In situ generation of palladium nanoparticles using agro waste and their use as catalyst for copper-, amine- and ligand-free Sonogashira reaction

Article abstract of DOI:10.1002/aoc.3646

The use of water extract of waste papaya bark ash for the in situ generation of palladium nanoparticles (Pd NPs) as an efficient and environmentally friendly basic medium for the Sonogashira reaction at room temperature is reported. This methodology follows green chemistry principles as the reaction is performed using agro waste (natural feedstock) for the generation of the Pd NPs as well as for providing a basic medium for the reaction in the absence of any additional organic or inorganic base, ligand and copper salt, giving excellent yield of cross-coupled product at room temperature. The reaction conditions are compatible with electronically diverse aryl iodides and electronically diverse alkyne derivatives.

Full text of DOI:10.1002/aoc.3646

Received: 27 August 2016
DOI 10.1002/aoc.3646
Accepted: 3 September 2016
F U L L PA P E R
In situ generation of palladium nanoparticles using agro waste and
their use as catalyst for copper‐, amine‐ and ligand‐free
Sonogashira reaction
Anindita Dewan | Manashi Sarmah | Utpal Bora | Ashim J. Thakur
Department of Chemical Sciences, Tezpur
The use of water extract of waste papaya bark ash for the in situ generation of
University, Napaam, Tezpur, 784028, Assam,
India
palladium nanoparticles (Pd NPs) as an efficient and environmentally friendly basic
medium for the Sonogashira reaction at room temperature is reported. This method-
ology follows green chemistry principles as the reaction is performed using agro
waste (natural feedstock) for the generation of the Pd NPs as well as for providing
a basic medium for the reaction in the absence of any additional organic or inorganic
base, ligand and copper salt, giving excellent yield of cross‐coupled product at room
temperature. The reaction conditions are compatible with electronically diverse aryl
iodides and electronically diverse alkyne derivatives.
Correspondence
Anindita Dewan, Department of Chemical
Sciences, Tezpur University, Napaam, Tezpur –
7
84028, Assam, India.
Email: ani_dewan@yahoo.co.in; adewan@tezu.
ernet.in
KEYWORDS
agro waste material, in situ Pd NPs, Sonogashira coupling
[
17]
[18]
[20]
1
|
INTRODUCTION
ligands such as N‐heterocyclic carbenes,
phase‐transfer reaction conditions,
Salen,
[
19]
aqueous conditions,
The Pd‐catalysed cross‐coupling reaction of terminal alkynes
with aryl or alkenyl halides, which was described for the first
time by Sonogashira et al. in 1975, is one of the most reliable
tools for the preparation of arylalkynes and conjugated enyne
etc. However, the principal drawbacks associated with the
majority of these systems are the availability, stability and
cost of the Pd complexes and related ligands and their sensi-
tivity to moisture.
[
1]
moieties. Generally, the Sonogashira reaction is carried
out in the presence of a catalytic amount of a Pd salt, a Cu
salt as co‐catalyst and a ligand using excess of amine as a
Considering the concept of green chemistry regarding
environmental impact, there has been continuous effort to
develop alternative processes that minimize pollution in
chemical synthesis using environmentally benign solvents
and biodegradable, non‐toxic chemicals. In this regard use
of agro waste‐stuffs as chemical surrogates has become
one of the most exciting research endeavours for organic
chemists. Here, we present a novel and green protocol for
the Sonogashira cross‐coupling reaction carried out in water
extract of papaya tree/bark ash that is discarded as waste.
Recently Bora and co‐workers established a novel route
using extract of burnt ash of waste papaya tree/bark as
[
2]
co‐solvent.
Typically, the importance of these alkyne
derivatives as structural components of numerous natural
[
3]
[1,4]
[5]
[6]
products, pharmaceuticals,
dyes, sensors, electron-
[7]
[8]
[9]
ics, polymers, guest–host constructs and heterocycle
[
10]
synthesis
has attracted enormous attention from the scien-
tific community. In recent years, a variety of modifications
have been reported for this reaction. The most important
modification is the elimination of the Cu salt, since it can
[
11]
produce undesired alkyne homocoupled side products.
As a result, several alternative protocols have been developed
solvent system as well as in situ base for the Suzuki–
[
12]
[13]
[14]
[21]
where the use of other additives such as Ag,
and salts
Zn,
Sn
Miyaura cross‐coupling reaction.
It is well documented
[
15]
as activators has been reported. A variety of
in the literature that burnt ash of agro‐waste dead papaya
tree/bark (Carica papaya) contains potassium, sodium,
carbonate and chloride as major constituents along with a
host of other trace elements like copper, magnesium and
modifications have been reported for this reaction, including
the design of different ligands like phosphine‐based ligands
[16]
such as electron‐rich bulky phosphanes,
nitrogen‐based
Appl Organometal Chem 2016; 1–5
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
DEWAN ET AL.
[
21]
calcium.
It is believed that carbonates of sodium and
of distilled water and stirred for half an hour at room
temperature. The suspension was then filtered and a
light‐yellow‐coloured extract was isolated. This water extract
was used for the in situ synthesis of palladium nanoparticles
(Pd NPs) as well as a basic medium for the Sonogashira reac-
tion. At first, we evaluated the efficiency of WEPBA as sol-
potassium of papaya bark act as bases and other salts can
act as promoters for Sonogashira reactions.
[15]
To the best
of our knowledge, base plays a significant role in
Sonogashira reactions. Thus, the feasibility of using the
inherent basicity of water extract of burnt ash of dead
papaya tree/bark in Pd‐catalysed Sonogashira reactions
deserves serious consideration. From the green chemistry
principles, agro waste papaya tree/bark‐generated water
extract promises great potential in green chemical processes
from the sustainability, environmental and economic points
of view. In this communication, we report the successful
utilization of water extract of agro waste papaya bark ash in
Pd‐catalysed, ligand‐ and Cu‐free Sonogashira cross‐coupling
reactions.
vent in Sonogashira reaction using Pd(OAc) as the catalyst
2
by taking 4‐nitroiodobenzene and phenylacetylene as the
model substrates in the absence of base, ligand or any other
additives. In this initial experiment, we were able to isolate
about 50% of cross‐coupling product at room temperature
(Table 1, entry 1). The lower yield of the cross‐coupled
product may be because of the poor solubility of
4‐nitroiodobenzene and phenylacetylene in water. It is well
known that the insolubility of most reactants in water restricts
their use in water as a solvent for the Sonogashira reaction.
Therefore, we carried out the reaction using WEPBAwith eth-
anol as co‐solvent. Under these reaction conditions, signifi-
cant improvement was observed with 75% yield of the
desired product (Table 1, entry 2). Moreover, several test reac-
tions were also carried out using various WEPBA‐to‐ethanol
ratios. It was observed that the WEPBA‐to‐ethanol ratio is
very important and we obtained 98% yield of cross‐coupling
product using WEPBA (4 ml) and ethanol (4 ml) (Table 1,
entries 2–5). Interestingly, isopropanol also afforded compara-
ble yields of the cross‐coupling product (Table 1, entry 6).
However, in the absence of WEPBA under the present reaction
conditions the coupling reaction did not proceed (Table 1,
entry 7). It was clear that WEPBA with other Pd(II) salts such
1
.1 | Results and discussion
Initially, we prepared water extract of papaya bark ash
WEPBA) by burning agro waste papaya bark to ashes
Figure 1). After that, 10 g of ash was suspended in 100 ml
(
(
as PdCl also exhibited high catalytic activity (Table 1, entry
2
8
). But, in the absence of Pd(OAc) , the reaction did not pro-
2
ceed at all under the same reaction conditions (Table 1, entry
0). These results establish the crucial role of WEPBA in the
1
high efficiency of this system. Again, literature reports reveal
that in many Pd(II) salt‐catalysed reactions, Pd(II) species
decompose to Pd NP species at the end of the reaction
FIGURE 1 Preparation of water extract of waste papaya bark ash
a
TABLE 1 Optimization of Pd(OAc) and solvent in Sonogashira cross‐coupling reaction
2
b
Entry
Catalyst (mol%)
Co‐solvent (ml)
WEPBA (ml)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
2
(1)
(1)
(1)
(1)
(1)
(1)
(1)
—
4
4
24
24
10
10
10
10
24
10
12
24
50
75
98
50
85
95
0
EtOH (2)
EtOH (4)
EtOH (2)
EtOH (3)
i‐PrOH (4)
EtOH (4)
EtOH (4)
EtOH (4)
EtOH (4)
4
2
3
4
—
4
PdCl
2
(1)
95
70
0
Pd(OAc)
2
(0.5)
4
1
0
—
4
a
Reaction conditions: 4‐nitroiodobenzene (0.5 mmol), phenylacetylene (1 mmol).
b
Isolated yield.

DEWAN ET AL.
3
[
22]
cycle. Therefore we could not exclude the possibility of in
situ formation of Pd NPs under the current reaction conditions.
Recently, there has been a renewed interest in using non‐toxic
natural substrates to act as both in situ base as well as reducing
agent. Transmission electron microscopy (TEM) analysis
results for the optimized Sonogashira reaction of 4‐
nitroiodobenzene and phenylacetylene in WEPBA–ethanol
at room temperature (Figure 2) showed the presence of Pd
NPs. The agro waste WEPBA may play a crucial dual role both
as the reducing agent for in situ generation of Pd NPs and pro-
viding the basic medium for the Sonogashira reaction. Under
the present reaction conditions it is believed that the Pd NPs
significantly enhance the reaction yields. The particle size
and shape of the in situ generated Pd NPs were characterized
using TEM measurements (Figure 2). Figure 2(a) presents
a global TEM image of in situ generated Pd NPs. It could
be observed from the TEM image that Pd NPs preferentially
crystallize in a spherical shape. The distribution of the in
situ generated Pd NPs from Figure 2(a) was analysed using
Gaussian fits and the resultant data are plotted in Figure 2
So, it is important to mention that 1 mol% Pd(OAc) in
2
WEPBA‐catalysed Sonogashira cross‐coupling reaction is
very efficient with excellent yields at room temperature under
mild reaction conditions.
We also investigated the substrate ratio for efficient cou-
pling. Favourable results were obtained with 1:1.5 equivalent
of iodobenzene to phenylacetylene (Table 2, entry 2 versus
entry 1 versus entry 3).
To further investigate the scope and limitation of the cur-
rent protocol, several test reactions were carried out using
electronically diverse aryl iodides and acetylenes under the
[
23]
optimized reaction conditions. The results are summarized
in Table 3.
It was noticed that aryl halides with electron‐withdrawing
substituents, like nitro group (Table 3, entry 2), afforded the
corresponding coupled products in excellent yield. No signif-
icant differences were observed in the yield with meta‐
substituted electron‐rich aryl iodide (Table 3, entry 3). But,
in the case of sterically hindered ortho‐nitroiodobenzne,
product yield was very low (Table 3, entry 4). Thus, to
investigate the effects of the new catalytic system with an
electron‐donating substrate, we performed the reaction
between 4‐iodotoluene and phenylacetylene under similar
reaction conditions. It was observed that 4‐iodotoluene gave
superior product formation compared to 4‐iodonitrobenzene
within shorter reaction time (Table 3, entry 5). It is important
to mention that, generally, aryl halides with electron‐
withdrawing groups at the para position are much more reac-
tive than aryl halides bearing electron‐donating groups. The
reaction also proceeds well in the case of meta‐substituted
iodotoluene (Table 3, entry 6). However, the reaction of aryl
halides with electron‐donating groups, such as methoxy and
amine, gave the coupling products with slightly lower yields
(b). It could be ascertained that more than 80% of the Pd
NPs lie in the size range 10–20 nm and the mean particle
diameter was about 10.9 nm.
To optimize the reaction conditions, the effect of the
amount of Pd(OAc) on the coupling reaction was also stud-
2
ied (Table 1, entries 3 and 9). From the results in Table 1, we
can see that an excellent yield of the desired product was
obtained in the presence of 1 mol% Pd(OAc) . Decreasing
2
the catalyst loading to 0.5 mol% gave moderate conversion
with 70% isolated yield of the product (Table 1, entry 9).
(Table 3, entries 7 and 8). No significant differences were
observed in the yield when phenylacetylene was replaced
by p‐tolylphenylacetylene (Table 3, entry 9). The new
catalytic system was also effective for cross‐coupling of elec-
tronically diverse arylacetylenes with aliphatic alkynes
(Table 3, entries 10–16). In all cases modest to good yields
of desired cross‐coupling products were observed. However,
iodobenzene gave superior product formation with aliphatic
alkynes (Table 3, entries 10, 13 and 15). Our next endeavour
was to extend the scope of this green protocol for coupling of
FIGURE 2 (a) High‐resolution TEM image of Pd NPs and (b) Pd NPs size
distribution
a
TABLE 2 Optimization of substrate ratio
b
Entry
A:B (mmol)
Time (h)
Yield (%)
1
2
3
1:1
8
4
4
70
98
98
1:1.5
1:2
a
Reaction conditions: 4‐nitroiodobenzene (A mmol), phenylacetylene (B mmol), WEPBA (4 ml), EtOH (4 ml).
b
Isolated yield.

4
DEWAN ET AL.
a
TABLE 3 Pd(OAc) and WEPBA–EtOH catalysed Sonogashira cross‐coupling reaction of aryl halides with acetylene
2
Entry
R1
R2
X
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
H
C
C
C
C
C
C
C
C
6
6
6
6
6
6
6
6
H
H
H
H
H
H
H
H
5
5
5
5
5
5
5
5
I
8
10
12
12
4
98
98
80
30
98
80
70
40
98
95
90
55
85
65
75
60
90
90
80
60
4‐NO
3‐NO
2‐NO
2
2
2
I
I
I
4‐CH
3‐CH
3
3
I
I
12
12
12
5
4‐OCH
4‐NH
3
I
2
I
4‐CH
H
3
4‐CH
3
C
6
H
4
I
10
11
12
13
14
15
16
17
18
19
20
Hexyne
Hexyne
Hexyne
I
10
12
12
10
12
12
12
8
4‐CH
3‐CH
H
3
3
I
I
Cyclohexyl
Cyclohexyl
Dodecyne
Dodecyne
I
4‐CH
H
3
3
I
I
4‐CH
H
I
c
c
c
c
C
6
6
H
H
5
5
Br
Br
Br
Br
4‐CH
4‐CH
3
3
C
8
4‐CH
3
C
6
H
4
8
4‐NO
2
C
6
H
5
8
a
2
Reaction conditions: aryl halide (1 mmol), phenylacetylene (1.5 mmol), Pd(OAc) (1 mol%), WEPBA (4 ml), EtOH (4 ml).
b
Isolated yield.
c
Reaction carried out at 80 °C.
aryl bromides with acetylene. But at room temperature we
observed low conversion with aryl bromides. Hence, we
performed the reaction at 80 °C using the same catalyst
loading and we were able to isolate excellent yield of the
cross‐coupling product (Table 3, entries 17–20). Aryl
of any organic or inorganic base. Electronically diverse aryl
iodides undergo the coupling reaction with various electroni-
cally diverse aryl or aliphatic acetylene in excellent yields.
ACKNOWLEDGMENTS
bromide having an electron‐donating substituent, 4‐CH3,
The authors are grateful to UGC, New Delhi for awarding Dr
D. S. Kothari Postdoctoral Fellowship to A.D. M.S. thanks
Tezpur University for providing an institutional fellowship.
rendered good yields of isolated cross‐coupling products
(Table 3, entries 18 and 19). However, the reaction of aryl
bromide containing electron‐withdrawing substituents such
as p‐bromonitrobenzene with phenylacetylene failed to com-
plete, rendering only 60% yield (Table 3, entry 20).
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2
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1
13
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How to cite this article: Dewan, A., Sarmah, M.,
Bora, U., and Thakur, A. J. (2016), In situ generation
of palladium nanoparticles using agro waste and their
use as catalyst for copper‐, amine‐ and ligand‐free
Sonogashira reaction, Appl Organometal Chem, doi:
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