Samarium Diiodide-Mediated Free Radical Decomposition of Lignin β-O-4-Keto Model Compounds

Abstract

The present invention discloses a method for obtaining ketone compounds and phenolic compounds by free radical cracking of lignin β-O-4 ketone model compounds mediated by samarium diiodide, and the application of samarium diiodide in mediating free radical cracking of lignin β-O-4 ketone model compounds to obtain ketone compounds and phenolic compounds. The present invention uses samarium diiodide as an inducer and utilizes the oxophilicity and single-electron donor properties of samarium diiodide to promote the addition of ketone carbonyl radicals, thereby initiating the ether bond free radical cracking of lignin β-O-4 ketone model compounds to obtain lignin platform monoaromatic chemicals such as phenol and acetophenone. The method of the present invention is simple to operate, has mild reaction conditions, has a high free radical degradation conversion rate, avoids free radical dimerization side reactions, and can obtain monoaromatic chemicals with high selectivity.

Description

Technical Field

[0001] The present invention relates to the technical field of lignin chemical cracking, and in particular to a samarium diiodide (SmI2)-mediated free radical cracking method for a lignin β-O-4 keto model compound, and application of samarium diiodide in mediating the free radical cracking of a lignin β-O-4 keto model compound. Background Art

[0002] As the third largest biomass, lignin has the characteristics of abundant resources and low price. Because it is rich in aromatic units, it can provide chemical platform raw materials such as aromatic chemicals under chemical degradation systems. Over the past few decades, people have developed many chemical degradation methods for lignin, including acidic degradation, oxidative degradation, and reductive degradation. Chinese invention patent application CN105712860A provides a method for preparing syringaldehyde by catalyzing the conversion of lignin with ionic liquids such as ethanolamines and guanidines. In this method, the reaction temperature is 180°C, and the yield of syringaldehyde can reach up to 60.3% after 6 hours of reaction. Chinese invention patent application CN106316804A discloses a method for catalytic cracking of lignin sulfonates and their model compounds using nickel-doped molybdenum sulfide as a catalyst. In this method, the reaction temperature is 300°C, the reaction time is 48 hours, and the conversion rate of the model compound is as high as 93%.

[0003] In recent years, a two-step oxidation-followed degradation strategy has been developed for chemical lignin degradation, enabling lignin degradation under mild conditions. Generally speaking, the oxidation step involved in this two-step strategy often uses oxidants such as DDQ and acetyl TEMPO to oxidize the lignin. Oxidized lignin significantly reduces the dissociation energy of the CO ether bond, enabling subsequent degradation reactions to occur under mild conditions. However, these oxidized lignin degradation methods often require expensive metal catalysts, severely limiting their application in industrial production. Summary of the Invention

[0004] In response to the above-mentioned technical problems and the shortcomings in the art, the present invention provides a samarium diiodide-mediated free radical cleavage method for lignin β-O-4 ketone model compounds, using samarium diiodide as an inducer and utilizing the oxophilicity and single-electron donor properties of samarium diiodide to promote the addition of ketone carbonyl radicals, thereby initiating the ether bond free radical cleavage of the lignin β-O-4 ketone model compound to obtain lignin platform monoaromatic chemicals such as phenol and acetophenone. The method of the present invention is simple to operate, has mild reaction conditions, a high free radical degradation conversion rate, avoids free radical dimerization side reactions, and can obtain monoaromatic chemicals with high selectivity. The present invention eliminates the use of additives such as acids and bases or precious metal catalysts, and can cleave oxidized lignin at room temperature and pressure to obtain lignin platform monoaromatic chemicals such as phenol and acetophenone.

[0005] The specific technical solutions are as follows:

[0006] A samarium diiodide-mediated free radical cleavage method for a lignin β-O-4-keto model compound comprises mixing samarium diiodide with a lignin β-O-4-keto model compound represented by formula (I) in a reaction solvent to react and obtain a ketone compound represented by formula (II) and a phenolic compound represented by formula (III);

[0007]

[0008] R1~R 10 Each independently selected from H, hydroxyl, C1-C11 alkoxy or -O-(CH2-O) n -CH3, n is 1 to 10;

[0009] R 11 Selected from H, C1-C11 alkyl, C1-C11 hydroxyalkyl (such as -CH2-OH, etc.) or -O-(CH2-O) m -CH3, m is 1 to 10.

[0010] In one embodiment, the samarium diiodide-mediated free radical cleavage method of lignin β-O-4 keto model compound is carried out under the protection of an inert gas.

[0011] Furthermore, the inert gas may include at least one of rare gas, nitrogen, etc.

[0012] In one embodiment, in the samarium diiodide-mediated free radical cleavage method of the lignin β-O-4 keto model compound, the molar ratio of samarium diiodide to the lignin β-O-4 keto model compound represented by formula (I) can be 1.5 to 6:1, preferably 6:1.

[0013] In one embodiment, in the samarium diiodide-mediated free radical cleavage method of lignin β-O-4-keto model compound, the reaction solvent may include at least one of tetrahydrofuran (THF), 1,4-dioxane, N,N-dimethylformamide, dimethyl sulfoxide (DMSO), toluene, n-heptane, etc., preferably tetrahydrofuran.

[0014] In one embodiment, in the samarium diiodide-mediated free radical cleavage method of lignin β-O-4 keto model compound, the reaction temperature can be room temperature to 100°C, preferably 60°C.

[0015] In one embodiment, in the samarium diiodide-mediated free radical cleavage method of lignin β-O-4-keto model compound, the reaction time can be 3 to 24 hours, preferably 12 hours.

[0016] As a general inventive concept, the present invention also provides the use of samarium diiodide in mediating the free radical cleavage of the lignin β-O-4 ketone model compound shown in formula (I) to obtain the ketone compound shown in formula (II) and the phenolic compound shown in formula (III).

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. The reaction operation is simple and easy to implement, avoiding the use of precious metal catalysts and facilitating large-scale application;

[0019] 2. The reaction conditions are mild and the degradation products can be obtained efficiently;

[0020] 3. The free radical reaction activity initiated under this reaction condition is moderate, which can avoid the occurrence of free radical dimerization side reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is the H NMR of acetophenone ( 1 H NMR) spectrum.

[0022] Figure 2 Phenol 1 H NMR spectrum.

[0023] Figure 3 1-(4-methoxyphenyl)acetophenone 1 H NMR spectrum.

[0024] Figure 4 For guaiacol 1 H NMR spectrum.

[0025] Figure 5 3,4-dimethoxyacetophenone 1 H NMR spectrum.

[0026] Figure 6 3-Hydroxy-(3',4'-dimethoxy)propiophenone 1 H NMR spectrum. DETAILED DESCRIPTION

[0027] The present invention will be further described below with reference to the accompanying drawings and specific examples. It should be understood that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0028] Example 1

[0029] Under a nitrogen atmosphere, a solution of 2-phenoxy-1-acetophenone (13.3 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in THF was added to a 25 mL Shrek tube. The mixture was heated to 60°C with stirring, allowed to react for 9 hours, and then cooled to room temperature. Gas chromatography was used to analyze the reaction using an internal standard (n-dodecane). The conversion of 2-phenoxy-1-acetophenone and the GC yield of the cleavage product were calculated according to the following formula.

[0030] Conversion rate [mol%] = (A0-A) / A0*100%

[0031] GC yield [mol%] = B i / A0*100%

[0032] In the formula, A0 is the amount of the lignin β-O-4 ketone model compound (2-phenoxy-1-acetophenone in this embodiment) added before the reaction [mol], A is the amount of the lignin β-O-4 ketone model compound (2-phenoxy-1-acetophenone in this embodiment) after the reaction [mol], B i is the amount of the substance (mol) of the ketone compound (acetophenone in this example) or the phenolic compound (phenol in this example) generated after the reaction.

[0033] The calculated conversion rate of 2-phenoxy-1-acetophenone was 87%, the yield of acetophenone was 42%, and the yield of phenol was 76%. The product hydrogen spectrum data are as follows:

[0034] acetophenone 1 H NMR (500MHz,CDCl3)δ8.12-7.84(m,2H),7.62-7.48(m,1H),7.47-7.31(m,2H),2.65-2.53(m,3H). Figure 1 shown.

[0035] phenol 1 H NMR (500 MHz, CDCl3) δ7.26-7.19 (m, 2H), 6.93 (tt, J = 7.4, 1.1 Hz, 1H), 6.86-6.77 (m, 2H), 5.32 (s, 1H). Figure 2 shown.

[0036] Example 2

[0037] Under nitrogen, add 2-phenoxy-1-acetophenone (13.3 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in THF to a 25 mL Shrek tube. Heat to 100°C with stirring, react for 9 hours, and then cool to room temperature. Product analysis was performed as in Example 1.

[0038] The calculated conversion rate of 2-phenoxy-1-acetophenone was 87%, the yield of acetophenone was 32%, and the yield of phenol was 74%.

[0039] Example 3

[0040] Under nitrogen, add 2-phenoxy-1-acetophenone (13.3 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in THF to a 25 mL Shrek tube. Heat to 60°C with stirring, react for 12 hours, and then cool to room temperature. Product analysis was performed according to the method in Example 1.

[0041] The calculated conversion rate of 2-phenoxy-1-acetophenone was 96%, the yield of acetophenone was 50%, and the yield of phenol was 90%.

[0042] Example 4

[0043] Under nitrogen, add 2-phenoxy-1-acetophenone (13.3 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (75.8 mg, 0.1875 mmol, 3.0 eq.) in THF to a 25 mL Shrek tube. Heat to 60°C with stirring, react for 12 hours, and then cool to room temperature. Product analysis was performed as in Example 1.

[0044] The calculated conversion rate of 2-phenoxy-1-acetophenone was 88%, the yield of acetophenone was 40%, and the yield of phenol was 66%.

[0045] Example 5

[0046] Under nitrogen, add 2-phenoxy-1-acetophenone (13.3 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in THF to a 25 mL Shrek tube. After addition, open the reaction system, remove the nitrogen blanket, and heat to 60°C with stirring. React for 12 h, then cool to room temperature. Product analysis was performed as in Example 1.

[0047] The calculated conversion rate of 2-phenoxy-1-acetophenone was 94%, the yield of acetophenone was 14%, and the yield of phenol was 83%.

[0048] Example 6

[0049] Under nitrogen, add 2-phenoxy-1-acetophenone (13.3 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in DMSO to a 25 mL Shrek tube. Heat to 60°C with stirring, react for 12 hours, and then cool to room temperature. Product analysis was performed as in Example 1.

[0050] The calculated conversion rate of 2-phenoxy-1-acetophenone was 95%, the yield of acetophenone was 11%, and the yield of phenol was 90%.

[0051] Example 7

[0052] Under nitrogen, add 1-(4-methoxyphenyl)-2-phenoxyethanone (15.1 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in THF to a 25 mL Shrek tube. Heat to 60°C with stirring, react for 12 hours, and then cool to room temperature. Product analysis was performed as in Example 1.

[0053] The calculated conversion of 1-(4-methoxyphenyl)-2-phenoxyethanone was >99%, the yield of 1-(4-methoxyphenyl)ethanone was 32%, and the yield of phenol was 96%. The product hydrogen spectrum data are as follows:

[0054] 1-(4-Methoxyphenyl)ethanone 1 H NMR (500MHz,CDCl3)δ7.96-7.87(m,2H),6.95-6.89(m,2H),3.85(s,3H),2.54(s,3H). Figure 3 shown.

[0055] Example 8

[0056] Under nitrogen, add 2-(2-methoxyphenyl)-1-acetophenone (15.1 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in THF to a 25 mL Shrek tube. Heat to 60°C with stirring, react for 12 hours, and then cool to room temperature. Product analysis was performed as in Example 1.

[0057] The calculated conversion rate of 2-(2-methoxyphenyl)-1-acetophenone was >99%, the yield of acetophenone was 59%, and the yield of guaiacol was 68%. The product hydrogen spectrum data are as follows:

[0058] Guaiacol 1H NMR (500MHz,CDCl3)δ7.04-6.99(m,1H),6.96-6.88(m,3H),5.90(s,1H),3.89(s,3H). Figure 4 shown.

[0059] Example 9

[0060] Under nitrogen, add 2-(2-methoxyphenyl)-1-(4-methoxyphenyl)ethanone (17.0 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in THF to a 25 mL Shrek tube. Heat to 60°C with stirring, react for 12 hours, and then cool to room temperature. Product analysis was performed as in Example 1.

[0061] The calculated conversion rate of 2-(2-methoxyphenyl)-1-(4-methoxyphenyl)ethanone was >99%, the yield of 1-(4-methoxyphenyl)ethanone was 59%, and the yield of guaiacol was 68%.

[0062] Example 10

[0063] Under nitrogen, add 2-(2-methoxyphenyl)-3',4'-dimethoxyacetophenone (18.9 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) in THF to a 25 mL Shrek tube. Heat to 60°C with stirring, react for 12 hours, and then cool to room temperature. Product analysis was performed as in Example 1.

[0064] The calculated conversion rate of 2-(2-methoxyphenyl)-3',4'-dimethoxyacetophenone was >99%, the yield of 3,4-dimethoxyacetophenone was 59%, and the yield of guaiacol was 68%. The product hydrogen spectrum data are as follows:

[0065] 3,4-Dimethoxyacetophenone 1 H NMR (500 MHz, CDCl3) δ7.53 (dd, J = 8.3, 2.0 Hz, 1H), 7.48 (d, J = 2.0 Hz, 1H), 6.84 (d, J = 8.3 Hz, 1H), 3.89 (d, J = 5.8 Hz, 6H), 2.52 (s, 3H). Figure 5 shown.

[0066] Example 11

[0067] Under a nitrogen atmosphere, a THF solution of 1-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propan-1-one (20.8 mg, 0.0625 mmol, 1.0 eq.) and samarium diiodide (151.6 mg, 0.375 mmol, 6.0 eq.) was added to a 25 mL Shrek tube. The mixture was heated to 60°C with stirring, allowed to react for 12 hours, and then cooled to room temperature. Product analysis and isolation were performed according to the method in Example 1. The reaction mixture was quenched with saturated aqueous NH4Cl solution (80 mL) and extracted with dichloromethane (DCM, 3 × 150 mL). The organic layers were combined, dried over anhydrous sodium sulfate, and concentrated in vacuo to obtain the crude product. The crude product was purified by column chromatography (MeOH:DCM volume ratio = 1:55) to obtain the pure product as a milky white crystalline powder. The results showed a conversion rate of >99% for 1-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propan-1-one, an isolated yield of 85% for 3-hydroxy-(3',4'-dimethoxy)propiophenone, and an 89% yield for guaiacol. The product's H-spectroscopic data are as follows:

[0068] 3-Hydroxy-(3',4'-dimethoxy)propiophenone 1 H NMR (500 MHz, CDCl3) δ7.59 (dd, J = 8.4, 2.1 Hz, 1H), 7.53 (d, J = 2.0 Hz, 1H), 6.90 (d, J = 8.4 Hz, 1H), 4.06-4.00 (m, 2H), 3.95 (d, J = 7.2 Hz, 6H), 3.20 (t, J = 5.3 Hz, 2H). Figure 6 shown.

[0069] In addition, it should be understood that after reading the above description of the present invention, those skilled in the art may make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the claims attached to this application.

Claims

1. A samarium diiodide-mediated free radical cleavage method for lignin β-O-4 keto model compounds, characterized in that: Samarium diiodide and a lignin β-O-4 ketone model compound represented by formula (I) are mixed and reacted in a reaction solvent to obtain a ketone compound represented by formula (II) and a phenolic compound represented by formula (III); R1~R 10 Each independently selected from H, hydroxyl, C1-C11 alkoxy or -O-(CH2-O) n -CH3, n is 1 to 10; R 11 Selected from H, C1-C11 alkyl, C1-C11 hydroxyalkyl or -O-(CH2-O) m -CH3, m is 1 to 10.

2. The samarium diiodide-mediated free radical cleavage method of lignin β-O-4-keto model compound according to claim 1, characterized in that: The reaction is carried out under the protection of inert gas.

3. The samarium diiodide-mediated free radical cleavage method of lignin β-O-4-keto model compound according to claim 2, characterized in that: The inert gas includes at least one of a rare gas and nitrogen.

4. The samarium diiodide-mediated free radical cleavage method of lignin β-O-4 keto model compound according to claim 1, characterized in that: The molar ratio of samarium diiodide to the lignin β-O-4 ketone model compound represented by formula (I) is 1.5 to 6:

1.

5. The samarium diiodide-mediated free radical cleavage method of lignin β-O-4 keto model compound according to claim 1, characterized in that: The reaction solvent includes at least one of tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide, dimethyl sulfoxide, toluene, and n-heptane.

6. The samarium diiodide-mediated free radical cleavage method of lignin β-O-4-keto model compound according to claim 1, characterized in that: The reaction temperature is room temperature to 100°C.

7. The samarium diiodide-mediated free radical cleavage method of lignin β-O-4-keto model compound according to claim 6, characterized in that: The reaction temperature was 60°C.

8. The samarium diiodide-mediated free radical cleavage method of lignin β-O-4-keto model compound according to claim 1, characterized in that: The reaction time is 3 to 24 hours.

9. The samarium diiodide-mediated free radical cleavage method of lignin β-O-4 keto model compound according to claim 8, characterized in that: The reaction time is 12 h.

10. Use of samarium diiodide in mediating free radical cleavage of a lignin β-O-4-keto model compound of formula (I) to obtain a ketone compound of formula (II) and a phenolic compound of formula (III); R1~R 10 Each independently selected from H, hydroxyl, C1-C11 alkoxy or -O-(CH2-O) n -CH3, n is 1 to 10; R 11 Selected from H, C1-C11 alkyl, C1-C11 hydroxyalkyl or -O-(CH2-O) m -CH3, m is 1 to 10.

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