Ag2CO3-catalyzed efficient synthesis of internal or terminal propargylicamines and chalcones via A3-coupling under solvent-free condition

Article abstract of DOI:10.1016/j.cclet.2021.04.026

Several simple, fast and practical protocols have been developed to synthesize internal or terminal propargylamines and chalcones via A³-coupling reaction of aldehydes, amines, and alkynes catalyzed by an easily available catalyst Ag₂CO₃ under solvent-free condition. The reaction proceeded smoothly to deliver various products in good-to-excellent yields with good functional group tolerance. Gram-scale preparation, bioactive molecule synthesis and asymmetric substrates have been demonstrated. Furthermore, plausible mechanisms for the synthesis of different products have been proposed.

Full text of DOI:10.1016/j.cclet.2021.04.026

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Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Ag2CO3-catalyzed efficient synthesis of internal or terminal
propargylicamines and chalcones via A3-coupling under solvent-free
condition
Ningbo Lia,c,1, Shitang Xub,1, Xueyan Wang b, Li Xua, Jie Qiaoa,c, Zhiwu Liangb, Xinhua Xub,∗
a Basic Medical College, Shanxi Medical University, Taiyuan 030001, China
b College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
c Key Laboratory of Cellular Physiology, Ministry of Education, Shanxi Medical University, Taiyuan 030001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Several simple, fast and practical protocols have been developed to synthesize internal or terminal propar-
gylamines and chalcones via A3-coupling reaction of aldehydes, amines, and alkynes catalyzed by an eas-
ily available catalyst Ag2CO3 under solvent-free condition. The reaction proceeded smoothly to deliver
various products in good-to-excellent yields with good functional group tolerance. Gram-scale prepa-
ration, bioactive molecule synthesis and asymmetric substrates have been demonstrated. Furthermore,
plausible mechanisms for the synthesis of different products have been proposed.
Received 23 February 2021
Revised 14 April 2021
Accepted 14 April 2021
Available online xxx
Keywords:
Ag-catalyzed
A3-coupling
Solvent-free
Propargylamines
Chalcones
© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia
Medica, Chinese Academy of Medical Sciences.
Propargylamines and chalcones are widely used as valuable
compounds in organic synthesis, medicinal chemistry, and mate-
rial science due to their importance as versatile synthetic blocks,
biological activities and functional materials [1]. As a consequence,
the preparation of propargylamines and chalcones has been inten-
sively studied in the past decades [2]. In general, there are three
main methods to prepare propargylamines (Scheme 1). The tradi-
tional method is nucleophilic addition of alkynyl–metal reagents to
imines for the preparation of propargylamines (Scheme 1a). How-
ever, highly reactive reagents such as n-butyllithium or Grignard
reagents used to prepare alkynyl–metal reagents are highly mois-
ture sensitive and require harsh reaction conditions [3]. Transition-
metal-catalyzed A3-coupling reaction of aldehydes, amines and
alkynes is perhaps more attractive strategy [4]. Several transi-
tion metal catalysts such as AgI, AuBr, InCl3, CoCl2(PPh3)2, FeCl3,
Mn(OAc)2, and metal nanometerials have been used in such three-
component reactions (Scheme 1b) [5]. Nonetheless, some draw-
backs such as high reaction temperature, using the toxic solvents,
utilizing inert gas protection, long reaction time, laborious prepa-
ration of metal nanomaterial and moderate yields are rather com-
mon among these methods. Furthermore, most of the synthesized
propargylamines are internal alkyne structures, and the synthe-
sis of terminal propargylamines was rarely reported [6]. Alter-
natively, Li’s group and other groups developed several methods
based on copper-catalyzed oxidative coupling of terminal alkyne
and amines followed by C−C bond formation to construct propar-
gylamine (Scheme 1c) [7]. But the peroxide tert–BuOOH is indis-
pensable during the catalytic process.
For the preparation of chalcones, the main methods are the
Claisen-Schmidt condensation or hydration-condensation of aro-
matic alkynes with aldehydes (Scheme 1d) [8]. However, low se-
lectivity and harsh reaction conditions limited their application.
Therefore, further development of a simple, fast and practical ap-
proach to synthesize internal or terminal propargylamines and
chalcones is very necessary in organic synthesis.
In recent years, silver salts as common catalysts are widely used
in organic synthesis and industry due to the advantages of high
catalytic activity and easy availability [9]. Furthermore, solvent-
free synthetic reactions have attracted increasing attention from
chemists [10]. In this paper, we reported Ag2CO3 as a commer-
cial catalyst for the efficient preparation of internal or terminal
propargylamines and chalcones via A3 coupling reaction of aldehy-
des, amines and alkynes under solvent-free condition (Scheme 1e).
Compared to the reported methods [5], the present protocols pro-
vided several simple, fast, cost-effective and practical ways for the
preparation of various propargylamines and chalcones.
∗
Corresponding author.
E-mail address: xhx1581@hnu.edu.cn (X. Xu).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.cclet.2021.04.026
1001-8417/© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Please cite this article as: N. Li, S. Xu, X. Wang et al., Ag2CO3-catalyzed efficient synthesis of internal or terminal propargylicamines and
chalcones via A3-coupling under solvent-free condition, Chinese Chemical Letters, https://doi.org/10.1016/j.cclet.2021.04.026

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Table 1
Optimization of reaction conditions.a
Catalyst
(mol%)
Temp.
Entry
Solvent
(°C)
Time
Yield (%)b
1
Ag2CO3(3)
Ag2CO3(3)
Ag2CO3(3)
Ag2CO3(3)
Ag2CO3(3)
Ag2CO3(3)
Ag2CO3(3)
Ag2CO3(3)
Ag2CO3(3)
Ag2CO3(2)
Ag2O(3)
Toluene
80
12 h
72
67
70
50
52
84
93
98
97
83
65
43
57
53
58
62
2
H2O
80
12 h
3
CHCl3
80
12 h
4
THF
80
12 h
5
CH3CN
80
12 h
6
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
80
2 h
7
100
110
120
110
110
110
110
110
110
30 min
10 min
10 min
10 min
10 min
10 min
10 min
10 min
10 min
10 min
8
9
10
11
12
13
14
15
16
AgBF4(3)
AgNO3(3)
Ag2SO4(3)
AgOTf(3)
AgI(3)
110
a
All reactions were performed with benzaldehyde (1a, 1.0 mmol), piperidine
(2a, 1.2 mmol), phenylacetylene (3a, 1.5 mmol) in above conditions.
b
Isolated yields.
ꢀ
5-bromothiophene-2-carbaldehyde, [1,1 -biphenyl]−4-carbaldehyde
and N-(4-formylphenyl)acetamide were subjected to this process,
the desired products 4i, 4j and 4k were obtained in 87%, 90% and
88% yield, respectively, showing good tolerance. Moreover, the cy-
clohexanecarbaldehyde was also effective to generate the target
product with 91% yield (4l).
Scheme 1. Synthesis of propargylamines and chalcones.
This study was commenced by optimizing the reaction condi-
tions using benzaldehyde 1a, piperidine 2a and phenylacetylene
3a as model substrates (Table 1). A mixture of 1a (1 mmol), 2a
(1.2 mmol), 3a (1.5 mmol) and Ag2CO3 (0.03 mmol) in toluene
(3.0 mL) was stirred at 80 °C for 12 h. The reaction took place
and gave 72% isolated a yield of 4a (entry 1). When we employed
H2O, CHCl3, THF and CH3CN as the solvents, lower yields were ob-
served (entries 2–5). Interestingly, the solvent-free reaction pro-
ceeded rapidly in good yields (entry 6). As the temperature con-
tinued to increase at 100 °C, the yield of 4a increased to 93%. The
highest yield was achieved during the temperature at 110 °C and
the reaction time was only 10 min (entries 7–9). When the loading
amount of Ag2CO3 was reduced, yield of 4a decreased to 83% (en-
try 10). We also examined other silver salts such as Ag2O, AgBF4,
AgNO3, Ag2SO4, AgOTf and AgI, but moderate yields were ob-
served (entries 11–16). Therefore, the optimal reaction conditions
were as follows: aldehydes (1.0 mmol), amines (1.2 mmol), alkynes
(1.5 mmol), and Ag2CO3 (0.03 mmol) at 110 °C under solvent-free
condition.
Next, a variety of terminal alkynes and secondary amines were
screened under optimal condition. As for electron-deficient and
electron-rich phenylacetylenes with different substituents (F, Cl,
OMe, Me, Et and n-C5H11 ), regardless of the location at the ortho-,
meta-, or para-position, the products were obtained in good to ex-
cellent yields (4m-4t). 3,3-Diethoxyprop-1–yne with acetal group
could also be tolerated to afford the desired product 4u in 85%
yield. However, It is important to point out that trimethyl(prop–2-
yn-1-yloxy)silane did not produce the corresponding product, in-
stead desilylation of the TMS group (4v). Furthermore, the reaction
using heterocyclic secondary amines such as pyrrolidine, azepane,
morpholine, and thiomorpholine proceeded smoothly resulting in
the desired products in high yields (4w-4z).
Guided by removing the silyl group of the product 4t, we in-
vestigated another A3-coupling reaction of benzaldehyde, piperi-
dine and trimethyl(phenylethynyl)silane utilizing Ag2CO3-catalyzed
activation of the C–Si bond (optimal conditions in Table S1, Sup-
porting information). Gratifyingly, above Ag2CO3 catalytic system
can promote the reaction to proceed smoothly producing the cor-
responding propargylamines. As illustrated in Scheme 3, both aro-
matic aldehydes with electron-withdrawing (F, NO2) and electron-
donating (OMe) groups can react smoothly with piperidine and
trimethyl(phenylethynyl)silane to obtain propargylamines (4a’–4c’).
Meanwhile, alkynylsilanes possessing chlorine and alkyl groups
at para- position underwent the reaction smoothly delivering the
target products in excellent yields (4d’–4f’). Alkyl pivalaldehyde
and hex–1-yn-1-yltrimethylsilane can be transformed to the cor-
responding products with 80% and 87% yield (4g’, 4h’). In addi-
tion, noncyclic secondary amines were also effective to generate
the target products in good yields (4i’, 4j’). Thus, the above results
prove that Ag2CO3 can be used as an effective catalyst to synthe-
With the optimal reaction conditions in hand, the substrate
scope of the A3 coupling reaction was investigated with respect
to aldehydes, amines and terminal alkynes (Scheme 2). First,
several aldehydes bearing electron-rich (Me, OMe and OH) and
electron-deficient (F, Cl) substituents on the aromatic ring were
successfully converted into the corresponding propargylamines in
up to 99% yield (4b-4h). Among them, slight decreases in reac-
tivity were observed for 4c and 4h, probably because the rel-
atively strong electron-donating effect and steric hindrance im-
peded the nucleophilic addition of phenylacetylene to an imine.
The reactivity of the ortho position is lower than that of para
and meta for chlorine-substituted benzaldehyde (4e-4g). When
2

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Scheme 3. Reaction scope of aldehydes, secondary amines or alkynylsilanes. Con-
ditions: Catalyst Ag2CO3 (3 mol%), aldehydes (1, 1.0 mmol), secondary amines (2,
1.2 mmol), alkynylsilanes (5, 1.5 mmol), solvent-free, 110 °C, isolated yields.
as good coupling partners for the preparation of terminal propar-
gylamines through A3-coupling reaction catalyzed by Ag2CO3 with-
out using excessive F− reagent [11].
Accidentally, we extended the reaction time over 2 h for the re-
action of benzaldehyde, piperidine and phenylacetylene catalyzed
by Ag2CO3 at 110 °C under solvent-free condition. The other prod-
uct was (E)-chalcone, instead of stopping at the A3-coupling step.
So we developed a novel method based on Ag2CO3-catalyzed the
reaction of aldehydes, piperidine and terminal alkynes to obtain
valuable chalcones (optimal conditions in Table S3, Supporting in-
formation). As shown in Scheme 4, we obtained moderate-to-good
yields of (E)-chalcones. Aromatic aldehydes and phenylacetylenes
with electron-withdrawing groups (CN, F, Cl and Br) show excel-
lent diastereoselectivity (6b–6f, 6h-6j). On the contrary, pheny-
lacetylenes with electron-donating groups (Me, Et, n-Pr, t-Bu) show
good diastereoselectivity except for CH3 at meta position (6k–6l).
In addition, the π-extended 2-naphthyl aldehyde is also a suitable
substrate for this transformation (6g). Unfortunately, aliphatic alde-
hydes and aliphatic alkynes were not active in this transformation.
As a practical synthetic protocol, both the operational simplicity
and scalability have great significance for the synthesis of propar-
gylamines and chalcones in the laboratory, and even in the field
of the pharmaceutical industry. Therefore, both large-scale prepa-
ration and bioactive molecule synthesis via A3-coupling reaction
were further investigated. Pleasingly, 10 mmol of benzaldehyde
(1a) was employed to react with piperidine (2a) and phenylacety-
lene (3a) under standard conditions and 92% yield of 4a was ob-
tained (Scheme 5a), which is a positive aspect for industrial ap-
plication. Importantly, the above catalysis system could be used
for bioactive molecule synthesis such as ethisterone and N-ethyl-
3-carbazolecarboxaldehyde to afford the corresponding products
Scheme 2. Reaction scope of aldehydes, secondary amines or terminal alkynes.
Conditions: Catalyst Ag2CO3 (3 mol%), aldehydes (1, 1.0 mmol), secondary amines
(2, 1.2 mmol), terminal alkynes (3, 1.5 mmol), solvent-free, 110 °C, isolated yields.
size propargylamines easily and quickly, and showed great advan-
tages over the reported catalytic systems [5].
Although the preparation of the propargylamines with inter-
nal alkyne structure has a certain practicability, terminal propar-
gylamines may provide a wider range of applications due to the
unique reactivity of sp C−H bond. Therefore, we chose TMS–
acetylene as the coupling reagent by direct sp C-Si cleavage with
aldehydes and amines to synthesize terminal propargylamines cat-
alyzed by Ag2CO3. As illustrated in Scheme 3, all the aldehydes
and secondary amines substrates showed high reactivity delivering
the terminal propargylamines in high yields (4k’–4p’). It was note-
worthy that alkynylsilane containing TES could also be converted
to the corresponding terminal propargylamine 4k’ (removing TES),
which is in sharp contrast with that AgI-catalyzed A3-coupling re-
action in the previous reported method (retaining TES) [5a]. Sur-
prisingly, when using 3-(trimethylsilyl)propiolic acid as a substrate,
the results showed the product 4k’ was also the terminal propar-
gylamine, which was first disclosed by us. Likewise, substrates
aldehydes and secondary amines were readily converted into valu-
able terminal alkynes in satisfied yields (4q’–4t’). Hence, both
TMS–acetylene and 3-(trimethylsilyl)propiolic acid could be used
3

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Scheme 5. (a) Large-scale synthesis of 4a. (b) Bioactive molecule synthesis. (c) In-
vestigation of substrate-controlled asymmetric Ag2CO3-catalyzed A3-coupling.
Scheme 4. Reaction scope of aldehydes or terminal alkynes. Conditions: Catalyst
Ag2CO3 (3 mol%), aldehydes (1, 1.0 mmol), piperidine (2a, 1.2 mmol), terminal
alkynes (3, 1.5 mmol); solvent-free; 110 °C, 5 h; Isolated yields; The ratio of E/Z
was estimated by the integral area of aromatic ring hydrogen atom of the product
in 1H NMR.
–
posed in Scheme 6. The Ag2CO3 activated the C H bond of ter-
minal alkyne or C-Si bond of alkynylsilane to generate the sil-
ver acetylide intermediate, which reacted with the iminium ion I
formed by aldehydes and piperidine to afford the propargylamines
and releases the silver ion for further reaction (Path A). The alkynyl
carbon bonded to 3-(trimethylsilyl)propiolate attacked the iminium
ion I to form intermediate II, which decarboxylated to give the
propargylamine with TMS group. At last, the Ag2CO3 activated C-
Si bond, delivering the desired terminal propargylamine (Path B).
Ag2CO3-catalyzed A3 coupling resulted in the formation of propar-
gylamine, which was deprotonated by excessive base piperidine in
the coordination of silver with the triple bond to form interme-
diate III. Subsequent protonation generated allenylamine interme-
diate IV, which was further hydrolyzed to give preference to the
thermodynamically more stable E configuration product 6 (Path C).
In conclusion, we have developed several simple, efficient and
practical methods for preparing internal or terminal propargy-
lamines and chalcones via A3 coupling of aldehydes, amines and
alkynes using easily available Ag2CO3 as a catalyst under solvent-
with yields of 88% and 92% (4u’, 4v’) (Scheme 5b), respectively.
Furthermore, we screened the compound 4v’ for antiproliferative
activities in two human cancer cell lines (colorectal carcinoma
HCT116 cells and hepatoma HepG2 cells) using the CCK-8 assay.
Consequently, compound 4v’ showed good inhibitory activity for
these two human cancer cell lines at the relatively low μmol/L
level, with IC50 values of 61.01 μmol/L and 72.58 μmol/L, re-
spectively (Supporting information). A further bioactivity investi-
gation is still underway in our laboratory. Moreover, the diastere-
oselectivity of Ag2CO3-catalyzed A3-coupling reaction was inves-
tigated by using benzaldehyde, (S)-N-benzyl-1-phenylethylamine
and phenylacetylene as model substrates. It was worth noting that
(S)-N-benzyl-1-phenylethylamine showed good diastereoselectivity
(2.5:1). Meanwhile, when (S)-N-benzyl-1-phenylethylamine was re-
acted with formaldehyde and phenylacetylene, the desired prod-
ucts 4x’, which had no racemization, were afforded in 98% yield
(Scheme 5c).
–
free condition. The Ag2CO3 can effectively activate not only C H
Besed on previously well-documented results [5a,6a,12], plau-
sible mechanisms for the synthesis of different products are pro-
bond in terminal alkyne, but C-Si bond in alkynylsilane to af-
ford the corresponding products. Both TMS–acetylene and 3-
Scheme 6. Plausible mechanisms.
4

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(trimethylsilyl)propiolic acid could be used as good coupling part-
ners delivering the terminal propargylamines. Besides, the above
methods showed broad substrate group with good functional group
tolerance and could be applied to synthesize 1-(1,3-diphenylprop-
2-yn-1-yl)piperidine 4a in gram scale. Multifunctional compounds
such as ethisterone and N-ethyl-3-carbazolecarboxaldehyde could
also achieve the corresponding transformation and compound 4v’
showed good inhibitory activity against CHT116 cells and HepG2
cells. The stereoselectivity of A3-coupling reaction was also inves-
tigated. Given the operational simplicity, easily available commer-
cial catalysts, short reaction time, high-efficiency and the diversity
of products, these developed methods are expected to be ideal for
organic intermediate synthesis and fine chemical production.
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Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgments
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We are grateful for the financial support from the National Nat-
ural Science Foundation of China (Nos. 21802093, 21536003), Sci-
entific and Technological Innovation Programs of Higher Education
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