A new bifunctional heterogeneous nanocatalyst for one-pot reduction-Schiff base condensation and reduction-carbonylation of nitroarenes

Article abstract of DOI:10.1039/C8RA10212K

In this work, synthesis of Pd-NHC-γ-Fe₂O₃-n-butyl-SO₃H and its activity as a bifunctional heterogeneous nanocatalyst containing Pd-NHC and acidic functional groups, are described. This newly synthesized nanomagnetic catalyst is fully characterized by different methods such as FT-IR, XPS, TEM, VSM, ICP and TG analysis. At first, the catalytic activity of Pd-NHC-γ-Fe₂O₃-n-butyl-SO₃H is evaluated for the reduction of nitroarenes in aqueous media using NaBH₄ as a clean source of hydrogen generation at ambient temperature. Using the promising results obtained from the nitroarene reduction, this catalytic system is used for two one-pot protocols including reduction-Schiff base condensation and reduction-carbonylation of various nitroarenes. In these reactions the in situ formed amines are further reacted with aldehydes to yield imines or carbonylated to amides. The desired products are obtained in good to high yields in the presence of Pd-NHC-γ-Fe₂O₃-n-butyl-SO₃H as a bifunctional catalyst. The catalyst is reused with the aid of a magnetic bar for up to six consecutive cycles without any drastic loss of its catalytic activity.

Full text of DOI:10.1039/C8RA10212K

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A new bifunctional heterogeneous nanocatalyst for
one-pot reduction-Schiff base condensation and
reduction–carbonylation of nitroarenes
Cite this: RSC Adv., 2019, 9, 1362
a
b
Farhad Omarzehi Chahkamalia and Jose Miguel Sansano
*
Sara Sobhani,
´
In this work, synthesis of Pd–NHC-g-Fe2O3-n-butyl-SO3H and its activity as a bifunctional heterogeneous
nanocatalyst containing Pd–NHC and acidic functional groups, are described. This newly synthesized
nanomagnetic catalyst is fully characterized by different methods such as FT-IR, XPS, TEM, VSM, ICP and
TG analysis. At first, the catalytic activity of Pd–NHC-g-Fe2O3-n-butyl-SO3H is evaluated for the reduction
of nitroarenes in aqueous media using NaBH4 as a clean source of hydrogen generation at ambient
temperature. Using the promising results obtained from the nitroarene reduction, this catalytic system is
used for two one-pot protocols including reduction-Schiff base condensation and reduction–
carbonylation of various nitroarenes. In these reactions the in situ formed amines are further reacted with
aldehydes to yield imines or carbonylated to amides. The desired products are obtained in good to high
yields in the presence of Pd–NHC-g-Fe2O3-n-butyl-SO3H as a bifunctional catalyst. The catalyst is reused
with the aid of a magnetic bar for up to six consecutive cycles without any drastic loss of its catalytic activity.
Received 12th December 2018
Accepted 2nd January 2019
DOI: 10.1039/c8ra10212k
rsc.li/rsc-advances
ligands in transition metal catalysis due to the stability in air and
moisture and also strong s-electron-donating properties, which
Introduction
allow effective and strong NHC-metal bonds with numerous
transition metals.5 NHC-chelated transition metals have been
used in several types of homogeneous catalytic reactions, such as
C–C coupling, reduction and C–N bond formation reactions.6
Since NHC-chelated transition metals contain expensive ligands
and heavy metals, heterogenized NHC metal complexes are
attractive because of their reusability and preventing of product
contamination with traces of heavy metals.7 One way to prepare
heterogeneous NHC metal complexes is their immobilization on
different solid supports through their nitrogen. For instance,
polystyrene, iron oxide, graphene oxide or silica have been re-
ported for the immobilization of NHC metal complexes.8 The
presence of nitrogen atoms in NHC ligands is also very useful for
functionalization of NHC ligands to obtain bifunctional NHC
metal complexes. NHC-based bifunctional catalysts with late-
transition metals like Ni, Rh, Ru and Ir have been developed
for a variety of important reactions like Michael addition, alkyl-
ations of amines with alcohols, hydration of organonitriles,
hydrogenation of esters and ketones, etc.9 Although, NHCs have
long become popular as ligands of choice in transition metal
mediated catalysis, their related utility in bifunctional catalysis
remains surprisingly overlooked. Therefore, bifunctional catal-
ysis of the NHC-based systems is still in its developmental stage
and thereby there is a leaving room for development of other
transition metals of NHC-based bifunctional catalysts for
important organic transformations.
Polyfunctional catalysis, that enables one-pot multistep reactions,
holds great potential for increasing the efficiency of chemical
synthesis. Performing multiple reactions in a single reaction vessel
is a sustainable process avoiding intermediate separation, puri-
cation steps, and minimizing energy and increased safety as well
as manipulation of equilibria.1 By mimicking biological systems,
some research groups have focused their attention in the design
and synthesis of catalysts that contain multiple types of active
centers to promote domino reactions.2 These functionalities act in
a cooperative way to provide reactivity and selectivity superior to
what can be obtained from monofunctionalized materials. This
strategy brings to mind enzymes as biological catalysts, which
accelerate and control the selectivity of chemical transformations
by different active centers.3 A series of examples have been recently
reported for the synthesis of acids and bases on solid supports for
dual activation of electrophiles and nucleophiles.4 However,
systematic investigations as well as the successful cohabitation of
metal and acid functionalities on a single solid support and the
evaluation of the catalytic activity of such systems in one-pot
reactions are rare in the literature.
Over the last few decades, N-heterocyclic carbenes (NHCs)
have emerged as an extremely useful, new and versatile class of
aDepartment of Chemistry, College of Sciences, University of Birjand, Birjand, Iran.
E-mail: ssobhani@birjand.ac.ir; sobhanisara@yahoo.com
b
´
´
´
Departamento de Quımica Organica, Facultad de Ciencias, Centro de Innovacion en
Compounds containing nitro groups are the most readily
available starting materials in organic synthesis. One of the
´
´
´
Quımica Avanzada (ORFEO-CINQA) and Instituto de Sıntesis Organica (ISO),
Universidad de Alicante, Apdo. 99, 03080-Alicante, Spain
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major applications of nitroarenes in organic transformations is
their hydrogenation to afford anilines.10 Anilines are highly
versatile building blocks for various substrates. One of the
aniline derivatives is imines, which are versatile intermediates
in organic synthesis and prepared generally by the reaction of
amines and carbonyl compounds. Recently, few methods for
domino nitroarene reduction and intramolecular Schiff base
condensation in the presence of Au/TiO2/H2,11 Fe/HCl,12 Se/
15
NaOAc/CO,13 Ni/SiO2/H2,14 CoOx@NC-800/H2 have been re-
ported for the one-pot synthesis of imines. These protocols start
from nitroarenes and require special equipment to handle high
pressure inammable hydrogen gas, hazardous carbon
monoxide or highly acidic conditions, elevated pressures and
high temperatures. These requirements made the reported
methods far from ideal for laboratory-scale synthetic chemistry.
Amides, as the other derivatives of amines, occupy pivotal
positions in organic synthesis. Generally, amides are produced
through the reaction of carboxylic acids or esters with amines.
As nitroarenes are readily available chemicals in industry,
conversion of nitroarenes directly to their corresponding
amides in one-pot reduction–carbonylation reactions is more
efficient than starting from the corresponding amine.16 This
protocol generally involves two steps; the rst step is the
reduction of nitroarenes to the corresponding anilines and the
second step is the carbonylation of anilines to the correspond-
ing amides. Indium/AcOH,17 Ni2B@Cu2O/NaBH4,18 Pd–C/
NaBH419 and Fe3O4@Cu(OH)x/NaBH420 are some of the systems
which have been reported for direct transformation of nitro-
arenes to amides. Although most of the published methods are
useful synthetic methods, they generally suffer from long reac-
tion times, unsatisfactory yields of the products and use of
unrecoverable catalysts. Thus, an alternative method which is
effective, simple, green, employing reusable catalysts would be
a subject of interest. Continuing with the aim of our recent
works on the introduction of new heterogeneous Pd-catalysts,21
herein, we wish to report the synthesis of a new Pd–NHC
complex containing an acidic functional group immobilized on
Scheme 1 Synthesis of Pd–NHC-g-Fe2O3-n-butyl-SO3H.
by 3-chlorotrimethoxypropylsilane and reacted with imidazole.
Aer neutralization with Et3N, g-Fe2O3-Im was obtained. Reac-
tion of g-Fe2O3-Im with 1,4-butanesultone and then acidica-
tion with HCl (0.2 M) produced g-Fe2O3-Im-n-butyl-SO3H.
Finally, Pd–NHC-g-Fe2O3-n-butyl-SO3H was obtained from the
reaction of g-Fe2O3-Im-n-butyl-SO3H with Pd(OAc)2.
Chemical structures of all the synthesized compounds were
conrmed through the FT-IR spectroscopic technique. Fig. 1
demonstrates FT-IR spectra of g-Fe2O3-Im (a), g-Fe2O3-Im-n-butyl-
SO3H (b) and Pd–NHC-g-Fe2O3-n-butyl-SO3H (c). These spectra
displayed a characteristic peak of Fe–O at around 555–676 cmꢀ1
and stretching vibrations of O–H bonds at 3436 cmꢀ1. The bands
observed around 1068, 2927, 3141 and 1560 cmꢀ1 are attributed to
the Si–O, aliphatic and aromatic C–H and C]C stretching vibra-
tions, which conrm the functionalization of the surface of the
MNPs. The FT-IR spectra of g-Fe2O3-Im-butyl-SO3H (b) and Pd–
NHC-g-Fe2O3-n-butyl-SO3H (c) exhibit a typical band at around
1040 cmꢀ1 attributed to S]O stretching vibrations (Fig. 1).
XPS study was carried out to check the surface chemical
compositions of Pd–NHC-g-Fe2O3-n-butyl-SO3H (Fig. 2). The
peaks corresponding to oxygen, carbon, nitrogen, silicon, iron
and sulfur are clearly observed in the XPS elemental survey of
the catalyst (Fig. 2a). Deconvolution of C1s region showed
a major peak at 284.6 eV corresponding to C–C and C]C
g-Fe2O3 (Pd–NHC-g-Fe2O3-n-butyl-SO3H) as
a bifunctional
heterogeneous nanocatalyst. Aer characterization of this
catalyst, its catalytic activity was evaluated for the reduction of
nitrobenzenes using NaBH4 in water as a green solvent at
ambient temperature. By the promising results obtained from
nitroarene reduction, this catalytic system was used for two one-
pot protocols including reduction-Schiff base condensation and
reduction–carbonylation of various nitroarenes. The process
involved reduction of the correspondent nitro compounds to
anilines in the presence of the Pd-catalyst and then reaction of
anilines with the carbonyl compounds or anhydrides catalyzed
by acidic functional group of Pd–NHC-g-Fe2O3-n-butyl-SO3H to
give imines and amides, respectively.
Results and discussion
Synthesis and characterization of Pd–NHC-g-Fe2O3-n-butyl-
SO3H
Scheme 1 describes the synthetic procedure to prepare Pd–
Fig. 1 FT-IR spectra of (a) g-Fe2O3-Im, (b) g-Fe2O3-Im-n-butyl-SO3H
NHC-g-Fe2O3-n-butyl-SO3H. At rst, g-Fe2O3 was functionalized and (c) Pd–NHC-g-Fe2O3-n-butyl-SO3H.
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more peaks at 399.4 and 400.3 eV related to Csp3–N and Csp2–N,
respectively.22 As shown in Fig. 2d, the peaks at 337.7 (3d5/2)
and 342.8 eV (3d3/2), correspond to Pd with II oxidation state.
The peaks at 335.1 (3d5/2) and 340.3 eV (3d3/2) indicate that
a small portion of Pd is in zero oxidation state.8c
The Pd content of Pd–NHC-g-Fe2O3-n-butyl-SO3H was quan-
tied by ICP. The ICP analysis showed that 0.21 mmol of Pd was
anchored onto 1 g of Pd–NHC-g-Fe2O3-n-butyl-SO3H. The particle
size distribution of Pd–NHC-g-Fe2O3-n-butyl-SO3H was evaluated
using TEM, which demonstrated that the average diameter of the
particles was 17 nm. TEM images also showed that the NPs are
spherical in shape and are relatively monodispersed (Fig. 3).
The magnetic properties of Pd–NHC-g-Fe2O3-n-butyl-SO3H
and g-Fe2O3 were evaluated by VSM at room temperature
(Fig. 4). The saturation magnetization value of Pd–NHC-g-
Fe2O3-n-butyl-SO3H was 65.9 emu gꢀ1, which is similar to that of
Fig. 3 (a and b) TEM and (c) particle size distribution histogram of Pd–
NHC-g-Fe2O3-n-butyl-SO3H.
Fig.
2 (a) XPS patterns of the Pd–NHC-g-Fe2O3-n-butyl-SO3H
nanomagnetic catalyst (b) C 1s (c) N 1s (d) Pd.
(Fig. 2b). In addition, the peaks observed at 286.2 eV and
2
3
288.0 eV refer to the Csp –N and Csp –N bonds, respectively.
Fig. 2c showed the nitrogen region of the XPS measured spec-
trum for the Pd–NHC-g-Fe2O3-n-butyl-SO3H. It revealed the
presence of a main peak at 401.4 eV related to C–N–C and two
Fig. 4 Magnetization curves g-Fe2O3 (red) and Pd–NHC-g-Fe2O3-n-
butyl-SO3H (black).
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one-pot reduction-Schiff base condensation and one-pot
reduction–carbonylation of various nitroarenes, we have
initially evaluated the efficiency of the catalyst for the reduction
of nitroarenes by NaBH4 as a clean source of hydrogen gener-
ation. For this purpose, the reduction of nitrobenzene at room
temperature was chosen as a model reaction to optimize the
reaction conditions such as solvent and the amount of the
catalyst (Table 1). As indicated in Table 1, aqueous media and
0.6 mol% of the catalyst are the best choice of these reactions
(Table 1, entry 3). Increasing the amount of the catalyst to 0.8
and 1 mol% did not produce any effect on the yield of the
desired product (Table 1, entries 4 and 5). The reaction pro-
ceeded smoothly in the presence of lower amounts of the
catalyst (Table 1, entries 1 and 2). Other organic solvents such as
EtOH and MeOH afforded the desired product in longer reac-
tion time and lower yields (Table 1, entries 6 and 9). It was
observed that the product was obtained in higher yield when
a mixture of EtOH and water (1 : 1 and 1 : 4) was used as solvent
(Table 1, entries 7 and 8). This phenomenon could be attributed
to the property of greater activity of NaBH4 in water. In the
absence of solvent, the reaction did not proceed at all, even aer
24 h (Table 1, entry 10).
We next examined the reduction of a variety of nitroarenes by
NaBH4 under the optimized reaction conditions. The results of
this study are depicted in Table 2. The reaction of different
nitroarenes with electron-releasing or electron-withdrawing
groups proceeded well in the presence of Pd–NHC-g-Fe2O3-n-
butyl-SO3H (0.6 mol%) and the desired products were obtained
in 70–99% yields in 20–60 min.
A signicant practical advantage of heterogeneous catalysis
is the ability to easily remove the catalyst from the reaction
mixture and reuse it for subsequent reactions. In this regard,
the recycling and reusing capability of the catalyst was investi-
gated in the reduction reaction of nitrobenzene with NaBH4,
under optimum reaction conditions, in the presence of Pd–
NHC-g-Fe2O3-n-butyl-SO3H. Aer reaction, the catalyst was
completely collected from the reaction mixture using an
Fig. 5 TGA curve of Pd–NHC-g-Fe2O3-n-butyl-SO3H.
g-Fe2O3 (68.6 emu gꢀ1). Small decrease in the saturated
magnetization
value
of
Pd–NHC-g-Fe2O3-n-butyl-SO3H
compared to that of g-Fe2O3 can be attributed to the slight
increase in mass owing to the immobilized Pd-complex on the
surface of g-Fe2O3. The magnetization curves showed no
hysteresis loop, which indicated superparamagnetic character-
istic of the nanoparticles. The strong magnetic properties of the
nanoparticles were sufficient for complete and easy magnetic
separation with attraction to a conventional magnet.
Fig. 5 shows the TG analysis of Pd–NHC-g-Fe2O3-n-butyl-
SO3H. The thermogravimetric analysis exhibited the rst weight
ꢁ
loss of 1.45% below 168 C, which might be due to the loss of
physically adsorbed water adhering to the sample surface and
surface hydroxyl groups. The second weight loss between 168
and 488 ꢁC (3%) was due to the breakdown and decomposition
of imidazole moieties. Thus, the TG curves also convey the
obvious information that the imidazole molecules are success-
fully graed onto the magnetic nanoparticles.
Catalytic performance of Pd–NHC-g-Fe2O3-n-butyl-SO3H
In order to investigate the catalytic activity of Pd–NHC-g-Fe2O3-
n-butyl-SO3H as a bifunctional heterogeneous catalyst for the
Table 1 Reduction of nitrobenzene to aniline with NaBH4 catalyzed by Pd–NHC-g-Fe2O3-n-butyl-SO3H under different conditionsa
Entry
Solvent
Catalyst (mol%)
Time (min)
Isolated yields (%)
1
2
3
4
5
6
7
8
9
H2O
H2O
H2O
H2O
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.3)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.5)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.6)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.8)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (1)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.6)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.6)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.6)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.6)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.6)
60
40
30
25
95
95
98
97
98
25
26
93
90
0
H2O
EtOH
EtOH : H2O (1 : 1)
EtOH : H2O (1 : 4)
MeOH
20
5 h
5 h
3 h
5 h
24 h
10
—
a
NaBH4 ¼ 3 equiv.
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Table 2 Reduction of nitroarenes by NaBH4 catalyzed by Pd–NHC-g-Fe2O3-n-butyl-SO3H in aqueous media at ambient temperature
Obtained
Entrya
Nitroarene
Time (min)
Isolated yield (%)
mp (ꢁC)
Reported mp (ꢁC) [ref.]
b
1
2
3
4
5
6
7
8
Nitrobenzene
4-Nitroaniline
4-Nitrophenol
3-Nitrophenol
30
30
30
30
60
30
20
20
20
30
25
35
98
97
95
92
92
70
98
97
99
98
99
95
—
—
142–143
187–189
123–124
172–173
42–43
65–67
63–64
55–56
166–167
80
138–142 [23]
186–187 [24]
120–121 [24]
171–173 [25]
43–44 [23]
67–70 [23]
60–62 [23]
57 [24]
2-Nitrophenol
1-Methyl-4-nitrobenzene
1-Chloro-4-nitrobenzene
1-Bromo-4-nitrobenzene
1-Methoxy-4-nitrobenzene
4-Nitrobenzenesulfonamide
(2-Nitrophenyl)methanol
5-Nitro-1H-indole
9
10
11
12
166 [24]
81–83 [23]
130 [24]
132–134
a
Reaction conditions: NaBH4 (3 equiv.), Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.6 mol%). b Liquid.
external magnet. This catalyst was exhaustively washed with nitrobenzene followed by Schiff base formation with benzal-
water and acetone in sequence. Then, it was dried and used dehyde in aqueous media catalyzed by Pd–NHC-g-Fe2O3-n-
directly for the next run under the same conditions. Other cycles butyl-SO3H as a bifunctional heterogeneous catalyst. As it is
were repeated with the same procedure. Aer being used six depicted in Table 3, the best yield of the imine was obtained in
cycles in successive reactions, still had a high catalytic perfor- the presence of 1 mol% of the catalyst (Table 3, entry 3). To
mance, such as it is depicted in Fig. 6. Furthermore, FT-IR demonstrate the catalyst function, similar reactions in the
spectrum (Fig. 7) and XPS analysis (Fig. 8) of the recycled presence of Pd–NHC-g-Fe2O3–Me and Pd–NHC-g-Fe2O3–SO3H
catalyst aer six times revealed that this catalyst has a very high were examined. In both cases, nitrobenzene was fully converted
stability. In another experiment, aer a ꢂ50% conversion of the to aniline aer stirring for 30 min. The imine was produced in
reaction, the catalyst was removed from the reaction mixture. higher yield in the presence of Pd–NHC-g-Fe2O3–SO3H (entry 5).
The analysis of reaction mixture showed that further perform- These results revealed that sulfonic acid can activate the alde-
ing of the reaction under optimum conditions in the absence of hyde to undergo the reaction with the amine and produced
the catalyst did not show any signicant progress. Also, ICP imine in the second step of the reaction. Moreover, Pd–NHC-g-
analysis of the reaction mixture demonstrated that the amount Fe2O3-n-butyl-SO3H exhibited a higher catalytic efficiency than
of Pd in the solution was less than the detection limit. There- Pd–NHC-g-Fe2O3–SO3H in the formation of imine, perhaps due
fore, we can conclude that the designed catalyst has a truly to the presence of alkyl linker, which makes the acidic func-
heterogeneous nature. The heterogeneous character of the tional group more available.
catalyst was further checked by a poisoning test using S8 as an
To show the versatility and the wide scope of this protocol,
efficient Pd scavenger. For this purpose, an aqueous mixture of a variety of nitroarenes were subjected to reduction-Schiff base
nitrobenzene (1 mmol), NaBH4 (3 mmol), Pd–NHC-g-Fe2O3-n- condensation with arylaldehydes, yielding the corresponding
butyl-SO3H (0.6 mol%) and S8 (0.05 g) was stirred at room imines. The results are summarized in Table 4. The reaction of
temperature. Any product formation aer 30 min clearly
demonstrated its heterogeneous nature.
Having the results of nitro reduction in hand, we next
examined the one-pot synthesis of imines by reduction of
Fig. 6 The recycling efficiency of Pd–NHC-g-Fe2O3-n-butyl SO3H in Fig. 7 FT-IR spectra of Pd–NHC-g-Fe2O3-n-butyl-SO3H after six
the reduction of nitrobenzene.
times reused.
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the aromatic ring, gave slightly lesser yields of the desired
product during the reaction with benzaldehyde (Table 4, entries
4 and 5). This can be attributed to the lower nucleophilicity of
these freshly formed amines. Other different substituted-
benzaldehydes (Table 4, entries 6–11), as well as heterocyclic
aldehydes (Table 4, entries 12 and 13) were appropriate
substrates to perform the reaction with nitrobenzene.
Further synthetic application of this mild reduction was
demonstrated in other one-pot reaction involving carbonylation
of the produced aniline derivatives with Ac2O or Boc2O. The
results of these studies are depicted in Table 5. The one-pot
reduction–carbonylation of nitroarenes catalyzed by Pd–NHC-
g-Fe2O3-n-butyl-SO3H under optimized reaction conditions
proceeded well and produced the desired products in good to
high yields, independently of the acylating agent.
Finally, the catalytic efficiency of Pd–NHC-g-Fe2O3-n-butyl-
SO3H was compared with those of a number of previously re-
ported catalysts for the reduction of nitroarenes, one-pot
reduction-Schiff base condensation and one-pot reduction–
carbonylation of nitroarenes (Table 6). The results showed that
the previously reported catalytic systems were associated with
several drawbacks such as high temperatures, the use of
hazardous organic solvents or reagents, unrecoverable catalysts
or inammable hydrogen gas. Our catalysis method can elimi-
nate almost all of these drawbacks, since Pd–NHC-g-Fe2O3-n-
butyl-SO3H acted as a magnetically recyclable catalyst, worked
at room temperature in the presence of NaBH4 in neat water
and without using any additive. Moreover, Pd–NHC-g-Fe2O3-n-
butyl-SO3H is the most effective catalyst for the nitro-group
reduction (Table 6, entries 1–5) for its operational simplicity,
one-pot reduction-Schiff base condensation and one-pot
reduction–carbonylation of nitroarene of TON (Table 6,
entries 6–14).
This promising result should be attributed to the bifunc-
tional catalysis of Pd–NHC-g-Fe2O3-n-butyl-SO3H that enabled
one-pot multistep reactions. The process involved reduction of
the correspondent nitro compounds to anilines in the presence
of the Pd catalyst and then reaction of anilines with the carbonyl
compounds or anhydrides catalyzed by acidic functional group
of Pd–NHC-g-Fe2O3-n-butyl-SO3H to give imines and amides,
respectively.
Experimental
General information
Chemicals were purchased from Merck Chemical Company.
The purity of the products and the progress of the reactions
were accomplished by TLC on silica gel polygram SILG/UV254
plates. FT-IR spectra were recorded on a Shimadzu Fourier
Transform Infrared Spectrophotometer (FT-IR-8300). The
content of Pd in the catalyst was determined by OPTIMA
7300DV ICP analyzer. TEM analysis was performed using TEM
microscope (Philips EM 208S). Thermo-gravimetric analysis
(TGA) was performed using a Shimadzu thermos-gravimetric
analyzer (TG-50). Vibrating Sample Magnetometer (VSM) anal-
ysis was performed using VSM (Lake Shore Cryotronics 7407).
Melting points were measured on an electrothermal 9100
Fig. 8 (a) XPS patterns of Pd–NHC-g-Fe2O3-n-butyl-SO3H after six
times reused (b) C 1s (c) N 1s (d) Pd.
nitroarenes, bearing electron-donating groups at the aromatic
ring, with benzaldehyde proceeded smoothly to afford the cor-
responding imines in high yields (Table 4, entries 1–3).
However, nitroarenes, having electron-withdrawing groups at
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Table 3 One-pot reduction-Schiff base condensation of nitrobenzene with benzaldehyde in aqueous media
Entrya
Catalyst (mol%)
Time (min)
Isolated yield (%)
1
2
3
4
5
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.6)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.8)
Pd–NHC-g-Fe2O3-n-butyl-SO3H (1)
Pd–NHC-g-Fe2O3–Me (1)
70
70
70
180
180
76
80
93
52
83
Pd–NHC-g-Fe2O3–SO3H (1)
a
Nitrobenzene, NaBH4 (3 equiv.) and catalyst was stirred in aqueous media at room temperature for 30 min. Benzaldehyde (1.2 equiv.) was added to
the stirring mixture.
apparatus. XPS analyses were performed using a VG-Microtech
Multilab 3000 spectrometer, equipped with an Al anode. The
deconvolution of spectra was carried out by using Gaussian–
Lorentzian curves.
Synthesis of n-butyl sulfonated g-Fe2O3-Im (g-Fe2O3-Im-n-
butyl-SO3H)
The synthesized g-Fe2O3-Im (1.5 g) was sonicated in dry
toluene (30 mL) for 30 min. 1,4-Butanesultone (3 mL) was
added dropwise to the dispersed mixture and stirred at 90 C
for 24 h. The solid was separated by an external magnet,
washed with H2O (3 ꢃ 10 mL) and acetone (3 ꢃ 10 mL) and
dried at 70 ꢁC in oven under vacuum. The synthesized solid
(1.5 g) was added to HCl (25 mL, 0.1 M) and sonicated for 24 h
at room temperature. The solid was separated by an external
magnet, washed with H2O to reach pH 6–7 and dried at 70 ꢁC
in oven under vacuum.
ꢁ
Synthesis of imidazole supported on g-Fe2O3 (g-Fe2O3-Im)
21e
The synthesized chloro-functionalized g-Fe2O3 (1.6 g) was
sonicated in dry toluene (30 mL) for 30 min. Imidazole
(0.204 g, 3 mmol) was added to the stirring mixture and
reuxed for 24 h. Aer cooling, the mixture was stirred with
Et3N (0.43 mL) for 30 min at room temperature. The solid was
separated by an external magnet, washed with H2O (3 ꢃ 10
mL) and acetone (3 ꢃ 10 mL) and dried at 70 ꢁC in oven under
vacuum.
Table 4 One-pot reduction-Schiff base condensation of nitroarenes with arylaldehydes catalyzed by Pd–NHC-g-Fe2O3-n-butyl-SO3H as
a bifunctional heterogeneous catalysta
Obtained
ꢁ
Entry
Aldehyde
Nitroarene
Time (min)
Isolated yield (%)
mp (ꢁ C)
Reported mp (ꢁC) [ref.]
1
2
3
4
5
6
7
8
Benzaldehyde
Benzaldehyde
4-Chlorobenzene
Benzaldehyde
Benzaldehyde
2-Hydroxybenzaldehyde
Benzaldehyde
4-Chlorobenzaldehyde
4-Cyanobenzaldehyde
4-Bromobenzaldehyde
2,4-Dichlorobenzaldehyde
Nicotinaldehyde
3-Nitrophenol
3-Nitrophenol
1-Methoxy-4-nitrobenzene
1-Bromo-4-nitrobenzene
1-Chloro-4-nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
100
110
110
110
110
130
70
100
90
100
180
180
240
94
98
87
74
71
92
93
84
86
75
87
90
60
151
149 [26]
192–194
124–125
61–62
58–60
51–52
47–49
59–61
94–95
75–77
74–76
19–20
128–130
197–198 [27]
121–123 [28]
62–63 [27]
58–61 [29]
52–54 [30]
48–50 [31]
60–61 [31]
97–98 [32]
71–74 [29]
78–80 [31]
21.5 [33]
9
10
11
12
13
Nitrobenzene
Nitrobenzene
Nitrobenzene
1H-Indole-3-carboxaldehyde
132 [34]
a
Nitroarene, NaBH4 (3 equiv.) and catalyst was stirred in aqueous media at room temperature for 30 min. Aldehyde (1.2 equiv.) was added to the
stirring mixture.
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Table 5 One-pot reduction–carbonylation of nitroarenes catalyzed by Pd–NHC-g-Fe2O3-n-butyl-SO3H as a bifunctional heterogeneous
catalysta
Obtained
Entry
Nitroarene
Anhydride
Time (min)
Isolated yield (%)
mp (ꢁC)
Reported mp (ꢁC) [ref.]
1
2
3
4
5
6
7
8
Nitrobenzene
Ac2O
Ac2O
Ac2O
Ac2O
Boc2O
Boc2O
Boc2O
Boc2O
110
120
130
75
100
80
99
74
95
93
92
98
98
80
106–107
174–176
153–155
125–126
134–135
92–94
108–111 [35]
176–177 [35]
152–154 [35]
127–128 [35]
134–136 [36]
92–93 [36]
4-Chloronitrobenzene
1-Methyl-4-nitrobenzene
1-Methoxy-4-nitrobenzene
Nitrobenzene
1-Methoxy-4-nitrobenzene
1-Methyl-4-nitrobenzene
1-Chloro-4-nitrobenzene
70
180
84–85
103–105
86–88 [36]
104–106 [36]
a
Nitroarene, NaBH4 (3 equiv.) and catalyst was stirred in aqueous media at room temperature for 30 min. Ac2O or Boc2O (1.2 equiv.) was added to
the stirring mixture.
external magnet, washed with H2O to reach pH 6–7 and dried at
70 C in oven under vacuum.
Synthesis of Pd–NHC complex of g-Fe2O3-Im-n-butyl-SO3H
(Pd–NHC-g-Fe2O3-n-butyl-SO3H)
ꢁ
The synthesized g-Fe2O3-Im-n-butyl-SO3H (1 g) was sonicated in
DMSO (15 mL) for 30 min. Pd(OAc)2 (0.5 mmol) was added to
the dispersed mixture under N2 atmosphere at room tempera-
ture. The mixture was stirred for 6 h at 50 ꢁC and then allowed to
Synthesis of Pd–NHC of g-Fe2O3-Im-SO3H (Pd–NHC-g-Fe2O3–
SO3H)
The synthesized g-Fe2O3-Im-SO3H (1 g) was sonicated in DMSO
(15 mL) for 30 min. Pd(OAc)2 (0.5 mmol) was added to the
dispersed mixture under N2 atmosphere at room temperature.
The mixture was stirred for 6 h at 50 ꢁC and then allowed to
proceed for an additional 4 h at 100 ꢁC. The solid was separated
by an external magnet, washed with Et2O (3 ꢃ 10 mL), acetone
ꢁ
proceed for an additional 4 h at 100 C. The resulting complex
was collected by an external permanent magnet and washed
with acetone (3 ꢃ 10 mL) and EtOH (3 ꢃ 10 mL) to remove the
unreacted Pd(OAc)2. The catalyst was obtained as dark-brown
ꢁ
solid aer drying at 70 C in oven under vacuum.
ꢁ
(3 ꢃ 10 mL) and dried at 70 C in oven under vacuum.
Methylation reaction of g-Fe2O3-Im to synthesis g-Fe2O3-Im-Me
The synthesized g-Fe2O3-Im (1 g) was sonicated in dry toluene General procedure for reduction of nitroarenes
(30 mL) for 30 min. Methyl iodide (2 mmol) was added to the
dispersed mixture and reuxed for 24 h. The solid was sepa-
Pd–NHC-g-Fe2O3-n-butyl-SO3H (0.028 g, 0.6 mol%) was added to
the stirring mixture of nitrobenzene (1 mmol) in H2O (2 mL).
NaBH4 (3 mmol) was added in two portions with the interval of
2 min between them, and the resulting mixture was continued
rated by an external magnet, washed with Et2O (3 ꢃ 10 mL) and
acetone (3 ꢃ 10 mL) and dried in oven under vacuum.
to stir at room temperature for the appropriate time (see Table
2). EtOAc (5 mL) was added to the reaction mixture. Pd–NHC-g-
Synthesis of Pd–NHC of g-Fe2O3-Im-Me (Pd–NHC-g-Fe2O3–Me)
Fe2O3-n-butyl-SO3H was separated by an external magnet and
washed with EtOAc (2 ꢃ 5 mL), H2O (3 ꢃ 10 mL) and acetone (3
ꢃ 10 mL). The combined organic layer was then dried over
anhydrous Na2SO4. Evaporation of the solvent under reduced
pressure gave the crude products. The pure products were iso-
lated by chromatography on silica gel eluted with n-hex-
ane:EtOAc (4 : 1).
The synthesized g-Fe2O3-Im-Me (1 g) was sonicated in DMSO
(15 mL) for 30 min. Pd(OAc)2 (0.5 mmol) was added to the
dispersed mixture under N2 atmosphere at room temperature.
The mixture was stirred for 6 h at 50 ꢁC and then allowed to
proceed for an additional 4 h at 100 ꢁC. The solid was separated
by an external magnet, washed with Et2O (3 ꢃ 10 mL), acetone
ꢁ
(3 ꢃ 10 mL) and dried at 70 C in oven under vacuum.
Synthesis of sulfonated g-Fe2O3-Im (g-Fe2O3-Im-SO3H)
General procedure for the one-pot reduction-Schiff base
formation from nitroarenes
The synthesized g-Fe2O3-Im (1 g) was sonicated in DMSO (30
mL) for 30 min. Chlorosulfonic acid (3 mL) was slowly added Pd–NHC-g-Fe2O3-n-butyl-SO3H (1 mol%) was added to the stir-
dropwise to the dispersed mixture in an ice-bath and stirred for ring mixture of nitrobenzene (1 mmol) in H2O (2 mL). NaBH4 (3
24 h at room temperature. The solid was separated by an mmol) was added in two portions with the interval of 2 min
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between them, and the mixture was stirred at room temperature
for 30 min. Next, aryl aldehyde (1.2 mmol) was added to the
mixture and stirred for the appropriate time (see Table 4). EtOAc
(5 mL) was added to the reaction mixture. Pd–NHC-g-Fe2O3-n-
butyl-SO3H was separated by an external magnet and washed
with EtOAc (2 ꢃ 5 mL), H2O (3 ꢃ 10 mL) and acetone (3 ꢃ 10
mL). The combined organic layer was then dried over anhy-
drous Na2SO4. Evaporation of the solvent under reduced pres-
sure gave the crude products. The pure products were isolated
by chromatography on silica gel eluted with n-hexane : EtOAc
(4 : 1).
General procedure for the one-pot reduction–carbonylation of
nitroarenes
Pd–NHC-g-Fe2O3-n-butyl-SO3H (1 mol%) was added to the
mixture of nitrobenzene (1 mmol) in H2O (2 mL). NaBH4 (3
mmol) was added in two portions with the interval of 2 min
between them, and the mixture was vigorously stirred at room
temperature for 30 min. Ac2O or Boc2O (1.2 mmol) was added to
the mixture and stirred for an appropriate time (see Table 5).
EtOAc (5 mL) was added to the reaction mixture. Pd–NHC-g-
Fe2O3-n-butyl-SO3H was separated by an external magnet and
washed with EtOAc (2 ꢃ 5 mL), H2O (3 ꢃ 10 mL) and acetone (3
ꢃ 10 mL). The combined organic layer was then dried over
anhydrous Na2SO4. Evaporation of the solvent under reduced
pressure gave the crude products. The pure products were iso-
lated by chromatography on silica gel eluted with n-hex-
ane : EtOAc (5 : 3).
Conclusions
We have demonstrated the successful synthesis of Pd–NHC-g-
Fe2O3-n-butyl-SO3H as a bifunctional heterogeneous nano-
magnetic catalyst containing Pd–NHC and acidic functional
groups. This newly synthesized catalyst was characterized by
different methods such as FT-IR, XPS, TEM, VSM, ICP and TG
analysis. Pd–NHC-g-Fe2O3-n-butyl-SO3H was successfully used
as a new heterogeneous catalyst for the reduction of nitroarenes
using NaBH4 as a clean source of hydrogen generation in
aqueous media at ambient temperature. The catalytic activity of
Pd–NHC-g-Fe2O3-n-butyl-SO3H as a bifunctional heterogeneous
catalyst was also evaluated in two one-pot protocols including
reduction-Schiff base condensation and reduction–carbonyla-
tion of various nitroarenes. In these two reactions, amines
formed in situ, using NaBH4 as a clean source of hydrogen
generation, in aqueous media at ambient temperature and
further reacted with aldehydes to yield imines or carbonylated
to amides. By this protocol, a variety of imines and amides were
obtained in good to high yields. The magnetic nature of the
catalyst allows its facile recovery using an external magnetic
eld. The catalyst was separated efficiently and reused for six
consecutive cycles without any drastic loss of its reactivity. Truly
heterogeneous nature of the catalyst was proved by removing
the catalyst aer 50% progress of the reaction and poisoning
test. The structure of the catalyst remained intact aer six
recoveries according to the FT-IR spectrum and XPS analysis of
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the used catalyst. Bifunctional catalysis of Pd–NHC-g-Fe2O3-n-
butyl-SO3H enabled one-pot multistep reactions without
requiring product isolation in each step, using water as a green
reaction media and NaBH4 as a clean source of hydrogen
generation, which is safer than hydrogen gas. True heteroge-
neous nature and simple and efficient recyclability of the cata-
lyst makes the present methodology sustainable and economic.
The employment of this NHC-based system is in its develop-
mental stage. The synthesis and properties of Pd–NHC-g-Fe2O3-
n-butyl-SO3H exhibited in this work offers valuable opportuni-
ties for further advancements in this eld of study.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
We gratefully acknowledge the University of Birjand Research
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