupling product significantly
the loading amount of 4 (0.005–0.1 mol %). Furthermore,
to get higher efficiency of 4, we have changed the
amounts of TBAB as an additive and base. The initial
reaction was performed without TBAB using 0.1 mol %
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Table 2. SMC of heteroaryl halides in water.a
4 (0.1 mol %)
+
(HO)2B
Heteroaryl
X
Heteroaryl
K3PO4, TBAB
85 °C, 1 h, Water
X = Br, Cl
Entry
1c
Aryl halide
Product
Yieldb (%)
0
Br
N
N
N
2
3
100 (98)
97 (95)
Br
N
Br
N
N
N
N
4
5
98 (96)
83 (81)
Br
Me
Me
Br
Br
N
N
6
7
96 (94)
85 (82)
N
N
N
N
NH2
NH2
r
B
N
N
8
9
91 (88)
81 (80)
r
H2N
B
H2N
N
N
r
B
N
N
10
100 (98)
100 (98)
99 (97)
86 (84)
H
r
H
O C
O
C
B
S
S
11
Br
S
S
12d
13d
r
B
r
B
S
S
H13
C6
H13
C6
r
B
r
B
S
S
14e
15e
100 (96)
95 (93)
l
C
N
N
l
C
S
S
16e
90 (88)
NH2
NH2
l
C
N
N
a
Reaction conditions: heteroarylhalide (1.0 mmol), PhB(OH)2 (1.2 mmol), K3PO4 (2.0 mmol), TBAB (0.5 mmol) and water (3.5 mL).
GC yields; isolated yield is given in parenthesis.
rGO used as catalyst.
PhB(OH)2 (2.4 mmol), K2CO3 (3.0 mmol) and 4 (0.2 mol%).
90ꢀC for 6 h.
b
c
d
e
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increased with a simultaneous decrease of the reaction time
and temperature when 0.1–0.5 mmol of TBAB was added
(entries 2–4). It was observed that high catalytic activity
still maintained at a very small (0.01–0.005 mol %)
amount of catalyst loading (entries 5–7). It is also note-
worthy that 4 efficiently promoted this coupling reaction
even at 55ꢀC to provide the corresponding product in 96%
yield within 3 h (entry 8).
Among the various bases screened in water, the relatively
weak and less expensive K3PO4 gave the best result (entry 4),
while many other inorganic bases showed only moderate
activities (entries 9–13). Organic bases showed lower activ-
ities than the inorganic bases used (entries 14–19). After suc-
cessful screening of the cross-coupling reaction, we carried
out SMC reaction of several less reactive heteroaryl halides,
and control experiment using rGO under very similar condi-
tions were applied, showing that the Pd-free material was
completely ineffective (Table 2, entry 1). Outstanding cata-
lytic efficiency was observed for the coupling reactions of
various N-heteroaryl bromides with boronic acid at 85ꢀC to
give the corresponding coupling products in excellent yields
(entries 2–9). It is therefore remarkable that catalyst 4 success-
fully induced the SMC reactions of heteroaryl halides as well
as heteroarylamino bromides. Further extension to the cou-
pling of heteroarylthiophenes with boronic acid also provided
the desired products in high yields (entries 10–13). Even suc-
cessful couplings of the less reactive 2,5-dibromothiophene
and 2,5-dibromo-3-hexylthiophene with both steric hindrance
and strong electron-donating hexyl side chain with boronic
acid were observed (entries 12, 13). It should be noted that
4 efficiently promoted the SMC reaction of heteroaryl chlor-
ides to afford the corresponding products in high yield
(entries 14–16). Moreover, catalyst 4 showed high efficiency
toward aryl chlorides but with a slightly longer reaction time
and higher temperature. This could be attributed to the differ-
ent bond strengths of C Br and C Cl with different
electron-withdrawing powers of the halogen substituents.31
Further, we turned our attention to catalyst reusability
under mild condition. The recycling of 4 was investigated
in the coupling of 2-bromopyridine with phenylboronic
acid (Figure 4). Catalyst 4 showed high catalytic activity
(100–98% conversion) and could be recycled more than
five times without significant loss of catalytic activity. The
possible leaching of Pd after completion of the reaction
was monitored by ICP-MS, proving a total loss of 0.9 wt
%. Comparative TEM studies of catalyst 4 before and after
the experiments are shown in Figure 1. When used catalyst
was compared with the fresh one, Pd nanoparticles dis-
persed on the rGO could be seen after several runs of the
reaction. The result of this study was in agreement with
those in the literature that the formation of Pd nanoparticles
on the rGO surface as an active species developed during
reaction.14,32 The extremely high stability and reusability of
the prepared catalyst could be attributed to the strong non-
covalent interaction between the pyrene moiety and the gra-
phitic surface. The FT-IR and XPS survey spectra of used
Figure 4. Recycling experiment for coupling 2-bromopyridine
using 4. The reaction conditions are the same as in Table 1
(entry 2).
4 were indistinguishable from those of the fresh catalyst 4 -
(Figures 2(c), (d), and S3), suggesting that no structural
variation or deformation of the catalyst occurred during the
reaction under the defined conditions.
Conclusion
In conclusion, we have designed and developed a highly
active nanoparticle-supported heterogeneous palladium cat-
alyst for coupling reactions in water. The catalyst demon-
strated that a wide range of less reactive heteroaryl halides
could be successfully coupled under mild reaction condi-
tions. In particular, the activity of the catalyst was fully
maintained during reuse. We strongly believe that this het-
erogeneous catalytic system, in combination with its effi-
ciency in coupling reactions of less reactive heteroaryl
substrates, may inspire future research in the field of
organic synthesis and catalytic engineering.
Acknowledgment. This work is supported by an Inha Uni-
versity Research Grant (2014).
Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
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A Highly Efficient and Recyclable Palladium Catalyst
KOREAN CHEMICAL SOCIETY
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