Atmospheric pressure aminocarbonylation of aryl iodides using palladium nanoparticles supported on MOF-5

Article abstract of DOI:10.1039/c2cc16808a

An efficient amide synthesis by atmospheric pressure aminocarbonylation using palladium nanoparticles supported on MOF-5 is reported. Interestingly, only 0.5 wt% palladium loading was required to achieve high yields. The catalyst is recyclable and offers negligible palladium leaching.

Full text of DOI:10.1039/c2cc16808a

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COMMUNICATION
Atmospheric pressure aminocarbonylation of aryl iodides using palladium
nanoparticles supported on MOF-5w
Tuan T. Dang, Yinghuai Zhu, Subhash C. Ghosh, Anqi Chen, Christina L. L. Chai and
Abdul M. Seayad*
Received 3rd November 2011, Accepted 2nd December 2011
DOI: 10.1039/c2cc16808a
An efficient amide synthesis by atmospheric pressure amino-
carbonylation using palladium nanoparticles supported on MOF-5
is reported. Interestingly, only 0.5 wt% palladium loading was
required to achieve high yields. The catalyst is recyclable and offers
negligible palladium leaching.
catalysts to carry out the reaction under mild conditions and
to address the contamination of the leached metal catalyst
impurities carried over to the product. Here we report palladium
nanoparticles on MOF-5 with low levels of palladium loading as
an efficient heterogeneous catalyst for the atmospheric pressure
aminocarbonylation of aryl iodides.
Metal–organic frameworks (MOFs) are a class of emerging
porous materials having promising applications in gas absorption
and separation, drug delivery and catalysis.7 Our interest in
exploring MOF-5 as a catalyst support is based on the promising
catalytic performance reported for metal nanoparticles supported
on MOFs in various cross coupling and hydrogenation reactions.8
Initial screening studies to find a suitable support for the
palladium metal catalyst were carried out by using p-iodotoluene
and morpholine as coupling partners in the presence of Et3N as
the base at atmospheric pressure of CO (Table 1).
Amides are one of the most important classes of compounds
necessary for life as well as for the chemical and pharmaceutical
industries.1 Classical methods of amide bond formation from the
corresponding carboxylic acids and amines are generally atom-
inefficient as these necessitate the use of superstoichiometric
amounts of coupling reagents as well as several protection–
deprotection strategies, thereby producing large amounts of toxic
waste materials posing environmental hazards.2 Moreover, the
carboxylic acid coupling partner needs to be synthesized beforehand
from the available feedstock. Hence, efficient catalytic synthesis of
amides from less toxic and readily available feedstock avoiding poor
atom economy reagents has been a challenge for both academia
and industry. Recently the ACS GCI Pharmaceutical Roundtable
has also projected amidation as the most preferred reaction that
requires efficient, green and sustainable alternatives.3 In this context,
aminocarbonylation offers an elegant and environmentally
attractive single-step catalytic method for the synthesis of
amides directly from inexpensive and easily available feedstock
such as carbon monoxide and organic halides in the presence of
suitable amines.4
Silica as a support provided only poor yield of the amidation
product by using 3 wt% of palladium at 80 1C after 8 h of
reaction (Table 1, entry 1). However, Al2O3 and AlO(OH) were
found to be promising as supports and gave up to 29% and
34% yields, respectively, under similar conditions (Table 1,
entries 2 and 6). To our delight the reaction using a palladium
catalyst supported on MOF-5 (3 wt%) gave 38% yield under
similar reaction conditions (Table 1, entry 7) which was further
increased up to 92% by increasing the reaction temperature to
120 1C and by using K2CO3 as the base (Table 1, entry 18).
Other common bases such as Na2CO3, Cs2CO3, NaHCO3,
K3PO4, and pyridine showed relatively low yields (21–56%)
(Table 1, entries 11–15) under these conditions. The MOF-5
support was found to be unstable with KOH as the base and no
appreciable amount of aminocarbonylation was observed
(Table 1, entry 17). Further optimization of palladium loading
on MOF-5 on the catalytic activity revealed that Pd loading can
be reduced to 0.5 wt% without significant changes in catalytic
activity (Table 1). Thus near quantitative isolated yield of 3a
was achieved by using 15 mg of 0.5 wt% PdNP–MOF-5
catalyst at 120 1C for 8 h at atmospheric pressure of CO
(Scheme 1).
Aminocarbonylation using homogeneous palladium catalysts
were reported to form various amide products in high selectivities
under moderate temperatures (90–120 1C) and CO pressure
conditions (1–15 bar).4 In order to address the catalyst-product
separation, heterogeneous palladium catalysts were reported by
Csajagi and co-workers5 under continuous flow conditions (100 1C,
30 bar) and Petricci and co-workers6 under microwave conditions
(130 1C, 9 bar) in the presence of 10 wt% Pd/C. However the main
challenges are still to develop active and stable heterogeneous
Institute of Chemical and Engineering Sciences (ICES), 8 Biomedical
Grove, #07-01 Neuros, Singapore 138665.
E-mail: abdul_seayad@ices.a-star.edu.sg; Fax: +65-64642102;
Tel: +65-67998527
w Electronic supplementary information (ESI) available: XRD, XPS,
ICP, SEM, TEM images of MOFs and supported catalysts, NMR
data of the amide products. See DOI: 10.1039/c2cc16808a
MOF-5 was prepared using a literature procedure9 under
anhydrous conditions and the palladium nanoparticles (PdNPs)
were supported on the surface of MOF-5 using a simple
modified procedure in which the adsorbed K2PdCl4 was reduced
using NaBH4 in dry DMF.10 The sharp peaks on the XRD
c
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Chem. Commun., 2012, 48, 1805–1807 1805

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Table 1 Optimization of aminocarbonylation
Weight/
mg
Temp/ Time/ Yielda
Entry Catalyst (wt%)
Base
1C
h
(%)
1
2
3
4
5
6
7
8
Pd/SiO2 (3)
Pd/Al2O3 (3)
Pd/Celite (3)
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
15
15
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
K3PO4
NaOAc 120
NaHCO3 120
Pyridine 120
Na2CO3 120
Cs2CO3 120
80
80
80
80
80
80
80
8
8
8
8
8
8
8
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
29
16
22
14
34
38
18
31
47
41
43
56
21
28
50
—
92
88
84
83
74
49
26
Fig. 1 TEM image of 0.5 wt% PdNP–MOF-5. The Pd particles have
a size distribution of 3 to 12 nm (see ESIw).
Pd/4A-MS (3)
Pd/Florisil (3)
Pd/AlO(OH) (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (3)
Pd/MOF-5 (1)
Pd/MOF-5 (1)
Pd/MOF-5 (0.5)
Pd/MOF-5 (0.5)
The BET and Langmuir specific surface areas are 718 m2 gÀ1 and
1043 m2 gÀ1 (see ESIw).9 The surface area of Pd/MOF-5 samples
was lower than that of the pure MOF-5, which may be due to
partial structural decomposition. No significant change in surface
morphology was observed in SEM images of MOF-5 and
Pd/MOF-5 samples (see ESIw). The HR-TEM images (Fig. 1)
indicated that palladium exists as nanoparticles, which are seen as
gray circles ranging from 3 to 12 nm (see ESIw). The PdNPs were
well dispersed on the surface of MOF-5 and no obvious aggrega-
tion was observed for Pd/MOF-5 with low levels of palladium
loading (0.25 and 1.0 wt%). On the other hand, aggregates
of PdNPs were observed with higher palladium loading of
>1 wt% (see ESIw).
80
9
100
120
120
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
KOH
120
120
120
120
120
120
120
120
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
7.5
Pd/MOF-5 (0.25) 30
Pd/MOF-5 (0.25) 15
With the optimized reaction conditions in hand, we extended
the scope of aminocarbonylation to different aryl iodides with
morpholine 2a as the amine (Scheme 1). In general, the amino-
carbonylation of various aryl iodides with morpholine afforded
the desired products 3a–3i in good to excellent yields. Iodobenzene
gave up to 98% isolated yield under optimized conditions. Similar
results were achieved with p-Cl, p-Me and m-Me substituted aryl
iodides (96%, 97% and 94% respectively), while slightly lower
yields were obtained with p-OMe, m-OMe and p-NO2 substituted
aryl iodides as given in Scheme 1. Sterically demanding ortho
substituted substrates such as o-iodotoluene required higher
catalyst loading and prolonged reaction time to obtain up to
72% yield of 3c. 1-Naphthyl iodide also reacted well giving up
to 83% yield of the amide 3i.
a
Yields were calculated by 1H-NMR using mesitylene as internal standard.
The effect of various amines on the aminocarbonylation was
investigated using p-iodotoluene 1a as the standard aryl
iodide. With cyclic amines such as piperidine and pyrrolidine,
the desired amides 3j and 3k were obtained in 95% and 84%
yields respectively. In the latter case 3 equivalents of pyrrolidine
and an extended reaction time of 12 h were required. Acyclic
secondary and primary amines, e.g. N-methylbenzylamine and
benzylamine also reacted well giving 80% and 73% yields of the
corresponding amides 3l and 3m, respectively, after 12 h of
reaction. Branched and linear aliphatic amines were also active
as substrates giving 3n and 3o in very good yields of 82–84%.
Aromatic primary amines showed lower reactivity and required
higher catalyst loading of 1 wt% and extended reaction time of
24 h. Unsubstituted aniline gave 85% yield of 3p, while lower
yields (3q, 74%; 3r, 71%) were observed with para substituted
anilines. Sterically bulky aniline such as 2,6-dimethylaniline
gave a promising yield of 53% (3t) after 36 h of reaction
(Scheme 2). It is noteworthy that indole also forms the
corresponding amide 3u in 81% yield under these conditions.
Scheme 1 Aminocarbonylation of various aryl iodides using morpholine
as the amine.
pattern showed good crystallinity of the MOF-5 lattice structure
similar to the literature reports (see ESIw).9–13 The XRD
pattern of 0.5–1 wt% Pd/MOF-5 also showed no significant
difference to the XRD pattern of pure MOF-5. The N2
adsorption isotherm measurement carried out at 77 K indicated
that the MOF-5 material has a type I isotherm of micropores.
c
1806 Chem. Commun., 2012, 48, 1805–1807
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attractive aminocarbonylation method for the synthesis of
various amides from readily available feedstock. Further
studies are currently in progress to expand the scope of this
catalyst.
This work was funded by ‘‘Singapore-EDB-GSK Partnership for
Green and Sustainable Manufacture’’ and the Institute of Chemical
and Engineering Sciences, Agency for Science, Technology
and Research, Singapore.
Notes and references
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Scheme 2 Aminocarbonylation of p-iodotoluene using various amines.
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achieving 94% yield under the standard conditions and was
successfully scaled up15 fourfold as shown in Scheme 3.
Recycle studies at atmospheric pressure of CO showed a
progressive decrease in catalytic activity. However, up to four
recycles of the 0.5 wt% PdNP–MOF-5 catalyst was efficiently
carried out at 2 atm of CO without any appreciable change in
the yield of 3a at 120 1C.15 It is remarkable that the Pd
leaching was negligible (o2 ppm in each recycle)16 and no
aminocarbonylation activity was observed using the super-
natant liquid after the isolation of the solid catalyst.
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In conclusion, Pd nanoparticles supported on MOF-5 is an
efficient catalyst for the atmospheric pressure aminocarbonyl-
ation of iodoarenes. Only low levels of palladium loading
(0.5–1.0 wt%) are required to achieve high yields of the amide
products. The catalyst can be efficiently recycled and offer a
negligible amount of palladium leaching. Thus, this hetero-
geneous catalyst has significant advantages over the corres-
ponding homogeneous catalysts as it is not only applicable for
the synthesis of a wide range of amides under mild conditions
but also reduces the residual palladium in the isolated product.
Hence, the present catalyst system offers an environmentally
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15 Scale up was conducted in a parr pressure reactor. See ESIw for
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16 See ESIw for the details of recycle studies.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1805–1807 1807