Preparation method of a negative monomer liquid crystal containing an ethane bridge bond

Abstract

The present invention relates to the field of liquid crystal materials, and specifically relates to a preparation method of a negative monomer liquid crystal containing an ethane bridge bond, as follows: S1. Under nitrogen protection, it undergoes Friedel-Crafts acylation with p-bromophenylacetyl chloride under the catalysis of an acid such as aluminum trichloride to obtain an intermediate; S2. Under nitrogen protection, it undergoes Suzuki coupling with to obtain an intermediate; S3. The is reduced to the target compound by Huang Minglong reduction. The preparation method has a short process route, cheap and easily available raw materials, avoids dangerous processes such as Grignard reaction and hydrogenation reaction, has a simple reaction type, and ensures the safe and smooth progress of production.

Description

Technical Field

[0001] The invention relates to the field of liquid crystal materials, and in particular to a method for preparing negative monomer liquid crystal containing ethane bridge bonds. Background Art

[0002] At present, TFT-LCD (thin-film transistor liquid crystal display) technology is developing rapidly, and the large-screen wide-viewing angle technology of various manufacturers is basically divided into two camps: VA (vertical alignment) mode camp and IPS (in-plane switching) mode camp. On this basis, various manufacturers have developed their own unique wide-viewing angle technology, which not only obtains a wide viewing angle (horizontally / vertically reaches or exceeds 170°C), but also has great progress in response time and color. The two technologies continue to learn from each other, compete, and improve together. The VA (vertical alignment) mode camp is further divided into: MVA, PVA, ASV, PSVA and ASM technologies; the IPS (in-plane switching) mode camp is further divided into: FFS, SFT, AFFS and other technologies.

[0003] The larger the size of the TV, the more the VA mode technology shows its technical advantages. The liquid crystal molecules used in the VA display mode are negative liquid crystal molecules. In today's world, among the two major display modes of IPS and VA used for large-screen TV displays, the VA display mode is more popular because of its high contrast, wide viewing angle range and fast response speed. Among them, the VA improved display mode is MERCK's PSVA technology, which has a better effect and therefore occupies a high proportion in large-screen displays. Because this PSVA display mode requires UV irradiation to cure PI after crystal filling, in order to achieve the fast response and wide viewing angle of VA-TFT-LCD, it is necessary to select an ethane bridge negative monomer liquid crystal that meets this requirement. Based on the particularity of its own molecular structure, when it exists as a component of a mixed liquid crystal, the ethane bridge negative monomer liquid crystal can reduce the viscosity and optical anisotropy of the mixed liquid crystal, broaden the use temperature range of the liquid crystal phase, and also has good optical stability and chemical stability. It plays an important role in the mixed crystal formula, and the content reaches between 6% and 10%. DIC Corporation's patent CN111417700B and patent application CN109643037A use this type of monomer liquid crystal in mixed liquid crystal formulas, and JNC Corporation's patent application JP200616072A also uses this type of monomer in mixed crystal formulas. With the widespread use and popularity of TV-LCDs, the global demand for TV-LCDs is growing year by year, and the liquid crystal materials used will increase significantly. Therefore, the demand for ethane bridged negative monomer liquid crystal compounds will inevitably increase along with the growth of liquid crystal materials.

[0004] Based on the importance of ethane-bridged negative monomer liquid crystals in liquid crystal materials for VA-mode TFT-LCDs, according to the development trends in the liquid crystal material industry and after consulting a large amount of information, there are currently several methods for preparing such monomer liquid crystals, but they all have many problems.

[0005] Method 1: DIC Corporation, CN105295947A, uses propylphenylacetylene as the starting material, and the preparation method is as follows:

[0006] ;

[0007] Existing technical problems: ① The raw materials are expensive and not easily available; ② The hydrogenation reaction belongs to a dangerous process.

[0008] In the literature doi: 10.3788 / YJYXS20173210.0794, it is synthesized by Witting reaction, coupling, and hydrogenation of propylbenzaldehyde and the triphenylphosphonium salt of p-bromobenzyl bromide, and the preparation method is as follows:

[0009] ;

[0010] Existing technical problems: ① The Grignard reaction belongs to a dangerous process; ② The raw materials are expensive and not easily available; ③ The hydrogenation reaction belongs to a dangerous process; ④ The synthesis route is long, the cost is high, and the efficiency is low. Summary of the Invention

[0011] In view of the deficiencies in the existing technology, the present invention provides a method for preparing an ethane-bridged negative monomer liquid crystal, and the preparation method is as follows:

[0012] ;

[0013] S1. Preparation of Intermediate I

[0014] Under an inert gas, in the first solvent system, after Reactant I and an acidic catalyst are stirred and dispersed, p-bromophenylacetyl chloride is added dropwise for a Friedel-Crafts acylation reaction. After the reaction is completed, Intermediate I is obtained through post-treatment;

[0015] S2. Preparation of Intermediate II

[0016] Under an inert gas, in the second solvent system, in the presence of a base and a Pd-based catalyst, Intermediate I and Reactant II are heated under reflux. After the reaction is completed, Intermediate II is obtained through post-treatment;

[0017] S3. Preparation of the ethane-bridged negative monomer liquid crystal

[0018] Under an inert gas, in the third solvent system, under strong base conditions, Intermediate II undergoes a Huang Minlong reduction reaction to generate the target liquid crystal compound;

[0019] Among them, R 1 is any one of C1-C6 straight-chain alkyl groups or alkoxy groups of C1-C6 straight-chain alkanes; R 2 is any one of C1-C6 straight-chain alkyl groups or alkoxy groups of C1-C6 straight-chain alkanes; R 3 is a C1-C6 straight-chain alkyl group.

[0020] Furthermore, in step S1, the molar ratio of bromophenylacetyl chloride, reactant I, and the acidic catalyst is 1:(1.0-2):(1-1.5).

[0021] Furthermore, in step S1, the temperature of the Friedel-Crafts acylation reaction is 10-20°C.

[0022] Furthermore, in step S1, the acidic catalyst is any one of aluminum trichloride, zinc chloride, or iron trichloride; the first solvent is any one of dichloromethane or dichloroethane.

[0023] Furthermore, in step S2, the molar ratio of intermediate I, reactant II, the Pd-based catalyst, and the base is 1:(0.9-1.2):(0.1‰-2‰):(2-3).

[0024] Furthermore, in step S2, the Pd-based catalyst is PdCl 2 (PPh 3 ) 2 , tetrakis(triphenyl)phosphine palladium, Pd-132, Pd(dppf)Cl 2 or any one of them.

[0025] Furthermore, in step S2, the second solvent system is any one of an aqueous system, a toluene / ethanol / water system, a toluene / water system, or a tetrahydrofuran / water system.

[0026] Furthermore, in step S2, the base is any one of potassium carbonate, sodium carbonate, potassium tert-butoxide, sodium tert-butoxide, tripotassium phosphate, or potassium acetate.

[0027] Furthermore, in step S3, the molar ratio of intermediate II, hydrazine hydrate, and the strong base used is 1:(1.5-2.5):(1.5-3).

[0028] Furthermore, in step S3, the strong base is one of sodium hydroxide or potassium hydroxide; the third solvent is any one of diethylene glycol or triethylene glycol.

[0029] The beneficial effects of the present invention are:

[0030] (1) In the preparation method of the present invention, a cheap and easily available intermediate, p-bromophenylacetyl chloride, is used as the starting material. Through Friedel-Crafts acylation reaction, Suzuki coupling reaction and Huang Minglong reduction reaction, the method is simple, easy to operate and has good stability. The purity of the final product reaches over 99%.

[0031] (2) The preparation method of the present invention avoids using dangerous processes such as Grignard reaction and hydrogenation reaction, ensuring the safe and smooth progress of production.

[0032] (3) The preparation method of the present invention has only three-step reactions, with a short process route, which shortens the production cycle, improves production efficiency, thus greatly reducing costs. Especially in the second reaction, it avoids using a large amount of organic solvents, is green and environmentally friendly, safe and energy-saving, and has a relatively low economic cost; the process conditions are mild, the post-treatment and purification methods are simple, meeting the requirements of green environmental protection and safe production, and facilitating industrial scale-up production. Description of the Drawings

[0033] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0034] Figure 1 GC-MS spectrum of compound A prepared in Example 1;

[0035] Figure 2 Gas chromatogram of compound A prepared in Example 1;

[0036] Figure 3 GC-MS spectrum of compound B prepared in Example 2;

[0037] Figure 4 Gas chromatogram of compound B prepared in Example 2. Detailed Description of the Embodiments

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments not specified by the manufacturer are all conventional products that can be obtained through commercial purchase.

[0039] The present invention provides a method for preparing a negative monomer liquid crystal containing an ethane bridge bond, and the flow chart is as follows:

[0040] ;

[0041] It includes the following steps:

[0042] S1. Prepare intermediate I

[0043] Under an inert gas, in the first solvent system, after reactant I and an acidic catalyst are stirred and dispersed, p-bromophenylacetyl chloride is added dropwise for a Friedel-Crafts acylation reaction. After the reaction is completed, intermediate I is obtained through post-treatment;

[0044] S2. Prepare intermediate II

[0045] Under an inert gas, in the second solvent system, under the action of a base and a Pd-based catalyst, intermediate I and reactant II are subjected to a heating reflux reaction. After the reaction is completed, intermediate II is obtained through post-treatment;

[0046] S3. Preparation of the negative monomer liquid crystal containing an ethane bridge bond

[0047] Under an inert gas, in the third solvent system, under strong base conditions, intermediate II is generated through a Huang-Minlon reduction reaction to obtain the target liquid crystal compound;

[0048] Among them, R 1 is any one of a C1-C6 straight-chain alkyl group or or an alkoxy group of a C1-C6 straight-chain alkane; R 2 is any one of a C1-C6 straight-chain alkyl group or an alkoxy group of a C1-C6 straight-chain alkane; R 3 is a C1-C6 straight-chain alkyl group.

[0049] It should be noted that the inert gas described in this article refers to a substance that does not react with the relevant substances in this experiment under experimental conditions, such as nitrogen, helium, etc. In the following examples, only nitrogen is taken as an example.

[0050] In the example of step S1 in this article, when adding p-bromophenylacetyl chloride, a mixture of p-bromophenylacetyl chloride and the first solvent is added dropwise, but p-bromophenylacetyl chloride can also be directly added dropwise; the post-treatment after the reaction in step S1 specifically may refer to adding a dilute hydrochloric acid solution for hydrolysis, standing, separating the liquid, washing the organic phase with saturated brine until neutral, concentrating under reduced pressure until there is no solvent, recrystallizing with toluene, and drying to obtain the required intermediate product; it can also be other conventional post-treatment processes.

[0051] In step S2, in the embodiments of the present invention, tetrabutylammonium bromide is added as a phase transfer catalyst, and other phase transfer catalysts can also be used; the post-treatment after the reaction is specifically to cool down the system, then add toluene for extraction, liquid separation, wash the organic phase with water until neutral, dry with anhydrous magnesium sulfate, pass through a silica gel column, concentrate under reduced pressure until there is no solvent, and then recrystallize and dry with a toluene-ethanol mixed solution to obtain a white crystal, which is the required intermediate product; it can also be other conventional post-treatment processes.

[0052] In step S3, after the raw materials are added, heat up to reflux for heat preservation reaction, install a water separation device, distill off the low-boiling substances at atmospheric pressure until the internal temperature of the system rises to 200 - 210 °C, and carry out heat preservation reaction; after the reaction is completed, cool down, add toluene to the system for extraction, liquid separation, separate out diethylene glycol, wash the organic phase twice with water first, then neutralize with a dilute acid solution, and then wash with water until neutral, concentrate under reduced pressure until there is no solvent, recrystallize with a toluene and ethanol mixed solution, and dry to obtain the target product; the post-treatment process in this step can also adopt other conventional post-treatment processes.

[0053] The preparation method of the present invention uses the inexpensive and easily available intermediate p-bromophenylacetyl chloride as the starting material, and through Friedel-Crafts acylation reaction, Suzuki coupling reaction and Huang Minglong reduction reaction, the method is simple, easy to operate, and has good stability, and the purity of the final product can reach more than 99.9%.

[0054] The preparation method described in the present invention avoids using dangerous processes such as Grignard reaction and hydrogenation reaction, ensuring the safe and smooth progress of production.

[0055] The preparation method described in the present invention has only three-step reactions, with a short process route, shortening the production cycle, improving production efficiency, and thus significantly reducing costs; the process conditions are mild, the post-treatment and purification methods are simple, meeting the requirements of green environmental protection and safe production, and facilitating industrial scale-up production.

[0056] Preferably, in step S1, the molar ratio of p-bromophenylacetyl chloride, reactant I and acidic catalyst is 1:(1.0 - 2):(1 - 1.5).

[0057] Specifically, in step S1, the temperature of the Friedel-Crafts acylation reaction is 10 - 20 °C.

[0058] Specifically, in step S1, the acidic catalyst is any one of aluminum trichloride, zinc chloride or ferric trichloride; the first solvent is any one of dichloromethane or dichloroethane.

[0059] Specifically, in step S2, the molar ratio of intermediate I, reactant II, Pd-based catalyst and base is 1:(0.9 - 1.2):(0.1‰ - 2‰):(2 - 3).

[0060] Specifically, in step S2, the Pd-based catalyst is PdCl 2 (PPh 3 ) 2 , tetrakis(triphenylphosphine)palladium, Pd-132, Pd(dppf)Cl 2 , or any one of them.

[0061] Specifically, in step S2, the second solvent system is any one of a water system, a toluene / ethanol / water system, a toluene / water system, and a tetrahydrofuran / water system.

[0062] Preferably, in step S2, the base is any one of potassium carbonate, sodium carbonate, potassium tert-butoxide, sodium tert-butoxide, tripotassium phosphate, and potassium acetate.

[0063] Specifically, in step S3, the molar ratio of intermediate II, hydrazine hydrate, and strong base used is 1:(1.5 - 2.5):(1.5 - 3).

[0064] Preferably, in step S3, the strong base is one of sodium hydroxide or potassium hydroxide; the third solvent is any one of diethylene glycol and triethylene glycol.

[0065] To further illustrate the present invention, the following examples are used to describe in detail a preparation method of a negative monomer liquid crystal containing an ethane bridge bond provided by the present invention, but they should not be construed as limiting the protection scope of the present invention.

[0066] The reagents used in the following examples are all commercially available reagents, and the specific CAS numbers are as follows:

[0067]

[0068] Example 1

[0069] The preparation process of the target liquid crystal compound A is shown in the following formula: ;

[0070] The specific preparation method is as follows:

[0071] S1. Preparation of intermediate I-A:

[0072] Under nitrogen protection, 720 g of dichloroethane, 276 g (3 mol) of reactant I-A, and AlCl 3293.7 g (2.2 mol), keep the temperature at 10 - 20 °C and stir for 2 h; then keep the temperature at 10 - 20 °C, dropwise add a dichloroethane solution of p-bromophenylacetyl chloride (467 g, i.e., 2 mol of p-bromophenylacetyl chloride and 360 g of dichloroethane), after dropping, keep the temperature and react for 3 - 4 h. After the reaction is completed, dropwise add dilute hydrochloric acid solution for hydrolysis, let it stand, separate the layers, wash the organic phase with saturated brine until neutral, concentrate under reduced pressure until there is no solvent, recrystallize with toluene and dry to obtain 530 g (1.83 mol) of intermediate Ⅰ-A, with a yield of 91.6% and a product purity of GC > 96%.

[0073] S2. Preparation of intermediate Ⅱ-A:

[0074] Under nitrogen protection, add the above-synthesized intermediate Ⅰ-A 361 g (1.25 mol), reactant Ⅱ-A 252.5 g (1.25 mol), tetrabutylammonium bromide 121 g (0.375 mol), potassium carbonate 345 g (2.5 mol), water 1800 g into a three-necked flask equipped with a stirrer and a condenser in sequence, and finally add 89 mg of Pd-132, heat up to reflux and react for 3 h; after the reaction is completed, cool down the system, then add toluene for extraction, separate the layers, wash the organic phase until neutral, dry with anhydrous magnesium sulfate, pass through a silica gel column, concentrate under reduced pressure until there is no solvent, and then recrystallize and dry with a toluene-ethanol mixed solution to obtain intermediate Ⅱ-A, 387 g (1.06 mol) of white crystals, with a yield of 84.5% and a product purity of GC > 99%.

[0075] S3. Preparation of the target liquid crystal compound A:

[0076] Under nitrogen protection, add the above-synthesized intermediate Ⅱ-A 83 g (0.5 mol), diethylene glycol 550 g, potassium hydroxide 70 g (1.25 mol) into a three-necked flask equipped with a stirrer and a condenser, stir and dropwise add hydrazine hydrate 50 g (1 mol), after dropping, heat up to reflux and keep the temperature and react for 1.5 h; install a water separation device, distill off the low-boiling substances under normal pressure until the internal temperature of the system rises to 200 - 210 °C, and keep the temperature and react for 1.5 h. After the reaction is completed, cool down, add toluene for extraction to the system, separate the layers, separate out diethylene glycol, wash the organic phase twice with water first, then neutralize with dilute acid solution, and then wash until neutral, concentrate under reduced pressure until there is no solvent, recrystallize with a toluene-ethanol mixed solution and dry to obtain 138 g (0.39 mol) of the target liquid crystal compound A, with a yield of 78.4% and a product purity of GC > 99.9%.

[0077] Example 2

[0078] The preparation process of the target liquid crystal compound B is shown in the following formula: ;

[0079] The specific preparation method is as follows:

[0080] S1. Preparation of Intermediate Ⅰ-B:

[0081] Under nitrogen protection, add 721 g of dichloroethane, 264.5 g (2.2 mol) of Reactant Ⅰ-B, and 320 g (2.4 mol) of AlCl 3 3 to a three-necked flask equipped with a stirrer and a condenser. Control the temperature at 10 - 20 °C and stir for 1 h; then control the temperature at 10 - 20 °C and dropwise add a dichloroethane solution of p-bromophenylacetyl chloride (467 g, i.e., 2 mol of p-bromophenylacetyl chloride and 309 g of dichloroethane). After dropping, keep the temperature and react for 3 - 4 h. After the reaction is completed, dropwise add dilute hydrochloric acid solution for hydrolysis, let it stand, separate the layers, wash the organic phase with saturated brine until neutral, concentrate under reduced pressure until there is no solvent, recrystallize with toluene and dry to obtain 546 g (1.72 mol) of Intermediate Ⅰ-B, with a yield of 86% and a product purity of GC > 96%.

[0082] S2. Preparation of Intermediate Ⅱ-B:

[0083] Under nitrogen protection, add 317 g (1 mol) of the above-synthesized Intermediate Ⅰ-B, 202 g (1 mol) of Reactant Ⅱ-B, 96.6 g (0.3 mol) of tetrabutylammonium bromide, 276 g (2 mol) of potassium carbonate, 1900 g of water, and finally 71 mg of Pd-132 to a three-necked flask equipped with a stirrer and a condenser. Heat up to reflux and react for 3 h; after the reaction is completed, cool down the system, then add toluene for extraction, separate the layers, wash the organic phase until neutral, dry with anhydrous magnesium sulfate, pass through a silica gel column, concentrate under reduced pressure until there is no solvent, and then recrystallize with a toluene-ethanol mixed solution and dry to obtain Intermediate Ⅱ-B as 327 g (0.83 mol) of white crystals, with a yield of 82.9% and a product purity of GC > 99%.

[0084] S3. Preparation of Target Liquid Crystal Compound B:

[0085] Under nitrogen protection, add 200 g (0.5 mol) of the above-synthesized Intermediate Ⅱ-B, 460 g of diethylene glycol, and 61 g (1.53 mol) of sodium hydroxide to a three-necked flask equipped with a stirrer and a condenser. Stir and dropwise add 50.7 g (1 mol) of hydrazine hydrate. After dropping, heat up to reflux and keep the temperature for 1.5 h; set up a water separation device, distill off low-boiling substances at atmospheric pressure until the internal temperature of the system rises to 200 - 210 °C, and keep the temperature for 1.5 h. After the reaction is completed, cool down, add toluene for extraction to the system, separate the layers, separate out diethylene glycol, wash the organic phase twice with water, then neutralize with dilute acid solution, and then wash until neutral, concentrate under reduced pressure until there is no solvent, recrystallize with a toluene-ethanol mixed solution, and dry to obtain 145 g (0.38 mol) of Target Liquid Crystal Compound B, with a yield of 75.2% and a product purity of GC > 99.9%.

[0086] Example 3

[0087] The preparation process of the target liquid crystal compound C is shown as follows:

[0088] ;

[0089] The specific preparation method is as follows:

[0090] S1. Preparation of intermediate Ⅰ-C:

[0091] Under nitrogen protection, add 700 g of dichloroethane, 245 g (2 mol) of reactant Ⅰ-C, and 272.6 g (2 mol) of ZnCl 2 2 into a three-necked flask equipped with a stirrer and a condenser, control the temperature at 10 - 20 °C and stir for 2 h; then control the temperature at 10 - 20 °C, and dropwise add a dichloroethane solution of p-bromophenylacetyl chloride (467 g, i.e., 2 mol of p-bromophenylacetyl chloride and 350 g of dichloroethane). After dropping, keep the temperature for reaction for 3 - 4 h. After the reaction is completed, dropwise add dilute hydrochloric acid solution for hydrolysis, let it stand, separate the layers, wash the organic phase with saturated brine until neutral, concentrate under reduced pressure until there is no solvent, recrystallize with toluene and dry to obtain 498 g (1.56 mol) of intermediate Ⅰ-C, with a yield of 78% and a product purity of GC > 97%.

[0092] S2. Preparation of intermediate Ⅱ-C:

[0093] Under nitrogen protection, add 319.2 g (1 mol) of the above-synthesized intermediate Ⅰ-C, 206.3 g (1.2 mol) of reactant Ⅱ-C, 96.6 g (0.3 mol) of tetrabutylammonium bromide, 168 g (1.5 mol) of potassium tert-butoxide, 900 g of water, 1915 g of toluene into a three-necked flask equipped with a stirrer and a condenser, and finally add 89 mg of PdCl 2 (PPh 3 ) 2 , heat up to reflux and react for 5 h; after the reaction is completed, cool down the system, then add toluene for extraction, separate the layers, wash the organic phase until neutral, dry with anhydrous magnesium sulfate, pass through a silica gel column, concentrate under reduced pressure until there is no solvent, and then recrystallize and dry with a toluene-ethanol mixed solution to obtain 374 g (1.02 mol) of white crystals of intermediate Ⅱ-C, with a yield of 85% and a product purity of GC > 99%.

[0094] S3. Preparation of the target liquid crystal compound C:

[0095] Under nitrogen protection, add the above-synthesized intermediate Ⅱ-C 183 g (0.5 mol), diethylene glycol 550 g, and sodium hydroxide 60 g (1.5 mol) to a three-necked flask equipped with a stirrer and a condenser. While stirring, add hydrazine hydrate 37.6 g (0.75 mol) dropwise. After the addition is complete, heat up to reflux and keep the reaction at this temperature for 1.5 h. Set up a water separation device and distill off the low-boiling substances under atmospheric pressure until the internal temperature of the system rises to 200 - 210 °C, and keep the reaction at this temperature for 1.5 h. After the reaction is completed, cool down, add toluene to the system for extraction, separate the layers, separate out the diethylene glycol. The organic phase is washed with water twice first, then neutralized with a dilute acid solution, and then washed with water until neutral. Concentrate under reduced pressure until there is no solvent, recrystallize with a toluene and ethanol mixed solution, and dry to obtain the target liquid crystal compound C 141 g (0.4 mol), with a yield of 80% and a product purity of GC > 99.9%.

[0096] Example 4

[0097] The preparation process of the target liquid crystal compound D is shown in the following formula:

[0098] ;

[0099] The specific preparation method is as follows:

[0100] S1. Preparation of intermediate Ⅰ-D:

[0101] Under nitrogen protection, add 742 g of dichloroethane, 324 g (2 mol) of hexylbenzene, and FeCl 3 243.3 g (1.5 mol) to a three-necked flask equipped with a stirrer and a condenser. Control the temperature at 10 - 20 °C and keep stirring for 2 h; then control the temperature at 10 - 20 °C and add dropwise a dichloroethane solution of p-bromophenylacetyl chloride (233.5 g, i.e., 1 mol of p-bromophenylacetyl chloride and 317 g, i.e., 3.2 mol of dichloroethane). After the addition is complete, keep the reaction at this temperature for 3 - 4 h. After the reaction is completed, add a dilute hydrochloric acid solution for hydrolysis, let it stand, separate the layers, and wash the organic phase with saturated brine until neutral. Concentrate under reduced pressure until there is no solvent, recrystallize with toluene, and dry to obtain intermediate Ⅰ-D 287.4 g (0.8 mol), with a yield of 80% and a product purity of GC > 96%.

[0102] S2. Preparation of intermediate Ⅱ-D:

[0103] Under nitrogen protection, 287.4 g (0.8 mol) of the above-synthesized intermediate Ⅰ-D, 175.7 g (0.72 mol) of reactant Ⅱ-D, 122 g (0.38 mol) of tetrabutylammonium bromide, 509 g (2.4 mol) of tripotassium phosphate, 580 g of water, 2300 g of toluene, and 580 g of ethanol were successively added to a three-necked flask equipped with a stirrer and a condenser. Finally, 1.85 g of a Pd-based catalyst, tetrakis(triphenylphosphine)palladium, was added. The temperature was raised to reflux and the reaction was carried out for 3 h. After the reaction was completed, the system was cooled, and then toluene was added for extraction. After liquid separation, the organic phase was washed with water until neutral, dried over anhydrous magnesium sulfate, passed through a silica gel column, concentrated under reduced pressure until the solvent was removed, and then recrystallized and dried from a toluene-ethanol mixed solution to obtain 268.8 g (0.56 mol) of intermediate Ⅱ-D as white crystals, with a yield of 78% and a product purity of GC > 99%.

[0104] S3. Preparation of the target liquid crystal compound D:

[0105] Under nitrogen protection, 239.3 g (0.5 mol) of the above-synthesized intermediate Ⅱ-D, 500 g of triethylene glycol, and 30 g (0.75 mol) of sodium hydroxide were added to a three-necked flask equipped with a stirrer and a condenser. Hydrazine hydrate 62.5 g (1.25 mol) was added dropwise with stirring. After the addition was completed, the temperature was raised to reflux and the reaction was kept at this temperature for 1.5 h. A water-separating device was installed, and low-boiling substances were distilled off at atmospheric pressure until the internal temperature of the system rose to 200 - 210 °C, and the reaction was kept at this temperature for 1.5 h. After the reaction was completed, the system was cooled, toluene was added to the system for extraction, and liquid separation was carried out to separate out diethylene glycol. The organic phase was first washed twice with water, then neutralized with a dilute acid solution, and then washed with water until neutral. It was concentrated under reduced pressure until the solvent was removed, recrystallized from a toluene-ethanol mixed solution, and dried to obtain 186 g (0.4 mol) of the target liquid crystal compound D, with a yield of 80% and a product purity of GC > 99.9%.

[0106] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

[0107] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a negative monomer liquid crystal containing ethane bridge bonds, characterized in that: The preparation method is as follows: ； S1. Preparation of Intermediate I Under an inert gas, in a first solvent system, reactant I and an acidic catalyst are stirred and dispersed, and then p-bromophenylacetyl chloride is added dropwise to carry out Friedel-Crafts acylation reaction. After the reaction is completed, intermediate I is obtained by post-treatment; S2. Preparation of Intermediate II Under an inert gas, in a second solvent system, in the presence of a base and a Pd-based catalyst, the intermediate I and the reactant II are subjected to a heating reflux reaction, and after the reaction is completed, the intermediate II is obtained by post-treatment; S3. Preparation of Negative Monomer Liquid Crystals Containing Ethane Bridge Bonds Under an inert gas, in a third solvent system, under a strong base condition, intermediate II is subjected to a Huang Ming Long reduction reaction to obtain a target liquid crystal compound; Wherein R1 is C1-C6 straight chain alkyl or or any one of the alkoxy groups of C1-C6 straight-chain alkanes; R2 is any one of the alkoxy groups of C1-C6 straight-chain alkyl groups or C1-C6 straight-chain alkanes; R3 is a C1-C6 straight-chain alkyl group; The inert gas mentioned here refers to a substance that does not react with the relevant substances in this experiment under the experimental conditions; In step S1, the p-bromophenylacetyl chloride is added dropwise as a mixture of p-bromophenylacetyl chloride and the first solvent, or p-bromophenylacetyl chloride is directly added dropwise; the post-treatment after the reaction in step S1 specifically refers to adding a dilute hydrochloric acid solution for hydrolysis, standing, separating the liquids, washing the organic phase with saturated brine until neutral, concentrating under reduced pressure until there is no solvent, recrystallizing from toluene, and drying to obtain the desired intermediate product; In step S2, tetrabutylammonium bromide is added as a phase transfer catalyst, or other phase transfer catalysts are used; the post-treatment after the reaction is completed specifically refers to cooling the system, then adding toluene for extraction, separating the liquids, washing the organic phase with water until it is neutral, drying it with anhydrous magnesium sulfate, passing it through a silica gel column, and concentrating it under reduced pressure until there is no solvent, and then recrystallizing and drying the toluene-ethanol mixed solution to obtain white crystals, which are the desired intermediate product; In step S3, after the raw materials are added, the temperature is raised to reflux for heat preservation reaction, a water separation device is set up, and low boiling point substances are separated at normal pressure until the temperature in the system rises to 200-210°C, and the heat preservation reaction is carried out; after the reaction is completed, the temperature is lowered, toluene is added to the system for extraction, and the liquid is separated to separate diethylene glycol. The organic phase is first washed with water twice, then neutralized with a dilute acid solution, and then washed with water until neutral, and concentrated under reduced pressure until there is no solvent, and the toluene and ethanol mixture is recrystallized and dried to obtain the target product.

2. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S1, the molar ratio of p-bromophenylacetyl chloride, reactant I and acidic catalyst is 1:(1.0-2):(1-1.5).

3. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S1, the Friedel-Crafts acylation reaction temperature is 10-20°C.

4. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S1, the acidic catalyst is any one of aluminum trichloride, zinc chloride or ferric chloride; and the first solvent is any one of dichloromethane or dichloroethane.

5. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S2, the molar ratio of intermediate I, reactant II, Pd-based catalyst and base is 1:(0.9-1.2):(0.1‰-2‰):(2-3).

6. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S2, the Pd-based catalyst is any one of PdCl2(PPh3)2, tetrakis(triphenyl)phosphine palladium, Pd-132, and Pd(dppf)Cl2.

7. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S2, the second solvent system is any one of a water system, a toluene / ethanol / water system, a toluene / water system, and a tetrahydrofuran / water system.

8. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S2, the base is any one of potassium carbonate, sodium carbonate, potassium tert-butoxide, sodium tert-butoxide, tripotassium phosphate, and potassium acetate.

9. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S3, the molar ratio of intermediate II, hydrazine hydrate and strong base is 1:(1.5-2.5):(1.5-3).

10. The method for preparing a negative monomer liquid crystal containing ethane bridge bonds according to claim 1, characterized in that: In step S3, the strong base is one of sodium hydroxide and potassium hydroxide; the third solvent is any one of diethylene glycol and triethylene glycol.

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