Magnetically recyclable palladium nanoparticles (Fe3O4-Pd) for oxidative coupling between amides and olefins at room temperature

Article abstract of DOI:10.1002/aoc.4985

A convenient method for the synthesis of magnetically recyclable palladium nanoparticles (Fe₃O₄-Pd) is described. The catalytic application of the Fe₃O₄-Pd nanoparticles was explored for the first time in oxidat

Full text of DOI:10.1002/aoc.4985

Received: 28 December 2018
DOI: 10.1002/aoc.4985
Revised: 10 April 2019
Accepted: 15 April 2019
C O M M U N I C A T I O N
Magnetically recyclable palladium nanoparticles (Fe3O4‐Pd)
for oxidative coupling between amides and olefins at room
temperature
Chandan Kumar Pal1 | Swagatika Sahu1,2 | Ranjan Kumar Sahu3 | Rajesh Kumar Singh1 |
Ashis Kumar Jena1
1 Organic Synthesis and Nanocatalysis
A convenient method for the synthesis of magnetically recyclable palladium
nanoparticles (Fe3O4‐Pd) is described. The catalytic application of the Fe3O4‐
Pd nanoparticles was explored for the first time in oxidative coupling between
amides and olefins. p‐Toluenesulfonic acid plays a significant role in the oxida-
tive amidation reaction. The reaction proceeds at room temperature, resulting
in (Z)‐enamides under ambient air in the absence of co‐catalyst and ligand.
The superparamagnetic nature of Fe3O4‐Pd facilitates easy, quantitative recov-
ery of the catalyst from a reaction mixture, and it can be reused for up to three
consecutive cycles with a slight decrease in catalytic activity.
Laboratory, Department of Chemistry,
North Orissa University, Baripada 757003
Odisha, India
2 Betnoti College, Betnoti 757025 Odisha,
India
3 National Metallurgical Laboratory,
Jamshedpur 831007, India
Correspondence
Ashis Kumar Jena, Organic Synthesis and
Nanocatalysis Laboratory, Department of
Chemistry, North Orissa University,
Baripada 757003, Odisha, India.
Email: jenaashis016@gmail.com
KEYWORDS
Fe3O4‐Pd nanoparticles, leaching redeposition, oxidative coupling, recyclable, (Z)‐enamides
1 | INTRODUCTION
catalyst to the nanometre scale enhances the reactivity
of the catalyst. Therefore, the use of magnetically recycla-
Over the past few years, transition‐metal‐catalysed oxida-
tive coupling reactions have received much attention due
to their being atom‐ and step‐economic approaches for
the preparation of complex molecules.[1] In oxidative cou-
pling reactions, two C―H and heteroatom―H bonds are
directly coupled to form C―CC heteroatom bonds in the
presence of oxidant and transition metal catalyst. Among
the various transition metals employed in oxidative cou-
pling, palladium has received considerable attention due
to its excellent reactivity, high level of regio‐, chemo‐
and stereoselectivity, as well as good functional group tol-
erance.[2] However, an expensive palladium catalyst
needs to be recovered from the reaction mixture and also
the product should be uncontaminated by metal species
since in the pharmaceutical industry it is essential to
remove all trace metal impurities.[3] Magnetization of a
metal catalyst, where the catalyst can be quantitatively
isolated using an external magnet, is an ideal solution to
this problem.[4] Moreover, decreasing the size of a metal
ble palladium nanoparticles in oxidative coupling reac-
tions represents an efficient and eco‐friendly method.
Enamide derivatives fall into a significant class of
nitrogen‐containing compounds and are versatile inter-
mediates for the synthesis of bioactive molecules, hetero-
cycles, amino acids and chiral amines.[5] The enamide
moiety also appears in a plethora of natural products
such as altamide, lansiumamide A, lansiumamide B,
salicyhalamide A, crocacin A, etc. (Figure 1).[6]
Considering the importance of the enamide scaffold,
several effective methods have been developed for its syn-
thesis.[7–12] Among them, transition‐metal‐catalysed oxi-
dative coupling between amides and olefins would be a
straight forward approach. Murahashi and co‐workers
reported the coupling of cyclic carbamates and amides
with electron‐deficient olefins using PdCl2(MeCN)2 as
catalyst and cuprous chloride as co‐catalyst.[13] Later,
Chang and co‐workers developed a Pd/Cu co‐catalysed
method for the selective synthesis of (Z)‐enamides from
Appl Organometal Chem. 2019;e4985.
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https://doi.org/10.1002/aoc.4985

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PAL ET AL.
FIGURE 1 Enamide‐containing natural
products
acyclic amides.[6] Liu and Hii reported a chloride‐free
Pd/Cu co‐catalysed system for the oxidative coupling of
electron‐rich amides with olefins.[14] Subsequently, Panda
et al. reported the stereoselective synthesis of (Z)‐
enamides by coupling amides with electron‐deficient ole-
fins employing 10 mol% palladium acetate as catalyst.[15]
However, use of homogeneous palladium catalysts in
these enamide syntheses is associated with a number of
disadvantages such as palladium metal contamination
in the final product, use of co‐catalyst as well as non‐
recoverability of the catalyst. These difficulties could be
overcome by employing a heterogeneous palladium
nanocatalyst which has its own merits of easy recyclabil-
ity and avoiding palladium contamination in products. To
the best of our knowledge, compared to homogeneous
counterparts, heterogeneous palladium catalysts in
enamide synthesis have not been reported yet. In this
context, we have synthesized palladium‐decorated Fe3O4
nanoparticles (Fe3O4‐Pd)[16] and employed them for the
first time in oxidative coupling between amides and ole-
fins. To our delight, the reaction resulted in thermody-
namically disfavoured (Z)‐enamides stereoselectively at
room temperature in the absence of a co‐catalyst.
FIGURE 2 XRD patterns of Fe3O4 and Fe3O4‐Pd
(JCPDS no. 82‐1533). No other impurity peaks were
detected. For Fe3O4‐Pd, all the peaks correspond to cubic
spinel Fe3O4. The absence of reflection peaks correspond-
ing to Pd metal deposited on Fe3O4 was probably due to
very low content of Pd or a low level of crystallization.
This observation regarding crystal structure is consistent
with that for Fe3O4‐CeO nanocatalyst prepared by
Gawande et al.[17]
The loading of Pd on the nanomaterial was deter-
mined using atomic absorption spectroscopy (AAS) as
2.4 wt%, corresponding to the expected value. The mor-
phology and size distribution of Fe3O4‐Pd were obtained
from FESEM and TEM analyses. The FESEM image
(Figure 3a) clearly reveals that the particles are spherical
and have a narrow size distribution of 30–45 nm. EDS
analysis confirms the presence of Fe and Pd in Fe3O4‐Pd
(Figure 3c). The TEM image of Fe3O4‐Pd (Figure 3b)
shows the formation of magnetic nanoparticles having
somewhat spherical morphology with a size range of
20–28 nm. It is observed that particles are slightly
agglomerated. The agglomeration observed using TEM
might occur during TEM sample preparation and drying
effect in addition to magnetic interaction under a high‐
energy electron beam. The appearance of dark dot‐like
2 | RESULTS AND DISCUSSION
The magnetic Fe3O4‐Pd was synthesized using a two‐step
process. Fe3O4 nanoparticles were synthesized by a chem-
ical co‐precipitation method. In the second step palla-
dium metal was deposited on the surface of Fe3O4 by a
wet impregnation method. The synthesized Fe3O4‐Pd
was characterized using several techniques such as pow-
der X‐ray diffraction (XRD), field emission scanning elec-
tron microscopy (FESEM), transmission electron
microscopy (TEM), energy‐dispersive X‐ray spectroscopy
(EDS), X‐ray photoelectron spectroscopy (XPS) and
vibrating sample magnetometry (VSM).
The crystal structure and the phase purity of Fe3O4
and Fe3O4‐Pd were determined using powder XRD
(Figure 2). The diffraction peaks of Fe3O4 can be easily
indexed to a crystalline cubic spinel structure of Fe3O4

PAL ET AL.
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FIGURE 3 (a) FESEM image of Fe3O4‐Pd at 500 nm. (b) TEM image of Fe3O4‐Pd at 20 nm. (c) EDS profile of Fe3O4‐Pd
structures on the surface of Fe3O4 nanoparticles clearly
indicates the deposition of PdO.
and +9000 Oe at 300 K (Figure 5a). The saturation mag-
netization (Ms) value for Fe3O4‐Pd was found to be
45 emu g−1. Fe3O4‐Pd shows a superparamagnetic nature.
The superparamagnetic nature and high Ms make Fe3O4‐
Pd an efficient and reusable catalyst for oxidative cou-
pling of amides with olefins. The complete magnetic sep-
aration of the Fe3O4‐Pd nanocatalyst using an external
magnetic field (Figure 5b) demonstrates the strong mag-
netic sensitivity of the catalyst.
To further examine the presence of Pd on the surface of
Fe3O4, XPS measurements were carried out. In a typical
survey spectrum (Figure 4a) of the nanoparticles, the
peaks at 57, 330–345, 530–538 and 705–730 eV correspond
to the binding energy of Fe 3p, Pd 3d, O 1s and Fe 2p
regions, respectively. The binding energies at 710 and
723 eV corresponding to Fe 2p3/2 and Fe 2p1/2 are consis-
tent with the Fe 2p binding energy of magnetite nanopar-
ticles (Figure 4b). The O 1s spectrum (Figure 4d) can be
deconvoluted into two peaks at 528.8 and 530.9 eV, which
correspond to M―O and O―H respectively. The high‐
resolution spectrum of Pd (Figure 4c) clearly indicates
the presence of Pd 3d5/2 (336.1 eV) and Pd 3d3/2
(341.3 eV), corresponding to Pd in oxidation state of +2.
The +2 oxidation state of Pd indicates the presence of
both PdO and Pd(OH)2, which may be formed during
the hydrolysis of PdCl2 under basic conditions.[16]
The nitrogen adsorption/desorption isotherm of the
synthesized Fe3O4‐Pd is of type IV with H3 hysteresis
(Figure 6a). This indicates the formation of aggregates
of plate‐like particles giving rise to slit‐shaped pores.
The Brunauer–Emmett–Teller surface area was found to
be 91.144 m2
g
−1, which is much higher than that
reported in the literature.[16] The high surface area
favours catalytic activity of the synthesized nanoparticles.
Furthermore, the corresponding Barrett–Joyner–Halenda
analyses indicate that the average pore size was 3.95 nm.
Most of the pores ranged from 3 to 22 nm (Figure 6b).
To investigate the catalytic efficiency of Fe3O4‐Pd for
enamide synthesis, we commenced our investigation
To examine the magnetic properties of the synthesized
Fe3O4‐Pd, magnetization curves were measured using
VSM with the magnetic field cycling between −9000

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PAL ET AL.
FIGURE 4 XPS survey spectrum (a) and high‐resolution scans of Fe3O4‐Pd corresponding to Fe 2p (b), Pd 3d (c) and O 1s (d)
summarized in Table 1. The reaction of 1a (0.826 mmol)
with 2a (2.478 mmol) was carried out in the presence of
Fe3O4‐Pd (30 mg, 2.3 wt% Pd), p‐toluenesulfonic acid
(PTSA) (1.652 mmol) and tert‐butyl hydroperoxide (TBHP)
(1.156 mmol) as oxidant under ambient air at room tem-
perature for 14 h using toluene as the solvent, resulting
in 3a in 72% yield (Table 1, entry 1). The formation of
(Z)‐enamide was proved from the 1H NMR spectrum
(Supporting information). The appearance of doublet at
11.49 ppm (for N―H) and 5.28 ppm (vinylic proton) with
coupling constant of 8.8 Hz confirmed the synthesis of (Z)‐
enamide.[6] We investigated the optimization reaction with
both polar and non‐polar solvents. Polar solvents such as
dimethylformamide (DMF), dimethylsulfoxide (DMSO)
FIGURE 5 (a) Room temperature magnetic hysteresis curve of
Fe3O4‐Pd and (b) photograph showing separation of catalyst
using benzamide (1a) and methyl acrylate (2a) as model
substrates. The results of the oxidative amidation are
FIGURE 6 (a) Nitrogen adsorption/desorption isotherm and (b) pore size distribution of Fe3O4‐Pd

PAL ET AL.
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TABLE 1 Optimization of reaction conditions
Entry
Catalyst
Solvent
Oxidant
Additive
Yield (%)a
1
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
—
Toluene
Dioxane
DCE
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
BQ
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
PTSA
AcOH
PhCOOH
PTSA
PTSA
—
72
2
35
3
15
4
DMF
< 5
< 5
< 5
15
5
DMSO
6
NMP
7
Hexane
Benzene
Xylene
—
8
16
9
18
10
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
n.r.
12
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Cu(OAc)2
AgOAc
DTBP
DDQ
15
n.r.
< 5
n.r.
30
K2S2O8
NaIO4
TBHP
TBHP
—
< 5
n.r.
n.r.
Trace
n.r.
n.r
10
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4‐Pd
Fe3O4
PivOH
PTSA
PTSA
PTSA
40b
50c
n.r.
aReaction conditions: benzamide (0.826 mmol), methyl acrylate (2.478 mmol), Fe3O4‐Pd (30 mg, 2.3 wt% Pd, 5.5 mol% Pd), oxidant (1.156 mmol), additive
(1.652 mmol), 4 ml of solvent, 14 h in air; n.r., no reaction.
bAdditive (0.826 mmol).
cTBHP (2.065 mmol).
and N‐methylpyrrolidone (NMP) resulted in very low yield
of the product. With non‐polar solvent, moderate to good
yields of (Z)‐enamide were obtained. In the absence of sol-
vent, the reaction failed to proceed (Table 1, entry 10).
Next, we investigated several oxidants such as benzoqui-
none (BQ), 2,3‐dichloro‐5,6‐dicyanoquinone (DDQ),
TBHP, di‐tert‐butyl perbenzoate (DTBP), sodium periodate
(NaIO4), copper acetate (Cu(OAc)2) and silver acetate
(AgOAc). Among the oxidants tested, TBHP was found
to be highly effective for promoting the reaction (Table 1,
entry 1). Also various additives such as PTSA, acetic acid
(AcOH) and pivallic acid (PivOH) were examined. The
optimization results revealed that 2.0 eqiv. of PTSA was
necessary to maintain the efficacy of the reaction
(Table 1, entry 1). Decreasing the additive concentration
to 1 equiv. decreased the yield of the reaction (Table 1,
entry 23). This is due to the strong coordination of
benzamide with metal catalyst leading to deactivation of
the catalyst. However, with excess PTSA (2.0 equiv.) the
competitive binding of amide with nanocatalyst may

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PAL ET AL.
TABLE 2 Oxidative coupling between amides and olefinsa
Entry
Substrate
Product
Time (h)
Yield (%)b
1
14
72
2
3
15
48
70
42
4
48
35
5
6
35
35
41
37
7
48
48
48
48
0
15
<5
0
8
9
10
11
48
26
(Continues)

PAL ET AL.
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TABLE 2 (Continued)
Entry
Substrate
Product
Time (h)
Yield (%)b
12
24
52
13
14
24
48
43
0
15
16
48
36
0
38
17
36
40
18
19
24
15
32
62
aReaction conditions: amide (0.826 mmol), olefin (2.478 mmol), Fe3O4‐Pd (30 mg, 2.3 wt% Pd, 5.5 mol% Pd), PTSA (1.652 mmol), TBHP (1.156 mmol) toluene (4
ml), r.t.
bIsolated yield.
decrease and hence deactivation of the nanocatalyst is
prevented, so higher yield was obtained.[18] In the
absence of the Fe3O4‐Pd nanocatalyst, no reaction pro-
ceeds, highlighting the important role of the
nanocatalyst in the coupling reaction (Table 1, entry
20). Performing the coupling reaction in the absence
of Pd with Fe3O4 failed to afford the desired enamide
(Table 1, entry 25). Increasing the catalyst loading did
not enhance the yield of the reaction, perhaps due to
possible polymerization. Furthermore, when the reac-
tion was conducted under nitrogen atmosphere, a very
low yield of the product was obtained, indicating the
crucial role played by molecular oxygen in the air.
With the optimized conditions in hand, the scope of the
reaction was investigated by coupling various amides with
olefins. The results of the reactions are summarized in
Table 2. Interestingly, moderate to good yields with high
level of (Z)‐selectivity were observed in almost all cases.
Under the standard reaction conditions, functional groups
such as chloro, nitro and methoxy were well tolerated on
the phenyl ring of amide. Reactions of p‐nitrobenzamide
with methyl acrylate and butyl acrylate resulted the corre-
sponding (Z)‐enamide in 42 and 35% yield, respectively
(Table 2, entries 3 and 4). However, with o‐nitrobenzamide,
a low yield of the product was observed (Table 2, entry 11).
This may be due to the steric hindrance by ortho substituent.

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PAL ET AL.
SCHEME 1 Oxidative coupling
between benzamide and methyl acrylate
in presence of TEMPO
TABLE 3 Reusability study of Fe3O4‐Pd nanocatalyst
Cycle
1
2
3
Yield (%)a
aIsolated yield.
72
71
53
We also examined the coupling of trifluoroacetamide with
butyl acrylate (Table 2, entry 7) and failed to observe any
expected product. It is important to mention that sterically
demanding secondary amides also reacted with methyl
acrylate and butyl acrylate, affording the product (E)‐
enamides in low yield (Table 2, entries 8 and 9). The absence
of formation of hydrogen bonding results the synthesis of
(E)‐enamide. In addition, low yield of the product may be
due to the partial deactivation of the nanocatalyst by
secondary amides. Heterocyclic amides such as furan‐2‐
carboxamide coupled with methyl acrylate and butyl acry-
late to afford the corresponding (Z)‐enamide in 52 and
43% yield (Table 2, entries 12 and 13). Also thiophene‐2‐
carboxamide reacted with butyl acrylate, affording the prod-
uct (15) in 43% yield (Table 2, entry 18). Electronically
deficient alkenes such as acrylamide and acrylonitrile were
found unsuitable for oxidative amidation reaction (Table 2,
entries 14 and 15). This may be due to the possible coordina-
tion of nitrogen atom of these olefinic substrates with the
nanocatalyst, leading to catalyst deactivation.
acrylate under the optimized conditions (Table 1, entry
1). After completion of the reaction, the catalyst was sepa-
rated magnetically, washed with water and dichlorometh-
ane (thrice), dried in a hot‐air oven at 50°C for 2 h and
reused for the next cycle. The yields of the reaction versus
the number of cycles are presented in Table 3. The catalyst
could be reused without loss of activity up to the second
cycle. In the third reuse, the yield was decreased (53%).
We were interested to investigate further as to whether
the reaction took place on the surface of the catalyst or
the active species were present in the reaction solution.
We conducted a hot filtration test using the standard condi-
tions to determine the exact nature of catalysis. The crude
reaction mixture was filtered through celite after 40 min
at 32% conversion and the reaction continued further for
14 h. The yield of the reaction was increased to 71%. AAS
analysis after 32% conversion indicated Pd in solution.[19]
Surprisingly, the Pd abundance in solution after reac-
tion does not seem to correlate with the observed results
of reusability. After the second cycle there is a small
decrease in catalytic activity. This may be due to the
adsorption of by‐products on the surface of the catalyst,
small loss of Pd metal and insufficient redeposition of
Pd on magnetite.[16] The change in Pd content in solution
indicates that soluble catalytically active species may be
formed and redeposited on the magnetite.[19] Reusability
study and hot filtration test hint at the occurrence of a
boomerang phenomenon.[20]
2.1 | Mechanism
To investigate the mechanism of the reaction, a radical
trapping reagent, 2,2,6,6‐tetramethylpiperidin‐1‐yloxy
(TEMPO), was added to the reaction in the optimized
condition (Scheme 1). No desired product (3a) was
formed, indicating that a radical process was most likely
involved. However, the exact mechanism of the reaction
is unclear and requires further investigation.
3 | CONCLUSIONS
We have demonstrated for the first time the application of
Fe3O4‐Pd nanocatalyst for oxidative coupling of amides
with olefins. This method allows the stereoselective syn-
thesis of (Z)‐enamides at room temperature in the
absence of co‐catalyst and ligands. The magnetic nature
of the Fe3O4‐Pd nanocatalyst is particularly advantageous
2.2 | Reusability study
The stability and catalytic activity of the Fe3O4‐Pd
nanocatalyst were investigated in recycling experiments
using the model reaction between benzamide and methyl

PAL ET AL.
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4 | GENERAL PROCEDURE FOR
OXIDATIVE COUPLING BETWEEN
AMIDES AND OLEFINS
A 15 ml screw‐cap tube was charged with a mixture of
amide (0.826 mmol), olefin (2.478 mmol), 70% aqueous
solution of TBHP (1.156 mmol), PTSA (1.652 mmol) and
Fe3O4‐Pd nanocatalyst (30 mg, 2.3 wt% Pd, 5.5 mol%
Pd), and stirred in toluene at room temperature under
ambient air. The progress of the reaction was monitored
by TLC; upon completion, dichloromethane was added
to the reaction mixture. The nanocatalyst was separated
using an external magnet. The organic layer was washed
repeatedly with water, dried over Na2SO4 and
concentrated using a rotary vacuum evaporator. The
crude product was then purified by silica gel column
chromatography (eluent: ethyl acetate–petroleum ether)
to afford the corresponding enamides.
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ACKNOWLEDGMENTS
[13] T. Hosokawa, M. Takano, Y. Kuroki, S. I. Murahashi, Tetrahe-
dron Lett. 1992, 33, 6643.
We are grateful for financial support from Science and
Engineering Research Board (file no. YSS/2014/001017),
Government of India and UGC, New Delhi (F.30‐378/
2017(BSR)).
[14] X. Liu, K. K. Hii, Eur. J. Org. Chem. 2010, 2010, 5181.
[15] N. Panda, A. K. Jena, M. Raghavender, ACS Catal. 2012, 2, 539.
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CONFLICT OF INTEREST
[18] N. Panda, S. A. Yadav, S. Giri, Adv. Synth. Catal. 2017, 359, 654.
The authors declare no conflict of interest.
[19] AAS analysis showed that at 32% conversion, 168 ppm Pd was
present in solution; after 72% conversion, 20 ppm Pd was present.
ORCID
[20] (a)D. B. Eremin, V. P. Ananikov, Coord. Chem. Rev. 2017, 346,
2. (b)M. Genelot, V. Dufaud, L. Djakovitch, Adv. Synth. Catal.
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Ashis Kumar Jena https://orcid.org/0000-0003-4272-1116
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in the Supporting Information section at the end of the
article.
How to cite this article: Pal CK, Sahu S, Sahu
RK, Singh RK, Jena AK. Magnetically recyclable
palladium nanoparticles (Fe3O4‐Pd) for oxidative
coupling between amides and olefins at room
temperature. Appl Organometal Chem. 2019;e4985.
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