Mesoporous poly-melamine-formaldehyde stabilized palladium nanoparticle (Pd@mPMF) catalyzed mono and double carbonylation of aryl halides with amines

Article abstract of DOI:10.1039/c4ra07554d

A new mesoporous poly-melamine-formaldehyde material supported Pd nano catalyst (mPMF-Pd⁰) has been synthesized and characterized by thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflection spectroscopy (DRS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and a N₂ adsorption study. The mPMF-Pd⁰ material showed very good catalytic activity in the field of mono and double amino carbonylation of aryl bromides/iodides. Moreover, the catalyst is easily recoverable and can be reused six times without appreciable loss of catalytic activity in the above reactions. So, the highly dispersed and strongly bound palladium(0) sites in the mPMF-Pd⁰ could be responsible for the observed high activities. Due to strong binding with the functional groups of the polymer, no evidence of leached Pd from the catalyst during the course of reaction occurred, suggesting true heterogeneity in the catalytic process. This journal is

Full text of DOI:10.1039/c4ra07554d

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Mesoporous poly-melamine-formaldehyde
stabilized palladium nanoparticle (Pd@mPMF)
catalyzed mono and double carbonylation of aryl
halides with amines†
Cite this: RSC Adv., 2014, 4, 48177
Rostam Ali Molla,a Md. Asif Iqubal,‡b Kajari Ghosh,‡a Anupam Singha Roy,a
b
a
*
*
Kamaluddin and Sk. Manirul Islam
A new mesoporous poly-melamine-formaldehyde material supported Pd nano catalyst (mPMF–Pd0) has
been synthesized and characterized by thermogravimetric analysis (TGA), powder X-ray diffraction (XRD),
scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), high-resolution
transmission electron microscopy (HRTEM), UV-vis diffuse reflection spectroscopy (DRS), Raman
spectroscopy, X-ray photoelectron spectroscopy (XPS) and a N2 adsorption study. The mPMF–Pd0
material showed very good catalytic activity in the field of mono and double amino carbonylation of aryl
bromides/iodides. Moreover, the catalyst is easily recoverable and can be reused six times without
appreciable loss of catalytic activity in the above reactions. So, the highly dispersed and strongly bound
palladium(0) sites in the mPMF–Pd0 could be responsible for the observed high activities. Due to strong
binding with the functional groups of the polymer, no evidence of leached Pd from the catalyst during
the course of reaction occurred, suggesting true heterogeneity in the catalytic process.
Received 24th July 2014
Accepted 22nd September 2014
DOI: 10.1039/c4ra07554d
www.rsc.org/advances
leached out of the matrix during the reaction and thus facili-
tates the overall reaction.
1. Introduction
Various supports such as silica, carbon, and zirconia have
been used for the immobilization of palladium nano parti-
cles.6–8 These supports, though advantageous in some cases,
retain some of the common drawbacks that pertain to the
traditional and cumbersome isolation procedure aer comple-
tion of reactions and most importantly are decient in the
reusability aspect. However, ordinary mesoporous materials are
poor in functional groups and the impregnation of metal
nanoparticles oen results in polydisperse nanoparticles on the
supportive materials. Even so, there is an ample scope in the
design of improved supported catalysts from the point of view of
benign and sustainable protocols.
Among the different type of mesoporous materials, the
organic mesoporous polymers are a class of advanced materials,
which possess the textural porosities of mesoporous materials
(well-ordered pores, large surface area and tunable structures)
and simultaneously the advantages of organic polymers (high
hydrophobicity, containing aromatic sections and higher
stability in acid or base media in comparison to silica based
materials).9,10 Recently, we have reported several mesoporous
organic polymer supported metal catalysts.11
Metal nanoparticles have gained substantial attention in recent
years in both chemistry and biology.1,2 The smaller size of the
metal nanoparticles leads to a high surface/volume ratio and
consequently, a large number of potential active sites would be
available to the substrates, thereby enhancing their catalytic
activity. However, metal nanoparticles, due to their high surface
energy, are not stable as such. For instance, palladium nano-
particles aggregate into Pd black, which possesses lower cata-
lytic activity.3 Hence immobilization of metal nanoparticles into
a suitable rigid matrix has come into practice. In particular, Pd
nanoparticles are stabilized by dispersing them in porous
materials, ionic liquids, surfactants, dendrimers, functional-
ized polymers, biopolymers, various ligands, silica in super-
critical CO2, etc.4,5 The stabilizing medium serves as
a
supporting material for protecting the nanoparticles from
aggregation, provides the desired chemical interface between
the nanoparticles and substrate, prevents the metal from being
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia 741235, W.B., India.
E-mail: manir65@rediffmail.com; Fax: +91-33-2582-8282; Tel: +91-33-2582-8750
bDepartment of Chemistry, IIT Roorkee, Roorkee 247667, Uttarakhand, India. E-mail:
kamalfcy@iitr.ac.in; Tel: +91-1332285796
In this context, mesoporous organic polymers carrying the
reactive functional groups at the surface of the mesopores12–16
can provide an ideal tethering agent for the active metals to
bind at its surface strongly. Due to extensive cross-linking,
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07554d
‡ These authors contributed equally to this manuscript.
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polymeric materials are highly desirable material for long term that exhibits superb activity in catalyzing the aryl bromides for
stabilization of the entrapped metal centres, which reduces the the synthesis of aryl amides and this novel Pd catalyst is also
possibility of leaching of the metal under reaction conditions.17 highly effective for the synthesis of a-ketoamides from aryl
On the other hand, the palladium-catalyzed carbonylation of iodides under phosphine free condition. The high activity of
aryl halides in the presence of carbon monoxide is an important mPMF–Pd0 is attributed to its condensed network structure
methodology for the preparation of carbonyl containing deriv- which contains large number of nitrogen atoms. Signicant
atives.18 The amides and a-ketoamides are important targets in amounts of nitrogen have been incorporated into comparable
organic synthesis because of their ability to serve as building inorganic framework materials25 and this is found to be bene-
blocks in a wide variety of functional group transformations.19 cial for the stabilization of Pd nano.
They have also been found as a substructure in numerous
bioactive natural products and pharmaceutically interesting
2. Experimental section
2.1. Chemicals
compounds.20 Therefore, the development of methods that can
construct these two classes of nitrogen containing compounds
in an efficient and regioselective manner from readily accessible
substrates continues to be a hot research topic.
Melamine and paraformaldehyde were purchased from Sigma-
Aldrich. Pd(OAc)2 was purchased from Universal Chemicals,
Several strategies have been reported in the literature to
achieve their synthesis21 among which the transition metal
catalysis approaches have proven to be the most effective for
providing access to the desired compounds in good yields and
high selectivities. Although, few examples have been reported
on mono and double carbonylation of aryl halides with amines
for the synthesis of amides and a-ketoamides respectively.22
Most of the works have been realised in the presence of
homogeneous Pd catalysts making the separation/purication
and recovery of the expensive metal catalysts very difficult, or
even impossible. These environmental and economic concerns
associated with the contamination issues of biologically active
compounds are particularly important when dealing with large
scale-synthesis and industrial processes. Although few exam-
ples have been reported for mono carbonylation, reports on
double carbonylation of aryl halides with amine catalysed by
heterogeneous palladium nano materials are still rare.23 Less
expensive aryl bromides are more attractive substrates for the
amino carbonylation but more expensive aryl iodides are
generally the substrate of choice in amino carbonylation cata-
lyzed by homo or heterogeneous Pd catalyst and most of these
reported methods require harsher conditions.22 It is thus
desirable to develop an efficient, one-pot, cost effective, phos-
phine free and green methodology to avoid all these difficulties.
Herein, we report a novel mesoporous poly-melamine-
formaldehyde (mPMF) material24 supported Pd nano catalyst
Scheme 2 Schematic diagram showing the formation of mesoporous
mPMF–Pd0 nanocatalyst.
Fig. 1 Powder XRD pattern of mPMF–Pd0 material.
Scheme 1 Schematic diagram showing the formation of mesoporous polymer.
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India. Sodium borohydride (NaBH4) was obtained from Spec- integrating sphere attachment for solid samples. Thermogra-
trochem, India and used as received. All other reagents and vimetric analysis (TGA) was carried out using a Mettler Toledo
substrates were purchased from Merck, India and were used TGA/DTA 851e. Surface morphology of the samples was
without further purication. Solvents were dried and distilled measured using a scanning electron microscope (SEM) (ZEISS
through standard procedure.
EVO40, England) equipped with EDX facility. Specic surface
area of the sample was measured by adsorption of nitrogen gas
at 77 K and applying the Brunauer–Emmett–Teller (BET)
calculation. Prior to adsorption, the samples were degassed at
250 ꢁC for 3 h. Powder X-ray diffraction (XRD) patterns of
different samples were analyzed with a Bruker D8 Advance X-ray
diffractometer using Ni-ltered Cu Ka (l ¼ 0.15406 nm)
2.2. Physical measurements
The FT-IR spectra of the samples were recorded from 400 to
4000 cmꢀ1 on a Perkins Elmer FT-IR 783 spectrophotometer
using KBr pellets. UV-vis spectra were taken using a Shimadzu
UV-2401PC doubled beam spectrophotometer having an
Fig. 2 HR-TEM images of mPMF–Pd0 material.
Fig. 3 TEM-EDS image of mPMF–Pd0 material.
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Fig. 4 SEM images of mPMF–Pd0 material.
radiation. Transmission electron microscopy (TEM) images of
the mesoporous polymer were obtained using a JEOL JEM 2010
transmission electron microscope operating at 200 kV.
Elemental analysis was carried out in a 2400 Series-II CHN
Analyzer, Perkin–Elmer, USA. NMR spectra were recorded on a
Varian Mercury plus NMR spectrometer in pure deuterated
solvents.
2.4. Synthesis of colloidal Pd nanoparticles
In a typical synthesis, 150 mg of Pd(OAc)2 was added to 10 mL of
distilled water containing 0.5 mmol Tris-buffer and was stirred
for 2 min. Then, aqueous solution of NaBH4 (37 mg, 1 mmol)
was added drop-wise under stirring. The stirring was continued
for another 10 min, and the resulting nanocolloid was stored at
ꢁ
4 C.
2.5. Synthesis of mPMF–Pd0 nanocatalyst
2.3. Synthesis of mesoporous poly-melamine-formaldehyde
polymer (mPMF)
A total of 600 mg of mPMF was dispersed in 10 mL of TRIS-
stabilized Pd-NPs and stirred for 1 h at room temperature.
The colour of the colloidal nanoparticles gradually disappeared
while stirring. The supernatant solution was colourless aer 1 h
of stirring at room temperature, whereas the colour of mPMF
changed to black, indicating the loading of Pd-NPs onto the
surface of mPMF. Aer centrifugation, black coloured Pd-NP
containing mesoporous polymer mPMF–Pd0 was obtained.
This mesoporous material was washed further with copious
amounts of water and dried at room temperature. Pd(0) loading
to the mPMF was determined to be 10.0% by AAS (Scheme 2).
Melamine (0.378 g, 3 mmol) and paraformaldehyde (1.8 eq.,
0.162 g, 5.4 mmol) were mixed with 3.36 mL (overall concen-
tration of 2.5 M) of dimethyl sulfoxide (DMSO) in a 15 mL Teon
container secured in a steel reactor. The reaction mixture was
heated to 120 ꢁC in an oven for 1 h. The reactor was then
carefully removed from the oven for stirring on a magnetic
stirrer plate to obtain a homogeneous solution. It was then
heated in the oven to 170 ꢁC for 72 h. The reaction was allowed
to cool to room temperature, and the obtained solid was
crushed, ltered, and washed with DMSO, acetone, tetrahy-
drofuran (THF) and CH2Cl2. The resulting white solid was dried
ꢁ
under vacuum at 80 C for 24 h (Scheme 1).
2.6. General procedure of mono-carbonylation of aryl
bromides with morpholine
A 50 mL high-pressure reactor was charged with toluene
(3.0 mL), K3PO4 (2.5 equiv.), aryl bromides (1 mmol), morpho-
line (1.5 mmol), mPMF–Pd0 (25 mg) and CO (3 atm) before
heating at 80 ꢁC for 10 h. Aer cooling the reactor, the CO was
vented from the reactor and the desired compound was puried
Fig. 5 Raman spectroscopy of mPMF–Pd0.
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Fig. 6 XPS of mPMF–Pd0.
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Scheme 3 Mono and double carbonylation of aryl halide with morpholine.
by chromatography on silica gel using a pentane/CH2Cl2 eluent
system. All the prepared compounds were conrmed by
1H NMR and 13C spectrum.
Raman spectroscopy. In addition to the optical spectroscopy,
Raman spectroscopy also provides useful information
regarding the interaction of PdNPs and mesoporous mPMF.
2.7. General procedure of double-carbonylation of aryl
iodides with amines
A 50 mL high-pressure reactor was charged with DMF (3.0 mL),
K2CO3 (2.5 equiv.), aryl iodides (1 mmol), morpholine
(1.5 mmol), mPMF–Pd0 (25 mg) and CO (20 atm) before heating
ꢁ
at 90 C for 12 h. Aer cooling the reactor, the CO was vented
Scheme 4 Mono carbonylation of bromobenzene with morpholine.
Table 1 Effect of base and solvent on mono carbonylationa
from the reactor and the desired compound was puried by
chromatography on silica gel using a pentane/CH2Cl2 eluent
system. All the prepared compounds were conrmed by 1H and
13C NMR spectrum.
Entry
Base
Solvent
Conversion (%)
3. Results and discussion
3.1. Characterization of Pd–mPMF material
1
2
3
4
5
6
7
8
9
K3PO4
K3PO4
K3PO4
K2CO3
K2CO3
K2CO3
Et3N
Toluene
Dioxane
DMF
Toluene
Dioxane
DMF
Toluene
Dioxane
DMF
92
37
45
86
28
31
21
NR
NR
X-ray diffraction. In Fig. 1 powder X-ray diffraction pattern of
mPMF–Pd0 is shown. The mesoporous polymer, mPMF shows
one broad diffraction peak cantered at 2q ¼ 21.70, which
correspond to characteristic peak of the mPMF. In the XRD
pattern of the mPMF–Pd0 shows the additional diffraction
peaks at 2q ¼ 38.80, 44.85 and 67.00 degree are attributed to Pd
nano.26
Et3N
Et3N
a
Reaction conditions: bromobenzene (1.0 mmol), morpholine (1.5
mmol), base (2.5 mmol), CO (3 atm), mPMF–Pd0 (25 mg), toluene (3.0
HR-TEM analysis. Fig. 2 demonstrates HR-TEM images of
mL), 80 ꢁC, 10 h.
Pd-NPs
supported
on
mesoporous
poly-melamine-
formaldehyde. HR-TEM images (Fig. 2a and b) suggested a
foam-like interconnected mesoporous network structure con-
tained high electron density dark spots of size 1.5 to 2 nm
(ref. 26) throughout the specimen, which are spherical in nature
(Fig. 2c). These spherical particles are assigned to
Pd-nanoparticles in mPMF–Pd0. TEM-EDS from the obtained
nanomaterials (Fig. 3) provided the presence of the expected
elements in the structure of the catalyst, namely palladium,
nitrogen and carbon.
SEM analysis. The scanning electron micrograph of the
mPMF–Pd0 is shown in Fig. 4. The FE-SEM image (Fig. 4A)
indicates uniform submicronsized spherical morphology of the
polymer mPMF–Pd0 material. Spherical particles get aggregated
among themselves to form large assembly of particles and these
are inter connected with each others (Fig. 4B).
Table 2 Effect of time and catalyst amount on mono carbonylationa
Entry
Time (h)
Catalyst (mg)
Yield (%)
1
2
3
4
5
6
7
8
5
8
25
25
25
25
10
15
20
30
34
68
92
92
29
45
67
92
10
12
10
10
10
10
a
Reaction conditions: bromobenzene (1.0 mmol), morpholine (1.5
mmol), K3PO4 (2.5 mmol), CO (3 atm), mPMF–Pd0 (25 mg), toluene
(3.0 mL) at 80 ꢁC for 10 h.
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Table 3 mPMF–Pd0 catalyzed mono carbonylation of aryl bromides with morpholinea
Entry
Substrate
Product
Yield (%)
1
92
2
3
4
94
71
83
5
6
7
97
88
70
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Table 3 (Contd.)
Entry
Substrate
Product
Yield (%)
8
89
9
88
10
85
11
92
12
91
13
80
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Table 3 (Contd.)
Entry
Substrate
Product
Yield (%)
14
82
15
16
17
90
92
94
18
95
a
Reaction conditions: aryl bromide (1.0 mmol), morpholine (1.5 mmol), K3PO4 (2.5 mmol), CO (3 atm), mPMF–Pd0 (25 mg), toluene (3.0 mL) at
80 ꢁC for 10 h.
Raman spectra of the mPMF–Pd0 shows two well documented D XPS spectrum of mPMF–Pd0 was shown in the Fig. 6. Two main
and G band at 1362 and 1549 cmꢀ1 respectively (Fig. 5). The D peaks related to Pd 3d5/2 and Pd 3d3/2 is centered at the XPS
band at 1362 cmꢀ1 is attributed due to the defect or disorder in binding energies of 337.07 eV and 342.57 eV respectively in the
carbon atom and G band at 1549 cmꢀ1 is attributed due to sp2 in XPS spectrum of mPMF–Pd0. This XPS data clearly reveals that
plane vibration of carbon atom. Raman spectroscopy of meso- the Pd(0) species have been generated during the course of
porous mPMF–Pd0 showed different intensity peaks from each reduction, which are embedded uniformly throughout the
component. The peak at 482 cmꢀ1 corresponded to Pd nano and mesoporous material. These binding energy values agree well
the peak at 1057 cmꢀ1 corresponded to the T band.27
with the values for palladium(0) state accounted into the liter-
XPS analysis. X-ray photoelectron spectroscopy provides the ature. No peak corresponding to Pd2+ in the XPS spectrum was
valuable information regarding the oxidation state of Pd. The found, which discloses the fact that the palladium(II) has been
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completely reduced into metallic palladium nanoparticles observed that inorganic bases are more effective than organic
during the reduction technique.28
base like Et3N. Within a short time the reaction proceeded in
high yield in presence of K3PO4 or K2CO3. We found that using
K3PO4 as base in toluene at 80 ꢁC gave the 92% conversion. Et3N
was not as effective as K3PO4, only affords to low yield of
coupling product. To verify the solvent effect, a series of solvents
were investigated for the mono carbonylation of bromobenzene
with morpholine as the model reaction. The reaction conducted
in nonpolar solvent medium like toluene, was found to be the
most effective. The use of DMF or dioxane as solvents led to
slower reactions. Consequently, toluene was chosen as the
medium of choice for this mono carbonylation.
This coupling reaction was found to be highly sensitive to the
catalyst amount. Profound increase in the yield of the desired
product was observed when the catalyst amount was increased
from 10 to 25 mg; maximum yield was obtained at 25 mg
mPMF–Pd0. Also the reaction was carried out for different time
ranging from 5 h to 12 h and it was found that at 10 h the
conversion was 92% at given conditions.
3.2. Catalytic activities
Since mesoporous supported palladium nano exhibit high
catalytic activity in a wide range of industrially important
processes and have been extensively studied for c–c coupling,
thus we decided to investigate the catalytic activity of the
mPMF–Pd0 in the eld of carbonylation reactions. Carbonyla-
tion of aryl halides with secondary amines usually gives rise to a
mixture of mono and dicarbonylated products. Chemo-
selectivity of the reaction is greatly dependent on the reaction
conditions: solvent, base, nature of the nucleophile, the catalyst
and most importantly the pressure of CO (Scheme 3).
Mono carbonylation of aryl bromides for the synthesis of
aryl amides. To test the catalytic activity of the present catalyst,
the carbonylation of aryl bromides were performed (Scheme 4).
In this reaction aryl bromide and secondary amine were con-
verted to aryl amide under the reaction conditions. Quantitative
conversion of aryl bromide to aryl amide can be achieved by
using the mPMF–Pd0 catalyst. The carbonylation reaction of aryl
bromides with morpholine was run in a 50 mL autoclave at 80 ꢁC
under 3 atm carbon monoxide atmospheres for 10 h reaction
time. In our initial studies, the bromobenzene was chosen for the
optimization of the mono carbonylation reaction.
Hence, the nalized reaction conditions were the following:
ꢁ
base, K3PO4; temperature, 80 C; solvent, toluene; time, 10 h;
and 3 atm of CO pressure. These reaction parameters were then
successfully applied for mono carbonylation of a variety of aryl
bromides with morpholine in toluene. The experimental results
are summarized in Table 3. With a dened protocol in hand, the
substrate scope with various aryl bromides was explored. As
shown in Table 3, the protocol well tolerated diverse electronic
and steric substituents of the aryl bromides, as well as hetero-
cyclic substrates, all resulting in good to excellent yields. The
relative position of substituents slightly impacted the coupling
efficiency: p-bromotoluene resulted in a higher yield than its m
or o-analogues (Table 3, entries 2–4). The substitution of elec-
tron withdrawing and electron donating groups on the phenyl
ring of aryl bromides did not have any appreciable inuence on
the outcome of the reaction. Aryl bromides bearing either
electron-donating or electron-withdrawing substituents, affor-
ded the corresponding cabonylative products in good to excel-
lent yields. To our surprise, we found that Strong electron-
donating groups such as –OMe and –NMe2 did not inuence
the activity signicantly compared to electron withdrawing
groups such as –F and –CF3 (Table 3, entries 8 and 9). To our
delight, electron-decient bromoarenes also provided good to
excellent yields (Table 3, entries 8–11). As expected, heterocyclic
and bulky bromoarenes all afforded excellent yields (Table 3,
entries 15–18).
The performance of palladium-catalyzed carbonylation
reaction is known to be governed by a number of reaction
parameters. To nd the appropriate reaction conditions various
bases and solvents were screened (Table 1). In order to identify
effective conditions for promoting the monocabonylation, the
reaction was carried out under various catalyst amount and
reaction time (Table 2).
The inuence of base on the catalytic performance of this
system was investigated by employing various bases. It was
Double carbonylation of aryl iodides for the synthesis of a-
ketoamides. In continuation of our interest in carbonylation
reactions, we have extended the application of our newly
prepared mPMF–Pd0 in eld of double carbonylation reactions
for the synthesis of a-ketoamides from aryl iodides. Our study of
double carbonylation reaction began on the basis of the work
reported by Marie et al.,29 the reaction was reported in DMF with
ꢁ
iodobenzene, morpholine and Et3N as the base at 60 C under
40 bar of CO. The coupling was carried out in the presence of 1
mol% [Pd] using PdCl2(PPh2)2@SBA-15 as the model catalyst.
Under these conditions, high selectivity, up to 74%, for the
double carbonylation product was reported. However, the
Fig. 7 Effect of CO pressure on carbonylation of iodobenzene with
morpholine. Reaction conditions: iodobenzene (1.0 mmol), morpho-
line (1.5 mmol), K2CO3 (2.5 mmol), mPMF–Pd0 (25 mg), DMF (5.0 mL)
at 90 ꢁC for 12 h.
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Table 4 mPMF–Pd0 catalyzed double carbonylation of aryl iodides with aminesa
Entry
Aryl iodide
Amine
Product
Yield (%)
1
98
2
92
3
87
4
89
5
98
6
95
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Table 4 (Contd.)
Entry
Aryl iodide
Amine
Product
Yield (%)
7
96
8
88
9
95
10
94
a
Reaction conditions: aryl iodides (1.0 mmol), amines (1.5 mmol), base (2.5 mmol), CO (20 atm), mPMF–Pd0 (25 mg), DMF (5.0 mL) at 90 ꢁC for
12 h.
method requires high CO pressure and long reaction time (1 atm to 25 atm) keeping the other parameters xed. The
(48 h). Since high-pressure conditions make them expensive for reaction mixtures were collected aer completion of reaction at
industry and difficult to handle in the laboratory and in different pressure. The products were identied and quantied
industry, this observation makes the present carbonylation by GC. All obtained results from each run were plotted in Fig. 7.
catalyst even more attractive. To improve this preliminary result It has been found that the selectivity of double carbonylation
and reaction conditions we have performed the similar reaction gradually increased at high pressure region.
catalyzed by our mPMF–Pd0 in DMF at 90 ꢁC and under the
To explore the generality and scope of the double carbonyl-
20 atm CO pressures. Delightfully, the desired product was ation reaction, the reaction of various aryl iodides and amines
obtained in excellent yield (98%). To optimize the reaction was carried out under optimum conditions (aryl iodide, 1.0
conditions we have screened various solvents and bases and mmol; K2CO3, 2.0 mmol; amines, 5.0 mmol; DMF, 5.0 mL;
maximum yield was obtained for K2CO3 in DMF. To get mPMF–Pd0 25 mg; P(CO) ¼ 20 atm, 90 ꢁC, 12 h) and the results
maximum selectivity towards double carbonylations we have are summarized in Table 4. Various aryl iodides afforded the
varied the CO pressure from 1 atm to 25 atm and maximum corresponding a-ketoamides in moderate to good yields. On the
double carbonylated yield was obtained at 20 atm CO pressure other hand, the steric bulkiness of amine has a good conversion
(Fig. 7). The reaction was carried out by varying the CO pressure and selectivity (Table 4, entries 2–4). Less basic amines such as
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halide to the Pd(0) species but the later course of the catalytic
reaction may vary depending on the nucleophiles and reaction
conditions employed. The most oen assumed process involves
migratory insertion of a coordinated carbon monoxide to give
an acylpalladium halide. It is found that when CO pressure is
low (3 atm) then the acylpalladium halide species reacts further
with a nucleophile (amine) and a base to liberate the amide
(mono carbonylative product). But in the presence of high CO
pressure (20 atm) acylpalladium halide species further coordi-
nates with CO and then it reacts with amine and base to give a-
ketoamide (double carbonylative product).
Catalyst reusability. Recovery and catalyst reuse are impor-
tant issues in the heterogeneous catalysis. We studied the
reusability of the present heterogeneous palladium nano cata-
lyst in the mono and double carbonylation of aryl halide with
morpholine under the optimized reaction conditions. Aer
completion of the reactions, the catalyst was recovered by
simple ltration and washed with ethyl acetate followed by
acetone then dried at 40 ꢁC. The recovered catalyst was
employed in the next run with further addition of substrates in
appropriate amount under optimum reaction conditions. The
catalyst show almost same activity up to six reaction cycles
(Fig. 8). No catalyst deterioration was observed, conrming the
high stability of the heterogeneous catalyst under the reaction
conditions.
Scheme 5 Proposed reaction mechanism of palladium catalyzed
carbonylation of aryl halide.
4. Conclusions
In conclusion, we have reported the preparation and charac-
terization of a highly stable, reusable nano catalyst, mPMF–Pd0
and its successful applications for the mono and double amino
carbonylations of aryl halides with morpholine, exploiting a
simple setup and reaction conditions. Both electron-rich and
electron-decient aryl halides could be applied and the nature
of the base, temperature and the solvent system proved crucial
for these transformations. The present system is highly air and
moisture stable and the catalyst can be synthesized readily from
inexpensive and commercially available starting materials.
Moreover, the catalyst was reused for six consecutive cycles with
consistent catalytic activity. Further work is in progress to
broaden the scope of this catalytic system for other organic
transformation. A protocol, which allows expansion of this
chemistry to other aromatic halides and exact mechanistic
pathway of this reaction are currently under investigation.
Fig. 8 Catalyst reusability test of the mPMF–Pd0 catalyst.
aniline did not afford any double carbonylated product. It is
noteworthy that the a-ketoamides were obtained with good
selectivities when using bulky amines such as 1,2,3,4-tetrahy-
droisoquinoline as nucleophilic reagents (Table 4, entries 3 and
6–10). In an attempt to extend the substrate scope to aryl
bromides, we also tested the double carbonylation reaction of
bromobenzene with diethylamine. However, no signicant
conversion was observed.
Proposed mechanism. The full mechanistic details of this
transformation have not been determined; however a plausible
reaction mechanism is shown in Scheme 5 based on previous
reports.30 Palladium nano particles are stabilized by nitrogen
containing mesoporous mPMF. These Pd nano particles are the
active centres which are responsible for both the carbonylation
reactions. It is generally accepted that an organopalladium
halide is produced by oxidative addition of an organic aryl
Acknowledgements
SMI acknowledges CSIR and DST, New Delhi for nancial
support. RAM acknowledges UGC, New Delhi, India for his
Maulana Azad National Fellowship (F1-17.1/2012-13/MANF-
2012-13-MUS-WES-9628/SA-III). ASR acknowledges CSIR, New
Delhi, India for his senior research fellowship.
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