Highly active and reusable hydrotalcite-supported Pd(0) catalyst for Suzuki coupling reactions of aryl bromides and chlorides

Article abstract of DOI:10.1016/j.tet.2017.12.056

A palladium(0) nanocluster supported on hydrotalcite has been prepared and tested for the Suzuki coupling reaction. The prepared catalyst showed very efficient catalytic activity for cross coupling of iodo- and bromoarenes under very mild reaction conditions, affording >90% yield. Under the optimized reaction conditions, chloroarenenes also showed very good reactivity. Transmission electron microscopic imaging data showed the formation of very small Pd(0)-nanoclusters (d = 2.2 ± 0.5 nm) well dispersed on the support, which enhanced the activity and stability of the catalyst for the Suzuki cross-coupling reaction. This catalytic system offers an easy method of preparation with good activity and reusability up to five cycles.

Full text of DOI:10.1016/j.tet.2017.12.056

Accepted Manuscript
Highly active and reusable hydrotalcite-supported Pd(0) catalyst for Suzuki coupling
reactions of aryl bromides and chlorides
Sangita Karanjit, Masaya Kashihara, Atsushi Nakayama, Lok Kumar Shrestha,
Katsuhiko Ariga, Kosuke Namba
PII:
S0040-4020(17)31340-6
10.1016/j.tet.2017.12.056
TET 29212
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 29 October 2017
Revised Date: 23 December 2017
Accepted Date: 28 December 2017
Please cite this article as: Karanjit S, Kashihara M, Nakayama A, Shrestha LK, Ariga K, Namba K,
Highly active and reusable hydrotalcite-supported Pd(0) catalyst for Suzuki coupling reactions of aryl
bromides and chlorides, Tetrahedron (2018), doi: 10.1016/j.tet.2017.12.056.
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Highly active and reusable Pd(0)
nanocluster on hydrotalcite is applied in
the Suzuki reaction of aryl halides.
Highly active and reusable hydrotalcite-
supported Pd(0) catalyst for Suzuki coupling
reactions of aryl bromides and chlorides
Sangita Karanjit a*, Masaya Kashihara a, Atsushi Nakayama a, Lok Kumar Shresthab, Katsuhiko Arigab,c and
Kosuke Nambaa*
aGraduate School of Pharmaceutical Sciences, Tokushima University, 1-78-1 Sho-machi, Tokushima, 770-8505,
Japan.
bInternational Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), 1-1
Namiki, Tsukuba Ibaraki, 305-0044 Japan
cDepartment of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-
1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan.

1
ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
Highly active and reusable hydrotalcite-supported Pd(0) catalyst for Suzuki coupling
reactions of aryl bromides and chlorides
Sangita Karanjit a*, Masaya Kashihara a, Atsushi Nakayama a, Lok Kumar Shrestha b, Katsuhiko Arigab,c
and Kosuke Nambaa*
aGraduate School of Pharmaceutical Sciences, Tokushima University, 1-78-1 Sho-machi, Tokushima, 770-8505,
Japan.
bInternational Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), 1-1 Namiki,
Tsukuba Ibaraki, 305-0044 Japan
cDepartment of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5
Kashiwanoha, Kashiwa, Chiba 277-8561, Japan.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A palladium(0) nanocluster supported on hydrotalcite has been prepared and tested for the
Suzuki coupling reaction. The prepared catalyst showed very efficient catalytic activity for cross
coupling of iodo- and bromoarenes under very mild reaction conditions, affording >90% yield.
Under the optimized reaction conditions, chloroarenenes also showed very good reactivity.
Transmission electron microscopic imaging data showed the formation of very small Pd(0)-
nanoclusters (d = 2.2 ± 0.5 nm) well dispersed on the support, which enhanced the activity and
stability of the catalyst for the Suzuki cross-coupling reaction. This catalytic system offers an
easy method of preparation with good activity and reusability up to five cycles.
Available online
Keywords:
Heterogenous catalyst
Suzuki coupling
Palladium nanocluster
Aryl chlorides
2009 Elsevier Ltd. All rights reserved
.
1. Introduction
reactions is very attractive for organic synthesis due to
environmental, economic, and safety reasons. But, the
incompatibility of various ligands with water and air limits their
utility in various solvents, including water, in synthetically
important Suzuki reactions.
The Suzuki cross-coupling reaction between aryl halide and
phenylboronic acid is a very common method for the C-C bond-
forming reaction in modern organic synthesis.1 Traditionally,
these reactions are usually carried out in organic solvents and
catalyzed by various soluble Pd complexes such as Pd/ligand
systems.1,2 Phosphine-based palladium catalysts are generally
used, but they are expensive, toxic, unrecoverable, and sensitive
to oxygen and water.3 Although homogeneous Pd catalysts are
usually effective in this reaction, their use often brings about the
typical problems associated with all homogeneous catalysts:
separation of the catalyst from the reaction products and catalyst
recovery. Moreover, homogeneous Pd catalysts easily lose their
catalytic activity because of aggregation and precipitation. These
are very serious problems for environmental and economic issues
in large-scale synthesis. To obtain high reactivity, the common
phosphine-mediated catalysis must be replaced by methods
involving either new types of ligands forming highly reactive but
stable complexes, or by new forms of palladium able to operate
in the absence of tightly coordinated ligands. These new methods
can be regarded as phosphine-free catalysis, and the development
of new ligands has brought some promising results. The use of
water as a reaction medium for transition-metal-catalyzed
Heterogenization of the existing homogeneous Pd catalyst is
an attractive approach to meeting the increasingly stringent
environmental requirements to minimize waste. Heterogeneous
catalysis has attracted more and more attention as an effective
catalyst by virtue of the large surface area,4 and continuous effort
has been applied to its development so that it can be reused
efficiently while keeping the inherent activity of the catalytic
centre. Metallic palladium particles are known to exhibit very
high catalytic activity towards a wide range of applications. In
recent years, supported Pd nanoparticles have provided excellent
results as catalysts for the Suzuki coupling reaction. Their
success lies in the combination of nanoparticles and suitable
support providing catalysts with special properties and enormous
catalytic potential. The nature of the support and the method of
preparation certainly affect particle morphology, size, and
distribution, all of which affect the catalytic properties. Various
heterogeneous catalysts have been designed to support active Pd
species on different materials such as silica, carbon, zeolites, and

2
Tetrahedron
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metal organic frameworks.5 Although these catalysts were
approaches for Pd(0) catalyst (>4 nm) on hydrotalcite11 using
optimized to effectively convert aryl iodides or bromides,6 they
exhibited limited scope for less-reactive but economical and
readily available aryl chlorides. Few heterogeneous catalysts on
various support and stabilizers has been developed for this
purpose and applied efficiently; and are still on the way of
developing new catalysts.7 Support plays an important role in
dispersing the active metals and giving stability to the metal in
addition to enhancing the catalytic properties. Even though many
inorganic oxide materials have been used to support various
metals, the development of high-performance catalysts is of
ongoing interest.
hydrazine hydrate and cyclohexene as reductants has been
reported at elevated temperatures and applied in the cross-
coupling reaction. However, the greatest challenge lies in
confining the size to smaller nanoclusters to achieve the high
dispersion and more number of catalytically active sites and to
realize high order of activity and selectivity in chemical reaction
which is a great demand of heterogeneous catalyst for industrial
purpose.
Herein, we report the synthesis of a catalyst consisting of very
small Pd(0) NPs supported on hydrotalcite (Pd-HT) and its
application in Suzuki coupling reactions of aryl bromides and
chlorides under environmentally benign conditions. One
important benefit of the proposed procedure is its easy and safe
preparation method under mild conditions for Pd(0) nanoparticles
from Pd(II) precursor using NaBH4 as a reductant.
Hydrotalcites (HTs), a new class of basic mixed hydroxides,
are being tried as supports for dispersing noble metals. They are
also known as “layered double hydroxides” and possess a
number of useful properties such as variable composition of
cations, tunable acidity, basicity of the surface and high metal-
adsorption abilities, which help in obtaining highly dispersed
nanoparticles on this support. Due to these properties, HTs have
shown a great potential in a number of scientific fields and
aroused much interest in recent years.8 Several catalysts,
including Pd, Cu, Au, and others, have been prepared and applied
to various catalytic reactions in recent years.9 Successive studies
on Pd (II) catalyst supported on HTs led us to utilize it for Suzuki
coupling and other reactions under milder conditions.10 However,
most of those reactions were performed under high temperature,
and additives such as surfactants are needed. Because the C-C
coupling reaction needs Pd(0) in the catalytic cycle, in situ
generation of catalytically active Pd(0) species and performance
of a catalytic cycle may lead to the enlargement of Pd
nanoparticles (NPs) under reaction conditions leading to
decreased catalytic activity after several cycles. So, small and
highly dispersed Pd(0) NPs on the support should be desirable for
prolonging the activity of the catalyst by adopting a suitable
preparation method. To the best of our knowledge, only two
2. Results and Discussion
We started our investigation of a Suzuki coupling reaction of
2-bromonapthalene with 4-methoxyphenylboronic acid at 70 C
o
using various Pd(0) and Pd(II) catalysts prepared by various
methods (supporting information). The additional ligand/capping
agent and polymers are sometimes very effective for controlling
the size and morphology of nanoclusters, resulting in better
catalytic performance in several reactions. In this context we
prepared Pd-PVP [Pd(0)], Pd-PVP/HT [Pd(0)], and Pd-HT
[Pd(II)] to test their catalytic performance in Suzuki coupling
reactions. Pd-PVP and Pd-PVP/HT show very low or moderate
activity for cross-coupling reactions of 1 and 2 (Table 1, entry 1-
3) as compared to Pd- HT [Pd(II)] prepared by a metal
impregnation method (entry 4). But, when the solvent was
changed to DMF/H2O, in Cs2CO3 the reaction gave 83% of the
desired product at 70 oC (entry 5), whereas the yield decreased to
o
74% when the temperature was decreased to 40 C (entry 6). To
Table 1 Optimization of Suzuki reaction between 2-bromonapthalene and 4-methoxyphenylboronic acida
aReaction conditions: 2-bromonapthalene (0.1 mmol), 4-methoxyphenylboronic acid (0.15 mmol), catalyst 3 atom%, solvent 1 ml, base
b
c
d
(0.15 mmol). NMR yield. Solvent 3 ml, Catalyst prepared by using NaBH4 as reductant. Solvent 3 ml, Catalyst prepared by ethanol
reduction method. Method A= metal impregnation method. Method B= NaBH4 reduction method.

3
Table 2 Suzuki-Miyaura cross coupling rAeaCctiConEa PofTarEyDl haMlidAe 1NwUitSh CphRenIyPlbToronic acid 2
aReaction conditions: aryl halide 1 (0.1 mmol), phenylboronic acid (0.15 mmol), catalyst 3 atom%, solvent 1 ml, base (0.15 mmol).
bIsolated yield.
our delight, the Pd(0) catalyst prepared on the same support by
NaBH4 as a reductant (Pd-HT’) showed excellent activity for the
reaction, giving a quantitative yield of the product in 3h (entry 7).
However, its precursor Pd(OAc)2 gave only 70% (entry 8). Other
inorganic bases, such as K2CO3, KOtBu, and KOH, also worked
efficiently yet less so than Cs2CO3, however, DBU did not
perform well (entry 9-12). The result of solvent screening
showed that the reaction went well in EtOH/H2O, requiring a
much longer reaction time; and in other solvents, such as
DMSO/H2O, toluene/H2O, THF, dioxane and others the reaction
did not proceed at all or gave very trace amount of product (entry
13, 14, 15 and table S3, supporting information).
optimized reaction conditions. Various aryl bromides with
electron-donating and electron-withdrawing substituents also
gave good yields of the cross-coupling products (3d, 90% and 3e,
88%). The o-substituted aryl bromides and heteroaryl bromides
also performed well (products 3f and 3g). Further, the scope of
boronic acid was also examined. Various substituents at the p-
position of boronic acid did not affect the reactivity or yield of
cross-coupling products (3h-j). Sterically hindered boronic acid
(3n), o- or m-substituted boronic acids neither electron donating
(3l) nor withdrawing (3m) had any negative effect on the reaction
and the reaction was completed within 24 h with quantitative
yields of the desired products (3k-n). However, 2-bromoaniline
and 2-methyl-1-bromonapthalene show no reaction under the
optimized reaction condition.
After optimization of the reaction conditions, the catalytic
activities of Pd-HT’ for various aryl halides and phenylboronic
acids were examined. Table 2 shows the results of Suzuki
coupling reactions of various aryl bromides and iodides with
boronic acids. The catalytic Suzuki coupling reaction was tested
with aryl iodides, which showed smooth reactions with excellent
catalytic activity even at room temperature (3a-c, 95%) under the
Next, we focused on the Suzuki coupling reaction of
chloroarenes and found that various chloro-compounds
successfully completed cross coupling reaction to give good
o
yields of biphenyls at 60 C. The reaction of chlorobenzene with
4-methoxyphenylboronic acid gave a moderate yield of desired

4
Tetrahedron
excellent result (entry 2, 97%). The o-substituents –CH3, -OMe
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Table 3 Suzuki-Miyaura cross coupling reactiona of aryl
and the p-substituent -F also gave satisfactory yields of the
desired product (entries 3-5). Electron withdrawing substituents
at the p-position of aryl chlorides decreased the desired cross-
coupling product with electron-rich arylboronic acid due to a
competing Ullmann-type coupling reaction at 60 oC, below which
the reaction was too slow (entry 6). However, with 4-
methylphenylboronic acid or 4-chlorophenylboronic acid, either
chlorobenzene or electron-rich chloro-substrates were tolerated to
give excellent yields of the product (entries 7-9), and 4-
chlorotoluene and 4-nitrochlorobenzene gave good yields with
electron-deficient phenylboronic acids (entries 10-11). In
addition, Heteroaryl chlorides, such as 2-chloropyridine also
reacted well with 4-methoxyphenylboronic acid giving good
yield of the desired biphenyl (entry 12). Further, the catalytic
activities of other commercially available heterogeneous catalysts
chlorides 1a with phenylboronic acids 2a
and the catalysts prepared by following method
B on
commercially available supports were tested for the reaction of 4-
chlorotoluene and 4-methoxyphenylboronic acid. However, none
of them showed better activity compared to Pd-HT’ (Table S4,
supporting information). The catalyst prepared by our method
showed high chemoselectivity toward the halogens when existing
in the same aromatic ring. For example, 3-bromochlorobenzene
afforded 97% of 3o as a sole product with phenylboronic acid by
o
activation of the C-Br bond only at 40 C under the reaction
condition. In addition, 4-chloroiodobenzene and 4-
bromoiodobenzene with 4-methoxyphenylboronic acid, as well
as 4-chloroiodobenzene with 4-acetylphenylboronic acid, showed
cross-coupling products selectively by activation of the C-I bond
at room temperature, leaving the C-Br and C-Cl bonds untouched
(Table 3, compounds 3p-3q). Similarly, 4-chlorobenzene gave a
very good yield of biphenyl 3ac by activating the C-Cl bond over
the C-F bond (Table 3, entry 3).
For the reusability test, the reaction of 2-bromonapthalene
with 4-methoxyphenylboronic acid was performed under the best
reaction conditions for 3 h. Then the catalyst was filtered,
washed, and dried after the first catalytic cycle and used for the
next run. The catalyst could be easily recovered and reused for
successive runs without any significant loss in activity when
tested for up to five successive catalytic tests (Figure 1)
(supporting information, Table S2). The Pd nanoclusters
collected after the coupling reaction for the second time showed
an average particle size of 2.8 ± 0.5 nm, which is almost the same
as that of the fresh catalyst (2.2 ± 0.5). This means there were no
significant changes in the size or morphology of the Pd particles
between the fresh catalyst and the recovered one (Fig. 2 and
supporting information, figure S2). The possibility of leaching
that might lead to homogeneous catalysis under the reaction
conditions was confirmed by the hot filtration test (supporting
information, table S1 and Figure S1). The reaction was stopped at
50% conversion of the starting material and, after the catalyst
was removed by filtration, the reaction was continued for another
24 h. No further conversion of the reactants was observed, and
the ratio of reactant to product remained the same even after 24
h. This suggests that a complete heterogeneous mechanism acted
on the Pd nanocluster surface, but we cannot completely exclude
the possibility of leaching and reclusterization of Pd species after
the catalytic cycle.
3. Conclusions
We reported the successful synthesis of highly dispersed
heterogeneous Pd catalyst on Mg/Al hydrotalcite by adopting a
very mild and convenient preparation method. The catalyst,
consisting of Pd(0) nanoparticles, showed its wide applicability
in Suzuki coupling reactions of aryl bromides and chlorides
aReaction conditions: aryl halide
phenylboronic acid (0.15 mmol), catalyst 5 atom%, solvent 1
ml, base (0.15 mmol). bIsolated yield. c4 days.
1
(0.1 mmol),
biphenyl (Table 3, entry 1, 70%), while 4-chlorotoluene gave an

5
under mild condition with good activity, selectivity, and
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reusability, making it a potential catalytic system for application
to industrial-scale synthesis and other synthetically important
reactions.
4. Experimental
4.1. Characterization of catalyst
a) X-ray powder diffraction
X-ray powder diffraction (XRD) studies were conducted to
investigate the phase, nature, and structural changes in
synthesized nanoclusters on the support. The XRD patterns of
the Mg/Al-layered double hydroxide (HT) and Pd-HT’ hardly
differ in the range 2θ = 3o- 65o, indicating that there is no effect
on the crystallinity of HT by the incorporation of Pd onto it
(supplementary information). Both HT and Pd-HT’ showed the
typical characteristics of layered crystalline material exhibiting
sharp symmetric signals at low 2θ values and less symmetric
signals at high 2θ.10
Fig. 1. Reusability test of Pd(0)-HT’ in the Suzuki cross-
coupling reaction of 2-Bromonaphthalene and 4-
methoxyphenylboronic acid
b) Transmission electron microscopy (TEM)
The TEM images of the catalyst show uniformly dispersed Pd-
NPs on the support, with average diameter of 2.2 ± 0.5 nm
(Figure 3). The histogram plot and TEM images of the
nanoparticles are given in supplementary information.
4.2. Catalyst test for Suzuki cross-coupling reaction
The Suzuki coupling reactions were carried out in a test tube
using an organic synthesizer. An aryl halide (0.1 mmol) base
(150 mol%), (0.15 mmol) arylboronic acid, and catalyst 3-5
atom% were placed in a test tube. Argon gas was supplied from a
balloon with argon exchange twice without degassing. Solvent
was added and stirred at 1000 rpm at a specified temperature for
the desired time. Completion of the reaction was monitored by
TLC. After completion, the reaction was cooled and quenched by
1M HCl and extracted with EtOAc. The organic layer was
evaporated and dried, and the yields were reported as NMR or
isolated yields after purification by silica gel column
Fig. 2. a) TEM b) STEM image (dark field) of Pd-HT’ before
reaction c) TEM and d) STEM image of Pd-HT’ after second
catalytic reaction
chromatography
(eluted
with
EtOAc/hexane).
Fig. 3. a) TEM micrographs and b) HR-TEM of Pd-HT’

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l) Yuan, B.; Pan, Y.; Li, Y.; Yin, B.; Jiang, H. Angew. Chem. Int.
Acknowledgments
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This work was supported by Japan Science and Technology
Agency (Funds for the Development of Human Resources in
Science and Technology) and partially supported by JSPS
KAKENHI Grant Numbers JP16H01156 in Middle Molecular
Strategy and JP16H03292.
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