Palladium nanoparticles supported on modified single-walled carbon nanotubes: A heterogeneous and reusable catalyst in the Ullmann-type N-arylation of imidazoles and indoles

Article abstract of DOI:10.1039/c4nj02108h

Air- and moisture-stable and recyclable palladium nanoparticles supported on modified single-walled carbon nanotubes (SWCNT-Met/Pd) behave as very efficient heterogeneous catalysts in the Ullmann coupling of imidazoles and indoles with aryl iodides to afford the corresponding C-N coupling reactions under aerobic conditions. These cross coupled products were produced in excellent yields at low palladium loading (～0.2 mol%) and the heterogeneous catalyst can be readily recovered by simple filtration and reused five times without a noticeable loss in its catalytic activity.

Full text of DOI:10.1039/c4nj02108h

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Palladium nanoparticles supported on modified
single-walled carbon nanotubes: a heterogeneous
and reusable catalyst in the Ullmann-type
N-arylation of imidazoles and indoles†
Cite this:DOI: 10.1039/c4nj02108h
Hojat Veisi*a and Nekoo Morakabatib
Air- and moisture-stable and recyclable palladium nanoparticles supported on modified single-walled
carbon nanotubes (SWCNT-Met/Pd) behave as very efficient heterogeneous catalysts in the Ullmann
coupling of imidazoles and indoles with aryl iodides to afford the corresponding C–N coupling reactions
under aerobic conditions. These cross coupled products were produced in excellent yields at low palladium
loading (B0.2 mol%) and the heterogeneous catalyst can be readily recovered by simple filtration and reused
five times without a noticeable loss in its catalytic activity.
Received (in Porto Alegre, Brazil)
22nd November 2014,
Accepted 30th January 2015
DOI: 10.1039/c4nj02108h
www.rsc.org/njc
for cost saving is tremendous from a material, equipment,
labour, and time standpoint. Higher selectivity means less
Introduction
The formation of aryl-nitrogen bonds via cross-coupling reac- waste and fewer impurities, which could lead to safer and
tions represents a powerful technique for the preparation reduced environmental impact. The study of application
of various compounds (e.g., arylpyrroles, arylpyrazoles, aryl- of metal nanoparticles in catalysis, particularly, on organic
imidazoles, aryltriazoles, arylindoles, arylcarbazoles, etc.) that transformations, has become a frontier area of research in
are of biological, biochemical, pharmaceutical and materials nanocatalysis.5 Among the different metal nanocatalysts,
interest.1–3 Traditionally, these compounds have been syn- immobilized Pd(II) complexes have gained a notable reputa-
thesized via nucleophilic aromatic substitution of N–H tion, because palladium is a versatile catalyst in modern
containing p-electron-rich nitrogen heterocycles with electron- organic synthesis and is widely used for a significant number
deficient aryl halides, which limits its scope,4 or by coupling of synthetic transformations6 such as, Heck, Suzuki, Stille,
with organometallic reagents. They have also been pre- Sonogashira cross coupling reactions and N-arylation of
pared by the classical Ullmann-type coupling with aryl halides, heterocycles.7–17
which sometimes suffers from limitations such as high reaction
Studies on the isolation, characterization and catalytic activities
temperatures (often 150 1C or as high as 200 1C), the use of functionalized carbon nanotubes (CNTs) have received par-
of stoichiometric amounts of copper reagents, insolubility of ticular attention during the last decade owing to their specific
copper(I) salts in organic solvents, moderate yields, sensitivity catalytic applications compared to homogeneous complexes.
of the substituted aryl halide to the harsh reaction conditions, Metal nanoparticles as well as various transition metal com-
and poor substrate generality. Therefore a mild, economic, and plexes such as polymers and porphyrins have been used for
efficient catalytic system is still desirable for this process.
carbon nanotubes’ functionalization.18–20 Schiff bases, which
Today, catalysts play a significant role in the production are an important class of ligands with extensive applications in
of chemicals and nanomaterials have the potential for improv- different fields,21 also showed excellent catalytic activity when
ing efficiency, selectivity, and yield of the catalytic process. grafted onto CNTs.22–24
The higher surface to volume ratio means that much more
We have already reported the preparation of a Pd catalyst
catalyst actively participates in the reaction. The potential supported on biguanide(metformin)-functionalized single-
walled carbon nanotubes (SWCNT-Met/Pd) and its catalytic
application in the Suzuki–Miyaura coupling reactions.25 In
order to further establish other heterogeneous palladium-
catalyzed C–N cross-coupling reactions using our catalyst, we
herein report the application of this catalyst in Ullmann
a Department of Chemistry, Payame Noor University (PNU), Tehran, Iran.
E-mail: hojatveisi@yahoo.com; Fax: +98 833 8380709
b Department of Chemistry, Tonekabon branch, Islamic Azad University, Tonekabon,
Iran
coupling of imidazoles and indoles with aryl iodides under
aerobic conditions (Scheme 1).
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4nj02108h
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Scheme 1 SWCNT-Met/Pd catalyzed N-arylation of indoles and imidazoles.
reaction conditions including Pd concentration, solvent, bases
and temperature without protection by an inert gas, and the
results are summarized in Table 1.
Results and discussion
First of all, after successful fabrication of the catalyst,25 we have
investigated again the surface morphology by FESEM, EDS and
TEM images (Fig. 1 and 2). FESEM and EDS images of the
SWCNT-Met/Pd (Fig. 1) show that the attachment of organic
components to the SWCNT materials has no distinct influence
on the morphology of composition and also indicated that
metformin and Pd were present in the catalyst. The concen-
tration of palladium in SWCNT-Met/Pd was 19 wt%, which was
determined by ICP-AES.
Transmission electron microscopy (TEM) investigations
are carried out to observe the morphology and distribution of
palladium nanoparticles supported on SWCNTs after their
reduction with hydrazine (Fig. 2). It can be seen that although
Pd-NPs are distributed on SWCNTs, they do not aggregate with
each other because the imine groups of metformin effectively
isolate adjacent Pd-NPs. The average size of Pd-NPs is B10 nm.
In our initial screening experiments, the N-arylation reaction
of indoles with iodobenzene was investigated to optimize the
Initially, various solvents such as DMF, EtOH, CH3CN,
toluene, CH2Cl2, DMSO and H2O were studied in the presence
of 0.3 mol% Pd catalyst and 2 equiv. of Et3N at 110 1C (Table 1,
entries 1–7). As could be seen in Table 1, the best result was
obtained in DMF (Table 1, entry 7). Next, the bases, including
Et3N, KOH, K3PO4, Na2CO3, NaHCO3 and K2CO3 were explored,
and Et3N gave the best yields (Table 1, entries 1 and 12–16).
However, a low yield was obtained without any base (Table 1,
entry 10). It was also found that the reaction temperature has a
great influence on this transformation (Table 1, entries 9, 17,
and 18). The obvious improvement in the conversion (98%) was
achieved for the reaction at 110 1C (Table 1, entry 9).
In addition, when the loading of Pd was reduced from
0.3 mol% to 0.2 mol%, an excellent yield was also afforded.
However, when 0.1 mol% of Pd was employed, the reaction
yield decreased to 60% (Table 1, entries 7–9). It is important to
mention that N-arylation of indoles did not take place in the
Fig. 1 FESEM and EDS images of SWCNT-Met/Pd.
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Table 2 Synthesis of N-aryl-indoles catalyzed by SWCNT-Met/Pda
Yieldb (%) Ref.
Entry Aryl halide Indole
Product
1
98
98
26a
26a
2
3
4
5
6
7
96
95
90
95
60
26a
26a
26a
26a
26a
Fig. 2 TEM image of SWCNT-Met/Pd.
Table 1 Optimization of reaction conditions for the N-arylation of indoles
with iodobenzenea
Entry Pd (mol%) Base
Solvent
T (1C) t (h) Yieldb (%)
1
2
3
4
5
6
7
8
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.1
0.2
0.2
0.0
0.2
0.2
0.2
0.2
0.2
0.2
0.2
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
—
Et3N
K2CO3
Na2CO3
K3PO4
KOH
CH3CN
EtOH
Toluene 110
82
78
10
10
10
10
10
10
6
50
45
75
30
35
90
95
60
98
Trace
0
75
50
55
75
55
Trace
45
H2O
100
40
CH2Cl2
DMSO
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
110
110
110
110
110
110
110
110
110
110
110
25
10
6
9
10
11
12
13
14
15
16
17
18
24
24
10
10
10
10
10
10
10
NaHCO3 DMF
Et3N
Et3N
DMF
DMF
70
a
Reaction conditions: indole (1.0 mmol), iodobenzene (1.1 mmol), Pd
b
catalyst, base (2.0 mmol), solvent (5.0 mL). Isolated yield.
8
90
85
90
26a
26c
26b
absence of a Pd catalyst (Table 1, entry 11). In summary, the
optimal conditions for the N-arylation of indoles with iodo-
benzene were 0.2 mol% of Pd catalyst and 2 equiv. of Et3N in
DMF at 110 1C for 6 h (Table 1, entry 9).
9
With these optimum reaction conditions in hand, the scope
and generality of the N-arylation method were investigated by
the variations of aryl iodides and indoles with different sub-
stituents and the results are summarized in Table 2.
We examined the reaction of indoles with substituted aryl
iodides (Table 3, entries 1–14). All the substituted aryl iodides
reacted with indoles to afford the coupling products in good to
high yield. We did not observe any limitation as for aryl iodide
derivatives. The reactivity of 2-methylindole was slightly lower
than that of indoles and 3-methyl-indole due to the steric effect
(Table 2, entries 1, 10, and 12).
10
11
92
27
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Table 3 Palladium-catalyzed C3 arylation of indolesa
Table 2 (continued)
Entry
Aryl halide
Indole
Product
Yieldb (%)
Ref.
28
Entry Aryl halide Indole
Product
Yieldb (%) Ref.
1
90
12
13
98
95
26a
26a
2
3
70
82
28
28
14
90
0
26a
15
No reaction
—
4
65
75
28
28
a
Reactions were carried out under aerobic conditions in 5 mL of DMF,
1.1 mmol aryliodides, 1.0 mmol indoles and 2 mmol Et3N in the
presence of a Pd catalyst (0.020 g, 0.2 mol% Pd) and 110 1C for
b
6 h. Isolated yield.
5
To ensure that no C-arylation took place in the reaction, the
reaction was carried out with 1-methyl indole and it was found
that there was no product formed (Table 2, entry 17). This
shows the high selectivity of SWCNT-Met/Pd as a catalyst in the
N-arylation of indoles using aryl iodides.
a
Reactions were carried out under aerobic conditions in 5 mL of DMF,
1.1 mmol arylhalide, 1.0 mmol indoles and 3 mmol K2CO3 in the
presence of a Pd catalyst (0.020 g, 0.2 mol% Pd) and 110 1C for
b
10 h. Isolated yield.
Interestingly, less reactive aryl halides such as bromoben-
zenes and chlorobenzenes also couple with indoles furnishing
C3-aryl indole in good to appreciable yields at longer reaction The results indicated that the reaction stops after the catalyst is
times (Table 3).
filtered off. Additionally the reaction mixture was extracted with
The results indicate that the selectivity of the reaction ether and the aqueous layer was analyzed for palladium by
is mainly controlled by the ‘‘electronic character’’ of the Pd(II)- ICP-AAS. The total palladium concentration was found to be
intermediate.29 Using an aryl bromide or aryl chloride under 150 Æ 10 ppb. The negative results of the hot filtration test
the same conditions would generate an electron deficient Pd(II)- indicate that the leaching of palladium is negligible and most
species (‘‘hard electrophile’’) more able to perform C3-arylation, probably the reaction is heterogeneous in nature. Based on
while using aryl iodide would generate electron rich Pd(II)- these results, it appears that the reaction studied has the same
species (‘‘soft electrophile’’) due to orbital overlapping between mechanism as that reported in other Pd-catalyzed N-arylations
the Pd(II)-centre and the iodide ligand, species involved in the (Scheme 3).32 According to the general mechanism depicted in
N1-arylation (Scheme 2).
Scheme 3, the reaction of Het-NH with the oxidative addition
We were pleased to find that the arylation reactions also product I in the presence of base leads to a novel II complex.
proceed with imidazoles in good yields under the same conditions When the latter complex undergoes a reductive elimination, the
(Table 4, entries 1–9). We found that aryl iodides containing coupled product N-aryl-Het is produced, and the catalyst is
electron-donating groups as well as those with electron- released to complete the catalytic cycle.
withdrawing groups reacted with imidazoles to give the corres-
ponding N-arylated imidazoles.
Another important point concerning heterogeneous catalysis
is the deactivation and recyclability of the catalyst. In order to
To probe the leaching of the catalyst, a hot filtration test was investigate this, a series of 5 consecutive runs of the coupling
performed. The reaction between indoles and iodobenzene was between indoles and iodobenzene (N-arylation) in the presence
stopped after 2 hours (40% yields) and the resin filtered off of SWCNT-Met/Pd was carried out (Fig. 3). These results demon-
while the solution was still hot. The filtrate was heated further strate that nanocatalysts could be reused five times without a
and the reaction mixture was analyzed by GC after 4 hours. noticeable loss of their catalytic activity. The stability and good
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Scheme 2 Selectivity study for N1-/C3-arylation of indoles in the presence of various aryl halides.
reusability of the catalyst should result from the chelating action the presence of the catalyst. Moreover, the catalyst could be
of bidentate metformin groups on palladium and the meso- reused for four consecutive cycles without a noticeable loss of its
porous structure of the SWCNT support.
catalytic activity. The stability of the palladium nanoparticles on
The XPS spectroscopic analysis of the heterogeneous catalyst the support was studied by XPS and ICP-AAS analysis and also
is a quantitative technique to indicate the electron properties of using the hot filtration test. These advantages make the process
the species immobilized on the surface, such as the oxidation highly valuable from the synthetic and environmental points of
state, the electron environment and the binding of the core view. Further efforts to extend the application of this system in
electron (E binding) of the metal. Fig. 4 displays the Pd binding other reactions are currently in progress in our laboratory.
energy of SWCNT-Met/Pd. The study of the SWCNT-Met/Pd at
the Pd 3p level shows peaks at 531.4 and 553.7 eV for Pd 3p3/2
,
which clearly indicates that the Pd nanoparticles are stable as
metallic states in the nanocomposite structure. In comparison
to the standard binding energy of Pd0, with Pd 3p3/2 of about
532.4 eV and Pd 3p1/2 of about 560.2 eV, it can be concluded
that the Pd peaks for SWCNT-Met/Pd shifted to lower binding
energy than Pd0 standard binding energy. The previous studies33,34
indicated that the position of the Pd 3p peak is usually influ-
enced by the local chemical/physical environment around Pd
species besides the formal oxidation state, and shifts to lower
binding energy when the charge density around it increases.
Therefore, the peaks at 553.7 and 531.4 could be due to Pd0
species bound directly to metformin groups in SWCNT-Met/Pd.
Experimental
N-Arylation of indoles and imidazoles in the presence of
SWCNT-Met/Pd under mild conditions
In a typical reaction, 20 mg of the catalyst (20 mg catalyst =
0.002 mmol Pd, 0.2 mol%) was placed in a 25 mL round-bottom
flask, 1.1 mmol of the aryl iodides in 3 mL of DMF was added to
1 mmol of indoles or imidazoles and 2 mmol of Et3N. The
mixture was then refluxed for an appropriate amount of time at
110 1C (see Tables 2 and 4). After completion of reaction
monitored by TLC (hexane/acetone, 4 : 1), the reaction mixture
was cooled to room temperature (the catalyst was recovered
using a centrifuge) and was extracted with diethyl ether three
times (3 Â 10 mL). The combined organic layers were washed
with brine solution and dried over anhydrous Na2SO4 and
concentrated in vacuo. The crude product was purified by
column chromatography on silica gels (60–120 mesh) to pro-
vide the N-aryl indoles and N-aryl imidazoles.
Conclusion
In conclusion, we have applied palladium nanoparticles sup-
ported on modified single-walled carbon nanotubes (SWCNT-
Met/Pd) as very efficient heterogeneous catalysts in the Ullmann
coupling of imidazoles and indoles with aryl iodides to afford
the corresponding C–N coupling reactions under aerobic condi-
C3-Arylation of indoles in the presence of SWCNT-Met/Pd
under mild conditions.
tions. The corresponding N-arylindoles and N-arylimidazoles In a typical reaction, 20 mg of the catalyst (20 mg catalyst =
were obtained in good yields and were applicable to various 0.002 mmol Pd, 0.2 mol%) was placed in a 25 mL round-bottom
indoles, imidazoles and a variety of aryl iodides. Less reactive flask, and 1.1 mmol of the arylbromides or arylchlorides in
aryl halides such as bromobenzenes and chlorobenzenes also 3 mL of DMF was added to 1 mmol of indoles and 3 mmol of
couple with indoles furnishing C3-aryl indoles in good yields in K2CO3. The mixture was then refluxed for 10 hours at 110 1C
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Table 4 SWCNT-Met/Pd-catalyzed N-arylation of imidazoles with aryl
iodidesa
Entry
1
Aryl iodides
Imidazole
Product
Yieldb (%)
Ref.
30
96
2
98
31
Scheme 3 Plausible mechanism for the N-arylation.
3
4
5
96
85
92
30
31
30
Fig. 3 Reusability of the SWCNT-Met/Pd catalyst for the N-arylation of
indoles under similar conditions.
6
7
8
96
90
31
30
96
85
31
31
Fig. 4 XPS spectra of SWCNT-Met/Pd.
temperature (the catalyst was recovered using a centrifuge) and
was extracted with ethyl acetate three times (3 Â 10 mL). The
combined organic layers were washed with brine solution and
dried over anhydrous Na2SO4 and concentrated in vacuo. The
crude product was purified by column chromatography on
silica gels (60–120 mesh) to provide the C3-aryl indoles.
9
a
Reactions were carried out under aerobic conditions in 3 mL of DMF,
1.1 mmol aryliodide, 1.0 mmol imidazole and 2 mmol Et3N in the
presence of a Pd catalyst (0.020 g, 0.2 mol% Pd) and 110 1C for
b
10 h. Isolated yield.
Analytical data for selected products
(see Table 3). After completion of reaction monitored by TLC
N-Phenylindole (Table 2, entry 1). Colorless oil. 1H NMR
(hexane/acetone, 5 : 1), the reaction mixture was cooled to room (400 MHz, CDCl3): d 6.71 (d, J = 3.0 Hz, 1H), 7.20 (t, J = 8.0 Hz, 1H),
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Tetrahedron Lett., 2000, 41, 481; (e) G. A. Grasa, M. S. Viciu,
J. Huang and S. P. Nolan, J. Org. Chem., 2001, 66, 7729.
12 H. Veisi, D. Kordestani and A. R. Faraji, J. Porous Mater.,
2014, 21, 141.
13 H. Veisi, R. Masti, D. Kordestani, M. Safaei and O. Sahin,
J. Mol. Catal. A: Chem., 2014, 384, 61.
7.23 (t, J = 8.0 Hz, 1H), 7.32–7.40 (m, 2H), 7.50–7.56 (m, 4H), 7.60
(d, J = 8.0 Hz, 1H), 7.70 (d, J = 8.0 Hz, 1H).
N-Phenylimidazole (Table 3, entry 1). Colorless oil. 1H NMR
(400 MHz, CDCl3): d 8.28 (s, 1H), 7.76 (t, 1H), 7.67 (q, J = 5.8 Hz,
2H), 7.53 (t, 2H), 7.37 (t, 1H), 7.14 (s, 1H).
14 H. Veisi, M. Hamelian and S. Hemmati, J. Mol. Catal. A:
Chem., 2014, 395, 25.
Acknowledgements
15 H. Veisi, Tetrahedron Lett., 2010, 51, 2109.
16 H. Veisi, J. Gholami, H. Ueda, P. Mohammadi and
M. Noroozi, J. Mol. Catal. A: Chem., 2015, 396, 216.
17 H. Veisi, P. Mohammadi and J. Gholami, Appl. Organomet.
Chem., 2014, 28, 868.
We are thankful to Payame Noor University (PNU) for financial
support.
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