Ligand-free Pd(0)/SiO2-catalyzed aminocarbonylation of aryl iodides to amides under atmospheric CO pressure

Article abstract of DOI:10.1039/c7ra04985d

An efficient and facile route for CO-based carbonylation of aryl iodides with amines to synthesize amides has been established by using SiO₂ supported Pd(0) as the catalyst in a mild basic environment (K₂CO₃). This ligand-free heterogeneous reaction model can afford amide products in good to excellent yields (up to 99%) under atmospheric CO pressure and moderate temperature. The supported catalyst also displayed a broad substrate scope, good functional group tolerance and good recyclability. These features render the as-provided carbonylation approach sustainable and applicable in organic synthesis.

Full text of DOI:10.1039/c7ra04985d

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Ligand-free Pd(0)/SiO2-catalyzed
aminocarbonylation of aryl iodides to amides under
Cite this: RSC Adv., 2017, 7, 37200
atmospheric CO pressure†
*
Qinhua Hu, Lele Wang, Chen Wang, Yubin Wu, Zhengxin Ding and Rusheng Yuan
An efficient and facile route for CO-based carbonylation of aryl iodides with amines to synthesize amides
has been established by using SiO2 supported Pd(0) as the catalyst in a mild basic environment (K2CO3). This
ligand-free heterogeneous reaction model can afford amide products in good to excellent yields (up to
99%) under atmospheric CO pressure and moderate temperature. The supported catalyst also displayed
a broad substrate scope, good functional group tolerance and good recyclability. These features render
the as-provided carbonylation approach sustainable and applicable in organic synthesis.
Received 3rd May 2017
Accepted 23rd July 2017
DOI: 10.1039/c7ra04985d
rsc.li/rsc-advances
atmospheric pressure aminocarbonylation of aryl iodides using
palladium nanoparticles supported on MOF-5 at 120 ꢀC.20
Introduction
Adolfsson group have used a highly dispersed nanopalladium
catalysts supported on mesocellular foam for the amino-
carbonylation reaction of aryl iodides in the presence of 1 atm of
carbon monoxide.21 Cai group have developed MCM-41-
supported bidentate phosphine palladium(II) complex, silica-
supported poly-g-diphenylarsinopropylsiloxane palladium
complex and SiO2-supported sulfur and phosphine mixed
bidentate palladium complex as catalysts for the carbonylation
of aryl haliꢀdes under atmospheric pressure of carbon monoxide
at 90–130 C.22–24 It should be mentioned that the addition of
ligands or cocatalysts was required in most supported palla-
dium catalyzed carbonylation reactions at ambient CO
pressure.
Herein, we report an efficient and ligand-free approach to
synthesize aromatic amides by using Pd(0) supported on silica
as a robust catalyst for the carbonylation of aryl iodides with
amines at atmospheric CO pressure and moderate tempera-
tures. This recycling catalytic system provides a convenient
access to a series of amides from a wide range of aryl iodides
and amines in good to excellent yields. When amine was
replaced by methanol and styrene, the formation of esters and
a,b-unsaturated ketones can also be realized through this car-
bonylative process.
Aromatic amides are fundamental structural motifs in
numerous natural products, materials, agrochemicals, and
pharmaceutical agents.1,2 An effective and straightforward
method to synthesize amides is transition-metal-catalyzed
carbonylation of aryl halides using CO gas as a carbonyl
group source because CO is not only inexpensive and readily
available, but also an excellent ligand for transition metals due
to its ability to act as a s-donor and a p-acceptor.3–5 The
conversion of CO to a carbonyl group is consistent with the
demands of green chemistry.6
The homogeneous palladium-catalyzed carbonylation of aryl
halide with CO is well-known as a direct and efficient protocol
for the synthesis of aromatic amides.7–11 Although these
homogeneous palladium catalysts exhibited excellent selectivity
and yields, the practical applications still suffer from the
problems of separation and reuse of expensive palladium
catalysts. In addition, the palladium residues in the product
could be a serious issue in pharmaceutical applications. This
gives rise to the development of recyclable and efficient
heterogeneous palladium catalysts in a variety of organic
chemical elds. Various heterogeneous palladium catalysts
have been reported for the synthesis of amides by amino-
carbonylation of aryl halide with CO gas using a support such as
silica, carbon, ZIF-8, MCM-41, ionic liquid phases and organic
polymers.12–19 However, these carbonylation reactions over
supported palladium catalysts proceeded at high CO pressure
and high reaction temperature. Dang group have reported the
Results and discussion
Initially we carried out the model reaction of iodobenzene (0.4
mmol) with aniline (2 mL) using Pd/SiO2 as catalyst and K2CO3
(2 equiv.) as base under 1 atm of CO and solvent-free conditions
State Key Laboratory of Photocatalysis on Energy and Environment, College of
Chemistry, Fuzhou University, Fuzhou 350116, P. R. China. E-mail: yuanrs@fzu.
edu.cn
ꢀ
at 80 C for 24 hours. To our delight, the 97% yield of desired
product 2c was obtained (Table 1, entry 1). In the absence of
catalyst, no product was obtained. In the case of only using Pd/
SiO2 without K2CO3, a small amount of the desired product
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra04985d
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Table 1 Screening of catalyst for carbonylation reactiona
supports provided the desired product in moderate to good
yields (from 42% to 84%) under similar reaction conditions
(entries 11–18). Therefore, Pd/SiO2 catalyst was found to be the
optimum catalyst for the carbonylation of iodobenzene.
The different reaction parameters such as solvent, base,
catalyst loading, temperature and time were tested to show their
effect on this reaction. The presence of solvents like DMSO,
acetonitrile and DMF provided the desired product in moderate
yields, and only trace amounts of product were detected when
using benzene and THF as solvents (Table 2, entries 1–5). From
the yields of the model reaction with and without K2CO3, we
found that the addition of base played a signicant role in the
formation of amide. Among the bases used, K2CO3 exhibited
a best yield of 97% for the nal product, while NaOH and CsCO3
were also effective with only a slight decrease in the yield (93%
and 92%) (entries 7 and 9). When Na2CO3 and KOH were used,
only trace amounts of product were detected (entries 8 and 10).
Given that NaOH is more readily subject to deliquescence and
CsCO3 is more expensive, K2CO3 is considered to be the optimal
base. The catalyst loading from 1 wt% to 2 wt% had no obvious
effect on the product yield. From the view of economy, 1 wt%
catalyst loading was chosen.ꢀ An obviꢀous increase in reaction
yields was observed from 50 C to 80 C, but a higher reaction
temperature resulted in a slight decrease in the yields (Fig. 1a).
Entry
Catalyst
Yield (%)
Entry
Catalyst
Yield (%)
1
2b
3
4
5
6
7
8
9
Pd/SiO2
Pd/SiO2
Ni/SiO2
Co/SiO2
Ru/SiO2
Ir/SiO2
Au/SiO2
Cu/SiO2
Pt/SiO2
97
10
Trace
Trace
Trace
—
—
—
—
10
11
12c
13
14
15
16
17
18
Ag/SiO2
Pd/Al2O3
Pd/C
Pd/Bi2O3
Pd/V2O5
Pd/WO3
Pd/CeO2
Pd/ZrO2
Pd/TiO2
—
75
42
84
64
77
76
67
76
a
Reaction conditions: iodobenzene (0.4 mmol, 1 equiv.), aniline (2 mL),
and CO (1 atm), 1 wt% Pd/SiO2 (0.5 mol%), K2CO3 (2 equiv.), no solvent.
b
Without K2CO3. c Active charcoal. Yields based on GC analysis.
(10%) was obtained (Table 1, entry 2). This indicates that the
catalyst and base are essential for the above carbonylation
reaction. Various SiO2-supported transition-metal catalysts were
screened. As shown in Table 1, different transition-metal cata-
lysts had a great inuence on the yields of 2c. Supported Ru, Co,
and Ni catalysts only provided a trace amount of the desired
product, and other transition-metal catalysts like Ir, Au, Cu, Pt
and Ag gave no product (Table 1, entries 3–10). Other catalyst
Table 2 Screening of base, solvent and catalyst loading for carbon-
ylation reactiona
Catalyst loading
Entry
Solvent
(mol%)
Base
Yield (%)
1b
2b
3b
4b
5b
6
7
8
9
DMSO
DMF
Acetonitrile
Benzene
THF
—
—
—
—
—
—
—
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.75
1
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOH
Na2CO3
CsCO3
KOH
74
54
78
Trace
Trace
97
93
Trace
92
10
11c
12d
Trace
96
91
K2CO3
K2CO3
a
Reaction conditions: iodobenzene (0.4 mmol, 1 equiv.), aniline (2 mL),
and CO (1 atm), reaction time: 24 h, reaction temperature: 80 ꢀC, 1 wt%
Pd/SiO2 (0.5 mol%), base (2 equiv.). b Solvent (2 mL), aniline (2 equiv.).
Fig. 1 Effect of (a) temperature and (b) time for carbonylation reac-
tion. Reaction conditions: iodobenzene (0.4 mmol, 1 equiv.), aniline (2
mL), and CO (1 atm), 1 wt% Pd/SiO2 (0.5 mol%), K2CO3 (2 equiv.). Yields
based on GC analysis.
c
1.5 wt% Pd/SiO2 (0.75 mol%). d 2 wt% Pd/SiO2 (1 mol%). Yields based
on GC analysis.
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Table 3 Palladium-catalyzed carbonylation reactions of aryl iodides with aminesa
Entry
1
Aryl iodides
Amines
2b
Products
Yield (%)
96
2
3
2b
2b
98
91
4
2b
99
5
6
7
2b
2b
2b
95
85
98
8c
2b
2b
56
98
9
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Table 3 (Contd.)
Entry
10
Aryl iodides
Amines
2b
Products
Yield (%)
99
11
2b
94
89
12b
2a
13
14b
15
16
2a
2a
2a
2a
92
84
99
98
17c
2a
2a
11
87
18b
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Table 3 (Contd.)
Entry
19d
Aryl iodides
2a
Amines
Products
Yield (%)
91
20b
21c
2a
2a
77
66
22c
23
24
2a
2a
2a
55
59
35
25c
2a
15
a
Reaction conditions: aryl iodides (0.4 mmol, 1 equiv.), amines (2 mL), and CO (1 atm), 1 wt% Pd/SiO2 (0.5 mol%), K2CO3 (2 equiv.). b Amines (2
equiv.), acetonitrile (2 mL) as solvent. c Yields based on GC-MS analysis. d Amines (2 equiv.), acetonitrile (2 mL) as solvent, yields based on GC-MS
analysis. Yields based on H NMR analysis.
1
The product yield increased gradually with reaction time, and high yields (56% to 99%) (entries 1–11). Sterically hindered o-
reached the highest yield (97%) at 24 h (Fig. 1b). At this iodophenol exhibited much lower reactivity (entry 8), as
moment, iodobenzene has been exhausted almost completely. compared with that of p-iodophenol (98%). The o-, m- and p-
A longer reaction time than 24 h would result in the precipita- substituted aryl iodides with –CF3 group also provided excellent
tion of N-phenylbenzamide 2C on the solid catalyst (Table S1†). yields (entries 4–6). Different substituted amines were also
Having optimized the reaction conditions for the synthesis tested. Good yields of 84% to 99% were obtained for the amines
of amide, the substrate scope was further investigated. As with electron-donating groups (entries 12–16). It is noteworthy
shown in Table 3, aryl iodides bearing electron-donating and that p-substituted amines gave a slightly decreased yield
electron-withdrawing substituents on its phenyl ring were well compared with that of o- and m-substituted amines. The
tolerated, which gave the corresponding amides in moderate to substituted amines with electron-withdrawing halogen groups
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were also tolerated, and gave 66% to 91% yields to the corre- in agreement with the literature reports.25–27 Aer the Pd/SiO2
sponding amides in the order I > Br > Cl (entries 18–21). When catalyst was recycled for ve cycles, the two peaks were still
the amines that have a substituent attached to the amino group located at the same binding energy. Further evidence for Pd was
were used, the desired products were obtained in moderate also provided by HRTEM images in Fig. 4. For both samples,
yields (entries 22). In the case of using N-methyl-m-methylani- well-resolved lattice fringes with an interplanar spacing of
line, the yield of 2aq was reduced greatly possibly due to the about 0.23 nm was observed, which corresponds to the (111)
steric hindrance effect (entry 17). Alkyl and cyclic amines were lattice plane of face-centered cubic Pd metal.28
also reactive, but relatively low yields were obtained (entries 23–
25) under the current conditions.
In order to determine the reaction mechanism, we con-
ducted a series of studies. The addition of 3 equiv. of 2,2,6,6-
The recycling of Pd/SiO2 catalyst was evaluated for the ami- tetramethylpiperidine-N-oxyl (TEMPO, a typical radical scav-
nocarbonylation of aryl iodides under the optimized reaction enger) cannot inhibit the reaction, whereas only trace amounts
conditions. Aer ve cycles, the results are shown in Fig. 2. of desired product was obtained with the addition of 3 equiv. of
There is little change in the yields of the desired product, which electron scavenger (1,4-benzoquinone, BQ) (Table 4). This
indicated that the recovered catalyst showed a good reaction indicated that the aminocarbonylation reaction proceeded via
activity. In order to check the leaching of palladium metal, the a single electron transfer process, rather than via a free radical
fresh catalyst, the third and h recycled catalyst were subjected pathway. When triethylamine was substituted for aniline in the
to the inductively coupled plasma optical emission spectrometry model aminocarbonylation reaction, no amide product was
(ICP-OES) technique. The palladium content of the above three found in the nal reaction mixture. This is different from the
samples were 1.06 wt%, 1.00 wt% and 0.94 wt%, respectively. case using primary or secondary amine as the substrate that has
That is to say, no obvious catalyst leaching was observed.
hydrogen atom bonded to N atom. Thus, we think that the base-
The oxidation state of palladium in Pd/SiO2 catalyst before promoted deprotonation of amine play an indispensable role in
and aer recycling was examined by XPS. As shown in Fig. 3, the the occurrence of the aminocarbonylation reaction. Aer the
peaks around 335.7 eV and 340.8 eV for the fresh Pd/SiO2 reaction of iodobenzene with aniline, the pH of the nal reac-
catalyst can be readily assigned to Pd(0) 3d5/2 and 3d3/2 bands, tion mixture decreased. Meanwhile, a negligible amount of
hydrogen was detected by gas chromatography. From these
results, we can conclude that the hydrogen in the nal reaction
solution exists mainly in the form of hydrogen ion. Ion chro-
matography analysis detected a small amount of iodide ions,
but potassium ions were not present. Thus, the base K2CO3 was
not dissolved in solution, and the combination of iodide ion
and hydrogen ion formed HI.
Fig. 2 Recycle study of Pd/SiO2 for the synthesis of amide. Reaction
conditions: iodobenzene (0.4 mmol, 1 equiv.), aniline (2 mL), and CO (1
atm), 1 wt% Pd/SiO2 (0.5 mol%), K2CO3 (2 equiv.). Yields based on GC
analysis.
Fig. 4 HRTEM images of (a) fresh Pd/SiO2 and (b) recycled Pd/SiO2 for
five cycles.
Table 4 Radical/electron trapping experimentsa
Entry
Catalyst
Additive
Yield (%)
1
2
3
Pd/SiO2
Pd/SiO2
Pd/SiO2
BQ
TEMPO
None
Trace
88
97
a
Reaction conditions: iodobenzene (0.4 mmol, 1 equiv.), aniline (2 mL),
and CO (1 atm), 1 wt% Pd/SiO2 (0.5 mol%), K2CO3 (2 equiv.). Yields
based on GC analysis.
Fig. 3 XPS spectra of (a) fresh Pd/SiO2 and (b) recycled Pd/SiO2 for five
cycles.
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pressure and moderate temperature, which avoids the need for
specialized pressure reactors and the dangers associated with
handling presurized CO. The supported catalyst can be easily
recycled and no obvious decrease in reaction activity was observed
aer ve cycles. This prorocol offered a green and sustainable way
to the construction of the amide products, and the reaction mode
can also be applied to other synthetic reactions, such as the
construction of esters and a,b-unsaturated ketones.
Experimental section
General information
All reagents were purchased from commercial suppliers and used
without further purication. All reactions were carried out in a dry
10 mL glass tube reactor, and the products were identied by GC,
GC-MS and 400 MHz instrument. GC analysis was performed in
a GC Agilent Technologies 7890A equipped with a ame ioniza-
tion detector by using an HP-5 5% phenyl methyl siloxane
column (30 m  0.32 mm  0.5 mm). GC-MS analysis was per-
formed in a GC Agilent Technologies 7890B that was tted with
a mass detector Agilent Technologies 5977A MSD. 1H NMR
spectra and 13C NMR spectra were recorded at 400 MHz and at
100 MHz in DMSO-d6 on a Bruker AVANCE III 400 MHz spec-
trometer. X-ray photoelectron spectroscopy (XPS) measurements
of Pd/SiO2 were performed on a PHI Quantum 2000 XPS system,
and the binding energies were calibrated to the C 1s peak at
284.8 eV of the surface adventitious carbon. Inductively coupled
plasma optical emission spectrometry (ICP-OES) technique of Pd/
SiO2 was performed on a OPTIMA 8000. High resolution trans-
mission electron microscopy (HRTEM) imaging was obtained on
a JEOL 2010F TEM at 200 kV accelerating voltage. The crude
products were monitored by analytical thin layer chromatography
using commercial thin layer chromatography, and puried by
exquisite column chromatography silica gel (100–200 mesh).
Scheme 1 Plausible reaction mechanism.
Scheme 2 Extended reactions.
On the basis of our results and previous reports.29,30 a plau-
sible reaction mechanism of Pd/SiO2 catalyzed carbonylation
process is proposed in Scheme 1. Initially, the complex C is
formed by oxidative addition of aryl iodide B to Pd(0) species A
under the reaction conditions. Next, the insertion of CO into the
phenyl–palladium bond of complex C forms acylpalladium
intermediate D. In the presence of a base, the nucleophilic
amine coordinates with intermediate D to give complex E via
the elimination of HI. Finally, reductive elimination from
complex E affords amide product F and Pd(0) species A.
In addition, the proposed carbonylation method is also
suitable for the formation of esters and a,b-unsaturated ketones
when amine was replaced by methanol and styrene (Scheme 2).
The reaction of iodobenzene with styrene did produce the
phenyltrans–styrylketone product in acetonitrile as solvent at
80 ꢀC for 40 h, albeit in low yield (<10%). When using methanol
as staring material, methyl benzoate was also constructed
effectively in 62% yield at 70 ꢀC for 6 h. Although the above
reaction conditions are not optimal, these results demonstrate
the feasibility of our system for the synthesis of esters and a,b-
unsaturated ketones.
Preparation of palladium catalyst
We adopted precipitation deposition method to prepare palla-
dium catalyst by using chloropalladic acid as the palladium
source. First of all, 2 g milled commercial silica powders (300–
400 mesh) was dissolved in 50 mL deionized water with
mechanical agitation for 0.5 h. Then different amounts of
chloropalladic acid solution was added into the as-obtained
SiO2-water to make the load is 1 wt%, 1.5 wt%, 2 wt%, respec-
tively. The pH of this solution was adjusted to 9 through slowly
adding KOH solution and stirred for 7 hours at room tempera-
ture, followed by drying at 80 ꢀC. Aer grinding, the dried solid
was dissolved in deionized water and reduced by sodium boro-
hydride solution under stirring at room temperature for 12 h.
Finally, the Pd/SiO2 catalyst was washed with deionized water
and centrifuged until the ionic degree less than 10, the obtained
solid product was dried in a vacuum at 60 ꢀC overnight. The
preparation of other catalysts are similar to that of Pd/SiO2.
Conclusions
General procedure of palladium catalyzed synthesis of amide
An efficient, ligand-free Pd(0)/SiO2 catalyst has been applied to the In a typical experiment, iodobenzene (0.4 mmol, 1 equiv.), aniline
carbonylation of aryl iodines with amines at atmospheric CO (2 mL), Pd/SiO2 (0.5 mol%), K2CO3 (2 equiv.) were added into
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a dry 10 mL glass tube reactor with a stirring bar. The tube reactor
was sealed and evacuated by a vacuum pump. Then CO gas was
introduced through a balloon being attached to the air inlet of
the reactor. Finally, the reactor with carbon monoxide balloons
was put into a stirred oil bath at 80 ꢀC for 24 h (the subsequent
reaction process is the same as that discussed above for the
carbonylation reaction). In the case of amines on solid phase,
acetonitrile was used as solvent. The amount of amine is 2 equiv.,
the reaction process remains unchanged. The reaction mixture
was separated from the catalyst by simple centrifugation, and the
liquid was analyzed by GC, GC–MS, and 1H NMR techniques.
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The recycling of palladium catalyst
The recyclability of Pd/SiO2 for the synthesis of amide was
carried out under the optimized reaction conditions. Aer
completing the reaction, the reactor was cooled to room
temperature and the remaining CO gas was carefully vented out
to a hood aer 24 hours. The catalyst was separated from the
mixture and washed with acetonitrile (2 mL) and deionized
water (2 mL) to remove the residual organic material. Aer
drying under vacuum, the obtained catalyst was used for a new
reaction cycle.
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Acknowledgements
This work was nancially supported by the National Nature
Science Foundation of China 21643009, the National Basic
Research Program of China 2014CB239303, the Natural Science
Foundation of Fujian Province of China 2015J01046, and the
Independent Research Project of State Key Laboratory of Pho-
tocatalysis on Energy and Environment 2014B01.
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