Highly efficient three-component (aldehyde-alkyne-amine) coupling reactions catalyzed by a reusable PS-supported NHC-Ag(I) under solvent-free reaction conditions

Article abstract of DOI:10.1016/j.tetlet.2008.09.026

The development of a highly efficient, polystyrene-supported NHC-Ag(I) catalyst for the three-component coupling reactions of aldehydes, alkynes, and amines (A³-coupling) was described. In the presence of PS-NHC-Ag(I) (2 mol %), the A³-reactions were carried out at room temperature under solvent-free reaction conditions and the corresponding propargylamines were generated in good to excellent yields. Furthermore, the catalyst could be reused at least 12 times without a significant loss of its catalytic activity.

Full text of DOI:10.1016/j.tetlet.2008.09.026

Tetrahedron Letters 49 (2008) 6650–6654
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Tetrahedron Letters
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Highly efficient three-component (aldehyde–alkyne–amine)
coupling reactions catalyzed by a reusable PS-supported
NHC–Ag(I) under solvent-free reaction conditions
a
a
Pinhua Li a, Lei Wang a,b, , Yicheng Zhang , Min Wang
*
a Department of Chemistry, Huaibei Coal Teachers College, Huaibei, Anhui 235000, PR China
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 June 2008
Revised 30 August 2008
Accepted 9 September 2008
Available online 11 September 2008
The development of a highly efficient, polystyrene-supported NHC–Ag(I) catalyst for the three-compo-
nent coupling reactions of aldehydes, alkynes, and amines (A3-coupling) was described. In the presence
of PS–NHC–Ag(I) (2 mol %), the A3-reactions were carried out at room temperature under solvent-free
reaction conditions and the corresponding propargylamines were generated in good to excellent yields.
Furthermore, the catalyst could be reused at least 12 times without a significant loss of its catalytic
activity.
Keywords:
Ó 2008 Published by Elsevier Ltd.
Three-component coupling reactions
Aldehyde–alkyne–amine
Propargylamines
PS-supported NHC–Ag(I)
Solvent-free
The transition-metal catalyzed multi-component reaction is a
powerful synthetic tool to access complex structures from simple
precursors.1 In comparison with other transition metals, silver
has been virtually untouched as a catalyst for coupling purposes
because silver species are commonly considered to have low activ-
ity and efficiency, and not to be as good as other late transition
metals. But very recently, silver has received more and more atten-
tion and is also used as a catalyst or co-catalyst in organic reac-
tions, such as addition,2 carbon–carbon bond coupling reaction,3
cyclization,4 and allylation5 In 2003, Li’s group developed the first
silver-catalyzed three-component coupling of aldehyde, alkyne,
and amine (A3-coupling),6 because A3-coupling reactions have
attracted much attention from organic chemists for the coupling
products, propargylamines, which are major skeletons or synthet-
ically versatile building blocks for the preparation of many nitro-
gen-containing biologically active compounds.7 There are several
transition-metal catalysts, which are able to promote this three-
component (A3-coupling) reactions. These include Cu(I) salts,8
the catalysts, Ag(I) and Cu(I) in ionic liquids have been developed
by Li et al.15 and Park et al.,16 respectively, hydroxyapatite-sup-
ported copper (Cu-HAP) and layered double hydroxide supported
gold (LDH-AuCl4) reported by Likhar et al.17 heteropolyacid-sup-
ported silver (Ag-HPA) observed by Reddy et al.18 and AgY zeolite
prepared by Maggi et al.19 were successfully used to catalyze A3-
coupling reactions under heterogeneous reaction conditions with
reusability of catalysts. Most recently, Kidwai et al.20 reported
gold- and copper-nanoparticles as reusable catalysts, and Corma
et al.21 developed CeO2- and ZrO2-stabilized Au(III) as efficient cat-
alysts for the A3-coupling reactions. However, almost all of these
A3-coupling reaction protocols were carried out under heating
reaction conditions. It is desirable to develop highly efficient cata-
lysts for A3-coupling reactions, which can be carried out at ambient
temperature.
In the past decades, N-heterocyclic carbenes (NHCs) are widely
used as ligands in inorganic and organometallic chemistry since
Arduengo and coworkers isolated the first stable N-heterocyclic
carbenes in 1991.22 NHCs were first considered as simple phos-
phine mimics in organometallic chemistry,23 and increasing exper-
imental data have clearly shown that NHC–metal complexes can
surpass their phosphine-based counterparts in both activity and
scope. Complexes of N-heterocyclic carbenes with virtually every
transition metal and many main group elements have been re-
ported.24 Meanwhile, Nolan’s group developed a lot of new NHC–
metal complexes, and which were subsequently used to catalyze
Ag(I) salts,6,9 Au(I)/Au(III) salts,10 Au(III)–salen complexes,11 Ir
13
complexes,12 Hg2Cl2
and Cu/Ru bimetallic systems14 under
homogeneous reaction conditions. But all of these catalytic sys-
tems suffer from the loss of the precious or hazardous catalysts
at the end of the reaction. In order to achieve the recyclability of
* Corresponding author. Tel.: +86 561 380 2069; fax: +86 561 309 0518.
E-mail address: leiwang@hbcnc.edu.cn (L. Wang).
0040-4039/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2008.09.026

P. Li et al. / Tetrahedron Letters 49 (2008) 6650–6654
6651
C–C, C–H, C–O, and C–N bond formation reactions efficiently.25
However, NHC–Ag complexes could be easily synthesized from sil-
ver oxide and the corresponding imidazolium salts, while their cat-
alytic activities and applications in organic reactions received little
attention.26
CH2Cl2. The results are summarized in Table 1. The influence of a
substituted group of imidazolium salts in NHC–Ag(I) on their cat-
alytic activity for the A3-coupling reaction is C6H5CH2 > C6H5 > t-
C4H9 > CH3. It is obvious that the catalytic activity of NHC–Ag(I)
decreases for the A3-coupling reaction in order: 2b > 2c > 2d > 2a
(Table 1, entries 1–4). This order was also found in the sequence
of PS–NHC–Ag(I) catalysts, 4a–4d (Table 1, entries 5–8). The effect
of polymer support on the catalytic activity was also investigated,
and the results showed that PS–NHC–Ag(I) with polystyrene back-
bone as support were bestowed with a little higher catalytic activ-
ity than their corresponding NHC–Ag(I) analogues (Table 1, 2a
versus 4a, 2b versus 4b, 2c versus 4c, and 2d versus 4d). As a result,
it is evident that 2b and 4b are the best catalysts for the A3-cou-
pling reaction, and especially, 4b exhibits the highest catalytic
activity. (Table 1, entries 2 and 6). Poor results were observed
when Ag2O or AgI was used as catalyst (Table 1, entries 9 and
10), and when the reaction was performed in the absence of any
silver source, no desired A3-coupling product was detected.
Environmental concerns associated with chemical processes
have encouraged the development of more environmentally
friendly methodology for organic reactions. In recent years, immo-
bilizing transition-metal catalysts have received considerable
interest. A great deal of efforts have been made for the develop-
ment of carbon–carbon bond formation reactions catalyzed by
supported NHC-metal complexes.27 As a part of our ongoing inter-
est in the synthesis of propargylamines with A3-coupling reactions,
we herein report polystyrene-supported NHC–Ag(I) complexes, as
well as NHC–Ag(I) complexes, as efficient catalysts for the three-
component coupling of aldehydes, alkynes, and amines (A3-reac-
tions) under solvent-free conditions at room temperature. The
reactions generated the corresponding propargylamines in good
to excellent yields. This methodology provided a wide range of
substrate applicability and could be applied to aromatic and ali-
phatic aldehydes, amines, and alkynes. To the best of our knowl-
edge, this is the first example of NHC–Ag(I) or PS–NHC–Ag(I)
catalyzed A3-coupling reactions (Scheme 1). It is noteworthy that
PS–NHC–Ag(I) could be reused at least 12 times without a signifi-
cant loss of its catalytic activity.
The effect of solvent on A3-coupling of para-formaldehyde,
phenylacetylene, and piperidine using 4b as catalyst was surveyed
(Table 2). Among the solvents tested in Table 2, acetone, CH3CN,
DMSO, and CH2Cl2 were the most suitable reaction media for A3-
coupling reaction (Table 2, entries 1–4). DMF was inferior and gen-
erated the corresponding product in 84% yield (Table 1, entry 5),
whereas toluene and THF afforded moderate yields of desired
products (Table 1, entries 6 and 7). Poor results were observed
when the reactions were carried out in C2H5OH and H2O (Table
1, entries 8 and 9). Surprisingly, 97% yield of the desired product
was isolated when the reaction was carried out under neat reaction
conditions at room temperature (Table 1, entry 10). To avoid the
use of volatile solvents and reduce the environmental pollution,
all the three-component coupling reactions were performed under
solvent-free reaction conditions. Thus, the optimized reaction
The synthesis of a series of NHC–Ag(I) and PS–NHC–Ag(I) cata-
lysts is illustrated in Scheme 1. They were readily prepared in good
yields through a straightforward one- or two-step procedure from
commercially available starting materials and reagents.28 Our ini-
tial investigation was focused on the catalytic activity of NHC–
Ag(I) and PS–NHC–Ag(I). The above-synthesized catalysts were
employed in the A3-coupling of para-formaldehyde, phenylacetyl-
ene, and piperidine (a model reaction) at room temperature in
Ag2O
2a: R = CH3
2b: R = C6H5CH2
2c: R = C6H5
2d: R = t-C4H9
N
CH2Cl2, r.t.
N
Cl
N
R
N
R
Ag
1
Cl
2
NHC Ag(I)
N
N
R
P
P
CH2Cl
Toluene, 80 oC
N
Cl
Polystyrene
N
R
3
P
Ag2O
N
4a: R = CH3
4b: R = C6H5CH2
4c: R = C6H5
CH2Cl2, r.t.
N
R
Ag
Cl
4d: R = t-C4H9
4
PS NHC Ag(I)
NHC-Ag(I) (2 mol%) or
PS-NHC-Ag(I) (2 mol%)
R1
NR2R3
R1CHO
HNR2R3
+
+
R
H
R
Solvent-free, r.t.
R = Aromatic, Aliphatic
R1 = H, Aromatic, Aliphatic
R2, R3 = Aromatic, Aliphatic
Scheme 1.

6652
P. Li et al. / Tetrahedron Letters 49 (2008) 6650–6654
Table 1
Effect of silver catalyst on A3-coupling reactiona
O
Catalyst
NH
+
CH2
N
+
H
H
Entry
Catalyst
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
4a
4b
4c
4d
Ag2O
AgI
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
61
80
74
68
69
90
80
75
37
31
10
a
Reaction conditions: para-formaldehyde (1.0 mmol), piperidine (1.1 mmol), phenylacetylene (1.1 mmol), silver catalyst (2 mol %), CH2Cl2 (0.5 mL), nitrogen atmosphere,
at room temperature for 24 h.
b
Isolated yields.
lent yields, and the reaction of aryl aldehydes was also tolerant of
ortho substitution (Table 3, entries 10–15).
Table 2
Effect of solvent on A3-coupling reaction using 4b as catalysta
Subsequently, a variety of alkynes were also examined for the
A3-coupling by using para-formaldehyde–N,N-dibenzylamine–al-
kynes manner (Table 3, entries 16–22). As can be seen from the
Table 3, the reactivity of both aliphatic and aromatic alkynes was
observed, in which aromatic alkynes were often more reactive than
aliphatic ones. Aromatic alkynes, including those bearing func-
tional groups, such as methyl, phenyl, chloro, and bromo, were able
to undergo three-component-coupling smoothly and generate the
corresponding products in excellent yields (Table 3, entries 16–19).
The reactions involving terminal aliphatic alkynes also gave both
higher conversions and isolated yields under the present reaction
conditions (Table 3, entries 20–23). However, no A3-reaction prod-
uct was isolated and starting materials were recovered when ethy-
nyltrimethylsilane was served as terminal alkyne substrate (Table
3, entry 24).
The recyclability of PS–NHC–Ag(I) catalyst 4b was also investi-
gated. After carrying out the reaction, the catalyst 4b was sepa-
rated by simple filtration and washed with acetone (2 mL Â 2).
After air-dried, it could be reused directly without further purifica-
tion. The recovered catalyst was used in the next run and almost
consistent activity was observed for 12 consecutive cycles (Table
4, entries 1–12). Meanwhile, silver leaching in polystyrene-sup-
ported NHC–Ag(I) catalyst 4b was determined. Inductively coupled
plasma (ICP) analysis of the clear filtrates obtained by filtration
after the reaction indicated that Ag content is less than 0.1 ppm.
However, only trace amount of the desired product was isolated
for the model reaction by adding substrates to a filtrate obtained
from the filtration of the supported catalyst after the reaction.
Representative preparation of NHC-Ag(I) catalyst 2b: In an oven-
dried Schlenk flask, 1,3-dibenzylimidazolium chloride 2a
(285 mg, 1.0 mmol) and silver(I) oxide (116 mg, 0.5 mmol) were
dissolved/suspended in dichloromethane (10 mL) and stirred at
room temperature in the dark for 12 h. The resulting solution
was filtered and concentrated, and the silver complex precipitated
by addition of excess diethyl ether as a pale crystal, 2b (243 mg,
62% yield).28 1HNMR (300 MHz, CDCl3): d 7.48–7.40 (m, 6H),
7.33–7.26 (m, 4H), 6.98 (s, 2H), 5.34 (s, 4H). 13C NMR (75 MHz,
CDCl3): d 151.1, 135.2, 129.1, 128.9, 127.9, 121.7, 55.8.
O
Catalyst 4b
CH2
N
NH
+
+
H
H
Entry
Catalyst
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
9
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
4b
Acetone
CH3CN
DMSO
CH2Cl2
DMF
Toluene
THF
C2H5OH
H2O
95
91
90
90
84
69
62
49
42
97
71c
10
11
Neat
Neat
a
Reaction conditions: para-formaldehyde (1.0 mmol), piperidine (1.1 mmol),
phenylacetylene (1.1 mmol), 4b (2 mol %), solvent (0.5 mL), nitrogen atmosphere at
room temperature for 24 h.
b
Isolated yields.
c
PS–NHC–Ag(I) (1 mol %) was used.
conditions for this A3-coupling reaction are PS–NHC–Ag(I) catalyst,
4b (2 mol %) in neat reaction conditions at room temperature for
24 h.
To examine the scope of this three-component coupling reac-
tion, we extended our studies to different combinations of amines,
aldehydes, and alkynes. The results are outlined in Table 3. At the
beginning of the search for the amine substrate scope, phenylacet-
ylene was used as a model substrate and various amines with dif-
ferent aldehydes were examined (Table 3, entries 1–15). The
results indicated that cyclic, heterocyclic, and acyclic secondary
aliphatic amines gave excellent yields of the desired products at
room temperature with the combination of phenylacetylene–
para-formaldehyde–amines under standard reaction conditions
(Table 3, entries 1–5). Fortunately, secondary aromatic amines also
reacted with benzaldehyde and phenylacetylene to give the corre-
sponding products in good yield (Table 3, entry 6).
In order to expand the scope of aldehyde substrates, a combina-
tion of penylacetylene–piperidine–aldehydes was chosen, and var-
ious aldehydes were examined. The aliphatic aldehydes, including
cyclic and acyclic ones, displayed high reactivity under present
reaction conditions (Table 3, entries 7–9). Aromatic aldehydes with
both electron-donating and electron-withdrawing functionalities
also afforded the corresponding propargylamines in good to excel-
Representative preparation of Merrifield resin supported ionic
liquid 3b: Under nitrogen atmosphere, benzylimidazole (395 mg,
2.5 mmol) and Merrifield resin (2.0 g, loading of benzylcholoride,
1.0 mmol gÀ1) were mixed in toluene (15 mL) in a round-bottomed
flask. The reaction was carried out at 80 °C for 24 h. Then the solu-
tion was filtered and the solid was washed with chloroform (5 mL),

P. Li et al. / Tetrahedron Letters 49 (2008) 6650–6654
6653
Table 3 (continued)
Table 3
A3-coupling reaction catalyzed by PS–NHC–Ag(I) catalyst 4ba
Entry
Alkyne
Aldehyde
CH2O
Amine
Yieldb (%)
85
R1
PS-NHC-Ag(I), 4b
R1CHO
+
NHR2R3
Ph
Ph
+
R
22
C6H5CH2OCH2C„CH
R
Solvent-free
HN
NR2R3
Entry
1
Alkyne
Aldehyde Amine
Yieldb (%)
Ph
Ph
23
C2H5O2CC„CH
(CH3)3SiC„CH
CH2O
CH2O
87
0
HN
HN
C6H5C„CH
CH2O
CH2O
97
94
HN
HN
Ph
Ph
2
3
C6H5C„CH
C6H5C„CH
24
O
Ph
Ph
a
Reaction conditions: aldehyde (1.0 mmol), amine (1.1 mmol), alkyne
(1.1 mmol), PS–NHC–Ag(I), 4b (2 mol %), nitrogen atmosphere at room temperature
for 24 h.
CH2O
CH2O
CH2O
95
97
98
HN
b
Isolated yields.
At 50 °C for 5 h.
c
4
5
C6H5C„CH
C6H5C„CH
N
H
HN
methanol (5 mL), and ethyl acetate (5 mL), respectively, and dried
under vacuo at 60 °C. 2.26 g of pale resin was obtained. The loading
of the Merrifield resin supported ionic liquid 3b was quantified via
CHN microanalysis and found to be 0.92 mmol gÀ1 based on nitro-
NHCH3
6
C6H5C„CH
C6H5CHO
85c
gen percentage. IR (film): 2851, 1353, 1077, 1028 cmÀ1
.
Representative preparation of PS–NHC–Ag(I) catalyst 4b: In an
oven-dried Schlenk flask, Ag2O (116 mg, 0.5 mmol); 3b (1.1 g),
and CH2Cl2 (10 mL) were added. The resulting suspension was stir-
red at room temperature for 10 h. Then the solution was filtered
and the solid was washed with methanol (4 mL) and acetone
(4 mL Â 2), respectively, and dried under vacuo at 60 °C for 3 h.
PS–NHC–Ag(I) catalyst 4b was obtained as a light gray resin
(1.16 g). IR (film): 2907, 1373, 1093, 1020 cmÀ1. The silver amount
of the catalyst 4b was found to be 0.78 mmol gÀ1 based on ICP
analysis.
7
8
C6H5C„CH
C6H5C„CH
94
91
HN
CHO
CHO
HN
HN
9
C6H5C„CH
90
CHO
10
11
12
13
14
15
C6H5C„CH
C6H5C„CH
C6H5C„CH
C6H5C„CH
C6H5C„CH
C6H5C„CH
C6H5CHO
92c
95c
92c
89c
96c
92c
HN
HN
HN
HN
HN
HN
Typical procedure of A3-coupling reaction catalyzed by NHC–Ag(I)
catalyst 2b: In an oven-dried Schlenk flask, catalyst 2b (8 mg,
0.02 mmol), phenylacetylene (110 mg, 1.1 mmol), para-formalde-
hyde (30 mg, 1.0 mmol), piperidine (94 mg, 1.1 mmol), and CH2Cl2
(1.0 mL) were added. The mixture was stirred at room temperature
for 24 h. After the reaction was completed, ethyl ether (5 mL) was
added. The organic layer was dried over Na2SO4, filtered, concen-
trated, and the residue was purified by flash chromatography on
silica gel (eluant:hexane/ethyl acetate = 3:1, V/V) to give the corre-
sponding A3-coupling product [1-(3-phenyl-prop-2-yn-1-yl)piper-
idine] as a colorless oil (160 mg, 80% yield).
p-ClC6H4CHO
m-ClC6H4CHO
o-ClC6H4CHO
p-CH3C6H4CHO
p-CH3OC6H4CHO
Typical procedure of A3-coupling reaction catalyzed by PS–NHC–
Ag(I) catalyst 4b: Under nitrogen atmosphere, PS–NHC–Ag(I) cata-
lyst 4b (32 mg, contains silver 0.02 mmol), phenylacetylene
(110 mg, 1.1 mmol), para-formaldehyde (30 mg, 1.0 mmol), and
piperidine (94 mg, 1.1 mmol) were added in a 10 mL of Schlenk
flask. The mixture was stirred at room temperature for 24 h. After
the reaction was completed, ethyl ether (3 mL Â 3) was added and
the slurry was stirred to ensure removal of the product from the
surface of catalyst, then filtered using a sintered glass funnel. The
residue was washed with ethyl ether. The combined organics were
dried over Na2SO4, filtered, concentrated, and the residue was puri-
fied by flash chromatography on silica gel (eluant:hexane/ethyl
acetate = 3:1, V/V) to give the corresponding A3-coupling product
Ph
Ph
16
17
p-CH3C6H4C„CH
p-C6H5C6H4C„CH
CH2O
CH2O
96
95
HN
HN
Ph
Ph
Ph
Ph
18
19
20
21
p-ClC6H4C„CH
p-BrC6H4C„CH
n-C8H17C„CH
n-C6H13C„CH
CH2O
CH2O
CH2O
CH2O
98
96
90
89
HN
HN
HN
HN
Ph
Ph
[1-(3-phenyl-prop-2-yn-1-yl)piperidine] as
(193 mg, 97% yield).
a
colorless oil
Ph
Ph
In summary, we have successfully developed a novel, practical
and environmentally friendly method for the syntheses of propar-
gylamines through three-component coupling of aldehydes,
amines, and alkynes by using PS–NHC–Ag(I) catalyst 4b (2 mol %)
at room temperature under solvent-free reaction conditions. The
reactions generated the corresponding propargylamines in high
Ph
Ph

6654
P. Li et al. / Tetrahedron Letters 49 (2008) 6650–6654
Table 4
Successive trials by using reused PS–NHC–Ag(I) catalyst 4ba
Reused
O
PS-NHC-Ag(I), 4b
NH
+
CH2
N
+
Solvent-free
H
H
Cycle
Yieldb (%)
Cycle
Yieldb (%)
1
2
3
4
5
6
97
94
96
95
94
94
7
8
9
10
11
12
93
92
93
92
90
91
a
Reaction conditions: para-formaldehyde (1.0 mmol), phenylacetylene (1.1 mmol), piperidine (1.1 mmol), reused PS–NHC–Ag(I) catalyst 4b (2 mol %), nitrogen atmo-
sphere at room temperature for 24 h.
b
Isolated yields.
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alkynes, and amines. In addition, this methodology offers a com-
1532.
petitive recyclability of the catalyst without a significant loss of
its catalytic activity and the catalyst could be readily recovered
and reused, thus making this procedure more environmentally
acceptable whilst no catalyst leaching was observed. Further inves-
tigation on the application of this kind of supported catalysts is still
underway in our laboratory.
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We gratefully acknowledge financial support by the National
Natural Science Foundation of China (Nos. 20572031 and
20772043).
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