A pd1-cu1 / b-cn double-atom catalyst and application thereof in electrocatalytic synthesis of carbazole compounds

Abstract

The application belongs to the field of organic electrosynthesis and green catalysis, and particularly relates to application of a body phase-like carbon nitride catalyst with in-plane palladium and inter-plane copper double monatomic sites in electrocatalytic oxidation dehydrogenation aromatization synthesis of carbazole compounds. The catalyst can efficiently catalyze dehydrogenation aromatization reaction of N-protected-1,2,3,4-tetrahydrocarbazole and its derivatives under electrochemical anodic oxidation conditions to generate corresponding carbazole compounds with excellent yield. The method uses electrons as clean oxidant and uses cheap lithium bromide as medium, and can realize reaction under mild room temperature conditions, completely abandoning the use of traditional stoichiometric oxidants and noble metal catalysts, and fundamentally eliminating toxic waste and heavy metal pollution. The method has wide substrate application range, good functional group compatibility, and the separation yield of the target product can reach 70-85%, and the generation of excessive bromination by-products is avoided.

Description

Technical Field

[0001] This invention belongs to the field of organic electrosynthesis and catalytic materials technology, specifically relating to a method for preparing a bulk carbon nitride (Pd1-Cu1 / b-CN) electrocatalyst based on in-plane palladium (Pd) and inter-plane copper (Cu) dual single-atom anchoring, and its application in the electrocatalytic oxidative dehydrogenation aromatization synthesis of carbazole compounds. This invention belongs to the interdisciplinary field of green electrochemical synthesis, catalytic materials, and heterocyclic compound synthesis. Background Technology

[0002] Carbazole and its derivatives are an important class of nitrogen-containing heterocyclic compounds, widely found in natural products, drug molecules, and functional materials. Many carbazole alkaloids exhibit significant biological activities such as antiviral, antitumor, antioxidant, anti-Alzheimer's disease, and anti-inflammatory effects. Furthermore, due to their excellent thermal stability, photoelectric properties, and hole transport capabilities, carbazole compounds have broad application prospects in fields such as organic light-emitting diodes, organic photovoltaic materials, and conductive polymers.

[0003] Among the synthetic methods for carbazole compounds, the Fischer-Borsche synthesis is one of the most classic and reliable strategies: arylhydrazine is condensed and rearranged with a ketone compound to obtain 1,2,3,4-tetrahydrocarbazole, which is then converted to carbazole via oxidative dehydrogenation aromatization. However, the latter step of this classic route—the oxidative aromatization of tetrahydrocarbazole—has long relied on stoichiometric strong oxidants, such as DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone), V₂O₅, MnO₂, etc., or requires reaction with I₂ / DMSO or CuCl₂ / DMSO systems at high temperatures. These methods have several insurmountable drawbacks: First, oxidants such as DDQ are highly toxic and costly, and produce equivalent amounts of toxic hydroquinone waste after the reaction, causing serious environmental pollution; second, high-temperature conditions (usually above 100°C) result in high energy consumption and may damage sensitive functional groups; third, the use of stoichiometric oxidants leads to poor atom economy, which does not conform to the principles of green chemistry.

[0004] Electrochemical synthesis utilizes electron transfer at the electrode surface to drive chemical reactions, replacing stoichiometric oxidants or reductants with clean electrons, representing an important direction for the greening of synthetic chemistry. In recent years, significant progress has been made in halogen-mediated anodic oxidation reactions. Electrolysis of N-acetyl-1,2,3,4-tetrahydrocarbazole in acetonitrile using lithium bromide as a mediator can yield corresponding carbazole products. However, this method generates a large amount of difficult-to-separate brominated byproduct (3-bromocarbazole) during electrolysis of the substrate N-methyltetrahydrocarbazole, severely affecting product yield and purity, and limiting its substrate applicability and practical application value. Summary of the Invention

[0005] The technical problem this invention aims to solve is to address the numerous shortcomings of existing methods for synthesizing carbazole via the oxidative dehydrogenation of tetrahydrocarbazole—such as reliance on stoichiometric toxic oxidants, harsh high-temperature conditions, narrow substrate applicability, and severe bromination byproducts in electrochemical methods (especially for substrates containing electron-donating groups). This invention provides a method for the electrocatalytic synthesis of carbazole compounds using Pd1-Cu1 / b-CN as the anolyte catalyst, electrons as a clean oxidant, and LiBr as the mediator. This method should possess characteristics such as high catalytic activity, good selectivity, a wide substrate applicability, mild conditions, no toxic waste, and effective suppression of bromination byproduct formation. In particular, it should solve the problem of over-bromination of substrates containing electron-donating groups such as alkoxy groups under electrochemical conditions.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: (I) Preparation of Pd1-Cu1 / b-CN catalyst As one aspect of the present invention, a method for preparing a Pd1-Cu1 / b-CN catalyst is provided, comprising the following steps: Step 1: Synthesis of in-plane Pd-coordinated supramolecular precursors: Palladium salts (such as PdCl2, Pd(NO3)2, or Pd(OAc)2) are dissolved in a solvent with melamine and cyanuric acid in a certain proportion. After hydrothermal or solvothermal treatment, Pd²⁺ ions coordinate with nitrogen atoms in the supramolecular framework to form in-plane Pd-coordinated supramolecular precursors. The hydrothermal reaction temperature is 120-200℃, and the reaction time is 4-24 hours.

[0007] Step 2, Intercalation of Cu Intercalation Composite: The precursor obtained in Step 1 is dispersed in a solution containing copper salt (such as CuCl2, Cu(NO3)2 or Cu(OAc)2), and Cu²⁺ is introduced into the interlayer by ion exchange or impregnation to obtain a supramolecular intermediate co-assembled with Pd / Cu.

[0008] Step 3: Thermal polymerization preparation of Pd1-Cu1 / b-CN: The intermediate obtained in Step 2 is heated to 500-650℃ at a heating rate of 2-10℃ / min under an inert atmosphere (argon or nitrogen) and held at this temperature for 2-6 hours to undergo thermal polycondensation, transforming the supramolecular precursor into bulk carbon nitride-like precursor. After natural cooling, the Pd1-Cu1 / b-CN catalyst with in-plane Pd and inter-plane Cu dual single-atom anchorage is obtained.

[0009] The loading of Pd and Cu in the prepared catalyst was determined by ICP-OES. The total loading of the two was 0.2-1.0 wt%, and the Pd / Cu molar ratio was 1:0.5-2.

[0010] (II) Electrocatalytic Synthesis of Carbazole Compounds In another aspect, the present invention provides a method for electrocatalytic synthesis of carbazole compounds using the above-mentioned Pd1-Cu1 / b-CN catalyst, comprising the following steps: Step 1: Preparation of electrolyte: Under inert gas protection, N-protected-1,2,3,4-tetrahydrocarbazole substrate, LiBr and AcOH are dissolved in a strictly dry aprotic organic solvent and stirred until completely dissolved to obtain the electrolyte.

[0011] The concentration of the substrate in the electrolyte is 0.02-0.10 M, preferably 0.04 M. The N-protecting group is selected from at least one of acetyl (Ac), methoxycarbonyl (Cb2), methyl (Me), and benzyl (Bn), preferably acetyl. The substrate may contain substituents at any position on the benzene ring, and the substituents are selected from at least one of C1-C6 alkyl, C1-C6 alkoxy, halogen (F, Cl, Br), cyano, and ester groups, with the number of substituents being 1-3.

[0012] The amount of LiBr used is 1.0-4.0 equivalents of the substrate molar amount, preferably 2.0 equivalents. The amount of AcOH used is 1.0-5.0 equivalents of the substrate molar amount, preferably 3.0 equivalents. The aprotic organic solvent is selected from at least one of acetonitrile, o-dichlorobenzene, tetrahydrofuran, and dichloromethane, preferably dried acetonitrile, and the solvent water content is less than 50 ppm.

[0013] Step 2: Construction of a three-electrode or two-electrode electrolysis system: Under inert gas protection, the electrolyte prepared in Step 1 is transferred to an electrochemical electrolytic cell. Using the above-prepared conductive substrate loaded with Pd1-Cu1 / b-CN (such as carbon cloth, carbon paper, nickel foam, or platinum sheet) as the working electrode (anode), and a platinum sheet or carbon rod as the counter electrode (cathode), an unseparated electrolytic cell system is constructed. The Pd1-Cu1 / b-CN loading at the anode is 1-5 mg / cm².

[0014] Step 3: Constant Current or Constant Potential Electrolysis: Electrolysis is performed under stirring conditions in constant current or constant potential mode. For constant current electrolysis, the current density is 2.0-15.0 mA / cm², preferably 5.0-10.0 mA / cm², and the applied charge is 3.0-6.0 F / mol (relative to the substrate). For constant potential electrolysis, the potential relative to the Ag / Ag⁺ non-aqueous reference electrode is 1.0-2.0 V. The electrolysis temperature is 15-50℃, preferably room temperature (20-30℃), and the electrolysis time is 2-12 hours. Throughout the electrolysis process, the electrolyte is continuously stirred to promote mass transfer. If necessary, an inert gas can be introduced into the cathode chamber to purge air.

[0015] In this electrocatalytic system, Pd1-Cu1 / b-CN plays a unique cascade catalytic role, and its mechanism is as follows: First, after a potential is applied at the anode, Br⁻ in the electrolyte is oxidized to active bromine species (Br⁺ or Br₂) on the electrode surface. Simultaneously, the interfacial Cu sites act as charge transfer channels, utilizing the rapid electron transport path provided by their four-coordinate structure (charge decay lifetime of only 3.07 ps) to quickly transfer electrons from the carbon nitride bulk phase to the surface, effectively reducing the interfacial charge transport resistance and thus lowering the reaction overpotential.

[0016] Secondly, the in-plane Pd site, acting as the catalytic active center, specifically adsorbs and activates the substrate N-protected-1,2,3,4-tetrahydrocarbazole molecule through its three-coordinate unsaturated structure. The long charge decay lifetime (95.6 ps) of the Pd site facilitates the residence of the substrate molecule on the electrode surface and the subsequent oxidative dehydrogenation reaction. The active bromine species selectively oxidize the substrate to the corresponding carbazole product near the Pd site, while releasing protons.

[0017] More importantly, the dual single-atom synergistic structure of Pd1-Cu1 / b-CN can precisely regulate the reaction pathway of the bromine mediator. In systems without catalysts or with only a single metal site catalyst, the active bromine species, in addition to participating in the target oxidative dehydrogenation reaction, also attack the electron-rich C3 position of tetrahydrocarbazole, generating bromination byproducts such as 3-bromocarbazole. In this invention, the in-plane Pd site, through specific interactions with the substrate molecule, causes the substrate to adsorb onto the electrode surface in a configuration favorable to CN-bond dehydrogenation. Simultaneously, the Cu site modulates the local electron density of the Pd site through electronic effects, causing the active bromine species to preferentially participate in the target oxidative dehydrogenation reaction rather than the electrophilic bromination reaction, thereby significantly suppressing the formation of bromination byproducts at the C3 position.

[0018] Step 4: Separation and Purification of the Product: After the electrolysis reaction, most of the solvent is removed by vacuum distillation. The residue is dissolved in dichloromethane or ethyl acetate and washed successively with saturated sodium thiosulfate solution and saturated brine. The organic phase is dried over anhydrous sodium sulfate and concentrated. The crude product is purified by silica gel column chromatography with 200-300 mesh silica gel as the stationary phase and petroleum ether / ethyl acetate or a mixed solvent of n-hexane / ethyl acetate as the eluent. Gradient elution separation yields the target carbazole product. If necessary, further purification can be achieved by recrystallization.

[0019] (III) Application and Recycling of Catalysts This invention also provides the application of the above-mentioned Pd1-Cu1 / b-CN catalyst in the electrocatalytic synthesis of carbazole compounds. The catalyst maintains structural stability during anodic oxidation, and the in-plane Pd and inter-plane Cu single-atom sites do not aggregate or leach during electrolysis.

[0020] The Pd1-Cu1 / b-CN catalyst can be scraped from the electrode after the reaction or the electrode can be cleaned and dried for reuse. After the catalyst is reused 5 times, the separation yield of the target product can still maintain more than 80% of the initial yield.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. Green and environmentally friendly, with no toxic waste: This invention uses electrons as a clean oxidant and lithium bromide as a catalytic medium, completely eliminating the use of traditional stoichiometric oxidants (such as DDQ, V2O5, MnO2, etc.). The reaction does not produce equivalent amounts of toxic hydroquinones or heavy metal waste; the only byproduct is a small amount of hydrogen bromide / acetic acid, which can be rendered harmless by simple alkaline washing, conforming to the core concepts of green chemistry and sustainable development. The electrolysis process is carried out at room temperature, without the need for high-temperature heating, significantly reducing energy consumption.

[0022] 2. High catalytic activity and significantly improved product yield: This invention introduces the concept of dual single-atom cascade catalysis into the electrochemical oxidative aromatization reaction of tetrahydrocarbazole for the first time. Compared with the traditional single bromine mediator electrochemical system, the Pd1-Cu1 / b-CN catalyst increases the yield of the target carbazole product from 53% to 73%, an increase of 38%; compared with the classic DDQ chemical oxidation method, the yield increases from 65% to 73%, and avoids the use of toxic reagents. Normalized to the noble metal Pd content (0.5wt%) in the catalyst, its catalytic efficiency is far superior to existing methods.

[0023] 3. Excellent selectivity and effective suppression of bromination byproducts: The most prominent advantage of this invention lies in the precise control of reaction selectivity. In traditional bromine mediator electrochemical systems, tetrahydrocarbazole substrates containing electron-donating groups (such as methoxy groups) or N-methyl protected groups are prone to C3-position over-bromination, producing difficult-to-separate bromination byproducts (yields can reach over 14%), severely limiting the substrate applicability and product purity. This invention utilizes the precise control of the reaction intermediates by the in-plane Pd sites of Pd1-Cu1 / b-CN and the synergistic electronic effects of the inter-plane Cu sites to significantly reduce the proportion of bromination byproducts from over 14% to below 5%, significantly improving the purity and separation efficiency of the target product, making this method widely applicable to various substituted tetrahydrocarbazole substrates.

[0024] 4. Wide substrate applicability and good functional group compatibility: The method of this invention exhibits good compatibility with different N-protecting groups (Ac, Cbz, Me, Bn) and substituents (alkyl, alkoxy, halogen, etc.) at different positions and with different charges on the benzene ring, with a stable separation yield between 65-75%. This substrate versatility is significantly better than traditional electrochemical methods (which are difficult to apply when using substrates containing electron-donating groups due to severe bromination byproducts), providing a powerful tool for the diverse synthesis of carbazole compounds.

[0025] 5. Excellent catalyst stability and reusability: The Pd1-Cu1 / b-CN catalyst of this invention uses bulk carbon nitride as a support, with Pd and Cu firmly anchored in-plane and interlayer at the atomic level, exhibiting excellent stability under electrochemical oxidation conditions. Experiments show that after five cycles, the catalyst retains over 89% of its catalytic activity, and the leaching rate of the precious metal Pd is less than 5%. The catalyst can be directly regenerated on the electrode without complex separation and recovery steps, significantly reducing the consumption of precious metal catalysts and overall costs.

[0026] 6. Mild reaction conditions and simple operation: This invention can be carried out efficiently at room temperature (20-30℃), without the need for harsh conditions such as low temperature (e.g., -78℃) or high temperature (above 100℃), nor does it require strictly anhydrous and oxygen-free Schlenk operation (the electrolytic cell can be simply assembled under argon protection). The unseparated electrolytic cell design simplifies the electrolysis device, and the constant current mode facilitates scale-up production. Product separation only requires conventional column chromatography, and the post-processing is simple, making it suitable for large-scale preparation.

[0027] 7. High atom economy and low overall cost: Compared with traditional stoichiometric oxidation methods, this invention uses electrons as the oxidant and LiBr as a regenerable mediator, significantly improving atom economy. The loading of Pd and Cu in the catalyst is extremely low (total loading ≤0.7wt%) and can be reused, resulting in far lower consumption of precious metals than traditional homogeneous palladium catalysis methods. The overall cost of the process is lower than existing methods, demonstrating promising prospects for industrial application.

[0028] 8. Pioneering Application of Dual Single-Atom Catalysts in Organic Electrosynthesis: This invention is the first to extend the application of Pd1-Cu1 / b-CN dual single-atom cascade catalysts from the field of photocatalysis to organic electrosynthesis, successfully realizing the synergistic effect of the cascade function of charge transfer channels (interplanar Cu) + catalytic active centers (in-plane Pd) in anodic oxidation reactions. This strategy not only provides an efficient solution for the carbazole synthesis involved in this invention, but also opens up new catalyst design ideas for the electrochemical dehydrogenation functionalization reactions of other heterocyclic compounds, possessing significant scientific value and broad technological radiation effects.

[0029] 9. High product purity, no special purification required: Benefiting from the advantages of few byproducts and high selectivity, the crude carbazole product obtained by this invention has high purity. A simple column chromatography process can yield chromatographically pure product (purity ≥98%), eliminating the need for expensive purification methods such as preparative HPLC. The product contains extremely low levels of residual metals (ICP-MS detection shows Pd and Cu residues are both <1 ppm), meeting the stringent purity requirements for pharmaceutical intermediates and optoelectronic materials.

[0030] 10. Low safety risk and environmentally friendly operation: This invention completely avoids the use of flammable, toxic, or irritating chemical reagents (such as DDQ, PMe3, I2, etc.). The electrolysis process only involves conventional organic solvents such as acetonitrile, inexpensive LiBr, and a small amount of AcOH, with no risk of spontaneous combustion, explosion, or high toxicity. The reaction scale is easily scalable (preliminary verification shows that the yield of the 1.0 mmol-level reaction is basically the same as that of the 0.4 mmol-level reaction), providing a safe and reliable technical foundation for industrial production.

[0031] The above summary provides a simplified overview of some concepts, which will be further described in detail in the following specific embodiments. The above summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to help determine the scope of the claimed subject matter. The claimed subject matter is not limited to embodiments that address any or all the shortcomings pointed out in the background art.

[0032] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by means of embodiments thereof. Attached Figure Description

[0033] The accompanying drawings, incorporated in and forming part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without any creative effort. These drawings and textual descriptions are not intended to limit the scope of the concept of this application in any way, but rather to illustrate the concepts of this application to those skilled in the art by referring to specific embodiments.

[0034] Figure 1 This is a scanning electron microscope image of the Pd1-Cu1 / b-CN catalyst of the present invention; Figure 2 A comparison of the electrocatalytic oxidation performance of different catalysts for N-acetyl-1,2,3,4-tetrahydrocarbazole; Figure 3 This is a graph showing the electrocatalytic oxidation results of different N-protecting groups and different substituted substrates using the method of the present invention; Figure 4 Performance and solution resistivity of different bromide mediators in the electrocatalytic synthesis of N-acetylcarbazole; Figure 5 This is a graph showing the recycling performance of the Pd1-Cu1 / b-CN catalyst of this invention in the electrocatalytic synthesis of N-acetylcarbazole. Detailed Implementation

[0035] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0036] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. It should be further understood that, as used herein, the singular forms "a," "an," and "the" are intended to also include the plural forms, unless the context indicates otherwise. Furthermore, the terms "or," "and / or," "including at least one of the following," etc., as used herein, can be interpreted as inclusive, or mean any one or any combination thereof. Exceptions to this definition only arise when combinations of elements, functions, steps, or operations are inherently mutually exclusive in some manner.

[0037] It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit the scope of this application. Unless otherwise specified, the materials, reagents, etc., used in the following embodiments are commercially available. Experimental methods not specifically described in the embodiments are generally performed under conventional conditions or as recommended by the manufacturer.

[0038] It is worth noting that single-atom catalysts (SACs) have attracted widespread attention in the field of catalysis due to their maximized atom utilization efficiency and unique electronic structure. Dual single-atom catalysts (DSACs), by introducing two different metal sites, can achieve functional synergy and cascade catalysis, exhibiting catalytic performance exceeding that of single-metal sites. Among them, bulk carbon nitride (b-CN), as a layered non-metallic semiconductor material, possesses abundant nitrogen coordination sites and good chemical stability, making it an ideal support for anchoring single atoms. Introducing both in-plane tricoordinated Pd and inter-plane tetracoordinated Cu dual single atoms into layered carbon nitride simultaneously forms a cascade-functional Pd1-Cu1 / b-CN catalyst. In this system, the in-plane Pd sites act as electron-trapping centers and catalytically active sites, exhibiting a long charge decay lifetime (95.6 ps), which is beneficial for substrate adsorption and activation; the inter-plane Cu sites act as charge transfer channels (with a charge decay lifetime of only 3.07 ps), significantly promoting interlayer electron transport. This synergistic design of the active center and transport channel enables Pd1-Cu1 / b-CN to exhibit excellent performance in photocatalytic CO2 reduction.

[0039] The technical solution of the present invention will be further described below through specific embodiments. The scope of protection of the present invention includes, but is not limited to, the following embodiments.

[0040] Example 1: Preparation of Pd1-Cu1 / b-CN catalyst The catalyst was prepared according to the following steps: (1) Synthesis of in-plane Pd-coordinated supramolecular precursor: 1.0 g melamine, 1.2 g cyanuric acid and 2.0 mg PdCl2 were dispersed in 60 mL deionized water and stirred at room temperature for 30 minutes. The mixture was then transferred to a 100 mL hydrothermal reactor and hydrothermally reacted at 180 °C for 12 hours. After natural cooling, the precipitate was collected by filtration and washed three times alternately with deionized water and anhydrous ethanol. The precipitate was then dried under vacuum at 60 °C for 12 hours to obtain the in-plane Pd-coordinated supramolecular precursor.

[0041] (2) Intercalation of Cu in planes: The precursor obtained in step (1) was dispersed in 20 mL of ethanol solution containing 0.5 mg CuCl2 and stirred at room temperature for 6 hours to allow Cu to... 2+ The product enters the interlayer via ion exchange. It is collected by filtration, washed three times with anhydrous ethanol, and dried under vacuum at 60°C.

[0042] 3) Thermal polymerization: The intermediate obtained in step (2) was placed in a tube furnace and heated to 600°C at 5°C / min under argon protection, and held at that temperature for 4 hours. After natural cooling to room temperature, the Pd1-Cu1 / b-CN catalyst was obtained. ICP-OES determined that the Pd and Cu loadings were 0.5 wt% and 0.2 wt%, respectively.

[0043] refer to Figure 1 The image shows a scanning electron microscope (SEM) image of the Pd1-Cu1 / b-CN catalyst of the present invention, with a scale bar of 10 μm. It can be seen that the catalyst has a regular cubic block supramolecular thermal polymerization morphology, with uniform particle dispersion, no obvious metal particle agglomeration, and complete lamellar structure. No morphological collapse or agglomeration caused by metal doping was observed, indicating that the introduction of Pd and Cu did not destroy the bulk structure of carbon nitride.

[0044] Example 2: Electrocatalytic synthesis of N-acetylcarbazole (representative substrate) Under argon protection, N-acetyl-1,2,3,4-tetrahydrocarbazole (1a, 0.4 mmol, 85.3 mg), LiBr (0.8 mmol, 69.5 mg), and AcOH (1.2 mmol, 72.0 μL) were dissolved in 10 mL of dry acetonitrile and stirred until completely dissolved. Pd1-Cu1 / b-CN-supported carbon cloth (geometric area 1 cm x 2 cm, catalyst loading approximately 3 mg / cm²) was then used. 2 Using a platinum sheet (2cm x 2cm) as the anode and a platinum sheet as the cathode, an electrolytic cell was used at room temperature with a current of 5.0 mA / cm². 2 Electrolysis was performed using a constant current density. The reaction process was monitored by TLC, and electrolysis was stopped when the applied charge reached 4.0 F / mol (approximately 6-8 hours).

[0045] After the reaction was complete, acetonitrile was removed by vacuum distillation. The residue was dissolved in 20 mL of dichloromethane and washed successively with 10 mL of saturated sodium thiosulfate solution and 10 mL of saturated brine. The organic phase was dried over anhydrous sodium sulfate and then concentrated. The crude product was purified by silica gel column chromatography (eluting with a gradient of petroleum ether / ethyl acetate from 8:1 to 4:1) to give a white solid N-acetylcarbazole in 73% yield. 1 H NMR and 13 The C NMR data are consistent with those reported in the literature.

[0046] Example 3: Performance Comparison of Different Catalytic Systems Under the same electrolysis conditions, blank carbon cloth, pure b-CN, Pd1 / b-CN, Cu1 / b-CN and Pd1-Cu1 / b-CN were used as anode catalysts to investigate the effect on electrocatalytic oxidation performance.

[0047] like Figure 2As shown, the product yields of the blank carbon cloth and pure b-CN system were 6% and 18%, respectively, with bromination byproduct proportions of 9% and 10%, respectively, indicating that the carbon nitride support alone had no significant promoting effect on electrocatalytic performance. Using a single Pd site catalyst (Pd1 / b-CN), the yield increased to 45%, and the bromination byproduct decreased to 7%; the yield of the single Cu site catalyst (Cu1 / b-CN) was 31%, and the bromination byproduct was 8%. Figure 2 The bar chart of Pd1-Cu1 / b-CN was significantly higher than that of the other groups, with a product yield of 74% and the proportion of bromination byproducts reduced to below 1%. Figure 2 The comparative results clearly reveal that while single Pd or Cu site catalysts can improve the yield to some extent, the problem of bromination byproducts remains unresolved (7-8%). Only the Pd1-Cu1 / b-CN dual single-atom catalyst simultaneously achieved a significant increase in yield and almost complete suppression of bromination byproducts. Figure 2 The “scissors difference” between the medium yield column and the by-product column fully demonstrates the synergistic cascade catalytic effect of the in-plane Pd and inter-plane Cu dual sites, rather than a simple superposition of their performance.

[0048] Example 4: Study on the applicable scope of the substrate Using the same reaction conditions as in Example 2, the substrate applicability of different N-protecting groups was investigated.

[0049] like Figure 3 As shown, the method of the present invention exhibits good compatibility with substrates of different N-protecting groups (Ac, Cbz, Me, Bn), and the separation yield is stable between 68-74%. Figure 3 In particular, for methoxy-containing substrates and N-methyl protected substrates that are prone to over-bromination in conventional electrochemical methods, the method of this invention controls the proportion of bromination byproducts to below 4% and 3% respectively, which is significantly better than the prior art (typically >14%), demonstrating the broad substrate adaptability of this method.

[0050] Comparative Example 1: Bromine-free mediator control experiment Under the conditions of Example 2, electrolysis was performed without LiBr. The results showed that even with the Pd1-Cu1 / b-CN catalyst, the substrate hardly transformed without a bromine mediator, and the excessively high anolytical potential led to solvent decomposition. This result is consistent with literature reports, indicating that a bromine mediator is necessary for achieving anodic oxidation under mild conditions, and that the role of Pd1-Cu1 / b-CN is to synergistically regulate the reaction pathway of the bromine mediator and suppress side reactions.

[0051] Comparative Example 2: Effect of Different Bromine Salts Under the conditions of Example 2, LiCl, LiI, and Bu4NBr were used instead of LiBr as the mediator, respectively. The experimental results are as follows: Figure 4 As shown. Figure 4 The left-hand bar chart shows that the LiCl system yields only about 30% (due to insufficient oxidizing power of the chlorine mediator); the LiI system produces almost no product (due to the iodine mediator causing other side reactions in the substrate); the Bu4NBr system yields about 50-55%; while the LiBr system achieves the highest yield (73%). Figure 4 The curve on the right (electrochemical impedance spectroscopy, EIS) further reveals the differences in solution resistance among different bromide salt systems: the LiBr system has the lowest solution resistance (about 80 Ω), the LiCl system is slightly higher, the Bu4NBr system has a significantly increased resistance (about 180 Ω), and the LiI system has an abnormal curve due to side reactions. Figure 4 Experimental data show that Li⁺ plays a key role in maintaining the conductivity of the solution and the stability of the cathode, while Br⁻ has a moderate oxidation potential. The combination of the two (LiBr) is the mediator combination to achieve the optimal electrolysis effect.

[0052] Comparative Example 3: Traditional Chemical Oxidation Method (DDQ Method) The substrate (0.4 mmol) was dissolved in 5 mL of dry dichloromethane, and 0.6 mmol of DDQ was added. The mixture was stirred overnight at room temperature. After the reaction was completed by TLC monitoring, DDQH2 was removed by filtration, and the filtrate was concentrated and purified by silica gel column chromatography. The separation yield was 65%, but it required 1.5 equivalents of toxic DDQ reagent and generated an equivalent amount of DDQH2 waste. In contrast, the method of this invention uses electrons as a clean oxidant and only uses a catalytic amount of bromine mediator, making it significantly more environmentally friendly than traditional chemical methods.

[0053] Comparative Example 4: Electrolysis of bromine mediator only (without Pd1-Cu1 / b-CN) Electrolysis was performed using blank carbon cloth as the anode under the conditions of Example 2 (containing LiBr and AcOH). The product yield was 53%, with approximately 9% of C3-position bromination byproducts observed. 1 (Confirmed by 1H NMR and MS). This result indicates that the pure bromine mediator system has a significant bromination side reaction, while the introduction of the Pd1-Cu1 / b-CN catalyst significantly reduces the bromination byproducts to <5%, demonstrating the key regulatory role of Pd1-Cu1 / b-CN in the reaction selectivity.

[0054] Example 5: Catalyst recycling performance The Pd1-Cu1 / b-CN / carbon cloth anode used in Example 2 was removed, washed three times alternately with acetonitrile and deionized water, and then vacuum dried at 60°C for 2 hours. The electrolysis experiment of Example 2 was then repeated under the same conditions. The experimental results are as follows: Figure 5 As shown. Figure 5The line graph shows that after 5 cycles, the yields were: 73% for the first cycle, 72% for the second cycle, 70% for the third cycle, 68% for the fourth cycle, and 65% for the fifth cycle. Figure 5 The yield curve after 5 cycles showed only a slow downward trend, with a catalytic activity retention rate of 89%, indicating that the catalyst has excellent reusability. ICP-OES analysis showed that the loadings of Pd and Cu decreased by approximately 5% and 8%, respectively, after 5 cycles, further confirming the excellent stability of the catalyst under electrochemical oxidation conditions. Figure 5 The slow decline curve, together with the ICP data, indicates that Pd and Cu are firmly anchored in the in-plane and interlayer of carbon nitride in an atomically dispersed form, and are not prone to aggregation or leaching under electrochemical oxidation conditions.

[0055] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0056] In this application, the same or similar terms, concepts, technical solutions and / or application scenario descriptions are generally described in detail only when they appear for the first time. When they appear again, they are generally not repeated for the sake of brevity. When understanding the technical solutions and other contents of this application, the same or similar terms, concepts, technical solutions and / or application scenario descriptions that are not described in detail later can be referred to their previous relevant detailed descriptions.

[0057] In this application, the descriptions of the various embodiments have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0058] The technical features of the present application can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of the present application.

[0059] The above are merely preferred embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method for electrocatalytic synthesis of carbazole compounds, characterized in that, Using a Pd1-Cu1 / b-CN catalyst as the anodic catalyst, N-protected-1,2,3,4-tetrahydrocarbazole substrates were subjected to anodic oxidation dehydrogenation aromatization under constant current or constant potential conditions in an aprotic organic solvent containing lithium bromide and acetic acid to generate the corresponding carbazole compounds. The Pd1-Cu1 / b-CN catalyst is a bulk carbon nitride-like compound anchored by in-plane tricoordinate palladium and inter-plane tetracoordinate copper.

2. The method according to claim 1, characterized in that, In the Pd1-Cu1 / b-CN catalyst, palladium is anchored in the plane of carbon nitride in an atomically dispersed state to form a tricoordinate structure, and copper is anchored in the interlayer of carbon nitride in an atomically dispersed state to form a tetracoordinate structure; the total loading of palladium and copper is 0.2-1.0 wt%, and the molar ratio of palladium to copper is 1:0.5-2.

3. The method according to claim 1 or 2, characterized in that, The Pd1-Cu1 / b-CN catalyst is prepared by a method comprising the following steps: (1) The palladium source is mixed with melamine and cyanuric acid in a solvent and hydrothermally reacted at 120-200℃ for 4-24 hours to form an in-plane palladium coordinated supramolecular precursor. (2) The precursor obtained in step (1) is dispersed in a solution containing a copper source, and copper ions are introduced into the interlayer through ion exchange to obtain a palladium / copper co-assembled supramolecular intermediate. (3) The intermediate obtained in step (2) is heated to 500-650°C at a heating rate of 2-10°C / min under an inert atmosphere and kept at the temperature for 2-6 hours for thermal polymerization treatment to obtain the Pd1-Cu1 / b-CN catalyst.

4. The method according to claim 1, characterized in that, The nitrogen atom of the N-protected-1,2,3,4-tetrahydrocarbazole substrate is attached with a protecting group, which is selected from at least one of acetyl, methoxycarbonyl, methyl, and benzyl; at least one benzene ring of the substrate optionally contains 1-3 substituents, which are selected from at least one of C1-C6 alkyl, C1-C6 alkoxy, halogen, cyano, and ester.

5. The method according to claim 1, characterized in that, The amount of lithium bromide used is 1.0-4.0 equivalents of the substrate molar amount, and the amount of acetic acid used is 1.0-5.0 equivalents of the substrate molar amount; the aprotic organic solvent is selected from at least one of acetonitrile, o-dichlorobenzene, tetrahydrofuran, and dichloromethane, preferably dried acetonitrile with a water content of less than 50 ppm.

6. The method according to claim 1, characterized in that, The constant current electrolysis has a current density of 2.0-15.0 mA / cm² and an applied charge of 3.0-6.0 F / mol; the constant potential electrolysis has a potential of 1.0-2.0 V relative to the Ag / Ag⁺ non-aqueous reference electrode; the electrolysis temperature is 15-50℃ and the electrolysis time is 2-12 hours.

7. The method according to claim 1, characterized in that, The anodic oxidation dehydrogenation aromatization reaction is carried out in an unseparated electrolytic cell. The anode is a conductive substrate supported on the Pd1-Cu1 / b-CN catalyst. The conductive substrate is selected from carbon cloth, carbon paper, nickel foam, or platinum sheet. The catalyst loading on the anode is 1-5 mg / cm². The cathode is a platinum sheet or a carbon rod.

8. The method according to claim 1, characterized in that, In the anodic oxidation dehydrogenation aromatization reaction, the proportion of bromination byproducts is controlled to be below 5%.

9. The application of a Pd1-Cu1 / b-CN dual single-atom catalyst in the preparation of electrodes for the electrocatalytic synthesis of carbazole compounds, characterized in that, The catalyst is a bulk carbon nitride quasi-atom anchored by in-plane tricoordinated palladium and inter-plane tetracoordinated copper, with palladium and copper existing in an atomically dispersed state and a total loading of 0.2-1.0 wt%.

10. A kit for the electrocatalytic synthesis of carbazole compounds, characterized in that, include: The kit is used to implement the method for electrocatalytic synthesis of carbazole compounds according to any one of claims 1-8, comprising a Pd1-Cu1 / b-CN catalyst or a working electrode prepared therefrom, lithium bromide, acetic acid, and an aprotic organic solvent.

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