We have developed visible-light photoredox-catalyzed C–O bond cleavage of diaryl ethers by an acidolysis with an aryl car-boxylic acid and a following one-pot hydrolysis. Two phenols are obtained from a diaryl ether at room temperature. The aryl carboxylic acid used for the acidolysis can be recovered. The key to success of the acidolysis is merging visible-light photore-dox catalysis with a new acridinium photocatalyst and Lewic acid catalysis with Cu(TMHD)2. Preliminary mechanistic studies indicate that the catalytic cycle occurs via a rare selective electrophilic attack of the generated aryl carboxylic radical on the electron-rich aryl ring of diphenyl ether. This transformation is applied to a gram-scale reaction and the model of 4-O-5 lignin linkages.
Article
Visible-Light Photoredox-Catalyzed C–O Bond Cleavage of Diaryl Ethers by Acridinium Photocatalysts at Room Temperature
https://doi.org/10.21203/rs.3.rs-52259/v1
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published 29 Nov, 2020
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We have developed visible-light photoredox-catalyzed C–O bond cleavage of diaryl ethers by an acidolysis with an aryl carboxylic acid and a following one-pot hydrolysis. Two phenols are obtained from a diaryl ether at room temperature. The aryl carboxylic acid used for the acidolysis can be recovered. The key to success of the acidolysis is merging visible-light photoredox catalysis with a new acridinium photocatalyst and Lewic acid catalysis with Cu(TMHD)2. Preliminary mechanistic studies indicate that the catalytic cycle occurs via a rare selective electrophilic attack of the generated aryl carboxylic radical on the electron-rich aryl ring of diphenyl ether. This transformation is applied to a gram-scale reaction and the model of 4-O-5 lignin linkages.
Biomass widely distributes in nature containing lignin as one of major components.1-3 In lignin, there are three major types of aryl ether bonds of α-O-4 (218 kJ/mol), β-O-4 (289 kJ/mol), and 4-O-5 (314 kJ/mol).1-3 Cleavage of C–O bonds in lignin can afford renewable aryl sources for fine chemicals. However, the high bond energies of these C–O bonds, especially the 4-O-5-type diary ether C–O bonds, make the cleavage very challenging.1-10 Therefore, in fundamental research, cleavage of aryl C–O bonds has attracted much attention.11-18
Cleavage of the α-O-4 and β-O-4 types of C–O bonds has been studied, even under mild conditions by visible-light photoredox catalysis.19-23 For cleavage of the 4-O-5-type diary ether C–O bonds, classical studies focused on the hydrolysis by supercritical water, the hydrogenolysis with poor selectivity using the model of 4-O-5 lignin linkages. 2,4-5,10 Many aryl skeletons were destroyed. In selective cleavage methods, the use of stoichiometric alkali metals24-26 or electrocatalytic hydrogenolysis27-29 limited the large-scale applications because of the associated high costs.
In recent years, the selective hydrogenolysis under comparatively milder conditions was developed by Hartwig, Grubbs, Wang, et al (Fig. 1A).30-41 The hydrogenolysis was accomplished by using [Ni], [Fe], or [Co] as catalyst, with H2 (1–6 bar) or LiAlH4 (2.5 equiv) as reductant at 120–180 °C.30-36 A higher than stoichiometric strong base, such as NaOtBu/KOtBu/KHMDS (2.5 equiv), is important for the selectivity (Fig. 1A, conditions a). Without using transition-metal catalyst, a combination of Et3SiH/NaH (³2.5 equiv) and KHMDS (2.5 equiv) at 140–165 °C achieved the selective hydrogenolysis (Fig. 1A, conditions b).37-38 More importantly, by MOF or Pd/C as catalyst without a base, the selective hydrogenolysis with H2 (10–30 bar) at 120–200 °C obtained no more than 55% yield (Fig. 1A, conditions c).39-41 In these studies, a large amount of reductant, strong base, and/or the low yield are/is still the limited factor(s) for the applications.
Thus, there exists a strong incentive to develop more practical methods for diaryl ether C–O bonds cleavage, toward the utilization of lignin as renewable aryl sources.
Herein, we describe visible-light photoredox-catalyzed C–O bond cleavage of diaryl ethers by an acidolysis with an aryl carboxylic acid and a following one-pot hydrolysis at room temperature (rt). Two phenols, with some advantages than arenes in transformations such as aminations, the functionalization, cross coupling reactions,42-43 are obtained. The aryl carboxylic acid used for the acidolysis can be recovered (Fig. 1B). The key to success of the acidolysis is merging visible-light photoredox catalysis with a new acridinium photocatalyst (PC) and Lewis acid catalysis with Cu(TMHD)2 (Fig. 1B). Inspired by the significant contributions of Fukuzumi,44-46 Nicewicz,47-49 Sparr,50-51 et al.52-54 on acridinium PCs, we investigated the use of a new acridinium catalyst. Notably, an aryl ring with electron-withdrawing groups, instead of mesitylene, typically used in other acridinium catalysts,44-54 on the 9-position resulted in obviously higher efficiency. Furthermore, with this method, the model of 4-O-5 lignin linkages afforded phenol and 2-methoxyphenol in high efficiency. Compared with the developed selective hydrogenolysis in recent years, using H2O instead of a large amount of reductant afforded two more valuable phenols at rt.
We developed the C–O bond cleavage of diaryl ethers containing a carboxylic acid group on the ortho position by a visible-light photoredox-catalyzed intramolecular aryl migration from an aryl ether to the ortho carboxylic acid group at rt, and a following one-pot hydrolysis.55 Thus, we envisioned the possibility of C–O bond cleavage of general diaryl ethers by an aryl acidolysis with an aryl carboxylic acid followed by a hydrolysis, which would expand the scope of the special diaryl ethers largely, even to the model of 4-O-5 lignin linkages.
Specifically, in the aryl acidolysis, photoredox catalysis affords an aryl carboxylic radical A, then followed by its electrophilic attack on the diphenyl ether, and a single electron transfer (SET) with a proton and an electron (Fig. 2). However, the two issues make the transformation more challenging. First, although some intramolecular electrophilic attack of aryl carboxylic acid radicals to arenes has been reported,55-60 the intermolecular electrophilic attack of aryl carboxylic acid radicals to arenes has not been successfully explored. The intermolecular electrophilic attack of aryl carboxylic acid radicals to arenes was proposed in thermal decomposition of substituted dibenzoyl peroxides in diphenyl ether, in which the corresponding aryl benzoates were obtained in less than 39% yields with low selectivity. In addition, in comparison with the produced aryl carboxylic acid radicals, the amount of arenes was huge because diphenyl ether was used as solvent.61 Second, similarly to the intramolecular reaction, the formation of the more stable ester C–O bond should be the driving force of the diaryl ether C–O cleavage, as the C–O bond energies of an aryl ether and an ester are about 78.8 and 87–93 kcal mol-1, respectively.62-63 However, the possible intermediate B is indicated to lack stronger driving force of C–O bond cleavage, compared with the six-membered ring intermediate in the intramolecular reaction.55-60 As mentioned below, less than 10% product with the remained starting material was obtained under the optimized reaction conditions for intramolecular reactions (Table 1, entries 1, 2). Perhaps a Lewis acid could activate the ether.
Optimization study. With these considerations in mind, diphenyl ether (1a) and 4-methylbenzoic acid (2a) were studied as model substrates. Under our developed conditions for the intramolecular C–O bond cleavage,55 using PDI64 or Acr+-Mes ClO4– (PC 1)44-46 as PC, with 10 mol% K2HPO4 as base, under 450–455 nm blue LEDs irradiation, only less than 10% of phenyl 4-methylbenzoate (3a) and phenol (4a) were obtained (Table 1, entries 1, 2). Thereafter, a series of Lewis acids such as Cu(OAc)2, Cu(acac)2, Cu(OTf)2, Ni(acac)2, Fe(acac)2, Zn(acac)2, and Cu(TMHD)2 were studied (Table S1, entries 1–6, Table 1, entry 3). Cu(TMHD)2 slightly promoted the transformation. Adjustment of the wavelength of the blue LEDs to the maximum absorption of PC 1 (425–430 nm) induced a slightly increased reactivity (Table 1, entry 4). Other solvents such as MeOH, DCE, EtOAc, and acetone did not give any better results (Table S1, entries 7–10). As Acr+-Mes ClO4– is susceptible to degradation in the presence of oxygen-centered radicals,48 and de-N-methylation is also possible,47 it is deduced that the generated carboxylic acid radical may induce the degradation of Acr+-Mes ClO4–.
Subsequently, PC 2 and PC 3 were tried,48 in which the N-phenyl were used instead of the N-methyl, and also with the tert-butyl on the 3- and 6-positions of the acridinium in the latter case. Both factors induced distinct higher efficiency (Table 1, entries 5 and 6, 3a in 29% and 55%, with 4a in 22% and 49%).
Furthermore, the influence of the substitutes on the 9-aryl ring was investigated. Since the complex procedure for synthesis of PC 3,48 PC 4–PC 9, with a similar skeleton to that of PC 3 but with different substituents on the 9-aryl ring, by a two-step synthetic procedure,52 were investigated (Table 1, entries 7–12). Notably, the aryl rings with electron-withdrawing groups instead of mesitylene, typically used in other acridinium catalysts,44-54 resulted in noticeably high efficiencies. To our delight, PC 9 with 2’-Cl and 4’-F on the 9-aryl ring resulted in 80% of 3a with 71% of 4a (entry 12). Although the smaller group of 2’-Cl compared with the methyl groups in mesitylene was used, X-ray crystallography of PC 9 unambiguously confirmed the angle of torsion between the 9-aryl ring and the acridinium ring (Fig. 1B), which is closely related with a longer fluorescence lifetime.44,47 In addition, variation of the tert-butyl groups from the 2,7-positions to the 3,6-positions resulted in decreased efficiency (entry 13). Further variation of the substituents on the 10-aryl ring revealed that the unsubstituted phenyl gave a slightly higher yield (entry 15). Decreasing the amount of Cu(TMHD)2 resulted in obviously lower efficiency (entry 16). The amount of base did not influence the reaction efficiency, even without base (entries 17, 18). Other Lewis acids, such as Cu(OAc)2, Cu(acac)2, and Fe(acac)2 instead of Cu(TMHD)2, were investigated once again (entries 19–22), as slight differences during the initial investigation. The distinctly positive influence of Cu(TMHD)2 was further confirmed. Without base and Cu(TMHD)2, no reactivity was observed (entry 23). Control experiments indicate that a PC and visible light irradiation are essential (entries 25, 26). The fluorescence lifetime and the redox potentials of PC 1–PC 12 were determined (Table S2). The data do not provide clear insight regarding the higher efficiency achieved using PC 9.
Table 1 Optimization of the Reaction Conditions.
Evaluation of substrate scope. With the optimized reaction conditions, the substrate scope was investigated (Fig. 3). First, the influence of various substituents on the benzoic acid was investigated. 4-Methoxy, 4-tert-butyl gave decreased reaction efficiencies (3ab, 3ac in 61–65% with 4a in 56–60%). The benzoic acid afforded 3ad in 55% with 4a in 50%. 4-Fluoro, 4-chloro, and 4-bromo induced excellent yields (3ae–3ag in 87–91% with 4a in 82–84%). 4-Nitro, 4-aldehyde also resulted in high efficiencies (3ah, 3ai in 71–77% with 4a in 65–70%). The substituents on the 3-position were also studied. As with the substituents on the 4-position, fluoro and chloro resulted in high yields (3ak, 3al in 81–86% with 4a in 74–76%). Methyl gave a decreased yield (3aj in 61%). When the methyl was installed on the 2-position, no product was observed. 2-Fluoro resulted in lower efficiency (3ap in 63%). The phenomenon should be influenced by the steric factor.
Next, the substituents on the aryl ring of the diphenyl ethers were investigated (Fig. 4). With electron-withdrawing groups such as methyl ester, trifluoromethyl, nitro, cyan, or acetyl on the 4-position of the aryl ring, 3a (65–74%) and the corresponding phenols with these electron-withdrawing groups (4b-4f, 62–72%) were selectively obtained in high yields. The results agree with the designed pathway of the electrophilic attack of the generated carboxylic acid radical, including the selective attack on the electron-rich aryl ring of the diphenyl ethers. 4-Bromo afforded two esters 3a (22%) and 3aq (64%), and two phenols 4g (21%) and 4a (54%). The reason for the result may be the synergic effect of induction and conjugation of the bromo. For the 4-phenyl group, 3ar and 4a were selectively obtained in 84% yields. The reason for the selectivity may be the 4-phenyl group stabilizing the generated radical intermediate after the electrophilic attack. 4-Methyl and 4-methoxy resulted in very low efficiency (<10%). Generally, it maybe be explained by their comparatively lower oxidation potentials, inhibiting generation of the aryl carboxylic radical (Table S3). Methyl, methoxyl, dimethyl substituents on other positions afforded 3as–3au in 68–80% yields, and comparable yields of 4a. Symmetric dimethyl, dimethoxyl, dibromo, and dichloro, and asymmetric dichloro on the 3- or 4-positions resulted in good to high efficiencies (3av, 3aw, 52–90%, 4h–4k, 50–86%). When the methyl and methoxy on 2- or 3-position, with 4¢-ester, 4¢-cynao or 4¢-trifluoro, were investigated, the esters with methyl or methoxyl, as well as phenols with these electron-withdrawing groups, were obtained selectively in high yields (3as–3ax, 72–82%, 4b, 4e, 4c, 70–82%).
Synthetic application. To demonstrate the potential application, a gram-scale reaction of 1a with 2e in a flow reactor and a following one-pot hydrolysis was conducted. 4a was obtained in 80% yield, with 2e in 88% recovery rate (Fig. 5A). Meanwhile, the model of 4-O-5 lignin linkage (1t)30 afforded 4a (71%) and 4l (75%) in high efficiency, with 2e in 82% recovery rate (Fig. 5B).
Mechanism studies. To gain insight into the reaction mechanism, a series of experiments were conducted. First, UV−vis absorption spectra of each component and the reaction mixture confirmed that PC 9 acts as a PC (Fig. 6A). Second, luminescent quenching experiments were conducted (Fig. 6B). The anion of 2a (4-MePhCO2–nBu4N+), 2a, and 1a quenched the excited state PC 9*. The anion of 2a displayed an obviously larger quenching rate. Third, the pH value of the reaction mixture was determined as 3.61 or 4.30, with or without 10 mol% Cu(TMHD)2. Based on these pH values, Cu(TMHD)2 should promote the ionization of 2a by the salt effect.65 In addition, under base free conditions, the high reactivity of 80% ester 3a with Cu(TMHD)2 (entry 18), and 13–56% ester 3a with Cu(OAc)2, Cu(acac)2, Ni(acac)2, Fe(acac)2 (entries 19−22) in comparison with no production of 3a without any these metal salts (entry 23), these results suggest that the function of Cu(TMHD)2 also as a Lewis acid to promote the transformation.
Furthermore, with addition of TEMPO as oxidant, compounds 5, instead of 3 and 4, were obtained via the possible intermediates C48 (Fig. 7). This result suggests the generation of intermediate B’ to afford 3a and 4a under the optimized conditions.
The thermodynamic feasibility of the photo-induced SET was analyzed based on the oxidation–reduction potentials. The oxidation potential of E4-CH3PhCO2•/4-CH3PhCO2–, E1a+•/1a, and the reduction potential of EPC 9/PC 9–• in CH3CN were determined as +1.45 V vs. SCE, +1.86 V vs. SCE66, and –0.47 V vs. SCE (Figures S9, S10, and S34), respectively. The excited-state energy E0,0 of PC 9 was determined as 2.63 eV (Fig. S22). Therefore, the reduction potential of EPC 9*/PC 9–• was calculated as + 2.16 V vs. SCE (EPC*/PC–•=EPC/PC–•+ E0,0) (Fig. S34). These reduction potentials indicate the prior formation of PC–• and the carboxylic acid radical55-60 by a SET between PC* and the carboxylic acid anion. Furthermore, a quantum yield value of φ=0.20 was determined. Thus, at this stage, whether the reaction proceeds via a photoredox catalytic pathway or a radical chain pathway could not be reached.67
Based on these results, the reaction mechanism is proposed as shown in Fig. 8. First, Cu(TMHD)2 promotes the ionization of 2a to afford 2a– and a proton. Meanwhile, irradiation of PC with blue LEDs leads to the excited state PC*. A SET occurs between PC* and 2a– to generate the carboxylic acid radical A¢ and PC–•. An electrophilic attack of A¢ occurs on the electron-rich aryl ring of diphenyl ethers to form intermediate B¢. A SET between B¢ and PC–• in the presence of a proton, and the promotion of Cu(TMHD)2 as a Lewis acid afford 3a, 4a, with the regeneration of PC.
In summary, we have developed visible-light photoredox-catalyzed C–O bond cleavage of diaryl ethers by an acidolysis and a following one-pot hydrolysis at rt. Two phenols are obtained from a diaryl ether in high efficiency. The aryl carboxylic acid used for the acidolysis can be recovered. The key to success of the acidolysis is merging visible-light photoredox catalysis with a new acridinium photocatalyst and Lewis acid catalysis with Cu(TMHD)2. The transformation is applied to a gram-scale reaction and the model of 4-O-5 lignin linkages. In comparison with the developed selective hydrogenolysis in recent years, using H2O instead of a large amount of reductant affords two more valuable phenols at rt. This approach would inspire further visible-light photoredox-catalyzed C–O bond cleavage of 4-O-5 lignin linkages in native biomass for utilization of lignin as renewable aryl sources.
General Procedure for the C-O Bond Cleavage of Diaryl Ethers. To a quartz tube equipped with a magnetic stirring bar, PC 9 (0.015 mmol, 3.0 mol%, 9.60 mg), compound 1 (0.50 mmol), compound 2 (0.60 mmol), (Cu(TMHD)2) (0.05 mmol, 10 mol%, 21.5 mg) were added. The tube was evacuated and filled with argon three times with each cycle in 15 minutes. Freshly distilled solvent was then added into the tube via a syringe under argon atmosphere, then stirred and irradiated with 425-430 nm blue LEDs at ambient temperature (19-21 °C) in a Wattecs Parallel Reactor (Figure S1) for 30-60 h. After the reaction, the solvent was removed in vacuo and the residue was purified by column chromatography (petroleum ether/EtOAc = 200/1 - 5/1) to afford compound 3 and 4.
Full experimental procedures are provided in the Supplemental Information.
Data availability
The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number 2004336. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/. The data supporting the findings of this study are available within the article and its Supplementary Information files. Any further relevant data are available from the authors on request.
Author Contributions
F.-F. Tan and X.-Y. He performed the experiments and analyzed the data, F.-F. Tan and W.-F. Tian conducted the theoretical studies. Y. Li directed the project and wrote the manuscript.
Conflict of interest
The authors declare no conflict of interest.
Acknowledgements
This work was supported by National Key R&D Program of China (Nos: 2018YFB1501601) and the Instrument Analysis Center of the Xi'an Jiaotong University. We thank Prof. Gang He, Xi’an Jiaotong University for the support on cyclic voltammetry experiments and emission-quenching experiments.
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There is NO Competing Interest.
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