Environment Friendly g-C₃N₄-Based Catalysts and Their Recent Strategy in Organic Transformations

Abstract

Organic molecules synthesized in an environmentally friendly manner have excellent therapeutic potential. The entire preparation technique was examined in the existence of a light source, implying that light has been replaced by heating and the usage of dangerous chemicals has decreased, resulting in less pollution of the environment. The advantages of these nanocarbon catalysts include high efficiency, environmentally friendly synthesis, eco-friendly, inexpensive, and non-corrodible. In organic transformations, solid metal base/metal-free catalysts produce better results. Here, the metal-free semiconductor g-C₃N₄ was used to demonstrate the catalytic behavior of organic conversions. g-C₃N₄ is a two-dimensional material and a p‑type semiconductor to enhance the photocatalytic activity. The excellent properties of g-C₃N₄ sheet lead to the support of metals to form metal-organic frameworks. Most of the reactions gained positive response under visible light irradiation. This review will inspire readers in widen the applications of g-C₃N₄ based catalyst in various organic transformation reactions.

INTRODUCTION

In the medical industry, every hour spent synthesizing a new chemical molecule is employed to treat ailments. Furthermore, the creation of new compounds produces toxic waste, which has negative consequences for our planet. Greener measures are employed to combat waste creation. Green synthesis is a type of synthesis that is both ecologically friendly and efficient. In this method, renewable raw materials and safer solvents were used. Also, this method ensures the avoidance of chemical accidents by preventing the hazardous substance production [1]. Having 12 principles of green chemistry in hand, we can rectify all the drawbacks in the preparation of organic compounds in the laboratory. The preparation needs some hazardous chemicals, heating equipment and conditions to get the final product. These conditions produce harmful effects on nature. Scientists have worked more to reduce these effects for the preparation of compounds [2]. So, they introduced green methods for the preparation of organic compounds such as catalysis and photocatalysis.

In the field of chemistry, catalysts play a vital role in enhancing the rate of reactions. Catalytic functions are essential for almost 90% of industrial processes [3]. All the times, many scientists were interested in developing chemical processes via catalytic approach. Such catalytic processes proceed through either homogeneous or heterogeneous medium. Homogeneous means the same phase and heterogeneous means the different phase of the reactants with respect to catalyst [4, 5]. Homogeneous catalysis has the advantage of high chemo-regio selectivity and high activity, but simultaneously, it has disadvantages such as reusability. The homogeneous reaction conditions were difficult to conduct at the laboratory level. To overcome the above-mentioned drawback, heterogeneous catalysts were used because they have merits such as catalyst recovery, excellent stability and easy accessibility. Also, heterogeneous catalysis has some demerits in the catalytic performance such that it takes more time to make contact with the catalyst and reactants [6]. Hence, researchers needed to develop a new combined system in catalysis. This new system has dynamics like homogeneous catalysts and effectively recoverable like heterogeneous catalysts [7].

Graphitic carbon nitride (g-C₃N₄) is a metal-free semiconductor that has a bandgap of 2.7 eV. g-C₃N₄ has high thermal stability and chemical properties suitable for work on organic reactions, degradation and water splitting. In 2009, g-C₃N₄ using photocatalysis was reported for hydrogen production [8]. g-C₃N₄ catalyst was used to significantly improve the catalytic behavior in increasing hydrogen production [9, 10]. g-C₃N₄ is a two-dimensional material and a p-type semiconductor to enhance the photocatalytic activity [11]. Many advanced synthetic methodologies were used to characterize g-C₃N₄ which led to the study of its activity in the reaction medium [12]. The catalytic properties of g-C₃N₄ have been improved by introducing metallic and nonmetallic elements such as boron and sulfur onto its surface [13]. Synthesis of g-C₃N₄ has been performed by co-polymerization of monomer with thermal condensation of nitrogen-rich forerunners such as thiourea [14, 15], dicyandiamide, melamine [16, 17], etc. g-C₃N₄ can be prepared from melamine monomer by the hydrothermal method to evaporate ammonia [18–21]. These already exist in many reviews highlighting the photocatalytic activity of g-C₃N₄ but only a few reviews focusing on g-C₃N₄ in catalyzing organic transformation reactions are reported. This review aims to explore various works regarding g-C₃N₄ to perform organic transformations in presence of visible light.

Vividly increasing attention is paid to g-C₃N₄ based photo-catalysts due to their exclusive electronic band structure and physicochemical properties. To improve the photocatalytic properties of g-C₃N₄, binding with any metal or doping with composite has to be done [22]. Such improved catalyst will be helpful in organic reactions. Generally, (organic reactions having reactive intermediates via activation energy to form the final products) the photogeneration of these reactive intermediates is mainly focused on organic synthesis [23–28]. However, the creation of these active species under mild reaction conditions and obeying the twelve principles of green chemistry is not an easy task [29, 30]. It was a green synthesized method because the light energy was directly used for the chemical reaction to occur [31]. In recent decades, researchers are interested in working on metal doped g-C₃N₄ polymer sheets to enhance the catalytic activity. Figure 1 shows the recent publications in g-C₃N₄, thus confirming it as an emerging catalyst in the field of catalysis. Pristine g-C₃N₄ has few restrictions because of its inadequate solar light absorption, decreased surface area and fast recombination of photo-generated electron hole-pairs in the field of photocatalysis [32–34]. Photons present in light deliver high energy into the molecules to form radicals. These radicals are recombined to give the products and no waste is generated in this process [35]. Applications in photochemical technique are that the reactants should get stabilized by the proton and go to the next singlet or triplet state via multiplicity of reactants. The above approach is applied to carbon intermediate species having wide applications in organic synthesis [36–38].

Fig. 1.

List of publications in g-C₃N₄ based on Scopus data on 31 October 2021.

g-C₃N₄ is a graphitic nanosheet having tunable properties with various applications such as easy synthesis [39], H₂ production from water [40], dye degradation [41], CO₂ exchanges [42] and organic synthesis. Two-dimensional structure functionalize the graphitic nature of g-C₃N₄ to reduce the band gap and slow the recombination of electrons at the valence band. The excellent properties of g-C₃N₄ sheet leads to support metals to form metal-organic frameworks [43–47]. If any heteroatom is doped in g-C₃N₄, it enhances the activity towards degradation applications. Nonmetallic elements such as sulfur, carbon, phosphorus, boron, fluorine, and iodine can also be bound with g-C₃N₄ to increase the light absorption and electron-hole recombination [48–51].

Heterocycles have core structures in biologically active compounds. Recently, plentiful efforts to engineer new organic molecules. New methods to prepare the heterocycles have been developed [52–58]. In recent decades, the synthesis of organic compounds in visible light has been difficult because of the photocatalyst deficit. It spoils the catalytic pathway of the reaction; hence, it need an upgrade to conduct the photo catalytic organic reaction. It tainted the activated catalytic system and then it became the most appropriate in the field of photocatalysis [59–66]. Photocatalysis in organic synthesis is more important because of its properties of redox transformation [67–70]. Inorganic semiconductors (TiO₂) are either UV or visible light active photocatalysts which are widely used to study the photocatalytic activity in hydrogen production from water [71]. Organic semiconductors are not given any special application because they vary in synthetic modularity thus changing their electronic and structural properties [72–74].

In 2019 Antonietti discussed g-C₃N₄ photocatalysis in a short review. Here they explain the activity of g‑C₃N₄ in catalysis via organic transformations. In the same year Savateev exposed the current scenario of g‑C₃N₄ catalyst and its applications in C−C bond forming reactions, dye degradation, and its semiconductor activities in catalysis field. In 2020, Luque reported a review of g-C₃N₄ catalyzed organic transformations [75–77].

All of the above reviews emphasized information on g-C₃N₄ catalyzed organic transformation, dye degradation, and semiconductor properties. In this review, the readers could able to understand current scenario on eco-friendly g-C₃N₄ based catalyst for organic reactions in the presence of light medium. The discussed schemes are effective in producing high yield products and reusability of the catalyst ensured the heterogeneity of g-C₃N₄.

APPLICATIONS OF g-C₃N₄

Oxidation

Recently, Lopez et al. reported the potassium containing C₃N₄ catalyst for the selective oxidation of 5-hydroxymethyl-2-furfural 1 to 2,5-furandicarboxaldehyde 2 [78]. Scheme 1 shows K-C₃N₄ catalyzed selective oxidation of 1 and selectively conversion to 2 in 30 min with water and natural solar light was used directly as the source. The authors prepared three K-‑C₃N₄ catalysts using two ambient preparative methodologies involving no hazardous chemicals. Metal-free catalysts have a dominant approach in catalyzing nonmetallic reactions. Xin et al. expressed the photocatalytic activity of g-C₃N₄ in Scheme 2 . Here, toluene was used and metal-free catalyst enhanced the oxidation of primary C−H bond to give 4 [79]. Toluene an active methylene group and it was converted into aldehyde form 3 in the presence of O₂. Preparation of g-C₃N₄ having a mesoporous structure was reported and it involved the straight oxidation of 3. In the absence of a catalyst, auto-oxidation occurred to form byproducts of acids and esters. The mechanism of these reactions showed heterogeneous oxidation of toluene to form superoxide radicals on the mesoporous surface. The superoxide radical anion plays a key role in the oxidation of alcohols. The sheets of g‑C₃N₄ were examined for 21 trails to check the selectivity to produce greater yield of 4.

Scheme 1 . Oxidation of 5-hydroxymethyl-2-furfural.

Scheme 2 . Oxidation of primary C−H.

Fangzheng et al. published Scheme 3 in 2010, in which the g-C₃N₄ polymer functions as a catalyst to produce 6 from the oxidation of 5 [80, 81]. This was the first report in the 21st century to synthesize mpg-C₃N₄. Mpg-C₃N₄ had good catalytic activity in the selective alcohols to generate aldehydes. Mpg-C₃N₄ catalyst was nontoxic, metal-free, economic, and easy to recover from the reaction mixture. Previously, nitroxyl radicals were used to convert aldehydes, which are much more harmful, but here, moderate experimental conditions are used to create benzaldehydes. Metal-free heterogeneous catalysis was used in this selective oxidation to generate higher yields of 6. The mechanism of Scheme 3 is shown in Fig. 2. Here light energy is used in mpg-C₃N₄ semiconductor to generate 6. Scheme 4 shows the oxidation of the C-H bond which was activated with O₂ to form 7 in the presence of g-C₃N₄ catalyst. g-C₃N₄ is best metal-free catalyst for activating oxygen at high pressure. The 20% of graphene and carbon nitride showed good activity to produce a maximum yield of 8a and a minor yield of 8b. The cyclohexane compound has only secondary C-H bonds of saturated alkanes, which react with oxygen gas to form ketones. g-C₃N₄ was an organic semiconductor that was is more active in the presence of O₂ to produce radicals for this selective oxidation [82]. Also, the polymeric catalyst has high stability and chemoselectivity to activate the C-H bonds in 7 to give good yields.

Fig. 2.

Schematic explanation of Oxidation of alcohols.

Scheme 3 . Selective oxidation of alcohols.

Scheme 4 . Selective oxidation of secondary C−H.

The photocatalyzed selective oxidation of benzyl alcohol to benzaldehyde is a simple organic reaction that occurs in visible light. Verma et al. have already reported the same aldol condensation reaction between alcohols and corresponding ketones/aldehydes with the presence of mesoporous g-C₃N₄ and di-oxygen in 2010. Several catalysts were examined for a green method. Verma et al reported Scheme 5 another visible light active VO@g-C₃N₄ system, which was used for the selective oxidation of 9 [83]. The main advantage of g-C₃N₄ is use of renewable energy like sunlight and wind. Energy consumption around the globe is gradually increasing and several environmental problems are caused by the burning of fossil fuels. These drawbacks can be overcome by the utilization of renewable energy sources. Here, in the first step VO@g-C₃N₄ absorbs light and gets the energy to activate superoxide radicals on the surface. Superoxide radicals are active and react with alcohol/diols to generate the corresponding carbonyl compounds. The activity of VO@g-C₃N₄ was checked for 13 samples to give more than 90% yield of 10 under visible light. Photocatalysis can be categorized into two groups; one is semiconductors (metal nanoparticles) and the other are metal complexes more active in UV light compared to visible light to conduct the photocatalytic process efficiently. Now semiconductor type metal-free carbon nanosheets are more active in a visible light region. Scheme 6 [84] explains the best semiconductor type catalyst for the removal of hydrogen atoms in 11 to form aromatic 12 and this was achieved with MnO₂@g-C₃N₄ in visible light. The secondary oxidizing agents (hydroxy radicals) directly react with 12 to form the product in the presence of light. 93% of products were obtained and recyclability of the catalyst was checked for several runs.

Scheme 5 . Selective oxidation of alcohols.

Scheme 6 . Oxidation of aromatic anilines.

Starting with a liquid precursor and creating the material using condensation technique, controlled porosity at the nanometric scale in the bulk of carbon nitride must improve its functionality. Scheme 7 describes the oxidation of 13 to produce 14 with noble metal-free catalyst g-C₃N₄. The reaction proceeds via the green route and O₂ acts as an oxidizing agent. Metal-free g-C₃N₄ is an excellent catalyst in visible light to give high yields of products. The high selectivity of g-C₃N₄ directs the esterification of 13 to form alcohol [85].

Scheme 7 . Selective oxidation of aldehydes.

Another example of the oxidation type of amidation process has been described by Wang et al. where Ag was anchored to g-C₃N₄ via condensation method [86]. The catalysts in an amidation reaction render the hydroxyl group free for activation before progressing to interact with an amine. The use of THF in place of amine led to the corresponding formation of an aldehyde. Most of the organic compounds are not active in the visible light region. Therefore, an external photocatalytic material is required for the organic transformation. Scheme 8 explains that Ag supported g-C₃N₄ exhibits excellent photocatalytic activity through the aerobic oxidative amidation of 15a and 15b under visible light irradiation. A 25W compact fluorescent light bulb was used for this reaction at room temperature. The reactions were green and mild reaction conditions to produce maximum yield of 16.

Scheme 8 . Oxidative amidation of aromatic aldehydes.

In 2017, Xu et al. worked on the catalysis study of CoPz supported g-C₃N₄ for the oxidation of 5-hydroxymethylfurfural [87]. Scheme 9 g-C₃N₄ was used to support the immobilized CoPz to generate a new hybrid catalyst for the selective oxidation of 5-hydroxymethylfurfural 17 into 2,5-furandicarboxylic acid. After 5 recycles, this catalyst showed good activity for performing the reaction.

Scheme 9 . Oxidation of 5-hydroxymethylfurfural.

Polyacrylonitrile (PAN) supported on silver nanoparticles and g-C₃N₄ was fabricated by electrospinning method by Shah et al (88) in 2020. In Scheme 10 spherical silver nanoparticles placed with g-C₃N₄ material were dispersed in PAN surface without agglomeration. 19a–c was converted into corresponding 20a in the presence of visible light. The light active mediated reaction was initiated on the surface of g-C₃N₄, because of its photoactive nature. The properties of the catalyst were mesoporous, enhanced activity, and reusability. Ag supported catalyst was developed with selective oxidation to give epoxide (99%). C−H activation reaction was also carried out by the same catalyst to give 99% of yields. Scheme 11 was the first report on the sulfide oxidation of 21 to give 22 in the presence of winkler g-C₃N₄ and and ¹O₂ catalysts. the wrinkled C₃N₄ nanosheets cause an efficient electron photoexcitation even in the presence of strong excitonic effects was confirmed in photoluminescence spectroscopy (89). The catalyst and cocatalyst show high selectivity towards sulfide oxidation.

Scheme 10 . Oxidation by PAN/AgNPs/g-C₃N₄.

Scheme 11 . Selective sulfide oxidation.

Reduction

In 2016, Nasir et al. reported new strategies for the hydrogenation of alkenes (Scheme 12) using photo-reactive catalysts [90]. Fe@g-C₃N₄ was introduced for the hydrogenation of 23 using NH₂NH₂.H₂O. Iron nanoparticles had good physical properties like magnetic separability and they were light active, which led to the formation of final products. The combination of the bandgap for 10%Fe@g-C₃N₄ with modification strategies is extremely fortified to find better ways to improve the performance of g-C₃N_4. Light was the only energy source required for this reaction to produce a 99% yield.

Scheme 12 . Hydrogenation of alkenes.

In 2015, authors reported the mpg-C₃N₄/Pd catalyst for the selective addition of hydrogen to nitrobenzene [91]. Scheme 13 shows mpg-C₃N₄/Pd catalyzed hydrogenation of 25 and selectively conversion to 26 within 2 min with water and methanol as solvent. The polymer catalyst produced 96% yield under ambient conditions. The catalysts were easily recoverable and reusability of the catalyst was also checked for several runs.

Scheme 13 . Hydrogenation of nitroarenes.

Hydroxylation

Verma et al. reported Scheme 14 where the bimetal supported CuAg@g-C₃N₄ system was prepared by the saturated mixture of metal nanoparticles and g-C₃N₄ [92]. Here, Cu and Ag nanoparticles were introduced on the g-C₃N₄ surface. Optimization of the catalyst with 21 trials was done and CuAg@g-C₃N₄ produced 99% yield of 28. The hydroxylation of 27 was performed under visible light with water and H₂O₂ to generate 28. The radicals formed on the surface of CuAg@g-C₃N₄ reacted with benzene to form intermediate ions, which reacted with benzene to form phenol.

Scheme 14 . Hydroxylation of benzene.

C-H Activation

In 2016, Verma et al. worked on the activation of hydrocarbons shown in Scheme 15 where methyl arenes were involved in C-H activation to oxidize 29 to form 30 [93]. They reported 12 different derivatives of methyl arenes and there was no change in yield with the introduction of the electron-withdrawing or electron-donating group of arene substituent. Graphically, Fig. 3 explains the VO@g-C₃N₄ , which was reactive to activate methylene group and oxygen insertion for the C-H activation reaction. Overall, the surface of g-C₃N₄ was activated by H₂O₂ to create superoxide and hydroxyl radicals in the presence of light. These radicals were more reactive, which attacked the methylene group in the arenes and generated the corresponding aldehydes.

Fig. 3.

Schematic reaction condition of arenes.

Scheme 15 . C-H activation of hydrocarbons.

Condensation

Xiang et al. conducted the benzoin synthesis from 31 in the UV region with potassium-supported g-C₃N₄ catalyst for the C-C coupling as shown in Scheme 16 [94]. Here, 31 undergoes C–C coupling through nucleophilic catalysis with the use of cyanide and methyl cyanide as organic solvents in light-mediated conditions. In the last decade, benzoin was synthesized via nucleophilic catalyst, which included cyanide and an organic medium like methyl cyanide, diethyl ether solvents. Benzoin synthesis was done by the green method. Here, water was used as solvent and K-gC₃N₄ acted as a catalyst in the presence of light and 90% yield was obtained. Benzyl alcohol was taken in an oxygenated atmosphere to produce benzaldehyde. Next, the addition of KCN and Xe lamp irradiation to the mixture 32 was obtained in 2 h.

Scheme 16 . Benzoin condensation.

Coupling Reactions

In Scheme 17, Palladium nanoparticles supported by polymeric g-C₃N₄ showed good photocatalytic properties. Many publications on Suzuki coupling reactions have been reported in the last decade. Recently, Li et al. [95] demonstrated a new path to perform the Suzuki coupling reaction. Secondary active radicals reacted with iodobenzene 33a and boronic acid 33b to give a high yield of 34. Here, light energy was used for the C−C bond formation reaction at ambient temperature using a green approach. In recent [96] Au/g-C₃N₄ nanocatalyst was used for stile cross coupling reaction under visible light medium (Scheme 18). To a allyltributylstannane liquid (3 mmol) 35a, (1-bromoethyl) benzene (1.2 mmol) 35b and Au/g-C₃N₄ (10 mg), DCM (1 mL) were added. The reaction mixture was irradiated under Xe lamp (300 W) using 420 nm cut-off filter. GC-MS was used to identify the formation of product 36. The reaction conversion took 10 h to complete the reaction at room temperature. In Scheme 19 [97] exposes the reaction between amines in the presence of cyano-decorated g‑C₃N₄ nanosheets (DCNNS). Here the authors describe the difference between DCNNS and g-C₃N₄ nanosheets. Cyano substituted catalyst showed good activity through the preparation of 38 in visible light medium.

Scheme 17 . Suzuki coupling reaction.

Scheme 18. Stille coupling reaction.

Scheme 19 . Oxidative coupling of amines.

Other Reactions

Metal-free catalyzed synthesis provides a new platform for organic transformations in the presence of visible light active nanosheets. In 2019, Camussi et al. reported new Diels-Alder and ene reactions with stereochemistry [98]. Oxidized g-C₃N₄ was used to promote chemoselective and unselective oxidation that was related to dienes and alkenes 39 as shown in Scheme 20 and 21.

Scheme 20. O₂ and acetonitrile solvent were used for oxidation of reactants to give 100% of selective and unselective products 44.

Scheme 21 . Ene reactions with singlet Oxygen.

Recently, Allahresani et al. worked on the study of SiO₂ nanoparticles immersed on the surface of g-C₃N₄ (Fig. 4) [99]. Scheme 22 displays one-pot three-component synthesis of 44 with the introduction of SiO₂@g-C₃N₄ nanocomposites using 43a, 43b, 43c at ambient temperature. A new way has been developed to prepare the spirooxindole 44 with the support of an equal ratio of ethanol and water. The catalyst has many advantages such as easy synthesis, easily recoverable, reusable for more than 5 times and low reaction time with high yield (95%).

Fig. 4.

Formation and reduction mechanism of two dimensional g-C₃N₄ sheet incorporated SiO₂ nanoparticles in spirooxindole organic compounds.

Scheme 22 . Green synthesis of spirooxindole.

Recently, Verma et al. prepared nanocomposites of Co@g-C₃N₄ to investigate the conversion of various aldehydes 45 to nitriles 46 under visible light irradiation [100]. The interaction between Co and g-C₃N₄ resulted in the tuned bandgap, quenching of photoluminescence (PL), and extended the lifetime of charge carriers for photocatalytic activity. In Scheme 23, polymer nanocomposites comprising Co@g-C₃N₄ were fabricated as a visible light active photocatalysts. The optimized nanocomposite demonstrated impressive performance and stability with a quantum efficiency and gave 94% yields under visible-light irradiation. The reaction continues for 5 more runs to reuse to get over 90% of conversion. Contrary to the conventional belief one-step four-electron process for photocatalytic organic reaction, this study put forward a new hypothesis of the two-step process; photocatalysis (two-electron) and chemical catalysis (two-electron).

Scheme 23 . Synthesis of nitriles from aldehydes.

Recently Cu/C₃N₄ catalyzed homocoupling of terminal alkynes 47 was reported by Xu et al. [101] in Scheme 24. The reaction was conducted at room temperature in the presence of isopropyl alcohol solvent and KOH base to produce 48. 14 derivatives and 12 trails were examined to optimize the reaction with maximum yield. The reusability of catalyst showed superior activity for 10 runs. In Scheme 25 [102] simple and efficient visible light-promoted heterogeneous g-C₃N₄-catalyzed switchable divergent synthetic procedure for the synthesis of dihydroquinoxalin-2(1H)-ones and tetrahydroimidazo[1,5-a] quinoxalin-4(5H)-ones using the low-cost and easily available quinoxalin-2(1H)-ones and N-aryl glycines as starting materials. White LED light was used for the irradiation. Here tetrahydroimidazo[1,5-a] quinoxalin-4(5H)-ones were obtained in good yields. This solvent-dependent system has enormous applications like easy operation, short reaction time, high chemoselectivity, a recyclable catalyst, metal-/base-/oxidant-free, and mild reaction conditions. In Scheme 26 [103] Pd supported g-C₃N₄ shows good activity to the preparation of Diethyl (2-(2-hydroxyphenyl) benzopyrano pyrimidinyl phosphonate at ambient conditions. The layers of g-C₃N₄ decorated with Pd⁰ prohibit excellent activity towards the formation of 52. The catalyst shows good activity even after four runs.

Scheme 24 . Homo-coupling of terminal alkynes.

Scheme 25. Hydroaminomethylation of Dihydroquinoxalin-2(1H)-ones.

Scheme 26 . Synthesis of Diethyl (2-(2-hydroxyphenyl)benzopy`rano pyrimidinyl phosphonate.

Scheme 27 and 28 were performed in the presence of visible light/ultrasonication in 2021 by Roopan SM [104]. Here the same reaction was worked up within 10mins in visible light medium (Scheme 27). Cu₃TiO₄/g-C₃N₄ (CNCT) has a bandgap of 2.68 shows good activity in visible light region. Quinoxaline and dihydro-quinazolinone were synthesized with (CNCT) nanocomposites in the presence of visible light/ultrasonication. Here, 12 derivatives of quinoxaline and 11 derivatives of quinazolinone were prepared in a short reaction time. The reusability of the catalyst was also conducted for four cycles.

Scheme 27 . Synthesis of quinoxalline.

Scheme 28 . Synthesis of dihydro- quinazolinone in visible light.

CONCLUSION

The reactions discussed in this review are the preliminary works in g-C₃N₄. We are looking forward to the heterocyclic compounds prepared using the green method using this eco-friendly catalyst. On comparison of the g-C₃N₄ with other traditional catalysts, we found g-C₃N₄ has many advantages like low cost, good physicochemical properties, reusability, visible light active, metal-free catalyst, etc. g-C₃N₄ has proved to be a promising heterogeneous catalyst having good stability for performing heterocyclic reactions. This review will inspire researchers to work on the visible light-mediated synthesis of heterocycles.

CONFLICT OF INTEREST

All author herewith stating that we don’t have any conflict to publish this work.