Electrochemical Oxidation Dearomatization of Anisol Derivatives toward Spiropyrrolidines and Spirolactones
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-03-25 |
| Journal | CCS Chemistry |
| Authors | Cui Zhang, Faxiang Bu, Chulin Zeng, Dan Wang, Lijun Lu |
| Institutions | Shanghai Institute of Organic Chemistry, Wuhan University |
| Citations | 49 |
Abstract
Section titled “Abstract”Open AccessCCS ChemistryCOMMUNICATION1 Apr 2022Electrochemical Oxidation Dearomatization of Anisol Derivatives toward Spiropyrrolidines and Spirolactones Cui Zhang†, Faxiang Bu†, Chulin Zeng, Dan Wang, Lijun Lu, Heng Zhang and Aiwen Lei Cui Zhang† College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 †C. Zhang and F. Bu contributed equally to this work.Google Scholar More articles by this author , Faxiang Bu† College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 †C. Zhang and F. Bu contributed equally to this work.Google Scholar More articles by this author , Chulin Zeng College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 Google Scholar More articles by this author , Dan Wang College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 Google Scholar More articles by this author , Lijun Lu College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 Google Scholar More articles by this author , Heng Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 Google Scholar More articles by this author and Aiwen Lei Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100860 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Spiro compounds are widely prevalent in biological activities and natural products. However, developing new strategies for their efficient synthesis and derivatization remains a challenge. Outstanding progress has been made in the synthesis of spiro compounds through dearomatization of aromatic compounds, most of them are mediated by the hypervalent iodine reagents. Herein, we report a method of anodic oxidation spiroamination and spirolactonization of anisole derivatives with concomitant cathodic reduction of protons in the absence of hypervalent iodine reagents. A wide variety of spiropyrrolidines and spirolactones with diverse functional groups made useful scaffolds in this transformation, with yields up to 97%. Moreover, hectogram-scale synthesis could supply target product with 83% yield in a flow electrochemical cell using carbon paper as the anode and nickel plate as the cathode, demonstrating the potential application of this method. Download figure Download PowerPoint Introduction Spiro compounds have drawn increasing attention due to their unique structural features, wide prevalence in biological activities and natural products, and broad application prospects in asymmetric catalysis.1,2 Although the high demand for spiro compounds in particular has stimulated the exploitation of new synthetic methods to obtain the important building blocks,3-5 developing novel and efficient strategies for their synthesis and derivatization remains urgent.1 Dearomatization strategy is one of the paramount methods to construct spiro compounds, since the easily available aryl compounds can be used as starting materials.6,7 In the early studies, the common methods to access spiro compounds from arenols were mediated by a stoichiometric amount of hypervalent iodine reagents (Scheme 1a).8-13 A significant breakthrough in this field was made by Kita and co-workers.14,15 In their work, only a catalytic amount of hypervalent iodine reagents was needed, which could be recycled by using m-chloroperbenzoic acid as cooxidant (Scheme 1b, Mode A). Inspired by this pioneering work, we developed various methods by combining a catalytic amount of hypervalent iodines, halogenated compounds, or noble metal species with a stoichiometric amount of oxidants.16-21 However, the substrate scope is usually limited to 1- and 2-naphthols, especially for the spiroamination and spirolactonization of arenols. Besides, Wirth and co-workers22 reported a novel continuous-flow electrochemical method with two steps in which hypervalent iodine reagents were initially generated from low valent aryl iodides in the reaction system, and then substrates were oxidized in the next step (Scheme 1b, Mode B). The flow electrochemical method avoids the utilization of hypervalent iodines or other chemical oxidants. Very recently, a significant photochemical method was developed by Habert and Cariou,23 in which only a catalytic amount of iodoarene and photocatalyst was needed (Scheme 1c). The active ArIIII species could be formed in situ in the presence of photocatalyst and dioxygen. It is worth noting that although cooxidants and photocatalysts were used, hypervalent iodines are still necessary as direct oxidants for these effective transformations. Scheme 1 | Spiro cyclization with (a) stoichiometric amount of hypervalent iodines as oxidants; (b) catalytic amount of hypervalent iodines and stoichiometric amount of cooxidants or in flow electrochemistry; (c) catalytic amount of iodoarene in photochemistry. (d) This work: electrochemical oxidation-induced spiro cyclization. Download figure Download PowerPoint In recent years, electrochemical synthesis has been widely recognized as an efficient and environmentally benign synthetic tool in synthetic chemistry.24-34 With sufficient potential bias, organic substrates can lose electrons at the anode to generate highly reactive intermediates, which is superior to the classical reactions involving chemical oxidants. Compared with the well-known chemical dearomative procedures, few sustainable electrochemical versions have been validated. In fact, there are several challenges associated with such a transformation under electrochemical conditions: (1) Without hypervalent iodines which can selectively oxidize arenols through aryl-λ3-iodane species,35 it is difficult to preferentially oxidize the aromatic rings in the presence of the amine groups (XH = NHR),36,37 which may lead to many unfavorable side reactions.38-42 (2) Direct oxidation of substrates can yield active radical or radical cation intermediates, which may go through self-coupling or react with polar solvent to produce unfavorable byproduct.43-47 To address this challenge, Amano and Nishiyama48 chose methoxyamides as substrates to be employed for direct anodic oxidation with limited substrate scope and yields up to 67%. The nitrenium ion that they produced could be localized and stabilized by the methoxyamide structures. In spite of the above difficulties, we report herein an electrochemical oxidation-induced dearomatization reaction toward C-N and C-O bonds formation with the generation of spiropyrrolidines and spirolactones in good to excellent yields (Scheme 1d). Results and Discussion We first attempted the dearomatization and spiro cyclization of 1-Ts in the presence of nBu4NBF4 as supporting electrolyte and MeOH as solvent (Table 1). When no additive was involved, no desired product was detected. Only 46% yield of benzyl oxide methoxylation byproduct was obtained (entry 1, see details in Supporting Information Scheme S1). After adding nBu4NOAc as a base, the desired product 2 was obtained with the concomitant formation of byproducts 2′ and 2″, and the yields were 32%, 6%, and 24%, respectively (entry 2). Other bases that are more basic than nBu4NOAc, such as PivONa and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), gave increased yields for both desired product 2 and undesired byproducts 2′ and 2″ (entries 3 and 4). However, the selectivity of 2 was low. Next, the extra addition of PivOH in the presence of PivONa improved the selectivity of 2, although there was no change in yield (entry 5). It was proposed that 2′ was produced through the addition of the N radical to the aryl ring, which could be caused by the oxidation of the amido group, and the byproduct 2″ was generated following the deep oxidation of 2′ (see details in Supporting Information Scheme S2). According to the analysis of the above experimental results, we considered that the inhibition of the oxidation of the amido group should benefit the formation of the desired product 2. Then Cs was chosen as the protective group that was more electron-deficient than Ts and should be beneficial to suppress the oxidation of the amido group. With nBu4NOAc as additive, the yield and selectivity were increased to 44% and 73%, respectively (entry 6). The yield and selectivity improved in the presence of PivONa and PivOH as additives (entry 7). Furthermore, decreasing the current from 10 to 6 mA promoted the desired transformation (entry 8). Finally, changing the supporting electrolyte to LiClO4 and increasing the reaction time further increased the yield and selectivity to 85% and 92%, respectively (entry 9). However, the free phenol derivatives were not compatible with this protocol (see details in Supporting Information Scheme S3). Table 1 | Optimization of the Reaction Conditions Entry Additive (equiv.) PG Yield (2/2′/2″) (%)a Conversion (%) 1 None Ts 0/0/0 (46b) 77 2 nBu4NOAc (1) Ts 32/6/24 56 3 PivONa (1) Ts 37/30/20 84 4 DBU (1) Ts 43/41/0 85 5 PivONa (1), PivOH (2) Ts 36/6/31 65 6 nBu4NOAc (1) Cs 44/0/0 60 7 PivONa (1), PivOH (2) Cs 63/7/10 82 8c PivONa (1), PivOH (2) Cs 69/0/9 78 9d PivONa (1), PivOH (2) Cs 85/0/6 91 Reaction conditions: 1 (0.2 mmol), nBu4NBF4 (0.02 M), MeOH (10.0 mL), carbon cloth anode (15 mm × 15 mm × 0.36 mm), Pt cathode (15 mm × 15 mm × 0.3 mm), constant current 10 mA, 75 min, room temperature. aIsolated yields are shown. bYield of benzyl oxide methoxylation byproduct. c6 mA, 2 h. dLiClO4 (0.015 M) as electrolyte, 6 mA, 4 h. With the optimized conditions in hand, we next explored the scope of the reaction (Scheme 2). First, when the two methyl groups on the alkyl chain, generally considered to promote intramolecular cyclization, were replaced with hydrogens, only a little change of the yield was observed ( 2b). Then, the substrates with various substituents on the aromatic rings ( 1c- 1h) were studied, and moderate to good yields were obtained. The electron-donating groups, such as methyl ( 1c) and methoxy groups ( 1d), and electron-withdrawing groups, such as chloro ( 1e), trifluoromethyl ( 1f), and fluoro groups ( 1g), were all compatible with this transformation. Interestingly, only para-ketones 2d was obtained for 1d with two methoxy groups. It is worth mentioning that dearomatization spiro cyclization reaction is normally applicable to electron-rich or neutral aromatic rings. The substrate with trifluoromethyl group-substituted aromatic core ( 1f), which was generally intolerable according to the previous literature reports,8-13,16-21 could be successfully transformed in this reaction. Besides, the substrates such as 4-methoxy naphthyl ( 1h) and 2-methoxy phenyl ( 1i) could also be tolerated with this method. Various substituents in the alkyl chain were also investigated. The substrates with ethyl ( 1j) or phenyl ( 1k) on the β position of the amido group could be transformed smoothly with good yields. In addition, multispiro compounds could be constructed in one step with good to excellent yields when three- to six-membered rings were introduced to the substrates ( 1l- 1p). When we explored the substrate with methyl substituted on the benzyl position, no spiro product was obtained, but the six-membered ring product was obtained with 76% yield (see details in Supporting Information Scheme S4). In addition, a five-membered cyclization product containing the benzyl C-N bond was obtained in 78% yield when Cs-protected 4-(4-methoxyphenyl)butan-1-amine was used as substrate (see details in Supporting Information Scheme S5). Finally, several other protecting groups were investigated. With para-trifluoromethyl ( 1q), fluoro ( 1r), and formylmethyl ( 1s)-substituted benzenesulfonyl or trifluoromethylsulfonyl ( 1t) as protecting groups, the transformation could still be carried out successfully with moderate to good yields. Scheme 2 | Reaction conditions: 1 (0.2 mmol), PivONa (1 equiv based on 1), PivOH (2 equiv based on 1), LiClO4 (0.015 M), MeOH (10.0 mL), room temperature, carbon cloth anode, Pt cathode, constant current, 6 mA, 4 h. Isolated yields are shown. a6 h. Download figure Download PowerPoint Based on the study of the above spiroamination reactions, this electrooxidaton strategy was also applied to the spirolactonization reactions (Scheme 3). Para-methoxyphenylpropionic acid 3a was converted to the corresponding product 4a with excellent yield. Similar to the spiroamination reactions, the substrates with electron-donating groups, such as the methoxy ( 3b) and methyl groups ( 3c), and the electron-withdrawing group, such as trifluoromethyl group ( 3d), could be compatible in this reaction system. The compound 3e with methoxy group on the ortho position of the aryl ring could also be carried out in this transformation. Interestingly, only ketal products were detected for substrates 3d and 3e. We were delighted to find carboxylic acids substituted with alkyl or aryl groups either at the α or β position, and ( 3f- 3i) could be transformed smoothly with good to excellent yields. Scheme 3 | Reaction conditions: 3 (0.5 mmol), nBu4NOAc (1 equiv based on 3), MeOH (5.0 mL), room temperature, carbon cloth anode, Pt cathode, constant current, 10 mA, 3 h. Isolated yields are shown. aOnly ketal products detected. b10 mA, 3.5 h. Download figure Download PowerPoint To further explore the practicability of this protocol, we expanded the synthesis of 2a on the gram scale in an electrochemical flow cell49 with carbon paper as the anode and nickel plate as the cathode (see details in Supporting Information Figure S1). To our delight, 1a could be transformed smoothly with 90% yield (Scheme 4a). However, the yield of product 2a decreased to 47% when the magnitude of the current was increased to 1 A (see details in Supporting Information Scheme S7). Moreover, product 4a could be synthesized on the gram scale with 82% yield (Scheme 4b, see details in Supporting Information Scheme S8). Importantly, the reaction could be successfully extended to the hectogram scale in the same flow cell at 1 A current with 83% yield (Scheme 4c, see details in Supporting Information Schemes S9 and S10). These results illustrate the great potential of this protocol in the synthesis of spiro compounds based on readily available arenes. Scheme 4 | Gram- and hectogram-scale synthesis using flow cell. Download figure Download PowerPoint To gain further insight into the reaction mechanism, cyclic voltammetry was tested to measure the oxidation potentials of the substrates in MeOH with nBu4NF as the base (see details in Supporting Information Figure S2). The choice of nBu4NF can be ascribed to the following two reasons: (1) No oxidation peak of nBu4NF could be observed below 2.3 V versus Ag/AgCl in this reaction system (Scheme 5a, black line). (2) The target product 2a could be obtained with 51% yield when employing nBu4NF as base (see details in Supporting Information Scheme S6). To determine whether the aromatic core or the amido group was more easily oxidized, p-propylanisole, nPrNHCs, and 1a were tested. No oxidation peak was observed below 2.3 V versus Ag/AgCl for nPrNHCs (Scheme 5a, green line), which indicated that the Cs-protected amido group could not be oxidized under this reaction system. The oxidation peak of p-propylanisole was about 1.66 V versus Ag/AgCl (Scheme 5a, blue line). 1a, bearing both the aromatic core and the amido group, was irreversibly oxidized at 1.73 V versus Ag/AgCl (Scheme 5a, red line), which resembled that of p-propylanisole. Based on the cyclic voltammetry results, we postulated that the aromatic core was the functional group that engaged in the initial oxidation. Besides, 1H NMR experiments were carried out to confirm the effect of the base (Scheme 5b, see details in Supporting Information Scheme S11). The chemical shift of the N-H in 1a was 4.64 ppm (Scheme 5b, blue line). When 1 equiv nBu4NOAc was added, the peak of the N-H in 1a disappeared (Scheme 5b, red line), which indicated that it was an active hydrogen. The above 1H NMR experiments showed that the N-H bond of 1a could be activated in the presence of the base. Scheme 5 | (a) Cyclic voltammetry of 1a, p-propylanisole, and nPrNHCs in MeOH. (b) 1H NMR experiments. (c) Possible mechanism. Download figure Download PowerPoint Based on the above experimental results and literature reports,50-55 a possible mechanism was proposed, as shown in Scheme 5c. First, 1a was oxidized at the anode to generate the intermediate I with radical cation focused on the aromatic core. Then with the help of the base, the nucleophilic amido group attacked the aromatic core to form the radical intermediate II which could be oxidized at the anode to produce cation intermediate III. Next, intermediate III was captured by MeOH and then hydrolyzed to give the target product 2a. Meanwhile, concomitant reduction of protons generated H2 at the cathode. However, radical intermediate II, generated through the addition of N radical intermediate V to the aromatic core, could not be excluded (see details in Supporting Information Schemes S12 and S13). Conclusions We have developed spiroamination and spirolactonization of anisole derivatives through anodic oxidation dearomatization iodine reagents. This synthetic strategy can be employed for a wide range of amines and carboxylic acids, not just 1- and 2-naphthols. Besides, anisole derivatives with electron-withdrawing groups were also applicable in this electrochemical system, which were extremely challenging under chemical oxidation conditions. Importantly, the electrosynthetic reaction could be easily scaled up to hectogram in a flow electrochemical cell, demonstrating the synthetic potential of this method. Based on the cyclic voltammetry experiments and density functional theory (DFT) calculations, a reasonable mechanism with the aromatic core oxidized preferentially has been proposed. Compared with other electrochemical lactone formation work,56-59 our protocol is compatible with a wide range of substrates and scales up easily. Supporting Information Supporting Information is available, including the general information, experimental methods, DFT calculation details, and copies of NMR spectra for products. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (no. 22031008) and the Science Foundation of Wuhan (no. 2020010601012192). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated. The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of Wuhan University. Acknowledgments Dedicated to P.H. Dixneuf for his outstanding contribution to organometallic chemistry and catalysis. References 1. Ding A.; Meazza M.; Guo H.; Yang J. W.; Rios R.New Development in the Enantioselective Synthesis of Spiro Compounds.Chem. Soc. Rev.2018, 47, 5946-5996. Google Scholar 2. D’yakonov V. A.; Trapeznikova O. A.; de Meijere A.; Dzhemilev U. M.Metal Complex Catalysis in the Synthesis of Spirocarbocycles.Chem. Rev.2014, 114, 5775-5814. Google Scholar 3. 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