Mild and Selective Synthesis of Secondary Amines Direct from the

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Mild and selective synthesis of secondary amines direct from the coupling of two aldehydes with ammonia Ziliang Yuan, Peng Zhou, Xixi Liu, Yanxin Wang, Bing Liu, Xun Li, and Zehui Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03887 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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Industrial & Engineering Chemistry Research

Mild and selective synthesis of secondary amines direct from the coupling of two aldehydes with ammonia Ziliang Yuan, Peng Zhou, Xixi Liu, Yanxin Wang, Bing Liu*, Xun Li, Zehui Zhang

Key Laboratory of Catalysis and Materials Sciences of the Ministry of Education, South-Central University for Nationalities, Wuhan, 430074, P. R. China.

ABSTRACT: Synthesis of secondary amines direct from the coupling of two aldehydes was firstly reported in the presence of NH3 and H2 molecules. A magnetic material supported palladium catalyst (γ-Fe2O3@HAP-Pd) was found to be active for this transformation at room temperature and atmospheric hydrogen pressure, providing structurally-diverse secondary amines with good to excellent yields (70.0~92.3%).

KEYWORDS: Secondary amines, Aldehydes, Coupling reaction, Ammonia, Mild conditions

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INTRODUCTION Secondary amine has been considered to be one kind of the important motifs in organic compounds, because they have been widely present in numerous bioactive molecules and their widespread pharmaceutical applications.1 Currently, several methods have been reported for the synthesis of secondary amines, mainly including amine-carbonyl reductive amination,2 alkylation of amines with alkyl halides,3 direct N-alkylation of amines with alcohols,4 reductive coupling of nitroarenes with organohalides,5 Buchwald-Hartwig5−6 as well as Ullman-type carbon-nitrogen cross-coupling reactions.8−9 Although these methods could produce good to high yields of secondary amines, some of these methods are limited to the practical applications, due to the inherent drawbacks such as the difficulty in the recycling of homogenous catalysts, the release of the high toxic wastes and the harsh reaction conditions. Reductive amination of carbonyl compounds with ammonia as a nitrogen source has recently been explored in the synthesis of primary amines either over homogeneous or heterogeneous catalysts.10-11 For example, Zhang and co-workers recently reported that the partially reduced Ru/ZrO2 could catalyse the reductive amination of carbonyl compounds with aqueous ammonia with high yields at 85 °C (Scheme 1). 10 However, this method required the use of a large amount of aqueous ammonia (14 equiv.) and high hydrogen pressure (20 bar). 10 The as-formed primary amines can be further transformed into the secondary amines via the reductive amination with carbonyl compounds in the presence of reducing agent.12−13 The pursuit of sustainable chemistry has required the development of new strategies to access organic compounds in a safe, compact and energy efficient manner.14 Thus, it is reasonable to develop the one-pot strategy for the synthesis of secondary amines direct from the coupling of two carbonyl compounds with ammonia (Scheme 1).


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Scheme 1 Synthesis of secondary amines via the coupling of two aldehydes with ammonia. In our previous work, a magnetic γ-Fe2O3@HAP-Pd catalyst was prepared by the following steps including the encapsulation of γ-Fe2O3 by hydroxyapatite (HAP) to get core-shell structured γ-Fe2O3@HAP, the exchange of Pd2+ with Ca2+ in γ-Fe2O3@HAP, followed by the reduction of the Pd2+ to metallic Pd nanoparticles, and this catalyst showed high catalytic activity for the oxidation of 5-hydroxymethylfurfural into 2, 5-furandicarboxylic acid.14 As palladium catalysts have also been widely used as the catalysts to activate molecular H2 for reductive chemical reactions,16−17 it is expected that this catalyst is active for the selective synthesis of secondary amines directly from two aldehydes in the presence of NH3 and H2 molecules. EXPERIMENTAL SECTION Materials All of the solvents were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the chemicals were purchased from Aladdin Chemicals Co. Ltd. (Beijing, China). All of the solvents and the chemicals were used directly without the purification. General Procedures of the Synthesis of Secondary Amines


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Typically, the γ-Fe2O3@HAP-Pd catalyst (20 mg), iso-propanol (10 mL), benzaldehyde (1 mmol) and 26 wt.% aqueous ammonia solution (0.3 mL, 4.2 mmol) was charged in to the reactor. The air in the reactor was expelled from the reactor by the purge of H2 for 5 times. Then the reactor was charged with 1 bar H2 at room temperature and then sealed. Then the reactions were carried out at a desired temperature for a certain time with a magnetic stirring at 1000 rpm. After reaction, the autoclave was depressurized. The reaction mixture was analyzed by gas chromatography (GC). Analytic Methods Products analysis was performed on Agilent 7890A GC with autosampler and a flame ionization detector. The products were separated by a HP-5 capillary column (30 m × 530 µm × 1.5 µm). The temperature of the column was initially kept at 80 °C for 3 min, and then increased at a rate of 20 °C min-1 to 220 °C. The temperature of FID and injector was set to be 270 °C and 300 °C respectively. Products were identified by the comparison of the retention time with the authentic chemicals, and further confirmed by GC-MS (Agilent 7890A GC/5973 MS, HP-5 column). The amounts of products were determined based on GC data using the internal standard method using toluene and phenyl ether as double internal standards to quantify primary amines, the imines and secondary amines, respectively. RESULTS AND DISCUSSION Optimization

of the Reaction Conditions for the Synthesis of N-dibenzylamine from

Benzaldehyde Synthesis of N-dibenzylamine directly from benzaldehyde was used as the model reaction. Firstly, the reaction was performed in different solvents at 25 oC, as different solvents have


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different properties, which would influence the catalyst performance, and the resulsts are listed in Table 1. Three products including N-benzylidenebenzylamine, N-dibenzylamine and benzyl alcohol were detected for the reaction of benzaldehyde in the presence of ammonia and hydrogen molecules in presence of the γ-Fe2O3@HAP-Pd catalyst. Obviously, benzyl alcohol was generated from the hydrogenation of benzaldehyde. N-dibenzylamine was the aim product, which was formed from the hydrogenation of N-benzylidenebenzylamine (the intermediate). As shown in Table 1, both benzaldehyde conversion and products distribution were greatly affected by the solvents. Generally speaking, low to moderate benzaldehyde conversions and low selectivity of N-dibenzylamine were observed in non-polar or weaker polar solvents (Table 1, Entries 1-4). While higher benzaldehyde conversions and higher selectivity of N-dibenzylamine were attained in the solvents with stronger polarity such as tetrahydrofuran (THF), methanol and iso-propanol (Table 1, Entries 5-7). The main reason should be the polarity of the reaction solvents. The non-polar or weak polar solvents showed poor solubility for the aqueous ammonia solution, and the γ-Fe2O3@HAP-Pd catalyst also had a poor dispersion in those solvents. Insolubility or poor solubility of aqueous ammonia solution in the non-polar or weaker polar solvents make it difficult for the condensation between ammonia molecules and benzaldehyde, which is the first step in the one-pot synthesis of N-dibenzylamine from benzaldehyde, and results in the low selectivity for N-dibenzylamine (Table 1, Entries 1-4). The poor dispersion of α-Fe2O3@HAP-Pd catalyst should be one of the reason for the lower conversion. Therefore, reactions in those solvents were much more sluggish. For all cases, benzylamine was not detected, because of the fast condensation of benzylamine with benzaldehyde to produce the other intermediate N-benzylidenebenzylamine. In addition, the reaction in water showed poor selectivity of N-dibenzylamine (Table 1, Entry 8). Iso-propanol was proved to be the best solvent


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for this reaction, which gave 100% benzaldehyde conversion and the highest N-dibenzylamine selectivity of 50.4% (Table 1, Entry 7). However, benzyl alcohol was still produced with a selectivity of 49.6% (Table 1, Entry 7). Table 1 The results of the synthesis of N-dibenzylamine from benzaldehyde in different solvents.a

a

Entry

Solvent

Conv. (%)

Sel. of 1(%)

Sel. of 2(%)

1

Hexane

48.0

91.6

8.4

2

Dichloromethane

26.2

96.8

-

3

Ethyl acetate

30.7

97.4

-

4

p-Xylene

65.1

77.4

22.6

5

THF

93.8

55.2

43.8

6

Methanol

100

58.2

41.8

7

iso-Propanol

100

49.6

50.4

8

H2O

100

67.2

14.0

Reaction conditions: benzaldehyde (1 mmol), solvent (10 mL), NH3·H2O (2.8 mmol), catalyst

(20 mg), 20 bar H2, and temperature (25 °C) for 2 h. Then the amount of catalyst was optimized and the results are shown in figure 1. As the amount of catalyst increased from 5 mg to 20 mg, the conversion of the reaction increased gradually and reach 100% when 20 mg catalyst was used. This is because the more catalytic active sites as the amount of catalyst increased. However, when the amount of catalyst increased to 25 mg, no more effect on the conversion or selectivity on the reaction was found. Moreover


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the amount of catalyst shown no effect on the selectivity on N-dibenzylamine. So, the optimized amount of catalyst is 20 mg.

Conversion or selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120

Conversion Selectivity of N-dibenzylamine

100 80 60 40 20 0

5

10

15

20

25

Catalyst amount (mg) Fig 1. The effect of amount of catalyst on the reaction. (reaction condition: i-PrOH (10 mL), 20 bar of H2, and 25 °C for 2h) With the aim to improve the selectivity of the N-dibenzylamine in iso-propanol, other reaction conditions were further optimized (Table 2). The selectivity of N-dibenzylamine greatly increased from 50.4% at 25 °C to 87.0% at 0 °C under the same reaction conditions, albeit the conversion of benzaldehyde decreased from 100% to 87.1% after 2 h (Table 2, Entries 1-2). These results suggested that the hydrogenation of benzaldehyde into benzyl alcohol was highly sensitive to the reaction temperature. Furthermore, the hydrogenation of benzaldehyde into benzyl alcohol was completely inhibited at -10 °C, but the conversion of benzaldehyde continuously decreased to 53.4% after 2 h (Table 2, Entry 3). Taking the conversion of benzaldehyde and the selectivity of N-dibenzylamine into hand, the reaction temperature was set at 0 °C, as it can produce both high conversion and high selectivity of N-dibenzylamine.


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Furthermore, the effect of hydrogen pressure and the amount of ammonia amount on this reaction was investigated. It was noted that benzaldehyde conversion gradually increased with the increase of the hydrogen pressure from 1 bar to 10 bar, and then almost kept stable at 20 bar (Table 2, Entries 2 & 4-8). The increase of benzaldehyde conversion at higher hydrogen pressure was due to the increase of the hydrogen concentration in the reaction solution, and it can be considered that hydrogen concentration in the reaction solution was almost constant when the hydrogen pressure beyond 10 bar. Interestingly, the product selectivity at 0 °C was almost not influenced by the hydrogen pressure (Table 2, Entries 2 & 4-8). The effect of ammonia amount was also studied. It was noted that both benzaldehyde conversion and N-dibenzylamine selectivity increased with the increase of ammonia amount (Table 2, Entries 2 & 8-10). For example, benzaldehyde conversion increased from 30.0% with 1.4 equiv. NH3 to 52.9% with 4.2 equiv. NH3 after 2 h at 0 °C and 1 bar H2, and the formation of benzyl alcohol was greatly inhibited with 4.2 equiv. NH3. This is due to the higher concentration of ammonia promoted the condensation step of benzaldehyde with ammonia, and thus inhibit the hydrogenation of benzaldehyde into benzyl alcohol. By prolonging the reaction time to 6 h, N-dibenzylamine was produced in an excellent yield of 96.6% at 0 °C and 1 bar H2 with 4.2 equiv. NH3. Moreover, this reaction shown no conversion without catalyst or with γ-Fe2O3@HAP as catalyst (Table 2, Entries 12 and 13), indicating that Pd catalyst is needed in this transformation. To the best of our knowledge, this is the first time for the synthesis of secondary amines directly from the coupling of two carbonyl compounds under such mild conditions. The high activity at low temperature and atmospheric H2 pressure enables the use of common glass reactors, demonstrating a promising potential in industrial application from an economical, environmental and safe viewpoint.


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Table 2 Optimization of the reaction conditions for the synthesis of N-dibenzylaminea

Entry

T. (°C)

H2 (bar)

NH3·H2O (mmol)

Conv. (%)

1

25

20

2.8

2

0

20

3

-10

4

Sel. (%) 1

2

100

49.6

50.4

2.8

87.1

13.0

87.0

20

2.8

53.4

0

98.0

0

10

2.8

88.1

10.2

89.8

5

0

7.5

2.8

73.8

11.2

88.2

6

0

5

2.8

55.2

14.6

85.4

7

0

2.5

2.8

46.1

12.4

87.6

8

0

1

2.8

38.1

11.2

88.8

9

0

1

1.4

30.0

53.8

46.2

10

0

1

4.2

52.9

2.0

98.0

11b

0

1

4.2

100

3.4

96.6

12c

0

1

4.2

0

0

0

13d

0

1

4.2

0

0

0

a

Reaction conditions: benzaldehyde (1 mmol), iso-propanol (10 mL), catalyst (2.5

wt.% Pd 20 mg), 2 h;

b

The reaction time was 6 h. c No catalyst was added.

d

γ-

Fe2O3@HAP was added as catalyst. Reaction Pathway of the Transformation of Benzaldehyde into N-dibenzylamine


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Benzaldehyde N-Benzylidenebenzylamine N-Diphenylamine

100

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

Benzylamine

60 40 20 0 -1

0

1

2

3

4

5

6

7

8

9

10 11

Time (h) Fig. 2 Time course of the products distribution for the synthesis of N-dibenzylamine from benzaldehyde. Reaction conditions: γ-Fe2O3@HAP-Pd (20 mg), benzaldehyde (1 mmol), NH3·H2O (4.2 mmol), -10 °C, iso-propanol (10 mL), H2 (1 bar). To better understand the reaction process, time course of the reaction pathway was recorded. The reaction was performed at a lower temperature of -10 oC and 1 bar H2 with the aim to give more information about the reaction intermediate. As shown in Fig. 2, the concenetraion of benzaldehyde gradually decreased during the reaction process, and the concentraton of Ndibenzylamine

gradually

increased.

Furthermore,

the

concentration

of

N-

benzylidenebenzylamine and benzylamine gradaully increased at an early reaction stage, and then decreased at the late reaction stage. Therefore, both N-benzylidenebenzylamine and benzylamine were be the intermediates for the transformation of benzaldehyde into Ndibenzylamine. Benzylamine should be produced from the condensation of benzaldehyde with NH3 to generate phenylmethanimine (Compound A, Scheme 2) and the subsequent hydrogenation of phenylmethanimine.17 However, phenylmethanimine was not detected due to


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its instability.18 The as-formed benzylamine underwent the condensation with benzaldehyde to generate the other intermediate N-benzylidenebenzylamine (Compound B, Scheme 2), which was a very fast step. Thus, the concentration of benzylamine was very low during the reaction process. N-benzylidenebenzylamine was further reduced to the final product of N-dibenzylamine (Compound C, Scheme 2). After 10 h, N-dibenzylamine was produced in a high yield of 95.6% with benzaldehyde conversion of 100% at -10 oC and 1 bar H2.

Scheme 2. Reaction pathway for the synthesis of N-dibenzylamine. Substrate Scope of the Developed Method The substrate scope of the developed method was finally studied (Table 3). The γFe2O3@HAP-Pd catalyst was not only active for the aromatic aldehydes (Table 3, Entries 1-14), but also for the less reactive cyclohexanecarboxaldehyde and aliphatic carbonyl compounds, affording good to excellent secondary amines (Table 3, Entries 15-16). The activity of the aromatic aldehydes was affected by the substituted groups and steric effect. First, substrates with electron-withdrawing groups were less active than those with electron-donating groups. For example, no conversions were observed for the substrates of 4-fluorobenzaldehyde and 4(trifluoromethyl)benzaldehyde with stronger electron-withdrawing at 0 oC and good yields can be attained with reaction temperature of 25 °C groups (Table 3, Entries 1-2). However, the dehalogenation was observed for 4-bromobenzaldehyde (Table 3, Entry 3), which produced a 78% yield of benzaldehyde and a 22% yield of N-benzylidenebenzylamine. Then, substrates with more steric groups were less active than those with less steric effect. For example, 4-


11


methylbenzaldehyde

shown

a

higher

conversion

and

Page 12 of 17

yields

compared

with

3-

methylbenzaldehyde. And for more steric 2-methylbenzaldehyde they become even lower. To our pleasure, the γ-Fe2O3@HAP-Pd catalyst was tolerant to other functional groups such as esters (Table 3, Entry 9) and carboxylic group (Table 3, Entry 10). This catalytic system was also effective

for

the

heterocyclic

aldehydes

(Table

3,

Entries

11-13)

and

fused-

ring aromatic aldehyde (Table 3, Entry 14), affording high yields of the corresponding secondary amines. For the less active aliphatic aldehydes, the reactions were performed at 25 oC, affording the corresponding secondary amine in good yields (Table 3, Entry 16). Unfortunately, we fail to extend our method to ketones because of its higher steric effect. Table 3 Substrate scope of the developed catalytic system.a

RCHO

H2 NH3•H2O

RCH2OH

+

R

1

Entry

Substrate

N H

R

+

R

Conv. (%)

N

R

3

2

1

Yield (%) 2

3

1b

100

11.0

89.0

0

2b

100

15.8

84.2

0

3

100

0

0

0

4

100

3.4

96.6

0

5

100

8.8

91.2

0

6

96.3

4.0

92.4

0

7

89.5

5.1

84.4

0

8

78.1

6.0

72.1

0

9

100

11.8

88.2

0

10

100

30.0

70.0

0

11

84.8

0

84.8

0


12

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12b

100

0

88.2

11.8

13

100

0

79.2

10.7

14

91.2

0

91.2

0

15

83.8

0

83.8

0

16b 100 0 81.8 3.1 a Reaction conditions: Substrate (1 mmol), iso-propanol (10 mL), γ-Fe2O3@HAPPd (2.5 wt.% Pd, 20 mg), 26.5 wt.% NH3·H2O (300 μL, 4.2 mmol), 6 h. 1 bar of H2 b The reaction was conducted at 25 oC.

100 80

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0

1

2

3

4

5

6

Recycling number Fig 3. Recycling experiments of the γ-Fe2O3@HAP-Pd catalyst. Reaction conditions: benzaldehyde (1 mmol), iso-propanol (10 mL), γ-Fe2O3@HAP-Pd (20 mg), 26.5 wt.% NH3·H2O (300 µL, 4.2 mmol), 0 oC, 6 h, 1bar of H2. Recycling of the γ-Fe2O3@HAP-Pd catalyst


13


The recycling experiments of the Fe2O3@HAP-Pd catalyst

Page 14 of 17

was finally studied. The

transformation of benzaldehyde into N-dibenzylamine was used as the model reaction, and the reaction was performed at 0 oC for 6 h. After reaction, the γ-Fe2O3@HAP-Pd catalyst was easily recovered from the reaction mixture by the assist of an external magnet, and then it was washed with a large amount of water and ethanol, respectively. And the content of Pd in reaction mixture is not detected in ICP-MS. Furthermore, the filtered solution did not exhibit any furtherreactivity. Then the recovered γ-Fe2O3@HAP-Pd catalyst was dried in an vacuum oven overnight. The spent catalyst was reused for the second run, and the reaction conditions were the same as the first run. As shown in Fig. 3, the γ-Fe2O3@HAP-Pd catalyst can be reused for 6 times without the loss of its catalytic activity. As shown in Fig. 3, the yields remained over 90% and the catalytic activity of the γ-Fe2O3@HAP-Pd catalyst was fully restored without apparent loss of activity and selectivity during the recycling experiments. CONCLUSION In conclusion, an unprecedented method was developed for the one-pot synthesis of secondary amines directly via the coupling of two carbonyl compounds in the presence of NH3 and H2 over the magnetic γ-Fe2O3@HAP-Pd catalyst. This method produced the structurediverse secondary amines with good to excellent yields from the readily available carbonyl compounds under very mild conditions. Furthermore, the γ-Fe2O3@HAP-Pd catalyst was highly stable without the loss of its catalytic activity during the recycling experiments. AUTHOR INFORMATION Corresponding Author


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*Tel.: +86-135-4529-5675. E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (21203252). REFERENCES [1] Ono, N. The Nitro Group in Organic Synthesis. Wiley, New York, NY, 2001. [2] Baxter, E. W.; Reitz, A. B. Org. React. 2002. [3] Salvatore, R. N.; Yoon, C. H.; Jung, K. W. Synthesis of secondary amines. Tetrahedron 2001, 57, 7785–7811. [4] Mastalir, M.; Stoger, B.; Pittenauer, E.; Puchberger, M.; Allmaier, G.; Kirchner, K. Air stable iron(ii) PNP pincer complexes as efficient catalysts for the selective alkylation of amines with alcohols. Adv. Synth. Catal. 2016, 358, 3824–3831. [5] Cheung, C. W., & Hu, X. (2016). Amine synthesis via iron-catalysed reductive coupling of nitroarenes with alkyl halides. Nature communications, 7, 12494. [6] Surry, D. S.; Buchwald, S. L. Biaryl phosphane ligands in Palladium-catalyzed amination. Angew. Chem. Int. Ed. 2008, 47, 6338–6361.


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Mild and Selective Synthesis of Secondary Amines Direct from the

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