Microwave assisted cross-coupling reactions using palladium nanoparticles in aqueous media

Article abstract of DOI:10.1080/00397911.2019.1576203

Glucose stabilized palladium nanoparticles (PdNPs) have been prepared and the application of NPs in catalyzing both Suzuki and Heck reactions has been explored in aqueous media under microwave conditions. Both electron-rich and electron-deficient aryl halides can be coupled with a variety of boronic acids and styrene to access a wide variety of biaryl compounds and substituted alkenes in good to excellent yields. The catalyst can be recycled and reused four times with minimally affecting the morphology and efficiency of the nanoparticles. A plausible reaction mechanism has been proposed.

Full text of DOI:10.1080/00397911.2019.1576203

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Microwave assisted cross-coupling reactions using
palladium nanoparticles in aqueous media
Kalyan Dhara, Bibudha Parasar, Avinash J. Patil & Jyotirmayee Dash
To cite this article: Kalyan Dhara, Bibudha Parasar, Avinash J. Patil & Jyotirmayee Dash (2019):
Microwave assisted cross-coupling reactions using palladium nanoparticles in aqueous media,
Synthetic Communications, DOI: 10.1080/00397911.2019.1576203
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https://doi.org/10.1080/00397911.2019.1576203
Microwave assisted cross-coupling reactions using
palladium nanoparticles in aqueous media
Kalyan Dharaa, Bibudha Parasara, Avinash J. Patilb, and Jyotirmayee Dasha
aDepartment of Organic Chemistry, Indian Association for the Cultivation of Science, Kolkata, India;
bCentre for Organized Matter Chemistry, University of Bristol, Bristol, UK
ABSTRACT
ARTICLE HISTORY
Received 14 December 2018
Glucose stabilized palladium nanoparticles (PdNPs) have been pre-
pared and the application of NPs in catalyzing both Suzuki and Heck
reactions has been explored in aqueous media under microwave
conditions. Both electron-rich and electron-deficient aryl halides can
be coupled with a variety of boronic acids and styrene to access a
wide variety of biaryl compounds and substituted alkenes in good
to excellent yields. The catalyst can be recycled and reused four
times with minimally affecting the morphology and efficiency of the
nanoparticles. A plausible reaction mechanism has been proposed.
KEYWORDS
Glucose; Heck coupling;
palladium nanoparticle;
recycling; Suzuki coupling
GRAPHICAL ABSTRACT
Introduction
In the past few decades, the major focus of catalysis has been shifted towards the devel-
opment of sustainable synthetic reactions that can generate bio-friendly by-products
and waste materials without compromising on yield. The persisting problem for indus-
trial scale synthesis of drug molecules, pesticides etc. is the disposal of organic solvents,
and therefore, efforts have been pursued to use eco-friendly alternative solvents; mainly
water. Water has always been avoided as a solvent of choice due to the poor solubility
CONTACT Jyotirmayee Dash
ocjd@iacs.res.in
School of Chemical Sciences, Indian Association for the Cultivation
of Science, 2B Raja S C Mullick Road, Kolkata 700032, India.
Supplemental data for this article is available online at on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
K. DHARA ET AL.
of reactants as well as the labile nature of reagents in aqueous media. In recent years,
there is significant interest in developing metal-catalyzed organic transformations in
aqueous media.[1] Increasing evidence further suggests that water, with its high dielec-
tric constant, is an ideal solvent for microwave-based synthesis[2–4] that can also be car-
ried out at a higher temperature.
The Suzuki coupling[5–9] is one of the most commonly used C–C bond forming reac-
tions for the construction of biaryl compounds.[10] These coupling reactions have also
been optimized in aqueous media[11] using nanoparticle-based catalysts. Colloidal nano-
particle-based catalysts provide a platform to carry out reactions with high efficiency
and catalyst reusability.[12] Given their large surface area, they provide an abundance of
local reaction centers that could result in high turnover numbers (TON). With a water-
soluble ligand/stabilizing agent, they can further be tuned to facilitate organic reactions
in water.[13,14]
In this context, we herein report glucose stabilized palladium nanoparticles for cross-
coupling reactions. We anticipated that the presence of five hydroxyl groups in glucose
would efficiently chelate palladium nanoparticles as well as render high aqueous solubil-
ity of the palladium catalyst. Liu et al. described the catalytic ability of starch-based pal-
ladium nanoparticle (PdNP) to catalyze Suzuki–Miyaura coupling.[15] In their report,
the particle size increased over successive recycling of the catalyst and the activity went
down to 20% after 6th cycle. The decrease in yield was possibly due to the agglomer-
ation of the nanoparticles. We envisaged that ligand might be useful to prevent the
aggregation and retain the catalytic efficiency of the nanoparticles. Starch, being poly-
meric has a higher propensity to interact with other particles in a cooperative manner
facilitating agglomeration. However, strategic use of a high binding monomer may pre-
vent such a side reaction, and retain the property and catalytic activity of nanopar-
ticles.[16,17] Herein, we report the catalytic activity of D-glucose stabilized Pd
nanoparticles for the Suzuki and Heck cross-coupling reactions.
Results and discussion
We have performed the Suzuki reaction with freshly prepared Pd nanoparticles stabi-
lized by D-glucose in neat water using microwave heating.[18–23] We first examined the
catalytic efficiency of our synthesized glucose stabilized palladium nanoparticles by
exploring the cross-coupling of iodobenzene 1a with 4-methoxy phenylboronic acid 2a
in the presence of 5 mol % PdNPs, Na2CO3 as a base and tetrabutylammonium brom-
ide to give the corresponding product 3a in 95% yield (Table 1, entry 1).[22] Notably,
under microwave conditions, the reaction underwent complete conversion within 5
min. The optimized reaction conditions were further applied for the cross-coupling of
both electron-rich and electron-deficient aryl halides 1a–1n with 4-methoxy phenylbor-
onic acid 2a, phenylboronic acid 2b, and 4-cyano phenylboronic acid 2c to obtain the
corresponding products 3a–3n in good to excellent yields with comparable reaction rate
(Table 1). Next, electron-deficient aryl halides 4-bromoacetophenone 1b, 4-bromo ben-
zaldehyde 1c and 4-bromo nitrobenzene 1f were used to cross-couple with electron-rich
4-methoxy phenylboronic acid 2a and the corresponding products 3b, 3c, and 3f were
obtained in 69–76% (Table 1, entries 2, 3, and 6). Subsequently, electron-rich aryl

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Table 1. Suzuki couplings under microwave conditions in aqueous mediaa.
R2
X
B(OH)2
PdNPs
TBAB, Na2CO3
H2O, µM
R2
R1
R1
3a-n
2a-c
2 = -OMe, -H, -CN
R1 = -COCH3, -CHO, -OH, -NH2, -NO2, -OMe
1a-h
X = Br, I
R
Entry
1
ArX, 1
Ar1B(OH)2, 2
Ar-Ar1, 3
Yield (%)
95%
I
MeO
B(OH)2
MeO
3a
2a
1a
O
O
Br
MeO
MeO
B(OH)2
B(OH)2
MeO
2
3
76%
69%
1b
3b
3c
2a
2a
O
CHO
MeO
MeO
Br
H
1c
OH
Br
OH
NO2
OMe
MeO
MeO
B(OH)2
B(OH)2
4
5
52%
88%
3d
1d
2a
2a
I
MeO
3e H2
N
NH2
1e
Br
MeO
MeO
B(OH)2
B(OH)2
MeO
O2N
6
7
70%
54%
3f
1f
2a
2a
MeO
Br
N
N
1g
3g
MeO
Br
MeO
B(OH)2
MeO
8
82%
93%
91%
88%
92%
93%
3h
1h
2a
B(OH)2
I
I
9
3i
2b
1a
CN
NC
B(OH)2
10
11
12
13
3j
2c
1a
O
O
B(OH)2
B(OH)2
B(OH)2
Br
Br
3k
3l
2b
1b
OH
HO
1d
2b
2b
I
NH2
3m
NH2
1e
Br
Br
O2N
B(OH)2
B(OH)2
14
15
O2N
73%
89%
3n
1f
2b
2b
MeO
MeO
3a
1h
aAryl halides (1.0 mmol), boronic acid (1.0 mmol), Na2CO3 (3 mmol), TBAB (1.0 mmol), 5 mol% water soluble
glucose stabilized Pd (0) nanoparticles as catalyst, 2 mL water, microwave, 150 ꢀC, 5 min.

4
K. DHARA ET AL.
Table 2. Heck couplings under Microwave conditions in aqueous mediaa.
X
Ph
PdNPs
Ph
TBAB, Na2CO3
H2O, M
m
R1
R1
5
1
4
X = Br, I R1 = -COCH3, -NH2
Entry
ArX, 1
Product, 5
Yield (%)
74%
I
Ph
1
2
3
5a
1a
O
Ph
47%
5b
Br
O
1b
Ph
I
45%
NH2
NH2
5c
1e
aAryl halides (1.0 mmol), Styrene (1.0 mmol), Na2CO3 (3 mmol), TBAB (1.0 mmol), 5 mol% water soluble glucose
stabilized Pd (0) nanoparticles as catalyst, 2 mL water, microwave, 150 ꢀC, 5 min.
halides and electron-rich boronic acids were examined for this coupling reaction. The
catalytic system also proved to be highly efficient for electron-rich aryl halides. When
the efficiency of the coupling reaction with electron-rich 4-bromo phenol 1d was tested,
the product was obtained in 52% yield (Table 1, entry 4).
However, the reaction with electron-rich aryl halides 4-bromoanisole 1h and 2-iodoa-
niline 1e underwent a coupling reaction with 4-methoxy phenylboronic acid 2a using
this protocol to afford corresponding products 3e and 3h in good yields (Table 1,
entries 5 and 8). Aromatic heterocyclic halide, 2-bromopyridine 1g also underwent reac-
tion with 4-methoxy phenylboronic acid 2a smoothly to give the desired product 3g in
54% yield (Table 1, entry 7). We subsequently compared the reactivity of all the aryl
halides with 4-methoxyphenylboronic acid 2a. It can be easily realized that, aryl bro-
mides bearing electron withdrawing group decreases the reaction yield (Table 1, entries
2, 3, and 6) whereas the presence of amino group in the ortho position and hydroxy
group in the para position resulted in high yield for phenyl boronic acid (Table 1,
entries 12 and 13). 4-Hydroxy bromobenzene 1d and 2-iodoaniline 1e reacted with phe-
nylboronic acid 2b to afford the corresponding products 3l and 3m in 92–93%
(Table 1, entries 12 and 13). Electron-deficient 4-cyanophenylboronic acid 2c also
reacted with iodobenzene 1a to give the corresponding product 3j in 91% yield
(Table 1, entry 10). Electron-deficient aryl halides 4-bromoacetophenone 1b and 4-
bromo nitrobenzene 1f were also efficiently reacted with phenylboronic acid 2b to
afford the coupling products 3k and 3n in 88% and 73%, respectively (Table 1, entries
11 and 14). The coupling reaction of iodobenzene 1a and 4-bromoanisole 1h were per-
formed with phenylboronic acid 2b. The corresponding products 3i and 3a were iso-
lated in synthetically useful yields (Table 1, entries 9 and 15).
To further explore the scope of our proposed methodology, we have performed the
Heck coupling[24–26] of different aryl halides with styrene. Applying the conditions
established for the Suzuki coupling, Heck coupling was carried out and the results are
summarized in Table 2. Both electron withdrawing (p-bromoacetophenone 1b) and
electron donating groups bearing aryl halides (2-iodoaniline 1e) reacted with styrene 4

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Table 3. Reusability of the glucose stabilized palladium nanoparticle for Suzuki and Heck cou-
pling reactiona.
Recycle Number
1
2
3
4
Yield for Suzuki reaction
Yield for Heck reaction
95%
74%
85%
65%
79%
63%
70%
60%
aAll reactions are carried out under optimized condition.
Figure 1. TEM images of PdNPs freshly prepared (a), after 1st cycle (b), after 4th cycle (c).
to afford the corresponding products in comparable yields whereas iodobenzene 1a
reacted with styrene 4 to generate the compound 5a in 74% yield.
Recycling of catalyst
Catalysts with durable lifetime and recyclability are highly desirable traits in industrial
scale synthesis. Keeping the broad applications in mind, catalytic recycling efficiency of
PdNPs was investigated for both Suzuki and Heck coupling reactions. The reaction
between iodobenzene 1a and 4-methoxyphenyl boronic acid 2a was chosen as the model
reaction for Suzuki coupling and that between iodobenzene 1a and styrene 4 was con-
sidered as the model reaction for Heck coupling reaction, to assess the efficiency of the
catalysis after successive reusability. In a typical procedure, both the reactions were car-
ried out under optimized conditions using glucose stabilized palladium nanoparticles.
After each catalytic cycle, the aqueous part was washed with diethyl ether, and then the
colloidal aqueous part was used in the subsequent catalytic reaction. The catalyst
remained active up to four consecutive cycles for both the reactions, albeit with a grad-
ual decrease in efficiency over repeated use (Table 3). Notably, the catalytic efficiency in
4th reuse was still higher than that observed in the starch stabilized PdNP observed
earlier.[15,27]
Transmission electron microscopy (TEM) images of the PdNPs were taken after suc-
cessive recovery of the catalyst for both Suzuki and Heck reactions. The size of the
freshly prepared glucose stabilized PdNPs was 10 nm (Fig. 1a). The TEM images
recorded after the catalytic runs revealed that both size and morphology of the PdNPs
remained same even after the fourth round of the catalytic cycle (Fig. 1b and c).
However, comparison of the images of PdNPs recovered after the first and fourth round
of catalysis revealed the increase in density of the aggregated clusters that were formed
post reaction. This gradual decrease of the catalytic efficiency may be attributed to
aggregation and formation of a supercluster of PdNPs over successive reuse of the

6
K. DHARA ET AL.
HX
Ar-Ar'
Ar
Ar'
discrete
Pd (0)
H
L
Pd(II) X
L
D
Ar
L
Pd(II) Ar'
L
D
Ar
Ar'
L
Pd(II) X
L
PdNPs
XB(OH)2
Suzuki
Heck
X
Oxidative
Insertion
r
-
B
reaction
reaction
A
C
Ar'-B(OH)2
Ar
Ar'
Pd(II) X
Ar
Ar
Pd(II) X
L
L
L
X
L
A
Ar
A
L
Pd(II) X
L
Ar'
Scheme 1. Proposed mechanism of Suzuki and Heck reaction. A: chemical etching, B: transmetalation,
C: b-hydride elimination, D: reductive elimination.
catalyst, which in turn may potentially have a critical effect on the catalytic efficiency.
PdNPs were washed with organic solvents and that may alter or remove the glucose
layer from the surface of the nanoparticles facilitating aggregation and decreasing the
catalytic efficiency.
Proposed mechanism
We believe both Suzuki and Heck reactions are initiated with oxidative addition of aryl
halide to the core of the palladium nanoparticles, involving both Pd (0) and Pd (II).
Involvement of both the oxidation states along with an abundance of ionic species cre-
ates a corrosive environment[28–30] around the active site that finally lead to chemical
etching of nanocluster to generate discrete Pd(II) catalytic center (Scheme 1). Our
hypothesis is supported by the fact that the homocoupling byproduct has not been
formed during the reaction. Suzuki reactions may further undergo transmetalation and
reductive elimination to release biaryl 3 and Heck reactions may undergo b-hydride
elimination followed by reductive elimination to generate substituted alkene 5 as the
product. Along with the desired product, both the reactions also generate discrete Pd
(0) species, which in turn is deposited back to Pd (0) nanocluster.[31] The palladium
nanoparticles aggregated with repeated cycling of the catalyst as a result of a process
similar to Ostwald ripening,[32] causing atomic rearrangement.
We believe, glucose being a monomer is a weaker ligand than starch and is less cor-
rosive. Hence, glucose stabilized nanoparticle would exhibit aggregation at a slower rate.
Given the dependence of reaction efficiency on the size of metal nanoparticles[33] and
glucose being a softer ligand than starch, palladium nanoparticles stabilized by glucose
would function better than the starch stabilized nanoparticles on the recovery and reuse
of the catalyst.[34]

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Experimental
General remarks
All experiments were carried out under an inert atmosphere of argon in flame-dried
flasks. Solvents were dried using standard procedures reported in Perrin, D. D.;
Armarego, W. L. F., Purification of Laboratory Chemicals, 3rd edition, Pergamon Press,
Oxford, 1988. All starting materials were obtained from commercial suppliers and used
as received. Products were purified by flash chromatography on silica gel (100–200
1
€
mesh, Merck). H NMR spectra were recorded at 500 MHz using Bruker ADVANCE
500 MHz (Bruker, Switzerland) and JEOL 400 MHz (JEOL, Japan) at 298 K. Signals are
quoted as d values in ppm using residual protonated solvent signals as internal standard
(CDCl3: d 7.26 ppm). Data is reported as follows: chemical shift, integration, multipli-
city (s = singlet, d = doublet, t = triplet, q = quartet, p = pentet, br = broad, m = multi-
plet), and coupling constants (Hz). 13C NMR spectra were recorded on either a JEOL-
€
400 (100 MHz) or a Bruker ADVANCE 500 MHz (125 MHz) with complete proton
decoupling. Chemical shifts (d) are reported in ppm downfield from tetramethylsilane
with the solvent as the internal reference (CDCl3: d 77.16 ppm). HRMS analyses were
performed with Q-TOF YA263 high resolution (Water Corporation, Australia) instru-
ments by þ ve mode electrospray ionization. All the reactions were carried out in a
Biotage Microwave synthesis system (Initiator EXP. EU, Biotage Sweden AB).
Preparation of glucose stabilized palladium nanoparticles
Reusable D-glucose stabilized Pd nanoparticles with controlled particle sizes were pre-
pared using the procedure described by Xu and coworkers.[27,35]
Typical procedure for Suzuki–Miyaura coupling reaction
Biphenyl derivatives were prepared from cross-coupling of commercially available aryl
halides and boronic acids using glucose stabilized Pd-nanoparticles (PdNPs), Na2CO3,
ꢀ
TBAB in water under microwave irradiation at 150 C for 5 min. In a 5 mL microwave
vial Na2CO3 (3 equiv.) and TBAB (1 equiv.) were added to a suspension of aryl halide
(1 equiv.) and boronic acid (1 equiv.) in 2 mL water. Then the mixture was degassed
with N2 for 5 min. Then a catalyticꢀamount of PdNPs (5 mol %) was added and the
resulting mixture was stirred at 150 C in the microwave for 5 min. Then the reaction
mixture was cooled and extracted with diethyl ether (3 ꢁ 20 mL). The combined
organic phases were washed with brine, dried over anhydrous Na2SO4, filtered, and con-
centrated in vacuum. The crude residue was then purified by column chromatography
on silica gel with ethyl acetate hexane (5/95 to 10/90) to provide compounds 3.
Typical procedure for Heck coupling reaction
In a 5 mL microwave vial containing Na2CO3 (3 equiv.) and TBAB (1 equiv.), aryl hal-
ide (1 equiv.) and styrene (1 equiv.) were added in 2 mL water. Then the mixture was
degassed with N2 for 5 min in room temperature. Then 5 mol% glucose stabilized Pd

8
K. DHARA ET AL.
ꢀ
nanoparticles (PdNPs) was added to it and stirred at 150 C in the microwave for 5
min. Then the reaction mixture was cooled to room temperature and extracted with
diethyl ether (3 ꢁ 20 mL). The combined organic phases were washed with brine, dried
over anhydrous Na2SO4, filtered and concentrated in vacuum. The crude residue was
then purified by column chromatography on silica gel with ethyl acetate-hexane (5/95
to 10/90) to provide compounds 5.
Conclusions
In conclusion, our results suggest that use of a monomeric ligand over polymeric may
increase reaction efficiency by controlling aggregation of the nano-catalysts over succes-
sive use. As a proof of principle, we have synthesized glucose stabilized palladium nano-
particles and showcased its efficiency for Suzuki and Heck coupling reactions in
aqueous media, with potential reusability of the catalyst and relatively minimal loss in
activity. The exact mechanism of aggregation induced decrease in reaction efficiency is
currently under investigation.
Funding
We thank the Department of Science and Technology, Ministry of Science and Technology
(DST) and CSIR- India for research funding. KD thanks UGC for a research fellowship.
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