Ligand-free C-C and C-N cross-couplings with Pd/Nf-G nanocomposite

Article abstract of DOI:10.1016/j.tetlet.2015.05.091

Abstract The catalytic activity of electrochemical deposited Pd nanoparticles on nafion-graphene support was examined for Buchwald-Hartwig amination reaction and Heck coupling reaction. The developed protocol is very efficient and ecofriendly, providing excellent product yield. The Pd/Nf-G catalyst can be used up to four cycles with slight decrease in catalytic activity.

Full text of DOI:10.1016/j.tetlet.2015.05.091

Accepted Manuscript
Ligand-free C-C and C-N cross-couplings with Pd /Nf-G nanocomposite
Radheshyam S. Shelkar, Suresh S. Shendage, Jayashree M. Nagarkar
PII:
S0040-4039(15)00924-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.05.091
TETL 46363
Reference:
Tetrahedron Letters
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Revised Date:
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22 May 2015
23 May 2015
Please cite this article as: Shelkar, R.S., Shendage, S.S., Nagarkar, J.M., Ligand-free C-C and C-N cross-couplings
with Pd /Nf-G nanocomposite, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.05.091
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Tetrahedron Letters
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Ligand-free C-C and C-N cross-couplings
with Pd/Nf-G nanocomposite
Radheshyam S Shelkara, Suresh S Shendageb* and Jayashree M Nagarkara
∗ Corresponding author. Tel.: +91-22-2163-1421/1423; fax: +91-22-2163-4262; e-mail: sureshsshendage@gmail.com

Tetrahedron Letters
1
Tetrahedron Letters
journal homepage: www.elsevier.com
Ligand-free C-C and C-N cross-couplings with Pd /Nf-G nanocomposite
Radheshyam S Shelkara Suresh S Shendageb* and Jayashree M Nagarkara*
aDepartment of Chemistry, Institute of Chemical Technology (Deemed University), Nathalal Parekh Marg, Matunga(E), Mumbai - 400 019, India
b
*KET’S Vinayak Ganesh Vaze College (“College with Potential for Excellence”), Mithagar Road, Mulund (E) ,Mumbai-400081, India
ARTICLE INFO
ABSTRACT
Article history:
Received
The catalytic activity of electrochemical deposited Pd nanoparticles on nafion-graphene support
was examined for Buchwald-Hartwig amination reaction and Heck coupling reaction. The
developed protocol is very efficient and ecofriendly, providing excellent product yield. The
Pd/Nf-G catalyst can be used up to four cycles with slight decrease in catalytic activity.
Received in revised form
Accepted
Available online
Keywords:
Heterogeneous catalysis
Electrochemical synthesis
Palladium nanoparticles
Graphene
Heck reaction
Buchwld-Hartwing aminaition
Palladium catalyzed reactions are important in synthetic
best of our knowledge, there is no report on application of
electrochemically prepared Pd/Nf-G catalyst for Buchwald–
Hartwig amination and Heck reactions.
organic chemistry, especially palladium catalyzed coupling and
amination reactions1-4. The C-C and C-N bond forming palladium
catalyzed reactions are one of the best alternatives for synthesis
natural products, pharmaceutically and chemically applicable
products5-8. These coupling reactions were carried out by various
homogeneous catalysts with an excellent activity9-14. However,
from industrial point of view, homogeneous systems are not
suitable because of issues related to recovery and reusability. To
address these issues, solid supported metal nano particles
(heterogeneous catalyst) are the best alternative for homogeneous
systems. The variety of heterogeneous solid supported palladium
catalysts were reported to accomplish these coupling reactions15-
18. Palladium is special metal due to its high catalytic activity at
the nanoscale. Now days, Pd-based supported catalysts have been
prepared and used as the electrocatalyst for electrochemical
oxidation of alcohols19, 20. In addition to this it finds applications
in carbon-carbon bond forming coupling reactions21, 22. Although
in case of some heterogeneous catalysts, there is difficulty in
reducing surface area with agglomeration of nano particles,
mechanical and thermal stability of catalyst, use of capping
agent, long and non-ecofriendly route of preparation, which have
an effect on catalyst activity and environment. In order to
overcome these issues, herein we explore catalytic activity of
Pd/Nf-G in continuation with our previous report23 for C-C and
C-N coupling reactions as heterogeneous catalyst. It is easiest to
control the size and morphology of nano particles by
electrochemical method. Electrochemical deposition of metal
nano particles is one of the most useful approaches to prepare
supported metal nano particles. Furthermore, compared with the
chemical reduction methods, no capping reagent is used in the
electro deposition method. Consequently, the synthetic
procedures can be simplified to a greater extent. Graphene is used
as catalyst support due to its good mechanical strength and
thermal stability, high conductivity and enormous surface area.
Moreover, graphene based composite with different metals like
Pd, Pt and Au has been used as electro catalyst for the different
kinds of oxidation and reduction reactions24- 26. However, to the
As synthesized nano Pd/Nf-G catalyst has been
extensively screened for its catalytic activity against the C-C and
C-N bond forming reactions. Initially, the Pd/Nf-G has been
employed to catalyzed Buchwald Hartwig amination reactions
The model Buchwald Hartwig amination reaction was selected
by taking a mixture of iodobenzene (1mmol) and morpholine
(1mmol) in DMSO using tBuONa as a base at 110 °C for 12h
using Pd/Nf-G as a catalyst (Scheme 1).
Scheme1. Buchwald-Hartwig amination reaction catalyzed by
Pd/Nf-G.
To fix the various reaction parameters for this model reaction we
have carried out a broad optimization study. Initially, variety of
solvents has been screened out such as DMSO, DMF, NMP,
dioxane and toluene. It was found that DMSO was the best
solvent for the reaction (Table 1, entries 1-5). We have also
carried out reaction in mixture solvents with water the product
yield was very low (Less than 10%) after 24 hrs. In selection of
appropriate base for this reaction, base study also carried out and
the obtained results showed that out of K2CO3, K3PO4, NEt3 and
tBuONa, tBuONa gave excellent yield of the product (Table1,

Tetrahedron Letters
2
entries 1, 6-8). Temperature effect was found significantly; 110
substituents on aromatic ring of aryl halides or by changing
°C was the required temperature for this transformation to
obtained desire product in high percent (Table 1, entries 1, 9, 10).
Then Pd loading was monitored get maximum yield of the
product. Pd loading, 0.2 mol% was found to be sufficient for
higher yield (Table 1, entries 1, 11, 12). Conversion of the
iodobenzene into product is 98 % with above reaction conditions.
In this reaction no homocoupling product was observed. The
progress of reaction has been monitored by GC and the graph of
GC yield versus time showed that after 12h, the yield of desire
product was high (Fig. 1).
Table 2. Buchwald-Hartwig amination reactions of aryl halides with
secondary amines by using Pd/Nf-Ga.
Entry
Aryl
Secondary
amines
Products
Yield
(%)b
halides
I
1.
93
95
84
86
89
87
84
77
75
69
94
92
89
95
91
2.
3.
4.
5.
6.
Fig. 1 The plot of GC yield (%) versus time (h)
7.
Table 1. Optimization of reaction condition for Buchwald-Hartwig
amination reaction of iodobenzene and morpholinea
8.
Entry Solvent
Base
Catalyst Temperature Yield
loading
tBuONa 0.2
mol%
tBuONa 0.2
mol%
tBuONa 0.2
mol%
Dioxane tBuONa 0.2
mol%
Toluene tBuONa 0.2
mol%
0.2
(%)b
9.
1.
2.
DMSO
DMF
110 °C
110 °C
110 °C
110 °C
110 °C
110 °C
110 °C
110 °C
80 C
97
N
S
I
I
93
10.
11.
12.
13.
14.
15.
3.
NMP
90
I
4.
79
I
5.
84
6.
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
K2CO3
K3PO4
Et3N
68
I
I
mol%
0.2
7.
76
mol%
0.2
8.
64
mol%
9.
tBuONa 0.2
75
I
mol%
tBuONa 0.2
mol%
tBuONa 0.1
mol%
tBuONa 0.3
mol%
Iodobenzene(1mmol),
10.
11.
12.
aReaction
60 °C
51
110 °C
110 °C
73
97
aReaction conditions: Aryl halides (1 mmol), Secondary amines (1 mmol),
tBuONa (1 mmol), DMSO (1 mL) at 110 °C for 12h. 0.7 mg of Pd/Nf-G (0.2
mol% or 0.21 mg of Pd) bIsolated yield.
conditions:
Morpholine(1mmol),
variety of secondary amines and the results are summarized in
Table 2. Initially the catalyst efficiency was evaluated for various
substituted iodobenzene and bromobenzene with morpholine and
moderate to excellent yield of products were obtained. The
Base(1mmol), Solvent(1mL) for 12h. bGC yield.
To investigate the catalytic activity of nano sized Pd/Nf-G, we
have showed wide substrate study by using different
a

Tetrahedron Letters
3
aReaction conditions: Iodobenzene (1mmol), Ethylacrylate (2mmol), Base
catalyst afforded good yield of product for both electron-
withdrawing substituents (-NO2, -COCH3 and -CN) and electron-
donating substituents (-OMe, -CH3 and -F) (Table 2, entries 1-8).
However, iodobenzene exhibit greater activity than
bromobenzene.
(2mmol), Solvent (1mL) for 2h. bGC yield. cYield after 24h
Table 4. Heck cross coupling reactions of aryl halides and olefins
by using Pd/Nf-Ga.
In addition to Buchwald-Hartwig amination reaction, we have also
studied catalytic application of nano Pd/Nf-G for Heck cross
coupling reactions. The Heck reaction of iodobenzene (1mmol) and
ethyl acrylate (2mmol), NEt3 as a base in DMF at 120 °C for 2h in
presence of Pd/Nf-G catalyst was found to be best model reaction
after employing various optimized reaction parameters (Scheme 2).
En Aryl
try halide
s
Alkenes
Tim Yield
Products
e (h)
(%)b
90
83
93
81
84
86
79
83
91
82
94
89
92
I
O
1.
2.
3.
4.
5.
6.
7.
2
O
O
2.5
2
O
Scheme 2. Heck cross coupling reaction catalyzed by Pd/Nf-G
O
To confirm various parameters we have screened
various solvents, reaction in DMF gave higher yield of the
product (Table 3, entries 1-4). We have tried reaction in mixture
of solvents with water the product yield was very low after 24
hrs. Base study showed that NEt3 was suitable base to increase
catalyst performance (Table 3, entries 1, 5-7). Variation in
temperature also affects the product yield, temp. 120 °C gave
higher yield of product (Table 3, entries 1, 8-11). To accomplish
this reaction to larger extent, Pd loading has been examined and
it showed that 0.3 mol% was enough to achieve maximum yield
of the product (Table 3, entries 1, 12, 13). Conversion of
iodobenzene into product was 100%. This indicates excellent
selectivity towards the reaction product. The graph shown in Fig.
2 is based on GC yield which explain the progress of reaction
with increasing time.
O
O
O
O
3
O
O
O
3
3
O
3
O
O
Table 3. Optimization of reaction conditions for Heck cross coupling
reaction of iodobenzene and ethylacrylatea.
8.
5
O
I
S
Entry Solvent Base
Catalyst
loading
0.3
Temperature Yield
(%)a
I
9.
2
1.
DMF
NMP
Et3N
Et3N
120 °C
120 °C
120 °C
120 °C
120 °C
120 °C
120 °C
100 °C
80 °C
95
87
69
88
75
64
51
81
60
38
<10c
73
95
mol%
0.3
10.
11.
12.
13.
3
2.
mol%
0.3
3.
Toluene Et3N
Et3N
3.5
2.5
3
mol%
0.3
4.
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
mol%
O
I
5.
K2CO3 0.3
mol%
K3PO4 0.3
mol%
NaOH 0.3
mol%
0.3
O
6.
I
7.
8.
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
14.
15.
4
88
mol%
0.3
9.
mol%
0.3
10.
11.
12.
13.
60 °C
4
90
mol%
0.3
aReaction conditions: Aryl halides (1mmol), Olefins (2mmol), Et3N (2mmol),
DMF (1mL) at 120 °C. 1mg of Pd/Nf-G (0.3 mol% or 0.32 mg of Pd)
bIsolated yield.
RT
mol%
0.2
120 °C
120 °C
mol%
0.4
mol%
∗ Corresponding author. Tel.: +91-22-2163-1421/1423; fax: +91-22-2163-4262; e-mail: sureshsshendage@gmail.com

Tetrahedron Letters
4
In conclusion we have explored catalytic activity of Pd/Nf-G
composite as catalyst for Buchwald-Hartwig amination and Heck
coupling reactions. The catalyst showed excellent activity against
these C-N and C-C bond forming reactions with simple
methodology. The stability of catalyst has been confirmed by
recyclability. The Pd/Nf-G catalyst can be used up to four cycles
with slight decrease in catalytic activity.
Acknowledgement
We gratefully acknowledge SAIF IIT, Bombay for the
TEM and ICP-AES analysis.
Fig. 2 The plot of GC yield (%) versus time (h) for scheme 2.
References
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Fig. 3 The plot of GC yield (%) versus Run
∗ Corresponding author. Tel.: +91-22-2163-1421/1423; fax: +91-22-2163-4262; e-mail: sureshsshendage@gmail.com