C-C cross-coupling reactions by palladium on barium and potassium polyoxotungstate supports

Article abstract of DOI:10.14233/ajchem.2018.21508

A heterogeneous catalyst system for Suzuki and Heck coupling reactions was developed using polyoxometalates as supports for palladium. The catalyst system was synthesized in a one step process using sodium phosphotungstate and palladium acetate. We used the supported catalyst in organic media without the need for special ligands. Substantial activity was observed after several recycles. The catalyst showed high product selectivity with yields up to > 95 % in both Suzuki and Heck reactions.

Full text of DOI:10.14233/ajchem.2018.21508

Asian Journal of Chemistry; Vol. 30, No. 10 (2018), 2357-2360
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21508
C-C Cross-Coupling Reactions by Palladium on Barium and Potassium Polyoxotungstate Supports
1,*
1
1
1
2
I. KARUME , E. TEBANDEKE , J. MBABAZI , H. SSEKAALO and O.F. WENDT
1Department of Chemistry, Makerere University, P.O. Box 7062 Kampala, Uganda
2Centre for Analysis and Synthesis, Lund University, P.O. Box 124 Lund, Sweden
*Corresponding author: Fax: +256 414 531081; Tel: +256 777 700961; E-mail: ikarume@cns.mak.ac.ug
Received: 11 June 2018;
Accepted: 4 August 2018;
Published online: 31 August 2018;
AJC-19074
A heterogeneous catalyst system for Suzuki and Heck coupling reactions was developed using polyoxometalates as supports for palladium.
The catalyst system was synthesized in a one step process using sodium phosphotungstate and palladium acetate. We used the supported
catalyst in organic media without the need for special ligands. Substantial activity was observed after several recycles. The catalyst
showed high product selectivity with yields up to > 95 % in both Suzuki and Heck reactions.
Keywords: Suzuki coupling, Heck coupling, Polyoxometalate, Heterogeneous, Palladium.
controlled by transition metal substitution and the counterion
used. They tolerate high temperatures, have versatile electronic
properties that stabilize nanoparticles and can produce hetero-
geneous systems depending on the choice of counter ion used
[14,15]. We sought a less sophisticated catalyst system that can
be employed under ordinary conditions. A heterogeneous POM
support for Pd provided a robust catalyst system that withstands
high temperature conditions demanded by coupling reactions.
INTRODUCTION
The ongoing demand for greener approaches has revolu-
tionized synthetic chemistry demanding efficient and environ-
mentally friendly methodologies. The interest in C-C cross-
coupling reactions was triggered by the high demand for biaryl
intermediates in pharmaceuticals. Palladium catalyzed Suzuki
cross-coupling of aryl and vinylboronic acids with aryl or vinyl
halides [1,2] is being employed. However the reactions need
exotic catalysts which are expensive and difficult to handle.
Catalyst systems based on palladium ranging from simple salts
like palladium acetate in the presence of quaternary ammonium
salts [3] or water [4] to palladium nanoparticles (Pd-NPs)
stabilized by ionic liquids [5],dendrimers [6], polyvinylpyridine
(PVP) [7], fluoro ligands [8], phosphine ligands [9], aqueous
solutions [10] as well as solid supports such as diatomite [11]
and on polymer supports like poly(N,N-dihexylcarbodiimide)
(PDHC) [12] have been employed. Most of the catalyst systems
reported require sophisticated reaction setups in addition to
homogeneity with the media making isolation of the product
tedious. Ligand decomposition at higher temperatures has also
been reported which tend to reduce the activity of Pd-catalyst
systems employed so far [11,13] making the entire process very
expensive. In the search for suitable supports, polyxoxometalates
(POMs), which possess excellent properties make POMs suitable
supports. The chemical properties of POMs can be largely
EXPERIMENTAL
Sodium phosphotungstate hydrate (Na3PO4·12WO3·xH2O),
Pd(OAc)2, aryl halides, inorganic bases and solvents were pur-
chased from Sigma-Aldrich.
Gas chromatography (GC) analyses were performed using
a Varian 3300 gas chromatograph with an FID and fitted with
a 30 m Varian CP-Sil 5 CB capillary column. The following
temperature program was used for the oven: 100 °C (initial,
hold time 10 min) to 250 °C (final, hold time 12 min) at a rate
of 20 °C/min. The injector temperature was 260 °C and the
detector temperature was 320 °C. ICP analyses were performed
on a Perkin-Elmer model OPTIMA 3000 DV instrument at
the Department of Ecology in Lund University.
General procedure for synthesis of Pd/BaPOM and Pd/
KPOM catalysts: Na3PO4·12WO3 (2.5 g) in a potassium acetate/
acetic acid buffer solution (50 mL) was heated to 80 °C and
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2358 Karume et al.
Asian J. Chem.
then adjusted to pH 7 by potassium carbonate. To the solution
was added palladium acetate (0.19 g) solution and the mixture
was refluxed at 95 °C for 2 h. Barium nitrate, (2.5 g) in mini-
mum quantity of water was added to the refluxing mixture and
the process continued for 1 h. The resulting mixture was evapo-
rated to half the volume and then maintained at 40 °C for 1 h.
The mixture was then filtered and the residue was kept at 10
°C overnight. The residue was then dried in air and at 120 °C
for 12 h to yield the Pd/BaPOM catalyst. The procedure was
modified by addition of ascorbic acid (0.43 g) during reflux
but before precipitation of the POM to produce the reduced
Pd/BaPOM catalyst. Synthesis of Pd/KPOM was similar to
Pd/BaPOM but instead of barium nitrate, potassium chloride,
KCl (1.0 g) was added as the precipitating agent. A portion of
the synthesized Pd/BaPOM was heated at 400 °C for 2 h to
yield calcined Pd/BaPOM catalyst.
General procedure for C-C cross-coupling reactions:
Suzuki reactions were performed using phenylboronic acid
(1.2 mmol), aryl halide (0.8 mmol), base (0.8 mmol), catalyst
(0.025 mg) and solvent (10 mL) under reflux. Heck reactions
were performed using the same moles of the olefin and aryl
halides as substrates. The reaction mixture was allowed to settle
and a portion of the supernatant (0.5 mL) was shaken with
mixture of diethyl ether (2.0 mL) and hydrochloric acid (1.0
mL, 2 M). The organic layer (2.0 µL) was analyzed by gas
chromatography.
the least activity likely because high formation of Pd back
which lowers loading of Pd on the reduced Pd/BaPOM (Table-2,
entry 3).
TABLE-2
EFFECT OF COUNTERION ON THE ACTIVITY
OF Pd/KPOM AND Pd/BaPOM CATALYSTS
Entry
Catalyst
Pd/KPOM
Pd/BaPOM
Selectivity (%)
Yield (%)
1
2
3
95
90
73
52
77
28
Reduced Pd/BaPOM
Unless stated, the reactions were performed using 4-bromoacetophe-
none (0.8 mmol), phenyl boronic acid (1.2 mmol), K2CO3 (1.6 mmol),
catalyst (0.025 g, 3.6 % Pd) in toluene (10 mL), 140 °C, 24 h reflux.
The efficiency of bases in activating phenylboronic acid
and the effect of temperature on the reaction was investigated
using Pd/BaPOM in coupling of the acid with 4-bromoaceto-
phenone.A significant increase in product yield and selectivity
(69 to 97 % yield and 85 to 97 % selectivity) was observed
when the temperature was increased from 120 to 150 °C (Table-
3, entries 1-3). The presence of base was necessary for catalyst
selectivity as no product was detectable in absence of the base.
The conversion was 15 % in the absence of a base suggest
very poor selectivity. The results clearly manifest increase in
the product yield and selectivity with increase in base strength
(Table-3, entries 3-7) and caesium carbonate emerged the best
base (> 99 yield, > 99 % selectivity).
RESULTS AND DISCUSSION
TABLE-3
Our initial step involved design and synthesis of POMs
with inorganic counter ion where potassium and barium were
used as counterions to precipitate the POM, forming KPOM
and BaPOM, respectively on which the Pd gets adsorbed. We
proceeded to determine the composition of the catalyst system
using inductively coupled plasma atomic emission spectroscopy
(Table-1). The results revealed a 6.7 times higher concentration
of palladium in Pd/BaPOM (4.1 %) than Pd/KPOM (0.6 %).
EFFECT OF BASES ON THE ACTIVITY OF Pd/BaPOM
CATALYST IN SUZUKI COUPLING OF PHENYL
BORONIC ACID AND 4-BROMO-ACETOPHENONE
Entry
Base
K2CO3
K2CO3
K2CO3
Cs2CO3
KHCO3
Na2CO3
CH3CO2K
–
Temp. (°C)
120
Selectivity (%)
Yield (%)
1
2
3
4
5
6
7
8
85
90
97
99
97
81
97
–
69
77
97
99
71
21
15
–
140
150
150
150
150
150
150
TABLE-1
ELEMENTAL ANALYSIS OF PALLADIUM
SUPPORTED Ba AND K POLYOXOTUNGSTATE
Unless stated, reactions were performed using 4-bromo-acetophenone
(0.8 mmol), phenylboronic acid (1.2 mmol), base (1.6 mmol),
Pd/BaPOM (0.025 g, 3.6 % Pd) in toluene (10 mL), 24 h.
Elemental comp (mg g-1)
Entry
Catalyst
Pd (%)
Ba
159
0
K
0
151
1
2
Pd/BaPOM
Pd/KPOM
4.1
0.6
The effect of the base on Heck reaction was investigated
by coupling styrene with iodobenzene where caesium carbo-
nate surpassed potassium carbonate giving yield of 61 % vs. 16
%, respectively with similar selectivity. We further proceeded
to check the effect of selected organic on the reaction. Conver-
sions (> 99 %) were recorded with dimethyl formamide and
N-methyl-2-pyrrolidone. However, the former has a slight edge
in selectivity (86 % vs. 84 %) (Table-4, entries 3 and 4). To the
contrary toluene gave the least yield (44 %) in the Heck coup-
ling (Table-4, entry 1).
Potassium carbonate and a temperature of 150 °C were
chosen as the optimal conditions for the Pd/BaPOM in coupling
of phenylboronic acid with selected aryl halides. The catalyst
system gave > 95 % with 4-bromo-acetophenone and > 50 %
yield with majority of the substrates. The low product yield by
This is attributed to the low solubility of barium in water
allowing immediately precipitation forming the BaPOM and
subsequent adsorption of more Pd on the precipitated support
system. High solubility of potassium results to poor precipita-
tion of the KPOM hence less amount of Pd adsorbed. We also
generated a reduced Pd/BaPOM in which ascorbic acid was
added in mild concentration to reduce some of the Pd2+ ions to
Pd0 before adsorption. The Pd/KPOM and Pd/BaPOMs were
compared in Suzuki coupling of phenylboronic acid and 4-
bromoacetophenone (Table-2, entries 1 and 2). The Pd/BaPOM
showed higher activity than Pd/KPOM attributed to the higher
loading of Pd (4.1 %) in the former as observed from the
elemental analysis (Table-1). The reduced Pd/BaPOM showed

Vol. 30, No. 10 (2018)
C-C Cross-Coupling Reactions by Palladium on Barium and Potassium Polyoxotungstate Supports 2359
TABLE-4
EFFECT OF ORGANIC SOLVENT ON THE
ACTIVITY OF Pd/BaPOM CATALYST SYSTEM IN
HECK COUPLING OF STYRENE AND IODOBENZENE
TABLE-6
HECK COUPLING OF SELECTED OLEFINS
AND ARYL HALIDES USING Pd/BaPOM
Selectivity
Yield
(%)
Entry
Olefin
ArX
C6H5-I
C6H5-Cl
p-MeO-C6H4-I
C6H5-I
Entry
Solvent
Toluene
THF
DMF
NMP
Selectivity (%)
Yield (%)
(%)
> 99
–
90
99
–
99
> 99
86
–
1
2
3
4
5
6
7
8
9
> 99a
–
1
2
3
4
92
87
86
84
44
61
86
84
CH2=CHCO2Me
90
99a
–
99
> 99
86a
–
C6H5-Cl
Unless stated, reactions were performed using iodobenzene (0.8
mmol), styrene (1.2 mmol), Cs2CO3 (1.6 mmol), Pd/BaPOM (0.025 g,
3.6 % Pd) in solvent (10 mL), 150 °C, 24 h.
CH2=CHCO2Bu
p-MeO-C6H4-I
p-AcO-C6H4-Br
C6H5-I
C6H5-Cl
p-MeO-C6H4-I
C6H5CH=CH2
some substrates could be improved at extended reaction time
(Table-5, entries 7 and 8). We were successful in coupling of
the poorly reactive aryl chloride after extended hours (Table-5,
entry 6).
10
81
81
Unless stated, reactions were performed using aryl halide (0.8 mmol),
olefin (1.2 mmol), Cs2CO3 (1.6 mmol), Pd/BaPOM (0.025 g, 3.6 % Pd)
in DMF (10 mL), 150 °C, 24 h; a 6 h.
TABLE-5
100
95
90
85
80
75
70
65
60
SUZUKI COUPLING OF PHENYLBORONIC ACID WITH
SELECTED ARYL HALIDES USING Pd/BaPOM
Entry
Aryl halide
C6H5-I
C6H5-Br
Time (h) Selectivity (%) Yield (%)
1
2
3
4
5
6
7
8
9
24
24
24
24
24
48
24
48
24
90
100
–
98
–
98
93
90
97
54
39
–
63
–
C6H5-Cl
p-Me-C6H5-Br
p-Me-C6H5-Cl
p-Me-C6H5-Cl
o-MeO-C6H5-Br
o-MeO-C6H5-Br
p-AcO-C6H5-Br
5
41
66
97
Unless stated, reactions were performed using aryl halide (0.8 mmol),
phenylboronic acid (1.2 mmol), K2CO3 (1.6 mmol), Pd/BaPOM (0.025
g, 3.6 % Pd), toluene (10 mL), 150 °C, 24 h. ND = not detected.
0
1
2
3
The nature and position of the substituent on the aryl halide
was found to greatly affect the reaction. Substituents with
positive inductive effect enhanced the reaction. The reactivity
of aryl halides increased in the order: bromobenzene < methyl-
benzene < acetylbenzene (Table-5, entries 2, 4 and 9). The
methyl group in the para position activated bromobenzene
significantly enhancing product yield. (Table-5, entries 2 vs. 4).
Caesium carbonate, DMF and temperature of 150 °C were
taken as optimal conditions for the Pd/BaPOM catalyzed Heck
coupling of selected olefins and aryl halides. Complete conver-
sion with high selectivity (> 80 %) was observed with majority
of the substrates except chlorobenzene where no yield was
detectable within reaction time employed (Table-6, entries 1-10).
The recyclability of the Pd/BaPOM catalyst system was
also investigated. It retained full activity after first recycle in
coupling of phenylboronic acid with 4-bromoacetonephenone.
Moderate yield (65 %) was obtained in the 3rd recycle (Fig. 1).
Nevertheless, the Pd/BaPOM maintained full activity giving
100 % conversions up to the 4th cycle of reuse in the coupling
of styrene with iodobenzene.
Number of cycles
Fig. 1. Activity of Pd/BaPOM during recycling
100
80
60
40
20
0
Suzuki reaction
Heck reaction
CA
CP
CA
CP
Fig. 2. Comparison leaching in the absence of catalyst (CA) and presence
of catalyst (CP)
Leaching of Pd from BaPOM support was investigated
by decanting a reaction mixture for 24 h. Fresh substrates were
then coupled in the filtrate without addition of the catalyst.
Very low yields were obtained in the absence of the catalysts in
both Suzuki and Heck reactions of the same substrates com-
pared to the same reaction with the catalyst added (Fig. 2).
The low yields were probably due to minimal leaching of
some Pd from the Pd/BaPOM in to the reaction solution used
as the medium for the reaction where the catalyst was not added
or more likely from the suspended Pd particles, which failed
to settle during decanting of the reaction mixture [16].

2360 Karume et al.
Asian J. Chem.
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CONFLICT OF INTEREST
The authors declare that there is no conflict of interests
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