The physical properties of two-dimensional nanosheet materials make them promising candidates as active materials in the areas of photoelectronics, fuel cells, sensors, water splitting, solar energy conversion, CO2 reduction, and heterogeneous catalysis. Among two-dimensional nanosheet materials, graphitic carbon nitride due to its electronic structure and high chemical and thermal stability possesses unique properties. Covalent functionalization of graphitic carbon nitride could be the key step in modifying its ability and significantly improving its properties. To this purpose, a novel strategy for the covalent functionalization of g-C3N4 nanosheets (CN) with vitamin B1 (VB1) by using 1,3-dibromopropane as a covalent linker for the first time is demonstrated. The obtained CN-Pr-VB1 exhibits increased thermal stability compared to the VB1 which is important in the practice application and can be easily dispersed in common organic solvents. The efficacy of the CN-Pr-VB1 as a heterogeneous organocatalyst was evaluated in the quinoxaline synthesis under solvent-free conditions and afforded good isolated yield with high purity. Moreover, the prepared catalyst could be facilely recycled and reused for seven consecutive cycles without a noticeable decrease in the catalytic activity. Extensive characterization confirmed the stability of morphology and chemical structure after recyclability of the CN-Pr-VB1.
The physical properties of two-dimensional nanosheet materials make them promising candidates as active materials in the areas of photoelectronics, fuel cells, sensors, water splitting, solar energy conversion, CO2 reduction, and heterogeneous catalysis. Among two-dimensional nanosheet materials, graphitic carbon nitride due to its electronic structure and high chemical and thermal stability possesses unique properties. Covalent functionalization of graphitic carbon nitride could be the key step in modifying its ability and significantly improving its properties. To this purpose, a novel strategy for the covalent functionalization of g-C3N4 nanosheets (CN) with vitamin B1 (VB1) by using 1,3-dibromopropane as a covalent linker for the first time is demonstrated. The obtained CN-Pr-VB1 exhibits increased thermal stability compared to the VB1 which is important in the practice application and can be easily dispersed in common organic solvents. The efficacy of the CN-Pr-VB1 as a heterogeneous organocatalyst was evaluated in the quinoxaline synthesis under solvent-free conditions and afforded good isolated yield with high purity. Moreover, the prepared catalyst could be facilely recycled and reused for seven consecutive cycles without a noticeable decrease in the catalytic activity. Extensive characterization confirmed the stability of morphology and chemical structure after recyclability of the CN-Pr-VB1.
Nowadays with the growing
severity of energy crisis and environmental
pollution, the development of a green sustainable catalytic process
is considered as one of the promising methods for a resolution of
these problems.[1] For this purpose, organocatalysts
and biocatalysts based on their advantages such as nontoxicity, ready
availability, low cost, stability, and biodegradability have been
the subject of considerable interest in chemical syntheses and transformations.[1−5] Since the pioneering work reported in 1960, vitamin B1 (VB1) has attracted much attention as a catalyst and
an important coenzyme in biochemical transformation of α-keto
acids into acetyl CoA in polyketide synthesis and formation of acyloins.[6] Moreover, VB1 that consists of multifunctional
groups such as hydroxyl and quaternary ammonium salt has already demonstrated
its effectiveness to catalyze organic reactions such as Knoevenagel
condensation, Michael addition, cyclization, and creating carbon–carbon
bonds and carbon–heteroatom bonds.[7−10] The focus during the past decades
has been on covalent and noncovalent functionalization of solid supports
with active biomolecules and organic molecules, which is an effective
strategy to overcome the problems of using homogeneous catalysts such
as difficult recovery from the reaction mixture and also to improve
catalytic performance and stability.[11] To
date, the development of two-dimensional (2D) nanosheet materials
with molecular thickness has attracted worldwide attention owing to
fascinating physicochemical properties.[12] Among various 2D layered systems, graphitic carbon nitride (g-C3N4) as an important class of metal-free conjugated
polymers with weak van der Waals forces between layers possesses unique
electronic band structure, high chemical and thermal stability, large
surface area, and environmentally friendly property and nontoxicity.[13,14] Therefore, g-C3N4 sheets has been investigated
in the areas of photoelectronics, fuel cells, sensors, water splitting,
solar energy conversion, CO2 reduction, and heterogeneous
catalysis.[15−25] Covalent functionalization of g-C3N4 sheets
has seldom been used in catalytic applications.[26−30] Thus, the utilization of g-C3N4 sheets as a potential support material is still worthy and desirable
to explore new heterogeneous catalysts.On the basis of the
above-mentioned properties of VB1, g-C3N4 sheets, and continuing our efforts
toward the design of greener and efficient heterogeneous catalysts
for the synthesis of organic compounds,[31−36] we report herein the first successful covalent functionalization
of g-C3N4 sheets with VB1 through
a facile and innovative strategy. VB1 covalently functionalized
g-C3N4 sheets exhibit high thermal stability
and much improved catalytic performance compared to VB1 and g-C3N4 sheets in organic reactions.Quinoxaline and their derivatives are a valuable and well-established
class of nitrogen-bearing heterocycles that have found prominence
in the area of dyes, pharmaceutical, and agricultural chemistry.[37−39] Some commercial antibiotics contain a quinoxaline nucleus as a fundamental
scaffold like Echinomycin, Levomycin, and Actinoleutin which are active
against Gram-positive bacteria and represent antitumor activity.[40,41] Moreover, they have diverse applications in several fields such
as significant intermediates in organic synthesis, efficient electron
luminescent materials, organic semiconductors, and DNA cleaving agent.[42−45] Because of the remarkable aforementioned properties of quinoxaline
derivatives, many protocols have been developed for the preparation
of these compounds. The conventional synthesis of quinoxalines involves
the reaction between aryl 1,2-diamines and 1,2-dicarbonyl compounds
in refluxing ethanolic medium or acetic acid.[46] The use of heterogeneous catalysts has become an essential inspiration
for chemists to develop sustainable methods for the synthesis of organic
compounds. In view of this, a variety of catalysts such as heterogeneous
carbon-based materials, glycerol, CAN, I2, polyaniline
sulfate, Montmorillonite K-10, oxalic acid, VB1, and Zn[(l)proline] have been employed to carry out this condensation.[47−55] Although some of these reported methods suffer from a number of
demerits including prolonged reaction times, the use of toxic organic
solvents, a low yield of the product, the use of costly catalysts,
and harsh reaction conditions. Hence, a great deal of interest has
been focused on the development of environmentally, efficient, and
recyclable benign catalytic methods for the synthesis of quinoxaline
derivatives. In this work, we demonstrate the application of CN-Pr-VB1 as a heterogeneous organocatalyst for the preparation of
quinoxalines via condensation of 1,2-diamines with 1,2-dicarbonyl
compounds under solvent-free condition in excellent yields (Scheme ).
Scheme 1
Synthesis of Quinoxaline
Derivatives Catalyzed by CN-Pr-VB1
Results and Discussion
Catalyst Characterizations
CN-Pr-VB1 was
prepared by a facile three-step process as shown in Scheme . At the initial step, two-dimensional
(2D) g-C3N4 nanosheets were synthesized through
the liquid exfoliation followed by sonication treatment. The crystal
and chemical structure of g-C3N4 nanosheets
were confirmed by X-ray diffraction (XRD) analysis and Fourier transform
infrared (FT-IR) spectroscopy. The XRD analysis indicated that the
layered g-C3N4 was successfully exfoliated into
2D nanosheets. As verified in the XRD analysis of bulk g-C3N4 (Figure ), two diffraction peaks which correspond to the interplanar stacking
of the conjugated aromatic systems and the in-plane structural packing
motif of repeated tri-s-triazine units in 2θ
= 27.4° for (002) reflection and 2θ = 13.1° for (100)
reflection can be observed. After exfoliation, the intensity of (002)
diffraction is weakened significantly and its related peak is shifted
from 27.4° to 27.8°. This change is related to shrinking
of the interplanar stacking distance while the typical reflection
peak of the bulk g-C3N4 at 2θ = 13.1°
disappeared.
Scheme 2
Preparation of CN-Pr-VB1
Figure 1
XRD patterns of bulk g-C3N4 and
g-C3N4 nanosheets.
XRD patterns of bulk g-C3N4 and
n class="Chemical">g-C3N4 nanosheets.
The FT-IR spectroscopy of the g-C3N4nanosheet
and bulk g-C3N4 is illustrated in Figure . As can be seen, the characterization
peaks of bulk g-C3N4 were similar to the g-C3N4 nanosheet. A strong and broad peak of the N–H
group (−NH2 or =NH groups) appeared at 3000–3500
cm–1, stretching vibration peaks of C=N were
observed at 1614 and 1550 cm–1, stretching peaks
of the C–N heterocycle were at around 1406, 1319, and 1234
cm–1, and a sharp peak at 808 cm–1 is due to the breathing vibration of tri-s-triazine
units.
Figure 2
FT-IR spectra of bulk g-C3N4 and g-C3N4 nanosheets.
FT-IR spectra of bulk g-C3N4 and n class="Chemical">g-C3N4 nanosheets.
Then, the g-C3N4nanosheet was modified
with
1,3-dibromopropane as a covalent linker for the first time. The chemical
structure of the new synthesized CN-Pr-Br was confirmed by FT-IR and
energy-dispersive X-ray spectroscopy (EDS) (Figure a,b). From FT-IR spectra, main adsorption
bands of g-C3N4 nanosheets can be observed in
CN-Pr-Br, suggesting that the basic g-C3N4 chemical
structure is largely retained. Energy-dispersive X-ray spectroscopy
(EDS) displays the successful incorporation of the Br atoms within
the framework.
Figure 3
(a) FT-IR spectra of g-C3N4 nanosheet
(CN)
and CN-Pr-Br, (b) EDS analysis of CN-Pr-Br.
(a) FT-IR spectra of g-C3N4nanosheet
(Cn class="Chemical">N)
and CN-Pr-Br, (b) EDS analysis of CN-Pr-Br.
Intensive efforts have been devoted to evaluate the best
conditions
for the loading of VB1 on CN sheets, and various amounts
of 1,3-dibromopropane as a linker were investigated. In this study,
two different amounts of 1,3-dibromopropane, 1.01 and 2.02 mL, were
studied under same conditions to investigate the immobilization of
VB1 on CN sheets. CHNS elemental analysis of CN-Pr-VB1 in the presence of 1.0 g of CN sheets, 1.01 mL of 1,3-dibromopropane,
and 1 mmol of VB1 shows 30.82% (C), 3.97% (H), 45.97% (N),
and 0.12% (S) percentage of contents. In the other study, in the presence
of 1.0 g of CN sheets, 2.02 mL of 1,3-dibromopropane, and 1 mmol of
VB1, the contents of C, H, N, and S surprisingly were changed
to 35.20% (C), 3.76% (H), 26.75% (N), and 2.97% (S). According to
these results, it is concluded that about 31% vitamin (VB1) molecules have grafted onto the CN sheets. To explain this difference,
it can be described that in the presence of 2.02 mL of linker and
by participation of nitrogen atoms with sp2-hybridization
in the g-C3N4 framework, 1,3-dibromopropanecan react with =N, −NH and −NH2 groups;
thus, the ratio of reacted functional groups increased and more of
VB1 can be attached to g-C3N4-Pr
with covalent bonds.The CHNS elemental analysis is in accordance
with the EDS spectra.
As shown in Figure , the presence of C, N, O, Br, S, and Cl elements in the prepared
CN-Pr-VB1 structure are proved. Therefore, it is concluded
that vitamin (VB1) molecules are grafted onto the CN sheets
via the nucleophilic reaction between the hydroxyl group of VB1 and Br atoms of CN-Pr-Br. In addition, also with the aid
of thermogravimetric analysis/differential thermal analysis (TGA/DTA),
the existence of organic moieties on the CN sheets has been proved.
Figure 4
EDS analysis
of CN-Pr-VB1.
EDS analysis
of CN-Pr-VB1.XRD data depicted the retention of the crystalline structure
of
g-C3N4 without any phase change in the structure
of CN-Pr-VB1 during the functionalization procedure. The
main characteristic peak of CN (g-C3N4 nanosheet)
did not change after anchoring of VB1 (Figure ).
Figure 5
XRD patterns of g-C3N4 nanosheets and CN-Pr-VB1.
XRD patterns of g-C3N4nanosheets and Cn class="Chemical">N-Pr-VB1.
Further evidence of covalent functionalization
of CN sheets with
VB1 molecules was also reflected inTGA/DTA (Figure ). Small amounts of adsorbed
water are lost in the initial heating step below 200 °C. The
DTA curve of CN-Pr-VB1 exhibited an obvious weight loss
within the temperature range of 200–400 °C, which indicates
the existence of organic moieties on the CN sheets. In the last step,
CN sheets start to decompose at >500 °C. Thus, the CN-Pr-VB1 as a catalyst retains its stability up to 200 °C in
organic reactions. TGA of the CN-Pr-VB1 were also used
to determine the content of organic functional groups (Pr-VB1) of the CN-Pr-VB1. The second step in the range of 200–400
°C indicates the presence of organic moieties for the CN-Pr-VB1, which correspond to the loss of the Pr-VB1 to
the surface of the g-C3N4 nanosheets (CN). Therefore,
it is concluded that the amount of Pr-VB1 grafted on the
surface of the g-C3N4 nanosheets (CN) was about
30% that estimated from the percentage of weight loss from the TGA
profile of CN-Pr-VB1 and CN.
Figure 6
Thermogravimetric and
differential thermogravimetric of (a) g-C3N4 nanosheets (CN) and (b) CN-Pr-VB1.
Thermogravimetric and
differential thermogravimetric of (a) g-C3N4nanosheets (Cn class="Chemical">N) and (b) CN-Pr-VB1.
As shown in Figure , the FT-IR spectra reveal that the CN-Pr-VB1 still
keeps
the original C–n class="Chemical">N network of g-C3N4 nanosheets
and the same chemical structure of VB1.
Figure 7
FT-IR spectra of the
g-C3N4 nanosheet (CN),
VB1, CN-Pr-Br, and CN-Pr-VB1.
FT-IR spectra of the
g-C3N4nanosheet (Cn class="Chemical">N),
VB1, CN-Pr-Br, and CN-Pr-VB1.
The morphology and microstructure of the synthesized
CN-Pr-VB1 was characterized via field emission scanning
electron microscopy
(Figure ). The FESEM
images obtained indicated that the layered structure of CN sheets
(g-C3N4 nanosheets) changes during functionalization
of CN sheets with VB1, suggesting that the surface of CN
sheets were immobilized with VB1.
Figure 8
FESEM images of (a,b)
CN (g-C3N4 nanosheets)
and (c,d) CN-Pr-VB1.
FESEM images of (a,b)
CN (n class="Chemical">g-C3N4 nanosheets)
and (c,d) CN-Pr-VB1.
To understand the colloidal stability of the CN-Pr-VB1 sample compared to Cn class="Chemical">N, zeta potential measurements of aqueous
solution
of CN (g-C3N4 nanosheet) and CN-Pr-VB1 were tested (Figure S1). The zeta potential
results indicate that the CN sheet suspension carries a negative charge
of −2.56 mV. After covalent functionalization of CN (g-C3N4 sheets) with VB1, the zeta potential
increased from −2.56 mV (CN) to −5.76 mV (CN-Pr-VB1) and possessed more negative polarity on its surface in the
solution attributed to the existence of the ion pairs (Cl– and Br–) in the structure of CN-Pr-VB1 (Figure S2). Therefore, the CN-Pr-VB1 demonstrates good dispersibility in water.
Catalytic Properties
The catalytic potential of CN-Pr-VB1 was explored for
the preparation of n class="Chemical">quinoxaline derivatives
via condensation between aryl 1,2-diamines and 1,2-dicarbonyl compounds.
To obtain the optimum reaction conditions regarding the solvent,
reaction temperature, and the amount of catalyst, the reaction of o-phenylenediamine and benzil was chosen as the model reaction
(Scheme ). In the
absence of the catalyst, no desired product was observed after the
prolonged reaction time of 24 h (Table , entries 1–3). The progress of the reaction
was investigated by using CN-Pr-VB1 as the catalyst in
different solvents. In CH3CN and MeOH, the reaction proceeded
slowly with low yield of the target product (Table , entries 4–7), whereas in EtOH, H2O, and H2O/EtOH, the product was obtained in moderate
yields 60–70% (Table , entries 8–13). Conducting the model reaction under
solvent-free condition enhanced significantly the yield of the desired
product in a shorter time period (Table , entry 14). In addition to this, we performed
the model reaction at varying temperatures (Table , entries 14–17). It was found that
increasing the temperature, from 80 to 100 °C, improves the yield
of the product. However, another increase in the reaction temperature
to 120 °C had no impact on the product yield. Further to get
the optimized amount of catalyst, the model reaction was carried out
at 10, 15, 20, and 25 mg of CN-Pr-VB1 at 100 °C under
solvent-free conditions (Table , entries 17–20). Therefore, 15 mg of the catalyst
is sufficient to achieve the higher yields. The efficiency of CN-Pr-VB1 was also compared with CN (g-C3N4 nanosheets)
and VB1 (Table , entries 21, 22). These observations reveal that the high
catalytic activity of CN-Pr-VB1 may be due to the synergetic
effect of CN (g-C3N4 nanosheets) and VB1.
Scheme 3
Model Reaction for the Optimization Reaction Conditions
Table 1
Optimization of the
Reaction Conditions
for the Synthesis of Quinoxalinesa
Reaction condition: o-phenylenediamine (1.0 mmol), n class="Chemical">benzil (1.0 mmol), solvent
(3.0 mL).
Isolated yield.Encouraged by the established
optimized reaction conditions, we
explored/expanded the library construction and scope of the reaction
involving various 1,2-diamines and 1,2-diketones (Table ). The results indicate that
substrates possessing both electron-donating as well as electron-withdrawing
groups afforded good-to-excellent yields of the desired products within
a very short period of time.
Table 2
CN-Pr-VB1-Catalyzed Synthesis
of Quinoxalines via the Condensation of 1,2-Diamines with 1,2-Diketones
under Solvent-Free Conditiona[56−67]
Reaction conditions:
1.0 mmol 1,2-diamine,
1.0 mmol 1,2-diketone, and 15.0 mg catalyst under solvent-free condition
at 100 °C.
Isolated
yields.
Reaction conditions:
1.0 mmol 1,2-diamine,
1.0 mmol n class="Chemical">1,2-diketone, and 15.0 mg catalyst under solvent-free condition
at 100 °C.
Isolated
yields.A plausible mechanism
for the formation of quinoxalines is presented
in Scheme . CN (g-C3N4 nanosheet) has a π-conjugated structure
with surface termination as defects and nitrogen atoms for electron
localization or anchoring inorganic/organic functional motifs as the
active sites. Because of the incorporation of amine groups in the
carbon architecture of CN, the free amino groups distributed on the
surface of CN-Pr-VB1 and also −NH proton of the
VB1, and the acidic CH proton (the carbenoid proton) of
VB1 activate the carbonyl groups of the 1,2-diketone through
hydrogen bonding for nucleophilic attack of 1,2-diamine to afford
the corresponding intermediate. Then, dehydration and elimination
of a proton were done to afford quinoxaline as the final product.
Scheme 4
Plausible Mechanism for the Synthesis of Quinoxalines Catalyzed by
CN-Pr-VB1
In view of the ion-pair structure of the catalyst (CN-Pr-VB1), we can postulate an important contribution of the catalyst
to the TS stabilization. As a matter of fact, the CN-Pr-VB1 provides dipolar environment, leading to stabilize the charge distribution
in the TS through specific interactions including H-bonding and dipole
charge interactions. These observations reveal that the high catalytic
activity of CN-Pr-VB1 may be due to the synergetic effect
of CN (g-C3N4 nanosheets) and VB1.In light of the significant features of heterogeneous catalysts
including recovery, reusability, and stability, we evaluated the recyclability
of the CN-Pr-VB1 for the condensation reaction among o-phenylenediamine and benzil under optimum reaction conditions.
The used CN-Pr-VB1 catalyst was separated from the reaction
mixture by simple filtration, washed thoroughly with ethanol and water,
and dried at 60 °C. The recovered CN-Pr-VB1 catalyst
can be reused over six runs without notable descent of catalytic activity
(Figure ). The FT-IR
spectra of the CN-Pr-VB1 after six cycles (Figure ) presented that the structure
of the catalyst does not change during the course of reaction.
Figure 9
Reusability
of the CN-Pr-VB1 in the synthesis of quinoxalines.
Figure 10
FT-IR spectra of fresh and used CN-Pr-VB1.
Reusability
of the CN-Pr-VB1 in the synthesis of n class="Chemical">quinoxalines.
FT-IR spectra of fresh and used CN-Pr-VB1.In order to further investigate
the catalytic activity of CN-Pr-VB1, a comparison with
other catalysts reported earlier in the
literature for this reaction was done and the results are summarized
in Table . According
to these data, our protocol has several advantages over the reported
methods in terms of the short reaction time, superior efficiency,
mild and environmentally benign conditions, high yield, and recyclability
of the catalyst.
Table 3
Comparison between the Catalytic Efficiency
of the CN-Pr-VB1 with the Reported Catalysts for the Condensation
of o-Phenylenediamine and Benzil
entry
reaction
conditions
time (min)
yielda (%)
refs
1
DMSO, I2 (10 mol %), rt.
35
95
(68)
2
nano-ZrO2 (5 mol %),
dichloroethane, 25 °C
45
89
(69)
3
ZnCl2 (4 mol %), EtOH/H2O (3/1, v/v), rt.
300
80
(70)
4
glycerol/H2O, 90 °C
240
90
(47)
5
17% ZrO2 4% Ga2O3/MCM-41, acetonitrile,
rt.
120
97
(71)
6
polyaniline sulfate, dichloromethane,
rt.
20
95
(72)
7
oxalic acid (20 mol %), EtOH/H2O (1/1), rt.
10
93
(52)
8
Zn[(l)proline] (10 mol %), HOAc, rt.
10
95
(53)
9
graphite (2 mmol), EtOH, rt.
60
92
(54)
10
montmorillonite K-10 (10 mol %), H2O rt.
150
100
(51)
11
CN-Pr-VB1 (15.0 mg), solvent-free, 100 °C
3
97
this work
Isolated yield.
Isolated yield.
Conclusions
Chemical
modification of g-C3N4can enhance
its properties as an advantageous 2D nanosheet material such as developing
solubility and stability. To enhance g-C3N4’s
properties, a new and straightforward synthetic strategy for the covalent
functionalization of the g-C3N4 framework was
developed through 1,3-dibromopropane as a linker for the first time.
The synthesized heterogeneous catalyst (CN-Pr-VB1) was
characterized by a range of techniques to confirm the covalent bond
formation of VB1 with CN sheets. This novel catalyst displayed
high thermal stability compared to the VB1. Additionally,
the CN-Pr-VB1 was found to be an efficient and biocompatible
heterogeneous organocatalyst for the synthesis of quinoxalines. This
novel protocol offers several advantages like easy preparation of
the catalyst from inexpensive and benign precursors, simple operational
procedure, short reaction time period, excellent yield of products,
and reusability and stability of the catalyst over several reaction
cycles. It can be expected that the g-C3N4 nanosheets
are a promising candidate for the catalyst support in organic synthesis
and transformation.
Experimental Section
Chemicals and Instruments
All reagents and solvents
were purchased from Merck or Fluka chemical companies and were used
without further purification. XRD analyses were obtained on a PANalytical
X’Pert-PRO MPD diffractometer with Cu Kα (λ = 1.5406
Å) irradiation in the 2θ = 10°–80° with
a 2θ step size of 0.02°. FESEM were acquired using MIRA3
TESCAN-XMU. Energy-dispersive X-ray spectroscopy (EDS) was used to
investigate the composition and structure of the samples and was recorded
on Oxford Instrument (England). C, H, N, and S elemental analyses
for CN-Pr-VB1 were performed using a Thermo Finnigan Flash
EA 1112 Element Analyzer. Thermogravimetric/differential thermal analyses
(TGA/DTG) were conducted using an STA504 analyzer with a heating rate
of 10 °C/min from 20 to 800 °C. Zeta potential measurements
of the samples were obtained using the dynamic light-scattering analysis
using the Zeta sizer Nano ZS (Malvern Instruments, England). The suspensions
of each sample in aqueous solution media (0.1 mg·mL–1) were prepared through sonication at room temperature. Fourier transform
infrared (FT-IR) spectra were obtained on a Shimadzu 800 IR 100 FT-IR
spectrometer with a standard KBr disk technique. Melting points were
determined on an Electrothermal 9100 apparatus without correction. 1HNMR and 13CNMR spectra were recorded on a Burker
DRX-500 Avance spectrometer (500 and 125 MHz) in DMSO-d6 using tetramethylsilane as the internal standard.
Preparation of Bulk g-C3N4
Bulk
g-C3N4 powder was synthesized according to the
previous reported article.[16] The n class="Chemical">melamine
was heated at 550 °C in a furnace at a ramp of 2.5 °C min–1 in static air for 4 h, and the resulted yellow solid
was grinded into powder in a mortar.
Preparation of g-C3N4 Nanosheets
g-C3N4nanosheets (CN) were prepared according
to the previous reported method.[17] First,
bulk g-C3N4 (1.0 g) was treated in the mixture
of 20.0 mL of H2SO4 and 20.0 mL of HNO3 at room temperature for 2 h. The mixture was diluted with 1.0 L
of deionized H2O, and the obtained precipitate was filtered
and washed several times with deionized water (filtered undispersed
g-C3N4) and dried at 60 °C. Then, treated
bulk g-C3N4 (100.0 mg) was dispersed in 100.0
mL of water/isopropanol (1:1) and sonicated for 6 h. Finally, to remove
the residual unexfoliated g-C3N4 nanoparticles,
the formed suspension was centrifuged (5000 rpm).
Synthesis of
CN-Pr-Br
The resulting g-C3N4nanosheets
(Cn class="Chemical">N) (1.0 g) were dispersed in 25.0 mL of
dry toluene and then, 1,3-dibromopropane (2.02 mL) and NaI (1.0 mmol)
were added to the dispersed solution, and the reaction mixture was
refluxed overnight under a nitrogen atmosphere. The resulted mixture
was cooled to room temperature and collected by centrifugation, washed
with ethyl acetate, and dried at room temperature.
Synthesis of
CN-Pr-VB1
As-prepared CN-Pr-Br
(1.0 g) was dispersed in n class="Chemical">toluene (25.0 mL) by ultrasonication for
30 min; then, thiamine hydrochloride (1.0 mmol), K2CO3 (1.0 mmol), and NaI (1.0 mmol) were added to this mixture.
The reaction mixture was heated at reflux temperature for 48 h under
a nitrogen atmosphere. After being cooled to room temperature, the
filtration product was washed several times with H2O and
EtOH to remove the unreacted substrate to obtain CN-Pr-VB1 and then dried at 50 °C overnight.
General Procedure for the
Synthesis of Quinoxaline Derivatives
To a round-bottom flask
containing 1,2-diamine (1.0 mmol) and 1,2-diketone
(1.0 mmol) was added CN-Pr-VB1 as the catalyst (15.0 mg)
and stirred at 100 °C in an oil bath, some minutes later the
reagents were melted and after appropriate reaction time (Table ), and the solid product
was achieved. The completion of the reaction was indicated by thin-layer
chromatography. After completion of the reaction, EtOAC (10 mL) was
added and the catalyst was filtered off; then, the solvent was evaporated
under vacuum. The obtained crude product was purified by recrystallization
from ethanol. The products were identified by 1HNMR, 13CNMR, IR spectroscopy, and melting points. The recovered
catalyst was rinsed with ethanol and reused in other fresh reactions
without a significant loss of activity for another six times.
Authors: Baris Kumru; Jesús Barrio; Jianrui Zhang; Markus Antonietti; Menny Shalom; Bernhard V K J Schmidt Journal: ACS Appl Mater Interfaces Date: 2019-02-22 Impact factor: 9.229
Scheme 1
Scheme 2
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Scheme 3
Scheme 4
Figure 9
Figure 10