Fabrication of porous ultrathin carbon nitride nanosheet catalysts with enhanced photocatalytic activity for N- And O-heterocyclic compound synthesis

Article abstract of DOI:10.1039/d0nj05101b

A simple and efficient photocatalytic method for the synthesis of dihydropyrimidinones (DHPMs) and their derivatives via porous ultrathin carbon nitride nanosheets (p-CNNs) without solvents was demonstrated. The yields of 3,4-dihydropyrimidin-2(1H)-ones/t

Full text of DOI:10.1039/d0nj05101b

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Fabrication of porous ultrathin carbon nitride
nanosheet catalysts with enhanced photocatalytic
activity for N- and O-heterocyclic compound
synthesis†
Cite this:DOI: 10.1039/d0nj05101b
Yancong Li, Jiliang Ma, * Zhendong Liu, Dongnv Jin, Gaojie Jiao, Yanzhu Guo,
Qiang Wang, Jinghui Zhou
and Runcang Sun*
A simple and efficient photocatalytic method for the synthesis of dihydropyrimidinones (DHPMs) and their
derivatives via porous ultrathin carbon nitride nanosheets (p-CNNs) without solvents was demonstrated. The
yields of 3,4-dihydropyrimidin-2(1H)-ones/thiones and their derivatives were up to 97.0%. Furthermore, the
yield of 5-ethoxycarbonyl-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H)-one in the 10th cycle retained
98.9% of its 1st value. Considering the environmental and economic benefits, this work exhibits various merits
including excellent yields, environmental friendliness, inexpensiveness, and avoiding the use of solvents and
metal-based photocatalysts. In addition, the excellent performance of the p-CNNs and environmentally
benign reaction system have also been checked by the photocatalytic synthesis of 12-phenyl-9,9-dimethyl-
Received 17th October 2020,
Accepted 18th November 2020
8,9,10,12-tetrahydrobenzo[a]xanthen-11-one
and
5-phenyl-1(4-methoxyphenyl)-3[(4-methoxy-phenyl)-
DOI: 10.1039/d0nj05101b
aminol]-1H-pyrrol-2(5H)-one. This work paves a new way for carrying out a three-component reaction using
rsc.li/njc
metal-free photocatalysts under mild reaction conditions.
organic synthesis.22 High cost and complex preparation processes
of other photocatalysts as compared with functional carbon nitride
Introduction
Green organic synthesis has become a matter of great concern in makes these photocatalysts inconsistent with green chemistry
recent years. To achieve this goal, various strategies have been development requirements. Thus, taking environmental and eco-
developed involving three criteria: solvent, catalyst, and synthesis nomic benefits into consideration, the development of various
method.1–3 Photocatalysis as a green synthesis method has metal-free, environmentally friendly and inexpensive functional
attracted much attention due to its high efficiency, easy operation, carbon nitride photocatalysts for use in organic synthesis is of
mild reaction conditions, wide application, low cost, low energy great significance.
consumption and so on.4–6 The key to proper design of a photo-
Dihydropyrimidinones (DHPMs) and their derivatives are a
catalytic reaction is to first design and prepare an appropriate series of heterocyclic compounds, which are widely distributed
photocatalyst. The traditional photocatalysts, such as metal oxides in nature. Since the DHPMs were discovered, they have
(ZnO, TiO2)7,8 and metal sulfides (CdS, ZnS),9,10 exhibited high attracted much attention due to their wide range of biological
photocatalytic performance. However, some of them suffered from activities, such as antiviral, antimitotic, anticarcinogenic and
the narrow absorption range of visible light or light corrosion, antihypertensive effects.23–25 Furthermore, some functiona-
which limited their application. Recently, a series of novel photo- lized DHPMs are the constituents of neuropeptide Y antago-
catalysts have been reported, such as, BiOX (X = Cl, Br, I), metal– nists, calcium channel modulators and a-1a antagonists.26–28
organic frameworks (MOFs), covalent organic frameworks (COFs) Especially Batzelladine A and B play an important role in the
or functional carbon nitride.11–16 These functionalized photocata- HIV gp-120-CD4 treatment.29 Therefore, developing a mild and
lysts have made great achievements in the fields of solar water efficient strategy is significant for the synthesis of DHPMs.
splitting,14 CO2 reduction,17–19 N2 fixation,20 biorefinery21 and
The first straightforward method for the synthesis of DMPHs
via condensation of aldehydes, b-dicarbonyl compounds and urea
or thiourea reported by P. Biginelli (the reaction was then named
as Biginelli reaction)30 inspired a new wave of research on the
synthesis and application of DHPMs. And after that, various
strategies for carrying out Biginelli reaction were developed.31–36
However, most of them suffered from longer reaction time,
Center for Lignocellulosic Chemistry and Biomaterials, College of Light Industry
and Chemical Engineering, Dalian Polytechnic University, Dalian, 116034, China.
E-mail: jlma@dlpu.edu.cn, rcsun3@dlpu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj05101b
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harsher reaction conditions, high cost, and tedious work-up or
purification. Currently, several new protocols with natural catalysts
(e.g. citric acid,37 baker’s yeast,38 phytic acid,39 D-xylonic acid,2 etc.)
for carrying out Biginelli reaction have been demonstrated, which
gave good yields. The only problem is that the recovery and reuse
of these catalysts are relatively complicated. In this regard, efficient
heterogeneous catalysts have aroused great interest among
researchers. Taking into account the advantages of photocatalysis,
developing a metal-free and efficient heterogeneous photocatalyst
for carrying out Biginelli reaction has vital significance.
In this study, p-CNN photocatalyst was prepared by direct
calcination of copper oxide nanobelts (CuO NBs) and melamine,
which could effectively synthesize 3,4-dihydropyrimidin-2(1H)-ones/
thiones and their derivatives photocatalytically without solvents
under mild conditions (Scheme 1). The performance of p-CNNs in
the photothermal mediated Biginelli reaction was assessed at
varying irradiation time (30–120 min), temperature (30–100 1C),
photocatalyst dosage (0.56–5.60 wt%) and feed ratios. Furthermore,
the stability and recycling of p-CNNs were also investigated. In
addition, the excellent performance of p-CNNs and the environmen-
tally benign reaction system were also checked by the photocatalytic
synthesis of 12-phenyl-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]
xanthen-11-one and 5-phenyl-1(4-methoxyphenyl)-3[(4-methoxy-
phenyl)-aminol]-1H-pyrrol-2(5H)-one (Schemes 2 and 3).
Scheme 3 p-CNNs photocatalytic for the synthesis of 12-phenyl-9,9-
dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one under solvent-
free conditions.
Industrial Corporation (Shanghai, China). Aldehydes and 1,3-
dicarbonyl compounds were purchased from Aladdin Indus-
trial Corporation. Urea, thiourea, ethanol and other reagents
were obtained from Dalian Chemical Reagent Factory, China.
All the chemicals were analytical grade and used directly with-
out further purification.
Preparation of photocatalyst
In a typical procedure, a certain amount of sodium hydroxide
(NaOH) was dissolved in a certain volume of deionized water under
ice water bath. Subsequently, cetyltrimethylammonium bromide
(CTAB) was added into the above solution with a mass ratio of
1 : 5.5 (CTAB : NaOH). The system was then heated from room
temperature to 60 1C under stirring to give a NaOH–CTAB solution.
In addition, a certain amount of copperdiacetatehydrate was dis-
solved in deionized water, and the solution was then added into the
NaOH–CTAB solution drop by drop. The obtained mixed system
was heated at 60 1C for 60 min. After the reaction was complete, the
reaction system was filtered while hot and washed three times with
deionized water and ethanol, respectively. The black solid products
were dried at 50 1C for 12 h. Finally, the CuO NB was calcined at
350 1C for 120 min under N2 atmosphere before use.
Experimental section
Materials
Melamine, sodium hydroxide, cetyltrimethylammonium bro-
mide and copperdiacetatehydrate were provided by Macklin
The method for the preparation of multilayer carbon nitride
(ml-CN) was based on the reported literature,40 with a slight
difference. First, 2.0 g of melamine and 3 mL of deionized water
were taken in a crucible; the mixed system was then stirred for
30 min. Subsequently, the mixture was calcined at 250 1C for
60 min under N2 atmosphere. After the calcination process was
complete, the system was heated to 560 1C with a rate of 5 1C
minÀ1 and then kept for 180 min. The obtained ml-CN powder
was calcined at 560 1C for another 115 min before use.
In a typical procedure: first, 2.0 g of melamine, 10 mg of CuO
NB and 3 mL of deionized water were taken in a crucible; the
mixed system was then stirred for 30 min. Subsequently, the
mixture was calcined at 250 1C for 60 min under N2 atmosphere.
After the calcination process was complete, the system was heated
to 560 1C with a rate of 5 1C minÀ1 and then kept for 180 min. The
obtained powder was fully ground and then calcined at 560 1C for
115 min under N2 atmosphere to give p-CNNs.
Scheme 1 p-CNNs photocatalytic synthesis of DHPMs under solvent-
free conditions.
Characterization
The investigation of morphology by scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and atomic force
microscopy (AFM) was conducted on Hitachis-4800 SEM, Nano-
scope III AFM and JEM-2100 CXII TEM, respectively. X-ray photo-
electron spectroscopy (XPS) was studied using a Kratos Axis Ultra
DLD spectrometer equipped with a monochromatic A1KR X-ray
Scheme 2 Photocatalytic p-CNNs for the synthesis of 5-phenyl-1(4-
methoxyphenyl)-3[(4-methoxyphenyl)-amino]-1H-pyrrol-2(5H)-one under
solvent-free conditions.
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source. X-ray diffraction (XRD) patterns of the ml-CN and p-CNNs p-CNNs (2.5 wt%) was added into a reactor. The mixture was
were explored by a Bruker D8 Focus diffractometer. Fourier trans- stirred at dark condition for 30 min. Subsequently, the reaction
form infrared (FT-IR) spectra of the catalysts were investigated by a system was performed at room temperature for 0.5 h under
Bruker Tensor 27 spectrophotometer in the range of 400–4000 cmÀ1
.
visible-light irradiation. Once the reaction was complete, 5 mL
N2 adsorption–desorption isotherm measurements were conducted of ethanol was added and the obtained reaction system was
using a physisorption analyzer, and the Brunauer–Emmett–Teller stirred for 3–4 min. Subsequently, the reaction system was
(BET) method was used to calculate the specific surface area, as well filtered and washed with ethanol and ether several times,
as the desorption branch of the isotherm in line with the Barrett– respectively. The pure product was finally obtained by drying
Joyner–Halenda method was utilized to evaluate the average pore at 80 1C for 10 h.
diameter and pore size distribution of p-CNNs. The ultraviolet-visible
diffuse reflectance spectra (UV-vis DRS) of ml-CN and p-CNNs were
recorded on a Cary 5000 spectrophotometer fitted with an integrat-
ing sphere attachment from 200 to 800 nm. Inductively coupled
Results and discussion
The characterization of samples
plasma atomic emission spectroscopy (ICP-MS) was performed using
1
an Agilent 7800 equipment. H NMR spectral measurements were The morphology of ml-CN and p-CNNs was investigated by scan-
performed at 400 MHz using TMS as the internal standard, and ning electron microscopy (SEM), transmission electron microscopy
13C NMR spectral measurements were carried out at 100 MHz with (TEM) and atomic force microscopy (AFM). The ml-CN prepared by
complete proton decoupling.
the direct calcination of the melamine slurry without the addition
of CuO NB displayed obvious multi-layer structure (Fig. 1A), while
the p-CNNs prepared by the simple calcination of CuO NB (Fig. S1,
ESI†) and melamine exhibited typical two-dimensional structure
General procedure for the synthesis of dihydropyridine-2(1H)-
ones/thiones by using p-CNN photocatalyst
In a typical procedure, a mixture of aldehydes, 1,3-dicarbonyl (Fig. 1B). This finding shows that CuO NB can assist the p-CNNs to
compounds, urea or thiourea and p-CNNs was added into a form nanosheet structure during the calcination process. The
reactor. The mixture was stirred at dark condition for 30 min. lamellar structure and amorphous structure of the p-CNNs was
Subsequently, the reaction system was conducted at different also observed by TEM results, as well as the amounts of holes
temperatures for different irradiation time. Once the reaction existing in the nanosheet (Fig. 1C and D). To further investigate the
was complete, ice water was added. The obtained crude product porous structure of p-CNNs, AFM measurement was performed,
was then washed with ice water several times, filtered and dried and the obvious pores were found (Fig. 1E). Meanwhile, the p-CNN
at 100 1C for 10 h. Finally, the pure products were obtained by was hundreds of nanometers in width and B1.5 nm in thickness
recrystallization of the crude products in ethanol.
(Fig. 1F). All the results indicated that the p-CNNs prepared in this
In the cycling experiment, the p-CNNs was recovered during work owns porous and flaky structure, which is beneficial for the
the recrystallization process. The obtained p-CNNs was washed adsorption and mass transfer of the reactants.
with hot ethanol several times and then dried at 100 1C for 10 h
before reuse.
To investigate the chemical composition and status of the
elements in p-CNNs, XPS measurement was carried out. As shown
in Fig. 2A, three peaks were observed in the C 1s spectra, which
were located at 288.3 eV, 286.5 eV and 284.8 eV, corresponding to
N = C–N, C–N and C–C, respectively.41 The narrow scan N 1s
spectra of p-CNNs are shown in Fig. 2B, and the peaks were
General procedure for the synthesis of 12-phenyl-9,9-dimethyl-
8,9,10,12-tetrahydrobenzo[a]xanthen-11-one using p-CNN
photocatalyst
In a typical procedure, a mixture of benzaldehyde (1.0 mmol), observed at 401.4 eV, 400.3 eV and 398.8 eV, attributed to N–C,
2-naphthol (1.0 mmol), trimethadione (1.2 mmol) and p-CNNs N–(C)3 and C–NQC, respectively.42 Meanwhile, no Cu related
(1.1 wt%) was added into a reactor. The mixture was stirred at species were detected in the XPS spectra. Furthermore, the results
dark condition for 30 min. Subsequently, the reaction system of ICP-MS show that there is no Cu loading in the p-CNNs. Fig. 2C
was performed at 40 1C for 90 min under visible-light irradia- displays the XRD patterns of ml-CN and p-CNNs, which is similar
tion. Once the reaction was complete, the system was cooled to to that of pure graphitic carbon nitride (JCPDS Card No. 87-1526).
room temperature and ethyl acetate was added. The reaction The only difference is that the intensity of the (100) and (002)
system was then filtered and washed with ethyl acetate several planes of p-CNNs was lower than that of ml-CN, leading to the
times, and the obtained filtrate was collected. The pure product reduction of interlayer distance of the graphitic like planar
was afforded by evaporation of the collected filtrate, followed by p-CNNs. As shown in Fig. 2D, the FT-IR spectra of ml-CN and
column chromatography on silica gel using ethyl acetate/petroleum p-CNNs were similar with those reported;43 besides the change in
ether as the eluent.
signal peak intensity, no obvious difference in the FT-IR spectra
of ml-CN and p-CNNs was observed. The N2 sorption isotherms of
p-CNNs exhibited type IV isotherm patterns, suggesting that lots of
mesopores exist in p-CNNs, and the surface area was 85.2 m2 gÀ1
(Fig. 2E). To investigate the visible-light absorption regions of
General procedure for the synthesis of 5-phenyl-1
(4-methoxyphenyl)-3[(4-methoxyphenyl)-amino]-1H-pyrrol-
2(5H)-one by using p-CNN photocatalyst
In a typical procedure, a mixture of benzaldehyde (1.0 mmol), ml-CN and p-CNNs, UV-vis DRS was performed. As shown in
p-methoxyaniline (2.0 mmol), ethyl pyruvate (1.5 mmol) and Fig. 2F, the spectrum of the sample showed blue-shift with the
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Fig. 1 SEM images of ml-CN (A) and p-CNNs (B); TEM images of p-CNNs (C and D); AFM images of p-CNNs (E) and its thickness distribution curves (F).
Fig. 2 XPS spectra of p-CNNs, (A): C 1s, (B): N 1s; (C): XRD patterns of ml-CN (magenta) and p-CNNs (orange); (D): FT-IR spectra of ml-CN (magenta)
and p-CNNs (orange); (E): N2 adsorption isotherm, inset in E shows the BJH pore size distribution; (F): UV-vis DRS of ml-CN and p-CNNs, inset in F shows
the plot of transformed Kubelka–Munk function versus photo energy for ml-CN and p-CNNs.
decrease of thickness, indicating that the excitations by photo- was named as 3a. When the reaction catalyzed by p-CNNs was
carriers with energy is lower as compared with the intrinsic band- carried out at 40 1C or 90 1C for 60 min in the dark conditions,
band transition. Furthermore, their bandgaps reduced with the only trace amounts of 3a was obtained. Once visible light
decrease in sample thickness, resulting in an enhanced photo- was applied in the above system at 40 1C, 19.0% yield of 3a
catalytic activity.
was observed, suggesting that visible light is beneficial for the
synthesis of 3a. Subsequently, the effects of reaction time and
reaction temperature on the synthesis of 3a photocatalyzed by
The photocatalytic activity of samples
Initially, the influence of visible light or dark condition on p-CNNs were also investigated, and the results are given in
Biginelli reaction catalyzed by p-CNNs was investigated. The Table 1. Interestingly, the yield of 3a increased at first and then
product of the Biginelli condensation reaction of benzaldehyde decreased (Table 1, entries 1–4). The maximum yield of 3a was
(5 mmol) with ethyl acetoacetate (5 mmol) and urea (5 mmol) achieved in 90 min, and thus, this period of time was used as
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Table 1 The effects of reaction time and temperature on the synthesis of
3a photocatalyzed by p-CNNsa
temperatures on the synthesis of 3a was also explored. As the
reaction temperature was increased from 30 to 90 1C, the yield
of 3a increased from 17.9 to 84.5% (Table 1, entries 3 and 5–8).
However, the yield of 3a appeared to decrease slightly when the
reaction temperature was increased from 90 to 100 1C (Table 1,
entries 8 and 9). This finding indicates that proper temperature
is instrumental in the synthesis of 3a in the p-CNN catalytic
system irradiated by visible light.
p-CNNs
Entry dosage (wt%) Time (min) Temperature (1C) Ratiob Yieldc (%)
To investigate the effects of the p-CNN photocatalyst dosage
and the stoichiometry of the reactants for the synthesis of 3a, a
series of experiments was performed and the results are presented
in Table 2. Firstly, the yield of 3a increased along with the increase
in the p-CNNs dosage from 0.56 to 1.68 wt% (Table 2, entries 1–3).
However, no obvious increase in 3a yield was observed when the
photocatalyst dosage was 2.80, 3.92, and 5.60 wt%, respectively
(Table 2, entries 4–6). This is probably because the adsorption of
benzaldehyde, ethyl acetoacetate and urea on the surface of
p-CNNs formed unstable species, leading to a lower activation
energy of the entire reaction. Table 2 also shows the influence of
the stoichiometry of the reactants on the synthesis of 3a. Appar-
ently, as the urea dosage increased, the yield of 3a increased
(Table 2, entries 3 and 8, 7 and 9). Conversely, the yield of 3a
decreased when the content of ethyl acetoacetate was increased
(Table 2, entries 7, 8, 9 and 10). According to the aforementioned
analysis, two points can be reflected. One is that the optimum
reaction condition was observed at 90 1C for 90 min with the
reaction ratio of 1 : 1.2 : 1.5 (benzaldehyde : ethyl acetoacetate :
urea) under the irradiation of visible light with the p-CNNs
photocatalyst (1.68 wt%). The other is that the Biginelli reaction
for the efficient synthesis of 3a was achieved with the assistance of
photo-thermo catalysis.
1
2
3
4
5
6
7
8
9
1.12
1.12
1.12
1.12
1.12
1.12
1.12
1.12
1.12
30
60
90
120
90
90
90
90
90
40
40
40
40
30
50
70
90
100
1 : 1 : 1 14.8
1 : 1 : 1 19.0
1 : 1 : 1 26.6
1 : 1 : 1 25.2
1 : 1 : 1 17.9
1 : 1 : 1 48.1
1 : 1 : 1 72.7
1 : 1 : 1 84.5
1 : 1 : 1 82.0
a
3a is 5-ethoxycarbonyl-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H)-
b
one. The order of the reactants ratio is benzaldehyde to ethyl aceto-
acetate to urea (5 mmol benzaldehyde, 1 equiv.). Isolated yield.
c
the optimum reaction time for further investigations. As
we know, temperature plays an important role in organic
synthesis. In this study, the influence of different reaction
Table 2 The effects of p-CNNs dosage and the stoichiometric ratio of
reactants on the synthesis of 3a photocatalyzed by p-CNNsa
To further understand the merits of the synthesis of 3a
photocatalyzed by p-CNNs, the system was compared with that
of the earlier reported ones. As shown in Table 3, p-CNNs
exhibited excellent activity in a shorter reaction time as com-
pared to D-xylonic acid, PPH3, B(C6F5)3 and SBNBSA.2,46–48 The
SFP catalyst also gave a good yield of 3a, but suffered from the
high reaction temperature.45 SBNPSA and ZrOCl2/mont K10 can
work at relatively lower temperature and shorter time.48,49
However, additional solvents or metal catalysts were employed.
The ZSPS catalyst exhibited excellent activity at shorter reaction
time, but the catalyst preparation process is complex and
expensive when compared with p-CNNs.45 Obviously, with
p-CNNs
Temperature
Entry dosage (wt%) Time (min) (1C)
Ratio
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
0.56
1.12
1.68
2.80
3.92
5.60
1.68
1.68
1.68
1.68
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
1 : 1 : 1
1 : 1 : 1
1 : 1 : 1
1 : 1 : 1
1 : 1 : 1
1 : 1 : 1
1 : 1.2 : 1.2 84.7
1 : 1 : 1.2 94.2
1 : 1.2 : 1.5 95.4
1 : 1.5 : 1.5 68.1
75.3
84.5
88.4
87.8
87.3
80.1
a
3a is 5-ethoxycarbonyl-6-methyl-4-phenyl-3,4-dihydropyrimidin-2(1H)-
one. Isolated yield.
b
Table 3 Various catalysts for the synthesis of 3a in their own appropriate reaction medium
Entry
Catalyst
Time (min)
Temperature (1C)
Solvent
Yield (%)
Ref.
1
2
3
4
5
6
7
8
p-CNNs (1.68 wt%)
D-Xylonic acid (6.5 mol%)
ZSPSa (5 mol%)
90
300
30
90
100
90
100
100
Reflux
80
—
95.4
87
96
94
70
95
95
78
This work
2
D-Xylonic acid
—
—
—
EtOH
EtOH
—
41
42
43
44
45
46
SFPb (5 wt%)
20
PPH3 (2.78 mol%)
B(C6F5)3 (2.78 mol%)
SBNPSAc (1 mol%)
ZrOCl2/mont K10 (40 wt%)
720
150
240
20
80
a
b
c
ZSPS represents zirconium(IV)–salophen perfluorooctanesulfonate. SFP represents sulfated polyborate. SBNPSA represents silica-bonded
N-propyl sulfamic acid.
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respect to the efficiency or environmental economic benefit, acetoacetate on the Biginelli reaction was investigated. No obvious
the p-CNN system for the synthesis of 3a is superior to most difference in the yields of corresponding products was observed
reported systems. To investigate the relationship between the when compared with ethyl acetoacetate (3f–3m, 3h–3n, 3d–3o,
structure of p-CNNs and its performance, the photocatalytic 3b–3p). In addition, the influence of thiourea on the reaction was
activities of ml-CN and p-CNNs on the synthesis of 3a were also investigated, and 89.7% yield of 3q was achieved. Other
investigated, and the photocatalytic performance of ml-CN was investigations (3r and S-3a–3e) were similar with the above ones,
lower than that of p-CNNs. According to the characterization of and all the results indicated that the photo-thermo catalysis system
SEM and AFM, the possible active sites of p-CNNs in the using p-CNNs is appropriate for the Biginelli reaction.
synthesis of 3a was the porous structure of the p-CNNs.
With regard to the excellent performance and mild reaction
To investigate the substrate universality, Biginelli reaction of conditions of the reaction system, it is worth exploring for the
various aldehydes with ethyl acetoacetate or methyl acetoacetate synthesis of N- and O-heterocyclic compounds. Pyrroles and their
and urea or thiourea was performed. From the perspective of green analogs are a series of N-heterocyclic compounds, which play an
chemistry, the stoichiometric ratio of the reactants was changed important role in the synthesis of pharmacologically significant
from 1 : 1.2 : 1.5 to 1 : 1 : 1.2, and the other optimum conditions molecules and natural products.50 Therefore, the synthesis of
were not changed. Firstly, the influences of aromatic aldehydes pyrroles is of great significance. In addition, benzoxanthene
with different electro-donating groups and electro-withdrawing derivatives are a class of O-heterocyclic compounds. They have
groups on the Biginelli reaction were studied. As shown in favorable biological and pharmaceutical properties of analgesic,
Fig. 3, excellent yields were achieved (3b–3h), and no obvious antiviral and antibacterial.51,52 Thus, the synthesis of benzox-
difference in the yields of different products was observed, indicat- anthene derivatives has vital significance. For 5-phenyl-1
ing that the electron-donating groups and electron-withdrawing (4-methoxyphenyl)-3[(4-methoxyphenyl)-amino]-1H-pyrrol-2(5H)-one
groups in the aromatic aldehyde (p-) are all beneficial for the synthesis, the reaction was performed at room temperature by
Biginelli reaction. Subsequently, the effect of spatial hindrance mixing p-CNNs, benzaldehyde, 4-methoxyaniline and ethyl pyru-
on the Biginelli reaction was explored. The yield of the corres- vate in the presence of visible-light irradiation, and the yield of the
ponding product was 90.3 (3c), 87.1 (3i) and 80.3% (3j) when product was up to 93%. However, for 12-phenyl-9,9-dimethyl-
p-methoxybenzaldehyde, m-methoxybenzaldehyde and o-methoxy- 8,9,10,12-tetrahydrobenzo[a]xanthen-11-one synthesis, the reaction
benzaldehyde were employed, respectively. This finding suggests was carried out at 50 1C by mixing p-CNNs, benzaldehyde,
that larger the steric hindrance of the aromatic aldehyde, the more 2-hydroxynaphthalene and 5,5-dimethyl-1,3-cyclohexanedione in
unfavorable the Biginelli reaction. As compared with aromatic the presence of visible-light irradiation, and the yield of the
aldehydes, fatty aldehydes used in the Biginelli reaction with ethyl product was up to 97%.
acetoacetate and urea gave good yields, (3k, 3l) but lower than that
Due to the excellent activity of p-CNNs, it is worth exploring
of the aromatic aldehydes. Furthermore, the effect of methyl its cycle stability. In this work, the cycling experiment was
Fig. 3 Photo-thermo catalysis for the synthesis of dihydropyrimidin-2(H)-ones and thiones photocatalyzed by p-CNNs.
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Polytechnic University (2020-07) and the Foundation (No.
KF201914) of State Key Laboratory of Biobased Material and
Green Papermaking, Qilu University of Technology, Shandong
Academy of Sciences.
Notes and references
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Fig. 4 Recycling of p-CNNs photocatalyst.
performed using benzaldehyde, ethyl acetoacetate and urea
(1 : 1 : 1.2) at 90 1C for 90 min in the presence of p-CNNs
(1.68 wt%). In the cycling experiment, the p-CNNs was recov-
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p-CNNs has excellent stability.
Conclusions
In summary, this work provides a novel and green photocatalytic
system for the synthesis of N- and O-heterocyclic compounds. The
p-CNN photocatalyst showed excellent activity for the ‘‘one-pot’’
three-component Biginelli reaction to give DHPMs and their
derivatives. The ease of use, environmental friendliness as well as
air, water, and substrate tolerances make it an excellent system
for carrying out Biginelli reaction. Meanwhile, 5-phenyl-1
(4-methoxyphenyl)-3[(4-methoxyphenyl)-amino]-1H-pyrrol-2(5H)-one
and 12-phenyl-9,9-dimethyl-8,9,10,12-tetrahydrobenzo[a]xanthen-
11-one were all obtained via this reaction system with 93% and
97% yields, respectively, under mild reaction conditions. This
work paves a novel way for the organic synthesis of heterocyclic
compounds, especially the N- and O-heterocyclic compounds.
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Acknowledgements
We wish to thank for the Foundation of NSFC-CONICFT Joint 18 X. W. Zhu, S. Q. Huang, Q. Yu, Y. B. She, J. M. Yang,
Project (No. 51961125207), Liaoning Province ‘‘Xingliao Talent
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