A method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride

Abstract

The application discloses a synthesis method of 3,6-di(trifluoromethyl)-pyromellitic dianhydride. The synthesis method comprises the following steps: 1,2,4,5-tetramethylbenzene is subjected to iodization reaction in the presence of iodine, persulfate, concentrated sulfuric acid and acetic acid to obtain 3,6-diiodotetramethylbenzene; 3,6-diiodotetramethylbenzene is subjected to trifluoromethylation reaction with TMSCF3 (trifluoromethyltrimethylsilane) in the presence of CuX and MF in a polar solvent to obtain 3,6-di(trifluoromethyl)-tetramethylbenzene; 3,6-diiodotetramethylbenzene and an oxidant are subjected to oxidation reaction to obtain 3,6-di(trifluoromethyl)-pyromellitic acid; and 3,6-di(trifluoromethyl)-pyromellitic acid is subjected to dehydration and cyclization with acetic anhydride to obtain 3,6-di(trifluoromethyl)-pyromellitic dianhydrate. The reaction condition of the application is relatively mild, the treatment is simple, the efficiency is relatively high, and the cost is relatively low.

Description

Technical Field

[0001] This invention belongs to the field of organic synthesis technology, specifically relating to a method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride. Background Technology

[0002] Fluorinated polyimides possess advantages such as low dielectric constant, good solubility, excellent optical properties, and low moisture absorption. They also exhibit corrosion resistance, radiation resistance, high and low temperature resistance, superior mechanical properties, and good adhesion. These characteristics have led to their widespread application in electronics, power, OLED, aerospace, and precision machinery, making them irreplaceable high-performance polymer materials with significant development value. This necessitates the development of new technologies to prepare fluorinated dianhydride monomers to meet the evolving needs of the fluorinated polyimide industry.

[0003] 3,6-Di(trifluoromethyl)-pyromellitic dianhydride, a white solid powder with a melting point of 239-240℃. Its molecular formula is C2. 12 F6O6, relative molecular weight 354.1164.

[0004] There is only one publicly disclosed method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride:

[0005] Reference 1 discloses a method for preparing 3,6-diiodotetramethylbenzene from tetramethylbenzene as a raw material by first iodizing it with periodic acid, then reacting it with trifluoroiodomethane (CF3I) in the presence of metallic copper to obtain 3,6-di(trifluoromethyl)-pyromellitic acid dianhydride, and finally oxidizing and dehydrating it to obtain 3,6-di(trifluoromethyl)-pyromellitic acid dianhydride.

[0006] The shortcomings of this method are as follows: First, the iodination step uses periodic acid, which is expensive, unstable, and easily decomposed, making it unsuitable for large-scale use. Second, the introduction of trifluoromethyl groups uses trifluoroiodomethane (CF3I), which is photosensitive, difficult to store, and a gas at room temperature, making quantitative use inconvenient. Furthermore, the reaction requires high temperature and pressure (150℃, 50 atm), making it unsuitable for large-scale industrialization.

[0007] Document 1: Tohru Matsuura et al., "Polyimides Derived from 2,2,-Bis(trifluoromethyl)-4,4-diaminobiphenyl.2.Synthesis and Characterization ofPolyimides Prepared from Fluorinated Benzenetetracarboxylic Dianhydrides" "Macromolecules", Volume 25, Issue 13, 1992, pp. 3540-3545. Summary of the Invention

[0008] The technical problem this invention aims to solve is to overcome the shortcomings of existing technologies in the preparation of 3,6-bis(trifluoromethyl)-pyromellitic dianhydride, such as high cost, unstable reactants, and harsh reaction conditions. This invention proposes a method for preparing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride. This method is highly efficient, convenient, and suitable for large-scale industrial production, demonstrating promising application prospects and value.

[0009] This invention provides a method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride, which includes the following steps:

[0010] Step ①: In the presence of iodine, persulfate, concentrated sulfuric acid, and acetic acid, 1,2,4,5-tetramethylbenzene undergoes an iodination reaction to yield 3,6-diiodotetramethylbenzene.

[0011]

[0012] Step ②: In a polar solvent, in the presence of CuX and MF, compound 3,6-diiodotetramethylene is reacted with TMSCF3 (trifluoromethyltrimethylsilane) via a trifluoromethylation reaction to obtain 3,6-di(trifluoromethyl)-meta-tetramethylene.

[0013]

[0014] Step ③: 3,6-Diiodotetramethylbenzene and an oxidizing agent undergo an oxidation reaction to yield 3,6-bis(trifluoromethyl)-pyromellitic acid.

[0015]

[0016] Step ④: 3,6-Di(trifluoromethyl)-pyromellitic acid reacts with acetic anhydride via dehydration cyclization to yield 3,6-di(trifluoromethyl)-pyromellitic dianhydride.

[0017]

[0018] In one embodiment, in step ①, the persulfate is selected from one or more of potassium persulfate, sodium persulfate, and ammonium persulfate; potassium persulfate is preferred.

[0019] In one embodiment, in step ①, the molar ratio of 1,2,4,5-tetramethylbenzene to iodine is 1:1 to 1:10, preferably 1:2 to 1:3.

[0020] In one embodiment, in step ①, the molar ratio of 1,2,4,5-tetramethylbenzene to persulfate is 1:1 to 1:10, preferably 1:2 to 1:3.

[0021] In one embodiment, in step ①, the mass ratio of 1,2,4,5-tetramethylbenzene to concentrated sulfuric acid is 1:1 to 1:3, preferably 1:1.46.

[0022] In one embodiment, in step ①, the mass ratio of 1,2,4,5-tetramethylbenzene to acetic acid is 1:20-1:60, preferably 1:39.12.

[0023] In one embodiment, in step ①, the reaction temperature is 0-100℃, preferably 0-60℃.

[0024] In one embodiment, in step ①, the progress of the reaction can be monitored using conventional methods for such reactions in the art (e.g., TLC tracking), and the reaction endpoint is generally defined as the complete reaction of the compound 1,2,4,5-tetramethylbenzene and the production of a single product spot.

[0025] In one embodiment, step ① further includes the following post-processing steps: recrystallization (e.g., using an excess of 5% aqueous Na2SO3), filtration, dissolving the filter cake in an organic solvent (e.g., ethyl acetate), drying (e.g., drying with anhydrous sodium sulfate), filtration, concentration, and drying to obtain 3,6-diiodotetramethylbenzene.

[0026] In one embodiment, in step ②, the MF is selected from one or more of NaF, KF, and CsF, preferably KF.

[0027] In one embodiment, in step ②, the molar ratio of TMSCF3 to CuX is 1:0.5-1:3; preferably 1:0.83 or 1:0.91.

[0028] In one embodiment, in step ②, the molar ratio of TMSCF3 to MF is 1:0.5-1:3; preferably 1:0.92 or 1:1.

[0029] In one embodiment, in step ②, the molar ratio of TMSCF3 to 3,6-diiodotetramethylbenzene is 1:0.1-1:4; preferably 1:0.28, 1:0.39 or 1:0.30.

[0030] In one embodiment, in step ②, the polar solvent is selected from one or more of DMF, DMSO, or NMP; preferably, it is DMF.

[0031] In one embodiment, in step ②, CuX is selected from one or more of CuCl, CuBr, CuI, CuSCN, or CuOTf; preferably CuCl.

[0032] In one embodiment, in step ②, the reaction temperature is 0-120℃; preferably 60℃ or 90℃.

[0033] In one embodiment, in step ②, the reaction time is 30h-54h; preferably 30h or 54h.

[0034] In one embodiment, in step ②, the progress of the reaction can be monitored using conventional methods for such reactions in the art (e.g., NMR tracing), and the reaction endpoint is generally considered to be the complete reaction of the compound 3,6-diiodotetramethylbenzene.

[0035] In one embodiment, step ② further includes the following post-processing steps: quenching (e.g., quenching in 800 mL ammonia water), dilution (e.g., dilution with 500 mL water), extraction (e.g., extraction three times with n-pentane), drying (with anhydrous sodium sulfate), filtration, concentration, and drying to obtain the product.

[0036] In one embodiment, in step ③, the oxidant is potassium permanganate, sodium dichromate, potassium dichromate, or nitric acid; preferably potassium permanganate or nitric acid (e.g., 25% nitric acid).

[0037] In one embodiment, when the oxidant in step ③ is potassium permanganate, sodium dichromate, or potassium dichromate, step ③ further includes an alkali and a solvent. The alkali is preferably one or both of sodium hydroxide and pyridine, more preferably sodium hydroxide and pyridine; the solvent is preferably deionized water.

[0038] In one embodiment, in step ③, the mass ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to pyridine is 1:20-1:60, preferably 1:43.65.

[0039] In one embodiment, in step ③, the molar ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to sodium hydroxide is 1:5-1:15, preferably 1:10.

[0040] In one embodiment, in step ③, the mass ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to the deionized water is 1:20-1:60, preferably 1:43.65.

[0041] In one embodiment, in step ③, the reaction temperature is 80-180℃, preferably 90℃ or 170℃.

[0042] In one embodiment, in step ③, the progress of the reaction can be monitored using conventional methods in the art for such reactions (e.g., NMR fluorine spectroscopy tracking), and the reaction endpoint is generally considered to be the complete oxidation of the compound 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene.

[0043] In one embodiment, step ③ further includes the following post-processing steps: quenching (e.g., consuming excess KMnO4 with ethanol), filtration (e.g., hot filtration), washing the filter cake (e.g., washing with hot water), concentrating the filtrate, acidifying to pH 2.0 (e.g., using concentrated hydrochloric acid), recrystallizing (cooling overnight in the upper part of a refrigerator to precipitate the product), filtering, and drying to obtain 3,6-bis(trifluoromethyl)-pyromellitic acid.

[0044] In one embodiment, in step ④, the mass ratio of 3,6-bis(trifluoromethyl)pyromellitic acid to acetic acid is 1:1.0-1:4.0, preferably 1:2.69.

[0045] In one embodiment, the reaction time in step ④ is 3-9 hours, preferably 6 hours.

[0046] In one embodiment, step ④ further includes the following post-processing steps: filtration (e.g., rapid filtration), washing the filter cake (e.g., washing with a small amount of cooled diethyl ether), collecting the solid, and drying (e.g., drying in a vacuum oven at 50°C for 10 hours) to obtain the product 3,6-bis(trifluoromethyl)-pyromellitic dianhydride.

[0047] In one embodiment, the reactants in step ① consist of the following substances: 1,2,4,5-tetramethylbenzene, iodine, persulfate, concentrated sulfuric acid, and acetic acid.

[0048] In one embodiment, in step ②, the reactants for the reaction consist of the following substances: the compound 3,6-diiodotetramethylbenzene, the TMSCF3, the polar solvent, the CuX, and the MF.

[0049] In one embodiment, in step ③, the reactants for the reaction are either embodiment one or embodiment two. Embodiment one consists of the following substances: the 3,6-diiodotetramethylbenzene, the oxidant, the base, and the solvent; embodiment two consists of the following substances: the 3,6-diiodotetramethylbenzene and the oxidant.

[0050] In one embodiment, in step ④, the reactants for the reaction consist of the following substances: 3,6-bis(trifluoromethyl)pyromellitic acid and acetic anhydride.

[0051] A method for preparing 3,6-di(trifluoromethyl)-meta-tetramethylene includes the following steps: in a polar solvent, in the presence of CuX and MF, the compound 3,6-diiodotetramethylene is reacted with TMSCF3 (trifluoromethyltrimethylsilane) to undergo a trifluoromethylation reaction to obtain 3,6-di(trifluoromethyl)-meta-tetramethylene.

[0052]

[0053] Preferably, the operation and conditions of the trifluoromethylation reaction are as described in any one of the present invention.

[0054] It should be understood that, within the scope of this invention, the above-described technical features of this invention and the technical features specifically described below (such as in the embodiments) can be combined with each other to form new or preferred technical solutions. Due to space limitations, they will not be described in detail here.

[0055] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0056] The reagents and raw materials used in this invention are all commercially available.

[0057] The positive and progressive effects of this invention are: the reaction conditions are relatively mild, the processing is simple, the efficiency is high, and the cost is low. Attached Figure Description

[0058] Figure 1 The 1H NMR spectrum (CDCl3) of 3,6-diiodotetramethylbenzene.

[0059] Figure 2 The 1H NMR spectrum (CDCl3) of 3,6-bis(trifluoromethyl)-methemoyltetramethylene.

[0060] Figure 3 Fluorine spectrum (CDCl3) of 3,6-bis(trifluoromethyl)-methemoyltetramethylene.

[0061] Figure 4The 1H NMR spectrum (DMSO-d6) of 3,6-bis(trifluoromethyl)-pyromellitic acid.

[0062] Figure 5 Fluorine spectrum of 3,6-bis(trifluoromethyl)pyromellitic acid (DMSO-d6).

[0063] Figure 6 Fluorine spectrum of 3,6-bis(trifluoromethyl)-pyromellitic dianhydride (DMSO-d6). Detailed Implementation

[0064] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. Percentages and parts are by weight unless otherwise stated.

[0065] Example 1

[0066] ① Under nitrogen protection, I₂ (200 mmol, 50.8 g), 500 mL of glacial acetic acid, and 19.6 g of concentrated sulfuric acid (98%) were added to a 1000 mL three-necked reaction flask and stirred. Potassium persulfate (202.5 mmol, 54.83 g) was slowly added, and the mixture was heated to 60 °C and maintained at that temperature for 30 min. Then, 1,2,4,5-tetramethylbenzene (100 mmol, 13.42 g) was slowly added, and the mixture was kept at 60 °C. TLC was monitored until the 1,2,4,5-tetramethylbenzene reacted completely and a single product spot was formed. The reaction was then stopped and cooled to room temperature. The reaction solution was poured into an excess of 5% Na₂SO₃ aqueous solution, and a colorless solid precipitated. The solid was collected by filtration and then dissolved in ethyl acetate. The solid was then dried over anhydrous sodium sulfate, filtered, concentrated, and dried to obtain 37.83 g of 3,6-diiodotetramethylbenzene, with a yield of 98%. 1 ¹H NMR (CDCl₃, 400MHz): δ 2.63 (s, 12H); see attached NMR spectrum for details. Figure 1 .

[0067] ②

[0068] 2.1 First step: Under nitrogen protection, add dry KF, activated CuCl and dry DMF to a 20ml reaction tube, stir at room temperature, and then slowly add TMSCF3 dropwise. After the addition is complete, react at room temperature for 24 hours.

[0069] In the second step, under nitrogen protection, 3,6-diiodotetramethylbenzene (substrate A) prepared in step ① was added, and the mixture was heated to the predetermined temperature for the specified reaction time. After stopping the reaction and cooling to room temperature, the internal standard PhOCF3 was added, and NMR analysis was performed on a sample. (Fluorine spectroscopy yield was determined using PhOCF3 as the internal standard.)

[0070] Specific reaction conditions are shown in Table 1.

[0071] Table 1

[0072]

[0073]

[0074] 2.2 Under nitrogen protection, dry KF (427.4 mmol, 24.83 g, 3.3 equiv.), activated CuCl (388.6 mmol, 38.47 g, 3.0 equiv.), and dry DMF (500 mL) were added to a 1000 mL three-necked reaction flask. The mixture was stirred at room temperature (20-35 °C), and then TMSCF3 (427.4 mmol, 60.78 g) was slowly added dropwise. After the addition was complete, the reaction was allowed to proceed at room temperature for 24 hours. Then, 3,6-diiodotetramethylbenzene (129.5 mmol, 50.0 g, 1.0 equiv.) prepared in step ① was added, and the temperature was raised to 90 °C, and the reaction was continued for 6 hours. NMR monitoring showed that the reaction was complete. The reaction was stopped, and the reaction solution was poured into 800 mL of ammonia water, followed by the addition of 500 mL of water. The mixture was then extracted three times with n-pentane. The organic phases were combined, dried with anhydrous sodium sulfate, filtered, concentrated, and dried to obtain 34.78 g of a light yellow product, with a yield of 99%. 1 HNMR (CDCl3, 400MHz): δ2.15 (s, 12H); 19 F NMR (CDCl3, 376MHz): δ -52.44(s); see attached NMR spectrum for details. Figure 2 and Figure 3 .

[0075] ③ Under nitrogen protection, add 40 mmol (10.81 g) of 3,6-bis(trifluoromethyl)-meshtemethylbenzene prepared in step ②, 480 mL of pyridine, and 80 mL of deionized water to a 1000 mL three-necked reaction flask. Heat to 90 °C, then add 56.88 g (360 mmol) of KMnO4 in portions, and continue the reaction at 90 °C overnight. Filter while hot, wash the filtered insoluble solid several times with hot water, combine the filtrates, and evaporate to dryness to obtain the solid. Add 400 mL of deionized water and 16 g of NaOH to the solid to form a solution. The solution was transferred to a 1000 mL three-necked reaction flask and heated to 90 °C. 27.20 g of KMnO4 (172 mmol) was added in portions, and the reaction continued, monitored by NMR spectroscopy until complete oxidation. [Note: The actual amount of KMnO4 used needs to be determined by NMR spectroscopy; if 27.20 g of KMnO4 (172 mmol) is insufficient to completely oxidize the starting material, additional KMnO4 may need to be added.] The reaction was then quenched with ethanol to consume excess KMnO4. The solution was filtered while hot, and the filter cake was washed several times with hot water. The filtrates were combined and concentrated to approximately 200 mL. The solution was acidified to pH 2.0 with concentrated hydrochloric acid, and then placed in the upper part of a refrigerator to cool overnight to precipitate the product. The product was filtered and dried to obtain 14.36 g of product, a yield of 92%. 19 F NMR (CDCl3, 376MHz): δ -55.21(s); see attached NMR spectrum for details. Figure 4 and Figure 5 .

[0076] ④ Under nitrogen protection, 11.70 g (30 mmol) of 3,6-bis(trifluoromethyl)-pyromellitic acid obtained in step ③ was added to 30 mL of acetic anhydride and heated under reflux for 6 h. The mixture was then rapidly filtered and washed with a small amount of cooled diethyl ether. The solid was collected and dried in a vacuum oven at 50 °C for 10 h to obtain 9.56 g of the product 3,6-bis(trifluoromethyl)-pyromellitic acid dianhydride, yield 90%; mp 239-240 °C. 19 F NMR (CDCl3, 376MHz): δ -54.645(s); see attached NMR spectrum for details. Figure 6 .

[0077] Example 2

[0078] ① Under nitrogen protection, I₂ (200 mmol, 50.8 g), 500 mL of glacial acetic acid, and 19.6 g of concentrated sulfuric acid (98%) were added to a 1000 mL three-necked reaction flask and stirred. Potassium persulfate (202.5 mmol, 54.83 g) was slowly added, and the mixture was heated to 60 °C and maintained at that temperature for 30 min. Then, 1,2,4,5-tetramethylbenzene (100 mmol, 13.42 g) was slowly added, and the reaction was continued at 60 °C under TLC monitoring until the 1,2,4,5-tetramethylbenzene reacted completely and a single product spot was formed. The reaction was then stopped and cooled to room temperature. The reaction solution was poured into an excess of 5% Na₂SO₃ aqueous solution, and a colorless solid precipitated. The solid was collected by filtration and then dissolved in ethyl acetate. The solid was then dried over anhydrous sodium sulfate, filtered, concentrated, and dried to obtain 37.83 g of 3,6-diiodotetramethylbenzene, with a yield of 98%.

[0079] ② Under nitrogen protection, dry KF (427.4 mmol, 24.83 g, 3.3 equiv.), activated CuI (388.6 mmol, 74.01 g, 3.0 equiv.), and dry DMF (500 mL) were added to a 1000 mL three-necked reaction flask. The mixture was stirred at room temperature, and then TMSCF3 (427.4 mmol, 60.78 g) was slowly added dropwise. After the addition was complete, the reaction was allowed to proceed at room temperature for 24 hours. Then, 3,6-diiodotetramethylbenzene (129.5 mmol, 50.0 g, 1.0 equiv.) prepared in step ① was added, and the temperature was raised to 90 °C and the reaction was continued for 6 hours. NMR monitoring showed that the reaction was complete. The reaction was stopped, and the reaction solution was poured into 800 mL of ammonia water, followed by the addition of 500 mL of water. The mixture was then extracted three times with n-pentane. The organic phases were combined, dried over anhydrous sodium sulfate, filtered, concentrated, and dried to obtain 34.29 g of a light yellow product, with a yield of 98%.

[0080] ③ Add 40 mmol (10.81 g) of 3,6-bis(trifluoromethyl)-meshtethene (prepared in step ②) and 160 mL of 25% nitric acid to a 500 mL high-pressure reactor. Heat to 170 °C and react for 17 h. After cooling to room temperature, a solid precipitates. Dissolve the solid in a small amount of hot water, and then place the aqueous solution in the upper part of a refrigerator overnight to precipitate the solid. Filter quickly and dry to obtain 13.26 g of product, with a yield of 85%.

[0081] ④ Under nitrogen protection, 11.70 g (30 mmol) of 3,6-bis(trifluoromethyl)-pyromellitic acid obtained in step ③ was added to 30 mL of acetic anhydride and heated under reflux for 6 h. The mixture was then rapidly filtered and washed with a small amount of cooled diethyl ether. The solid was collected and dried in a vacuum oven at 50 °C for 10 h to obtain 9.56 g of the product 3,6-bis(trifluoromethyl)-pyromellitic acid dianhydride, with a yield of 90%.

Claims

1. A method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride, characterized in that, It includes the following steps: ① In the presence of iodine, persulfate, concentrated sulfuric acid, and acetic acid, 1,2,4,5-tetramethylbenzene undergoes an iodination reaction to yield 3,6-diiodotetramethylbenzene. ； ② In a polar solvent, in the presence of CuX and MF, compound 3,6-diiodotetramethylene was subjected to a trifluoromethylation reaction with TMSCF3 to obtain 3,6-di(trifluoromethyl)-meta-tetramethylene. The reaction temperature was 90°C. In step ②, the molar ratio of TMSCF3 to CuX is 1:0.5-1:3; In step ②, the molar ratio of TMSCF3 to MF is 1:0.5-1:3; In step ②, the molar ratio of TMSCF3 to 3,6-diiodotetramethylbenzene is 1:0.1-1:4; ； ③ 3,6-Diiodotetramethylbenzene reacts with an oxidizing agent to yield 3,6-bis(trifluoromethyl)-pyromellitic acid. ； ④ 3,6-Di(trifluoromethyl)-pyromellitic acid reacts with acetic anhydride via a dehydration cyclization reaction to yield 3,6-di(trifluoromethyl)-pyromellitic dianhydride. 。 2. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 1, characterized in that, Step ① must satisfy one or more of the following conditions: (1) In step ①, the persulfate is selected from one or more of potassium persulfate, sodium persulfate and ammonium persulfate; (2) In step ①, the molar ratio of 1,2,4,5-tetramethylbenzene to iodine is 1:1 to 1:10; (3) In step ①, the molar ratio of 1,2,4,5-tetramethylbenzene to persulfate is 1:1 to 1:10; (4) In step ①, the mass ratio of 1,2,4,5-tetramethylbenzene to concentrated sulfuric acid is 1:1 to 1:3; (5) In step ①, the mass ratio of 1,2,4,5-tetramethylbenzene to acetic acid is 1:20-1:60; In step ① of (6), the reaction temperature is 0-100℃.

3. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 2, characterized in that, Step ① satisfies one or more of the following conditions. (1) In step ①, the persulfate is potassium persulfate; (2) In step ①, the molar ratio of 1,2,4,5-tetramethylbenzene to iodine is 1:2-1:3; (3) In step ①, the molar ratio of 1,2,4,5-tetramethylbenzene to persulfate is 1:2-1:3; (4) In step ①, the mass ratio of 1,2,4,5-tetramethylbenzene to concentrated sulfuric acid is 1:1.46; (5) In step ①, the mass ratio of 1,2,4,5-tetramethylbenzene to acetic acid is 1:39.12; In step ① of (6), the reaction temperature is 0-60 ℃.

4. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 1, characterized in that, Step ② satisfies one or more of the following conditions. (1) In step ②, the MF is selected from one or more of NaF, KF and CsF; (2) In step ②, the polar solvent is selected from one or more of DMF, DMSO or NMP; In step ② of (3), CuX is selected from one or more of CuCl, CuBr, CuI, CuSCN and CuOTf.

5. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 4, characterized in that, Step ② satisfies one or more of the following conditions. (1) In step ②, MF is KF; (2) In step ②, the molar ratio of TMSCF3 to CuX is 1:0.83 or 1:0.91; (3) In step ②, the molar ratio of TMSCF3 to MF is 1:0.92 or 1:1; (4) In step ②, the molar ratio of TMSCF3 to 3,6-diiodotetramethylbenzene is 1:0.28, 1:0.39 or 1:0.30; (5) In step ②, the polar solvent is DMF; In step ② of (6), CuX is CuCl.

6. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 1, characterized in that, Step ③ satisfies one or more of the following conditions. (1) In step ③, the oxidant is potassium permanganate, sodium dichromate, potassium dichromate or nitric acid; (2) In step ③, when the oxidant is potassium permanganate, sodium dichromate or potassium dichromate, step ③ also includes an alkali and a solvent; In step ③ of (3), the reaction temperature is 80-180 ℃.

7. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 6, characterized in that, Step ③ satisfies one or two of the following conditions. (1) The base is one or both of sodium hydroxide and pyridine; (2) The solvent is deionized water.

8. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 7, characterized in that, Step ③ satisfies one or more of the following conditions. (1) In step ③, the mass ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to pyridine is 1:20-1:60; (2) In step ③, the molar ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to sodium hydroxide is 1:5-1:15; In step ③ of (3), the mass ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to the deionized water is 1:20-1:

60.

9. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 8, characterized in that, Step ③ satisfies one or more of the following conditions. (1) In step ③, the oxidant is potassium permanganate or nitric acid; (2) In step ③, the alkali is sodium hydroxide and pyridine; (3) In step ③, the mass ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to pyridine is 1:43.65; (4) In step ③, the molar ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to sodium hydroxide is 1:10; (5) In step ③, the mass ratio of 3,6-bis(trifluoromethyl)-meta-tetramethylbenzene to the deionized water is 1:43.65; In step ③ of (6), the reaction temperature is 90 ℃ or 170 ℃.

10. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 9, characterized in that, In step ③, the nitric acid is 25% nitric acid.

11. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 1, characterized in that, If one or more of the following conditions are met, (1) In step ④, the mass ratio of 3,6-bis(trifluoromethyl)pyromellitic acid to acetic anhydride is 1:1.0-1:4.0; (2) In step ④, the reaction time is 3-9 h; (3) In step ①, the reactants of the reaction consist of the following substances: 1,2,4,5-tetramethylbenzene, iodine, persulfate, concentrated sulfuric acid and acetic acid; (4) In step ②, the reactants of the reaction consist of the following substances: the compound 3,6-diiodotetramethylbenzene, the TMSCF3, the polar solvent, the CuX and the MF; In step ④ of (5), the reactants of the reaction consist of the following substances: 3,6-bis(trifluoromethyl)-pyromellitic acid and acetic anhydride.

12. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 11, characterized in that, If one or two of the following conditions are met. (1) In step ④, the mass ratio of 3,6-bis(trifluoromethyl)pyromellitic acid to acetic anhydride is 1:2.69; (2) In step ④, the reaction time is 6 h.

13. The method for synthesizing 3,6-bis(trifluoromethyl)-pyromellitic dianhydride as described in claim 10, characterized in that, In step ③, the reactants for the reaction are either Scheme 1 or Scheme 2. Scheme 1 consists of the following substances: the 3,6-diiodotetramethylbenzene, the oxidant, the base, and the solvent. Scheme 2 consists of the following substances: the 3,6-diiodotetramethylbenzene and the oxidant.

14. A method for preparing 3,6-bis(trifluoromethyl)-meshtethene, characterized in that, It includes the following steps: in a polar solvent, in the presence of CuX and MF, the compound 3,6-diiodotetramethylene is reacted with TMSCF3 (trifluoromethyltrimethylsilane) to obtain 3,6-di(trifluoromethyl)-meta-tetramethylene, wherein the reaction temperature is 90 °C. ； In step ②, the molar ratio of TMSCF3 to CuX is 1:0.5-1:3; In step ②, the molar ratio of TMSCF3 to MF is 1:0.5-1:3; In step ②, the molar ratio of TMSCF3 to 3,6-diiodotetramethylbenzene is 1:0.1-1:4; The operation and conditions of the trifluoromethylation reaction are as described in any one of claims 4, 5 and 11.

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