A method for synthesizing 3,3'-dihydroxyazobenzene and its derived hyperbranched azobenzene

Abstract

The application belongs to the technical field of organic synthesis and polymer synthesis, and particularly relates to a synthesis method of 3,3'-dihydroxy azobenzene and hyperbranched azobenzene derived therefrom. The method uses 3-aminophenol as a starting material. Because phenol is unstable under oxidation conditions, the hydroxyl group is first protected by using a dimethyl tertiary butyl silyl group to obtain 3-(tertiary butyl dimethyl silyloxy) aniline. Then, under the catalysis of cuprous bromide, 3,3'-di(tertiary butyl dimethyl silyloxy) azobenzene is efficiently synthesized by using air as an oxidant. Finally, the silicon group protection of 3,3'-di(tertiary butyl dimethyl silyloxy) azobenzene is removed, and the target product with high purity is obtained through recrystallization purification. Then, the substitution reaction of 3,3'-dihydroxy azobenzene and N-Boc-bromoethylamine, the DBOC protection of the amino group, and the substitution reaction of the amino group and the active polybromide are performed to prepare hyperbranched azobenzene. The derivative has the advantages of novel structure and simple synthesis.

Description

Technical Field

[0001] This invention belongs to the field of organic synthesis and polymer synthesis technology, and particularly relates to a method for synthesizing 3,3′-dihydroxyazobenzene and its derived hyperbranched azobenzene. Background Technology

[0002] Aromatic azo compounds are an important class of compounds whose aromatic rings are linked by nitrogen-nitrogen double bonds (-N=N-). They are widely used as dyes, food additives, and free radical reaction initiators. Azobenzene and its derivatives exist in two isomers, cis and trans. When external conditions such as light and temperature change, reversible structural isomerization transformations occur, accompanied by changes in color and stereostructure, thus enabling their application in molecular motors, molecular switches, energy storage, and many other fields. Furthermore, azo compounds show promising prospects in electronics and pharmaceuticals. They can also be used as phototriggers for the design and synthesis of various photoresponsive systems. Compared to small-molecule azobenzene compounds, azobenzene polymers possess excellent thermal stability, film-forming properties, and processability, making them ideal matrix materials. The introduction of aromatic azobenzene structures into polymer chains has significant application value as photofunctional materials in fields such as photomodulation materials, photoorientation materials, and surface-undulation gratings.

[0003] Hydroxyazobenzene is an important intermediate in the synthesis of functional aromatic azobenzene compounds. 3,3′-Dihydroxyazobenzene, as one of these intermediates, is difficult to synthesize using conventional methods due to the meta position of its hydroxyl group on the azo group, resulting in a relatively high price for commercially available 3,3′-dihydroxyazobenzene. A reported method for synthesizing 3,3′-dihydroxyazobenzene from 3-nitrophenol (J. Mater. Chem., 1999, 9, 673–681) involves steps such as tetrahydropyran protection of the hydroxyl group, reductive azotization with lithium aluminum hydride, and detetrahydropyran protection. However, this method is costly, and the reaction control and post-processing purification are cumbersome, making industrial-scale production difficult. Direct oxidation of 3-aminophenol using different oxidants results in low product yields (Chemical Paper, 2019, 73(2), 375-385) or makes it difficult to obtain the product (Australian Journal of Chemistry (1984), 37(4), 845-55). Azobenzene polymers are mainly classified into side-chain type, main-chain type, and polymers with other special structures. Their synthesis methods mainly include free radical polymerization, condensation polymerization, and post-modification methods. In this invention, a novel azobenzene polymer with multiple functional groups is synthesized by introducing flexible alkyl groups and highly active amino groups into 3,3′-dihydroxyazobenzene through derivatization, followed by substitution reaction with highly active polybrominated toluene. Summary of the Invention

[0004] The purpose of this invention is to provide a method for synthesizing 3,3′-dihydroxyazobenzene and its derived hyperbranched azobenzene. This method uses 3-aminophenol as the starting material. Since phenol is unstable under oxidizing conditions, the hydroxyl group is first protected with a dimethyl tert-butylsilyl group to obtain 3-(tert-butyldimethylsiloxy)aniline. Then, under the catalysis of cuprous bromide and with air as the oxidant, 3,3′-di(tert-butyldimethylsiloxy)azobenzene is synthesized efficiently. Finally, the silyl group protection of 3,3′-di(tert-butyldimethylsiloxy)azobenzene is removed, and the product is purified by recrystallization to obtain the target product with high purity.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for synthesizing 3,3′-dihydroxyazobenzene, the synthetic route of which is shown in Formula I: The synthesis method includes the following steps: using 3-aminophenol as the starting material, firstly, the hydroxyl group is protected with dimethyl tert-butylsilyl group to obtain 3-(tert-butyldimethylsiloxy)aniline; secondly, under the catalysis of cuprous bromide and with air as the oxidant, 3,3′-di(tert-butyldimethylsiloxy)azobenzene is synthesized; finally, the silyl group protection of 3,3′-di(tert-butyldimethylsiloxy)azobenzene is removed, and the product 3,3′-dihydroxyazobenzene is obtained by recrystallization and purification to obtain the target product 3,3′-dihydroxyazobenzene with high purity.

[0007] Furthermore, the synthesis steps of compound 2 in Formula I are as follows: 3-aminophenol (compound 1), 4-dimethylaminopyridine, dimethyltert-butylchlorosilane, and triethylamine are reacted at room temperature, and the reaction is monitored by TLC until the reaction is complete. 3-(tert-butyldimethylsiloxy)aniline (compound 2) is obtained by vacuum distillation; the molar ratio of 3-aminophenol, 4-dimethylaminopyridine, dimethyltert-butylchlorosilane, and triethylamine is 10:1:30:30.

[0008] Furthermore, the synthesis steps of compound 3 are as follows: compound 2, pyridine, and CuBr are reacted in toluene and exposed to air at 60°C, and compound 3, namely 3,3′-bis(tert-butyldimethylsiloxy)azobenzene, is obtained by vacuum distillation; the molar ratio of compound 2, pyridine, and CuBr is 100:3:9.

[0009] Furthermore, the synthesis steps of compound 4 in Formula I are as follows: compound 3, tetrabutylammonium fluoride, and tetrahydrofuran are added to a flask and reacted at room temperature. Hydrochloric acid is then added to make the reaction system acidic. After removing tetrahydrofuran and water by vacuum distillation, DMF is added to fully dissolve the solid in the flask. Subsequently, 100 ml of saturated brine is added to precipitate the solid. After filtration and vacuum drying, compound 4, namely 3,3′-dihydroxyazobenzene, is obtained. The molar ratio of compound 3 to tetrabutylammonium fluoride is 2:3.

[0010] The synthetic route for preparing hyperbranched azobenzene polymers derived from 3,3′-dihydroxyazobenzene is as follows:

[0011] The synthesis method includes the following steps: 3,3′-dihydroxyazobenzene, N-Boc-bromoethylamine, K2CO3, and DMF are added sequentially to a round-bottom flask and reacted at 80°C for 12 hours. The reaction solution is then poured into a saturated saline solution to precipitate the product. After filtration, washing with water, and vacuum drying, the product 3,3′-bis(N-Boc-ethoxy)azobenzene, i.e., compound 5, is obtained. 3,3′-bis(N-Boc-ethoxy)azobenzene, dichloromethane, and trifluoroacetic acid are then added sequentially... The product was added to a round-bottom flask and reacted at room temperature for 2 hours. Dichloromethane and trifluoroacetic acid were removed by vacuum distillation. NaOH solution was added to adjust the pH to alkaline, and ethyl acetate was added for extraction. The product was dried over anhydrous sodium sulfate, vacuum distilled, and recrystallized from acetonitrile to obtain 3,3′-bis(β-amino-ethoxy)azobenzene, i.e., compound 6. Compound 6 and polybromomethylbenzene were dissolved in ethanol and stirred at room temperature for 2 hours. Distilled water was added, and the target compound, branched polymeric azobenzene, precipitated out. The structural formula is [insert structural formula here]. It contains an azo group, various amino groups (including quaternary ammonium), and a reactive bromine atom, allowing it to participate in a variety of reactions. Besides serving as a photoresponsive functional compound, it can also act as a reaction intermediate for introducing functional azophenyl and amino groups. Its synthetic steps mainly include the substitution reaction of 3,3′-dihydroxyazobenzene with N-Boc-bromoethylamine, the deBOC protection of the amino group, and the substitution reaction of the amino group with the reactive polybrominated derivative.

[0012] Furthermore, the molar ratio of 3,3′-dihydroxyazobenzene, N-Boc-bromoethylamine, and potassium carbonate is 2:7:20.

[0013] Furthermore, the polybromomethylbenzene is one of 1,4-dibromomethylbenzene, 1,35-tribromomethylbenzene, and 1,3,4,6-tetrabromomethylbenzene. Adjusting its type or amount can change the microstructure of branched azobenzene and regulate the type of functional groups.

[0014] The advantages of this invention are: the method for synthesizing 3,3′-dihydroxyazobenzene provided by this invention has low synthesis cost, few synthesis steps, and simple post-processing. Using inexpensive 3-aminophenol as the starting material, the overall reaction yield is 53.9%. 3-Aminophenol has a phenolic structure, which makes it easy to generate other oxidation byproducts during its oxidative azotization, resulting in low reaction efficiency and difficulty in purification. This invention uses a dimethyl-tert-butylsilyl group, which is easily protected and deprotected, to protect the phenolic hydroxyl group, thereby enabling the oxidative azotization of aniline to proceed with high efficiency. Furthermore, oxygen in the air acts as an oxidant, resulting in mild and clean reaction conditions. The final target product can be purified by recrystallization, making it suitable for large-scale synthesis. A novel azobenzene polymer was rapidly and efficiently synthesized at room temperature by introducing an aminoethyl group into 3,3′-dihydroxyazobenzene to increase molecular flexibility and reactivity, along with polybrominated methylbenzene. In this polymer, the hydroxyl group is located at the meta position of the azo group, which leads to a larger steric osmotic transition during cis-trans isomerization, thereby increasing photosensitivity. The introduction of alkyl groups increases molecular flexibility. Furthermore, the structure and functionality of the polymer can be adjusted by varying the type and amount of polybrominated toluenes added, allowing for different functionalities based on practical applications and research needs. The polymer (active bromine atom or primary amino group) can be further modified. Attached Figure Description

[0015] Figure 1 It is the 1H NMR of 3,3′-bis(tert-butyldimethylsiloxy)azobenzene (CDCl3, 400MHz).

[0016] Figure 2 It is the 1H NMR of 3,3′-dihydroxyazobenzene (DMSO-D6, 400MHz).

[0017] Figure 3 It is the 13C NMR of 3,3′-dihydroxyazobenzene (DMSO-D6, 100MHz).

[0018] Figure 4 It is the 1H NMR of 3,3′-bis(N-Boc-ethoxy)azobenzene (CDCl3, 400MHz).

[0019] Figure 5 It is the 1H NMR of 3,3′-bis(β-amino-ethoxy)azobenzene (CDCl3, 400 MHz).

[0020] Figure 6 This is the infrared spectrum of a branched polymeric azobenzene. Detailed Implementation

[0021] A method for synthesizing 3,3′-dihydroxyazobenzene includes the following steps:

[0022] Synthesis of 3-(tert-butyldimethylsiloxy)aniline compound 2: 5.00 g m-hydroxyaniline, 0.55 g 4-dimethylaminopyridine (DMAP, 0.1 equiv), 20.70 g dimethyltert-butylchlorosilane (3 equiv), and 18.3 ml triethylamine (3 equiv) were added sequentially to a 500 ml round-bottom flask. Then, 150 ml dichloromethane was added as the reaction solvent. The reaction was carried out by stirring at room temperature. The reaction was monitored by TLC until the reaction was complete. The dichloromethane was removed by vacuum distillation to obtain the crude product. The crude product was purified by recrystallization to obtain 9.22 g of the product (yield 90%).

[0023] Synthesis of 3,3′-bis(tert-butyldimethylsiloxy)azobenzene compound 3: 9.22 g of compound 2, 0.344 g of cuprous bromide (0.06 equiv), 0.713 g of pyridine (0.18 equiv), and 150 ml of toluene were sequentially added to a 500 ml round-bottom flask. After stirring at 60 °C for 20 hours with the flask open, toluene was removed by vacuum distillation. 150 ml of water was added to the reaction flask, and the aqueous phase was extracted with ethyl acetate. The organic phase was dried over anhydrous sodium sulfate, and the crude product was obtained by vacuum distillation. Recrystallization and purification yielded 6.67 g of product 3 (73% yield).

[0024] Synthesis of 3,3′-dihydroxyazobenzene compound 4: 6.67 g of compound 3, 14.26 g of tetrabutylammonium fluoride (3 equiv), and 100 ml of tetrahydrofuran were added sequentially to a 250 ml round-bottom flask. After reacting at room temperature for 1 h, hydrochloric acid was added to make the reaction system acidic. After removing tetrahydrofuran and water by vacuum distillation, 10 ml of DMF was added to fully dissolve the solid in the flask. Then, 100 ml of saturated brine was added to precipitate the solid. After filtration and vacuum drying, product 4 was obtained as 2.65 g (yield 82%).

[0025] Synthesis of 3,3′-bis(N-Boc-ethoxy)azobenzene compound 5: 2.65 g of compound 4, 9.70 g of N-Boc-bromoethylamine (3.5 equiv), 17.13 g of K2CO3 (10 equiv), and 50 ml of DMF were added sequentially to a 100 ml round-bottom flask. After reacting at 80 °C for 12 h, the reaction solution was poured into 200 ml of saturated saline solution to precipitate the product. After filtration, washing with water, and vacuum drying, 2.41 g of product 5 was obtained.

[0026] Synthesis of 3,3′-bis(β-amino-ethoxy)azobenzene compound 6: 2.41 g of compound 5, 30 ml of dichloromethane, and 15 ml of trifluoroacetic acid were added sequentially to a 100 ml round-bottom flask. After reacting at room temperature for 2 h, the dichloromethane and trifluoroacetic acid were removed by vacuum distillation. Then, 50 ml of 1 mol / L NaOH solution was added to the round-bottom flask, and ethyl acetate was added for extraction. After drying with anhydrous sodium sulfate, vacuum distillation and recrystallization with acetonitrile were used to obtain 1.03 g of product 6.

[0027] Synthesis of branched polymeric azobenzene: 0.3 g of compound 6 and 0.48 g of 1,3,5-tris(bromomethyl)benzene were dissolved in ethanol and stirred at room temperature for 2 h. Distilled water was added, and the target compound was precipitated. The product was filtered and dried under vacuum to obtain 0.65 g of a yellow solid.

Claims

1. A method for preparing hyperbranched azobenzene polymers derived from 3,3′-dihydroxyazobenzene, characterized in that, Its synthetic route is as follows: ， The synthesis method includes the following steps: 3,3′-dihydroxyazobenzene, N-Boc-bromoethylamine, K2CO3, and DMF are added sequentially to a round-bottom flask and reacted at 80°C for 12 hours. The reaction solution is then poured into a saturated saline solution to precipitate the product. After filtration, washing with water, and vacuum drying, the product 3,3′-bis(N-Boc-ethoxy)azobenzene, i.e., compound 5, is obtained. 3,3′-bis(N-Boc-ethoxy)azobenzene, dichloromethane, and trifluoroacetic acid are added sequentially to a round-bottom flask and reacted at room temperature for 2 hours. Dichloromethane and trifluoroacetic acid are removed by vacuum distillation. NaOH solution is added to adjust the pH to alkaline, and ethyl acetate is added for extraction. The product 3,3′-bis(β-amino-ethoxy)azobenzene, i.e., compound 6, is obtained by drying with anhydrous sodium sulfate, vacuum distillation, and recrystallization with acetonitrile. Compound 6 and polybromomethylbenzene are dissolved in ethanol and stirred at room temperature for 2 hours. Distilled water is added to precipitate the target compound, i.e., branched polymeric azobenzene.

2. The method for preparing the hyperbranched azobenzene polymer as described in claim 1, characterized in that: The molar ratio of 3,3′-dihydroxyazobenzene, N-Boc-bromoethylamine, and potassium carbonate is 2:7:

20.

3. The method for preparing the hyperbranched azobenzene polymer as described in claim 1, characterized in that: The polybrominated methylbenzene is one of 1,4-dibromomethylbenzene, 1,3,5-tribromomethylbenzene, and 1,3,4,6-tetrabromomethylbenzene.

4. The application of the hyperbranched azobenzene polymer prepared by the method described in claim 1, characterized in that, It can be used as a photoresponsive functional compound or as a reaction intermediate for introducing functional azophenyl and amino groups.

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