Preparation method of high-purity 3, 7-diamino-2, 8-dimethyl dibenzothiophene sulfone

Abstract

The invention relates to a preparation method of high-purity 3, 7-diamino-2, 8-dimethyl dibenzothiophene sulfone, and belongs to the technical field of medicines and chemistry. The invention provides a preparation method of high-purity 3, 7-diamino-2, 8-dimethyl dibenzothiophene sulfone, and the reaction is as follows: # imgabs0 # comprises the following steps: step 1, in an organic solvent, reacting a compound as shown in a formula 1, concentrated sulfuric acid and a sulfonating reagent under the action of a Lewis acid catalyst to obtain a compound as shown in a formula 2; the route provided by the invention is simple to operate, free of harsh reaction conditions, high in yield, high in purity, simple to operate, low in energy consumption, good in selectivity, good in reproducibility, capable of effectively reducing the cost and beneficial to industrial large-scale production, thereby having better market value and profound practical significance.

Description

Technical Field

[0001] The invention relates to a method for preparing high-purity 3,7-diamino-2,8-dimethyldibenzothiophene sulfone, and belongs to the technical field of medicine and chemistry. Background Art

[0002] 3,7-Diamino-2,8-dimethyldibenzothiophene sulfone (o-tolidine sulfone) is a key organic compound and one of the most important frameworks in the field of materials science. It is widely used in products such as organic photodiodes, displays, photocatalysts, and thin film materials.

[0003] The main component of o-tolidine sulfone is 3,7-diamino-2,8-dimethyldibenzothiophene sulfone (Formula 2), which usually contains impurities of 3,7-diamino-2,6-dimethyldibenzothiophene sulfone (Formula 2A) and 3,7-diamino-4,6-dimethyldibenzothiophene sulfone (Formula 2B). The structure is shown below:

[0004]

[0005] The main component of the o-tolidine sulfone currently sold on the market is about 70% 3,7-diamino-2,8-dimethyldibenzothiophene sulfone, and the content of the other isomers is about 30%. There is a problem of low main component, which will have a great impact on the selectivity of oxygen and nitrogen in the downstream product polyimide film, affecting the oxygen separation effect and is not conducive to the final downstream product application.

[0006] JP5648327B2 discloses the use of o-tolidine to react with oleum and sulfur trioxide to obtain an o-tolidine sulfone mixture. A large amount of oleum is used in the patent method. Oleum is a hazardous chemical and a precursor chemical, has strong oxidizing properties, is easily corrosive to metals, and contains a byproduct of formula 2C of persulfonic acid. The post-processing is cumbersome and difficult to control, and is not suitable for industrial scale-up production. Formula 2C is shown below:

[0007]

[0008] CN118005604A discloses a method for obtaining 3,7-diamino-2,8-dimethyldibenzothiophene sulfone (compound of formula 2) and isomer impurity compound of formula 2A by reaction in the presence of o-tolidine and concentrated sulfuric acid and chlorosulfonic acid, the structure of which is shown below:

[0009]

[0010] The yield in this patent ranges from 12% to 94%, with most yields being around 60%. The patent states that the structure of the prepared monomer has been confirmed by nuclear magnetic resonance analysis, and the ratio of the isomers of Formula 2 to Formula 2A is 84:16. The impurity content in this patent is high, and the amount of acid used is very large. For example, the amount of chlorosulfonic acid used in Example 9 is 7 to 10 molar equivalents, and the amount of concentrated sulfuric acid used is 25 molar equivalents. Concentrated sulfuric acid is highly corrosive, and a large amount of alkali is required for post-treatment. The wastewater treatment pressure limits the development of industrialization, and it is dangerous during the specific operation process, and is not conducive to commercial scale-up production.

[0011] No other literature in the prior art reports a method for obtaining a high-purity compound of formula 2. The present invention provides a method for preparing high-purity 3,7-diamino-2,8-dimethyldibenzothiophene sulfone, which has the advantages of simple operation, good reproducibility, high conversion rate, simple post-treatment, extremely high purity, environmental friendliness, and suitability for industrial production, and therefore has good market value and far-reaching practical significance. Summary of the invention

[0012] In view of the above technical background, the present invention provides a method for preparing high-purity 3,7-diamino-2,8-dimethyldibenzothiophene sulfone, which has the advantages of simple operation, high yield, good selectivity, high conversion rate, short time consumption, reduced cost, high purity, and is conducive to industrial production.

[0013] The present invention provides a method for preparing high-purity 3,7-diamino-2,8-dimethyldibenzothiophene sulfone, and the reaction is as follows:

[0014]

[0015] The following steps are involved:

[0016] Step 1: In an organic solvent, the compound of formula 1, concentrated sulfuric acid and a sulfonating agent react under the action of a Lewis acid catalyst to obtain a compound of formula 2;

[0017] As a further improvement of the present invention, including but not limited to, the sulfonating reagent in step 1 is selected from one of oleum and chlorosulfonic acid, preferably chlorosulfonic acid.

[0018] As a further improvement of the present invention, including but not limited to, the inventors found that under the same conditions, when chlorosulfonic acid is used as the sulfonating agent, the effect is best; relative to the conditions of using oleum, the purity of the compound of formula 2 is increased by at least 6% or more, reaching more than 98%. It is well known to those skilled in the art that the purity of the compound is very difficult to achieve.

[0019] As a further improvement of the present invention, including but not limited to, the Lewis acid catalyst in step 1 is selected from one of aluminum trichloride, zinc dichloride, ferric trichloride, and titanium tetrachloride, preferably one of aluminum trichloride and zinc dichloride, and more preferably aluminum trichloride.

[0020] As further improvements of the present invention, including but not limited to, in the present invention, in the concentrated sulfuric acid and sulfonating agent system, a Lewis acid catalyst (such as aluminum chloride or zinc dichloride) is added, and the selectivity of the compound of formula 2 is improved by 7.7% to 15% compared with the preparation method of the prior art CN118005604A, and it is well known to those skilled in the art that it is difficult to improve the selectivity of the compound of formula 2; the method of the present invention significantly improves the purity of the compound of formula 2 and the selectivity of ring closure, and the present invention has good reproducibility, simple post-treatment, and is suitable for industrial production.

[0021] As a further improvement of the present invention, including but not limited to, in step 1, the molar ratio of the compound of formula 1 to concentrated sulfuric acid is 1:(0.3-1), preferably 1:(0.3-0.8).

[0022] As a further improvement of the present invention, including but not limited to, in some embodiments, the molar ratio of the compound of formula 1 to concentrated sulfuric acid is 1:0.5.

[0023] As a further improvement of the present invention, including but not limited to, the molar ratio of the compound of formula 1 to the sulfonation reagent and the Lewis acid catalyst in step 1 is 1:(1.5-3):(0.02-0.5), preferably 1:(1.5-2.5):(0.08-0.2).

[0024] As a further improvement of the present invention, including but not limited to, in certain embodiments, the molar ratio of the compound of formula 1 to the sulfonation reagent and the Lewis acid catalyst in step 1 is 1:2:0.1.

[0025] As a further improvement of the present invention, including but not limited to, the organic solvent in step 1 is selected from halogenated alkanes.

[0026] As a further improvement of the present invention, including but not limited to, the halogenated alkane in step 1 is selected from one or more of 1,2-dichloroethane, chloroform, and carbon tetrachloride, preferably 1,2-dichloroethane.

[0027] As a further improvement of the present invention, including but not limited to, the reaction temperature of step 1 is 45-80°C, preferably 50-70°C.

[0028] As a further improvement of the present invention, including but not limited to, in some embodiments, the reaction temperature of step 1 is 60°C.

[0029] As a further improvement of the present invention, including but not limited to, the reaction of the present invention can be carried out at 45°C to 80°C, and the purity of the compound of formula 2 is significantly improved when the reaction is carried out at 60°C compared with other temperatures, specifically, it is increased by more than 10% compared with 80°C and more than 8% compared with 45°C, the generation of isomer impurities is significantly reduced, and the yield is equivalent, while reducing energy consumption, a higher yield and purity are achieved, which is conducive to industrial production.

[0030] As a further improvement of the present invention, including but not limited to, the reaction time of step 1 is 5 to 52 hours, preferably 5 to 48 hours.

[0031] As a further improvement of the present invention, in certain embodiments, the compound of formula 1 reacts with concentrated sulfuric acid, aluminum chloride, and chlorosulfonic acid in an organic solvent to obtain a compound of formula 2.

[0032] As a further improvement of the present invention, in certain embodiments, the compound of formula 1 reacts with concentrated sulfuric acid, aluminum chloride, and chlorosulfonic acid in 1,2-dichloroethane to obtain the compound of formula 2.

[0033] As a further improvement of the present invention, in certain embodiments, the compound of formula 1 is reacted with concentrated sulfuric acid, aluminum chloride, and chlorosulfonic acid in 1,2-dichloroethane at a temperature of 60° C. to obtain the compound of formula 2.

[0034] As a further improvement of the present invention, in certain embodiments, the compound of formula 1 (1 eq) and concentrated sulfuric acid (0.5 eq), aluminum chloride (0.1 eq), and chlorosulfonic acid (2 eq) are dissolved in 1,2-dichloroethane, heated to 60° C., and reacted to obtain the compound of formula 2.

[0035] As a further improvement of the present invention, in certain embodiments, after the reaction of step 1 is completed, the reaction solution is poured into ice water, filtered, the pH of the filter cake is adjusted to alkaline with sodium hydroxide, filtered, the filter cake is washed with water, and dried to obtain the compound of formula 2.

[0036] As a further improvement of the present invention, including but not limited to, obtaining a compound of formula 2 by step 1, wherein the purity of the compound of formula 2 is ≥ 90%, the content of the isomer impurity compound of formula 2A is less than 10%, and does not contain the following impurities of formula 2B and 2C:

[0037]

[0038] The product 3,7-diamino-2,8-dimethyldibenzothiophene sulfone (including 2,6-dimethyl isomer) obtained by the preparation and separation method of the present invention has a main component ≥90% (H-NMR and HPLC), which is much larger than 70% on the market and 84% in the prior art; in addition, the residual solvent is low and the metal ion content is better than that in the prior art. The synthesis and application of this product are particularly beneficial to the improvement of the oxygen selectivity of the downstream product polyimide film in gas separation.

[0039] Beneficial technical effects of the present invention:

[0040] 1. The preparation method of the compound of formula 2 of the present invention has the advantages of high selectivity, high purity, few impurity types and low impurity content compared with the prior art method. Obtaining a high-purity compound of formula 2 is beneficial to the control of the next reaction and the production of subsequent products.

[0041] 2. After the inventors added a Lewis acid catalyst to the reaction, the selectivity and purity were significantly improved. Compared with the method of CN118005604A, the purity of the compound of formula 2 was increased by at least 7.7%, and no excessive sulfonated by-product shown in formula 2C was found in the method of the present invention. The method of the present invention has good reproducibility, simple post-processing, and is suitable for industrial production.

[0042] 3. The method of the present invention reduces the molar amount of concentrated sulfuric acid in the prior art by more than 25 times, and reduces the molar amount of the sulfonation reagent by more than 3 times, thereby saving materials, reducing costs, simplifying post-processing, and achieving higher purity and better selectivity than the prior art.

[0043] 4. More importantly, the present invention, through creative labor, finally determines the appropriate reaction temperature, time, solvent, etc., as well as the important process parameters of the creative Lewis acid catalyst and the sulfonating agent, such as the selection of aluminum chloride as the Lewis acid catalyst, the selection of chlorosulfonic acid as the sulfonating agent, and the selection of the equivalents of the Lewis acid catalyst and the sulfonating agent, etc. On this basis, a creative purification technology is further adopted for purification, so that the present invention provides a compound of formula 2 that can be prepared under a variety of different conditions.

[0044] 5. The method of the present invention not only has a low content of the isomeric impurity compound of formula 2A, but also does not contain the impurities of the following formulas 2B and 2C:

[0045]

[0046] 6. The reaction conditions of Example 3 and Example 9 in CN118005604A are consistent. Example 3 is kilogram-level with a yield of 63%, and Example 9 is gram-level with a yield of 94%. The method of this patent has poor reproducibility and is not conducive to amplification. The yields of the embodiments of the present invention are both above 90% at the hundred-gram level and the kilogram level, with good reproducibility. The route proposed by the present invention is simple to operate, free of harsh reaction conditions, and has high yield, high purity, simple operation, low energy consumption, good selectivity, good reproducibility, effective cost reduction, and is conducive to industrial amplification production, so it has good market value and far-reaching practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 HPLC spectrum of the compound of formula 2 prepared in Example 1 of the present invention;

[0048] Figure 2 HPLC spectrum of the compound of formula 2 prepared in Example 2 of the present invention;

[0049] Figure 3 HPLC spectrum of the compound of formula 2 prepared in Example 3 of the present invention;

[0050] Figure 4 HPLC spectrum of the compound of formula 2 prepared in Example 4 of the present invention;

[0051] Figure 5 HPLC spectrum of the compound of formula 2 obtained in Example 5 of the present invention;

[0052] Figure 6 HPLC spectrum of the compound of formula 2 obtained in Example 6 of the present invention;

[0053] Figure 7 HPLC spectrum of the compound of formula 2 prepared in Example 7 of the present invention;

[0054] Figure 8 HPLC spectrum of the compound of formula 2 prepared in comparative example 1 of the present invention;

[0055] Fig. 9 This is the H-NMR spectrum of the compound of formula 2 prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0056] The present invention will be further described in conjunction with specific examples. It should be understood that the following detailed description of the technical solution of the present invention with examples will help to further understand the advantages and effects of the technical solution of the present invention, and the examples do not limit the protection scope of the present invention, which is determined by the claims.

[0057] The experimental methods in the following examples without specifying specific conditions are usually carried out under conventional conditions or conditions recommended by the manufacturers.

[0058] Unless otherwise specified, the raw materials and reagents used in the examples are commercially available.

[0059] Unless otherwise specified, the reagents were used directly without purification. All solvents were purchased from commercial suppliers and used without further treatment.

[0060] Embodiment 1:

[0061]

[0062] 3,3'-dimethylbenzidine (1kg, 4.7mol) was added to the reaction kettle, and 1,2-dichloroethane (3L), concentrated sulfuric acid (230g, 2.3mol, 0.5eq), aluminum chloride (63g, 0.47mol, 0.1eq), and chlorosulfonic acid (1090g, 9.4mol, 2eq) were added. The temperature was raised to 60°C and the reaction was kept warm for 13h. After the reaction, the reaction solution was slowly poured into ice water and filtered. The pH of the filter cake was adjusted to alkaline with sodium hydroxide aqueous solution, filtered, and the filter cake was washed with water and dried to obtain the compound of formula 2 (1228g, yield 95%, purity 98.99%, containing 0.58% isomer compound 2A). The HPLC spectrum is shown as follows Figure 1 As shown, the H-NMR spectrum is as Fig. 9 shown.

[0063] Embodiment 2:

[0064]

[0065] 3,3'-Dimethylbenzidine (1000g, 4.7mol) was added to the reactor, and 1,2-dichloroethane (3L), concentrated sulfuric acid (230g, 2.3mol, 0.5eq), aluminum chloride (63g, 0.47mol, 0.1eq), and chlorosulfonic acid (1090g, 9.4mol) were added. The temperature was raised to 80°C and kept for 9h. After the reaction, the reaction solution was slowly poured into ice water and filtered. The filter cake was adjusted to alkaline pH with sodium hydroxide aqueous solution, filtered, washed with water, and dried to obtain the compound of formula 2 (1202g, yield 93%, purity 88.54%, containing 10.96% of isomer compound 2A). The HPLC spectrum is shown as follows: Figure 2 shown.

[0066] Embodiment 3:

[0067]

[0068] 3,3'-Dimethylbenzidine (1000g, 4.7mol) was added to the reaction kettle, and 1,2-dichloroethane (3L), concentrated sulfuric acid (230g, 2.3mol), aluminum chloride (63g, 0.47mol), chlorosulfonic acid (1090g, 9.4mol) were added, and the temperature was raised to 45°C. The reaction was kept warm for 48h. After the reaction, the reaction solution was slowly poured into ice water and filtered. The filter cake was adjusted to alkaline pH with sodium hydroxide aqueous solution, filtered, and the filter cake was washed with water and dried to obtain the compound of formula 2 (1215g, yield 94%, purity 90.65%, containing 8.78% of isomer compound 2A). The HPLC spectrum is shown as follows: Figure 3 shown.

[0069] The reaction time, yield, purity and impurity content of Examples 1 to 3 are shown in Table 1 below;

[0070] Table 1:

[0071] Example Reaction temperature Yield Purity of compound of formula 2 Content of isomeric impurity compound 2A 1 60℃ 95％ 98.99％ 0.58％ 2 80℃ 93％ 88.54％ 10.96％ 3 45℃ 94％ 90.65％ 8.78％

[0072] As shown in Table 1, the reaction of the present invention can be carried out at 45°C to 80°C, and the selectivity of the compound of formula 2 is significantly improved when the reaction is carried out at 60°C compared with other temperatures, and is increased by 10.45% and 8.34% compared with the temperatures of Examples 2 and 3, respectively, which significantly reduces the generation of isomeric impurities.

[0073] Embodiment 4:

[0074]

[0075] 3,3'-Dimethylbenzidine (100 g, 0.47 mol) was added to the reaction kettle, and 1,2-dichloroethane (0.3 L), concentrated sulfuric acid (23 g, 0.23 mol), aluminum chloride (6.3 g, 47.3 mmol, 0.1 eq), and chlorosulfonic acid (140 g, 1.2 mol, 2.5 eq) were added. The temperature was raised to 60°C and the reaction was kept warm for 13 hours. After the reaction, the reaction solution was slowly poured into ice water and filtered. The filter cake was adjusted to alkaline pH with sodium hydroxide aqueous solution, filtered, washed with water, and dried to obtain the compound of formula 2 (119.7 g, yield 92.6%, purity 87.09%, containing 12.33% of isomer compound 2A). The HPLC spectrum is shown as follows: Figure 4 shown.

[0076] Embodiment 5:

[0077]

[0078] 3,3'-dimethylbenzidine (100g, 0.47mol) was added to the reactor, 0.3L 1,2-dichloroethane was added, concentrated sulfuric acid (23g, 0.23mol), aluminum chloride (3.09g, 23.2mmol, 0.05eq), chlorosulfonic acid (109g, 0.94mol) were added, the temperature was raised to 60°C, and the reaction was kept warm for 48h. After the reaction, the reaction solution was slowly poured into ice water and filtered. The filter cake was adjusted to alkaline pH with sodium hydroxide aqueous solution, filtered, and the filter cake was washed with water and dried to obtain the compound of formula 2 (120g, yield 93%, purity 91.84%, containing 7.70% of isomer compound 2A), HPLC spectrum as shown Figure 5 shown.

[0079] Embodiment 6:

[0080]

[0081] 3,3'-dimethylbenzidine (100 g, 0.47 mol) was added to the reactor, and 1,2-dichloroethane (0.3 L) was added, and concentrated sulfuric acid (23 g, 0.23 mol), aluminum chloride (3.09 g, 0.024 mol), and 30% fuming sulfuric acid (250 g, 764.7 mmol, 1.6 eq) were added. The temperature was raised to 60°C and the reaction was kept warm for 13 hours. After the reaction, the reaction solution was slowly poured into ice water and filtered. The filter cake was adjusted to alkaline pH with sodium hydroxide aqueous solution, filtered, and the filter cake was washed with water and dried to obtain the compound of formula 2 (119.6 g, yield 92.6%, purity 92.66%, containing 6.98% of isomer compound 2A). The HPLC spectrum is shown as follows: Figure 6 shown.

[0082] Embodiment 7:

[0083]

[0084] 3,3'-dimethylbenzidine (100g, 0.47mol) was added to the reaction kettle, and 1,2-dichloroethane (0.3L), concentrated sulfuric acid (23g, 0.235mol), zinc dichloride (6.4g, 0.047mol, 0.1eq) were added, and chlorosulfonic acid (109g, 0.94mol) was added. The temperature was raised to 60°C and the reaction was kept warm for 18h. After the reaction, the reaction solution was slowly poured into ice water and filtered. The pH of the filter cake was adjusted to alkaline with sodium hydroxide aqueous solution, filtered, and the filter cake was washed with water and dried to obtain the compound of formula 2 (120g, yield 92%, purity 91.69%, containing 8.02% of isomer compound 2A). The HPLC spectrum is shown as follows: Figure 7 shown.

[0085] Comparative Example 1:

[0086]

[0087] 3,3'-dimethylbenzidine (100 g, 0.47 mol) was added to the reaction kettle, 1,2-dichloroethane (3 L) was added, concentrated sulfuric acid (23 g, 0.23 mol) was added, chlorosulfonic acid (109 g, 0.94 mol) was added, the temperature was raised to 80°C, and the reaction was kept warm for 15 hours. After the reaction was completed, the reaction solution was slowly poured into ice water and filtered. The filter cake was adjusted to alkaline pH with sodium hydroxide aqueous solution, filtered, the filter cake was washed with water, and dried to obtain the compound of formula 2 (110 g, yield 85%, purity 72.89%, containing 25.11% of isomer compound 2A), HPLC spectrum as shown Figure 8 shown.

[0088] The Lewis acid catalysts of Examples 1, 9 and Comparative Example 1 and their yields, purities and impurity contents are shown in Table 2 below;

[0089] Table 2:

[0090] Lewis acid catalyst Yield Purity of compound of formula 2 Content of isomeric impurity compound 2A Example 1 Aluminum chloride 95％ 98.99％ 0.58％ Example 9 Zinc Dichloride 92％ 91.69％ 8.02％ Comparative Example 1 N / A 85％ 72.89％ 25.11％

[0091] As shown in Table 2, compared with the preparation method of the prior art CN118005604A, the purity of the compound of formula 2 is increased from 84% to 91.69% to 98.99%, which is increased by more than 7.7% by adding Lewis acid catalyst aluminum trichloride or zinc dichloride in the present invention; compared with comparative example 1, the purity of the compound of formula 2 obtained by adding Lewis acid catalyst is increased by more than 18.8%, and the yield is increased by more than 7%; the method of the present invention significantly improves the purity of the compound of formula 2 and the selectivity of ring closure, and the present invention has good reproducibility, simple post-treatment, and is suitable for industrial production.

[0092] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for preparing high-purity 3,7-diamino-2,8-dimethyldibenzothiophene sulfone, characterized in that: The reaction is as follows: The following steps are involved: Step 1: In an organic solvent, the compound of formula 1 reacts with concentrated sulfuric acid and a sulfonating agent in the presence of a Lewis acid catalyst to obtain a compound of formula 2.

2. The preparation method according to claim 1, characterized in that: The Lewis acid catalyst in step 1 is selected from one of aluminum trichloride, zinc dichloride, ferric trichloride and titanium tetrachloride, preferably one of aluminum trichloride and zinc dichloride, more preferably aluminum trichloride.

3. The preparation method according to claim 1 or 2, characterized in that: The sulfonating reagent in step 1 is selected from oleum and chlorosulfonic acid, preferably chlorosulfonic acid.

4. The preparation method according to claim 1 or 2, characterized in that: In the step 1, the molar ratio of the compound of formula 1 to concentrated sulfuric acid is 1:(0.3-1), preferably 1:(0.3-0.8).

5. The preparation method according to claim 1 or 2, characterized in that: In the step 1, the molar ratio of the compound of formula 1 to the sulfonation reagent and the Lewis acid catalyst is 1:(1.5-3):(0.02-0.5), preferably 1:(1.5-2.5):(0.08-0.2).

6. The preparation method according to claim 1 or 2, characterized in that: The organic solvent in step 1 is selected from halogenated alkanes; preferably, the halogenated alkanes are selected from one or more of 1,2-dichloroethane, chloroform, and carbon tetrachloride, preferably 1,2-dichloroethane.

7. The preparation method according to claim 1 or 2, characterized in that: The reaction temperature of step 1 is 45-80°C, preferably 50-70°C, more preferably 60°C.

8. The preparation method according to claim 1 or 2, characterized in that: The reaction time of step 1 is 5 to 52 hours, preferably 5 to 48 hours.

9. The preparation method according to claim 1 or 2, characterized in that: The preparation method of step 1 satisfies one or more of the following conditions: (1) In step 1, the molar ratio of the compound of formula 1 to concentrated sulfuric acid is 1:0.5; (2) In step 1, the molar ratio of the compound of formula 1 to the sulfonating agent and the Lewis acid catalyst is 1:2:0.1; (3) The compound of formula 2 is obtained by step 1, wherein the purity of the compound of formula 2 is ≥ 90%, the content of the isomeric impurity compound of formula 2A is less than 10%, and the compound does not contain the impurities of the following formulas 2B and 2C:

10. The preparation method according to claim 1 or 2, characterized in that: After the reaction of step 1 is completed, the reaction solution is poured into ice water and filtered. The pH of the filter cake is adjusted to alkaline with sodium hydroxide, filtered, and the filter cake is washed with water and dried to obtain the compound of formula 2.

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