Palladium nanoparticles supported on a titanium dioxide cellulose composite (PdNPs@TiO2-Cell) for ligand-free carbon-carbon cross coupling reactions

Article abstract of DOI:10.1039/c5ra20680d

Well-dispersed non-spherical PdNPs with a diameter of 39-45 nm supported on a TiO₂-cellulose composite (PdNPs@TiO₂-Cell) can be synthesized by a simple and clean route. The catalyst was well characterized by XRD, FE-SEM, EDS, and TEM techniques. The PdNPs have good dispersity on the TiO₂-Cell support. This results in excellent catalytic activities for the synthesis of biphenyls, acrylates, acetylenes and prochiral ketones using low Pd loading (1 mol%) at comparatively low temperature. The effects of the nature and amount of bases, nature of solvents, amount of catalyst and the reaction temperature on the activity of PdNPs@TiO₂-Cell were thoroughly investigated. The catalyst showed at least four times reusability without decrease in catalytic activity.

Full text of DOI:10.1039/c5ra20680d

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Palladium nanoparticles supported on titanium dioxide cellulose composite
(PdNPs@TiO2-Cell) for ligand-free carbon-carbon cross coupling reactions
Sanjay Jadhava, Ashutosh Jagdaleb, Santosh Kamblec, Arjun Kumbharb*, Rajshri Salunkhea*
aDepartment of Chemistry, Shivaji University, Kolhapur, 416004, M.S., India
bDepartment of Chemistry, Padmabhushan Dr. Vasantraodada Patil College, Tasgaon,
(Sangli) Maharashtra, 416312, India
cDepartment of Chemistry, Yashvantrao Chavan Institute of Science, Satara, Maharashtra,
415001, India
E-mail: arjun2win@yahoo.co.in
Abstract: Well-dispersed non-spherical PdNPs with a diameter of 39-45 nm supported on
TiO2-cellulose composite (PdNPs@TiO2-Cell) can be synthesized by a simple and clean
route. The catalyst was well characterized by XRD, FE-SEM, EDS, and TEM techniques. The
PdNPs have good dispersity on the particles of support. This results in excellent catalytic
activities for the synthesis of biphenyls, acrylates, acetylenes and prochiral ketones using
low Pd loading (1 mol %) at comparatively low temperature. The effects of nature and
amount of bases, nature of solvents, amount of catalyst and the reaction temperature on
the activity of PdNPs@TiO2-Cell were thoroughly investigated. The catalyst showed at least
four times reusability without decrease in catalytic activity.
Keywords: Cellulose, Pd nanoparticles, biopolymeric-inorganic hybrid material, TiO2, cross-
coupling reactions.
1. Introduction
Transition metal catalysis especially palladium catalyzed cross coupling reactions of
aromatic halides in the presence of various nucleophiles is strategically important in
organic synthesis. It has been widely used for the synthesis of a diverse array of biphenyls,
acrylates, acetylenes and prochiral ketones by C-C cross coupling reactions. These
compounds have profound importance in chemical, pharmaceutical and biochemical
industries1. Additionally, such compounds are also present in many natural as well as
biologically active compounds2, and are especially interesting in applications for organic
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light-emitting diodes and chemiluminescence detection systems3. These compounds have
been mostly synthesized by palladium catalyzed Suzuki-Miyaura4, Mizoroki-Heck5, Heck-
Matsuda6, Sonogashira-Hagihara7, and carbonylative cross-coupling reactions8. Recently,
this area of research has attracted great interest because of its high compatibility to a wide
variety of functional groups under mild reaction conditions.
Though, most of these transformations have been extensively investigated by
homogeneous palladium complexes in solution9. The separation of metal catalysts from the
reaction mixture and their reuse is highly desirable from economical and environmentally
point of view10. Additionally, the homogeneous Pd complexes also undergo deactivation
due to the aggregation of Pd during the reactions. In this context, heterogeneous catalysts
particularly, the PdNPs supported on suitable solid support has found immense importance
for many cross coupling reactions11. This strategy increases the catalytic activity of Pd and
also reduces the amount of metal required for the reaction12. Several oxides have been used
as a support for PdNPs13, because moderate to high dispersions was obtained on these
oxides due to favorable metal–support interactions14. Out of these oxides TiO2 based
materials have found potential applications across many different areas15. In recent years
much like the noble metal nanoparticles, PdNPs supported TiO2 and Pd supported TiO2 core
shell catalysts have seen an extensive amount of research in methanol reforming16,
hydrogenation17, and photocatalysis18.
Biopolymers such as alginate, chitosan, starch, and cellulose has been developed as
a most attractive support for immobilization of many Pd catalysts19. The extensive number
of -OH groups present in cellulose can facilitate the complexation of TiO2 to the molecular
matrix, and play a significant role in guiding the organization of TiO2 among cellulose
molecules. In addition to this, the use of cellulose has several key advantages. No additional
reducing agents are required20. Cellulose also avoids the aggregations of PdNPs, as it acts
as the protecting agent similar to other biopolymers21. There is binding interaction
between cellulose and the metal nanoparticles which provides a platform to PdNPs and
helps to stabilize Pd as that of hydroxyl group containing solvents22-24. As a result of this
there is very less leaching of metal into solution. In addition it is possible to carry out
reactions in aqueous medium due to the fine dispersibility of cellulose in water25. Hybrid
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organic-inorganic composites have received much attention during recent years because of
their unique properties26. Due to these properties there is easy diffusion of reactants and
reagents inside the pores of materials. They can be modified in the form of hybrid
composites by easy processing with conventional techniques with good thermal and
chemical stability.
Recently we showed that Pd supported on alumina cellulose composite (Pd@Al2O3-
Cell catalyst) yields various biphenyls in good to excellent yields in aqueous medium27. In
continuation of our efforts to use natural feedstock such as carbon28, agrose29 and
chitosan30 for immobilization of Pd catalyst, herein we report an efficient method for the
synthesis of PdNPs dispersed on TiO2 cellulose composite (PdNPs@TiO2-Cell). The
prepared catalyst was well characterized by different techniques and applied for the
synthesis of various biphenyls, acrylates, prochiral ketones and acetylenes using low Pd
loading (1 mol %) at comparatively low temperatures.
2. Results and discussion
2.1 The preparation of PdNPs@TiO2-Cell catalyst
The preparation of PdNPs@TiO2-Cell catalyst is quite straightforward. TiO2-Cell
composite31 obtained from cellulose and TiCl4 was stirred with Pd(OAc)2 in ethanol at 50 °C
temperature as described in the experimental procedure affording the immobilized PdNPs
on TiO2-Cell composite. The composite turned light gray after 4 h indicating the formation
of PdNPs.
2.2 The characterization of PdNPs@TiO2-Cell catalyst
The amount of Pd in the catalyst was 2.9 wt % as given by ICP-AES. The field emission
scanning electron microscopy (FE-SEM) images of PdNPs@TiO2-Cell is shown in Fig. 1a and
1b. The SEM image shows the fibrous morphology of the catalyst. In EDS images (Fig.1c and
1d) the bright points on the surface are due to the characteristic emission of the Ti. The
EDS metal mapping shows that TiO2 was uniformly covered on the fibrous surface of the
cellulose. EDS spectrum (Fig. 1e) also shows that palladium is exists in the composite along
with titanium, oxygen. This also confirmed the successful grafting of the Pd and TiO2 on
cellulose.
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Fig. 1. FE-SEM images of PdNPs@TiO2-Cell, SEM images; (a, b), back scattering images (c,
d), and EDS spectrum (e).
The TEM micrographs of the catalyst are depicted in Fig. 2. It can be seen that non-
spherical shaped PdNPs of 39-45 nm were homogeneously dispersed on composite. As
shown in figure, the PdNPs are non-spherical and contain a large number of sharp corners,
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and edge sites. The surface atoms at these sites are physically unstable and are chemically
active.
Fig. 2. TEM analysis of PdNPs@TiO2-Cell (a) fresh catalyst, and (b) catalyst after 4th recycle.
The XRD analysis of cellulose and catalyst (Fig. 3) shows that the coating of oxide is
exclusively in amorphous. This was indicated by no other diffraction peaks in XRD
spectrum of the catalyst. The X-ray diffraction pattern also confirmed the formation of
highly crystalline PdNPs.
Fig. 3. XRD pattern of cellulose and PdNPs@TiO2-Cell
2.3 The activity of PdNPs@TiO2-Cell catalyst for Suzuki-Miyaura coupling reaction:
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To test the activity of catalyst and effect of various parameters, we first carried out
Suzuki-Miyaura cross-coupling reaction between 4-bromobenzophenone and
phenylboronic acid (Scheme 1).
O
O
B(OH)2
PdNPs@TiO2-Cell
+
Br
Solvent, Base,
Temperature
Scheme 1. Suzuki-Miyaura reaction catalyzed by PdNPs@TiO2-Cell
To examine the effect of bases, a series of bases were taken into consideration for
the coupling between 4-bromobenzophenone and phenylboronic acid in 95 % ethanol at
80°C temperature (Table 1) using 1 mol % catalyst. The most common and inexpensive
base K2CO3 was found to be most effective (Table 1, entry 7). Surprisingly, K3PO4, NaOAc,
Na2CO3, and Cs2CO3 were found to be less effective (Table 1, entries 1-4). Though organic
base Et3N showed good conversion (Table 1, entry 6), pyridine was completely failed to
convert 4-bromobenzophenone into corresponding coupling product (Table 1, entry 5). We
then studied the effect of quantity of base by carrying reactions using 0.5 to 3 equivalents
of K2CO3 (Table 1, entries 7-10). When the reaction is carried out without K2CO3, no
product was observed indicating that the base is known to activate32 phenylboronic acid
(Table 1, entry 11). However, our results indicated that for this catalytic system base is not
needed for the activation of phenylboronic acid, and its only role is to neutralize the boric
acid33 as one equivalent base showed good conversion (90 % yield) in 1 h (Table 1, entry
8). Thus, either a surface hydroxyl group of TiO2 hydrolyzes the boronic acid to boric acid
as reported for gold34 and/or to the cerium oxide35.
Table 1.Screening of various bases for Suzuki–Miyaura coupling reactiona
Entry Base (mmol)
Time (h)
Yieldb (%)
74 (55)
70(50)
1
2
3
4
K3PO4 (2)
NaOAc (2)
Na2CO3(2)
Cs2CO3 (2)
6
5
5
6
82(70)
80 (65)
6

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5
6
Pyridine (2)
Et3N (2)
10
7
Trace
85(75)
7
K2CO3 (2)
K2CO3 (1)
K2CO3 (0.5)
K2CO3 (3)
Base free
5
90 (90)
90(90)
8
5
9
6
50(50)
10
6
90(90)
11
12
no reaction
aReaction conditions: 4-bromobenzophenone (1.0 mmol), phenyl
boronic acid (1.1 mmol), catalyst (1 mol %), base (0 to 3 mmol),
95 % ethanol (5 mL), at 80 °C. Values in the parentheses are
yields after 1 h. bIsolated yields after column chromatography.
The screening of the solvents for model reaction showed that catalyst gave
quantitative yield (90 %) in 95 % ethanol within 1 h (Table 2, entry 1). Unfortunately
catalysts was less active in water (Table 2, entry 4), but showed moderate activity in
EtOH:H2O mixture (Table 2, entries 2 and 3). We then examined the activity of catalyst in
DMF and DMF:H2O mixture. The results showed that PdNPs@TiO2-Cell was less active as
compared to our previous reported Pd@Al2O3-Cell catalyst27 in DMF and DMF:H2O mixture.
Use of 95 % ethanol renders this protocol quite practical to large scale synthesis
considering its economic and environmental point of view.
Table 2. Effect of solvents on Suzuki–Miyaura coupling reactiona
Entry
Solvent
Time
(h)
1
Yieldb
(%)
90
1
2
3
4
5
6
7
95% EtOH
EtOH:H2O (80:20)
EtOH:H2O (50:50)
H2O
6
6
6
6
6
6
70
40
25
80
60
34
DMF
DMF:H2O (80:20)
DMF:H2O (50:50)
aReaction conditions: 4-bromobenzophenone (1.0 mmol), phenyl boronic acid (1.1
mmol), catalyst (1 mol %), K2CO3 (1.0 mmol), solvent (5 mL) at 80 °C. bIsolated yields
after column chromatography.
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In order to evaluate the potential of the catalyst, the model reaction was run with
changing the amount of catalyst. Lowering the palladium concentration might effectively
suppress agglomeration of Pd as observed in ligand-free Suzuki–Miyaura coupling reaction
by Pd supported zeolite36. The results shown in Table 3 indicated that among the different
loading 1 mol% proved to be the best and was chosen as the optimum loading for further
study.
Table 3.Optimization of catalyst loading for Suzuki–Miyaura coupling reactiona
Entry Amount of catalyst
(mol %)
Temperature
Time
(h)
1
Yieldb
(%)
10
(°C)
80
1
2
3
4
5
6
0.25
0.50
0.75
1.00
1.25
1.50
80
80
80
80
80
1
1
1
1
1
70
70
90
90
90
7
8
2.00
1.00
80
r.t.
1
90
20
12
9
1.00
1.00
1.00
1.00
40
60
80
80
12
12
12
12
45
80
10
11
12
20c
15d
aReaction conditions: 4-bromobenzophenone (1.0 mmol), phenyl boronic acid (1.1
mmol), catalyst (0.25 to 2 mol %), K2CO3 (1.0 mmol), 95 % ethanol (5 mL), room
b
c
temperature to 80 °C. Isolated yields after column chromatography. Reaction using
Pd@TiO2. dReaction using Pd@cellulose.
To study the effect of temperature on catalytic activity, the model reaction was
carried out at different temperatures (Table 3, entries 8-10). The results indicated that the
reaction temperature is crucial to the catalytic efficiency. Increasing the reaction
temperature from room temperature to 60 °C, the yield of the product gradually increases
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from 20 to 80%. While, significant acceleration in rate was observed at 80 °C, affording a
quantitative yield of the desired product in 1h (Table 3, entry 4). It was observed that
PdNPs@TiO2-Cell is more active as compared to Pd/TiO2 and Pd/Cellulose (Table 3, entries
11 and 12). This may be due to more diffusion of substrate and distribution of PdNPs over
composite as compared to pure support.
To expand the substrate scope of the arylation, we performed a number of
corresponding transformations using substituted aryl halides and aryl boronic acids using
K2CO3 as base at 80 °C in 95 % ethanol using 1 mol % catalyst. Several functional groups
such as OCH3, COCH3, PhCO, and heterocyclic moieties were reacted smoothly giving the
corresponding coupling products in good yields (Table 4). The position of substituents on
the aromatic ring of aryl halides did not have any appreciable effect on progress of the
reaction. We next turned our attention towards ortho substitution by coupling sterically
hindered bromomesitylene with phenyl boronic acid. In this case good yield (72 %) could
be achieved in 6 h (Table 4, entry 5).The C–X bond strength affects the activity towards the
Suzuki-Miyaura reaction as the general cross coupling reactions follows reactivity order of
C–I>C–Br > C–Cl. The main drawback of the many Pd mediated Suzuki-Miyaura cross-
coupling reaction is that only aryl iodides and aryl bromides can be used efficiently. While
aryl chlorides are either inert or gave very less conversion, because of the stronger C-Cl
bond which is in fact responsible for the slower reaction rate of aryl chlorides37.
Unfortunately, coupling of chlorobenzene as well as 4-chlorobenzophenone with phenyl
boronic acid was inefficient under optimized reaction conditions (Table 4, entries 14 and
15).
Table 4.The Suzuki-Miyaura reactions of various aryl halides with aryl boronic acidsa
Entry
1
Aryl halide
Arylboronic
acid
Product
Time
(h)
1
Yieldb
(%)
90
O
B(OH)2
O
Br
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O
O
2
3
B(OH)2
O
2
4
85
S
Br
Br
S
O
81
89
O
B(OH)2
S
S
O
B(OH)2
4
Br
2.5
O
O
B(OH)2
5
6
Br
Br
6
72
90
3.5
O
B(OH)2
S
S
O
B(OH)2
B(OH)2
7
8
4
6
82
80
Br
Br
9
Br
B(OH)2
B(OH)2
6
86
MeO
OMe
OMe
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B(OH)2
10
Br
1
90
MeO
OMe
Br
B(OH)2
B(OH)2
OMe
11
12
1
1
85
80
OMe
MeO
Br
OMe
Br
B(OH)2
B(OH)2
B(OH)2
13
14
15
2
88
50
36
Cl
12
12
O
O
Cl
aReaction conditions: aryl halides (1.0 mmol), arylboronic acids (1.1 mmol), catalyst (1 mol %), K2CO3 (1.0 mmol), 95
% ethanol (5 mL) at 80 °C. bIsolated yields after column chromatography.
2.4 The activity of PdNPs@TiO2-Cell catalyst for Suzuki-Miyaura coupling reactions
of arenediazonium salt
There are two main problems associated with use of aryl halides in many Pd
catalyzed cross-coupling reactions. Firstly, most of aryl halides are insoluble in water hence
it is very difficult to carry out reactions in aqueous medium. Secondly aryl halides undergo
homocoupling38 many times hence separation and purification of products become tedious.
One of the best ways to overcome these drawbacks is the use of diazonium salts as
substitute for aryl halides as they showed excellent reactivity towards many cross coupling
reactions39. The diazonium salts are generally crystalline solids and show high solubility in
water. They can be easily prepared from aromatic amines by diazotization in water40.
Furthermore arenediazonium salts undergo easy oxidative addition41 during the
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mechanistic cycle to generate active cationic aryl-Pd(II) intermediate. In addition the
ligand-free palladium-catalyzed coupling reactions of diazonium salts do not undergo N2
elimination as observed in triphenylphosphine assisted cross-coupling reactions42. In this
connection we used arenediazonium salts for ligand-free Suzuki-Miyaura reactions using 1
mol % catalyst, K2CO3 as a base in water at room temperature (Table 5). Regardless of
electron-rich or electron-poor substituents present on the phenyl rings, all the substrates
could react smoothly to give the desired cross-coupling products with good to excellent
yields (84-90 %) in 2 to 5 h.
Table 5.The Suzuki-Miyaura coupling reactions of various arenediazonium salts with
arylboronic acidsa
+
-
B(OH)2
N2 BF4
PdNPs@TiO2-Cell
H2O, K2CO3, r. t.
+
FG
FG
FG
FG
Entry Arenediazoniu
Aryl boronic
acid
Product
Time
(h)
2
Yieldb
(%)
88
m salt
-
N+BF4
B(OH)2
1
-
-
N+BF4
B(OH)2
B(OH)2
2
3
3
90
86
Cl
Cl
Cl
Cl
N+BF4
3.5
S
S
-
-
N+BF4
B(OH)2
4
5
2.5
3
90
85
O2N
O2N
O2N
O2N
N+BF4
B(OH)2
S
S
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-
-
N+BF4
N+BF4
6
7
5
84
87
O
O2N
B(OH)2
S
O
S
O2N
O2N
B(OH)2
4
O2N
-
N+BF4
8
9
B(OH)2
B(OH)2
2
90
89
MeO
MeO
MeO
N+BF4
-
3.5
MeO
-
N+BF4
B(OH)2
10
2.5
90
aReaction conditions: arenediazonium salts (1 mmol), aryl boronic acids (1.1 mmol), catalyst (1 mol %),
K2CO3 (1.0 mmol), water (5 mL) at room temperature. bIsolated yields after column chromatography.
2.5 The activity of PdNPs@TiO2-Cell catalyst for Mizoroki-Heck and Heck-Matsuda
coupling reactions:
Encouraged by above satisfying results of Suzuki-Miyaura reaction, we studied the
applicability of this catalytic system for Mizoroki-Heck and Heck-Matsuda cross-coupling
reactions. Since, 1968 the Mizoroki-Heck coupling reaction43 has been emerged as excellent
catalytic tool for the formation of carbon–carbon bonds, since it has enormous efficiency to
tolerate different functional groups44. We explored this catalytic system for Mizoroki-Heck
coupling reaction of various aryl bromides with different olefins in DMF using K2CO3 as a
base at 100 °C. The results shown in Table 6 indicated that all substrates could smoothly
react with different olefins in good to excellent yields (80-95%) in 3.5 to 9 h.
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Table 6. The Mizoroki-Heck coupling reactions of various aryl bromides with olefinsa
Br
PdNPs@TiO2-Cell
DMF, K2CO3, 100 oC
+
FG
FG
FG
FG
Time
Entry
1
Aryl
Yieldb
(%)
Olefin
Product
Halide
(h)
OMe
Br
4
88
95
O
O
COOMe
OBu
2
Br
3.5
O2N
COOBu
NO2
Br
OBu
OMe
3
4
6
5
89
90
MeO
O
O
COOBu
OMe
Br
COOMe
OBu
OMe
5
6
Br
6
91
85
O
O
COOBu
COOMe
Br
F
4.5
F
F
F
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OBu
7
8
Br
F
5
7
82
86
F
F
O
F
COOBu
Br
O
O
OBu
OBu
7
8
9
80
83
Br
O
O
O
COOBu
O
O
O
10
Br
COOBu
aReaction conditions: aryl bromides (1.0 mmol), olefins (1.1 mmol), catalyst (1 mol %), K2CO3 (1.0 mmol),
DMF (5 mL) at 100 °C. bIsolated yields after column chromatography.
It has been known that Pd-catalyzed coupling reaction of arenediazonium salts by
Heck-Matsuda reaction can easily carried out in water under base-free conditions45. The
first use of arenediazonium salt for Mizoroki-Heck reaction was reported by Matsuda46 and
hence known as ‘Heck-Matsuda reaction’. This reaction has been emerged as an alternative
to aryl halides for Pd catalyzed synthesis of olefinic compounds. Considering the possible
use of water as universal solvent and excellent reactivity of diazonium salts as arylating
agent herein we report use of PdNPs@TiO2-Cell catalyst for base-free Heck-Matsuda
reaction in water at room temperature. The results shown in Table 7 clearly demonstrates
the utility of this system in Heck-Matsuda reaction in which different arenediazonium salts
were coupled with varies olefin acceptors under specified reaction conditions without
base.
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Table 7. The Heck-Matsuda coupling reactions of various arenediazonium salts with
olefinsa
PdNPs@TiO2-Cell
+
FG
FG
FG
+
-
Water, r. t.
N2 BF4
FG
Entry Arenediazonium
Olefins
Product
Time Yieldb
salts
(h)
(%)
1
-
N+BF4
O2N
2
89
OMe
O
O2N
COOMe
-
N+BF4
2
3
4
1
2.5
3
90
85
92
OMe
OMe
OBu
O
O
O
COOMe
COOMe
COOBu
N+BF4
-
N+BF4
-
O2N
O2N
Cl
N+BF4
-
5
Cl
3.5
90
OBu
O
COOBu
COOBu
COOBu
N+BF4
-
6
7
2
4
88
90
OBu
OBu
O
O
N+BF4
-
MeO
O2N
MeO
O2N
N+BF4
-
8
5
80
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-
N+BF4
9
6
6
78
75
N+BF4
-
10
MeO
MeO
aReaction conditions: arenediazonium salts (1 mmol), olefins (1.1 mmol), catalyst (1 mol %), water (5 mL) at
room temperature. bIsolated yields after column chromatography.
2.6 The activity of PdNPs@TiO2-Cell catalyst for carbonylative Suzuki-Miyaura
coupling reaction:
Carbonyl compounds including prochiral ketones are of considerable interest due to
their existence in a wide range of biologically active molecules47. It also acts as a promising
synthetic fragment for the construction of structural core of many pharmacologically active
heterocycles48. Palladium-catalyzed carbonylative cross-coupling of benzoyl chloride with
aryl boronic acid is opened as alternative way to the classical Friedel–Crafts reaction49 to
synthesize carbonyl compounds. Thus using above catalyst range of corresponding ketone
derivatives were prepared from the different benzoyl chlorides in good to excellent yields
using 1 mol % catalyst, K2CO3 as a base in 95 % EtOH at room temperature (Table 8).
Table 8. The Suzuki-Miyaura coupling reactions of various benzoyl chlorides with
arylboronic acidsa
O
B(OH)2
O
PdNPs@TiO2-Cell
Cl
+
FG
FG
FG
FG
95 % EtOH, K2CO3
r.t.
Entry
1
Benzoyl
chlorides
O
Aryl boronic
Product
Time
(h)
1
Yieldb
(%)
92
acids
B(OH)2
O
Cl
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B(OH)2
O
2
3
2
3
88
O
Cl
O
O
91
87
Cl
Cl
B(OH)2
O2N
NO2
NO2
O
O
B(OH)2
4
5
4
O
O2N
B(OH)2
O
2.5
3.5
90
85
Cl
O
O
B(OH)2
O
6
O
Cl
O
O
7
8
B(OH)2
B(OH)2
1.5
2.5
91
90
Cl
S
S
S
O
O
Cl
O2N
S
O2N
aReaction conditions: benzoyl chlorides (1 mmol), arylboronic acids (1.1 mmol), catalyst (1 mol %), K2CO3
(1 mmol), 95 % EtOH (5 mL) at room temperature. bIsolated yields after column chromatography.
2.7. The activity of PdNPs@TiO2-Cell catalyst for Sonogashira coupling reaction
Compounds containing acetylene motifs are prevalent in many important natural
products and have been prepared by Sonogashira reaction. Such compounds have several
important biological as well as pharmaceutical properties50. Besides these compounds,
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oligomers and polymers of acetylenes have also been prepared via the Sonogashira
reaction. Considering the importance of this reaction in synthesis of such important
compounds51, we next attempted to perform the Cu-free Sonogashira coupling52 of various
aryl bromides with terminal alkynes using 1 mol % catalyst, K2CO3 as a base in DMF at 80
°C. Thus we are succeeded to carry out the reactions of terminal acetylenes with number of
aryl bromides under Cu-free conditions (Table 9). The results showed that the nature of the
substituents did not exert significant effect on the yield and reaction time. The reactions
were rapid giving good to excellent yields (85-95%) of the corresponding products in 1 to
7h.
Table 9.Cu-free Sonogashira coupling reactions of aryl bromides with terminal alkynesa
PdNPs@TiO2-Cell
Br
+
FG
FG
K2CO3, DMF, 80 0C
FG
FG
Time Yieldb
Entry Aryl bromides
Product
Phenyl acetylenes
(h)
(%)
Br
1
1
85
O
O
2
4
94
Br
Br
O2N
3
4
3
6
95
90
NO2
Br
O2N
NO2
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CHO
CHO
5
5
88
Br
N
6
7
6
2
85
94
Br
N
Br
Br
NC
H2N
O
CN
Br
8
9
2
6
92
90
NH2
Br
O
O
Br
O
10
7
89
aReaction conditions: aryl bromides (1.0 mmol), acetylenes (1.1 mmol), catalyst (1 mol %), K2CO3 (1 mmol),
DMF (5 ml), at 80 °C. bIsolated yields after column chromatography.
2.8. Leaching and Heterogeneity
To ascertain the heterogeneous nature of the catalyst we performed hot filtration
and leaching test. This study was carried out by coupling 4-bromobenzophenone and
phenyl boronic acid under optimized reaction condition of Suzuki-Miyaura reaction. After
30% conversion of the aryl halide (analyzed by HPLC), the catalyst was separated at the
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reaction temperature and the reaction mixture was continued to stirred at that
temperature for another 2 h. There was no further increase in the product concentration,
indicated heterogeneous nature of catalyst. This was further supported by Pd leaching test.
Possible Pd leaching during the reaction was tested by analyzing Pd concentration in the
reaction mixture by ICP-AES. It was observed that only 3 ppm of Pd was detected. The
activity of the leached Pd was found to be negligible by carrying out an experiment in 3
ppm of Pd in the form of Pd(OAc)2 as catalyst and P(Ph3)3 ligand.
2.9. Recyclability of catalyst
The feasibility of recycling the catalyst was also examined for coupling 4-
bromobenzophenone with phenylboronic acid. After completion of reaction, the supported
catalyst was separated by centrifugation washed with ethyl acetate, water, dried under
vacuum and used directly for the next cycle without further treatment. The recycling
results shown in Table 10 indicated that the catalyst was recycled for at least four times
without decrease in product yield. TEM analysis of reused catalysts (Fig. 2b) showed
particle size of PdNPs are 45 nm this indicated that TiO2-Cell composite avoids
agglomeration of PdNPs.
Table 10.Recyclability of catalyst in Suzuki–Miyaura coupling reaction
Cycle
Yield (%)
Fresh catalyst
1st cycle
2ed cycle
3rd cycle
4thcycle
90
90
89
89
89
3. Conclusions
In summary the Pd nanoparticles dispersed on the TiO2 particles supported on
cellulose was prepared by a simple procedure. The immobilization of Pd and TiO2 on
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cellulose was confirmed by FE-SEM and EDS as well as XRD analyses. Furthermore the
formation of PdNPs of 39-45 nm was confirmed by TEM. This methodology allows the
synthesis of various substituted biphenyls and acrylates from aryl halides as well
diazonium salts in environmentally benign solvents like 95% ethanol and water at low
temperature in good to excellent yields. We have also demonstrated that the catalyst
showed good activity for Sonogashira coupling of aryl bromides in DMF under Cu-free
conditions. In almost all conversions 1 equivalent K2CO3 was required. Also, efficient cross-
coupling could be accomplished with benzoyl chlorides for the synthesis of ketones at
room temperature. The use of this catalytic system has many advantages such as easy
separation, recyclability without decrease in yield, and the reaction does not require an
inert atmosphere.
. Supporting Information.
1H and 13C NMR data of representative compounds.
Author information
Corresponding authors
Department of Chemistry, Padmabhushan Dr. Vasantraodada Patil College, Tasgaon,
416312, M.S., India.
. E-mail: arjun2win@yahoo.co.in. Tel. +91 2346 250575; Fax: +91 2346 250665.
Acknowledgment
This work was supported by the Science and Engineering Research Board,
Department of Science and Technology (SERB-DST), Government of India, New Delhi,
under the scheme of Start-Up research grants for Young Scientists (SB/FT/CS-153/2013).
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Palladium nanoparticles supported on titanium dioxide cellulose composite (PdNPs@TiO2-
Cell) for ligand-free carbon-carbon cross coupling reactions
Sanjay Jadhava, Ashutosh Jagdaleb, Santosh Kamblec, Arjun Kumbharb*, Rajshri Salunkhea*
aDepartment of Chemistry, Shivaji University, Kolhapur, 416004, M.S., India
bDepartment of Chemistry, Padmabhushan Dr. Vasantraodada Patil College, Tasgaon, (Sangli)
Maharashtra, 416312, India
cDepartment of Chemistry, Yashvantrao Chavan Institute of Science, Satara, Maharashtra,
415001, India
E-mail: arjun2win@yahoo.co.in
Graphical Abstract