Copper-Catalyzed Transfer Hydrogenation of N-Heteroaromatics with

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Article Cite This: ACS Omega 2019, 4, 8487−8494

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Copper-Catalyzed Transfer Hydrogenation of N‑Heteroaromatics with an Oxazaborolidine Complex Yuanhai Zhong,† Taigang Zhou,*,†,‡ Zhuohua Zhang,† and Ruiqing Chang† †

College of Chemistry and Chemical Engineering and ‡State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Xindu Road 8, Chengdu, Sichuan 610500, China

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ABSTRACT: The first-time use of the oxazaborolidine complex in transfer hydrogenation was accomplished. It was prepared without difficulty from cheap materials: ethanolamine and BH3·THF. A general and efficient method for copper-catalyzed transfer hydrogenation of a variety of Nheteroaromatics with an oxazaborolidine−BH3 complex under mild reaction conditions afforded the corresponding hydrogenated products in up to 96% yield. Mechanistic studies indicate that the hydrogen source originated from water and borane that coordinate with the nitrogen atom of oxazaborolidine. Accordingly, a plausible mechanism for this reaction was proposed. This method was successfully used in the key step synthesis of natural products (±)-angustureine and (±)-galipinine in three steps.

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INTRODUCTION Chiral oxazaborolidines have proven to be powerful and effective catalysts not only for asymmetric reduction of ketones but also for Diels−Alder reactions, giving excellent enantioselectivity (Scheme 1).1 In 1981, Itsuno and co-workers first reported the asymmetric reduction of achiral aromatic ketones with a mixture of chiral amino alcohols and a borane− tetrahydrofuran complex in 60% ee and high yield.2 In 1987, Corey and co-workers discovered that chiral amino alcohols

and borane led to the formation of chiral oxazaborolidines, which were considered to be rapid and highly enantioselective catalysts for the reduction of achiral ketones.3 Later, Corey and Loh reported that chiral oxazaborolidines were highly effective not only for the Mukaiyama aldol reaction but also for the Diels−Alder reaction.4 Recently, Bach and Poplata have reported the highly enantioselective (82−96% ee) intermolecular [2 + 2] photocycloaddition reaction of cyclic enones promoted by the chiral oxazaborolidine−AlBr3 complex.5 Although great progress has been made since the discovery of this new reaction, the type of reaction is still limited.1b,6 It is highly desirable to expand the application of oxazaborolidines to different types of reactions. Saturated N-heteroaromatics are important building blocks for biologically active natural alkaloids and in the manufacture of commercial drugs.7 Direct hydrogenation of N-heteroaromatics has emerged as an efficient and straightforward strategy for this preparation. Despite this, the hydrogenation of N-heteroaromatics remains a challenge in organic chemistry. This is because of the high resonance stability of heteroarenes or the potential poisoning and/or deactivation of the catalyst caused by the nitrogen atom in heteroarene or their hydrogenated products.8 During the past decade, a number of homogeneous and heterogeneous metal catalysts, such as Ir,9 Ru,10 Rh,11 Co,12 Mo,13 and Fe,14 have been used in the hydrogenation of N-heteroaromatics. These methods usually require high H2 pressures, high reaction temperatures, and expensive catalysts or ligands. Transfer hydrogenation offers an alternative method in avoiding the use of flammable hydrogen gas.15 To the best of our knowledge, oxazaborolidine

Scheme 1. Application of Oxazaborolidine

Received: April 2, 2019 Accepted: May 3, 2019 Published: May 14, 2019 © 2019 American Chemical Society

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Table 1. Reaction Optimizationa

complexes have not been applied in transfer hydrogenation of N-heteroaromatics. Can we direct the use of borane (BH3) in the reaction? Borane is a common reagent in synthetic organic chemistry and has been widely used in many different types of reactions. However, the BH3 group coordinates to the nitrogen atom of quinoline and generates stable monoborane complexes.16 The question now is, how do we solve this problem? In 2017, Shi and co-workers reported B(C6F5)3-promoted hydrogenation of N-heterocycles with ammonia borane (BH3·NH3) at 80 °C with 50−87% yields.15h However, BH3·NH3 is an expensive reagent compared to the commercially available BH3·THF. Thus, we have designed a simple oxazaborolidine−BH3 complex based on previous work with chiral oxazaborolidine, for the reduction of ketone. The complex was synthesized from relatively inexpensive materials: ethanolamine and BH3·THF. The advantage is that chiral oxazaborolidine is easily formed for asymmetric reduction. Herein, we describe the first successful application of an oxazaborolidine complex in the copper-catalyzed transfer hydrogenation of N-heteroaromatics under mild reaction conditions.

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RESULTS AND DISCUSSION To initiate our study, 2-methyl quinoline (1a) was chosen as a model substrate. First, various boron reagents were screened as the hydrogen source and generated in situ from BH3·THF with 1,2-diols, 1,2-amino alcohols, 1,2-diamines, 2-aminobenzyl alcohol, and 1,2-dihydroxybenzene as the hydrogen source precursor (Table 1, HSP1−6). Cuprous chloride was chosen as the catalyst. The reaction gave the desired product 2a in 50% yield with HSP2 (Table 1, entry 2). We also evaluated different copper catalysts (entries 7−12). When copper(II) perchlorate hexahydrate was used as a catalyst, the yield increased to 88% (entry 8). However, when THF was replaced with a different solvent (entries 13−16), the yield decreased. To our delight, when the temperature increased to 40 °C, the reaction gave 2a in 94% yield (entry 17). When we decreased the reaction time to 12 h, the yield was still acceptable (entry 18). When the reaction proceeded without the addition of metal salts, the desired product 2a was not obtained (entry 19). The results indicate that the metal salt is instrumental in activating the heterocyclic ring and is a hydrogen source for the reduction to the desired product. Lastly, the reaction was carried out under anhydrous conditions with anhydrous copper trifluoromethanesulfonate and dry BH3·THF (entry 20), which had been generated from sodium borohydride. However, only the starting material but no desired products were obtained. A controlled experiment was also done without HSP under standard conditions (entry 21), and only the 2-methyl quinoline boron complex was observed. This indicated that water and HSP are necessary for the reaction, which has assisted us in understanding the reaction mechanism. With the optimized reaction conditions established, we extended our study to various quinoline substrate derivatives. The results are summarized in Table 2. First, quinolines bearing a methyl at position 2, 3, or 4 on the pyridine ring (Table 2, 1a, 1c, 1d), or a methyl at position 6, 7, or 8 on the benzene ring (1e−1g) were tested. The reaction proceeded smoothly under mild conditions, and the corresponding 1,2,3,4-tetrahydroquinolines were obtained in 87−95% yield. Furthermore, quinolines bearing the sterically hindered phenyl at position 2 on the pyridine ring (1i) gave the desired product

a

Reaction conditions: substrate (0.5 mmol), boron reagents (2.0 equiv), metal (0.2 equiv), solvent (2.0 mL); boron reagents prepared in situ: hydrogenation source precursor (2.0 equiv), BH3·THF (4.0 equiv). bIsolated yield. c40 °C. d12 h. eAnhydrous THF was used in the reaction. fBH3·THF (4.0 equiv) was used without HSP.

in 82% yield. Interestingly, even with the phenyl at position 4 on the pyridine ring, which is a challenging substrate, it was successfully hydrogenated to 2h in excellent yield (95%). In addition, we were glad to discover that quinolines bearing furyl (1j) or thienyl (1k) afforded the desired products in good yield. Next, we evaluated quinolines bearing electron-withdrawing groups on the benzene ring, including F (1l, 1r) and Cl (1m). The desired products were obtained in good yields. Quinolines bearing electron-donating groups, such as OMe (1o, 1p) and Me (1q), were also tested, and the desired products were obtained in good to excellent yields. However, quinoline bearing the strong electron-donating group of NMe2 (1n) had a significant influence on the reaction, with only 70% yield being obtained. Finally, this method was further applied to the 2,3-disubstituted quinolines (1s, 1t), affording the corresponding products in good yields (2s, 95%; 2t, 88%) and moderate diastereoselectivity (2s, d.r. = 2.96:1; 2t, d.r. = 2.73:1). Fused quinolines (1u, 1v) were hydrogenated to afford the corresponding products in 88−89% yields (2u, d.r. = 1.36:1; 2v, d.r. = 1.32:1). Encouraged by these results, we applied this method in the reduction of a few important N-heteroaromatics, including quinoxaline (Table 3, 3a−3f), acridine (3g), and phenanthroline (3h). Quinoxaline, with a mono- or disubstituted methyl, 8488

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were observed. However, the yield improved with the addition of cesium carbonate, and the partially hydrogenated major product 4h was obtained in 65% yield. To understand the mechanism, we performed deuteriumlabeled experiments (Scheme 2). We replaced THF with D2O,

Table 2. Transfer Hydrogenation of Substituted Quinolinesa,b

Scheme 2. Deuterium-Labeled Experiments

Reaction conditions: substrate (0.5 mmol), oxazaborolidine complex (2.0 equiv), Cu(ClO4)2·6H2O (0.2 equiv), THF (2.0 mL); oxazaborolidine complex prepared in situ: HSP2 (2.0 equiv), BH3· THF (4.0 equiv). bIsolated yield. cd.r. was determined by GC analysis. dCs2CO3 (2.0 equiv).

and the desired hydrogenated product was obtained in 23% yield, showing high deuterium incorporation at position 3 (eq 1). When the deuterium oxazaborolidine complex and D2O were used in the reaction, trideuterated 6 was obtained (28%) with high deuterium incorporation. Because BH3 or BD3 was generated from NaBH4 or NaBD4 and its content was much lower than 1 M solution, it resulted in a lower yield. These results demonstrated that H2O and BH3 are the H-donors for this transfer hydrogenation reaction. Water offers the H-atom at position 3 on the product and borane offers H-atoms at positions 2 and 4 on the product. On the basis of experimental results, we have proposed a plausible mechanism (Scheme 3). First, the copper catalyst and oxazaborolidine complex A forms the active catalytic species B. Then, B coordinates to quinoline (1b) to afford complex C. Subsequent transfer of the hydride from BH3 to position 2 on quinoline forms intermediate D and releases catalytic species E. Active catalytic species B regenerates from catalytic species E and oxazaborolidine complex A. Intermediate D undergoes hydrolysis to give 1,2-dihydroquinoline F, which isomerizes to 1,4-dihydroquinoline G. 1,4-Dihydroquinoline G isomerizes to enamine H,15f which is reduced by a similar catalytic cycle to form intermediate J. Eventually, the hydrolysis of intermediate J gives tetrahydroquinoline (2b). This explains the reason for the formation of the product that was only deuterated at position 3 of 2-methyl quinoline 5 in the deuterium-labeled experiments (Scheme 2, eq 1). We went on further to apply our method in the key step synthesis of natural products. (±)-Angustureine17 and (±)-galipinine,18 which have been isolated from the bark of the tropical American shrubby tree Galipea officinalis, have multiple biological activities.19 The reaction begins with 2aminobenzyl alcohol 7 and various ketones 8 to afford quinolines 9.20 This is followed by copper-catalyzed transfer hydrogenation with oxazaborolidine complex under mild conditions, giving 10 in high yields. Methylation21 easily gave the final natural product 11 in a total of 3 steps (Scheme 4).

was reduced to give high yields under mild conditions. Acridine reacted smoothly, affording the desired product in 88% yield. Interestingly, phenanthroline, which is a challenging substrate, was reduced to yield either partially or completely hydrogenated products. When phenanthroline was subjected to the standard reaction conditions, two products, in low yield,

CONCLUSIONS In conclusion, we have successfully expanded the use of oxazaborolidine complex in the copper-catalyzed transfer hydrogenation of N-heteroaromatics under mild reaction conditions. The oxazaborolidine complex was prepared without difficulty from cheap materials: ethanolamine and

a

Reaction conditions: substrate (0.5 mmol), oxazaborolidine complex (2.0 equiv), Cu(ClO4)2·6H2O (0.2 equiv), THF (2.0 mL); oxazaborolidine complex prepared in situ: HSP2 (2.0 equiv), BH3· THF (4.0 equiv). bIsolated yield. cd.r. was determined by GC analysis.

Table 3. Transfer Hydrogenation of N-Heteroaromaticsa,b

a

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Scheme 3. Plausible Mechanism

multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, and br = broad. Column chromatography was performed using silica gel (200−300 mesh). Visualization of the developed chromatogram was performed by UV absorbance (254 nm) unless otherwise noted. High-resolution mass spectral (HRMS) data were recorded on a SHIMADZU LCMS-ITTOF by analytical center at Sichuan University. N-Heteroaromatic substrates 1h−1i,22 1j−1k,20a 1s−1t,20a 1v,20a 3b,23 3c,24 and 9a−9b20a were synthesized according to reported procedures, and other substrates were obtained from commercial suppliers and were used without further purification. General Procedure A for Copper-Catalyzed Transfer Hydrogenation of N-Heteroaromatics. To a 25 mL Schlenk tube equipped with a magnetic stirrer bar were added ethanolamine (1.0 mmol, 61.08 mg) and BH3·THF (1.0 M in THF, 2 mL, 2.0 mmol) at 0 °C under an argon atmosphere. The tube was sealed and the mixture was allowed to stir at room temperature for 24 h. The oxazaborolidine complex was prepared after the solvent was removed under vacuum. To the Schlenk tube with the oxazaborolidine complex (1.0 mmol) were added Cu(ClO4)2·6H2O (0.1 mmol), N-heteroaromatics (0.5 mmol), and THF (2.0 mL) under an argon atmosphere. The tube was sealed and the mixture was allowed to stir at 40 °C for 24 h. Then, the reaction was cooled to room temperature and concentrated under reduced pressure. The product was purified by column chromatography on silica gel using petroleum ether/ethyl acetate (15:1−8:1, v/v) as eluent. 2-Methyl-1,2,3,4-tetrahydroquinoline (2a). Shallow yellow oil (94%, 69.0 mg). 1H NMR (400 MHz, CDCl3): δ 7.03−6.97 (m, 2H), 6.67−6.62 (m, 1H), 6.52−6.48 (m, 1H), 3.70 (br, 1H), 3.48−3.38 (m, 1H), 2.93−2.83 (m, 1H), 2.81−2.73 (m, 1H), 2.01−1.93 (m, 1H), 1.68−1.58 (m, 1H), 1.25 (d, J = 6.3 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 144.9, 129.4, 126.8, 121.2, 117.1, 114.1, 47.3, 47.3, 30.2, 26.7, 22.7. Analytical data were identical to those reported in literature.25 1,2,3,4-Tetrahydroquinoline (2b). Shallow yellow oil (93%, 64.3 mg). 1H NMR (400 MHz, CDCl3): δ 7.03−6.96 (m, 2H), 6.67−6.62 (m, 1H), 6.52−6.48 (m, 1H), 3.82 (br, 1H), 3.35− 3.30 (m, 2H), 2.80 (t, J = 6.4 Hz, 2H), 2.02−1.94 (m, 2H).

Scheme 4. Synthesis of Natural Product Precusor 10 under Mild Conditions

BH3·THF. A variety of N-heteroaromatics were successfully converted to the corresponding reduced products with good to excellent yields. Deuterium-labeled experiments were also undertaken and showed that water and borane, which coordinate to the nitrogen atom of oxazaborolidine, are the hydrogen sources. Furthermore, this method was used in the key step synthesis of natural products (±)-angustureine and (±)-galipinine in three steps. We are currently investigating the application of the oxazaborolidine complex in different heteroaromatics, including asymmetric transfer hydrogenation.

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EXPERIMENTAL SECTION General Information. All reactions were set up using standard Schlenk techniques under argon gas. Reactions were monitored by thin-layer chromatography carried out at 0.2 ± 0.03 mm using UV light as the visualizing agent in silica gel as developing agents. 1H and 13C NMR spectra were recorded on a Bruker AVANCE III HD 400 spectrometer (1H: 400 MHz. 13 C: 101 MHz). CDCl3 was used as the solvent, and tetramethylsilane (TMS) was used as the internal standard. Chemical shifts (δ) for 1H and 13C NMR spectra are given in ppm relative to TMS. The residual solvent signals were used as references for 1H and 13C NMR spectra and the chemical shifts converted to the TMS scale (CDCl3: δH = 7.26 ppm. δC = 77.16 ppm). Coupling constants (J) are expressed in Hz. The following abbreviations were used to designate chemical shift 8490

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(m, 1H), 2.20−2.12 (m, 1H), 13C NMR (101 MHz, CDCl3): δ 157.0, 143.9, 141.7, 129.4, 127.0, 121.1, 117.7, 114.5, 110.3, 105.3, 49.8, 27.0, 25.6. Analytical data were identical to those reported in literature.27 2-(Thiophen-2-yl)-1,2,3,4-tetrahydroquinoline (2k). Yellow oil (82%, 87.7 mg). 1H NMR (400 MHz, CDCl3): δ 7.15−7.11 (m, 1H), 6.95−6.85 (m, 4H), 6.61−6.55 (m, 1H), 6.54 (d, J = 8.1 Hz, 1H), 4.65 (dd, J = 9.2 Hz, 3.6 Hz, 1H), 4.06 (br, 1H), 2.88−2.78 (m, 1H), 2.73−2.63 (m, 1H), 2.16− 2.07 (m, 1H), 2.04−1.93 (m, 1H). 13C NMR (101 MHz, CDCl3): δ 148.9, 144.0, 129.3, 126.9, 126.6, 124.0, 123.5, 120.9, 117.7, 114.3, 52.0, 31.8, 26.1. Analytical data were identical to those reported in literature.27 6-Fluoro-1,2,3,4-tetrahydroquinoline (2l). Colorless oil (88%, 66.7 mg). 1H NMR (400 MHz, CDCl3): δ 6.71−6.64 (m, 2H), 6.43−6.37 (m, 1H), 3.66 (br, 1H), 3.27 (t, J = 5.2 Hz, 2H), 2.74 (t, J = 6.4 Hz, 2H), 1.96−1.88 (m, 2H). 13C NMR (101 MHz, CDCl3): δ [156.8, 154.4 (d, J = 242.4 Hz)], [141.1, 141.0 (d, J = 1.0 Hz)], [122.9, 122.9 (d, J = 6.1 Hz)], [115.8, 115.6 (d, J = 22.2 Hz)], [115.1, 115.0 (d, J = 8.1 Hz)], [113.4, 113.2 (d, J = 23.2 Hz)], 42.2, 27.2, 22.1. Analytical data were identical to those reported in literature.28 6-Chloro-1,2,3,4-tetrahydroquinoline (2m). Colorless oil (83%, 69.7 mg). 1H NMR (400 MHz, CDCl3): δ 6.94−6.88 (m, 2H), 6.39 (d, J = 5.6 Hz, 1H), 3.65 (br, 1H), 3.28 (t, J = 3.6 Hz, 2H), 2.73 (t, J = 4.4 Hz, 2H), 1.95−1.88 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 143.3, 129.1, 126.6, 123.0, 121.3, 115.2, 42.0, 27.0, 21.8. Analytical data were identical to those reported in literature.9b N,N-Dimethyl-1,2,3,4-tetrahydroquinolin-8-amine (2n). Yellow oil (70%, 61.7 mg). 1H NMR (400 MHz, CDCl3): δ 6.87 (d, J = 5.2 Hz, 1H), 6.76 (d, J = 5.2 Hz, 1H), 6.62−6.58 (m, 1H), 4.63 (br, 1H), 3.34 (t, J = 3.6 Hz, 2H), 2.80 (t, J = 4.4 Hz, 2H), 2.65 (s, 6H). 2.0−1.94 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 133.7, 133.3, 118.8, 118.8, 114.6, 114.5, 48.4, 45.8, 20.0. HRMS (ESI+): calculated for C11H17N2+ [M + H]+, 177.1386; found, 177.1393. 6-Methoxy-1,2,3,4-tetrahydroquinoline (2o). Colorless oil (95%, 77.1 mg). 1H NMR (400 MHz, CDCl3): δ 6.66−6.56 (m, 2H), 6.49−6.45 (m, 1H), 3.74 (s, 3H), 3.50 (br, 1H), 3.34−3.20 (m, 2H), 2.77 (t, J = 4.4 Hz, 2H), 1.97−1.91 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 152.0, 138.9, 123.0, 115.7, 115.0, 113.0, 55.9, 42.4, 27.3, 22.5. Analytical data were identical to those reported in literature.25 6-Methoxy-2-methyl-1,2,3,4-tetrahydroquinoline (2p). Colorless oil (96%, 84.9 mg). 1H NMR (400 MHz, CDCl3): δ 6.64−6.55 (m, 2H), 6.46 (d, J = 5.6 Hz, 1H), 3.74 (s, 3H), 3.58−3.22 (m, 2H), 2.90−2.80 (m, 1H), 2.75−2.69 (m, 1H), 1.96−1.89 (m, 1H), 1.63−1.54 (m, 1H), 1.21 (d, J = 4.0 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 152.0, 139.0, 122.6, 115.5, 114.8, 113.0, 55.9, 47.6, 30.4, 27.0, 22.6. Analytical data were identical to those reported in literature.27 2,6-Dimethyl-1,2,3,4-tetrahydroquinoline (2q). Colorless oil (89%, 71.6 mg). 1H NMR (400 MHz, CDCl3): δ 6.87−6.82 (m, 2H), 6.48−6.44 (m, 1H), 3.51 (br, 1H), 3.46−3.36 (m, 1H), 2.92−2.82 (m, 1H), 2.79−2.71 (m, 1H), 2.27 (s, 3H), 2.01−1.93 (m, 1H), 1.69−1.57 (m, 1H), 1.25 (d, J = 6.4 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 142.5, 129.9, 127.3, 126.3, 121.3, 114.3, 47.4, 30.4, 26.7, 22.7, 20.5. Analytical data were identical to those reported in literature.25 6-Fluoro-2-methyl-1,2,3,4-tetrahydroquinoline (2r). Colorless oil (84%, 69.1 mg). 1H NMR (400 MHz, CDCl3): δ 6.71−6.64 (m, 2H), 6.43−6.37 (m, 1H), 3.57 (br, 1H), 3.40−

C NMR (101 MHz, CDCl3): δ 144.9, 129.6, 126.8, 121.5, 117.0, 114.3, 42.1, 27.1, 22.3. Analytical data were identical to those reported in literature.25 3-Methyl-1,2,3,4-tetrahydroquinoline (2c). Shallow yellow oil (87%, 64.3 mg). 1H NMR (400 MHz, CDCl3): δ 7.05−6.95 (m, 2H), 6.68−6.63 (m, 1H), 6.54−6.50 (m, 1H), 3.84 (br, 1H), 3.33−3.27 (m, 1H), 2.94 (t, J = 10 Hz, 1H), 2.86−2.78 (m, 1H), 2.48 (dd, J = 10 Hz, 10 Hz, 1H), 2.17−2.03 (m, 1H), 1.09 (d, J = 6.8 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 144.4, 129.6, 126.8, 121.2, 117.0, 113.9, 48.9, 35.6, 27.3, 19.1. Analytical data were identical to those reported in literature.25 4-Methyl-1,2,3,4-tetrahydroquinoline (2d). Shallow yellow oil (88%, 64.5 mg). 1H NMR (400 MHz, CDCl3): δ 7.09−7.05 (m, 1H), 7.01−6.95 (m, 1H), 6.68−6.62 (m, 1H), 6.51−6.47 (m, 1H), 3.84 (br, 1H), 3.39−3.25 (m, 2H), 2.97−2.88 (m, 1H), 2.05−1.95 (m, 1H), 1.74−1.65 (m, 1H), 1.31 (d, J = 7.0 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 144.4, 128.6, 126.8, 126.7, 117.1, 114.3, 39.1, 30.4, 30.0, 22.8. Analytical data were identical to those reported in literature.25 6-Methyl-1,2,3,4-tetrahydroquinoline (2e). Shallow yellow oil (90%, 65.9 mg). 1H NMR (400 MHz, CDCl3): δ 6.83−6.79 (m, 2H), 6.46−6.41 (m, 1H), 3.69 (br, 1H), 3.30 (t, J = 7.2 Hz, 2H), 2.77 (t, J = 6.4 Hz, 2H), 2.24 (s, 3H), 2.00−1.92 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 142.5, 130.2, 127.3, 126.3, 121.7, 114.5, 42.3, 27.0, 22.6, 20.5. Analytical data were identical to those reported in literature.25 7-Methyl-1,2,3,4-tetrahydroquinoline (2f). Shallow yellow oil (88%, 65.0 mg). 1H NMR (400 MHz, CDCl3): δ 6.89 (d, J = 7.6 Hz, 1H), 6.51−6.47 (m, 1H), 6.35 (s, 1H), 3.75 (br, 1H), 3.32 (t, J = 5.5 Hz, 2H), 2.77 (t, J = 6.5 Hz, 2H), 2.27 (s, 3H), 2.01−1.92 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 144.7, 136.4, 129.5, 118.6, 118.0, 114.8, 42.1, 26.7, 22.5, 21.2. Analytical data were identical to those reported in literature.25 8-Methyl-1,2,3,4-tetrahydroquinoline (2g). Shallow yellow oil (95%, 70.0 mg). 1H NMR (400 MHz, CDCl3): δ 6.91−6.85 (m, 2H), 6.57 (t, J = 7.2 Hz, 1H), 3.63 (br, 1H), 3.39 (t, J = 5.6 Hz, 2H), 2.80 (t, J = 6.4 Hz, 2H), 2.09 (s, 3H), 2.00−1.94 (m, 2H). 13C NMR (101 MHz, CDCl3): δ 142.8, 128.0, 127.5, 121.3, 121.0, 116.5, 42.5, 27.4, 22.3, 17.3. Analytical data were identical to those reported in literature.25 4-Phenyl-1,2,3,4-tetrahydroquinoline (2h). White solid (95%, 99.3 mg). mp 72−74 °C. 1H NMR (400 MHz, CDCl3): δ 7.31−7.25 (m, 2H), 7.22−7.16 (m, 1H), 7.15−7.10 (m, 2H), 7.03−6.97 (m, 1H), 6.76−6.71 (m, 1H), 6.58−6.51 (m, 2H), 4.13 (t, J = 6.4 Hz, 1H), 3.90 (br, 1H) 3.31−3.17 (m, 2H), 2.25−2.15 (m, 1H), 2.08−1.99 (m, 1H). 13C NMR (101 MHz, CDCl3): δ 146.8, 145.1, 130.6, 128.8, 128.4, 127.4, 126.2, 123.5, 117.1, 114.3, 42.9, 39.3, 31.2. Analytical data were identical to those reported in literature.26 2-Phenyl-1,2,3,4-tetrahydroquinoline (2i). Colorless oil (82%, 85.8 mg). 1H NMR (400 MHz, CDCl3): δ 7.46−7.36 (m, 4H), 7.36−7.30 (m, 1H), 7.09−7.02 (m, 2H), 6.70 (t, J = 7.2 Hz, 1H), 6.57 (d, J = 8.0 Hz, 1H), 4.48 (dd, J = 9.2 Hz, 3.2 Hz, 1H), 4.07 (br, 1H), 3.02−2.91 (m, 1H), 2.82−2.73 (m, 1H), 2.21−2.12 (m, 1H), 2.09−1.97 (m, 1H). 13C NMR (101 MHz, CDCl3): δ 144.9, 144.8, 129.4, 128.7, 127.6, 127.0, 126.7, 121.0, 117.3, 114.1, 56.4, 31.1, 26.5. Analytical data were identical to those reported in literature.25 2-(Furan-2-yl)-1,2,3,4-tetrahydroquinoline (2j). Yellow oil (81%, 80.9 mg). 1H NMR (400 MHz, CDCl3): δ 7.40 (s, 1H), 7.06−6.99 (m, 2H), 6.71−6.66 (m, 1H), 6.57 (d, J = 5.2 Hz, 1H), 6.36 (s, 1H), 6.23 (s, 1H), 4.56 (d, J = 5.6 Hz, 1H), 4.16 (br, 1H), 2.93−2.85 (m, 1H), 2.81−2.75 (m, 1H), 2.27−2.20 13

8491

DOI: 10.1021/acsomega.9b00930 ACS Omega 2019, 4, 8487−8494

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1.84−1.69 (m, 4H), 1.68−1.52 (m, 5H), 1.37−1.29 (m, 1H). 13 C NMR (101 MHz, CDCl3): δ 145.0, 129.1, 126.7, 122.1, 117.1, 113.7, 59.8, 40.5, 36.6, 35.6, 31.9, 27.4, 25.8, 23.1. cis: 1H NMR (400 MHz, CDCl3): δ 6.99−6.92 (m, 2H), 6.60−6.55 (m, 1H), 6.46 (d, J = 5.2 Hz, 1H), 3.69 (br, 1H), 3.58−3.54 (m, 1H), 2.87 (dd, J = 10.4 Hz, 3.2 Hz, 1H), 2.50− 2.45 (dd, J = 10.4 Hz, 4.4 Hz, 1H), 2.09−2.03 (m, 1H), 1.80− 1.65 (m, 5H), 1.59−1.53 (m, 2H), 1.49−1.43 (m, 3H). 13C NMR (101 MHz, CDCl3): δ 144.5, 129.1, 126.8, 120.8, 116.4, 113.0, 54.3, 36.2, 35.1, 34.8, 30.1, 29.3, 26.0, 23.5. Analytical data were identical to those reported in literature.27 1,2,3,4-Tetrahydroquinoxaline (4a). White solid (89%, 60.0 mg). mp 94−96 °C. 1H NMR (400 MHz, CDCl3): δ 6.61−6.57 (m, 2H), 6.52−6.48 (m, 2H), 3.63 (br, 2H), 3.42 (s, 4H). 13C NMR (101 MHz, CDCl3): δ 133.8, 118.8, 114.8, 41.5. Analytical data were identical to those reported in literature.25 2-Methyl-1,2,3,4-tetrahydroquinoxaline (4b). White solid (88%, 65.0 mg). mp 70−71 °C. 1H NMR (400 MHz, CDCl3) δ 6.61−6.57 (m, 2H), 6.53−6.48 (m, 2H), 3.60 (br, 2H), 3.55−3.49 (m, 1H), 3.34−3.30 (m, 1H), 3.07−3.02 (m, 1H), 1.19 (d, J = 4.2 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 133.7, 133.3, 118.8, 118.8, 114.6, 114.5, 48.4, 45.8, 20.0. Analytical data were identical to those reported in literature.25 2,3-Dimethyl-1,2,3,4-tetrahydroquinoxaline (4c). White solid (86%, 70.0 mg, d.r. = 1.38:1). trans: mp 100−101 °C. 1H NMR (400 MHz, CDCl3): δ 6.60−6.56 (m, 2H), 6.52−6.48 (m, 2H), 3.52 (br, 2H), 3.05− 2.99 (m, 2H), 1.17 (d, J = 2.8 Hz, 6H). 13C NMR (101 MHz, CDCl3): δ 133.6, 118.7, 114.0, 52.2, 19.2. cis: mp 112−113 °C. 1H NMR (400 MHz, CDCl3): δ 6.61− 6.56 (m, 2H), 6.52−6.47 (m, 2H), 3.85−3.52 (br, 2H), 3.52− 2.46 (m, 2H), 1.13 (d, J = 4.0 Hz, 6H). 13C NMR (101 MHz, CDCl3): δ 132.8, 118.6, 114.5, 49.1, 17.4. Analytical data were identical to those reported in literature.29 5-Methyl-1,2,3,4-tetrahydroquinoxaline (4d). Yellow oil (82%, 60.7 mg). 1H NMR (400 MHz, CDCl3): δ 6.57−6.50 (m, 2H), 6.46−6.40 (m, 2H), 3.70−3.38 (m, 6H), 2.18−2.08 (m, 3H). 13C NMR (101 MHz, CDCl3): δ 133.3, 131.7, 122.2, 120.5, 118.1, 113.0, 41.9, 41.3, 17.0. Analytical data were identical to those reported in literature.25 6-Methyl-1,2,3,4-tetrahydroquinoxaline (4e). White solid (81%, 60.0 mg). mp 99−101 °C. 1H NMR (400 MHz, CDCl3): δ 6.43−6.36 (m, 2H), 6.32 (s, 1H), 3.70−3.34 (m, 6H), 2.17 (s, 3H). 13C NMR (101 MHz, CDCl3): δ 133.9, 131.3, 128.5, 119.2, 115.6, 115.1, 41.7, 20.8. Analytical data were identical to those reported in literature.15e 6-Bromo-1,2,3,4-tetrahydroquinoxaline (4f). White solid (89%, 94.3 mg). mp 117−118 °C. 1H NMR (400 MHz, CDCl3): δ 6.70−6.53 (m, 2H), 6.32 (d, J = 8.4 Hz, 2H), 3.68 (br, 2H), 3.37 (s, 4H). 13C NMR (101 MHz, CDCl3): δ 135.2, 132.7, 120.9, 116.7, 115.7, 110.2, 41.1. Analytical data were identical to those reported in literature.15e 9,10-Dihydroacridine (4g). White solid (88%, 79.5 mg). mp 153−155 °C. 1H NMR (400 MHz, CDCl3): δ 7.15−7.08 (m, 4H), 6.91−6.86 (m, 2H), 6.68 (d, J = 5.3 Hz, 2H), 5.95 (br, 1H), 4.08 (s, 2H). 13C NMR (101 MHz, CDCl3): δ 140.2, 128.7, 127.1, 120.8, 120.2, 113.6, 31.5. Analytical data were identical to those reported in literature.12a 1,2,3,4-Tetrahydro-1,10-phenanthroline (4h). Yellow oil (65%, 59.4 mg), 1H NMR (400 MHz, CDCl3): δ 8.70−8.67

3.29 (m, 1H), 2.88−2.77 (m, 1H), 1.96−1.87 (m, 1H), 1.62− 1.50 (m, 1H), 1.20 (d, J = 6.4 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ [156.8, 154.5 (d, J = 236.3 Hz)], [141.1, 141.1 (d, J = 1.0 Hz)], [122.6, 122.6 (d, J = 7.1 Hz)], [115.6, 115.4 (d, J = 21.2 Hz)], [114.9, 114.8 (d, J = 7.1 Hz)], [113.4, 113.2 (d, J = 22.2 Hz)], 47.4, 30.0, 26.9, 22.6. Analytical data were identical to those reported in literature.27 3-Methyl-2-phenyl-1,2,3,4-tetrahydroquinoline (2s). Colorless oil (95%, 105.9 mg, d.r. = 2.96:1). trans: 1H NMR (400 MHz, CDCl3): δ 7.41−7.27 (m, 5H), 7.09−7.01 (m, 2H), 6.72−6.65 (m, 1H), 6.59 (d, J = 8.0 Hz, 1H), 4.54 (d, J = 3.6 Hz, 1H), 4.14 (br, 1H), 3.00 (dd, J = 16.0 Hz, 4.8 Hz, 1H), 2.54 (dd, J = 16 Hz, 6.8 Hz, 1H), 2.39−2.29 (m, 1H), 0.86 (d, J = 6.8 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 144.6, 143.6, 129.2, 128.6, 127.7, 127.7, 126.9, 121.0, 117.1, 113.4, 63.4, 35.5, 33.9, 18.6. cis: 1H NMR (400 MHz, CDCl3): δ 7.41−7.27 (m, 5H), 7.09−7.01 (m, 2H), 6.72−6.65 (m, 1H), 6.52 (d, J = 8.0 Hz, 1H), 4.14 (br, 1H), 3.98 (d, J = 8.8 Hz, 1H), 2.84 (dd, J = 16.4 Hz, 4.8 Hz, 1H), 2.65 (dd, J = 16.4 Hz, 10.8 Hz, 1H), 2.16− 2.02 (m, 1H), 0.87 (d, J = 6.8 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 144.2, 143.0, 129.8, 128.2, 127.3, 127.2, 127.0, 120.1, 117.2, 113.8, 59.5, 33.5, 32.0, 15.3. Analytical data were identical to those reported in literature.27 2-Ethyl-3-methyl-1,2,3,4-tetrahydroquinoline (2t). Colorless oil (88%, 77.2 mg, d.r. = 2.73:1). trans: 1H NMR (400 MHz, CDCl3): δ 6.91−6.84 (m, 2H), 6.54−6.46 (m, 1H), 6.42−6.37 (m, 1H), 3.71 (br, 1H), 2.87− 2.76 (m, 1H), 2.70−2.63 (m, 1H), 2.43−2.33 (m, 1H), 1.77− 1.66 (m, 1H), 1.63−1.51 (m, 1H), 1.47−1.39 (m, 1H), 0.95− 0.81 (m, 6H). 13C NMR (101 MHz, CDCl3): δ 144.5, 129.3, 126.8, 120.9, 116.7, 113.6, 58.4, 34.7, 30.5, 26.9, 18.5, 9.4. cis: 1H NMR (400 MHz, CDCl3): δ 6.91−6.84 (m, 2H), 6.54−6.46 (m, 1H), 6.42−6.37 (m, 1H), 3.71 (br, 1H), 3.11− 3.06 (m, 1H), 2.89−2.78 (m, 1H), 2.43−2.33 (m, 1H), 2.07− 1.97 (m, 1H). 1.47−1.39 (m, 2H), 0.95−0.81 (m, 6H). 13C NMR (101 MHz, CDCl3): δ 144.1, 130.0, 126.7, 120.4, 117.0, 113.8, 56.4, 34.6, 29.0, 25.0, 13.8, 10.7. Analytical data were identical to those reported in literature.27 1,2,3,4,4a,9,9a,10-Octahydroacridine (2u). White solid (89%, 82.8 mg, d.r. = 1.36:1). trans: mp 82−84 °C. 1H NMR (400 MHz, CDCl3): δ 7.02− 6.92 (m, 2H), 6.63 (t, J = 7.2 Hz, 1H), 6.50 (d, J = 8.0 Hz, 1H), 3.70 (br, 1H), 2.89 (td, J = 10.4 Hz, 4.4 Hz, 1H), 2.70 (dd, J = 16.4 Hz, 5.2 Hz, 1H), 2.56−2.46 (m, 1H), 1.96−1.74 (m, 4H), 1.61−1.26 (m, 4H), 1.43−0.99 (m, 1H). 13C NMR (101 MHz, CDCl3): δ 144.7, 129.3, 126.7, 121.6, 117.0, 113.8, 56.2, 37.7, 34.7, 33.7, 32.1, 26.0, 24.8. cis: mp 68−70 °C. 1H NMR (400 MHz, CDCl3): δ 7.04− 6.95 (m, 2H), 6.62 (t, J = 7.6 Hz, 1H), 6.49 (d, J = 8.0 Hz, 1H), 3.69−3.48 (m, 2H), 2.96 (dd, J = 16.4 Hz, 5.6 Hz, 1H), 2.56 (dd, J = 16.4 Hz, 4.0 Hz, 1H), 2.05−1.96 (m, 1H), 1.80− 1.60 (m, 4H), 1.59−1.35 (m, 4H). 13C NMR (101 MHz, CDCl3): δ 144.0, 129.8, 126.7, 119.3, 116.5, 113.3, 50.1, 33.1, 32.7, 31.9, 27.4, 24.9, 20.8. Analytical data were identical to those reported in literature.27 5a,6,7,8,9,10,10a,11-Octahydro-5H-cyclohepta[b]quinoline (2v). Colorless oil (88%, 88.3 mg, d.r. = 1.32:1). trans: 1H NMR (400 MHz, CDCl3): δ 6.99−6.94 (m, 1H), 6.64−6.59 (m, 1H), 6.50 (d, J = 5.2 Hz, 1H), 3.61 (br, 1H), 2.99−2.93 (m, 1H), 2.70−2.64 (m, 1H), 2.62−2.55 (m, 1H), 8492

DOI: 10.1021/acsomega.9b00930 ACS Omega 2019, 4, 8487−8494

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Notes

(m, 1H), 8.02−7.99 (m, 1H), 7.31−7.29 (m, 1H), 7.16 (d, J = 5.2 Hz, 1H), 6.98 (d, J = 5.6 Hz, 1H), 5.94 (br, 1H), 3.53 (t, J = 3.6 Hz, 2H), 2.92 (t, J = 4.4 Hz, 2H), 2.10−2.04 (m, 2H). 13 C NMR (101 MHz, CDCl3): δ 147.1, 140.8, 137.6, 136.0, 129.2, 127.5, 120.7, 116.7, 113.2, 41.4, 27.2, 22.0. Analytical data were identical to those reported in literature.15g 2-Pentyl-1,2,3,4-tetrahydroquinoline (10a). Colorless oil (91%, 92.5 mg). 1H NMR (400 MHz, CDCl3): δ 7.02−6.93 (m, 2H), 6.64−6.58 (m, 1H), 6.51−6.47 (m, 1H), 3.77 (br, 1H), 3.29−3.21 (m, 1H), 2.88−2.70 (m, 1H), 2.02−1.94 (m, 1H), 1.68−1.56 (m, 1H), 1.55−1.47 (m, 2H), 1.47−1.22 (m, 6H), 0.93 (t, J = 8.8 Hz, 3H). 13C NMR (101 MHz, CDCl3): δ 144.9, 129.4, 126.8, 121.5, 117.0, 114.1, 51.7, 36.8, 32.1, 28.3, 26.6, 25.5, 22.8, 14.2. Analytical data were identical to those reported in literature.27 2-(2-(Benzo[d][1,3]dioxol-5-yl)ethyl)-1,2,3,4-tetrahydroquinoline (10b). White solid (76%, 106.2 mg). mp 67−68 °C. 1 H NMR (400 MHz, CDCl3): δ 7.00−6.92 (m, 2H), 6.77− 6.58 (m, 4H), 6.49−6.44 (m, 1H), 5.93 (s, 2H), 3.76 (br, 1H), 3.33−3.25 (m, 1H), 2.87−2.70 (m, 2H), 2.67 (t, J = 7.6 Hz, 2H), 2.03−1.95 (m, 1H), 1.83−1.76 (m, 1H), 1.73−1.61 (m, 1H). 13C NMR (101 MHz, CDCl3): δ 147.8, 145.9, 144.6, 135.8, 129.4, 126.9, 121.4, 121.2, 117.2, 114.3, 108.9, 108.4, 100.9, 51.1, 38.6, 32.0, 29.8, 28.1, 26.3. Analytical data were identical to those reported in literature.27 General Procedure B for the Synthesis of 6. In a round-bottom flask fitted with a rubber stopper and an argon inlet, NaBD4 (6 mmol) was suspended in 5 mL of diglyme. This flask was connected through a plastic cannula to a second flask containing a solution of ethanolamine (2 mmol) in 4 mL of anhydrous THF at 0 °C. The second flask was vented through a plastic cannula bubbling into THF. Iodine (6.2 mmol) in diglyme (5 mL) was added into the first flask dropwise over 1 h by syringe. At the end of the addition of I2, the temperature of the second flask was brought to room temperature and the stream of argon was continued for 2 h. Deuterium oxazaborolidine was prepared after the solvent in the second flask was removed under vacuum. To a 25 mL Schlenk tube equipped with a magnetic stirrer bar were added deuterium oxazaborolidine complex (1.0 mmol), Cu(CF3SO3)2 (0.1 mmol), D2O (1.2 mL), and 2-methlyquinoline (0.5 mmol). The tube was sealed and the mixture was allowed to stir at 40 °C for 24 h. Then, the reaction was cooled to room temperature and concentrated under reduced pressure. The product was purified by flash column chromatography on silica gel using petroleum ether/ethyl acetate (10:1, v/v) as eluent.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (no. 21602184), Youth Science and Technology Innovation Team of SWPU (no. 2017CXTD05), and the Scientific Research Starting Project of SWPU (no. 2015QHZ012).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00930. Optimization of reaction conditions and 1H NMR and C NMR spectra for all products (PDF)

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Taigang Zhou: 0000-0002-3208-067X 8493

DOI: 10.1021/acsomega.9b00930 ACS Omega 2019, 4, 8487−8494

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DOI: 10.1021/acsomega.9b00930 ACS Omega 2019, 4, 8487−8494