Graphitic Carbon Nitride Nanosheets Covalently Functionalized with Biocompatible Vitamin B1: Synthesis, Characterization, and Its Superior Performance for Synthesis of Quinoxalines.

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.

Introduction

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 n class="Chemical">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 n class="Chemical">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 n class="Chemical">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 n class="Chemical">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.

The FT-IR spectroscopy of the g-C3N4 nanosheet and bulk n class="Chemical">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.

Then, the g-C3N4 nanosheet was modified with n class="Chemical">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.

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 n class="Chemical">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-dibromopropane can 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 n class="Chemical">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.

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 (n class="Chemical">g-C3N4 nanosheet) did not change after anchoring of VB1 (Figure ).

Figure 5

XRD patterns of g-C3N4 nanosheets and CN-Pr-VB1.

Further evidence of covalent functionalization of CN sheets with VB1 molecules was also reflected in TGA/n class="Chemical">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.

As shown in Figure , the FT-IR spectra reveal that the CN-Pr-VB1 still keeps the original C–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.

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 (n class="Chemical">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.

To understand the colloidal stability of the CN-Pr-VB1 sample compared to CN, 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 (n class="Chemical">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 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 n class="Chemical">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

entry	solvent	catalyst (mg)	temperature (°C)	time (min)	yieldb (%)
1	EtOH		rt.	24 h	trace
2	EtOH		reflux	24 h	trace
3			100	24 h	trace
4	CH3CN	CN-Pr-VB1 (15)	rt.	70	35
5	CH3CN	CN-Pr-VB1 (15)	reflux	70	40
6	MeOH	CN-Pr-VB1 (15)	rt.	70	40
7	MeOH	CN-Pr-VB1 (15)	reflux	70	45
8	EtOH	CN-Pr-VB1 (15)	rt.	30	65
9	EtOH	CN-Pr-VB1 (15)	reflux	30	70
10	H2O	CN-Pr-VB1 (15)	rt.	30	60
11	H2O	CN-Pr-VB1 (15)	reflux	30	68
12	EtOH/H2O	CN-Pr-VB1 (15)	rt.	30	73
13	EtOH/H2O	CN-Pr-VB1 (15)	reflux	30	75
14		CN-Pr-VB1 (15)	80	3	85
15		CN-Pr-VB1 (15)	100	3	97
16		CN-Pr-VB1 (15)	110	3	97
17		CN-Pr-VB1 (15)	120	3	97
18		CN-Pr-VB1 (10)	100	3	90
19		CN-Pr-VB1 (20)	100	3	97
20		CN-Pr-VB1 (25)	100	3	97
21		CN (g-C3N4 nanosheets) (15)	100	3	45
22		VB1 (2 mol %)	100	3	60

Reaction condition: o-phenylenediamine (1.0 mmol), 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 n class="Chemical">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.

A plausible mechanism for the formation of quinoxalines is presented in Scheme . CN (n class="Chemical">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 (n class="Chemical">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 n class="Chemical">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.

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.

Conclusions

Chemical modification of g-C3N4 n class="Chemical">can 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 TESn class="Chemical">CAN-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. 1H NMR and 13C NMR 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 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-C3N4 nanosheets (CN) were prepared according to the previous reported method.[17] First, bulk n class="Chemical">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-C3N4 nanosheets (CN) (1.0 g) were dispersed in 25.0 mL of dry n class="Chemical">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 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 n class="Chemical">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 1H NMR, 13C NMR, 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.

Selected Spectral Data for Product

6-Nitro-2,3-diphenylquinoxaline

mp: 185–188 °C; FT-IR (KBr) νmax (cm–1): 3055, 2922, 1615, 1515, 1500, 1469, 1446, 1340; 1H NMR (500 MHz, CDCl3): δ (ppm): 7.35–7.44 (m, 6H), 7.6 (t, 4H, J = 6.5 Hz), 8.33 (d, 1H, J = 9.1 Hz), 8.56 (dd, 1H, J = 9.1, 1.4 Hz), 9.10 (d, 1H, J = 1.3 Hz).

2,3-Bis(4-methoxy-phenyl)-6-methylquinoxaline

mp: 124–126 °C; FT-IR (KBr) νmax (cm–1): 3050, 2900, 1655, 1604, 1511, 1343, 1246, 1027; 1H NMR (500 MHz, CDCl3): δ (ppm): 2.62 (s, 3H), 3.85 (s, 6H), 6.90 (d, 4H, J = 9 Hz), 7.50 (dd, 4H, J = 2, 8.75 Hz), 7.58 (dd, 1H, J = 2, 8.5 Hz), 7.92 (s, 1H), 8.03 (d, 1H, J = 8.5 Hz); 13C NMR (125 MHz, CDCl3): δ (ppm): 20.8, 54.2, 112.7, 126.8, 127.5, 130.2, 130.8, 138.5, 138.9, 140.1, 151.1, 151.8, 159.0.

Acenaphtho[1,2-b]quinoxaline

mp: 229–232 °C; FT-IR (KBr) νmax (cm–1): 3047, 2312, 1425, 1298, 1093, 754; 1H NMR (500 MHz, CDCl3): δ (ppm): 7.74–7.81 (m, 4H), 8.05 (d, 2H, J = 8 Hz), 8.18–8.20 (m, 2H), 8.37 (d, 2H, J = 7 Hz).

2,3-Diphenylquinoxaline

mp: 116–118 °C; FT-IR (KBr) νmax (cm–1): 3056, 1548, 1342, 767, 730, 696; 1H NMR (DMSO-d6, 500 MHz): δ (ppm): 8.15 (dd, J = 3.4, 6.4 Hz, 2H), 7.88 (dd, J = 3.4, 6.4 Hz, 2H), 7.48 (d, 4H), 7.39 (m, 6H).

2,3-Bis(4-methoxyphenyl)quinoxaline

mp: 147–149 °C; FT-IR (KBr) νmax (cm–1): 3004, 2935, 1604, 1510, 1344, 1026, 833; 1H NMR (DMSO-d6, 500 MHz): δ (ppm): 8.10 (dd, J = 3.5, 6.25 Hz, 2H), 7.82 (dd, J = 3.5, 6.3 Hz, 2H), 7.45 (d, J = 8.75 Hz, 4H), 6.89 (d, J = 8.7 Hz, 4H), 3.78 (s, 6H).

6-Methyl-2,3-diphenylquinoxaline

mp: 110–113 °C; FT-IR (KBr) νmax (cm–1): 3063, 1660, 1592, 1210, 874, 719, 640; 1H NMR (DMSO-d6, 500 MHz): δ (ppm): 8.05 (d, J = 8.5 Hz, 1H), 7.94 (s, 1H), 7.74 (dd, J = 2.1, 8.45 Hz, 1H), 7.45 (m, 4H), 7.35 (m, 6H), 2.5 (s, 3H).

5,6-Diphenylpyrazine-2,3-dicarbonitrile

mp: 210–212 °C; FT-IR (KBr) νmax (cm–1): 3060, 1596, 1512, 1375, 1228, 771, 698; 1H NMR (DMSO-d6, 500 MHz): δ (ppm): 7.40–7.43, 7.48–7.50 and (m, Ph, 10 H).

9-Methylacenaphtho[1,2-b]quinoxaline

mp: 233–235 °C; FT-IR (KBr) νmax (cm–1): 3047, 2360, 1691, 1512, 1425, 1296, 1095, 912, 825, 773; 1H NMR (DMSO-d6, 500 MHz): δ 2.59 (s, 3H), 7.68 (d, J = 8 Hz, 1H), 7.83 (td, J = 7.4 Hz, 2H), 7.99 (s, 1H), 8.10 (d, J = 8.35 1H), 8.30 (q, J = 4.1 Hz, 2H), 8.39 (t, J = 6.65 Hz, 2H).