A simple procedure for polymer-supported N-heterocyclic carbene silver complex via click chemistry: An efficient and recyclable catalyst for the one-pot synthesis of propargylamines

Article abstract of DOI:10.1039/c2dt31609a

A series of polymer-supported N-heterocyclic carbene silver complexes were prepared using a simple procedure via click chemistry. These complexes were tested as catalysts in the synthesis of propargylamines by the one-pot three-component coupling reaction

Full text of DOI:10.1039/c2dt31609a

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PAPER
A simple procedure for polymer-supported N-heterocyclic carbene silver
complex via click chemistry: an efficient and recyclable catalyst for the
one-pot synthesis of propargylamines†
Ying He, Mei-fang Lv and Chun Cai*
Received 19th July 2012, Accepted 21st August 2012
DOI: 10.1039/c2dt31609a
A series of polymer-supported N-heterocyclic carbene silver complexes were prepared using a simple
procedure via click chemistry. These complexes were tested as catalysts in the synthesis of
propargylamines by the one-pot three-component coupling reaction of aldehydes, amines and alkynes
(A3-coupling). The reaction was preceded smoothly to afford the corresponding products in good to
excellent yields at room temperature under low catalyst loading of 0.1 mol%. Furthermore, the catalysts
were stable and recoverable for six runs without a significant loss in their activity.
reusability and high catalytic activities have been widely investi-
Introduction
gated for A3 coupling reactions.12
Multi-component reactions in one-pot syntheses play a signifi-
cant role in modern synthetic chemistry since they not only
exhibit a higher atom economy and selectivity, but allow the
construction of molecules of more diversity and complexity.
Furthermore, in many cases, multi-component reactions are easy
to perform leading to simple experimental procedures as well as
lower costs, time and energy.1 Three-component coupling of an
aldehyde, amine and alkyne (A3-coupling) is one of the best
examples and has received much attention in recent years. The
resultant propargylamines obtained by the A3-coupling reaction
are versatile intermediates for organic synthesis2 and important
structural elements of natural products and potential drug
molecules.3
Traditionally, propargylamines are synthesized by the nucleo-
philic attack of lithium acetylides or Grignard reagents to imines
or their derivatives.4 However, these methods require the use of
stoichiometric amounts of organometallic reagents and strictly
controlled reaction conditions which limited their use. So, in the
following years, further modifications have been made for the
synthesis of propargylamines and their derivatives. Correspond-
ingly, homogeneous catalysts such as copper,5 silver,6 gold,7
iridium,8 zinc,9 nickel10 and cobalt11 have been applied for the
A3 coupling reaction efficiently. However, most of these catalytic
systems suffer from the loss of the precious or hazardous cata-
lysts at the end of the reaction. In order to overcome this draw-
back, heterogeneous catalysts due to their thermal stability,
Since the isolation of the first free carbene by Arduengo et al.
in 1991,13 N-heterocyclic carbenes (NHCs) have become an
important class of ligands in organometallic chemistry and cata-
lysis due to their excellent σ-donor properties, ease of synthesis,
and variable steric bulk. NHC-based catalysts feature robust
carbon–metal bonds that provide high thermal stability and low
dissociation rates, and consequently better resistance against oxi-
dation or leaching phenomena, making the use of excess ligand
unnecessary.14 Among the NHC–metal complexes, NHC–Ag(I)
complexes are well-established for the preparation of various
transition metal–NHCs complexes and proven to be a convenient
method under certain conditions.15 Other than transmetallation,
the use of Ag(I)–NHCs was also realized in homogeneous
and heterogeneous catalysis such as in the preparation of
1,2-bis(boronate) esters,16 ring-opening polymerization of
lactides,17 olefin polymerization,18 and also for the synthesis
of propargylamines.19
The copper-catalyzed azide–alkyne cycloaddition reaction,20
which was coined as “click chemistry”, has been proven to be a
powerful tool for conjugation between appropriately functiona-
lized partners of azides and alkynes. The process is modular,
wide in scope, high yielding, and requires only simple reaction
and purification conditions. Taking advantage of the outstanding
reaction profile, the chemical orthogonality of the formation con-
ditions which do not require activating reagents and the stability
toward strong bases, new “click families” incorporating a 1,2,3-
triazole moiety for the fusion of linker functionality and the resin
backbone have been reported.21 Very recently, we also success-
fully synthesized polymer-supported NHC–Rh complexes via
click chemistry. The complex could catalyze the addition of
arylboronic acids to aldehydes smoothly in good to excellent
yields.22 Based on our earlier work, herein we present the
preparation of a series of polymer-supported carbene–silver
Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing 210094, China. E-mail: c.cai@mail.njust.edu.cn;
Fax: +86-25-84315030; Tel: +86-25-84315514
†Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic data, images and the copies of NMR for the
products. See DOI: 10.1039/c2dt31609a
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Scheme 1 Polymer-supported NHC–Ag(I) for the synthesis of
propargylamines.
Fig. 1 TGA curves of the polymer-supported NHC–Ag(I) complexes
4a–4c.
Table 1 Optimization of the A3 coupling reactiona
Scheme 2 Preparation of the polymer-supported NHC–Ag(I)
complexes.
complexes through click chemistry, and their application as cata-
lysts in one-pot A3 coupling reactions at room temperature
(Scheme 1).
Entry
Cat.
Catalyst loading (%)
Solvent
Yield (%)b
Results and discussion
1
2
3
4
5
6
7
8
4a
4b
4c
4a
4a
4a
4a
4a
4a
4a
4a
—
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.5
—
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
DMF
CH3OH
Toluene
AcOEt
H2O
92
92
89
87
81
20
68
67
57
89
92
—
The synthesis of this series of polymer-supported NHC–Ag(I)
catalysts is illustrated in Scheme 2. They were readily prepared
via click chemistry in several steps. First of all, we tried to syn-
thesize a series of NHC–triazolyl bidentate ligands. As expected,
access to the NHC–triazolyl bidentate ligands was accomplished
by reacting N-aryl imidazoles with propargyl bromide in toluene
at a certain temperature, providing the alkynyl–imidazolium salts
in near quantitative yields, either by precipitation from or by
concentration of the reaction mixture. The preparation of
azidomethyl polystyrene was started from Merrifield resin
(1.0 mmol g−1) with a nucleophilic substitution reaction with a
large excess of NaN3. The product could be isolated and purified
by simple filtration and was washed with methanol and dichloro-
methane. The loading of the N3 group was determined to be
0.79 mmol g−1 by means of the nitrogen content calculated by
elemental analysis.
With the propargyl imidazolium bromides and azidomethyl
polystyrene in hand, we next constructed the “click chemistry
families”. Firstly, we tried to use copper(II) sulfate and sodium
ascorbate to form the 1,2,3-triazolyl moiety. However, this reac-
tion did not yield the desired products as determined by infrared
spectroscopy, which still showed a strong absorbance of the N3
group at 2096 cm−1. On the other hand, reaction of an excess of
propargyl imidazolium bromides using CuI and N,N-diisopropyl-
ethylamine (DIPEA) as a catalyst system led to complete
conversion of all azide groups, which is the suitable system to
form the 1,2,3-triazolyl moiety. The synthesis of the polymer-
supported NHC–Ag(I) complexes was then carried out. The pre-
ference for this reaction is understandable, as the desired com-
plexes are often air stable, and generally no solvent pretreatment
or additional base is required when silver(I) oxide is used to
generate the carbene complexes.23 The supported NHC–Ag(I)
9
10
11
12
Neat
CH2Cl2
CH2Cl2
a Reaction conditions: cyclohexanecarboxaldehyde (1.0 mmol),
phenylacetylene (1.1 mmol), piperidine (1.1 mmol), catalyst, CH2Cl2
(1 mL), room temperature for 24 h. b GC yield based on
cyclohexanecarboxaldehyde.
complexes 4a–4c were readily prepared by the reaction of 3 with
1/2 equivalent of Ag2O in dichloromethane using Lin’s proce-
dure.23a After stirring for 24 h at room temperature, the mixture
was filtered, washed and dried in vacuo to yield the target
products.
Then, thermogravimetry analysis (TGA) of the polymer-
supported NHC–silver(I) complexes was carried out to investi-
gate their stability (Fig. 1). The weight loss of the complexes on
heating from 40 to 150 °C are all less than 1% and might be
attributed to the release of physisorbed water and loosely bound
volatiles on the surface. Thermal degradation occurred after
250 °C, which implied good stability and use within a broad
temperature scale.
Next, the effect of different substituted groups of imidazolium
salts in polymer-supported NHC–Ag(I) complexes on their cata-
lytic activity for the A3 coupling reaction were investigated
using cyclohexanecarboxaldehyde, phenylacetylene and
piperidine as a model reaction. As can be seen from Table 1,
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entries 1–3, the activity of this series of catalysts was not very
notably affected by the substituted group of imidazolium salts.
Correspondingly, the reaction could proceed smoothly at room
temperature with the catalysts in yields of 89–92%, and from the
economic point of view, we chose 4a as the best catalyst for the
A3 coupling reaction.
catalyst was used in the next run, and almost consistent activity
was observed for next five consecutive cycles (Fig. 2).
A hot-filtration experiment was run to investigate whether the
reaction proceeded in a heterogeneous or homogeneous fashion.
The catalyst was then removed by filtration after 1 h at room
temperature. The filtrate was reacted for a further 24 h. The reac-
tion continued, although the conversion did not reach the level
obtained in the normal manner, which means that at least a part
of the catalytic activity of the catalyst was assigned to a homo-
geneous manner. Moreover, after carrying out the reaction, the
catalyst was filtered. ICP analysis of the clear filtrates obtained
by filtration showed that the silver content is less than 1.0 ppm
which implied the stability of the catalyst.
In order to optimize the conditions of the A3 reaction, the
effect of solvent was surveyed (Table 1, entries 4–10). Among
the solvents tested in Table 1, CH2Cl2, CH3CN, DMF and
solvent-free conditions all gave excellent yields of 81–92%
(Table 1, entries 1, 4, 5 and 10). However, solvents such as
CH3OH, toluene, AcOEt and H2O were not suitable for the reac-
tion and no satisfactory yields were obtained (Table 1, entries
6–9). When the amount of the catalyst loading was increased to
0.5 mol%, there was no obvious increase in the yield of the reac-
tion (Table 1, entry 11), and the reaction did not occurr under
catalyst-free conditions (Table 1, entry 12). Thus, the optimized
conditions for the A3 reaction are 0.1 mol% catalyst 4a and
CH2Cl2 as solvent at room temperature. It noteworthy that the
high yields of the A3 reaction obtained only with a catalyst
loading of 0.1 mol% are very competitive to other catalyst
systems.
Under the optimized conditions, the one-pot, three-component
reactions of various aldehydes, amines and alkynes were studied
using 4a as catalyst in CH2Cl2. The results are summarized in
Table 2. First, the aldehyde substrate was kept constant, and it
was reacted with different alkynes (Table 2, entries 1–7). As can
be seen in Table 2, a variety of terminal aromatic alkynes with
substituted groups in para- and meta-positions all reacted
smoothly at room temperature within 12–24 h (Table 2, entries
1–5). Moreover, the aliphatic alkynes could also be utilised,
which gave the expected products in yields of 82–85% (Table 2,
entries 6 and 7). This finding opens up the possibility of synthe-
sizing a variety of propargylamine motifs containing long-chain
acetylenic subunits in their molecular structures.
Further, we examined the effect of amines in the reaction.
Phenylacetylene and cyclohexanecarboxaldehyde was used as
model substrates, and various amines such as cyclic, heterocyclic
and acyclic secondary aliphatic amines were investigated
(Table 2, entries 1 and 8–11). As can be seen from Table 2,
cyclic amines gave high yields of 92–93% (Table 2, entries 1
and 8). However, when heterocyclic or acyclic secondary ali-
phatic amines were substrates, yields of 81–90% were obtained
for the corresponding propargylamines but a reaction temp-
erature of 50 °C was required (Table 2, entries 9–11).
Finally, various structurally divergent aldehydes were investi-
gated and the results were presented in Table 2, entries 12–18.
The aliphatic aldehydes displayed high reactivity at room temp-
erature within 12–24 h (Table 2, entries 1 and 12–15). However,
aromatic aldehydes were less reactive than aliphatic aldehydes,
and the reaction could proceed at 60 °C in yields of 65–74%
(Table 2, entries 16–18).
Conclusions
In summary, we have successfully synthesized a novel and prac-
tical series of polymer-supported NHC–Ag(I) complexes. The
complexes could efficiently catalyze the one-pot three-com-
ponent coupling reaction of aldehydes, amines and alkynes
(A3-coupling). The reaction generated the corresponding propar-
gylamines at room temperature and provided a wide range of
substrate applicability. In addition, this methodology offers a
competitive amount of the catalyst (0.1 mol%), and the catalyst
could be readily recovered and reused for several times without a
significant loss of its catalytic activity, which make this pro-
cedure more environmentally acceptable.
Experimental
General remarks
All chemicals were reagent grade and used as purchased. Chloro-
methylated polystyrene resin (1% divinylbenzene, 1.0 mmol g−1
of Cl, grain size range: 100–200 mesh) was obtained from GL
Biochem Ltd. (Shanghai, China). Elemental analyses were per-
formed on a Vario EL III recorder. GC analyses were performed
on an Agilent 7890 instrument. GC/MS analyses were performed
on a Saturn 2000GC/MS instrument. 1H NMR spectra were
measured with a Bruker Avance III 500 analyzer. Silver content
of the catalyst was measured by inductively-coupled plasma
atomic absorption spectroscopy (ICP-AAS) on
a Varian
AA240FS analyzer. Thermogravimetric analysis was carried out
under nitrogen with a flow rate of 30 mL min−1 using a TGA/
SDTA851e instrument.
Synthesis of N-mesitylimidazole
The preparation of N-mesitylimidazole was performed according
to the literature method.24 Glacial acetic acid (10 mL), aqueous
formaldehyde (3 mL), and aqueous glyoxal (4.6 mL) were trans-
ferred to a round-bottom flask (50 mL) and heated at 70 °C. A
solution of glacial acetic acid (10 mL), ammonium acetate in
water (3.08 g in 2 mL), and mesitylamine (5.6 mL) was then
added dropwise over a period of 30 min. The solution was con-
tinuously stirred and heated at 70 °C for 18 h. The reaction
mixture was then cooled to room temperature, and the reaction
For a heterogeneous catalyst, it is important to examine its
ease of separation, recoverability and reusability. The recyclabil-
ity of our polymer-supported NHC–Ag(I) 4a was also investi-
gated. After carrying out the reaction, the mixture was filtered.
The catalyst was separated by simple filtration and washed with
acetone several times. The catalyst was then air dried, and can be
reused directly without further purification. The recovered
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Table 2 A3 coupling reaction catalyzed by the catalysta
Entry
1
Alkyne
Amine
Aldehyde
Temperature (°C)
rt
Time (h)
24
Yields (%)b
92
2
3
4
5
rt
rt
rt
rt
24
24
24
12
92
91
89
91
6
rt
24
24
12
24
24
24
85
82
93
84
81
90
7
rt
8
rt
9
50
50
50
10
11
12
13
14
15
16
17
18
rt
12
12
12
12
24
24
24
92
93
91
94
74
65
71
rt
rt
rt
60
60
60
a Reaction conditions: aldehyde (1.0 mmol), alkyne (1.1 mmol), amine (1.1 mmol), catalyst (0.1 mol%), CH2Cl2 (1 mL). b GC yield based on
aldehydes.
Synthesis of propargyl imidazolium bromides
mixture was added dropwise to a stirred solution of NaHCO3
(29.4 g) in water (300 mL) when the target product precipitated.
The precipitate was isolated on a filter frit, washed with water
(3 × 20 mL), and air dried to obtain a brown-yellow solid
(5.18 g). The crude product was re-crystallized by using ethyl
acetate to obtain the target product in a yield of 65%.
N-Methylimidazole, N-mesitylimidazole or N-benzylimidazole
(17.8 mmol) was added dropwise to an 80% solution of pro-
pargyl bromide (2.33 mL, 21.4 mmol, 1.2 equiv, 80% in
toluene) in toluene (5 mL) at room temperature. The reaction
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the reaction was completed, ethyl ether (5.0 mL) was added. The
organic layer was dried over Na2SO4. The solvents were evapor-
ated and the residue was purified by flash chromatography to
give the corresponding products. Most of the products were
known compounds, and were identified by comparison with
their physical, GC/MS and 1H NMR and 13C NMR
spectroscopy.
Recycling of polymer-supported NHC–Ag(I) complex
Fig. 2 Recycling experiment.
The reaction of cyclohexanecarboxaldehyde, phenylacetylene
and piperidine was chosen as a model reaction. At the end of the
process, the reaction mixture was filtered, and the catalyst
residue was washed to completely remove any remaining pro-
ducts and reactants. The solid catalyst was filtered off, washed
with ether, dried under air atmosphere, and then reused for the
next cycle.
was heated to 50 °C, toluene (15 mL) was added, and then the
mixture was reacted overnight. The solvent was removed
in vacuo to yield a target product.
Preparation of azidomethyl polystyrene
Merrifield resin (1.0 g, 1.0 mmol g−1) was reacted in a round-
bottom flask with NaN3 (5 eq) in DMSO (10 mL) at 70 °C for
48 h. After being cooled to room temperature, the suspension
was filtrated and the resin was washed alternatively with MeOH
(5 × 6 mL) and CH2Cl2 (5 × 6 mL) to give azidomethyl poly-
styrene 2. Found (%): C, 82.41; H, 7.328; N, 3.319.
Notes and references
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collected by filtration and washed alternatively with pyridine
(5 × 6 mL), MeOH (5 × 6 mL) and CH2Cl2 (5 × 6 mL). Drying
of the residue in vacuum gave 3.
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Preparation of the polymer-supported NHC–Ag(I) complex
In a round-bottom flask, imidazolium-loaded polymeric support
3 (1.0 g) and silver oxide (0.4 mmol, 0.093 g) were added in
CH2Cl2 (10 mL). The mixture was stirred at room temperature
for 24 h. After the completion of the reaction, the mixture was
filtered and washed alternatively with NH3·H2O (5 × 6 mL),
MeOH (5 × 6 mL) and CH2Cl2 (5 × 6 mL) in order to wash the
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One-pot synthesis of propargylamines catalyzed by
polymer-supported NHC–Ag(I) complexes
In
a
closed Schlenk flask, cyclohexanecarboxaldehyde
(0.112 mg, 1.0 mmol), phenylacetylene (0.112 g, 1.1 mmol),
piperidine (0.094 g, 1.1 mmol), polymer-supported catalyst
(1.0 mmol) and CH2Cl2 (1.0 mL) was added. The mixture was
stirred at room temperature for 24 h. The reaction mixture was
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