Bimetallic Cobalt–Rhodium Nanoparticle-Catalyzed Reductive

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Bimetallic Cobalt−Rhodium Nanoparticle-Catalyzed Reductive Amination of Aldehydes with Nitroarenes Under Atmospheric Hydrogen Isaac Choi, Supill Chun, and Young Keun Chung* Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 08826, Korea S Supporting Information *

ABSTRACT: A cobalt−rhodium heterobimetallic nanoparticle (Co2Rh2/C)-catalyzed tandem reductive amination of aldehydes with nitroaromatics to sec-amines has been developed. The tandem reaction proceeds without any additives under mild conditions (1 atm H2 and 25 °C). This procedure can be scaled up to the gram scale, and the catalyst can be reused more than six times without loss of activity.

A

romatic amines are ubiquitous building blocks of functional materials, additives, dyes, biologically active natural products, and pharmaceuticals.1 Much effort has been devoted to the development of useful synthetic methods for aromatic amines. Traditional methods for N-alkylation of amines involve the use of alkylating agents such as alkyl halides and aldehydes.2 However, controlled alkylation of primary amines using alkyl halides is unsatisfactory in many cases3 and produces a stoichiometric amount of halogen waste. N-alkylation is often limited to the synthesis of tertiary amines. Thus, the synthesis of aromatic amines depends on the reductive amination of carbonyl compounds with anilines;4 the reaction is usually carried out under high H2 pressure or in the presence of hydride-based reagents.5 Recently, the use of nitroarenes instead of anilines in reductive amination has been disclosed.6 The use of nitroarenes has gained increasing attention because they are stable, inexpensive, and readily available. Numerous catalytic systems have been developed for the above-mentioned reductive amination;7 however, they have some drawbacks, such as poor stability, formation of toxic byproducts, and requirement of acidic conditions.8 Therefore, it is still challenging to develop an effective, facile, and reusable method for the synthesis of aryl amines using nitroarenes as the nitrogen source under mild reaction conditions. The use of heterogeneous transition-metal nanoparticle catalysts in reductive amination has attracted much attention, presumably because of their high activity.9 Recently, reductive amination of aldehydes with nitroarenes in the presence of heterogeneous Pd nanoparticle-based catalysts was reported.9a−d However, most nanocatalysts have a separation problem. To solve the problem, they are supported by a magnetic material such as Fe3O4.9e,f © 2017 American Chemical Society

Of late, heterobimetallic nanoparticles have gained importance because of their improved electronic, optical, and catalytic performance as compared to that of a single nanometal.10 Several years ago, Co/Rh heterobimetallic nanoparticles were developed as a catalyst for the Pauson−Khand reaction (PKR),11 wherein Co2Rh2 (derived from Co2Rh2(CO)12) was immobilized on charcoal (Co2Rh2/C). The resulting black Co2Rh2/C was ferromagnetic and could be magnetically separated from the reaction mixture. Co2Rh2/C showed a high catalytic activity for the PKR.11c Furthermore, Co2Rh2/C acted as a catalyst in many useful reactions such as carbonylation,11a aminocarbonylation,11e,g and hydroformylation.11f A recent study showed the usefulness of Co2Rh2/C in the hydrogenation of nitroarenes, reductive amination of aldehydes and ketones with amines in the presence of carbon monoxide,12 and reductive cyclization of 2-(2-nitroaryl)acetonitriles to indoles.13 To find a new catalytic system for the reductive amination of aldehydes with nitroarenes, we used Co2Rh2/C as the catalyst in the presence of atmospheric hydrogen without any external base or additive. The catalytic system was found to be effective in methanol as the solvent at 25 °C. Herein, we report that the heterogeneous Co2Rh2/C catalyst shows excellent activity for one-pot reductive amination in methanol at room temperature under a hydrogen atmosphere. The catalyst can be easily recovered by centrifugation and recycled at least six times without loss of activity. Received: August 10, 2017 Published: November 2, 2017 12771

DOI: 10.1021/acs.joc.7b02019 J. Org. Chem. 2017, 82, 12771−12777

Note

The Journal of Organic Chemistry First, the reaction of benzaldehyde (1a) with nitrobenzene (2a) in methanol using Co2Rh2/C under 1 atm of hydrogen (balloon) at ambient temperature for 16 h to afford Nbenzylaniline (3aa) was selected as the model reaction. After the work up, N-benzylaniline was isolated in 60% yield. Notably, the secondary amine was readily formed under mild reaction conditions. Encouraged by this result, we proceeded to optimize the reaction conditions for the amination of 1a with 2a under 1 atm H2 (Table 1). The presence of anhydrous

Scheme 1. Reductive Amination in the Presence of Monometallic Nanoparticles

Table 1. Screening the Reductive Amination of Benzaldehyde with Nitrobenzene a

Isolated yield.

aldehydes to afford the secondary benzylaniline products in good-to-excellent yields. entry

solvent

1 2 3 4 5b 6b 7b 8b 9b 10b 11b 12c

MeOH MeOH MeOH MeOH MeOH EtOH n-BuOH toluene xylene MeOH EtOH EtOH

a

additives MgSO4 3 Å molecular sieves B(OEt)3

time (h)

temp (°C)

yielda (%)

16 16 16 16 16 16 16 16 16 24 24 24

25 25 25 25 25 25 25 100 100 25 25 25

60 56 60 48 92 63 51 49 41 93 80 92

Isolated yield. b1.5 equiv of benzaldehyde used. benzaldehyde used.

c

Scheme 2. Reductive Amination of Various Aldehydes with Nitrobenzene

2 equiv of

reagents such as MgSO4 or molecular sieves (3 Å) did not improve the yield (entries 2 and 3, 56% and 60% yields, respectively). Addition of a Lewis acid, B(OEt)3, decreased the yield of the reaction (entry 4, 48%). When the amount of 2a was increased from 1.0 equiv to 1.5 equiv, the yield significantly increased to 92% (entry 5). Other solvents such as ethanol, butanol, toluene, and p-xylene were also screened at the appropriate temperatures (entries 6−9). Reasonable yields were observed for the solvents used, but the highest yield (92%) was observed with methanol. An increase in the reaction time from 16 to 24 h slightly improved the yield, affording the product in 93% yield (entry 10). Interestingly, an increase in both the reaction time (entry 11) and the amount of 2a (entry 12), in ethanol, significantly increased the yields to 80% and 92%, respectively. While optimizing the reaction conditions, we examined the feasibility of using other metal nanoparticles such as Co/C and Rh/C as the catalyst with 1 equiv of nitrobenzene and 1.5 equiv of benzaldehyde (Scheme 1). The reaction did not proceed in the presence of Co/C. In the presence of Rh/C, only 23% of Nbenzylaniline was isolated. The optimized reaction conditions were as follows: 5 mol % Co2Rh2/C (45 mg), 1 atm H2 (balloon), 3 mL of methanol, room temperature, 24 h. With the optimized reaction conditions in hand, we investigated the substrate scope of the reaction using structurally diverse aldehydes and nitroarenes (Scheme 2). Nitrobenzene reacted smoothly with various aryl and alkyl

a

Isolated yield.

To our delight, broad functional compatibility was observed among the aryl aldehydes. Both electron-donating (3ba−3ga and 3na) and electron-withdrawing substituents (3ka−3ma) were tolerated under the reaction conditions. In the transformation of methoxy nitrobenzenes, no significant steric effect was observed (3da−3fa, 84%, 92%, and 91% yields, respectively). When a hydroxyl group and a methoxy group were present on the same arene ring, the latter had a greater effect on the yield (3fa and 3ga, 91% and 72% yields, respectively). The lower rate of o- and m-methyl analogues 12772

DOI: 10.1021/acs.joc.7b02019 J. Org. Chem. 2017, 82, 12771−12777

Note

The Journal of Organic Chemistry relative to p-analogues indicated a steric effect (3ca−3da). Interestingly, in the case of hydroxy-substituted benzaldehydes, the obtained yields differed slightly depending on the position of the hydroxyl substituent on the aromatic ring (3ga−3ja, 72%, 71%, 78%, and 80% yields, respectively). ortho-Hydroxy benzaldehydes participated in intramolecular hydrogen bonding, thus decreasing the yields (3ga and 3ha). When the reaction was performed with benzaldehydes bearing a halogen, reductive amination occurred with F-, Cl-, and Br-substituted materials (3ka−3ma). No dehalogenation of the fluoro or chloro group was observed. However, with p-bromobenzaldehyde, a relatively low yield (3ma, 38%) was obtained because of debromination. In terms of steric hindrance, various substituted benzaldehydes (3ca, 3fa, 3ga, and 3ha) also participated in the reductive amination. A highly sterically hindered 1-naphthyl aldehyde group was well tolerated under the reaction conditions (3oa, 91% yield). Notably, a lower yield (54%) was observed for 4-(dimethylamino)benzaldehyde (1n), presumably because of the coordinative interaction with the catalytic species. In addition, the reductive amination of aliphatic aldehydes such as isobutylaldehyde, 3-phenylbutanal, and butyraldehyde gave the corresponding amines in high yields (3pa−3ra, 92%, 89%, and 85% yields, respectively). Next, the effect of substitution on the nitrobenzene ring on the reductive amination of benzaldehyde was investigated (Scheme 3). Diverse substrates underwent reductive amination under the experimental conditions. Nitrobenzenes bearing either an electron-donating group (3ab and 3ac) or an electron-withdrawing group (3ae−3ah) gave excellent isolated yields. However, among halonitroarenes, a profound effect of

the substituent on the yield was observed (3ae, 3ag, and 3ah, 66%, 71%, and 84% yields, respectively). This is probably because the presence of an electron-withdrawing group on the nitroarene deactivates the nucleophilic nitrogen and thus hinders the reaction.14 The position of an electron-withdrawing substituent (F) on the aromatic ring did not have any notable effect on the yield (3ae and 3af, 66% and 69% yields, respectively). In the case of halogen substituents, some interesting observations were made. No dehalogenation was observed for halonitroarenes (3ae−3ah) and N-benzyl-4-(2bromoethyl)aniline (2o), but a fully debrominated product was observed when 1-(bromomethyl)-4-nitrobenzene (2i) was used as a reactant. It might be related to the facile contact with the surface of the heterogeneous catalyst. The vinyl group was also reduced when 3-nitrostyrene (2j) was used as a reactant (3aj). The reaction could also be extended to sterically demanding reactants (3ak−3an). The reductive amination of benzaldehyde with a nitrobenzene derivative containing an amide group proceeded well (3ap). Note that no reaction occurred on the amide group. To better understand the substitution effect of different functional groups, competitive reactions were carried out (Scheme 4). When nitrobenzene was allowed to react with 4methoxybenzaldehyde and 4-chlorobenzaldehyde, N-(4-methoxybenzyl) aniline and N-(4-chlorobenzyl) aniline were isolated in 34% and 61% yields, respectively. This indicates that an electron-withdrawing group on the benzaldehyde activated the carbonyl group more easily than did an electron-donating group.14 Another competitive reaction of benzaldehyde with 1-methoxy-4-nitrobenzene and 1-chloro-4nitrobenzene afforded N-benzyl-4-metoxyaniline and N-benzyl4-chloroaniline in 56% and 37% yields, respectively. Thus, the presence of an electron-donating group on a nitroarene favors the reduction of nitroarene and the coupling of the resulting aniline with a benzaldehyde. However, an electron-withdrawing group on the aniline deactivates the nucleophilicity of nitrogen and thus hinders the reaction.14 On the basis of these observations, a reaction between 4fluorobenzaldehyde and 1-methoxy-4-nitrobenzene was carried out, and an excellent yield (96%) was obtained, as expected (eq 1).

Scheme 3. Reductive Amination of Various Nitroarenes with Benzaldehydea

To verify the practicality of the developed method, a gramscale reaction was carried out: 1.37 g of N-benzylaniline was isolated in 93% yield from 8 mmol of nitrobenzene and 12 mmol of benzaldehyde (eq 2).

The reusability of Co2Rh2/C was investigated by performing the reductive amination of nitrobenzene with benzaldehyde (Table 2). Considering the physical loss, a high level of activity was maintained even after seven cycles of reuse.

a

Isolated yield. bThe major product was debrominated. cThe vinyl group was reduced. 12773

DOI: 10.1021/acs.joc.7b02019 J. Org. Chem. 2017, 82, 12771−12777

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The Journal of Organic Chemistry Scheme 4. Competition Experimentsa

a

Isolated yield. on Merck silica gel 60 F254 TLC plates. TLC plates were visualized by ultraviolet light and treated with the acidic p-anisaldehyde stain followed by gentle heating. Flash column chromatography was carried out on Merck 60 silica gel (230−400 mesh). 1H NMR and 13C NMR were recorded with an Agilent 400-MR DD2 (400 and 100 MHz, respectively) or a Varian (500 and 125 MHz, respectively) spectrometer. 1H NMR spectra were taken in CDCl3 or DMSO-d6, referenced to residual TMS (0 ppm), and reported as follows: chemical shift (δ ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet), coupling constant (Hz), and integration. Chemical shifts of the 13C NMR spectra were measured relative to CDCl3 (77.16 ppm) and DMSO-d6 (39.52 ppm). Highresolution mass spectra were obtained at the Korea Basic Science Institute (Daegu, South Korea) on a Jeol JMS 700 high-resolution mass spectrometer. High-resolution mass spectra were obtained using the electronic impact (EI) mode using a magnetic sector−electric sector focusing mass analyzer. Immobilization of Co/Rh Nanoparticles on Charcoal. To a two-neck flask were added o-dichlorobenzene (10 mL), oleic acid (0.2 mL), and trioctylphosphine oxide (0.4 g). While the solution was heated at 180 °C, a solution of metal carbonyl Co2Rh2(CO)12 (0.8 g) in 25 mL of o-dichlorobenzene was injected into the flask. The resulting solution was heated to 180 °C for 2 h and then concentrated to a volume of 5 mL. The concentrated solution was cooled to room temperature. To the cooled solution was added 30 mL of THF. After the solution was well-stirred for 10 min, flame-dried charcoal (1.6 g) was added to the solution. After the resulting solution had been refluxed for 12 h, the precipitates were filtered and washed with diethyl ether (20 mL), dichloromethane (20 mL), acetone (20 mL), and methanol (20 mL). Vacuum drying gave a black solid. General Procedure. Reactions were performed in a tube-type Schlenk flask equipped with a stir bar and capped with a rubber septum. The following were placed in the flask in order: nitroarenes (0.5 mmol), aldehydes (0.75 mmol), 5 mol % Co2Rh2 (45 mg of the immobilized Co2Rh2/C), and methanol (3 mL). The solution was bubbled by hydrogen gas for 20 min. The solution was stirred for 24 h and then was filtered and concentrated by a rotary evaporator. The crude product was purified by flash column chromatography. Gram-Scale Experiment. Reactions were performed in a 50 mL Schlenk flask equipped with a stir bar and capped with a rubber septum. The following were placed in the flask in order: nitrobenzene (8 mmol), benzaldehyde (12 mmol), 5 mol % Co2Rh2, and methanol (25 mL). The solution was bubbled by hydrogen gas for 1 h. The

Table 2. Reuse of Co2Rh2/C in the Reductive Amination of Benzaldehyde with Nitrobenzenea entry

catalyst

yield (%)b

1 2 3 4 5 6 7

first run second run third run fourth run fifth run sixth run seventh run

93 93 89 94 90 87 80

a

Conditions: nitrobenzene (0.5 mmol), benzaldehyde (0.75 mmol), Co2Rh2/C (10 mol %), MeOH (3 mL), 24 h, 25 °C. bIsolated yield.

The mechanism of a reductive amination occurs through two consecutive reduction reactions. First, an imine intermediate was formed from benzaldehyde and aniline reduced from nitrobenzene. Then, the reductive amination is caused by the catalyst in the presence of hydrogen gas. Identification of the intermediates of amines and imines by GC−MS supports the mechanism. In summary, we have demonstrated that Co2Rh2/C can be effectively used in methanol for the selective reductive amination of aldehydes. The reaction tolerates diverse functional groups and is a feasible alternative to traditional reductive amination methods. The major advantages of the method are as follows: (i) No harmful byproducts are formed. (ii) Co2Rh2/C can be easily separated from the reaction mixture by centrifugation and reused. (iii) No high-pressure reaction vessels are needed. (iv) Complex hydrides commonly used in reductive aminations are not required. (v) It is not necessary to isolate the imine intermediate.

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EXPERIMENTAL SECTION

Workup procedures were done in air. All solvents were used without further purification. Unless otherwise noted, all commercial materials were used without purification. Reagents were purchased from chemical companies and were used as received. High purity H2 (99.999%) was used. TLC analysis of reaction mixtures was performed 12774

DOI: 10.1021/acs.joc.7b02019 J. Org. Chem. 2017, 82, 12771−12777

Note

The Journal of Organic Chemistry

3-((Phenylamino)methyl)phenol (3ia).9e Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 5/1) afforded a yellow solid product (78% yield, 78 mg): mp 103−104 °C; 1H NMR (500 MHz, CDCl3) δ 7.16−7.04 (m, 3H), 6.85 (d, J = 7.5 Hz, 1H), 6.74 (s, 1H), 6.65 (m, 2H), 6.55 (m, 2H), 4.29 (s, 2H); 13C NMR (125 MHz, CDCl3) δ 155.9, 148.2, 141.6, 130.0, 129.4, 119.9, 117.9, 114.3, 114.3, 113.1, 48.2; HRMS (EI) m/z M+ calcd for [C13H13NO]+ 199.0997, found 199.0996. 4-((Phenylamino)methyl)phenol (3ja).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 5/1) afforded a yellow liquid product (80% yield, 80 mg): 1H NMR (400 MHz, CDCl3) δ 7.19−7.09 (m, 4H), 6.75−6.61 (m, 4H), 6.57 (m, 2H), 4.45 (s, 1H), 4.16 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 154.9, 148.2, 131.5, 129.4, 129.2, 117.9, 115.6, 113.2, 48.0; HRMS (EI) m/z M+ calcd for [C13H13NO]+ 199.0997, found 199.0999. N-(4-Fluorobenzyl)aniline (3ka).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (81% yield, 81 mg): 1H NMR (500 MHz, CDCl3) δ 7.23 (m, 2H), 7.08 (m, 2H), 6.93 (m, 2H), 6.63 (m, 1H), 6.52 (m, 2H), 4.19 (s, 2H), 3.90 (s, 1H); 13C NMR (125 MHz, CDCl3) δ 162.1 (d, J = 245.0 Hz), 148.1, 135.2 (d, J = 3.1 Hz), 129.4, 129.1 (d, J = 8.0 Hz), 117.8, 115.5 (d, J = 21.4 Hz), 113.0, 47.7; HRMS (EI) m/z M+ calcd for [C13H12FN]+ 201.0954; found 201.0951. N-(4-Chlorobenzyl)aniline (3la).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow solid product (63% yield, 68 mg): mp 46−47 °C; 1H NMR (500 MHz, CDCl3) δ 7.23−7.16 (m, 4H), 7.07 (m, 2H), 6.63 (m, 1H), 6.50 (m, 2H), 4.19 (s, 2H), 3.93 (s, 1H); 13C NMR (125 MHz, CDCl3) δ 147.9, 138.1, 132.9, 129.4, 128.8, 128.8, 117.9, 113.0, 47.7; HRMS (EI) m/z M+ calcd for [C13H12ClN]+ 217.0658, found 217.0658. N-(4-Bromobenzyl)aniline (3ma).9b Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (38% yield, 50 mg): 1H NMR (500 MHz, CDCl3) δ 7.38 (m, 2H), 7.17 (m, 2H), 7.09 (m, 2H), 6.65 (m, 1H), 6.53 (m, 2H), 4.22 (s, 2H), 3.97 (s, 1H); 13C NMR (125 MHz, CDCl3) δ 147.9, 138.7, 131.8, 129.4, 129.2, 121.1, 118.0, 113.0, 47.8; HRMS (EI) m/z M+ calcd for [C13H12BrN]+ 261.0153, found 261.0155. N,N-Dimethyl-4-((phenylamino)methyl)aniline (3na).9e Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 20/1) afforded a yellow solid product (54% yield, 61 mg): mp 58−60 °C; 1H NMR (500 MHz, CDCl3) δ 7.17 (m, 2H), 7.10 (m, 2H), 6.65 (m, 2H), 6.57 (m, 2H), 4.12 (s, 2H), 3.79 (s, 1H), 2.86 (s, 6H); 13C NMR (125 MHz, CDCl3) δ 150.2, 148.6, 129.3, 128.9, 127.2, 117.4, 112.9, 48.1, 40.9; HRMS (EI) m/z M+ calcd for [C15H18N2]+ 226.1470, found 226.1473. N-(Naphthalen-1-ylmethyl)aniline (3oa).15e Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow solid product (91% yield, 106 mg): mp 65−67 °C; 1H NMR (500 MHz, CDCl3) δ 7.98−7.91 (m, 1H), 7.81−7.73 (m, 1H), 7.69 (m, 1H), 7.43−7.37 (m, 3H), 7.31 (m, 1H), 7.10 (m, 2H), 6.64 (m, 1H), 6.56 (d, 2H), 4.59 (s, 2H), 3.84 (s, 1H); 13C NMR (125 MHz, CDCl3) δ 148.3, 134.4, 134.0, 131.6, 129.4, 128.9, 128.3, 126.4, 126.1, 125.9, 125.7, 123.7, 117.7, 112.8, 46.5; HRMS (EI) m/z M+ calcd for [C17H15N]+ 233.1204, found 233.1203. N-Isobutylaniline (3pa).9a Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (92% yield, 69 mg): 1H NMR (500 MHz, CDCl3) δ 7.09 (m, 2H), 6.60 (t, J = 7.3 Hz, 1H), 6.52 (m, 2H), 3.60 (s, 1H), 2.85 (d, J = 6.8 Hz, 2H), 1.86−1.76 (m, 1H), 0.90 (d, J = 6.7 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 148.7, 129.3, 117.1, 112.8, 51.9, 28.2, 20.6; HRMS (EI) m/z M+ calcd for [C10H15N]+ 149.1204, found 149.1206. N-(3-Phenylbutyl)aniline (3qa). Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (89% yield, 100 mg): 1H NMR (500 MHz, CDCl3) δ 7.24−7.19 (m, 2H), 7.12 (m, 3H), 7.05 (m, 2H), 6.58 (m, J = 7.3 Hz, 1H), 6.41 (m, 2H), 3.39 (s, 1H), 2.97−2.88 (m, 2H), 2.79−2.72 (m, 1H), 1.81 (m, 1H), 1.21 (d, J = 7.0 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 148.4, 146.8, 129.3, 128.6, 127.0, 126.3, 117.2, 112.8, 42.4,

resulting solution was reacted for a specified time. After the reaction, the solution was filtered and concentrated by a rotary evaporator. The crude product was purified by flash column chromatography. N-Benzylaniline (3aa).9e Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (93% yield, 85 mg): 1H NMR (400 MHz, CDCl3) δ 7.32− 7.18 (m, 5 H), 7.09 (m, 2H), 6.63 (t, J = 7.2 Hz, 1H), 6.55 (m, 2H), 4.24 (s, 2H), 3.92 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 148.2, 139.5, 129.4, 128.7, 127.6, 127.3, 117.7, 113.0, 48.4; HRMS (EI) m/z M+ calcd for [C13H13N]+ 183.1048, found 183.1046. N-(4-Methylbenzyl)aniline (3ba).9e Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow solid product (91% yield, 90 mg): mp 45−47 °C; 1H NMR (400 MHz, CDCl3) δ 7.16 (m, 2H), 7.12−7.02 (m, 4H), 6.61 (t, J = 6.9 Hz, 1H), 6.52 (m, 2H), 4.16 (s, 2H), 3.85 (s, 1H), 2.24 (s, 3H); 13 C NMR (100 MHz, CDCl3) δ 148.3, 136.9, 136.4, 129.4, 129.3, 127.6, 117.6, 112.9, 48.1, 21.2; HRMS (EI) m/z M+ calcd for [C14H15N]+ 197.1204, found 197.1206. N-(2,5-Dimethylbenzyl)aniline (3ca).15a Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (86% yield, 91 mg): 1H NMR (400 MHz, CDCl3) δ 7.13−7.05 (m, 3H), 7.00 (d, J = 7.6 Hz, 1H), 6.93 (d, J = 7.6 Hz, 1H), 6.63 (t, J = 7.3 Hz, 1H), 6.54 (m, 2H), 4.12 (s, 2H), 3.67 (s, 1H), 2.23 (s, 3H), 2.21 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 148.4, 136.8, 135.7, 133.2, 130.4, 129.3, 129.2, 128.1, 117.4, 112.7, 46.5, 21.0, 18.5; HRMS (EI) m/z M+ calcd for [C15H17N]+ 211.1361, found 211.1359. N-(4-Methoxybenzyl)aniline (3da).7g Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (84% yield, 90 mg): 1H NMR (400 MHz, CDCl3) δ 7.18 (m, 2H), 7.07 (m, 2H), 6.77 (m, 2H), 6.61 (t, J = 7.3 Hz, 1H), 6.52 (m, 2H), 4.13 (s, 2H), 3.83 (s, 1H), 3.68 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 158.9, 148.3, 131.5, 129.3, 128.9, 117.5, 114.1, 112.9, 55.4, 47.8; HRMS (EI) m/z M+ calcd for [C14H15NO]+ 213.1154, found 213.1156. N-(3-Methoxybenzyl)aniline (3ea).15b Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow solid product (92% yield, 98 mg): mp 63−64 °C; 1H NMR (400 MHz, CDCl3) δ 7.16 (t, J = 7.8 Hz, 1H), 7.08 (m, 2H), 6.89− 6.82 (m, 2H), 6.73 (d, J = 8.2 Hz, 1H), 6.63 (t, J = 7.3 Hz, 1H), 6.54 (d, J = 7.9 Hz, 2H), 4.20 (s, 2H), 3.94 (s, 1H), 3.69 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 160.0, 148.2, 141.3, 129.8, 129.4, 119.8, 117.7, 113.1, 112.9, 112.7, 55.3, 48.4; HRMS (EI) m/z M+ calcd for [C14H15NO]+ 213.1154, found 213.1153. N-(2,5-Dimethoxybenzyl)aniline (3fa). Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (91% yield, 111 mg): 1H NMR (500 MHz, CDCl3) δ 7.07 (m, 2H), 6.84 (s, 1H), 6.72 (d, J = 8.8 Hz, 1H), 6.67 (d, J = 8.6 Hz, 1H), 6.61 (t, J = 7.2 Hz, 1H), 6.56 (m, 2H), 4.22 (d, J = 5.7 Hz, 2H), 4.03 (s, 1H), 3.73 (s, 3H), 3.64 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 153.7, 151.7, 148.5, 129.3, 128.7, 117.5, 115.4, 113.2, 112.3, 111.3, 56.0, 55.8, 43.6; HRMS (EI) m/z M+ calcd for [C15H17NO2]+ 243.1259, found 243.1258. 4-Methoxy-2-((phenylamino)methyl)phenol (3ga).15c Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/ 1) afforded a yellow solid product (72% yield, 83 mg): mp 136−137 °C; 1H NMR (500 MHz, CDCl3) δ 7.88 (s, 1H), 7.14 (m, 2H), 6.80 (t, J = 7.3 Hz, 1H), 6.72 (m, 3H), 6.67 (m, 1H), 6.64 (s, 1H), 4.25 (s, 2H), 3.88 (s, 1H), 3.66 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 153.2, 150.5, 147.4, 129.4, 123.9, 120.7, 117.2, 115.8, 114.5, 114.0, 55.9, 48.7; HRMS (EI) m/z M+ calcd for [C14H15NO2]+ 229.1103, found 229.1105. 2-((Phenylamino)methyl)phenol (3ha).15d Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 5/1) afforded a brown solid product (71% yield, 71 mg): mp 110−111 °C; 1H NMR (500 MHz, CDCl3) δ 8.28 (s, 1H), 7.19−7.11 (m, 3H), 7.07 (d, J = 7.4 Hz, 1H), 6.86−6.77 (m, 3H), 6.75 (m, 2H), 4.31 (s, 2H), 3.90 (s, 1H); 13C NMR (125 MHz, CDCl3) δ 156.8, 147.3, 129.5, 129.3, 128.8, 123.1, 120.9, 120.2, 116.7, 116.0, 48.8; HRMS (EI) m/z M+ calcd for [C13H13NO]+ 199.0997, found 199.0995. 12775

DOI: 10.1021/acs.joc.7b02019 J. Org. Chem. 2017, 82, 12771−12777

Note

The Journal of Organic Chemistry 38.0, 38.0, 22.7; HRMS (EI) m/z M+ calcd for [C16H19N]+ 225.1517, found 225.1520. N-Butylaniline (3ra).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (85% yield, 63 mg): 1H NMR (500 MHz, CDCl3) δ 7.13− 7.06 (m, 2H), 6.61 (t, J = 7.3 Hz, 1H), 6.53 (m, 2H), 3.51 (s, 1H), 3.04 (t, J = 7.1 Hz, 2H), 1.53 (m, 2H), 1.36 (m, 2H), 0.89 (t, J = 7.4 Hz, 3H); 13C NMR (125 MHz, CDCl3) δ 148.7, 129.4, 117.2, 112.8, 43.8, 31.8, 20.5, 14.1; HRMS (EI) m/z M+ calcd for [C10H15N]+ 149.1204, found 149.1202. N-Benzyl-4-methylaniline (3ab).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (86% yield, 85 mg): 1H NMR (400 MHz, CDCl3) δ 7.32−7.22 (m, 4H), 7.18 (t, J = 6.8 Hz, 1H), 6.90 (m, 2H), 6.48 (m, 2H), 4.21 (s, 2H), 2.15 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 146.0, 139.8, 129.9, 128.7, 127.6, 127.3, 126.9, 113.1, 48.8, 20.5; HRMS (EI) m/z M+ calcd for [C14H15N]+ 197.1204, found 197.1206. N-Benzyl-4-methoxyaniline (3ac).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (83% yield, 88 mg): 1H NMR (400 MHz, CDCl3) δ 7.32−7.23 (m, 4H), 7.20 (d, J = 6.5 Hz, 1H), 6.70 (m, 2H), 6.52 (m, 2H), 4.20 (s, 2H), 3.69 (s, 1H), 3.66 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 152.3, 142.6, 139.8, 128.7, 127.7, 127.3, 115.0, 114.2, 55.9, 49.4; HRMS (EI) m/z M+ calcd for [C14H15NO]+ 213.1154, found 213.1154. 4-(Benzylamino)phenol (3ad).9e Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 5/1) afforded a yellow solid product (77% yield, 77 mg): mp 84−85 °C; 1H NMR (400 MHz, CDCl3) δ 7.32−7.22 (m, 4H), 7.22−7.19−7.15 (m, 1H), 6.62 (m, 1H), 6.48 (m, 1H), 4.20 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 147.9, 142.5, 139.7, 128.7, 127.7, 127.3, 116.3, 114.5, 49.5; HRMS (EI) m/z M+ calcd for [C13H13NO]+ 199.0997, found 199.1000. N-Benzyl-4-fluoroaniline (3ae).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (66% yield, 66 mg): 1H NMR (400 MHz, CDCl3) δ 7.35−7.23 (m, 4H), 7.22−7.16 (m, 1H), 6.79 (m, 2H), 6.50−6.43 (m, 2H), 4.20 (s, 2H), 3.82 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 156.0 (d, J = 235.0 Hz), 144.6 (d, J = 1.8 Hz), 139.4, 128.8, 127.6, 127.4, 115.8 (d, J = 22.3 Hz), 113.8 (d, J = 7.4 Hz), 49.1; HRMS (EI) m/z M+ calcd for [C13H12FN]+ 201.0954, found 201.0952. N-Benzyl-2-fluoroaniline (3af). Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (69% yield, 69 mg): 1H NMR (400 MHz, CDCl3) δ 7.34−7.25 (m, 4H), 7.21 (m, 1H), 6.94−6.83 (m, 2H), 6.64−6.52 (m, 2H), 4.28 (s, 1H), 4.25 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 151.6 (d, J = 238.6 Hz), 139.1, 136.7 (d, J = 11.0 Hz), 128.8, 127.5, 127.5, 124.7 (d, J = 3.5 Hz), 116.9 (d, J = 7.0 Hz), 114.5 (d, J = 18.3 Hz), 112.4 (d, J = 3.3 Hz), 48.0; HRMS (EI) m/z M+ calcd for [C13H12FN]+ 201.0954, found 201.0953. N-Benzyl-4-chloroaniline (3ag).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow solid product (71% yield, 77 mg): mp 39−40 °C; 1H NMR (400 MHz, CDCl3) δ 7.31−7.25 (m, 3H), 7.23−7.16 (m, 1H), 6.80 (m, 2H), 6.48 (m, 2H), 4.21 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 146.8, 139.1, 129.2, 128.8, 127.5, 127.5, 122.2, 114.0, 48.5; HRMS (EI) m/z M+ calcd for [C13H12ClN]+ 217.0658, found 217.0659. N-Benzyl-4-bromoaniline (3ah).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded colorless liquid product (84% yield, 110 mg): 1H NMR (400 MHz, CDCl3) δ 7.27−7.23 (m, 4H), 7.22−7.16 (m, 1H), 7.14 (m, 2H), 6.40 (m, 2H), 4.19 (s, 2H), 3.97 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 148.3, 139.6, 129.4, 128.8, 127.6, 127.4, 117.7, 113.0, 48.4; HRMS (EI) m/z M+ calcd for [C13H12BrN]+ 261.0153, found 261.0154. N-Benzyl-4-methylaniline (3ai).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (61% yield, 60 mg): 1H NMR (400 MHz, CDCl3) δ 7.31−7.21 (m, 4H), 7.20−7.13 (m, 1H), 6.90 (d, J = 8.0 Hz, 2H), 6.48 (d, J = 8.0 Hz, 2H), 4.22 (s, 2H), 3.80 (s, 1H), 2.15 (s, 3H); 13C NMR (100 MHz, CDCl3) δ 146.0, 139.8, 129.9, 128.7, 127.6, 127.3, 126.8,

113.1, 48.8, 20.5; HRMS (EI) m/z M+ calcd for [C14H15N]+ 197.1204, found 197.1203. N-Benzyl-3-ethylaniline (3aj). Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (73% yield, 77 mg): 1H NMR (400 MHz, CDCl3) δ 7.34− 7.24 (m, 4H), 7.22−7.17 (m, 1H), 7.02 (m, 1H), 6.51 (m, 1H), 6.45− 6.38 (m, 2H), 4.25 (s, 2H), 2.49 (q, J = 7.6 Hz, 2H), 1.13 (t, J = 7.6 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 148.3, 145.6, 139.6, 129.4, 128.7, 127.7, 127.3, 117.5, 112.7, 110.3, 48.6, 29.2, 15.7; HRMS (EI) m/z M+ calcd for [C15H17N]+ 211.1361, found 211.1361. N-Benzylnaphthalen-1-amine (3ak).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (81% yield, 94 mg): 1H NMR (400 MHz, CDCl3) δ 7.70−7.60 (m, 2H), 7.33−7.28 (m, 3H), 7.27−7.21 (m, 3H), 7.19−7.17 (m, 2H), 7.14−7.12 (m, 1H), 6.48 (d, J = 7.3 Hz, 1H), 4.52 (s, 1H), 4.32 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 143.3, 139.2, 134.4, 128.9, 128.8, 127.9, 127.5, 126.7, 125.9, 124.9, 123.5, 120.0, 117.8, 104.9, 48.8; HRMS (EI) m/z M+ calcd for [C17H15N]+ 233.1204, found 233.1206. N-Benzyl-[1,1′-biphenyl]-2-amine (3al).7 Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a white solid product (77% yield, 100 mg): mp 86−87 °C; 1 H NMR (400 MHz, CDCl3) δ 7.40−7.32 (m, 4H), 7.29−7.19 (m, 5 H), 7.17−7.13 (m, 1H), 7.11−7.06 (m, 1H), 7.02 (d, J = 7.4 Hz, 1H), 6.68 (t, J = 7.4 Hz, 1H), 6.56 (m, 1H), 4.31 (s, 1H), 4.23 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 145.0, 139.6, 139.6, 130.3, 129.5, 129.1, 128.8, 128.7, 127.7, 127.4, 127.1, 117.3, 110.8, 48.2; HRMS (EI) m/z M+ calcd for [C19H17N]+ 259.1361, found 259.1360. 5′-(Benzylamino)-[1,1′:3′,1″-terphenyl]-2′-ol (3am). Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 5/1) afforded a pale-yellow sticky solid product (72% yield, 126 mg): mp 115−117 °C; 1H NMR (499 MHz, CDCl3) δ 7.45 (m, 4H), 7.37 (m, 4H), 7.31 (m, 3H), 7.29−7.25 (m, 3H), 7.22−7.16 (m, 1H), 6.55 (s, 2H), 4.24 (s, 2H); 13C NMR (125 MHz, CDCl3) δ 142.1, 141.8, 139.6, 138.2, 129.6, 129.4, 128.9, 128.8, 127.8, 127.7, 127.4, 114.9, 49.4; HRMS (EI) m/z M+ calcd for [C25H21NO]+ 351.1623, found 351.1626. 3-(Benzylamino)-2-methylphenol (3an). Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 5/1) afforded a yellow solid product (78% yield, 83 mg): mp 109−111 °C; 1H NMR (500 MHz, CDCl3) δ 7.33−7.25 (m, 4H), 7.23−7.19 (m, 1H), 6.88− 6.84 (m, 1H), 6.19 (m, 1H), 6.14 (m, 1H), 4.58 (s, 1H), 4.29 (s, 2H), 3.83 (s, 1H), 1.98 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 153.7, 147.6, 139.6, 128.8, 127.7, 127.4, 127.0, 107.8, 105.1, 103.6, 48.7, 8.9; HRMS (EI) m/z M+ calcd for [C14H15NO]+ 213.1154, found 213.1153. N-Benzyl-4-(2-bromoethyl)aniline (3ao). Purification by flash chromatography on silica gel (n-hexane/ethyl acetate = 100/1) afforded a yellow liquid product (80% yield, 116 mg): 1H NMR (500 MHz, CDCl3) δ 7.27 (m, 4H), 7.20 (m, 1H), 6.93 (m, 2H), 6.52 (m, 2H), 4.24 (s, 2H), 4.00 (s, 1H), 3.42 (t, J = 7.8 Hz, 2H), 2.96 (t, J = 7.8 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 147.2, 139.4, 129.7, 128.8, 128.0, 127.6, 127.4, 113.1, 48.6, 38.9, 33.8; HRMS (EI) m/z M+ calcd for [C15H16BrN]+ 289.0466, found 289.0464. 5-(Benzylamino)isoindoline-1,3-dione (3ap). Purification by flash chromatography on silica gel (dichloromethane/methanol = 10/1) afforded pale-yellow solid product (43% yield, 54 mg): mp 209−211 °C; 1H NMR (400 MHz, DMSO-d6) δ 10.75 (s, 1H), 7.56 (t, J = 5.7 Hz, 1H), 7.46 (d, J = 8.2 Hz, 1H), 7.36−7.32 (m, 3H), 7.29−7.20 (m, 1H), 6.88−6.81 (m, 2H), 4.41 (s, 2H); 13C NMR (100 MHz, DMSOd6) δ 169.8, 169.4, 154.0, 138.9, 135.4, 128.6, 127.3, 127.1, 124.7, 118.3, 115.9, 105.3, 46.1; HRMS (EI) m/z M + calcd for [C15H12N2O2]+ 252.0899, found 252.0895.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02019. 1 H and 13C NMR spectra of compounds (PDF) 12776

DOI: 10.1021/acs.joc.7b02019 J. Org. Chem. 2017, 82, 12771−12777

Note

The Journal of Organic Chemistry

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Young Keun Chung: 0000-0002-2837-5176 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (2014R1A5A1011165 and 2007-0093864). I.C. and S.C. are recipients of the BK21 Plus Fellowship.

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DOI: 10.1021/acs.joc.7b02019 J. Org. Chem. 2017, 82, 12771−12777