A method for the selective electro-oxidation of polyols

Abstract

This invention relates to the field of electrochemistry, specifically disclosing a method for the selective electrooxidation of polyhydroxy compounds. More specifically, it involves the selective electrooxidation of compound (Ⅰ) to obtain compound (Ⅱ). This invention reduces the large number of byproducts caused by the simultaneous oxidation of multiple pairs of hydroxyl groups in traditional oxidation methods. By selectively electrooxidizing to obtain compound (Ⅱ), and then reducing compound (Ⅱ) to obtain compound (Ⅲ), for example, selectively electrooxidizing hydroxycitronellol (3,7-dimethyl-1,7-octanediol, compound 1a) to obtain (7-hydroxy-3,7-dimethyloctanoic acid, compound 2b), and then reducing it with compound 1b to obtain compound 1c. Compound 1c is hydroxycitronellol (CAS: 107-75-5), which has a strong floral fragrance. By changing the number of carbon atoms in hydroxycitronellol without changing the functional groups, a series of small molecule compounds with different fragrances can be produced efficiently and with low pollution.

Description

Technical Field

[0001] This invention relates to the field of electrochemical technology, and specifically discloses a method for the selective electrooxidation of polyhydroxy compounds. Background Technology

[0002] The oxidation of alcohols to form corresponding carbonyl compounds has attracted widespread research attention. As important industrial chemicals, carboxylic acids are widely used not only as intermediates in pharmaceuticals and agrochemicals, but also as dyes and food additives. In existing technologies, traditional oxidation methods of carboxylic acids, especially those for polyhydroxy compounds, often produce a large number of unwanted byproducts.

[0003] For example, in the paper "Aerobic oxidation of isoprene glycol with platinum-bismuthnanoparticles catalysts supported on metal oxides", 2-methyl-2,5-pentanediol (CAS: 1462-10-8) was selectively oxidized to 4-hydroxy-4-methylglutaric acid (CAS: 23327-19-7) under a pressure of 0.9 MPa in a mixed solvent of n-butanol and water with a yield of only 46%, and a large amount of wastewater containing 5,5-dimethyl-dihydro-furan-2-one (CAS: 3123-97-5) as a byproduct was generated.

[0004] Electrocatalytic oxidation is a promising green process due to its mild reaction conditions and environmental friendliness. Known methods for the electrochemical oxidation of alcohols to the corresponding carbonyl compounds utilize NiOOH in an alkaline system. In this system, carboxylates are formed, requiring the addition of large amounts of acid to adjust the pH to obtain the product acid, which generates significant amounts of saline wastewater.

[0005] Therefore, it is essential to develop a green and sustainable selective electro-oxidation method for polyhydroxy compounds. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention specifically designs a method for the selective electrooxidation of polyhydroxy compounds to produce carboxylic acids under an acidic electrolysis system.

[0007] The method of this invention oxidizes specific hydroxyl groups on specific compounds in a highly efficient manner under acidic conditions using specific electrodes and voltages, without significantly affecting other hydroxyl groups. This method generates compounds containing both carboxyl and hydroxyl groups from dihydroxy compounds, without the need to adjust the pH of the system, and the acidic system can be recycled.

[0008] The first aspect of this invention provides a method for the selective electro-oxidation of polyhydroxy compounds, as shown in the formula: ; The substrate of formula (I), acid, and solvent are mixed to form an electrolyte. Electrodes are inserted and electrolysis is performed to obtain the substrate of formula (II). The R is -(CHR). n -, n=1~6, where R is selected from H, C1~6 alkyl.

[0009] In some specific embodiments of the first aspect of the present invention, the compound of formula (I) specifically comprises:

[0010] In some specific embodiments of the first aspect of the present invention, the compound of formula (II) is specifically:

[0011] In some specific embodiments of the first aspect of the present invention, the acid is one or more selected from sulfuric acid, nitric acid, or hydrochloric acid, and / or the mass fraction of the acid in the electrolyte is 5-25 wt%, specifically 5 wt% in some embodiments, 8 wt% in some embodiments, 11 wt% in some embodiments, 14 wt% in some embodiments, 17 wt% in some embodiments, 20 wt% in some embodiments, and 25 wt% in some embodiments. In some specific embodiments of the first aspect of the present invention, the solvent is water.

[0012] In some specific embodiments of the first aspect of the present invention, the electrode comprises an anode and a cathode.

[0013] In some specific embodiments of the first aspect of the present invention, the anode material is PbO2 / Ti.

[0014] In some specific embodiments of the first aspect of the present invention, the cathode material is selected from graphite, copper, or platinum.

[0015] In some specific embodiments of the first aspect of the present invention, the current density is 20~200 mA / cm². 2 In some specific embodiments of the first aspect, it is 20 mA / cm 2 In some specific embodiments of the first aspect, it is 20 mA / cm 2 In some specific embodiments of the first aspect, it is 20 mA / cm 2 In some specific embodiments of the first aspect, it is 40 mA / cm 2In some specific embodiments of the first aspect, it is 60 mA / cm 2 In some specific embodiments of the first aspect, it is 80 mA / cm 2 In some specific embodiments of the first aspect, it is 100 mA / cm 2 In some specific embodiments of the first aspect, it is 120 mA / cm 2 In some specific embodiments of the first aspect, it is 140 mA / cm 2 In some specific embodiments of the first aspect, it is 160 mA / cm 2 In some specific embodiments of the first aspect, it is 180 mA / cm 2 In some specific embodiments of the first aspect, it is 200 mA / cm 2 . In some specific embodiments of the first aspect, the current is a constant current.

[0016] In some specific embodiments of the first aspect, the electrolysis temperature is room temperature.

[0017] In some specific embodiments of the first aspect of the present invention, the molar concentration of the compound of formula (I) in the electrolyte is 1.67 mmol / g to 2.5 mmol / g, specifically 1.67 mmol / g in some specific embodiments of the first aspect, 1.80 mmol / g in some specific embodiments of the first aspect, 2.00 mmol / g in some specific embodiments of the first aspect, 2.20 mmol / g in some specific embodiments of the first aspect, 2.40 mmol / g in some specific embodiments of the first aspect, and 2.50 mmol / g in some specific embodiments of the first aspect.

[0018] A second aspect of the present invention provides a method for synthesizing formula (Ⅲ), comprising: electrolyzing a compound of formula (Ⅱ) using the method described in the first aspect, and reducing the compound of formula (Ⅱ) to obtain a compound of formula (Ⅲ). .

[0019] In some specific embodiments of the second aspect of the present invention, the method by which the compound of formula (II) is synthesized into the compound of formula (III) is as follows:

[0020] Compound (II) was added to an organic solution of 1-pyridine-4-piperidine (PPDP) under an inert atmosphere, followed by the sequential addition of trifluoromethanesulfonylpyridine salt to obtain compound (III).

[0021] In some embodiments, the room temperature is 5~40°C; in some embodiments, the room temperature is 10~35°C; in some embodiments, the room temperature is 15~30°C; in some embodiments, the room temperature is 20~25°C; and in some embodiments, the room temperature is 25°C.

[0022] All reagents used in this invention are purchased from the open and legal market, such as Aladdin, Xilong, Sinopharm Shanghai Test, and McLean, and have not undergone further purification.

[0023] In this invention, "RuO2-IrO2 / Ti" represents a titanium-based ruthenium-iridium oxide coated electrode, indicating that a coating composed of ruthenium oxide (RuO2) and iridium oxide (IrO2) is coated on a titanium substrate.

[0024] Conversion rate Where n b n is the molar amount of the substrate fed; a This represents the remaining molar amount of substrate. yield n a n is the theoretically maximum molar quantity of the target product that can be obtained. d The actual molar amount of the target product obtained; Advantages of this invention: This invention uses a specific electrode combination and current density to selectively electro-oxidize compound (I) to obtain compound (II), avoiding the large number of byproducts caused by the low selectivity when polyhydroxy compounds are oxidized in traditional oxidation methods.

[0025] In the case of obtaining compound (II), reduction yields compound (III). For example, selective electrooxidation of hydroxycitronellol (compound 1a) yields 7-hydroxy-3,7-dimethyloctanoic acid (compound 1b). Then, reduction of compound 1b yields compound 1c, which is hydroxycitronellol (CAS: 107-75-5) and has a special aroma.

[0026] By changing the number of carbon atoms in the carbon chain of compound (I), selective electro-oxidation and reduction were attempted on multiple specific substances with structure (I). The aroma was adjusted by changing the number of carbon atoms, and a series of small molecule compounds with different aromas were produced with high efficiency and low pollution. Detailed Implementation

[0027] Through the following detailed description of the embodiments of the present invention in conjunction with specific implementation methods, those skilled in the art will gain a clearer and more thorough understanding of the further features, advantages, and effects of the present invention.

[0028] Example 1:

[0029] According to the reaction conditions in Table 1, the substrate 3,7-dimethyl-1,7-octanediol (1a, 100 mmol), 40-60 g of 5-25 wt% sulfuric acid, and then a magnetic flux and insertion electrodes (anode: PbO2 / Ti or RuO2-IrO2 / Ti; cathode: graphite, copper, or platinum) were added to the reaction tube. The reaction was carried out at a current density of 20-200 mA / cm². 2 Under constant current electrolysis conditions, 4 F of charge was electrolyzed at room temperature. The reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield (yield%) of compound 1b and the conversion (conv.%) of substrate compound 1a in each group were measured. Table 1: Electrolysis conditions and yield of compound 1b Group Sulfuric acid concentration (%) Electrolyte mass (g) Anode material cathode material Current density mA / cm2 Conversion rate (conv.%) Yield% 1 25 60 PbO2 / Ti copper 40 mA / cm2 92.1 80.3 2 25 60 PbO2 / Ti platinum 40 mA / cm2 93.5 85.5 3 25 40 PbO2 / Ti graphite 20 mA / cm2 91.8 88.7 4 10 60 PbO2 / Ti graphite 100 mA / cm2 94.2 82.0 5 5 60 PbO2 / Ti graphite 200 mA / cm2 95.7 70.4 6 35 60 PbO2 / Ti graphite 40 mA / cm2 93.3 82.9 7 25 40 PbO2 / Ti graphite 40 mA / cm2 96.3 85.8 8 25 40 <![CDATA[RuO2-IrO2 / Ti]]> graphite 20 mA / cm2 85.7 11.4 Comparative Example 1: The difference between Comparative Example 1 and Example 1 Group 7 is that the electrolyte also contains 2 wt% sodium chloride based on the mass of sulfuric acid, and the yield of compound 1b is 7.2%. Under these conditions, the corrosion of the lead dioxide electrode is very obvious, and the lead dioxide on the surface is detached.

[0030] Comparative Example 2: The difference between Comparative Example 2 and Example 1 (Group 7) is that the current density is increased from 40 mA / cm². 2 Change to 300mA / cm 2 Under the same conditions, after electrolysis, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The conversion of compound 1a was 92.3% (conv.%), and the yield of compound 1b was 40% (yield%). Comparative Example 3: The difference between Comparative Example 3 and Group 7 in Example 1 is that 3,7-dimethyl-1,7-octanediol (1a) was replaced with compound 2a. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 2b was 22.1%, and the conversion of starting compound 2a was 92.1%.

[0031] Comparative Example 4: The difference between Comparative Example 4 and Group 7 in Example 1 is that 3,7-dimethyl-1,7-octanediol (1a) was replaced with compound 3a. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 3b was 5.7%, and the conversion of starting compound 3a was 94.5%.

[0032] Comparative Example 5: The difference between Example 5 and Group 7 in Example 1 is that the feedstock 3,7-dimethyl-1,7-octanediol (1a) was replaced with compound 4a. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 4b was 4.7%, and the conversion of the starting material compound 4a was 96.3%.

[0033] Example 6: The difference between Example 6 and Group 7 in Example 1 is that the feed 3,7-dimethyl-1,7-octanediol (1a) was replaced with compound 5a. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 5b was 90.7%, and the conversion of the starting compound 5a was 94.9%.

[0034] Example 3: The difference between Example 3 and Group 7 in Example 1 is that the feed 3,7-dimethyl-1,7-octanediol (1a) was replaced with compound 6a. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 6b was 89.7%, and the conversion of the starting compound 6a was 97.1%.

[0035] Example 4: The difference between Example 4 and Group 7 in Example 1 is that 3,7-dimethyl-1,7-octanediol was replaced with compound 7a as the feedstock. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 7b was 84.1%, and the conversion of the starting compound 7a was 96.8%.

[0036] Example 5: The difference between Example 5 and Group 7 in Example 1 is that the feedstock 3,7-dimethyl-1,7-octanediol was replaced with compound 8a. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 8b was 88.1%, and the conversion of the starting material compound 8a was 97.2%.

[0037] Example 6: The difference between Example 6 and Group 7 in Example 1 is that the feedstock 3,7-dimethyl-1,7-octanediol was replaced with compound 9a. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 9b was 89.2%, and the conversion of the starting compound 9a was 94.7%.

[0038] Example 7: The difference between Example 7 and Group 7 in Example 1 is that the feedstock 3,7-dimethyl-1,7-octanediol was replaced with compound 10a. Electrolysis was performed using the same parameters, the reaction solution was extracted with ethyl acetate, the organic phase was dried with anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to remove the solvent. The yield of compound 10b was 84.5%, and the conversion of the starting material compound 10a was 96.6%.

[0039] Example 8: The electrolyte after extraction in Group 7 of Example 1 was recycled multiple times. Each time, the amount of substrate 3,7-dimethyl-1,7-octanediol (1a) added, as well as the electrode material, current density, and electrolytic charge, were the same as in Example 1. The yield percentage of carboxylic acid was tested in each cycle, as shown in Table 2. Table 2: Electrolyte cycle count and yield Number of times sulfuric acid is recycled 1 2 3 4 Yield% 83.2% 82.7% 83.1% 84.3% Example 10: The carboxyl-containing compounds obtained from the above oxidation were reduced using existing techniques: Under a nitrogen atmosphere, 1-pyridine-4-piperidine (PPDP, 1.6 mmol) and 5 mL of dichloromethane were added to 25 mL Schlenk tubes, respectively. Compound 1b or 5–10b (1.0 mmol) was added to each Schlenk tube, followed by the sequential addition of trifluoromethanesulfonylpyridinium salt PPDP-Tf (1.7 mmol) and HBpin (1.5 mmol) at short time intervals, and the mixture was stirred for 10 min. After the reaction was complete, aqueous solution and dichloromethane (3 × 5 mL) were added to the system for extraction. Column chromatography was used to separate the aldehyde compounds 1c and 5–10c. The yields (yield%) are shown in Table 3. Table 3: Reduction of carboxylic acid molecules to aldehydes in Examples 1-8 (1-10 c) Group Feeding product Yield% 1 1b 1c 90.2 5 5b 5c 85.3 6 6b 6c 88.6 7 7b 7c 87.1 8 8b 8c 89.9 9 9b 9c 82.6 10 10b 10c 85.2 Structure of compound 1c, 5~10c:

[0040] This invention obtains compound (II) by selective electrochemical oxidation of compound (I), reducing the large amount of byproducts caused by the simultaneous oxidation of multiple hydroxyl groups in traditional oxidation methods. Compound (II) is then reduced to obtain a series of small fragrance molecules. For example, selective oxidation of hydroxycitronellol (3,7-dimethyl-1,7-octanediol, compound 1a) yields (7-hydroxy-3,7-dimethyloctanoic acid, compound 2b), and reduction of compound 1b yields compound 1c, which is hydroxycitronellol (CAS: 107-75-5), possessing a strong floral fragrance. By changing the number of carbon atoms while maintaining the same functional groups, a series of small molecule compounds with different fragrances are produced efficiently and with low pollution, expanding the synthetic pathway for small molecule fragrances.

[0041] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A method for the selective electro-oxidation of polyhydroxy compounds, as shown in the formula: ； The substrate of formula (I), acid, and solvent are mixed to form an electrolyte. Electrodes are inserted and electrolysis is performed to obtain the substrate of formula (II). The R is -(CHR). n -, n=1~6, where R is selected from H, C1~6 alkyl.

2. The method according to claim 1, characterized in that, The compound of formula (Ⅰ) is specifically: 。 3. The method according to claim 1, characterized in that, The compound of formula (II) is specifically: 。 4. The method according to any one of claims 1 to 3, characterized in that, The acid is one or more of sulfuric acid, nitric acid or hydrochloric acid, and / or the mass fraction of the acid in the electrolyte is 5 to 25 wt%, preferably 20 wt%.

5. The method according to any one of claims 1 to 4, characterized in that, The solvent is water.

6. The method according to any one of claims 1 to 5, characterized in that, The electrode comprises an anode, a cathode, and / or the anode material is PbO2 / Ti, and / or the cathode material is selected from graphite, copper, or platinum.

7. The method according to any one of claims 1 to 6, characterized in that, The current density is 20~200mA / cm² 2 20mA / cm is preferred 2 And / or, the current is a constant current, and / or, the electrolysis temperature is room temperature.

8. The method according to any one of claims 1 to 7, characterized in that, The molar concentration of the compound of formula (I) in the electrolyte is 1.67 mmol / g to 2.5 mmol / g, preferably 2.5 mmol / g.

9. A method for synthesizing formula (Ⅲ), comprising: The compound of formula (II) is obtained by electrolysis using the method described in claim 1, and the compound of formula (II) is reduced to obtain the compound of formula (III). .

10. The synthesis method of formula (Ⅲ) according to claim 8, characterized in that, The method for synthesizing compound (III) from compound (II) is as follows: Compound (II) was added to an organic solution of 1-pyridine-4-piperidine (PPDP) under an inert atmosphere, followed by the sequential addition of trifluoromethanesulfonylpyridine salt to obtain compound (III).

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