key: cord-0052104-o6jsudj2
authors: Luo, NianHua; Zhong, YuHong; Wen, HuiLing; Luo, Renshi
title: Cyclometalated Iridium Complex-Catalyzed N-Alkylation of Amines with Alcohols via Borrowing Hydrogen in Aqueous Media
date: 2020-10-19
journal: ACS Omega
DOI: 10.1021/acsomega.0c04192
sha: f422b318bc9f14da737c7a5bb28947fe2765ac5f
doc_id: 52104
cord_uid: o6jsudj2

[Image: see text] This paper develops a methodology for cyclometalated iridium complex-catalyzed N-alkylation of amines with alcohols via borrowing hydrogen in the aqueous phase. The cyclometalated iridium catalyst-mediated N-alkylation of amines with alcohols displays high activity (S/C up to 10,000 and yield up to 96%) and ratio of amine/imine (up to >99:1) in a broad range of substrates (up to 46 examples) using water as the green and eco-friendly solvent. Most importantly, this transformation is simple, efficient, and can be performed at a gram scale, showcasing its potential for industrially synthesizing N-alkylamine compounds.

Amines and amine skeletons exist in numerous natural products, biologically active molecules, and agrochemicals. 1 They are also the most fundamental and significant organonitrogen compounds for the preparation of other organonitrogen blocks as starting materials. Research on this kind of structure has gained immense interest, resulting in many synthetic methods. 1d Among them, the one-pot N-alkylation of amines with alcohols via borrowing hydrogen represents one of the most effective synthetic methods, which contrasts sharply with classical alkylation methods, which tend to cause overalkylation. 2 In particular, this method has only water as a byproduct, which has the advantages of atom economy and environmental friendliness and allows alcohols to replace more conventional but often toxic alkyl halides as an alkylating agent. 3 Since the introduction of this kind of synthetic method by Grigg and Watanabe in 1981, 4 it has aroused great interest in chemists as well as become one of the research hotspots. 5 For example, the nickel-, 6 iron-, 7 cobalt-, 8 manganese-, 9 silver-, 10 and copper-11 atalyzed N-alkylation of amines with alcohols via borrowing hydrogen was extensively reported. Besides, the ruthenium 12 and iridium complexes 13 are also employed in this transformation of N-alkylation.

Although there are many reported methods and catalysts for N-alkylation of amines with alcohols via borrowing hydrogen, there is still room for further improvement. For example, the currently reported transformation was usually carried out in organic solvents. Furthermore, the existing N-alkylation reactions are limited to high temperatures as well as large catalyst loading, which limits practical applications to a certain extent. 5

The importance of cyclometalated iridium compounds for N-alkylation of amines with alcohols has been well studied and documented (Scheme 1a,b). 13 Recently, we realized the transfer hydrogenation reduction of aldehydes and ketones and the hydroxylation of silane using a series of cyclometalated iridium catalysts, 14 which showed a high catalytic efficiency. Encouraged and inspired by the results of our previous studies, we envisioned whether these cyclometalated iridium complexes can be used for N-alkylation of amines with alcohols via borrowing hydrogen to develop an efficient, mild, and environmentally friendly catalytic methodology. In this report, we established a general and efficient N-alkylation of amines with alcohols via borrowing hydrogen using cyclometalated iridium complexes (Scheme 1c). The utilization of an electrondonating ligand of pyridyl 4,5-dihydro-1H-imidazole is crucial to realize the N-alkylation of amines with alcohols, achieving the N-alkylation products in high amine/imine ratios and yields. Meanwhile, the employment of water as the solvent promises lower reaction temperatures and higher reactivity of this iridium complex-catalyzed N-alkylation. Meanwhile, a gram-scale practical synthesis was achieved to demonstrate the utility of this method.

At the outset of the study on iridium complex-catalyzed Nalkylation, we selected benzyl alcohol (1a) and aniline (2a) as the templates for the versatility of this method. As depicted in Table 1 , the employment of the cyclic metal iridium complex TC-1, which was efficient for the transfer hydrogenation reduction of aldehydes and ketones, led to 88% yield and 90:10 ratio of amine/imine using toluene as the solvent and t-BuOk as the base under reflux conditions (Table 1, entry 1) . Meanwhile, the employment of the catalyst TC-2 bearing an electron-donating substituent as well as of TC-3 and TC-4 bearing electron-withdrawing substituents promised almost the same ratio of amine/imine and activity (Table 1 , entries 2−4). Catalysts TC-5 or TC-6 bearing the sterically hindered methoxy group was then designed in order to achieve enhanced selectivity and activity (Table 1, entries 5 and 6). Encouragingly, the obviously improved ratio of amine/imine (>99:1) and activity (>99 yield), compared with catalysts TC-2−TC-4, verified our speculation. More importantly, the use of the catalyst TC-6 led to a complete conversion and an excellent ratio of amine/imine (Table 1 , entry 6), indicating the importance of the methoxy group in the ligand structure. Further research of the use of 0.1−0.01 mol % TC-6 catalyst loading afforded almost the same activity and ratio of amine/ imine only by prolonging the reaction time (Table 1, entries 7−9). In stark contrast, it was detrimental to the N-alkylation of amines with alcohols as the lack of iridium catalyst afforded sharply diminished activity and ratio of amine/imine (Table 1 , entry 10).

In order to achieve more practical reaction conditions, we further looked into the effects of different bases on the reaction with the abovementioned optimal catalyst TC-6 ( Table 2) .

Generally, the reaction with a strong base can achieve a better ratio of amine/imine and activity (Table 2 , entries 1, 2, 8−11). For example, when KOH was used as the base, the reaction can achieve the most ideal reaction results (Table 2, entry 9). However, the employment of weak bases did not achieve the desired catalytic results ( Table 2 , entries 3−7). Interestingly, the base equivalent has a great impact on the ratio of amine/ imine and the activity of the reaction. For example, with 0.8 mmol base equivalent, the reaction only achieved 96:4 ratio of amine/imine and 81% yield ( The N-alkylation reaction can be proceeded in a variety of solvents. Generally, the activity and ratio of amine/imine of the catalyst are better in nonpolar organic solvents such as toluene or xylene (Table 3 , entries 1 and 2), which is in contrast to polar organic solvents such as DMF (Table 3 , entries 3−5). Our previous studies have shown that the cyclometalated iridium complex reveals good water solubility. For example, efficient catalytic results of the transfer hydrogenation reaction of aldehydes and ketones have been achieved in the aqueous solution. Inspired by our recent work, we envisage whether to use water as the solvent to achieve the N-alkylation reaction of alcohol and amine. To our delight, excellent activity and ratio of amine/imine can be achieved at 80 or 100°C in aqueous media. Meanwhile, the reaction time can be greatly shortened (Table 3 , entries 6 and 7). Unfortunately, ideal results were not achieved when the reaction temperature was down to 60°C (Table 3 , entry 8).

To further demonstrate the generality of this iridiumcatalyzed N-alkylation reaction, a variety of amines and alcohols were investigated under the optimal reaction conditions ( Table 4 ). The results show that (1) using aniline as the reaction substrate, excellent ratio of amine/imine and yield can be achieved with different primary alcohols such as substituted benzyl alcohol (Table 4 , entries 1−8). For example, N-alkylation can occur smoothly using electron-donating groups, such as methoxy and methyl groups, or electronwithdrawing groups such as chlorine or fluorine-substituted benzyl alcohol. Besides, the activity and ratio of amine/imine of the reaction are not affected by different secondary alcohols such as substituted phenylethanol (Table 4 , entries 9−15). For example, the reaction achieves a good yield and ratio of amine/ imine using an electron-donating group such as methoxy or an electron-withdrawing group such as chlorine in different positions of phenylethanol. (2) With 4-methylaniline as the substrate, ideal catalytic results can be achieved by employing primary alcohols such as a variety of substituted benzyl alcohol (Table 4 , entries 16−20). At the same time, the yield and ratio of amine/imine of the reaction are not affected using different secondary alcohols such as substituted phenylethanol (Table 4 , entries 21−28). (3) Using different primary alcohols such as substituted benzyl alcohol as the reaction substrate, Nalkylation products with a high ratio of amine/imine and a high yield can be obtained with different substituted anilines (Table 4 , entries 29−39). (4) Aniline as the reaction substrate, the N-alkylation reaction of different types of alcohols and amines was investigated (Table 4 , entries 40−45). Studies have shown that with naphthalene methanol, heterocyclic alcohols such as furan methanol and thiophene methanol, aliphatic alcohols such as n-butanol and cyclohexanol as alcohols, the corresponding N-alkylated products are also efficiently achieved. Interestingly, the use of sterically hindered amines such as N-ethylaniline can also undergo N-alkylation reactions with alcohols to efficiently achieve the corresponding addition products (Table 4 , entry 46).

In order to look into the practicality of the catalytic system, we also targeted the reduction of imine, aldehydes, and ketoester via a cyclometalated iridium-catalyzed borrowing hydrogen reaction with benzyl alcohol as the hydrogen source (Scheme 2). The corresponding reduction products were achieved in high yields (91−94%) with the imine, aldehyde, and ketoester as the substrates separately (Scheme 3c,d,f) . Surprisingly, the substrate with an unsaturated aldehyde was fully hydrogenated and led to 95% yield (Scheme 2e).

To further explore this catalytic system's activity, a gramscale experiment was also performed under similar conditions in the presence of 0.1 mol % TC-6. The desired product 3aa was acquired in 93% yield (Scheme 3), demonstrating the potential application for large-scale production.

Based on the above results and previous reports, we proposed a possible reaction mechanism of a "borrowing hydrogen" pathway (Scheme 4). 15 First, Int-I was gained via interaction of TC-1 with benzyl alcohol (BnOH) through extrusion of KCl under the base of KOH. Subsequently, β-H elimination of Int-I formed Int-II and aldehyde. 16 At the same time, the Ir−H complex Int-II will lead to proton-exchange in water (deuterium-labeling experiments support this, which is given in the Supporting Information). Then, condensation of aldehydes and amines was conducted smoothly to give imines, which were trapped by Int-II and generated an intermediate Int-III. Finally, ligand exchange of Int-III of BnOH delivered the desired products and Int-I for the next catalytic cycles.

A general, practical, and efficient iridium complex-catalyzed Nalkylation of amines with alcohols via borrowing hydrogen was established, displaying high activity (S/C up to 10,000 and yield up to 96%) and ratio of amine/imine (up to >99:1) in a broad range of substrates. The advantage of this method was the employment of water as the solvent, which may be attributed to the excellent water solubility of the iridium catalysts. Most importantly, application of this transformation is exemplified by a gram-scale synthesis with the template reaction, showcasing the potential for industrially synthesizing N-alkylamine compounds.

General Information. All the commercially available chemical reagents were purchased from Beijing Innochem Science & Technology Co., Ltd. and used directly in the reaction without purification. All the solvents employed in the reaction were purchased from Beijing Innochem Science & Technology Co., Ltd. and used directly without degassing. 1 H and 13 C NMR spectra were recorded using a Bruker DRX-400 spectrometer and the chemical shifts were referenced to signals at 7.26 (for 1 H) and 77.0 (for 13 C) ppm, respectively. GC analyses were performed on an Agilent GC-7900. All the products were purified by column chromatography with silica gel (200−300 mesh). General Procedure for N-Alkylation of Amine and Alcohol. In a 10 mL Schlenk tube, a mixture of 1 (1.1 mmol), 2 (1.0 mmol), KOH (1.1 mmol), H 2 O (2 mL), and TC-6 (0.1 mol %) was reacted at 80°C protected by nitrogen. After completion of the reaction, the mixture was extracted with ethyl acetate (3 × 10 mL). The combined ethyl acetate layer was then dried over magnesium sulfate and concentrated in vacuum. The resulting crude product was purified by silica gel chromatography using a mixture of EtOAc/petroleum ether (1/20) as an eluent to afford the amine product. Procedure for the Gram Scale of N-Alkylation of Aniline and Benzyl Alcohol. In a 100 mL Schlenk tube, a mixture of benzyl alcohol 1a (11 mmol), aniline 2a (10 mmol), KOH (11 mmol), H 2 O (20 mL), and TC-6 (0.1 mol %) was reacted at 80°C protected by nitrogen. After completion of the reaction, the mixture was extracted with ethyl acetate (3 × 50 mL). The combined ethyl acetate layer was then dried over magnesium sulfate and concentrated in vacuum. The resulting crude product was purified by silica gel chromatography using a mixture of EtOAc/petroleum ether (1/20) as an eluent to afford the product benzyl-phenyl-amine 3aa.

Benzyl-phenyl-amine (3aa). 9e Colorless oil, yield: 174.0 mg, 95%. 1 H NMR (400 MHz, CDCl 3 4, 139.7, 129.5, 128.9, 127.7, 127.4, 117.8, 113.1, 48.5 .

(2-Methoxy-benzyl)-phenyl-amine (3ba). 17 148.5, 129.3, 129.0, 128.4, 127.4, 120.6, 117.4, 113.2, 110.3, 55.4, 43.5 .

(3-Methoxy-benzyl)-phenyl-amine (3ca). 11b Colorless oil, yield: 193.9 mg, 91%. 1 H NMR (400 MHz 148.3, 141.4, 129.8, 129.4, 119.9, 117.7, 113.2, 113.0, 112.7, 55.3, 48.4 .

(4-Methoxy-benzyl)-phenyl-amine (3da 148.4, 131.6, 129.4, 128.9, 117.6, 114.2, 113.0, 55.4, 47.9 .

(4-Methyl-benzyl)-phenyl-amine (3ea 129.5, 129.5, 127.7, 117.7, 113.1, 48.2, 21.4 .

(3-Chloro-benzyl)-phenyl-amine (3fa). 11b Colorless oil, yield: 201.9 mg, 93%. 1 H NMR (400 MHz, CDCl 3 ): δ 7.47 (br, 1H), 7.36−7.28 (m, 5H), 6.86 (t, J = 7.3 Hz, 1H), 6.71 (d, J = 8.1 Hz, 2H), 4.38 (s, 2H), 4.14 (s, 1H); 13 C NMR (101 MHz, CDCl 3 ): δ 147. 9, 141.9, 134.6, 130.0, 129.5, 127.5, 127.5, 125.5, 117.9, 113.0, 47.8 .

(4-Chloro-benzyl)-phenyl-amine (3ga 2, 132.9, 129.4, 128.9,128.8, 117.9, 113.0, 47.7 .

(4-Fluoro-benzyl)-phenyl-amine (3ha 2, 135.4, 135.4, 129.5, 129.2, 129.1, 117.9, 115.7, 115.5, 113.1, 47.7 .

Phenyl-(1-phenyl-ethyl)-amine (3ia). 9e Colorless oil, yield: 185.3 mg, 94%. 1 H NMR ( 5, 129.4, 116.9, 113.3, 51.8, 336, 26.1, 25.2. Benzyl-ethyl-phenyl-amine (3ai). 31 Colorless oil, yield: 187.9 mg, 89%. 1 H NMR (400 MHz, CDCl 3 ): δ 7.43−7.30 (m,7H), 6.84−6.78 (m, 3H), 4.64 (s, 2H), 3.60 (q, J = 7.0 Hz, 2H), 1.33 (t, J = 7.1 Hz, 3H); 13 C NMR (101 MHz, CDCl 3 ): δ 148. 6, 139.4, 129.4, 128.7, 126.9, 126.7, 116.1, 112.3, 54.0, 45.2, 12.2. 1-Benzyl-4-(4-methoxy-phenyl)-piperazine (3aj). 32 Colorless oil, yield: 256.8 mg, 91%. 1 H NMR (400 MHz, CDCl 3 ): δ 7.38 (br, 5H), 7.35−7.30 (m, 2H), 6.93−6.85 (m, 2H), 4.68 (s, 3H), 3.79 (s, 2H), 3.60 (s, 1H), 3.26−2.95 (m, 3H), 2.67− 2.42 (m, 4H); 13 C NMR (101 MHz, CDCl 3 ): δ 153. 9, 145.7, 141.0, 137.7, 129.4, 128.6, 127.6, 127.0, 118.4, 114.4, 65.2, 63.1, 55.6, 53.1, 50.6. ■ ASSOCIATED CONTENT

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04192. 1 H and 13 C NMR spectra for all compounds prepared (PDF)

2-Methoxy-phenyl)-ethyl]-phenyl-amine (3ja

Hz, 1H), 6.65 (d, J = 7.7 Hz, 2H)

(3-Methoxy-phenyl)-ethyl]-phenyl-amine (3ka). 19 Colorless oil, yield: 209.0 mg, 92%. 1 H NMR (400 MHz

19 Colorless oil

Chloro-phenyl)-ethyl]-phenyl-amine (3ma). 20 Colorless oil, yield: 210.3 mg, 91%. 1 H NMR (400 MHz

(3-Chloro-phenyl)-ethyl]-phenyl-amine (3na). 21 Colorless oil, yield: 214.9 mg, 93%. 1 H NMR (400 MHz

−7.34 (m, 4H), 7.17 (t, J = 7.9 Hz, 2H), 6.74 (t, J =

21 (d, J = 8.2 Hz, 2H), 6.76 (d, J = 8.4 Hz, 2H), 4.47 (s, 2H), 4.01 (s, 1H), 2.47 (s, 3H); 13 C NMR (101 MHz

CDCl 3 ): δ 7.42 (q, J = 8.7 Hz, 4H), 7.15 (d, J = 8.2 Hz, 2H), 6.67 (d, J = 8.4 Hz, 2H), 4.38 (s, 2H), 4.02 (s, 1H), 2.41 (s, 3H); 13 C NMR (101 MHz

Colorless oil

98 (s, 3H), 2.30 (s, 3H), 1.59 (d

20.5. HRMS-ESI (m/z): calcd for C 16 H 20 NO

26 Colorless oil, yield: 221.9 mg, 92%. 1 H NMR (400 MHz

HRMS-ESI (m/z): calcd for C 15 H 17 NCl

29 (s, 3H), 1.55 (d, J = 6.7 Hz, 3H); 13 C NMR (101 MHz

Benzyl-(4-chloro-phenyl)-amine (3ac)

Benzyl-(4-fluoro-phenyl)-amine (3ad)

37 (s, 2H), 3.89 (s, 1H); 13 C NMR (101 MHz

−7.48 (m, 1H), 7.30 (dd, J = 12.9, 7.1 Hz, 2H), 6.89 (t, J = 7.4 Hz, 1H)

−7.46 (m, 1H), 7.31 (t, J = 7.7 Hz, 1H), 6.81 (d, J =

Benzyl-(3-bromo-4-methyl-phenyl)-amine (3ag). 28 Colorless oil

Benzyl-(4-methoxy-phenyl)-amine (3ah). 9e Colorless oil, yield: 198.2 mg, 93%. 1 H NMR (400 MHz

Colorless oil, yield: 232.3 mg, 94%. 1 H NMR (400 MHz, CDCl 3 ): δ 7.50−7.45 (m, 2H), 7.29−7.26 (m, 2H), 6.90−6.82 (m, 2H), 6.68−6.62 (m, 2H)

Colorless oil, yield: 237.2 mg, 96%. 1 H NMR (400 MHz, CDCl 3 ): δ 7.37 (dd, J = 8.3, 5.6 Hz, 2H), 7.06 (t, J = 8.7 Hz, 2H)

Colorless oil, yield: 218.8 mg, 90%. 1 H NMR (400 MHz, CDCl 3 ): δ 7.39 (d, J = 7.4 Hz, 1H), 7.36−7.30 (m, 1H), 7.03− 6.95 (m, 2H)

Yellow solid, yield: 223.7 mg, 92%, mp 93−94°C. 1 H NMR

CDCl 3 ): δ 7.38 (d, J = 8.5 Hz, 2H), 6.98 (d

Yellow solid, yield: 206.7 mg, 91%, mp 66−67°C. 1 H NMR

CDCl 3 ): δ 7.37 (d, J = 7.8 Hz, 2H), 7.26 (d, J = 7

Naphthalen-1-ylmethyl-phenyl-amine (3pa). 6i Grey solid

Naphthalen-2-ylmethyl-phenyl-amine (3qa)

CDCl 3 ): δ 8.05−7.89 (m, 4H)

−7.43 (m, 1H), 7.28−7.24 (m, 2H)

Phenyl-thiophen-2-ylmethyl-amine (3sa). 11b Colorless oil, yield: 168.3 mg, 89%. 1 H NMR (400 MHz

67 (d, J = 8.3 Hz, 2H), 3.64 (s, 1H), 3.17 (t, J = 7.1 Hz, 2H), 1.72−1.61 (m, 2H), 1.54−1.44 (m, 2H), 1.03 (t, J = 7.3 Hz, 3H); 13 C NMR (101 MHz

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The authors declare no competing financial interest.