Electrochemical Oxidative [4+2] Annulation of Different Styrenes toward the Synthesis of 1,2-Dihydronaphthalenes
Kailun Liang, Shengchun Wang, Hengjiang Cong, Lijun Lu, Aiwen Lei
Abstract
Open AccessCCS ChemistryCOMMUNICATION1 May 2022Electrochemical Oxidative [4+2] Annulation of Different Styrenes toward the Synthesis of 1,2-Dihydronaphthalenes Kailun Liang, Shengchun Wang, Hengjiang Cong, Lijun Lu and Aiwen Lei Kailun Liang College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 , Shengchun Wang College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 , Hengjiang Cong College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan, Hubei 430072 , Lijun Lu *Corresponding author: 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 and Aiwen Lei *Corresponding author: 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 National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang, Jiangxi 330022 https://doi.org/10.31635/ccschem.021.202100933 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail A [4+2] annulation of two different styrenes to construct polysubstituted 1,2-dihydronaphthalenes was achieved. This transformation proceeded smoothly under electrochemical oxidative conditions without metal catalysts and external oxidants. A series of polysubstituted 1,2-dihydronaphthalenes were obtained with high regioselectivity and diastereoselectivity. Moreover, polysubstituted 1,2-dihydronaphthalenes were further transformed to polysubstituted 1,2,3,4-tetrahydronaphthalenes and polysubstituted naphthalenes, which show great potential in synthetic applications. Download figure Download PowerPoint Introduction 1,2-Dihydronaphthalene compounds are important structures in pharmaceuticals and natural products.1–3 This motif can be synthesized by the elimination reaction of halo- or hydroxyl-substituted tetrahydronaphthalenes, intramolecular annulation of phenyl alkenes or alkynes, partial hydrogenation of naphthalene, and partial oxidation of tetrahydronaphthalenes (Scheme 1a).4–12 However, these methods usually suffer from substrate scope limitations and harsh reaction conditions. Regarding atom and step economy, the direct utilization of readily available styrenes to construct 1,2-dihydronaphthalenes is a promising route. Scheme 1 | (a) General syntheses of 1,2-dihydronaphthalenes. (b) Electrochemical synthesis of 1,2-dihydronaphthalenes. Download figure Download PowerPoint Compared with the transition-metal-catalyzed alkene insertion,13–16 the single-electron oxidation of alkenes is another strategy to achieve alkene functionalization. Nicewicz and colleagues developed a series of Anti-Markovnikov alkene hydrofunctionalization, in which the key step was alkene oxidation by photocatalysts to afford the corresponding alkene cation radical.17–19 In 2016, our group introduced the Anti-Markovnikov oxidation of alkenes to construct carbonyl compounds with H2 evolution.20 Subsequently, several studies reported the functionalization of alkenes by using alcohol and alkyne as nucleophiles.21,22 Additionally, Ooi and Wu reported photoredox-induced allylic C–H alkylation.23,24 The single-electron oxidation of alkenes can also be achieved by electrochemical oxidation.25–28 Currently, electrochemical synthesis attracts increasing attention due to its simple and mild reaction conditions.29–34 In 2019, Ye and his colleagues35 reported an electrochemical [4+2] annulation of styrenes to access naphthalene derivatives under mild conditions. The transformation of styrenes, such as difunctionalization,36–42 carboxylation,43–44 epoxidation,46 and hydrogenation47–48 showed great synthetic potential. Electrochemical oxidation provides a good opportunity to control the reactivity of different styrenes based on their oxidation potentials. Hence, selecting suitable styrenes and using an anodic oxidation method could achieve the annulation of different styrenes. Herein, we report an electrochemical oxidative [4+2] annulation of styrenes to synthesize 1,2-dihydronaphthalenes (Scheme 1b). Results and Discussion We initially explored the process with α-methylstyrene ( 1a) and trans-anethole ( 2a) as model substrates (Table 1). With carbon cloth as anode, Pt plate as cathode, tris(4-bromophenyl)amine [(4-BrPh)3N] as mediator, nBu4NClO4 as electrolyte, acetonitrile (CH3CN) and hexafluoroisopropanol (HFIP) as mixed solvents, 71% yield of the desired product was obtained under 7.5 mA current at room temperature (Entry 1). The presence of hydrogen was tested by gas chromatography with a thermal conductivity detector (GC-TCD) (see Supporting Information). The yield decreased slightly with acetone instead of CH3CN (Entry 2), and HFIP was important for the smooth transformation (Entry 3). Replacing the carbon cloth or Pt plate with other electrodes did not increase the yields (Entries 4 and 5). (4-BrPh)3N was useful for the repeatability of this system (Entry 6). Other electrolytes, such as n-Bu4NBF4 and n-Bu4NPF6, were also compatible in this electrochemical reaction (Entries 7 and 8). The yields obtained were lower when the amount of 2a or current was decreased (Entries 9 and 10). Air atmosphere was tolerated in this process (Entry 11). No desired product was produced without electricity (Entry 12). Table 1 | Investigation of the Reaction Conditionsa Entry Variation From the Standard Conditions Yield [%]b 1 None 71 (67) 2 Acetone instead of CH3CN 59 3 No HFIP 28 4 Graphite rod instead of carbon cloth 38 5 Nickel plate instead of platinum plate 53 6 No (4-BrPh)3N 64 7 n-Bu4NBF4 instead of n-Bu4NClO4 64 8 n-Bu4NPF6 instead of n-Bu4NClO4 62 9 2.0 equiv 2a 63 10 5 mA, 6 h 63 11 Under air atmosphere 59 12 No electricity n.d. aStandard conditions: carbon cloth anode (15 mm × 15 mm), platinum plate (15 mm ×15 mm × 0.3 mm) cathode, constant current of 7.5 mA, 1a (0.2 mmol), 2a (3.0 equiv based on 1a), (4-BrPh)3N (20 mol % based on 1a), n-Bu4NClO4 (0.5 equiv based on 1a), CH3CN (6.0 mL), HFIP (50 μL), room temperature (r.t.), N2 atmosphere, 4 h, undivided cell. bYields were determined by GC analysis, calibrated using biphenyl as the internal standard (isolated yield in parentheses; n.d. = not determined). With the optimized reaction conditions in hand, we turned our attention to investigate the substrate scope of α-methylstyrene derivatives (Scheme 2). α-Methylstyrene, 4-methyl, 3-methyl, and 3,5-dimethyl (structure was confirmed by X-ray single-crystal diffraction) substituted α-methylstyrenes gave 56–72% yields of desired products, respectively ( 3aa– 3da). p-Ethyl- and p-tert-butyl-α-methylstyrene afforded the corresponding products in moderate yields ( 3ea and 3fa). α-Methylstyrene derivatives containing halogen groups, such as Cl and Br, were well tolerated ( 3ga and 3ha). A 78% yield of product was obtained when using 4-benzyl-α-methylstyrene as substrate ( 3ia). 6-(Prop-1-en-2-yl)-1,2,3,4-tetrahydronaphthalene and 2-(prop-1-en-2-yl)naphthalene (structure was confirmed by X-ray single-crystal diffraction) were also produced in this electrochemical system ( 3ja and 3ka). Replacing methyl with ethyl, isopropyl, cyclopentyl, and cyclohexyl at the α-position did not influence the transformation of substrates ( 3la–3oa). 1,1-Diphenylethylene afforded the target product in 71% yield ( 3pa). The functionalized methyl, F, phenyl-substituted 1,1-diphenylethylenes all proceeded smoothly under standard conditions ( 3qa– 3ta). Scheme 2 | Substrate scope of α-methylstyrenes. Reaction conditions: Carbon cloth anode (15 mm × 15 mm), platinum plate (15 mm × 15 mm × 0.3 mm) cathode, constant current of 7.5 mA, 1 (0.2 mmol), 2a (3.0 equiv based on 1), (4-BrPh)3N (20 mol % based on 1), n-Bu4NClO4 (0.5 equiv based on 1), CH3CN (6.0 mL), HFIP (50 μL), room temperature (r.t.), N2 atmosphere, 4 h, undivided cell. Isolated yields are shown, and diastereomeric ratio (d.r.) values were determined by gas chromatography analysis. PMP = (4-methoxy-phenyl). a2 mmol scale, CH3CN/HFIP = 60 mL/0.5 mL, 40 h. Download figure Download PowerPoint Next, we focused on the substrate scope of anetholes (Scheme 3). Although a mixture of Z/E alkenes were used as substrates, excellent diastereoselectivity was observed in all cases. Longer carbon chains, such as ethyl and n-propyl, gave 53% and 58% yields of desired products, respectively ( 3ab and 3ac). Regarding anisole substrates, replacing methyl with isobutyl, cyclopentyl, and cyclohexyl was tolerated ( 3ad– 3af). Benzyl-substituted substrate, whose structure was confirmed by X-ray single-crystal diffraction, gave moderate yield ( 3ag). Alkenyl and alkynyl groups were also compatible in this electrochemical transformation ( 3ah and 3ai). β-Methylstyrene only afforded the dehydrogenated product because the desired product was susceptible to further oxidization. Moreover, electron-deficient styrenes, such as 4-trifluoro-β-methylstyrene, were not compatible in this reaction because it was difficult to be oxidized to afford the radical cation. Scheme 3 | Substrate scope of anetholes. Reaction conditions: Carbon cloth anode (15 mm × 15 mm), platinum plate (15 mm × 15 mm × 0.3 mm) cathode, constant current of 7.5 mA, 1a (0.2 mmol), 2 (3.0 equiv based on 1a), (4-BrPh)3N (20 mol % based on 1a), n-Bu4NClO4 (0.5 equiv based on 1a), CH3CN (6.0 mL), HFIP (50 μL), room temperature (r.t), N2 atmosphere, 4 h, undivided cell. Isolated yields are shown, and diastereomeric ratio (d.r.) values were determined by gas chromatography analysis. Download figure Download PowerPoint To gain insight into the reaction mechanism, we did cyclic voltammetry (CV) experiments and electron paramagnetic resonance (EPR) experiments . First, CV measurements were conducted to study the redox behavior of substrates (Figure 1a). As shown, the oxidation peak potential of (4-BrPh)3N is 1.20 V. The oxidation peak potential of 1a (1.91 V) is higher than 2a (1.41 V), which indicated that 2a is oxidized easier than 1a. Furthermore, EPR experiments were performed to investigate the radical intermediate produced by the oxidation of trans-anisole (Figure 1b). With the addition of radical spin trapping reagent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), a signal of mixed DMPO trapping carbon radicals (g = 2.0063, AN = 15.20 G, AH = 21.50 G) and hydrogenated DMPO (g = 2.0063, AN = 14.87 G, AH1 = 19.62 G, AH2 = 19.62 G) was identified. The results support the possible existence of a carbon radical intermediate according to the coincidence of the molecular weight of DMPO trapping products. Figure 1 | Mechanism studies. (a) CV measurements. (b) EPR experiments. Download figure Download PowerPoint Based on the results above, a plausible mechanism is proposed in Scheme 4a. First, (4-BrPh)3N is oxidized at the anode to afford N radical cation, which oxidizes 2a to produce alkene radical cation I through single-electron oxidation. Then, intermediate I is attacked by nucleophile 1a to give intermediate II. Finally, the desired product 3aa is obtained by successive radical cyclization, oxidation, and deprotonation. Scheme 4 | (a) Proposed mechanism. (b) Derivatization of products. Download figure Download PowerPoint The desired product 3aa was further transformed to get useful chemicals (Scheme 4b). For example, a polysubstituted naphthalene was obtained under 2,3-dicyano-5,6,-dichlorobenzoquinone (DDQ) oxidation conditions, and the methoxy group was converted to triflate for further transformation. Hydrogenation using Pd/C as catalyst under H2 atmosphere produced 83% yield of polysubstituted 1,2,3,4-tetrahydronaphthalenes. Conclusion In summary, we developed an electrochemical oxidative annulation of styrenes to construct 1,2-dihydronaphthalenes with good regioselectivity and diastereoselectivity. This methodology provides a powerful synthetic tool for the formation of multisubstituted six-membered ring compounds from commercially available materials. Supporting Information Supporting Information is available, including the general information, experimental methods, detail descriptions, and 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 (22031008) and Science Foundation of Wuhan (2020010601012192). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated. Acknowledgments Dedicated to Prof. Christian Bruneau for his outstanding contribution to catalysis. References 1. Azhar, Ul. H.; Malik A.; Anis I.; Khan S. B.; Ahmed E.; Ahmed Z.; Nawaz S.A.; Choudhary M. I.Enzyme Inhibiting Lignans from Vitex Negundo.Chem. Pharm. Bull.2004, 52, 1269–1272. Google Scholar 2. Jordan V.C.Antiestrogens and Selective Estrogen Receptor Modulators as Multifunctional Medicines. 2. Clinical Considerations and New Agents.J. Med. Chem.2003, 46, 1081–1111. 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