PdCuCeO-TPAB: A new catalytic system for quasi-heterogeneous Suzuki-Miyaura cross-coupling reactions under ligand-free conditions in water

Article abstract of DOI:10.1039/c7nj02759a

In this contribution, we report a simple and clean method for preparation of a single-phase Pd_0.04Cu_0.04Ce_0.92O_2-δ (PdCuCeO) solid-solution oxide and its application in quasi-heterogeneous Suzuki-Miyaura cross-coupling reactions. The catalyst was characterised fully and all the characterisation techniques strongly suggest that the Pd²⁺ and Cu²⁺ ions were successfully incorporated into the lattice of ceria. The as-prepared PdCuCeO solid-solution oxide was tested on Suzuki-Miyaura cross-coupling reactions under ligand free conditions using water as a sole solvent and tetrapropylammonium bromide (TPAB) as a phase transfer catalyst. It was discovered that TPAB acts as scavenger of Pd and the resulting material (Pd/TPAB) is able to catalyse the Suzuki-Miyaura coupling reaction. The Suzuki-Miyaura coupling of aryl iodides, bromides as well as activated aryl chlorides was efficiently performed by this PdCuCeO-TPAB catalytic system. The PdCuCeO-TPAB catalytic system also displayed good functional group tolerance and good to excellent isolated yields were obtained. Catalyst leaching and recyclability studies revealed that PdCuCeO acts as a Pd reservoir and that the reactions are essentially quasi-heterogeneous occurring over the recoverable Pd/TPAB aggregates. Only a negligible amount of palladium (<0.1 ppm) was detected by ICP-OES in the product solution.

Full text of DOI:10.1039/c7nj02759a

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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PdCuCeO-TPAB: A new catalytic system for quasi-heterogeneous
Suzuki-Miyaura cross-coupling reaction under ligand-free
conditions in water.
Received 00th January 20xx,
Accepted 00th January 20xx
Philani P. Mpungosea, Neo I. Sehlokoa, Glenn E. Maguirea and Holger B. Friedricha*.
DOI: 10.1039/x0xx00000x
In this contribution, we report a simple and clean method for preparation of a single-phase Pd0.04Cu0.04Ce0.92O2-δ (PdCuCeO)
solid-solution oxide and its application in quasi-heterogeneous Suzuki-Miyaura cross-coupling reactions. The catalyst was
characterised fully and all the characterisation techniques strongly suggest that the Pd2+ and Cu2+ ions were succesfully
incorporated into the lattice of ceria. The as-prepared PdCuCeO solid-solution oxide was tested on Suzuki-Miyaura cross-
coupling reactions under ligand free conditions using water as a sole solvent and tetrapropylammonium bromide (TPAB) as
a phase tranfer catalyst. It was discovered that TPAB acts as scavenger of Pd and the resulting material (Pd/TPAB) is able to
catalyse the Suzuki-Miyaura coupling reaction. The Suzuki-Miyaura coupling of aryl iodides, bromides as well as activated
aryl chlorides was efficiently performed by this PdCuCeO-TPAB catalytic system. The PdCuCeO-TPAB catalytic system also
displayed good functional group tolerance and good to excellent isolated yields were obtained. Catalyst leaching and
recyclability studies revealed that PdCuCeO acts as a Pd reservoir and that the reactions are essentially quasi-
heterogeneous occuring over the recoverable Pd/TPAB aggregates. Only a negligible amount of palladium (<0.1 ppm) was
detected by ICP-OES in the product solution.
www.rsc.org/
reactions.14-19 However, we think it is of equal importance to
determine whether palladium catalysts can be better utilised
to broaden their impressive reactivity.20 Notably, Pd catalysts
Introduction
The Suzuki–Miyaura (SM) cross-coupling reactions are
characterized by the cross coupling of an organohalide or
organotriflate and an aryl boronic acid to give a biaryl
compound.1-3 The SM cross-coupling reaction has become a
useful strategy for the preparation of substituted biaryl
compounds, which are essential building blocks for
agricultural, pharmaceutical and engineering material
industries.4-10 The 2010 Nobel Prize in Chemistry, awarded to
Akira Suzuki, Richard Heck and Ei-ichi Negishi, further
demonstrated the significance of Pd-catalyzed cross coupling
reactions.11 Given the applicability of the SM reactions, the
design of a more sustainable procedure that uses inexpensive
and easily prepared catalysts, environmentally friendly
solvents and avoids the use of expensive ligands remains a
goal of high practical value.12
often display a higher reactivity than their alternatives, which
enable them to easily catalyze less reactive substrates and
achieve high catalyst turnover numbers (TONs).21
Furthermore, bimetallic nanoparticles supported on carbon
(Pd-Cu/C, Pd-Ni/C and Pd-Ag/C) have also shown activity in SM
coupling reactions.22 The catalytic efficiency based on the yield
of the desired product decreased in the following order: Pd-
Cu/C > Pd/C > Pd-Ag/C > Pd-Ni/C.22,23 Thathagar et al. also
observed that the combined Pd/Cu nanoclusters displayed the
highest activity among all the other combinations (Pd/Pt and
Pd/Ru nanoclusters).14
Herein, we further investigated the Pd and Cu combination
using the Pd0.04Cu0.04Ce0.92O2-δ (PdCuCeO) solid-solution oxide,
instead of Pd-Cu/C or Pd/Cu nanoclusters, as a bimetallic
catalyst for SM reactions. We have previously shown that
substitution of Pd2+ and Cu2+ ions within the CeO2 lattice allows
for complete palladium and copper dispersion, which, as a
result, increased the catalyst activity in CO oxidation.24,25 In
addition, we also showed that the synergetic effect between
palladium, copper and cerium ions improves the catalyst
reducibility and as a result, it enhanced its performance in CO
oxidation.24 For this contribution, we wanted to further exploit
the synergetic effect between Pd-Cu-Ce to hopefully enhance
the reactivity of PdCuCeO in SM cross-coupling reactions.
There is also
a growing interest to finding a suitable
replacement for palladium with cheaper metals such as iron,
copper and nickel.13 Moreover, Cu(I) salts and Cu2O have been
reported as co-reagents and co-catalysts in SM coupling
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Recently,
a wealth of literature has been reported on The STEM-EDX (Figure 1, insert) confirmed the presence of Pd
supported Pd catalysts that catalyze SM cross-coupling and Cu in the as-prepared PdCuCeO and also shows that both
reactions with high reactivity, good recyclability and minimal metals are homogeneously dispersed in the ceria lattice.
metal leaching.6-11,26-41 However, the nature of the true
catalyst is still unclear and the dominant view at present is that We also synthesised two monometallic ionic solid-solution
the mechanism of most C-C cross-coupling reactions is largely oxides (Pd0.04Ce0.96O2-δ [PdCeO] and Cu0.04Ce0.96O2-δ [CuCeO])
homogeneous irrespective of the type of catalyst precursor as control materials. The chemical composition and some
used.42-49 Hence, the
heterogeneous from
challenge of distinguishing physicochemical properties of the studied materials are
homogeneous
catalysis
when summarised in Table 1 and the rest of the characterisation
critical data (XRD, XPS, TEM, SEM, EDS, TGA, BET, TPR and Raman
spectroscopy) is enclosed in the electronic supplementary
information (ESI). The Williamson-Hall method was used to
heterogeneous systems are developed is still
topic.7,9,42,50
a
In this regard, we now report a “greener” and highly efficient estimate the average crystallite sizes of the prepared solid-
procedure that can quasi-heterogeneously catalyse SM solution oxides.24 The three materials have similar physical
coupling using an easily prepared PdCuCeO solid-solution properties, their surface areas ranges between 31-34 m2/g,
oxide that can be recovered and reused at end of the reaction. while their crystallite sizes range from 11-14 nm (Table 1).
In addition, we have eliminated the need for a ligand and
Table 1: Physicochemical properties of prepared materials.
utilise water as the sole solvent, since it is cheap, safe and
environmentally friendly.27,31 We also investigated the true
nature of catalysis for this SM reaction by doing selective
poisoning and hot-filtration tests.
Metal loading
Crystallite Surface area
(m2/g)
(wt%)
Catalyst
size (nm)
Pd
-
Cu
1.8
-
CuCeO
PdCeO
14
14
11
31
33
34
Results and discussion
2.9
3.1
The solution combustion synthesis method was used to
prepare the bimetallic Pd0.04Cu0.04Ce0.92O2-δ (PdCuCeO) solid-
solution oxide. This synthesis method was preferred because it
is a relatively simple, quick and inexpensive method for
synthesizing highly crystalline and pure noble metal
substituted oxides.51-54 We have recently shown the structural
and electronic properties of this material using X-ray
diffraction (XRD), XPS, XANES, Raman spectroscopy, EXAFS and
high-resolution transmission electron microscopy (HR-TEM).24
The Rietveld refinement fits of XRD data for PdCuCeO
correspond only to the ceria phase with the fluorite structure
(JCPDS 34-0394). The absence of the PdO and CuO phases in
the X-ray pattern of PdCuCeO supports its phase purity and
also strongly suggests that Pd2+ and Cu2+ ions were successfully
incorporated into the ceria lattice (Figure 1).
PdCuCeO
1.7
Assessment of catalyst activity
The catalyst testing studies began by assessing the effect of
several variables on catalyst activity using iodobenzene and
phenylboric acid as model coupling partners. First, water was
chosen as the reaction medium, since one of our aims was to
eliminate organic impurities and use water as sole solvent. We
then investigated commonly used inorganic bases (K2CO3,
Na2CO3, Cs2CO3, NaOH and t-BuOK) in the Suzuki-Miyaura
coupling reactions. All the tested bases, except NaOH, gave
identical results and Na2CO3 was chosen since it is the most
commonly used base.55 It was later realized that for
bromoarenes and chloroarenes, the reactions don’t proceed
efficiently in the absence of
a phase transfer catalyst,
tetrapropyl ammonium bromide (TPAB).56-59 Hence, all the
reactions were carried out in the presence of TPAB to allow for
a good comparative study (Table 2).
Pd Cu Ce
Temperature and catalyst loading were also optimized and the
optimum reaction conditions found are summarized in Table 3.
There was no reaction at room temperature, while the
reaction went to completion in an hour at 60 and 100 °C (Table
2). Further testing was carried out at 100 C to allow for better
°
miscibility of less water-soluble substrates and better
reactivity for difficult substrate such as chloroarenes.
Figure 1: The Rietveld refined XRD patterns of PdCuCeO and its
STEM-EDX image (insert)
2 | J. Name., 2012, 00, 1-3
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Table 2 Optimisation of reaction conditions
secondly, it suggests that there is a synergetic effect between
palladium and copper ions, since PdCuCeO was more active
than PdCeO, while CuCeO was inactive. Hence, the synergetic
effect between palladium and copper in PdCuCeO improves its
catalyst performance. Further catalytic testing was thus carried
out using the PdCuCeO solid-solution oxide.
Catalyst
Table 4 Investigating the most robust catalyst for SM cross-coupling
reactions under optimum reaction condition.
loading
(mol%
Pd)
Temp.
(°C)
Reaction Yield
(%)c
Entrya
Base
time (h)
1
2
t-BuOK
K2CO3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
0.05
100
100
100
100
100
100
60
1
1
100
100
100
100
100
10
Entry
Catalyst
No catalyst
CuCeO
% Yield
No reaction
No reaction
20
3
Cs2CO3
Na2CO3
NaOH
1
1
2
3
4
4
1
5
3
PdCeO
PdCuCeO
48
6
TPAB
8
7
Na2CO3
Na2CO3
Na2CO3
Na2CO3
1
100
0
To establish the versatility of the developed PdCuCeO
catalyzed SM coupling reactions, the reactions of
phenylboronic acids with iodoarenes, bromoarenes, and
chloroarenes were further studied; the results are summarized
in Tables 5 and 6. To allow for a good comparative study, the
reaction time was fixed at 30 minutes, since using water as a
sole solvent meant that we couldn’t reliably and quantitatively
monitor the progress of the reaction due to limited solubility
of some subtrates and products in the reaction medium.
8
25
12
2
9b
100
100
100
100
10b
2
aVolume of water = 2 mL
bVolume of water = 5 mL
cGC yield
Table
5 SM cross-coupling of iodo- and bromoarenes with
phenylboric acid to give substituted biphenyls under our optimum
reaction conditions
Table 3 Optimum reaction conditions
TOF /h-1
2797
2010
2338
3308
3308
3308
3308
2630
2402
1670
2890
3308
3308
3308
3308
Reaction conditions
Entry
1
R1—
X
% Yield
85
Solvent
Base
H2O (5 mL)
Na2CO3 (3 eq.)
100 °C
H
I
2
4-NH2
4-Me
4-COH
4-COMe
3-CF3
4-NO2
H
I
62
Temperature
Catalyst loading (Pd based)
Additive
3
I
72
0.05 mol%
TPAB (1 eq.)
4
I
100
100
100
100
80
5
I
6
I
Under the obtained optimum reaction conditions (Table 3),
several control reactions were carried out using 2-
bromoanisole and phenylboric acid as model coupling
partners. 2-Bromoanisole is a highly deactivated coupling
partner, thus it allows for a more reliable determination of the
most robust catalytic system. The results showed that no
reaction occurs in the absence of a palladium containing
catalyst (Table 4). In addition, the palladium and copper
containing catalyst (PdCuCeO) was the superior catalyst.
Firstly, the data suggests that palladium is the active metal;
7
I
8
Br
Br
Br
Br
Br
Br
Br
Br
9
4-Me
2-OMe
4-F
73
10
11
12
13
14
15
48
88
4-COH
4-COMe
4-CF3
4-NO2
100
100
100
100
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The results show that the PdCuCeO catalysed SM cross- Furthermore, the present catalyst system gives superior TONs
coupling reactions are efficient over a wide range of ortho-, and TOFs than most reported catalytic systems that use water
meta- and para-substituted haloarenes and that various as a sole solvent (Tables 5 and 7).12,59-63
functional groups of the substituents are well tolerated. The
Table
7 Performance comparison of catalytic systems in SM
PdCuCeO catalyst was highly active, achieving TOFs of up-to
3300 h-1 (Table 5). It was observed that electron-withdrawing
substituents speed-up the reaction (Table 5, Entries 4-7 and
12-15), while electron-donating substituents slow down the
catalytic activity (Table 5, Entries 2-3 and 9-10). The highly
activated substrates reacted have identical TOFs since their
reaction times are less than 30 minutes (which is fixed) and
each reaction only yields the desired product.
coupling of 4-bromoacetophenone to phenylboric acid under
‘heterogeneous’, ligand-free conditions that use water as a sole
solvent.
Catalyst
[mol% Pd]
Reaction
conditions
H2O, CTAB,
K2CO3, @60
°C
H2O, K3PO4,
@100 °C
H2O, K2CO3,
TBAB,
Time
/h
Yielda
/%
Entry
Ref.
[60]
[61]
Due to the success gained from the activation of aryl iodides
and bromides, the study was extended to develop a procedure
that facilitates efficient SM coupling of chloroarenes (Table 6).
The synthesis of fine chemicals and phamaceuticals should
benefit highly from the activation of chloroarenes, since these
can serve as cheap and readily available starting materials.
Unfortunately, chloroarenes are notoriously more difficult to
activate than iodo- and bromoarenes. In our initial study we
focused on the coupling of unactivated chloroarenes (Table 6,
Entry 1), however, sluggish results were obtained even when
using 4.6 mol% Pd. Moderate to excellent yields were only
obtained from coupling of activated chloroarenes with various
phenylboric acids (Table 6, Entry 2-5). The activated
chloroarenes also allowed for lower catalyst loading.
89
(445)
1
2
Pd/C [0.05]
4
5
Pd/polymer
[2]
93
(9)
Pd/porous
glass [0.45] MW(300W),
99
(12941)
3
4
0.17
1
[62]
@150 °C
H2O, TBAB,
Pd@SBA-
97
(1212)
K2CO3, @80
15(C) [0.08]
°C
[63]
[12]
H2O, K2CO3,
Pd/HAP
TBAB, @80
[0.33]
94
(71)
5
6
7
4
°C
PdNPs/ZrO2 H2O, TBAOH,
90
(71)
14
0.5
[59]
This
Table 6 SM cross-coupling of chloroarenes with various
[0.1]
@90 °C
H2O, Na2CO3,
TPAB, @100
°C
phenylboric acids to give substituted biphenyls
PdCuCeO
[0.05]
100
(3308) paper
aParentheses = TOF/h-1
Entry
1a
Chloroarene
R2
%Yield
<5
TOF /h-1
<4
Catalyst leaching and recyclability studies
4-H
The SM cross-coupling reaction is often used by
pharmaceutical industries in the synthesis of medicinal
compounds. Thus it is important to quantify the residual metal
in the product when developing heterogeneous systems. The
requirement is that the residual metal must be kept below 5
ppm.57,64 To distinguish homogeneous from heterogeneous
catalysis and possibly determine the true nature of catalysis
for these reactions in the present study we conducted a series
of investigations: catalyst recovery and recyclability, hot-
filtration, selective poisoning tests and elemental analysis of
the fresh and used catalyst. Phenylboric acid and 4-
bromoacetophenone were used as model coupling partners
for all the leaching and recyclability studies.
2a
4-Me
42
15
3b
4b
5b
4-H
100
100
100
34
34
34
4-Me
4-OMe
aCatalyst loading = 4.6 mol%
bCatalyst loading = 2.3 mol%
To evaluate the performance of the developed catalytic
system, the product yield (4-acetylbiphenyl) of the present
catalytic system was compared to other reported
“heterogeneous” and ligand-less palladium catalyzed SM
cross-coupling reactions that use water as sole solvent (Table
7). The yield of 4-acetylbiphenyl in this work was found to be
comparable or better to most reported yields in literature.
The catalyst was recovered quantitatively at the end of the
reaction and reused three times. It was observed that the
catalyst activity dropped with each subsequent recycle (Figure
2). Catalyst deactivation is usually caused by chemical,
mechanical and/or thermal degradation of the heterogeneous
catalyst. Herein, we only investigated chemical degradation
(leaching) of the catalyst, since it is the most common catalyst
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deactivator for these types of reactions and its study could filtration a significant quantity of TPAB precipitated out of the
lead to a better understanding of the true nature of the solution, hence the filtrate contained little to no TPAB. The
catalysis.44,57 To investigate the cause of deactivation we results in Figure 2 show negligible activity in the filtrate,
conducted elemental analysis of the reaction solution, fresh suggesting that there is very little soluble Pd present in the
and used catalyst (Table 8). The metal content of the fresh and solution. The ICP elemental analysis of the filtrate also
used catalyst were compared and it was observed that the revealed that there was a very low amount of Pd (<0.1 ppm)
fresh catalyst lost about 50% of Pd and Cu in its first use. left in the filtrate (Table 8). Hence, lack of activity is due to an
However, only about 6% of the leached Pd was found in the insufficient amount of Pd in the filtrate to catalyse the Suzuki
reaction solution (Table 8).
reaction.
Table 8 ICP-OES elemental analysis of the fresh and used PdCuCeO
catalyst
It was suspected that the Pd might have precipitated out with
TPAB, since the ethylpyridine poison test confirmed the
presence of Pd in the TPAB-water mixture. This then led us to
recover TPAB and test its activity. The recovered TPAB gave
about 40% conversion to the desired product. This suggest
that TPAB acts as a scavenger of Pd and its aggregates are able
to generate, in situ, highly dispersed palladium nanoparticles
(Pd/TPAB) from simple PdCuCeO. Hence, the catalyst
recyclability and leaching tests performed in the current work
agree very well with the findings of other studies using Pd-
doped metal oxides as precatalysts, and substantiate the
hypothesis that the precatalysts act as a palladium reservoir,
that slowly releases extremely reactive Pd species.37,60 In our
case, the released Pd is captured by TPAB and generates, in
situ, a highly active quasi-heterogeneous Pd/TPAB catalyst
(Scheme 1). We have termed the active catalyst system as
PdCuCeO-TPAB. The catalytic cycle is then initiated by the
oxidative addition of the aryl halide to palladium to form the
organopalladium complex. In the presence of a base, this
STEM-EDX image
Pd loading
(wt%)
Cu loading
(wt%)
of used PdCuCeO
Fresh
PdCuCeO
Used
3.1
1.4
1.7
0.72
PdCuCeO
Pd Cu Ce
Reaction
solution
<0.1 ppm
3 ppm
Catalyst poisoning tests were thus performed to further
investigate the possible presence of leached, soluble palladium
from the PdCuCeO pre-catalyst. This test was performed using
ethylpyridine as poison, since pyridines are reported to bind
strongly to Pd(II).44,65 Ethylpyridine was added along with the
other reagents at the start of the reaction and then the
reaction was stirred under similar conditions. The results in
Figure 2 revealed that ethylpyridine addition did partially
deactivate the catalyst, since the 4-bromoacetophenone
conversion dropped to 40%. Hence, this test confirmed the
presence of leached palladium in the TPAB-water mixture.
organopalladium complex reacts with boric acid via
a
transmetallation step and the desired product is then obtained
via the reductive elimination step (Scheme 1). This special type
of the Pd release/capture mechanism proved very
advantageous, since it has potential to resolve the difficulties
of catalyst separation and recovery and it greatly minimizes
metal contamination of the products.
The hot filtration test was then performed to assess the
activity of any leached palladium in the TPAB-water mixture. In
Ar1
X
the hot filtration test,
a
mixture of PdCuCeO, 4-
As prepared Pd0.04Cu0.04Ce0.92O2ꢀδ preꢀcatalyst
bromoacetophenone, phenylboric acid, TPAB and Na2CO3 in
water was stirred at 100 °C for 5 minutes. The catalyst was
filtered off from the reaction mixture after 5 min and the hot
filtrate was then allowed to react further for 25 minutes under
similar conditions. It was, however, noticed that during the hot
Reaction conditions
induced Pd/Cu dissolution
X
O
O
O
O
Pd0/TPAB
Pd2+
Ce3+
Pd2+
Ce4+
C u2+
Ce4+
Ar1
Pd/Cu reservoir
Ar1Ar2
Ar2B(OH)2
Scheme
1
proposed
reaction mechanism for the PdCuCeO-TPAB catalysed
quasi-heterogeneous SM cross-coupling reactions.
Conclusions
In summary, we have demonstrated that the PdCuCeO-TPAB
catalytic system can catalyse the Suzuki-Miyaura cross-
coupling under milder and more environmentally friendly
reaction conditions. The reactions were conducted in water as
sole solvent and the addition of a ligand was unnecessary. The
PdCuCeO-TPAB catalytic system also displayed good functional
group tolerance and good to excellent isolated yields were
Figure 2 Catalyst leaching and recyclability studies
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obtained. Catalyst leaching and recyclability studies revealed
that PdCuCeO acts as a Pd reservoir and that the reactions are
essentially quasi-heterogeneous, occuring over recoverable
Pd/TPAB aggregates. The TPAB aggregates generate, in situ,
highly dispersed palladium nanoparticles from simple
PdCuCeO. As as result, the PdCuCeO catalytic activity declined
with each subsequent recycle. However, only a negligible
amount of palladium (<0.1 ppm) was detected by ICP-OES in
the product solution because most of the palladium is
removed from the reaction solution by TPAB. Hence, this
General procedure for catalyst recovery and
recyclability
The catalyst used in the first run was separated by
centrifugation, washed with
5 mL toluene and water,
respectively, dried at 60 °C and reused as described for the
fresh catalyst.
Acknowledgements
PdCuCeO-TPAB catalytic system has potential to resolve the We would like to express our gratitude to Mintek and the
difficulties of catalyst separation and recovery, since it greatly Department of Science and Technology South Africa (Advanced
minimises metal contamination of the products.
Metals Initiative program) for financial support. The authors
would also like to thank Dr M Shozi, Dr S Mohamed, Dr S Singh
and the rest of the UKZN catalysis research group for helpful
discussions.
Experimental
Cerium ammonium nitrate [(NH3)Ce(NO3)6, 99.9%], palladium
nitrate [Pd(NO3)2], copper nitrate [Cu(NO3)2], and urea
[CH4N2O, 99.9%] were obtained from Sigma-Aldrich and were
used without further purification.
Notes and references
1
2
N. Miyaura, A. Suzuki, Chemical Reviews, 1995, 95, 2457.
N. Miyaura, A. Suzuki, Journal of the Chemical Society,
Chemical Communications, 1979, 20, 866.
3
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5
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The monophasic Pd0.04Cu0.04Ce0.92O2-δ solid-solution oxide was
prepared using a one-step urea-assisted solution combustion
synthesis method described earlier.26 The catalyst synthesis
method involved preparation of a redox combustion mixture
composed of stoichiometric amounts of metal precursors
[(NH4)2Ce(NO3)6, Pd(NO3)2 and Cu(NO3)2] and urea [NH2CONH2]
in the ratio of 1.0:3.68 respectively. The solution was then
stirred at 150 °C for 10 minutes to evaporate water and reduce
its volume to ≈20 mL. The boiling solution was then introduced
into a muffle furnace pre-heated at 400 °C and was kept in the
furnace for 5 hours. A light grey solid was obtained.
,
7
8
9
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,
Catalyst testing: general procedure for coupling
reactions
A dry two necked pear-shaped flask containing a stirrer bar, a
condenser and 5 mL of H2O was charged with an aryl halide
(4.6 mmol), boric acid (2 eq.), Na2CO3 (3 eq.) and catalyst
Pd0.04Cu0.04Ce0.92O2-δ (0.05 mol% Pd). The reaction mixture was
stirred and (usually) heated to 100 °C and its progress was
monitored by GC and GC-MS (the reaction mixture was
homogenized with 5 mL of toluene). The aryl halide conversion
was used to estimate the catalytic activity, using
benzophenone as an internal standard. After the reaction had
gone to completion, the reaction mixture was filtered and the
filtrate was extracted with ethyl acetate and brine. Pure
products were obtained by simple filtration through silica gel
using ethyl acetate as solvent. The structure of the coupling
1
product was confirmed by H and 13C NMR spectroscopy and
the results were consistent with those reported in literature
for substituted biphenyls.8
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6 | J. Name., 2012, 00, 1-3
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New Journal of Chemistry
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DOI: 10.1039/C7NJ02759A
Highlights
•
PdCuCeO was applied for Suzuki-Miyaura coupling in pure water. The
catalyst was highly active (TOF >3000 h-1) and could be reused.
Ar1X
As prepared Pd0.04Cu0.04Ce0.92O2ꢀδ preꢀcatalyst
Reaction conditions
induced Pd/Cu dissolution
X
O
O
O
O
Pd0/TPAB
Pd2+
Ce3+
Pd2+
Ce4+
Cu2+
Ce4+
Ar1
Pd/Cu reservoir
Ar1Ar2
Ar2B(OH)2