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Synthesis of secondary amines from one-pot reductive amination with formic acid as the hydrogen donor over an acid-resistant cobalt catalyst Liang Jiang, Peng Zhou, Zehui Zhang, Shiwei Jin, and Quan Chi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03621 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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Industrial & Engineering Chemistry Research

Synthesis of secondary amines from one-pot reductive amination with formic acid as the hydrogen donor over an acid-resistant cobalt catalyst Liang Jiang, Peng Zhou, Zehui Zhang*, Shiwei Jin, Quan Chi Key Laboratory of Catalysis and Materials Sciences of the Ministry of Education, South-Central University for Nationalities, Wuhan, 430074, P. R. China.

ABSTRACT: Developing new heterogeneous non-noble metal catalysts to replace noble-metal catalysts in organic transformations is of high importance in modern synthetic chemistry. Herein, nitrogen-doped carbon embedded Co catalysts (abbreviated as Co@CN-T-AT, in which T represents the pyrolysis temperature, AT represents the acid-leaching process) were prepared through the simple pyrolysis of graphene oxide-supported cobalt-based zeolitic

imidazolate-

frameworks, followed by acid-leaching process. The Co@CN-600-AT catalyst demonstrated the highest catalytic activity among the synthesized Co catalyst towards the reductive amination of carbonyl compounds with nitro compounds by transfer hydrogenation with formic acid as the hydrogen donor, which integrated three consecutive steps into one-pot reaction. By applying this catalyst, structurally-diverse secondary amines were produced in good to excellent yields without

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the reduction of other functional groups. The transfer hydrogenation of the imines (C=N bonds) was the rate determining step. Furthermore, the catalyst was highly stable and could be reused without the decrease of the catalytic activity. KEYWORDS: Nitrogen-doped carbon; Cobalt catalysts; Transfer hydrogenation; Reductive amination; Secondary amines


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INTRODUCTION Amines serve as a very useful and irreplaceable class of compounds, which are of great significance in the synthesis of fine chemicals such as polymers, dyes, pigments, agrochemicals and pharmaceuticals.1 Nitro compounds are one of the most abundant starting materials, which have been used for the synthesis of primary amines for a long history.2 The as-formed primary amines can be further transformed into other valuable chemicals such as secondary/tertiary amines, imines and azo compounds.3-5 In current, great attention has been shifted to the synthesis of secondary amines either from primary amines or directly from nitro compounds, which mainly includes amine-carbonyl,6 Buchwald-Hartwig7 and Ullman-type carbon-nitrogen cross-coupling,8 direct alkylation of amines with alkyl halides9 as well as the direct N-alkylation of amines with alcohols.10 Among different methods, the amine-carbonyl reductive amination has been considered to be an sustainable and economical protocol, as the substrates are readily available and the processes are high atom-economy without toxic wastes. The reductive amination of carbonyl compounds with primary amines have been extensively studied with plentiful results in different catalytic systems.11-13 From the viewpoints of green and sustainable chemistry, it is a highly important alternative to pursue the one-pot reductive amination of carbonyl compounds directly from nitro compounds in a domino fashion. The one-pot reaction strategy avoids the tedious separation and purification steps, which saves energy and time. However, the one-pot reductive amination has been rarely studied in recent years. The currently reported methods for the one-pot reductive amination of carbonyl compounds with nitro compounds were mainly performed over noble-metal catalysts with H2 as the hydrogen source14-18 inexpensive and clean hydrogen source, the applications of these procedures


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are constrained by the use of explosive H2 especially for non-noble metal catalysts under harsh conditions (high temperature and high H2 pressure). In additions, some of the reported methods are limited to functional-group tolerance. Therefore, the development of new catalytic methods is highly appreciated for the one-pot reductive amination. In recent years, catalytic transfer hydrogenation (CTH) of nitro compounds has emerged as attractive protocols due to simple experimental setups.19 During the CTH process, nitro compounds can be reduced to amines by “extracting” hydrogen from hydrogen-donor molecules, which has been considered as promising alternatives to catalytic hydrogenation methods with H2.20 Among the common hydrogen donors, formic acid is abundant and cheap, which is currently produced industrially by the hydrolysis of methyl formate and also easily accessible via CO2 hydrogenation.21 Of particular note is that formic acid can also be produced from biomass resources with high yields.22 Hence, formic acid represents an economical, safe and easy-tohandle liquid hydrogen carrier for transfer hydrogenation. However, the one-pot reductive amination with formic acid as the hydrogen donor has been rarely studied,23-25 and the reported methods were performed over noble metal catalysts. In addition, a base was usually required for formic acid-mediated transfer hydrogenation. From the viewpoints of green and sustainable chemistry, it is highly recommended to perform the one-pot reductive amination by the use of heterogeneous non-noble metal catalyst with formic acid as the hydrogen donor in the absence of any additive. Recently, nitrogen-doped carbon materials based non-noble metal (particular cobalt or iron) catalysts, have been intensively studied for the electrochemical oxygen reduction reaction (ORR), and some of them even demonstrated better electrocatalytic performance than the conversional Pt/C catalysts.26 Currently, these non-noble metal catalysts have also received great


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attention in organic transformation, demonstrating great potential for replacing expensive noblemetal-based catalysts.27, 28 The obstacle to the use of formic acid with base metal catalysts is that formic acid can react with the base metals, which is unlike the noble-metal catalysts such as Au and Pd. Fortunately, the base metal nanoparticles embedded in carbon layer were found to be resistant to acid.29 In our previous work, a cobalt-nitrogen (Co-Nx) carbon catalyst was discovered to be active for the transfer reductive amination of nitro compounds with formic acid 30

and the subsequent reductive amination.31 The precursor for the preparation of Co-Nx catalyst

is a macrocyclic compound (cobalt phthalocyanine), which is high cost to purchase or difficult to prepare in laboratory. Thus, it is still necessary to develop new acid-resistant nitrogen doped carbon embedded cobalt catalyst for the synthesis of secondary amines via one-pot reductive amiantion. ZIF-67 as an important kind of metal organic framework, which is composed of Co2+ and imidazole ligands, has been considered as attractive catalyst precursors for the preparation of nitrogen-doped Co catalysts.16 Herein, a graphene-directed assembly route was used to prepare the nitrogen-doped carbon based cobalt catalysts, in which Co nanoparticle were embedded in the nitrogen-doped carbon structure. The as-prepared catalysts were found to be highly active and stable for one-pot reductive amination of carbonyl compounds with nitro compounds with formic acid as the hydrogen donor.

RESULTS AND DISCUSSION Catalyst Synthesis and Characterization


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Fig. 1 Schematic illustration of the synthesis of Co@CN-600-AT catalyst. The procedure of the preparation of the Co@CN-600-AT catalyst is schematically illustrated in Fig. 1. GO was prepared as described in our previous work,32 which was used to deposit ZIF67. Initially, GO was subjected to be ultrasonication for 1 h to give a clear solution, and then 2methylimidazole was added to the mixture. After the addition of Co(NO3)2 solution to the above solution, ZIF-67 was immediately formed at room temperature. Due to the coordination interactions between the Co2+ from the ZIF-67 cluster and oxygen containing groups (such as epoxy and hydroxyl groups) from the GO sheet, ZIF-67 was deposited on the GO layer to give a ZIF-67/GO composite. The XRD patterns of the composite of ZIF-67/GO (Fig. S1) were consistent with its standard XRD pattern of ZIF-67, which revealed that ZIF-67 was successfully deposited on the surface of the GO.33 In the second step, the ZIF-67/GO composite was pyrolyzed under a N2 atmosphere at 600 °C for 3 h. During this process, the continuous decomposition of ZIF-67 polyhedrons was accompanied by the release of gaseous molecules,34 and Co2+ was reduced to metallic cobalt nanoparticles. At the same time, the GO was converted into reduced GO (rGO) at a high temperature by the release of reducing gases. After pyrolysis, there were two types of Co nanoparticles in the sample of Co@CN-600. Some Co nanoparticles were loosely deposited on the surface of the material, which could be washed off by the acid. Some Co nanoparticles were embedded in the carbon layer, which were resistant to acid. In the


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third step, the as-made powder was treated with 6 M H2SO4 for 12 h to wash off the surface Co nanoparticles, but Co nanoparticles embedded in the carbon layer were remained. Similar, the Co@CN-750-AT and Co@CN-900-AT catalysts were prepared under the same procedure except at different pyrolysis temperature.

Co@CN-600-AT Co@CN-750-AT Co@CN-900-AT

Intensity (a.u.)

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Co-Nx Co (111) Co (220)

Co (200) C

10

20

30

40

50

60

70

80

2θ/o

Fig. 2 Powder XRD pattern of the Co@CN-T-AT catalysts. After carbonization, the characteristic peaks of ZIF-67 were disappeared for the Co@CN-TAT catalysts. As far as the XRD patterns of the Co@CN-T-AT catalysts, three peaks at 2θ = 44.2, 51.5 and 75.8° were observed (Fig. 2), which were corresponding to the (111), (200) and (220) plans of metallic Co nanoparticles (JCPDS No. 15–0806).34 In addition, a peak at 2θ around 43.0° was observed in Co@CN-900-AT, which was the characteristic peak for Co-Nx (JCPDS No. 41-0943).35, 36 It is worth noting that a peak at 2θ = 25.7° (JCPDS No.01-0646) was observed in the XRD patterns of the Co@CN-T-AT catalysts, which was assigned to a turbostratic ordering of the carbon and nitrogen atoms in the graphite layers.37 Particularly, the peak intensity at 25.7° for the Co@CN-900-AT catalysts was much stronger due to the lowest cobalt content (Table S1). In addition, it was also noted that the half-peak width of Co


7


nanoparticles for the Co@CN-900-AT catalysts decreased with the increase of pyrolysis temperature, indicating that Co nanoparticles became larger with the increase of the pyrolysis temperature, and the particle size was calculated to be 7.6, 9.5 and 15.2 nm by the Scherrer equation for Co@CN-600-AT, Co@CN-750-AT and Co@CN-900-AT, respectively.

Average diameter = 8.2 nm

30

Percentage(%)

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25 20 15 10 5 0 4-6

6-8

8-10

10-12

12-14

Diameter (nm)

Fig. 3 TEM images of the Co@CN-600-AT catalysts and the size distribution of Co nanoparticles. The morphology of the as-prepared Co@CN-T-AT catalysts was characterized by TEM. It was observed that Co nanoparticles were embedded in the carbon matrix of the Co@CN-600-AT, and the average size of the Co nanoparticles was estimated to be 8.2 nm (Fig. 3). Since the rGO is homogenously and completely coated with ZIF-derived carbon particles, the existence of rGO can be only found at some edges of TEM image of the Co@CN-600-AT catalyst. The TEM image of the Co@CN-750-AT catalyst was also similar with that of the Co@CN-600-AT catalyst, but the average size of Co nanoparticles was much larger of 14.2 nm (Fig. S2). However, much fewer Co nanoparticles were observed in the TEM images of the Co@CN-900AT catalyst (Fig. S3). In addition, the particle size of Co nanoparticle aggregated to a very large size of 28.1 nm. The weight percentage of cobalt in the Co@CN-T-AT decreased with the increase of pyrolysis temperature (Table S1). 19.6 wt.% of cobalt was determined for the


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Co@CN-600-AT catalyst, while those were 16.8 wt.% and 3.1 wt.% for the Co@CN-750-AT and Co@CN-900-AT catalysts, respectively. On the meanwhile, the atom percentage of nitrogen detected by XPS also decreased with the increase of pyrolysis temperature. These results indicated that a higher pyrolysis temperature promoted the cleavage of the carbon-nitrogen bonds, and much more Co nanoparticles were exposed on the surface of the Co@CN-T-AT, which were released off by acid-leaching. As shown in Fig. 3, porous structure was observed in the TEM images of the catalysts. Then texture properties of the Co@CN-T-AT catalysts were characterized by N2 sorption isotherms. Nitrogen adsorption isotherm curves of the Co@CN-TAT catalysts displayed a type-V curve and H3-type hysteresis loop, which is characteristic of mesoporous structure (Fig. 4). Pore size distribution (Fig. 5) demonstrated the Co@CN-T-AT catalysts had a wide size distribution centring around 2 nm and 4 nm. The texture parameters of the Co@CN-T-AT catalysts are listed Table S2. Interestingly, it was noted that the surface area of the Co@CN-T-AT catalysts decreased with the increase of the pyrolysis temperature, while the pore volume of the Co@CN-T-AT catalysts were close to each other.

700

Adsorbed volume (cc/g)

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



600 500 400 300 200 100 0

0.0

0.2

0.4

0.6

0.8

1.0

P/Po

Fig. 4 Nitrogen adsorption-desorption isotherms of the Co@CN-T-AT catalysts.


9


0.12 0.10

dV/logd (cc/g)

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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0.08 0.06 0.04 0.02 0.00 1

10

100

Pore diameter (nm)

Fig. 5 Pore size distribution of the as-prepared Co@CN-T-AT catalysts. XPS was used to analyze the valence state of elements of the as-prepared Co@CN-600-AT catalyst. The high resolution Co 2p3/2 spectrum of the Co@CN-600-AT catalyst could be fitted into two peaks with with the binding energies (BEs) around 780.3 and 778.3 eV (Fig. 6). The peak at 778.3 eV was attibuted to the metallic Co, while the peak at 780.3 was assigned metallic cobalt, which should be due to the oxidation of surface mettalic Co during the storage in the air 34

. Similarly, the Co 2p3/2 spectra of the Co@CN-750-AT and Co@CN-900-AT could also be

fitted into the same two peaks (Fig. S4). Interestingly, it was noted that the relative peak area of metallic Co increased with the increase of the pyrolysis temperature, and the reason should be caused by the fact that the surface of Co nanoparticles with a larger size was much more difficult to be oxidized. The high resolution N 1s XPS spectrum of Co@CN-600-AT can be fitted into three peaks at about 398.5, 399.5 and 400.8 eV, which were corresponded to pyridinic nitrogen, pyrrolic nitrogen and graphitic nitrogen, respectively (Fig. 6). The pyridinic and pyrrolic nitrogen may serve as coordination sites with the metal Co nanoparticles. In addition, the C 1s spectrum of the Co@CN-600-AT catalyst can be fitted the four peaks including C=C (284.6 eV), C–N (285.0 eV), C–O (286.2 eV), and O–C=O (289.2 eV), respectively (Fig. 6).


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Intensity (a.u.)

32000 Co 2p 3/2

31000

780.3

30000 29000

Co satellite (785.9)

28000 778.3

27000 26000 25000 790

788

786

784

782

780

778

776

Binding Energy (eV)

24000 N 1s

Intensity (a.u.)

22000

398.5 399.5

20000 400.8

18000 16000 14000

404

402

400

398

396

Binding Energy (eV)

80000

Intensity (a.u.)

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


70000

284.6

C 1s

60000 50000 40000 30000

286.2 285.0

20000

289.2

10000 0 294

292

290

288

286

284

282

280

278

Binding Energy (eV)

Fig. 6 The Co 2p3/2 , N 1s, and C 1 s XPS spectra of the Co@CN-600-AT catalyst. Catalyst Screening for the Transfer Hydrogenation of Nitrobenzene


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Table 1 Catalyst screening for the transfer hydrogenation of nitrobenzene.a

Entry

Catalyst

Time (h)

Con. (%)

Sel. 1 (%)

1

ZIF-67

10

0

-

2

GO

10

0

-

3

Co@CN-600-AT

3

43.1

93.3

6.7

4

Co@CN-750-AT

3

14

76.5

23.5

5

Co@CN-900-AT

3

6.3

66.7

32.3

6

Co@CN-600AT-Air

3

2.8

50.8

49.2

7

Co+ AC

3

1.4

44.6

55.4

Sel. 2 (%)

a

Reaction conditions: Nitrobenzene (1 mmol), THF (10 ml), formic acid (3 mmol), catalyst (40 mg), 110 °C. The catalytic activity of the as-prepared Co@CN-T-AT catalysts was firstly evaluated by the transfer hydrogenation of nitrobenzene, as this step determines the success of the one-pot reductive amination of carbonyl compounds with nitro compounds. Reactions were performed in tetrahydrofuran (THF) at 110 oC with 3 equiv. of formic acid. No conversion was observed either in the presence of ZIF-67 or GO (Table 1, Entries 1 & 2). These results suggested that the coordinated Co cations with 2-methylimidazole and the GO support were inactive towards this reaction. To our pleasure, the Co@CN-600-AT catalysts were active for the transfer hydrogenation of nitrobenzene towards this reaction (Table 1, Entries 3-5). The Co/CN-600-AT catalyst demonstrated the best catalytic activity among the three kinds of the Co@CN-T-AT catalysts with the same catalyst weight. As discussed later, both nitrogen atoms and Co nanoparticles were of significant importance towards the transfer hydrogenation reactions with


12

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formic acid as the hydrogen donor over the Co@CN-T-AT catalysts. Both cobalt and nitrogen content were noted to decrease with the increase of the pyrolysis temperature for the Co@CN-TAT catalysts. The Co@CN-600-AT catalyst had the highest content of cobalt and nitrogen. Therefore, it showed highest catalytic activity (Table 1, Entries 3-5). In addition, the average size of Co nanoparticles in the Co@CN-600-AT catalyst was the smallest. Generally, metallic nanoparticles with a smaller size demonstrated higher catalytic activity due to exposure of more active sites. For all cases, aniline and N-phenylformamide were detected to be the products, and N-phenylformamide was formed by the subsequent condensation of aniline with formic acid (Scheme S1). In addition, it was noted that the selectivity of N-phenylformamide was higher at lower nitrobenzene conversion, as much more formic acid remained in the reaction solution at lower nitrobenzene conversion, which was beneficial to the condensation of aniline with formic acid. The transfer hydrogenation of nitrobenzene was also performed over the Co@CN-600-ATAir catalyst, which was prepared by the oxidation of Co@CN-600-AT in the air at 250 oC for 4 h. XRD pattern of the Co@CN-600-AT-Air revealed that the metallic Co in the Co@CN-600AT catalyst was completely transformed into Co3O4 (Fig. S5). Interestingly, nitrobenzene conversion greatly decreased from 43.1% over the Co@CN-600-AT catalyst to 2.8% over the Co@CN-600-AT-Air catalyst (Table 1, Entry 6). Therefore, metallic Co nanoparticles should be the active sites for this reaction. In addition, a physical mixture of commercial metallic cobalt (30 nm) with the same cobalt weight by the use of the Co@CN-600-AT catalyst and activated carbon were almost no active towards this reaction (Table 1, Entry 7). These results again indicated that both nitrogen atoms and Co nanoparticles were determinant on the activity of the Co@CN-T-AT catalysts towards the transfer hydrogenation reactions with formic acid as the hydrogen donor.


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Table 2 The reductive amination of benzylaldehyde with nitrobenzene at different temperatures.

Entry

FA amount (mmol)

T. (°C)

Time (h)

Con. (%)

Sel. 1(%)

Sel. 2 (%)

Sel. 3 (%)

1

4.5

110

10

64.5

6.6

83.9

9.4

2

4.5

130

10

89.1

23.2

54.8

21.7

3

4.5

150

10

100

16.8

49.6

34.0

4

4.5

170

10

100

14.8

31.8

53.2

5

4.5

190

10

100

9.2

20.2

69.0

6

4.5

190

15

100

5.1

10.2

84.5

7

6

190

15

100

0

3.1

96.5

Furthermore, the effect of the solvent on the catalytic performance of the Co@CN-600-AT catalyst towards the hydrogenation of nitrobenzene with formic acid as the hydrogen donor was also studied. As shown in Table S3, the reaction solvents greatly affected the activity of the catalyst and the products selectivity. The reaction was stopped in the solvents of dimethyl sulphoxide (DMSO) and N,N-dimethylformamide (DMF), possibly due to the coordination ability of sulfur or nitrogen atoms in the solvents with the Co sites (Table S3, Entries 1 & 2). Alcohols and THF produced higher aniline selectivity than other solvents especially the solvents without oxygen atoms (Table S3, Entries 3~5 vs 6~8). The possible reasons should be due to the fact that alcohols and THF can stabilize formic acid with hydrogen bonds, thus the condensation of formic acid with aniline can be inhibited to some degree. THF was the best solvent for this reaction, which could produce the highest selectivity of aniline at high nitrobenzene conversion.


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Effect of the Reaction Temperature on the One-pot Reductive Amination With Co@CN-600-AT as the best catalyst and THF as the best solvent in hand, the one-pot reductive amination of benzylaldehyde with nitrobenzene was studied as a model reaction. The reaction at 110 °C produced nitrobenzene conversion of 64.5% after 10 h. With the aim to further improve the catalytic efficiency, the one-pot reductive amination was performed at high reaction temperatures. Nitrobenzene conversion increased to 89.1% after 10 h at 130 °C, and that was 100% beyond 150 °C. As far as products, three products including aniline, the intermediate of Nbenzylideneaniline and N-benzylaniline were observed after 10 h at different reaction temperatures. As shown in Table 2, the transfer hydrogenation of the imine (C=N bonds) was sensitive to the reaction temperature. Generally speaking, the selectivity of the intermediate decreased with the increase of the reaction temperature, while the selectivity of the Nbenzylaniline gradually increased with the increase of the reaction temperature (Table 2, Entries 1~5). These results indicated that the transfer hydrogenation of the imine (C=N bond) was the rate-determining step for the one-pot reductive amination of carbonyl compounds with nitro compounds. In addition, it was noted that aniline was present in the reaction system after 10 h for all tested temperatures. The reason should be that the intermediate of N-benzylideneaniline can be reversely transformed into benzylaldehyde and aniline in the presence of water and formic acid, which was different from the one-pot reductive amination with H2 as the hydrogen source. In fact, we have subjected the condensation of aniline with benzylaldehyde at 150 °C by the use of fresh distilled THF, which produced nearly quantitative yield of N-benzylideneaniline. Prolonging the reaction time could enhance the yield of N-benzylaniline (Table 2, Entries 5 vs 6), but N-benzylaniline was still not obtained in a very high yield after 15 h at 190 °C. To our


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delight, an excellent N-benzylaniline of 96.5% was obtained after 15 h at 190 oC by the use of 6 equiv. of formic acid (Table 2, Entry 7). Substrate scope Table 3. Substrate scope of the developed catalytic system. O R-NO 2

Entry

Nitro compounds

+

R'

NHR R''

R'

R''

Carbonyl compounds

Con. (%)

Sel. (%)

1

100

95.1

2

100

84.8

3

100

96.1

4

100

93.1

5

100

93.6

6

100

86.3

7

100

71.1

8

100

91.2

9

100

89.6

10

100

92.8

11

100

93.5

12

100

96.7

13

100

83.4


16

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14

100

94.7

15

100

84.0

16

100

99.0

17

100

100

18

100

67.8

19

100

96.3

20

100

95.1

21

100

90.4

22

100

98.3

23

100

98.1

24

100

91.3

25

100

97.2

26

100

76.3

27

100

87.8

28

100

76.2

29

100

84.5

30

100

78.3

31

100

32.5


17


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a

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32

100

91.7

33

100

52.1

Reaction conditions: nitrobenzene (1 mmol), aldehydes or ketones (2 mmol), Co@CN-600-

AT (40 mg), THF (10 mL), formic acid (6 mmol), 190 °C and 15 h.

Having achieved the optimal conditions to get the excellent N-benzylaniline yield from the one-pot reductive amination of benzylaldehyde with nitrobenzene, we set out to examine the scope of the developed catalytic method. Firstly, the reductive amination of different aldehydes with nitrobenzene was investigated. Despite of the electronic nature of the substituent groups, aromatic aldehydes reacted smoothly to give the corresponding secondary amine products with good to excellent yields (Table 3, Entries 1~13). More importantly, this catalytic system was tolerant to other functional groups. For example, no dehalogenation was observed for the reactions with 4-chlorobenzaldehyde and 4-fluorobenzaldehyde (Table 3, Entries 4 & 5), albeit a little amount of the debromination and deiodination was observed by the use of 4brorobenzaldehyde and 4-idio-nitroebenzene as the substrate (Table 3, Entry 6 & 7). In addition, other groups such as esters (Table 3, Entry 8), amides (Entry 9) and nitrile (Entry 10) were also remained intatct. Of particlualr note is that this catalytic system was also effective for other kinds of aldehydes. The reductive amination of a fused-ring aromatic aldehyde (2-naphthaldehyde) was also successful, and the conrresponding secondary amine yield was up to 93.5% after 15 h (Table 3, Entry 11). 4-Pyridinecarboxaldehyde and furfural as heteroaromatic aldehydes, (Table 3, Entry 12 & 13), also underwent the reductive amination with nitrobenzene smoothly to give the corresponding secondary amine with a high yield of 96.7% and 83.4%. It is more challenging to


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promote the reductive amination of aliphatic aldehydes. As listed in Table 3, our method was also effective for these transformations with non-aromatic aldehydes (Entries 14–16). Next, the reductive amination of different kinds of nitro compounds with benzaldehydes was also studied. Again, nitrobenzene substituted with electron-donating or -withdrawing groups could undergo reductive coupling to the corresponding secondary amines with high yields (Table 3, Entries 17-25). However, the catalytic activity of the nitroarenes was greatly affected by the position of the substituted groups. For instance, the reductive amination of para- or metamethylnitrobenznen with benzylaldehyde produced much higher yield of the desired secondary amine than the ortho-methylnitrobenznen (Table 3, Entries 18–20), which should be due to the smaller steric hindrance. Again, this method was also tolerant with different functional groups for the substituent nitroarenes. In addition, the non-aromatic nitro compounds also sucessfully underwent the reductive amination with benzylaldehyde to produce the conresponding secondary amines yields with high yields (Table 3, Entries 26 & 27). This catalytic system was even active for the reductive amination with both non-aromatic substrates (Table 3, Entries 26~28). The excellent results of the reductive amination of nitro compounds with aldehydes promoted us the to perform the use of ketons for this reaction. The reductive amination of nitrobenzen with cyclohexanone only produced the corresponding secondary amine with a yield of 32.5% (Table 3, Entry 31). The other main byproducts were aniline with a yield of 50.2% and Nphenylformamide with a yield of 16.4%. These results indicated that the difficulty in the reductive amination of nitrobenzene with cyclohexanone was the condensation step, which was due to the steric hindrance of the ketons as compared with the aldehydes. Interestingly, the reductive amination of the acetophenone with nitrophenylmethane yielded the corresponding secondary amine with high yield of 91.7% under the same reaction conditions (Table 3, Entry


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32), which was due to the smaller hindrance of the nitrophenylmethane as compared with nitrobenzene (Table 3, Entries 31 vs 30). Furthermore, dinitro compounds also sucessfully underwent the reductive amination with benzylaldehyde to produce the conresponding secondary amines in moderate yield (Table 3 Entry 32). Fig. 7 recorded the products distribution of the reductive amination of benzaldehyde with nitrobenzene at different reaction time points. As shown in Fig. 7, Nitrobenzene conversion reached 91.3% after 1 h, and full conversion was noted after 2 h. Then, the condensation of aniline with benzaldehyde generated the intermediate of N-benzylideneaniline, which was a fast reaction and underwent spontaneously without the catalyst. As shown in Fig. 7, the transfer hydrogenation of the intermediate of N-benzylideneaniline was much slower than the transfer hydrogenation of nitrobenzene over the Co@CN-600-AT catalyst. The yield of N-benzylaniline gradually increased during the reaction process, and it reached 96.5% after 15 h. The data revealed that the transfer hydrogenation of the imines (C=N bonds) was the rate determining step for the one-pot reductive amination.

Nitrobenzene Aniline N-Benzylideneaniline N-benzylaniline

100

Percentage (%)

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80 60 40 20 0 0

2

4

6

8

10

12

14

Time (h)


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Fig. 7 Time course of the products distribution during the reductive amination. Reaction conditions: Nitrobenzene (1 mmol), benzaldehyde (2 mmol), THF (10 mL), Co@CN-600-AT (40 mg), formic acid (6 mmol) and 190 °C. Catalyst recycling experiments Finally, the stability of the as-prepared Co@CN-600-AT catalyst was studied. After reaction, the Co@CN-600-AT catalyst was easily collected by the assist of an external magnet, and then the catalyst was washed off with lots of water and ethanol, respectively. Then the spent catalyst was dried in a vacuum oven at 60 °C over night. The spent catalyst was used for the next run under the same reaction conditions. As shown in Fig. S6, the yield of N-benzylaniline from the reductive amination of benzaldehyde remained stable remained stable during the six runs. These results suggested that the Co@CN-600-AT catalyst was highly stable, and the loss of its catalytic activity was avoided. In addition, the reaction solution was also analyzed by ICP-AES, which revealed that there was no leach of cobalt during the recycling experiments. The high stability of Co@CN-600-AT should be due to the encapsulation of cobalt nanoparticles in the carbon layer. CONCLUSION In summary, we have developed a new catalytic system for one-pot reductive amination of carbonyl compounds with nitro compounds for the synthesis of secondary amines in water by using easily handled non-noble metal catalyst with renewable formic acid as a hydrogen source. The Co@CN-600-AT catalyst was highly active and selective for this reaction, affording diverse secondary amines with high yields. The transfer hydrogenation of imines was the ratedetermining step. In addition, the as-prepared Co@CN-600-AT catalyst was highly stable without the loss of its catalytic activity. By saving energy and additional steps, our process


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illustrates the general potential for conducting gas-free reductive condensation reactions in a less costly, simpler and greener manner. 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). Catalyst Preparation Graphene oxide (GO) was prepared and characterized as described in our previous work.32 GO (30 mg) was added into 30 mL of water, and the mixture was subjected to be ultrasonication for 1 h. Then, 5.5 g of 2-methylimidazole was dissolved in this solution, and 0.45 g of Co(NO3)2·6H2O in 3 mL of water was added dropwise to mixture. After addition, the precipitation was collected by centrifugation (10000 rpm for 5 min) and washed with excessive water until the filter was colourless. After drying at 80 oC for 12 h in a vacuum oven, the composite of ZIF-67 and GO (ZIF-67/GO) was obtained. ZIF-67/GO was then calcined at three representative temperatures (600, 750 and 900 °C) for 3 h under a nitrogen atmosphere from room temperature at a heating rate of 3 °C/min to obtain Co@CN-T samples, in which T represents the pyrolysis temperature. To remove the Co particles, the Co@CN-T samples were treated with 6 M H2SO4 solution for 12 h and then collected by centrifugation and washing with water. After drying at 80 oC, Co@CN-T-AT was prepared. Catalyst Characterization Transmission electron microscope (TEM) was performed on an FEI Tecnai G2-20 instrument. X-ray powder diffraction (XRD) measurements were conducted on a Bruker advanced D8


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powder diffractometer (Cu Kα), operating with 2θ range of 10–80° at a scanning rate of 0.016 °/s. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo VG scientific ESCA MultiLab-2000 spectrometer with a monochromatized Al Kα source (1486.6 eV) at constant analyzer pass energy of 25 eV. The cobalt content was determined by inductively coupled plasma atomic emission spectrometer (ICP-AES) on an IRIS Intrepid II XSP instrument (Thermo Electron Corporation). Nitrogen physisorption measurements were conducted at 77 K on a quantachrome Autosorb-1-C-MS instrument. General Procedures for the Transfer Hydrogenation of Nitro compounds All of the reactions were performed in a 50 mL autoclave. In a typical run, nitrobenzene (1 mmol), formic acid (3 mmol) and Co@CN-600-AT catalyst (40 mg), and tetrahydrofuran (THF, 10 mL) were added into the autoclave, which was equipped with a temperature controller. Then the air in the autoclave was exchanged with N2 and the autoclave was charged with 1 MPa N2 at room temperature. The transfer hydrogenation of nitrobenzene was performed at 110 °C with a magnetic stirring at 1000 rpm. After cooling to room temperature, the mixture was quantified based on the internal standard method using toluene as the internal standard. General Procedure for the One-pot Reductive Amination The procedure of the one-pot reductive of nitro compounds with carbonyl compounds were almost the same as the above. In this type of reaction, THF (10 mL), formic acid (4.5 mmol), nitrobenzene (1 mmol), benzaldehyde (2 mmol) and the Co@CN-600-AT catalyst (40 mg) were added to the autoclave. The one-pot reductive amination was performed at a higher reaction temperature of 170 oC for 10 h. In addition, toluene and phenyl ether were added as the two internal standards, in which phenyl ether was used to quantify the imines and secondary amines. The peaks of the products were identified by comparison of the retention time of the unknown


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compounds with those of standard compounds and quantified based on the internal standard method. Analytic Methods An agilent 7890A gas chromatography (GC) was used to analyze the chemicals. This instrument was equipped with a flame ionization detector and a crosslinked capillary HP-5 column (30 m×0.32 mm×0.4 mm). The detail conditions were listed as follows: The flow rate of the N2 carrier gas was 40 mL·min-1, the injection port temperature was 300 °C. The GC oven temperature program was as follows: 50 °C ramp 10 °C /min to 280 °C and the detector temperature was set to 300 °C. The content of each compound was determined based on the internal standard. Recycling Experiments After reaction, the Co@CN-600-AT catalyst was collected from the reaction mixture by an external magnet. The spent catalyst was exhaustively washed with water, ethanol and THF in sequence. Then the spent catalyst was used directly for the next run under the same conditions without any treatment or drying. Other cycles were repeated with the same procedure. AUTHOR INFORMATION Corresponding Author * Tel. +86-27-67842572. Fax: +86-27-67842572. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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