Method for synthesizing aryl ketone compound through direct oxidation of aryl alkane under visible light room temperature catalysis

Abstract

The invention belongs to the field of organic synthesis methodology, and particularly relates to a method for synthesizing aryl ketone compounds by catalyzing aryl alkane to be directly oxidized at room temperature through visible light. A method for synthesizing aryl ketone compounds by catalyzing direct oxidation of aryl alkane at room temperature through visible light comprises the following steps: adding aryl alkane serving as a reaction substrate into a solvent, and reacting at room temperature by taking oxygen as an oxidizing agent and crystalline phase carbon nitride as a catalyst to obtain the aryl ketone compounds. Stirring and reacting for a certain time at room temperature under the irradiation of visible light with the wavelength of 425nm, and then selectively oxidizing aryl alkane to prepare the aryl ketone compound. Crystalline phase carbon nitride is used as a catalyst to catalyze and activate oxygen under visible light irradiation to generate superoxide radicals to grab hydrogen on aryl alkane, free radical activation of inert Csp3-H bonds is achieved, reaction conditions are mild and can be carried out at room temperature, a reaction solvent is green, environmentally friendly, low in price and easy to obtain, the yield of a target product is high, and the method is suitable for industrial production. The driving force of the reaction is visible light, and meets the development requirements of green chemistry.

Description

Technical Field

[0001] The invention belongs to the field of organic synthesis methodology, and specifically relates to a method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature. Background Art

[0002] Visible light, as an important component of sunlight, is a green and clean energy source. Compared with traditional thermal reactions, introducing visible light into organic synthesis to achieve chemical bond transformation is more in line with the concept of green and sustainable development. Aromatic ketone compounds are a widely existing natural product. They are widely used as food additives, drug molecules and their precursors, organic small molecule catalysts, etc., and are also an important organic synthesis intermediate. There are many methods for the synthesis of this type of compound. Among them, the most promising is to directly oxidize the CH bonds of aromatic alkanes, a bulk raw material in petrochemical production, to synthesize aromatic ketone compounds with high added value. This method not only has a wide range of raw materials, but also has short reaction steps and high atomic utilization, which is very suitable for large-scale industrial production. At present, there has been some progress in the direct oxidation of CH bonds to ketones by photocatalysis. In 2014, Oisaki et al. (Kojima M, Oisaki K, Kanai M. Tetrahedron Lett, 2014, 55(34): 4736-4738.) reported a coordinated photocatalytic system based on Ru(bpy)3Cl2 and Cu(I). Using oxygen as an oxidant, under the irradiation of visible light, fluorene compounds can be oxidized to generate ketones. However, this type of reaction requires the use of expensive photocatalysts and high-pressure oxygen (1MPa). In 2015, Professor Lei Aiwen et al. (Yi H, Bian C, Hu X, et al. Chem Commun, 2015, 51(74): 14046-9.) used 9-phenyl-10-methylpyridinium salt as a photocatalyst and oxygen as an oxidant to oxidize diphenylmethane compounds to produce ketones under visible light irradiation, with a yield of 28% to 77%. This reaction still faces the defects of expensive and non-recyclable photocatalysts and low yields; in 2020, Fu Hua et al. (Zhu X, Liu Y, Liu C, et al. Green Chem, 2020, 22(13): 4357-4363) used cheap sodium trifluoromethylsulfinate as a catalyst (25 mol%) to achieve oxidation of aromatic alkanes under blue light irradiation, but the reaction used a large amount of catalyst and was not suitable for large-scale industrial production. Heterogeneous photosensitizers can solve the problem of difficult recycling of photocatalysts. Recently, carbon nitride (C3N4) materials have received widespread attention as a cheap heterogeneous photocatalyst. In 2022, Teixeira et al. (Da Silva M a R, Silva IF, Xue Q, et al. Applied Catalysis B: Environmental, 2022, 304: 120965) reported that carbon nitride-loaded Fe was a single-atom photocatalyst, which achieved the oxidation of aromatic alkanes. This reaction requires the addition of sulfuric acid to assist oxidation, which is easy to corrode equipment in industrial production, limiting its large-scale application.Therefore, finding a new method for photocatalytic aerobic oxidation of aromatic alkanes with simple and mild reaction conditions and cheap and easy-to-prepare photosensitizers not only has important theoretical value, but is also very important in industrial production. Summary of the invention

[0003] In order to overcome the shortcomings and deficiencies in the prior art, the primary purpose of the present invention is to provide a method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes under visible light catalysis at room temperature. The present invention uses crystalline carbon nitride with high reactivity as a catalyst to directly oxidize aromatic alkanes to aromatic ketone compounds by using visible light to activate oxygen under room temperature. The method is simple and easy to control.

[0004] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme:

[0005] A method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature, comprising the following steps: adding aromatic alkanes as reaction substrates into a solvent, using oxygen as an oxidant and crystalline carbon nitride as a catalyst, stirring and reacting at room temperature for a certain period of time under 425 nm visible light irradiation, and selectively oxidizing aromatic alkane to prepare aromatic ketone compounds; wherein the chemical structure of aromatic alkanes is generally expressed as: Where R 1 is hydrogen, tert-butyl, methoxy, cyano, nitro, chlorine, bromine, phenyl, trifluoromethyl, acetyl, methoxycarbonyl, tert-butoxycarbonyl; is naphthyl, quinolyl, benzofuranyl; R 2 One of methyl, ethyl, propyl, benzyl, methoxycarbonyl, 2-bromoethyl, phenyl, naphthyl and pyridyl.

[0006] Furthermore, the solvent is at least one of water and ethanol.

[0007] Furthermore, the stirring reaction time is 12h to 16h.

[0008] Furthermore, the crystalline carbon nitride is synthesized according to the following steps:

[0009] (1) placing 5 g of melamine powder in a crucible and calcining it in an air atmosphere at 550° C. for 4 hours to obtain a carbon nitride precursor;

[0010] (2) placing the carbon nitride precursor obtained in step (1) and the inorganic salt in a mortar in proportion, grinding them evenly, and calcining them in a nitrogen atmosphere at 550° C. for 4 hours to obtain a crystalline carbon nitride precursor;

[0011] (3) removing excess inorganic salts in the crystalline carbon nitride precursor obtained in step (2) by washing with water, and obtaining the crystalline carbon nitride after drying.

[0012] Furthermore, the mass ratio of the carbon nitride precursor to the inorganic salt in the above step (2) is 1:10.

[0013] Furthermore, the inorganic salt in step (2) is a mixture of potassium chloride and lithium chloride.

[0014] Furthermore, the mass ratio of potassium chloride to lithium chloride is 11:9.

[0015] In addition, in the present invention, conventional organic synthesis operations such as termination of reaction, extraction, and drying of solvent, including the final operation of obtaining the corresponding aromatic ketone compounds by rapid column chromatography, etc., are themselves prior art, and the extractants and eluents used therein are also prior art. Those skilled in the art can select appropriate reagents according to the properties of the final product.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] The present invention uses crystalline carbon nitride (K-PHI) as a catalyst to generate superoxide radicals by catalytic activation of oxygen under visible light irradiation to capture hydrogen from aromatic alkanes, thereby achieving inert Csp 3 The free radical activation of -H bonds is carried out under mild reaction conditions and can be carried out at room temperature. The reaction solvent (water, ethanol) is green, environmentally friendly, cheap and easy to obtain. The yield of the target product is high. The driving force of the reaction is visible light, which meets the development requirements of green chemistry. The preparation method disclosed in the present invention is universal for a variety of aryl alkane substrates, and the reaction synthesis method is simple, easy to obtain, low in cost, simple to operate, and is conducive to the industrial production of aryl ketone compounds. DETAILED DESCRIPTION

[0018] The present invention is described in detail below with reference to the examples. The examples are only preferred embodiments of the present invention and are not intended to limit the present invention.

[0019] The crystalline carbon nitride catalysts used in the embodiments of the present invention are synthesized according to the following steps:

[0020] (1) placing 5 g of melamine powder in a crucible and calcining it in an air atmosphere at 550° C. for 4 hours to obtain a carbon nitride precursor;

[0021] (2) placing the carbon nitride precursor obtained in step (1) and the inorganic salt in a mortar in proportion, grinding them evenly, and calcining them in a nitrogen atmosphere at 550° C. for 4 hours to obtain a crystalline carbon nitride precursor;

[0022] (3) removing excess inorganic salts in the crystalline carbon nitride precursor obtained in step (2) by washing with water, and obtaining the crystalline carbon nitride after drying.

[0023] Example 1

[0024]

[0025] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1a (0.1 mmol, 10.6 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated saline was added to quench, and extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The obtained crude product was subjected to simple column chromatography to obtain the oxidation product 2a with a yield of 70%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ7.96-7.94(m,2H),7.58-7.53(m,1H),7.47-7.44(m,1H). 13 C{ 1 H}NMR(101MHz, CDCl3): δ198.3,137.2,133.2,128.6,128.4,26.7.HRMS(ESI):calcd.for C8H9O + [M+H] + :121.0653; found:121.0658.

[0026] Example 2

[0027]

[0028] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1b (0.1 mmol, 15.1 mg), and ethanol (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated brine was added to quench, and the mixture was extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried using a rotary evaporator. The obtained crude product was subjected to simple column chromatography to obtain the oxidation product 2b with a yield of 85%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ8.27 (d, J = 8.4Hz, 2H), 8.09 (d, J = 8.4Hz, 2H), 2.66 (s, 3H). 13 C{ 1 H}NMR(101MHz, CDCl3): δ196.4,150.0,141.4,129.4,123.9,27.0.HRMS(ESI):calcd.for C8H7NO3Na + [M+Na] +:188.0324; found:188.0320.

[0029] Example 3

[0030]

[0031] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1c (0.1 mmol, 14.0 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated brine was added to quench, and the mixture was extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The crude product was subjected to simple column chromatography to obtain the oxidation product 2c with a yield of 81%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ7.86 (d, J = 8.6 Hz, 2H), 7.40 (d, J = 8.6 Hz, 2H), 2.56 (s, 3H). 13 C{ 1 H}NMR(101MHz, CDCl3): δ196.6,139.4,135.3,129.6,128.7,26.4.HRMS(ESI):calcd.for C8H8ClO + [M+H] + :155.0264; found:155.0255.

[0032] Example 4

[0033]

[0034] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1d (0.1 mmol, 18.5 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated saline was added to quench, and the mixture was extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The obtained crude product was subjected to simple column chromatography to obtain the oxidation product 2d with a yield of 87%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ7.82 (d, J = 8.5 Hz, 2H), 7.61 (d, J = 8.5 Hz, 2H), 2.59 (s, 3H). 13 C{ 1H}NMR(101MHz, CDCl3): δ197.0,135.8,131.9,129.8,128.3,26.5.HRMS(ESI):calcd.for C8H8BrO + [M+H] + :198.9753; found:198.9748.

[0035] Example 5

[0036]

[0037] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1e (0.1 mmol, 18.2 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated brine was added to quench, and the mixture was extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried using a rotary evaporator. The crude product was subjected to simple column chromatography to obtain the oxidation product 2e with a yield of 83%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ7.97(d,J=8.4Hz,2H),7.62(d,J=8.5Hz,2H),7.56(d,J=7.4Hz,2H),7.41(t,J=7.5Hz,2H),7.33(t,J=7.3Hz,1H),2.57(s,3H). 13 C{ 1 H}NMR(101MHz, CDCl3): δ197.8,145.8,139.9,135.9,129.0,128.9,128.2,127.3,127.3,26.7.HRMS(ESI):calcd.for C 14 H 13 O + [M+H] + :197.0961; found:197.0957.

[0038] Example 6

[0039]

[0040] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1f (0.1 mmol, 13.1 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated saline was added to quench, and extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The crude product was subjected to simple column chromatography to obtain the oxidation product 2f with a yield of 78%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ8.04 (d, J = 8.4Hz, 2H), 7.78 (d, J = 8.2Hz, 2H), 2.65 (s, 3H). 13 C{ 1 H}NMR(101MHz, CDCl3): δ196.52,139.86,132.50,128.67,117.91,116.38,26.77.HRMS(ESI):calcd.for C9H8NO + [M+H] + :146.0600; found:146.0596.

[0041] Example 7

[0042]

[0043] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1 g (0.1 mmol, 17.4 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated saline was added to quench, and extracted with ethyl acetate (5 mL × 3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The crude product was subjected to simple column chromatography to obtain 2 g of oxidation product with a yield of 71%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ8.06 (d, J = 8.1 Hz, 2H), 7.73 (d, J = 8.2 Hz, 2H), 2.64 (s, 3H). 13 C{ 1 H}NMR(101MHz, CDCl3): δ196.96,139.65,134.41(q,J=32.7Hz),128.60,125.66(q,J=3.3Hz),123.57(q,J=273.3Hz),26.76.HRMS(ESI):calcd.for C9H8F3O + [M+H] +:189.0522; found:189.0517.

[0044] Example 8

[0045]

[0046] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1h (0.1 mmol, 14.8 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated saline was added to quench, and extracted with ethyl acetate (5 mL × 3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The obtained crude product was subjected to simple column chromatography to obtain the oxidation product 2h, with a yield of 85%. The product was characterized as follows: 1 HNMR(400MHz, CDCl3): δ8.04(s,4H),2.65(s,6H). 13 C{ 1 H}NMR(101MHz,CDCl3):δ197.49,140.13,128.49,26.92.HRMS(ESI):calcd.for C 10 H 11 O2 + [M+H] + :163.0754; found:163.0752.

[0047] Example 9

[0048]

[0049] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1i (0.1 mmol, 13.6 mg), and water (1.0 mL) were added in sequence. After irradiation with 460 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated saline was added to quench, and the mixture was extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The crude product was subjected to simple column chromatography to obtain the oxidation product 2i with a yield of 70%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ7.91 (d, J = 8.6 Hz, 2H), 6.91 (d, J = 8.6 Hz, 2H), 3.84 (s, 3H), 2.53 (s, 3H). 13 C{ 1H}NMR(101MHz, CDCl3): δ196.8,163.6,130.6,130.4,113.7,55.5,26.4.HRMS(ESI):calcd.for C9H 11 O2 + [M+H] + :151.0759; found:151.0752.

[0050] Example 10

[0051]

[0052] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1j (0.1 mmol, 13.4 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated saline was added to quench, and extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The obtained crude product was subjected to simple column chromatography to obtain the oxidation product 2j with a yield of 73%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ7.96(d,J=7.5Hz,2H),7.55(t,J=7.4Hz,1H),7.45(t, J=7.7Hz,2H),2.95(t,J=7.3Hz,2H),1.80-1.74(m,2H),1.00(t,J=7.4Hz,3H). 13 C{ 1 H}NMR(101MHz, CDCl3): δ200.4,137.1,132.8,128.5,128.0,40.5,17.7,13.9.HRMS(ESI):calcd.for C 10 H 13 O + [M+H] + :149.0961; found:149.0958.

[0053] Embodiment 11

[0054]

[0055] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (5 mg), compound 1k (0.1 mmol, 13.2 mg), and water (1.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 16 hours, 4 mL of saturated saline was added to quench, and extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The obtained crude product was subjected to simple column chromatography to obtain the oxidation product 2k with a yield of 85%. The product was characterized as follows: 1 HNMR (400MHz, CDCl3): δ7.96(d,J=7.5Hz,2H),7.55(t,J=7.4Hz,1H),7.45(t, J=7.7Hz,2H),2.95(t,J=7.3Hz,2H),1.80-1.74(m,2H),1.00(t,J=7.4Hz,3H). 13 C{ 1 H}NMR(101MHz, CDCl3): δ200.4,137.1,132.8,128.5,128.0,40.5,17.7,13.9.HRMS(ESI):calcd.for C 10 H 11 O + [M+H] + :147.0804; found:147.0798.

[0056] Example 12

[0057]

[0058] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (15 mg), compound 11 (0.1 mmol, 16.6 mg), and 1,2-dichloromethane (2.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 30 hours, 4 mL of saturated brine was added to quench, and the mixture was extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried using a rotary evaporator. The obtained crude product was subjected to simple column chromatography to obtain the oxidation product 21 with a yield of 80%. The product was characterized as follows:

[0059] 1 HNMR (400MHz, CDCl3): δ7.53-7.51(m,2H),7.36-7.51(m,4H),7.17-7.14(m,2H). 13 C{ 1H}NMR(101MHz, CDCl3): δ193.9,144.4,134.7,134.2,129.1,124.3,120.4.HRMS(ESI):calcd.for C 13 H9O + [M+H] + :181.0653; found:181.0644.

[0060] Embodiment 13

[0061]

[0062] In a clean reaction bottle equipped with a magnetic rod of appropriate size, K-PHI (15 mg), compound 1m (0.1 mmol, 16.8 mg), and 1,2-dichloromethane (2.0 mL) were added in sequence. After irradiation with 425 nm light under an oxygen atmosphere at room temperature for 30 hours, 4 mL of saturated saline was added to quench, and the mixture was extracted with ethyl acetate (5 mL×3). The organic phases were combined and dried with anhydrous sodium sulfate, and then the solvent was dried by rotary evaporation. The obtained crude product was subjected to simple column chromatography to obtain the oxidation product 2m with a yield of 75%. The product was characterized as follows:

[0063] 1 HNMR (400MHz, CDCl3): δ7.82-7.80(m,4H),7.61-7.57(m,2H),7.50-7.46(m,4H). 13 C{ 1 H}NMR(101MHz,CDCl3):δ196.3,137.3,132.2,129.8,128.1.HRMS(ESI):calcd.forC 13 H 11 O + [M+H] + :183.0810; found:183.0809.

[0064] The above-mentioned embodiments merely express the implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the patent scope of the present invention. Any technical solution obtained in the form of equivalent replacement or equivalent transformation shall fall within the protection scope of the present invention.

Claims

1. A method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature, characterized in that: The method comprises the following steps: adding an aryl alkane as a reaction substrate into a solvent, using oxygen as an oxidant and crystalline carbon nitride as a catalyst, stirring and reacting at room temperature for a certain period of time under 425 nm visible light irradiation, and selectively oxidizing the aryl alkane to prepare an aryl ketone compound; wherein the chemical structure of the aryl alkane is: Where R 1 is hydrogen, tert-butyl, methoxy, cyano, nitro, chlorine, bromine, phenyl, trifluoromethyl, acetyl, methoxycarbonyl, tert-butoxycarbonyl; is naphthyl, quinolyl, benzofuranyl; R 2 One of methyl, ethyl, propyl, benzyl, methoxycarbonyl, 2-bromoethyl, phenyl, naphthyl and pyridyl.

2. The method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature according to claim 1, characterized in that: The solvent is at least one of water and ethanol.

3. The method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature according to claim 2, characterized in that: The stirring reaction time is 12h to 16h.

4. The method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature according to claim 3, characterized in that: The crystalline carbon nitride is synthesized according to the following steps: (1) placing 5 g of melamine powder in a crucible and calcining it in an air atmosphere at 550° C. for 4 hours to obtain a carbon nitride precursor; (2) placing the carbon nitride precursor obtained in step (1) and the inorganic salt in a mortar in proportion, grinding them evenly, and calcining them in a nitrogen atmosphere at 550° C. for 4 hours to obtain a crystalline carbon nitride precursor; (3) removing excess inorganic salts in the crystalline carbon nitride precursor obtained in step (2) by washing with water, and obtaining the crystalline carbon nitride after drying.

5. The method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature according to claim 4, characterized in that: The mass ratio of the carbon nitride precursor to the inorganic salt in the step (2) is 1:

10.

6. The method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature according to claim 5, characterized in that: The inorganic salt is a mixture of potassium chloride and lithium chloride.

7. The method for synthesizing aromatic ketone compounds by direct oxidation of aromatic alkanes catalyzed by visible light at room temperature according to claim 6, characterized in that: The mass ratio of potassium chloride to lithium chloride is 11:9.

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