Solvent-free coupling of aldehyde, alkyne, and amine over a versatile catalyst: Ag-functionalized mesoporous S, P-doped g-C3N4

Article abstract of DOI:10.1007/s11164-021-04453-3

In the present study, we introduce mesoporous g-C₃N₄/Ag co-doped with P and S which was designed and acquired by using mesoporous silica (SBA-15) as a hard templating agent and thiourea and chitosan phosphate as the dopants. The prep

Full text of DOI:10.1007/s11164-021-04453-3

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04453-3
Solvent‑free coupling of aldehyde, alkyne, and amine
over a versatile catalyst: Ag‑functionalized mesoporous S,
P‑doped g‑C3N4
Azra Bagherzade1 · Firouzeh Nemati1
Received: 15 September 2020 / Accepted: 29 March 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
In the present study, we introduce mesoporous g-C3N4/Ag co-doped with P and S
which was designed and acquired by using mesoporous silica (SBA-15) as a hard
templating agent and thiourea and chitosan phosphate as the dopants. The prepared
catalyst was completely identifed by FT-IR (Fourier-transform infrared spectros-
copy), XRD (X-ray powder difraction), FE-SEM (feld-emission scanning electron
microscopy), EDX (energy-dispersive spectroscopy), TEM (transmission electron
microscopy), and Raman spectroscopy. The efciency of the synthesized catalyst
was evaluated toward the three-component coupling reaction, frequently named as
A3 coupling. The higher activity of the prepared catalyst is because of the syner-
gistic efects of phosphorus and sulfur co-doped together with Ag deposition. The
desired products were achieved by an environmentally safe catalyst under the opti-
mized conditions in high yields and short reaction time ranges.
Keywords Porous materials · Three-component reactions · Propargylamines · Ag
NPs · G-C3N4 nanocatalyst
Introduction
During the last decades, MCRs (multicomponent reactions) have been extensively
employed in target-oriented synthesis (TOS) to design and prepare a vast group of
organic and medical materials [1–3]. These ideal and unchallenging reactions from
the point of amiable chemistry can be run a via one-pot procedure. The A3 coupling
reaction (coupling reaction of terminal alkynes-aldehydes-amines) is referred to as one
of the most popular MCRs and leads to the efcient preparation of propargylamines
and water (the only by-product) [4–12]. The propargylamine derivatives are known to
*
Firouzeh Nemati
fnemati@semnan.ac.ir
1
Department of Chemistry, Semnan University, Semnan, Iran
Vol.:(0123456789)
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A. Bagherzade, F. Nemati
be multipurpose building blocks to form many organic compounds such as nitrogen-
containing heterocycles and pharmaceutical products [13–16]. Alzheimer’s and Par-
kinson’s disorders can be treated by some of these medical active materials as well
[17–21].
The 2D, stable and layered polymer, graphitic carbon nitride (g-C3N4) with the
bandgap of 2.7 eV has been extensively employed in many research scopes includ-
ing photocatalysis and catalysis [22]. G-C3N4 as an efcient catalyst has been used for
A3 coupling reaction [23], crossed-aldol condensation [24], Friedel–crafts reaction
[25], Knoevenagel condensation [26], C–C couplings [27], oxidation reaction [28],
Claisen–Schmidt condensation [29], reduction reaction [30], and so on. Commonly,
the g-C3N4 can be prepared through the calcination of nitrogen-containing precursors
such as melamine [31], urea [32], thiourea [33], and cyanamide [34]. Unfortunately,
despite features like non-toxic nature, low price, and efortless preparation method
[35], the bulk g-C3N4 sufers from the low surface area. So far, numerous papers have
been published modifying the pristine g-C3N4 and introducing porosity into the struc-
ture to obtain more active sites [36–38]. Its nitrogen-rich content as well as polymeric
character enables the texture to be doped with foreign atoms and this would afect and
enhance the catalytic activity.
Over the years, diferent transition metal compounds based on copper [39], nickel
[40], iron [41], cobalt [42], silver [43], gold [44], mercury [45], and even copper/ruthe-
nium salts [46] were implemented to catalyze the C–C and C–N coupling reaction.
However, previous literature validated the coinage metals including copper, silver, and
gold are the most intriguing transition metals to catalyze A3 coupling [47–49]. These
metals may potentially boost the production of the target molecule through coordinat-
ing to the terminal alkyne and creating π complexes.
In connection with the points previously mentioned and based on our group’s goal in
modifying g-C3N4, and designing new catalysts [50–55], we report the design of novel
mesoporous S, P-doped g-C3N4/Ag catalyst by using Pluronic P123 surfactant. To dope
into the structure of the mesoporous g-C3N4, economical, natural, and accessible mate-
rials, thiourea, and chitosan phosphate were utilized. The prepared catalyst possesses
a high surface area and exhibits acceptable catalytic performance for the A3 coupling
reaction. The preparation of the recoverable catalyst was supported by IR, XRD, FE-
SEM, EDX, TEM, and RAMAN analytical techniques.
Experimental
Materials
All the chemical reagents and solvents were used as acquired from Merck and Sigma-
Aldrich companies with the highest analytical grade.
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Solvent-free coupling of aldehyde, alkyne, and amine over…
Characterization techniques
FT-IR (Fourier transformed infrared) spectra were determined on the Shimadzu spec-
trophotometer (8400 s) in the 400–4000 cm−1 range. The powder XRD (crystalline
structure) of the designed catalyst was carried out on Philips (PW1730) using Cu Kα
source of irradiation (wavelength of 1.54 Å). Scanning electron microscopy (FE-SEM)
images and elemental investigation (EDAX) were employed by the FE-SEM TESCAN
MIRA3 instrument. Transmission electron microscopy (TEM) micrographs were taken
on TEM Philips (EM 208 s) microscope. The Raman spectra were conducted using
TESCAN Raman Analyzer with a neodymium-doped yttrium aluminum garnet laser,
which produces a wavelength of 532 nm.
Synthesis of the mesoporous S, P‑doped g‑C3N4/Ag catalyst
Synthesis of the SBA‑15 template (mesoporous silica)
Four grams of P123 copolymer surfactant was dissolved in 30 mL purifed water and
80.5 mL HCl solution (2 M). After adding 8.5 g of tetraethyl orthosilicate (TEOS) in a
◦
dropwise manner to the above solution, the mixture was stirred at 40 C for 8 h. Shortly
◦
after that, the mixture was transferred into an autoclave and heated to 100 C. After
20 h, the autoclave was cooled to the room temperature, and the resultant product was
◦
obtained by fltering, washing, and eventually drying at 65 C in the oven [56].
Synthesis of chitosan phosphate
Two grams of chitosan, 10 g of urea, and 2 g of orthophosphoric acid were added to the
round-bottom fask containing 20 mL of DMF solvent. The mixture was heated to 150
◦C for 1 h. After cooling, the precipitate was centrifuged, washed with ultrapure water
several times, and freeze-dried [57].
Synthesis of mesoporous S, P‑doped g‑C3N4
A mixture of SBA-15 (1 g), chitosan phosphate (2 g), thiourea (4 g), and deionized
water (40 mL) was heated for almost 12 h. After removing the solvent under reduced
pressure, the obtained sample was dried in the oven during the night. The grounded
◦
solid was transferred into the crucible and calcined at 500 C for 2 h with a heating rate
◦
of 20 C∕min. Finally, to leach out the silica template NaOH solution (2 M, 1:1 EtOH:
◦
H2O) was added to the calcined sample from the previous step at 80 C and stirred
for 4 h. As the last step, the mesoporous S, P-doped g-C3N4 was obtained by fltering,
◦
washing, and drying the resultant material at 80 C.
Synthesis of mesoporous S, P‑doped g‑C3N4/Ag
To load Ag NPs on the surface of graphitic support, 470 mg of mesoporous S,
P-doped g-C3N4 was added to DI water (100 mL) and sonicated. Afterward,
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A. Bagherzade, F. Nemati
4.72 mL of AgNO3 solution (10 mg mL−1) was added to the reaction mixture. As
the last step, 15 mL of the reducing agent (NaBH4) (210 mg) was injected into the
suspension under mechanical stirring. After 12 h, the synthesized catalyst was iso-
◦
lated through the centrifuge, washed with H2O, and dried at 80 C.
The common procedure for A3 coupling reaction catalyzed by mesoporous S,
P‑doped g‑C3N4/Ag
As a model method, 0.02 g of mesoporous S, P-doped g-C3N4/Ag catalyst was added
to the RB-fask containing 1 mmol of aromatic aldehyde, 1.2 mmol of piperidine,
◦
and 1.5 mmol of terminal alkyne. The reaction mixture was stirred at 100 C and
under solvent-free condition. The progress of the coupling reaction was followed
by TLC (thin layer chromatography) until the complete conversion of substrates to
propargylamine derivatives. Thereafter, the catalyst was removed by centrifuge and
washed with ethanol. The pure target compounds were obtained using silica gel col-
umn chromatography (ethyl acetate/hexane 4:6). Eventually, the reaction products
1
were recognized by the gas chromatography (GC) technique. HNMR spectra of
some selected compounds are given in SI.
1
Spectral data of N-(1,3-diphenyl-2-propynyl) piperidine: H NMR (400 MHz,
CDCl3) δ(ppm): 7.28–7.67 (m, 10H), 4.81 (s, 1H), 2.59 (s, 4H), 1.58–1.65 (m, 4H),
1.45–1.49 (m, 2H).
Results and discussion
Catalyst fabrication
The synthetic path for preparing the mesoporous S, P-doped g-C3N4/Ag was
mentioned in the previous section. In brief, after synthesis of mesoporous silica
and chitosan phosphate they were reacted with thiourea to form mesoporous S,
P-doped g-C3N4. As a fnal step, Ag NPs were immobilized on the surface of the
prepared mesoporous S, P-doped g-C3N4 (Scheme 1).
Catalyst characterization
To underline the existence of corresponding functional groups, FT-IR investiga-
tion was done. Figure 1 depicts Fourier-transform infrared spectra of mesoporous
S, P-doped g-C3N4 and mesoporous S, P-doped g-C3N4/Ag samples. The
appeared peaks in the spectrum of both samples are following the reported litera-
ture for mesoporous S, P-doped g-C3N4 [58]. The observed bands at the range of
1200–1600 cm−1 are related to the CN stretching modes of g-C3N4. The broad peak
centered at 3350 cm−1 refers to the free NH groups of the graphitic structure as
well as hydroxyl vibration of adsorbed water. After co-doping of P and S atoms,
all the characteristic peaks in connection with g-C3N4 could also be observed in the
mesoporous S, P-doped g-C3N4/Ag, proving that the original graphitic CN network
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Solvent-free coupling of aldehyde, alkyne, and amine over…
Scheme 1 General route for the synthesis of mesoporous S, P-doped g-C3N4/Ag catalyst
was maintained intact. Obviously, the peak intensity for mesoporous S, P-doped
g-C3N4/Ag sample was decreased due to the successful assembly of Ag NPs on the
surface of the graphitic support.
In this study, XRD analysis was surveyed to confrm the phase purity and crys-
talline texture of the samples. The X-ray difraction patterns corresponding to the
mesoporous S, P-doped g-C3N4, and mesoporous S, P-doped g-C3N4/Ag samples
are demonstrated in Fig. 2. Two diagnostic peaks at 13.1° and 27.3° (100 and 002
planes) are recognized for graphitic carbon nitride (g-C3N4) based on the JCPDS
card No. 87–1526 [59]. The peaks of S, P-doped CN show positions similar to those
of g-C3N4 but rather broadened, suggesting that more defects in the structure appear
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A. Bagherzade, F. Nemati
Fig. 1 FT-IR spectra of mesoporous S, P-doped g-C3N4 sample and mesoporous S, P-doped g-C3N4/Ag
catalyst
along with phosphorus and sulfur doping [60]. Moreover, the peak location of the
(002) plane is shifted from 27.3° to 26.45° (mesoporous S, P-doped g-C3N4) and
26.95° (mesoporous S, P-doped g-C3N4/Ag), which is because of the heteroatoms
(phosphorus and sulfur) doping into the lattices of g-C3N4, leading to greater lattice
Fig. 2 XRD patterns of the mesoporous S, P-doped g-C3N4 and mesoporous S, P-doped g-C3N4/Ag sam-
ples
1 3

Solvent-free coupling of aldehyde, alkyne, and amine over…
distortions and interplanar distance [61]. XRD patterns indicated that the S, P co-
doping has no signifcant infuence on the original crystal structure of g-C3N4. No
evidence for any impurity was detected. After immobilization of Ag NPs, four
distinct peaks (38.12° (111), 44.30° (200), 64.40° (220), and 77.26° (311)) were
noticed which were assigned to the cubic structure of silver, Ag (0) based on the
ICCD card No. 04–0783 [62]. Furthermore, the average crystalline size of the Ag
NPs in their zero oxidation state was calculated to be 25 nm (based on the Scherrer’s
equation D=kλ/βcosθ).
FE-SEM as well as mapping analysis was carried out to reveal the surface
morphology and syntax of the prepared mesoporous S, P-doped g-C3N4/Ag sam-
ple (Fig. 3). As can be realized from Fig. 3 the porous nature of the synthesized
mesoporous S, P-doped g-C3N4 did not alter after Ag nanoparticles sedimentation.
Figure 3a–e exhibits the uniform distribution of C, N, P, S, and Ag elements over the
surface of the g-C3N4 support. Besides, successful immobilizing of these NPs on the
surface of the g-C3N4 support can be verifed by TEM images (Fig. 4).
To pursue the chemical composition of the prepared samples (chitosan phosphate
and mesoporous S, P-doped g-C3N4, and mesoporous S, P-doped g-C3N4/Ag), EDS
analysis was performed. The illustrated spectrum in Fig. 5 represents corresponding
peaks to C, N, O, and P elements, which is related to chitosan phosphate. For the
mesoporous S, P-doped g-C3N4 sample, the presence of C, N, partially S (8.11 W%)
and P (1.43 W%) peaks in the spectrum declares the satisfactory preparation of the
graphitic support (Fig. 6). For the mesoporous S, P-doped g-C3N4/Ag catalyst, the
presence of Ag (7.2 W%) peaks in the spectrum declares the convincing immobili-
zation of silver NPs on the graphitic support (Fig. 7).
To study the vibrational feature of carbon-containing material, the Raman spec-
troscopy was conducted. The Raman spectra of mesoporous S, P-doped g-C3N4/
Ag catalyst is exhibited in Fig. 8 and in the range of 1000–2000 cm−1. The typi-
cal D and G bands for the mesoporous S, P-doped g-C3N4/Ag catalyst is positioned
at nearly 1361 and 1536 cm−1, respectively. The Raman spectra of excited doped
carbon-containing materials show two characteristic resonances (D and G bands)
located at about 1320 and 1590 cm−1 [63]. The in-plane stretching modes of carbon
(G-band) are related to graphene structure, while the out of plane modes of phonon
(D band) refer to the amorphous carbon structures [64]. Also, the IG/ID intensity
ratio is calculated to be 1.13, proving the coordination of metal nanoparticles (Ag)
to the ꢀ electrons of the graphitic support [65].
Evaluating the efciency of mesoporous S, P‑doped g‑C3N4/Ag catalyst in the A3
coupling reaction
Once the structure of the designed catalyst has been verifed, the catalytic ability
of mesoporous S, P-doped g-C3N4/Ag was determined in the A3 (three-component)
coupling reaction of aromatic aldehydes, secondary amines, and terminal alkynes.
Benzaldehyde, piperidine, and phenylacetylene were chosen as substrates to estab-
lish the title reaction.
1 3

A. Bagherzade, F. Nemati
Fig. 3 FE-SEM images of mesoporous S, P-doped g-C3N4/Ag catalyst with diferent magnifcations and
EDS-mapping pictures of the catalyst a–e
Diverse factors such as catalyst amount, temperature, and solvent were tested
and the summarized outcomes are given in Table 1. Originally, the efect of cata-
lyst loading was examined and due to Table 1, the expected product was achieved
in the presence of 20 mg of catalyst. The model reaction did not proceed in
the absence of the catalyst even after 10 h. This matter emphasizes the manda-
tory rule of mesoporous S, P-doped g-C3N4/Ag catalyst in the reaction progress
(Table 1, entry 1, 2, 3). To explore the infuence of the solvent’s nature on this
conversion, diferent solvents including protic (H2O, EtOH) and aprotic ones
(Toluene, THF) were applied to run the model test. From the entry 3, it is clear
that under the solvent-free condition the generation yield of the product is ideal.
Eventually, to investigate the impact of temperature, the A3 coupling reaction was
managed with 20 mg of catalyst and under the solvent-free condition at dissimilar
◦
temperatures. 100 C was chosen as the superior temperature to accomplish the
1 3

Solvent-free coupling of aldehyde, alkyne, and amine over…
Fig. 4 TEM image of mesoporous S, P-doped g-C3N4/Ag catalyst
Fig. 5 EDS analysis of chitosan phosphate
conversion. Entry 3 mentions the optimized condition for the production of prop-
argylamine derivatives.
Following optimizing the reaction condition, the addition of terminal alkynes to
secondary-type amines and aromatic aldehydes to form propargylamine derivatives
in a solvent-free condition by using a silver-based catalyst has also been investi-
gated. The summarized results in Table 2 reveal the ability of the prepared catalyst
1 3

A. Bagherzade, F. Nemati
Fig. 6 EDS analysis of mesoporous S, P-doped g-C3N4 support
Fig. 7 EDS analysis of mesoporous S, P-doped g-C3N4/Ag catalyst
1 3

Solvent-free coupling of aldehyde, alkyne, and amine over…
Fig. 8 Raman spectra of mesoporous S, P-doped g-C3N4/Ag catalyst
Table 1 Optimization conditions for three-component coupling reaction of benzaldehyde, piperidine, and
phenylacetylene
◦
Entry
Catalyst (mg)
T ( C)
Solvent
Time (min)
Con. %
1
2
3
4
5
6
7
8
–
100
100
100
100
100
100
refux
refux
refux
refux
r. t
80
130
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
toluene
THF
EtOH
H2O
10 h
10 h
10 h
120
75
120
120
120
120
120
120
120
120
trace
trace
trace
80
97
95
20
trace
10
trace
trace
85
m-g-C3N4 (10)
m-S, P-doped g-C3N4 (10)
10
20
30
20
20
20
20
20
20
20
9
10
11
12
13
Solvent-free
Solvent-free
Solvent-free
95
to be applied to aromatic aldehydes with diferent functional-groups (electron-rich
and electron-withdrawing) in diverse ring-positions. The substrates converted to the
desired product and water (the only green by-product) in excellent yields and accept-
able time ranges between 75 and 90 min.
The efectiveness of mesoporous S, P-doped g-C3N4/Ag catalyst was compared
with other published strategies for the Benzaldehyde/ Piperidine/ Phenylacetylene
1 3

A. Bagherzade, F. Nemati
Table 2 Coupling reaction of aromatic aldehyde, terminal alkyne, and amine catalyzed by the silver-
based catalyst
1 3

Solvent-free coupling of aldehyde, alkyne, and amine over…
Table 2 (continued)
10
75
75
94
88
11
12
13
14
15
16
80
80
75
75
75
87
90
96
87
89
Condition: Benzaldehyde (1 mmol), Amine (1.2 mmol), phenylacetylene (1.5 mmol), 0.02 g of catalyst,
and no solvent. The isolated yield is based on the conversion or completion of the reaction in given time.
coupling reaction (Table 3). According to the data in Table 3, our prepared green
and unchallenging catalyst showed much more catalytic efciency.
Based on the reported mechanistic studies in past literature [75], we propose a
tentative mechanism for the generation of propargylamines in the presence of the
mesoporous S, P-doped g-C3N4/Ag catalyst (Scheme 2). The feasible mechanism for
the coupling reaction over Ag nanoparticles is suggested that involves the C–H bond
of alkyne activation. After in situ generations of iminium ion from the initial amine
1 3

A. Bagherzade, F. Nemati
Table 3 Three-component coupling reaction catalyzed by diferent catalysts
Entry
Catalyst
Condition
Time (h)/Yield Refer-
ences
1
2
3
4
5
6
7
8
9
Silver–graphene nanocomposite
ZnO-IL/Ag NPs
Ag3PW12O40 (AgTPA)
AgY zeolite
CuO/GNS
Ag/EOs orange NPs
Ag-CIN-1
(AgxNi(1−x))yRGO(100−y)
Ag-Co/Hi-ZSM-5
Present study
CH2Cl2, 60 °C
H2O, refux
Acetonitrile, 80 °C
100 °C
Acetonitrile, 82 °C
Toluene, 80 °C
H2O, 40 °C
Acetonitrile, 100 °C
H2O, 75 °C
24/86
3/92
5/92
15/81
5/89
8/96
[66]
[67]
[68]
[69]
[70]
[71]
[72]
[73]
[74]
–
3/98
12/88
15/84
75 min/97
10
Solvent-free, 100 °C
Scheme 2 Tentative mechanism for the silver-catalyzed coupling reaction
and aldehyde, it reacted with the silver-acetylide intermediate to form the correspond-
ing product (propargylamine) and reproduce the silver-based catalyst to complete the
reaction.
The recyclability of the mesoporous S, P-doped g-C3N4/Ag catalyst was studied for
model reaction in seven consecutive runs (Fig. 9). The catalyst was regained through
centrifuge by the completion of each cycle. The catalyst was then washed with ethanol,
dried in the oven, and reused for another run. The catalyst can be easily recycled for
seven tests with inconsiderable loss of activity. As illustrated in Fig. 10 the XRD pat-
terns also afrms the recycling ability of the mesoporous S, P-doped g-C3N4/Ag cata-
lyst after 7 successive runs.
1 3

Solvent-free coupling of aldehyde, alkyne, and amine over…
Fig. 9 Reusability test of the catalyst for the model reaction
Fig. 10 XRD patterns of the catalyst and the recycled one
Conclusion
In conclusion, an exceptionally cost-efective silver-catalyzed A3 (three-compo-
nent) coupling reaction of aromatic aldehyde, terminal alkyne, and amine has
been accomplished through C-H activation. The catalyst preparation, procedure,
and work-up were plain, and no organic solvent or activator was needed. The cat-
alyst managed a broad range of starting materials for the generation of the propar-
gylamine derivatives with excellent yields. Recyclability of the catalyst was up to
seven repetitive runs with no appreciable loss of activity.
1 3

A. Bagherzade, F. Nemati
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04453-3.
Acknowledgment We are grateful to the Semnan University Research Council for the fnancial support
of this research.
Declaration
Conficts of interest There are no conficts to declare.
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