Recyclable Cu/C3N4 composite catalyzed AHA/A3 coupling reactions for the synthesis of propargylamines

Article abstract of DOI:10.1039/c8ra06613b

The heterogeneous Cu/C₃N₄ catalyst was found to be efficient for the synthesis of propargylamines using a three-component coupling reaction of alkynes, CH₂Cl₂ and amines (AHA) without additional base. Moreover,

Full text of DOI:10.1039/c8ra06613b

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Recyclable Cu/C3N4 composite catalyzed AHA/A3
coupling reactions for the synthesis of
propargylamines†
Cite this: RSC Adv., 2018, 8, 32942
Hang Xu,a Jun Wang,a Peng Wang,a Xiyu Niu,a Yidan Luo,b Li Zhub
a
*
and Xiaoquan Yao
The heterogeneous Cu/C3N4 catalyst was found to be efficient for the synthesis of propargylamines using
a three-component coupling reaction of alkynes, CH2Cl2 and amines (AHA) without additional base.
Moreover, the catalyst also showed highly catalytic activity in the synthesis of C1-alkynylated
tetrahydroisoquinolines (THIQs) via an A3 reaction of alkynes, aldehydes and THIQ. The Cu/C3N4-
catalyzed multicomponent reactions exhibited good functional group tolerance in most examples.
Furthermore, the easily prepared Cu/C3N4 catalyst could be recovered and reused conveniently over 5
times without losing catalytic activities.
Received 6th August 2018
Accepted 18th September 2018
DOI: 10.1039/c8ra06613b
rsc.li/rsc-advances
in CH2Cl2 solution.15 Although a highly catalytic activity was
observed, additional strong base (Cs2CO3) was also required in the
Introduction
reaction and only low recyclability of the Cu NPs catalyst was
observed. On the other hand, a metal-free AHA reaction was also
developed, but only moderate yields were obtained.16
Multicomponent reactions (MCRs) are pretty important and
effective in carbon–carbon bond formation because of their
considerable economic and ecological merits.1 A lot of organic
compounds could be produced via MCRs, for example,
propargylamines.
Moreover, as a special secondary amine, tetrahydroisoquino-
lines (THIQs) are found widely existed in natural products and
have many biological activities.17 Thus, many novel and valid
methods were developed to synthesize THIQ derivatives, and
most of them focused on the activation of THIQ's C1 atom.18
Recently, an efficient C1-alkynylation method for THIQs was
developed through the A3 coupling reaction of THIQ, aldehydes
and alkynes, in which AgOAc,19 CuI,20 and CuBr21 were reported
as efficient catalysts. To develop greener approaches for the
reaction, some efforts on the catalyst's immobilization were also
tried, and polymers22 or magnetic materials23 supported copper
species were reported. However, both of them have to suffer
lowered catalytic efficiency as well as complicated preparing
procedure for immobilized catalysts.
N-doped carbon materials are a kind of novel superior mate-
rials and have been wildly used as electrode materials because of
its porous structure and amine-containing molecules.24 The
materials have also come into the view of organic chemists in
recent years due to their superior performance and easy prepara-
tion.25 Very recently, we reported a Cu/C3N4 composite-catalyzed
homo- & cross-coupling reaction of terminal alkynes,26 in which
the easily prepared composite catalyst showed much higher cata-
lytic activity than Cu NPs with excellent recyclability.
Propargylamines are very essential in pharmaceuticals due to
their special nitrogen-containing biologically active structure, and
have wide application in synthetic chemistry.2 Traditionally,
propargylamines were synthesized by stoichiometric nucleophilic
attack of metal acetylides on imines3 or through the amination of
propargylic halides4 and propargylic triates.5 In the last decade,
a catalytic three-component coupling reaction of alkynes, alde-
hydes and amines (A3 reaction) was developed as an efficient
synthetic methodology for propargylamines, and many metal
catalysts were utilized successfully.6 Meanwhile, another multi-
component reaction for the synthesis of propargylamines was also
achieved through the coupling of alkynes, dihalomethane and
amines (AHA reaction). Remarkable efforts on this method have
been made by utilizing effective catalysts such as CuCl,7 nano-
In2O3,8 FeCl3,9 AgOAc,10 CoBr2,11 Nipy4Cl2,12 Au NPs13 and K
[AuCl4].14 However, in most of the examples above, additional
strong bases and additional solvents have to be required. In our
previous work, a Cu NPs-catalyzed AHA reaction was accomplished
aDepartment of Applied Chemistry, School of Material Science and Technology,
Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China.
E-mail: yaoxq@nuaa.edu.cn
Since the Cu/C3N4 composite shows highly catalytic activity
in Glaser–Hay reaction involving the activation of terminal
alkynes, herein, we hope to investigate its application in AHA
reaction of alkynes, dihaloalkanes and amines, and A3 coupling
reaction of THIQ, aldehydes and alkynes (Scheme 1). The Cu/
bDepartment of Chemistry, School of Pharmacy, Nanjing Medical University, Nanjing
211166, PR China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra06613b
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ether, and then, dried under vacuum at 60 ꢀC. The recovered
catalyst was reused for the next cycle with fresh starting mate-
rials and solvent.
The experiments of catalyst recycle in A3 reaction
Phenylacetylene (2.0 mmol), benzaldehyde (1.0 mmol), THIQ
ꢀ
(1.5 mmol), 5% Cu/C3N4 (32.0 mg), 4 A MS (125 mg), and 2 mL
of toluene were added into a 30 mL sealed tube under N2. The
mixture was stirred at 70 C for 12 hours. Aer completion of
Scheme 1 Synthesis of propargylamines via AHA/A3 reaction.
ꢀ
the reaction, the catalyst was separated by centrifugation and
washed by water, ethanol and ether, and then, dried under
vacuum at 60 ꢀC. The recovered catalyst was reused for the next
cycle with fresh starting materials and solvent.
C3N4 catalyst exhibited excellent catalytic activities as well as
good functional group tolerance in most examples. Further-
more, excellent recyclability of the catalyst was also achieved.
Results and discussion
Experimental
Synthesis of Cu/C3N4 compounds
With the Cu/C3N4 composite as catalyst, the AHA coupling
reaction of p-bromophenylacetylene, dichloromethane and
piperidine was selected as the prototype to start our investiga-
tion for the optimized reaction conditions, and the results were
summarized in Table 1.
At the beginning, the reaction was carried out with Cu/C3N4
as catalyst and Cs2CO3 as base under N2 atmosphere at 50 ꢀC for
24 hours, and a moderate yield was observed (entry 1, Table 1).
When DBU was used as base, 87% of 2e was obtained (entry 2).
The Cu/C3N4 catalyst was synthesized following the reported
method. Typically, melamine (2 g) was uniformly mixed with
copper(II) acetate (625 mg). The resulting mixture was then heated
to 550 ꢀC with 2 ꢀC minꢁ1 in a tube furnace under N2 atmosphere
and kept for 2 h. Aer cooling to room temperature, the nal solid
product (Cu/C3N4) was collected without further purication.
Typical procedure for Cu/C3N4-catalyzed AHA reaction of
dichloromethane, alkynes and amines
Table 1 The optimization of Cu/C3N4 catalyzed AHA coupling
reactiona
The mixture of p-bromophenylacetylene (0.15 mmol) and
piperidine (0.45 mmol), 20% Cu/C3N4 (9.6 mg, 20 mol%) in
ꢀ
anhydrous DCM (0.5 mL) was heated at 50 C for 24 h under
nitrogen atmosphere. Aer completion of the reaction, the
mixture was concentrated to yield the crude product, which was
further puried by ash chromatography (silica gel, petroleum
ether/ethyl acetate ¼ 5 : 1) to give the desired product.
The detailed characterization data for 2a–2u are provided in
the ESI.†
Entry
Catalyst
T/ꢀC
Base
Yieldb (%)
Typical procedure for Cu/C3N4-catalyzed A3 reaction of
alkynes, aldehydes and THIQ
1
20% Cu/C3N4 (20 mol%)
20% Cu/C3N4 (20 mol%)
20% Cu/C3N4 (20 mol%)
10% Cu/C3N4 (20 mol%)
5% Cu/C3N4 (20 mol%)
20% Cu/C3N4 (10 mol%)
20% Cu/C3N4 (5 mol%)
20% Cu/C3N4 (20 mol%)
20% Cu/C3N4 (20 mol%)
20% Cu/C3N4 (20 mol%)
20% Cu/C3N4 (20 mol%)
Cu NPs (20 mol%)
50
50
50
50
50
50
50
50
50
30
60
50
50
Cs2CO3
DBU
74
87
92
77
57
85
75
61
69
35
92
30
Trace
2
3c
None
None
None
None
None
None
None
None
None
None
None
4c
The mixture of benzaldehyde (0.2 mmol), phenylacetylene (0.3
mmol), THIQ (0.25 mmol), 5% Cu/C3N4 (6.4 mg, 2.5 mol%) and
5c
ꢀ
6c
4 A MS (25 mg) in anhydrous toluene (0.5 mL) was heated at
7c
70 ꢀC for 12 h under nitrogen atmosphere. Aer completion of
the reaction, the mixture was concentrated to yield the crude
product, which was further puried by ash chromatography
(silica gel, petroleum ether/ethyl acetate ¼ 30 : 1) to give the
desired product.
8d
9e
10c
11c
12c
13c
None
The detailed characterization data for 4a–4o are provided in
the ESI.†
a
Reaction conditions: p-bromophenylacetylene (0.15 mmol), piperidine
(0.15 mmol), base (0.3 mmol) and dichloromethane 0.5 mL as solvent
b
c
under N2 at 50 ꢀC for 24 h. Isolated yield. Reaction conditions: p-
bromophenylacetylene (0.15 mmol), piperidine (0.45 mmol) and
dichloromethane 0.5 mL as solvent under N2 at 50 ꢀC for 24 h.
The experiments of catalyst recycle in AHA reaction
d
p-Bromophenylacetylene (1.0 mmol), piperidine (3.0 mmol),
20% Cu/C3N4 (64.0 mg), and 2 mL of CH2Cl2 were added into
a 30 mL sealed tube under N2. The mixture was stirred at 50 ꢀC
for 24 hours. Aer completion of the reaction, the catalyst was
separated by centrifugation and washed by water, ethanol and
Reaction conditions: p-bromophenylacetylene (0.15 mmol),
piperidine (0.45 mmol) and dichloromethane (0.3 mmol) in 0.5 mL
e
CH3CN under N2 at 50 ꢀC for 24 h. Reaction conditions: p-
bromophenylacetylene (0.15 mmol), piperidine (0.45 mmol) and
dichloromethane (0.3 mmol) in 0.5 mL toluene under N2 at 50 ꢀC for
24 h.
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Noteworthy, an up to 92% of yield was achieved with 3.0 equiv. observed (2h). Moreover, the AHA coupling reaction of alkynes,
of piperidine introduced as reactant & base (entry 3). The copper dichloromethane and pyrrole was also investigated, and good
loading amount in catalyst was then evaluated. As can be seen, yields were obtained (2i–2m). Delightfully, acyclic secondary
decreasing the copper loading resulted in worse yields (entries 4 amine like diethylamine was also efficiently converted into the
and 5 vs. entry 3). The yields were also decreased with reducing corresponding propargylamines in good yields (2n–2s).
catalyst loading (entries 6 and 7). Other solvents, such as However, morpholine and diisopropylamine seem not be good
CH3CN and toluene, were ineffective in this catalytic system substrates in current reaction (2t–2u).
(entries 8 and 9).
As known to all, one of the advantages of heterogeneous
Moreover, the inuence of temperature was also investi- catalysts is their easy separation from the reaction mixture.
gated. The yield of 2e decreased sharply at 30 ꢀC, while the yield Without the distribution from additional base, the catalyst
was not changed when the reaction temperature was increased could be separated and recovered easily by centrifugation from
ꢀ
to 60 C (entries 10 and 11 vs. entry 3).
the reaction mixture, and then, fresh substrates were added to
To understand the effect of C3N4 support on the catalytic set up a new reaction. Following this procedure, the recyclability
activity, a controlled experiment was carried out, in which Cu of Cu/C3N4 was also investigated. It can be seen from Fig. 1 that
NPs was used as catalyst. Obviously, without additional strong the yields were kept in 91–88% in 5 recycles. There was only ca.
base,15 much worse result was observed (entry 12). This result 1–4% yield decrease compared with the fresh reaction.
might indicate that the C3N4 support increased the catalytic
As can be seen from above results, the Cu/C3N4 catalyst
activity of copper species successfully under current conditions. showed great catalytic activities in AHA coupling reaction as
On the other hand, there was no progress of reaction under well as good recyclability without additional base. With this
a catalyst-free condition (entry 13).
Thus, the optimal conditions involved the following coupling reaction of alkynes, aldehydes and THIQ.
parameters: 20 mol% of 20% Cu/C3N4 and 3.0 equiv. of amines At the beginning of the investigation, excess amounts of THIQ
catalyst in hand, therefore, we also examined its utilization in A3
in 0.5 mL of dichloromethane at 50 ꢀC under nitrogen for 24 h. and phenylacetylene were used in order to consume all of benz-
Under the optimized reaction conditions, we then tested the aldehyde, which was inseparable from products 4a and 5a by
scope of AHA reaction of alkynes, dichloromethane, and column chromatography. Firstly, the A3 reaction was carried out in
amines. As shown in Table 2, aromatic acetylene derivatives various solvents, with 5% Cu/C3N4 (2.5 mol%) as catalyst, under N2
ꢀ
underwent the coupling with dichloromethane and piperidine atmosphere at 70 C for 24 h. When the reaction was carried in
smoothly to afford the respective propargylamines in excellent water, a moderate yield of 5a was obtained as the single product
yields of 85–94% (2a–2g). Less reactive aliphatic alkyne, 1-hex- (entry 1, Table 3). Both EtOH and isopropanol (IPA) could be
yne, was also suitable for this reaction, and a moderate yield was processed effectively to afford 4a as the major product (entries 2
and 3). Apparently, an enhanced reaction yield and regioselectivity
Table 2 The scope of the AHA reactions a
was obtained in the presence of additional 4 A MS (entry 4 vs. 3). To
ꢀ
ꢀ
our delight, in toluene with 4 A MS, a regiospecic reaction to 4a
was accomplished with excellent yield (entry 5).
From these results above, water is the key factor affecting the
reaction selectivity. Following the mechanism described in
Scheme 2,21 iminium B was formed rstly following the dehy-
drogenation of intermediate A, and then, B was isomerized to
give iminium C as the key intermediate to the desired product
4a. Obviously, in the presence of water, the formation of imi-
nium B might be hindered, as a result, the isomerization
a
Reaction conditions: alkynes (0.15 mmol), amines (0.45 mmol) and
dichloromethane 0.5 mL as solvent with 20% Cu/C3N4 (20 mol%)
under N2 at 50 ꢀC for 24 h; isolated yield.
Fig. 1 AHA reaction catalyzed by recycled Cu/C3N4 catalyst.
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Table 3 The optimization of Cu/C3N4 catalyzed A3 coupling reactiona
Entry
Catalyst
T/oC
Solvent
Yieldb (%) 4a + 5a
Ratioc 4a/5a
1
2
5% Cu/C3N4 (2.5 mol%)
5% Cu/C3N4 (2.5 mol%)
5% Cu/C3N4 (2.5 mol%)
5% Cu/C3N4 (2.5 mol%)
5% Cu/C3N4 (2.5 mol%)
20% Cu/C3N4 (2.5 mol%)
Cu NPs (2.5 mol%)
5% Cu/C3N4 (1.25 mol%)
5% Cu/C3N4 (5 mol%)
5% Cu/C3N4 (2.5% mol%)
5% Cu/C3N4 (2.5% mol%)
5% Cu/C3N4 (2.5 mol%)
5% Cu/C3N4 (2.5 mol%)
70
70
70
70
70
70
70
70
70
50
25
70
70
H2O
EtOH
IPA
EtOH
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
46
54
50
62
97
90
70
85
97
76
35
97
78
0 : 1
3 : 1
1.6 : 1
7 : 1
1 : 0
1 : 0
1 : 0
1 : 0
1 : 0
1 : 0
1 : 0
1 : 0
1 : 0
3
4d
5d
6d
7d
8d
9d
10d
11d
12d,e
13d,f
a
Reaction conditions unless otherwise noted: benzaldehyde 3a (0.2 mmol), phenylacetylene 1a (0.3 mmol), THIQ (0.25 mmol), solvent (0.5 mL),
b
c
d
ꢀ
reaction time 24 h in cube with N2. Isolated combined yield of 4a and 5a. Ratio was determined by NMR prior to purication. 4 A MS (25
mg) was added. e 12 h. f 8 h.
procedure would be prohibited as well. Therefore, 5a was
observed as the single product in water solution.
Table 4 The scope of the A3 reactions a
Other heterogeneous copper catalysts, such as 20% Cu/C3N4
and Cu NPs were also tested, the yields were decreased obvi-
ously (entries 6 and 7).
Decreasing the loading of catalyst resulted in worse yield, but
the yield was not changed when the loading of catalyst
enhanced (entries 8 and 9). Lowering the reaction temperature
to 50 ꢀC or 30 ꢀC resulted in decreased yields (entries 10 and 11).
When the reaction time was shortened to 12 h, the yield was not
changed. However, when the reaction time was further short-
ened to 8 h, only 78% of 4a was obtained (entries 12 and 13).
Thus, the optimized reaction conditions for A3 coupling
ꢀ
reaꢀction were 5% Cu/C3N4 (2.5 mol%) in toluene with 4 A MS at
70 C for 12 h under N2 atmosphere.
Under the optimized reaction conditions, a series of
terminal alkynes and aldehydes were explored. The results were
a
Reaction condition: aldehydes (0.2 mmol), alkynes (0.3 mmol), THIQ
ꢀ
(0.25 mmol), 4 A MS (25 mg), toluene (0.5 mL), reaction time 12 h,
70 ꢀC; isolated yield.
Scheme 2 The proposed mechanism of A3 coupling reaction.
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Acknowledgements
We are grateful to the nancial support from National Natural
Science Foundation of China (21472092 to XY). This is a project
funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
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Fig. 2 A3 reaction catalyzed by recycled Cu/C3N4 catalyst.
listed in Table 4. Aromatic aldehydes with electron-donating
groups and electron-withdrawing groups worked well to
generate the corresponding products in excellent yields (4a–4g).
Aliphatic aldehyde like n-caprylic aldehyde could be converted
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´
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Conflicts of interest
There are no conicts to declare.
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