Cobalt nanocomposites on N-doped hierarchical porous carbon for highly selective formation of anilines and imines from nitroarenes

Tao Song a, Peng Ren ab, Yanan Duan a, Zhaozhan Wang a, Xiufang Chen a and Yong Yang *a
aCAS Key Laboratory of Bio-Based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China. E-mail: yangyong@qibebt.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, China

Received 2nd May 2018 , Accepted 5th July 2018

First published on 6th July 2018


The search for active, inexpensive, and stable heterogeneous catalysts for organic transformation still remains challenging in both academia and industry. Herein, we present the facile fabrication of non-noble cobalt nanocomposites on N-doped hierarchical porous carbon derived from naturally renewable biomass together with Earth-abundant and low-cost cobalt salts. The whole preparation process is operationally simple and straightforward, cost-effective and environmentally benign and can be used in mass production for practical applications. The most active cobalt nanocomposite catalyst, that is, CoOx@NC-800, demonstrated outstanding activity and excellent selectivity for the hydrogenation of functionalized nitroarenes to aniline derivatives with molecular hydrogen in water as a green medium. It also showed high activity for the direct and expedient synthesis of imines from the reductive amination of nitroarenes and aldehydes with excellent selectivity and good tolerance to a variety of functional groups. More importantly, a new straightforward one-pot cascade synthesis of benzimidazole was developed from the reductive amination of ortho-amino substituted nitrobenzene with aldehyde followed by sequential cyclization in a green and sustainable manner, further highlighting the utility of the catalyst. In addition, the catalyst can be easily separated for successive reuses without a significant loss in both activity and selectivity.


Introduction

Anilines and their derivatives are an important class of compounds used as building blocks in the manufacture of dyes, agrochemicals, pigments, pharmaceuticals, and polymers.1 Owing to their immense utilization, the development of more cost-effective and convenient methods for their synthesis and functionalization starting from easily available feedstocks continues to be an active and important task for organic synthesis and the chemical industry. In this context, direct catalytic reduction of nitroarenes with molecular hydrogen represents the most straightforward, atom-economical, and environmentally benign method that is used widely in research laboratories and industries.2 A number of well-defined homogeneous metal complexes3 and stable heterogeneous metal-based catalysts4 have been developed for this process, while chemoselective hydrogenation of nitroarenes still remains challenging, especially for functionalized nitroarenes bearing diverse and readily reducible functional groups.5 The key to this challenge is to develop appropriate catalysts enabling highly efficient and selective hydrogenation of nitroarenes.

From both economic and sustainable perspectives, Earth-abundant metal-based heterogeneous catalysts have attracted considerable attention due to their high abundance, low cost, low toxicity, high stability and easy separation. In this regard, Beller and co-workers developed a novel heterogeneous nanohybrid catalyst, in which cobalt or iron nanoparticles (NPs) are embedded in a N-doped carbon matrix by direct carbonization of non-volatile molecularly defined cobalt or iron-amine ligated complexes for a general reduction of all kinds of nitroarenes with outstanding catalytic performance in 2013.6 After their seminal work, several independent research groups, including Wang,7 Li,8 Kempe,9 Jiang,10 Zhang,11 Gascon,12et al., have developed numerous N-doped carbon modified cobalt-based catalysts, respectively, for selective hydrogenation of functionalized nitroarenes. Despite the great achievements, in the preparation process of such nanocomposites, the necessity of either the use of complicated and expensive organic ligands or tedious and multi-step synthesis of sacrificial template materials accompanied by their uneconomical and inapplicable nature in mass production, or concomitant emission of a large amount of wastes constitutes a serious drawback. To this end, in recent times, attention has turned to the naturally renewable and available biomass, e.g., chitosan (a polymer of D-glucosamine) as a carbon and nitrogen source, in combination with cheap and Earth-abundant base metal salts to prepare metal modified N-doped porous carbon nanocomposites in a straightforward and simple manner, which has been recently demonstrated by Beller and co-workers.13

Our recent work on the fabrication of heteroatom-doped porous carbon derived from biomass, e.g., bamboo shoots, and their applications as the support for heterogeneous catalysts inspired us to continue investigating its potential further.14 Bamboo shoots, as a naturally available and renewable biomass, have an appealing feature because of their ingredients containing nearly 8 wt% of N coming from their intrinsic proteins and amino acids, which can serve as ideal binding sites for metal species. Bearing this in mind, we envision that the complexation of a cobalt salt with bamboo shoots can be used for in situ generation of cobalt nanoparticles dispersed on N-doped carbon after pyrolysis under an inert atmosphere, which we expect to be an active catalyst capable of catalysing hydrogenation of nitroarenes. To our delight, we found that the as-prepared cobalt nanocomposites showed high activity and excellent selectivity for the hydrogenation of nitroarenes including functionalized ones. More importantly, such cobalt nanocomposites also demonstrated expedient synthesis of versatile imines from the reductive amination of various nitroarenes with benzaldehydes in high yields. Herein, we wish to report the preparation of cobalt nanocomposites on N-doped porous carbon and its application in catalytic reduction of nitroarenes and reductive amination of nitroarenes with benzaldehydes as well.

Results and discussion

Cobalt nanocomposites on N-doped hierarchical porous carbon were prepared by a facile tandem hydrothermal-pyrolysis process. Typically, fresh bamboo shoots were first cut into small slices, dried and ground into powder followed by the hydrothermal carbonization (HTC) process in a Teflon-inner stainless-steel autoclave with deionized water at 180 °C. In this step, the bamboo shoots were converted into biochar. The resulting brown biochar was filtered, washed thoroughly with deionized water and dried under vacuum at room temperature. Next, the obtained solids were homogeneously mixed with a CoCl2 solution at 60 °C for 2 h followed by drying and pyrolysis at 700, 800, and 900 °C under a N2 atmosphere for 2 h with a heating rate of 5 °C min−1, respectively. The obtained catalysts were denoted by CoOx@NC-T, where T represents the pyrolysis temperature. The Co content in the catalysts was determined to be 5.39–8.30 wt% by coupled plasma optical emission spectrometry (ICP-OES) (Table 2). For comparison, the catalyst CoOx@C-T with 4.96 wt% Co content was also prepared by the direct pyrolysis of the mixture of CoCl2 with commercially available activated carbon, and the bare support NC without metal loading was also prepared by a similar procedure, respectively.

The as-prepared catalysts were investigated for the catalytic hydrogenation of nitrobenzene as a benchmark reaction to demonstrate their usefulness and the representative results are compiled in Table 1. The reaction was initially performed in the presence of CoOx@NC-800 in THF at 110 °C with 5.0 MPa of H2. To our delight, 45% GC conversion of 1a with perfect selectivity to aniline (2a) was observed (entry 3). To further improve the reaction efficiency, a set of factors, including solvents, types of catalysts, pressure of H2, and reaction temperatures, was screened. The screening of several common organic solvents and even water shows that the reaction efficiency had a profound dependence on the polarity of the solvent (Table S1 in the ESI). Nonpolar solvents such as hexane and toluene exhibited a relatively poor conversion of 1a (entries 1 & 2). Upon increasing the polarity of the solvents, conversion was gradually enhanced. Among the solvents investigated, H2O, as a green and sustainable solvent, showed the best catalytic performance with complete conversion and exclusive selectivity to 2a after 5 h, highlighting its high atom-economical nature and environmental benignness (entry 7). A decrease of the H2 pressure led to a lower catalytic activity towards 2a, while no reaction proceeded under an atmosphere of hydrogen (hydrogen balloon) (Table S2 in the ESI). Similarly, the lower reaction temperature gave poorer activity (Table S3 in the ESI). Subsequently, the resultant catalysts pyrolyzed at 700 and 900 °C were employed for the reaction, respectively, under otherwise identical conditions (entries 7–9). It turned out that the pyrolysis temperature of 800 °C gave superior activity and the reaction efficiency followed the order of CoOx@NC-800 > CoOx@NC-700 > CoOx@NC-900. For comparison, the catalyst CoOx@C-800, which was prepared by the direct pyrolysis of the mixture of CoCl2 and activated carbon without any dopant, showed considerably lower conversion than that of CoOx@NC-800 under identical conditions (entry 10). Such finding indicates that the N-dopant in the carbon framework played an important role in activating molecular hydrogen and the nitro group in the substrate, which was further confirmed by control experiments of the catalyst CoOx@C-800 with an external addition of a N source (Table S4 in the ESI). Moreover, the catalyst CoOx@NC-T is essential for the success of the hydrogenation reaction. A considerably lower reactivity or even no reactivity was observed in the absence of the catalyst or in the presence of CoCl2, pure Co3O4, or pure nano Co3O4 (100 nm) as a catalyst (entries 11–14). Notably, the kinetic profile of the reaction shown in Fig. S1 indicates that nitrobenzene could be converted smoothly into aniline and no intermediates could be detected by gas chromatography during the entire reaction process.

Table 1 Optimization of reaction conditionsa

image file: c8gc01374h-u1.tif

Entry Catalyst Solvent Conversionb (%) Selectivityb (%)
a Reaction conditions: Nitroarene (0.5 mmol), catalyst (10 mol% of Co), solvent (2 mL), H2 (5 MPa), 110 °C. b Determined by GC and GC-MS using dodecane as an internal standard sample and confirmed with their corresponding authentic samples.
1 CoOx@NC-800 Hexane Trace >99
2 CoOx@NC-800 Toluene 25 >99
3 CoOx@NC-800 THF 45 >99
4 CoOx@NC-800 EtOH 40 >99
5 CoOx@NC-800 CH3CN 55 >99
6 CoOx@NC-800 MeOH 73 >99
7 CoOx@NC-800 H2O 100 >99
8 CoOx@NC-700 H2O 86 >99
9 CoOx@NC-900 H2O 72 >99
10 CoOx@C-800 H2O 50 >99
11 CoCl2 H2O 5 >99
12 Co3O4 H2O 6 >99
13 Nano Co3O4 H2O 8 >99
14 Blank H2O 0 0


With such impressive results in hand, we next characterized the catalysts by means of comprehensive techniques. X-ray diffraction (XRD) characterization of the catalyst CoOx@NC-T discloses the formation of a mixture of Co phases, including metallic Co, CoO, and Co3O4, accompanied by a characteristic bump diffraction peak at around 25° assignable to the (002) plane of graphitic carbon as shown in Fig. 1. Further analysis shows that the variable pyrolysis temperatures caused a certain difference in the diffraction peaks of each cobalt phase. For example, the catalyst CoOx@NC-800 shows a much stronger and sharper metallic Co diffraction peak, while stronger diffraction peaks assignable to Co3O4 are observed for the catalyst CoOx@NC-900. Such a discrepancy clearly indicates that this pyrolysis reduction strategy could manufacture cobalt phases with different crystallinities derived from biomass as C and N sources.


image file: c8gc01374h-f1.tif
Fig. 1 XRD pattern for the catalysts CoOx@NC-T (T = 700, 800, and 900 °C).

High resolution transmission electron microscopy (HR-TEM) observation of CoOx@NC-800 demonstrates that CoOx NPs, consisting of Co and/or CoO as a core and Co3O4 as a shell with an average size of 25 nm (particle sizes ranging from 15 to 80 nm), were uniformly dispersed on the N-doped carbon supports (Fig. 2A). The well-resolved lattice spacing of 0.202, 0.260, and 0.247 nm corresponding to the metallic cobalt (111), CoO (111), and Co3O4 (311) plane can be perceived, respectively (Fig. 2B), in good accordance with the XRD observation. Elemental mapping by using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy-dispersive X-ray (EDX) maps clearly shows a homogeneous distribution of Co, N, O and C through the entire carbon framework (Fig. 2C & D). Magnetic measurements also confirm the formation of metallic cobalt NPs (Ms = 12.2 emu g−1, T = 300 K), which is particularly helpful for simple separation with an external magnet after the reaction (Fig. S2).


image file: c8gc01374h-f2.tif
Fig. 2 Representative HRTEM images (A & B), HAADF-STEM image (C & D) and the corresponding mapping of C, N, O and Co for the catalyst CoOx@NC-800.

Raman spectroscopy (Fig. 3A) shows the formation of graphitic carbon in the catalysts, in which the G band at 1590 cm−1 indicates the in-plane vibrations of the sp2 carbon atoms, while the D band at 1350 cm−1 is a defect-induced non-perfect crystalline structure.15 A higher pyrolysis temperature leads to a relatively higher degree of graphitization. Besides, the characteristic Raman peaks that correspond to the Eg (465 cm−1), F2g (507 cm−1), and A1g (672 cm−1) vibration modes of the CoOx crystalline phase are distinctly observed,16 firmly confirming the formation of the mixed cobalt oxide phases and are consistent with HRTEM and XRD observations. N2 adsorption/desorption measurements clearly demonstrate that the catalysts prepared in this strategy possess hierarchically micro-, meso-, and macro-pores with large specific surface areas and high pore volumes as shown in Fig. 3B and Table 2. Remarkably, the catalyst CoOx@NC-900 possesses much larger surface area and pore volume. The large surface areas, the hierarchical porous structure, and high pore volumes are expected to be favorable for the rapid mass-transfer and can provide sufficient active sites for catalysis.


image file: c8gc01374h-f3.tif
Fig. 3 (A) Raman spectra and (B) N2 sorption isotherms and pore size distribution calculated using a nonlocal density functional theory (NLDFT) method for the catalysts CoOx@NC-700, 800, and 900.
Table 2 Chemical composition and texture properties of the catalyst CoOx@NC-T
Sample Co contenta (wt%) Elemental analysis BET analysis
C (wt%) N (wt%) S BET[thin space (1/6-em)]b (m2 g−1) Pore volume (cm3 g−1)
a Determined by ICP-OES. b Specific surface areas were determined by the BET multipoint method.
CoOx@NC-700 5.48 67.34 4.00 462.9 0.307
CoOx@NC-800 5.39 66.67 2.97 471.7 0.319
CoOx@NC-900 8.30 64.21 1.74 763.3 0.589


X-ray photoelectron spectroscopy (XPS) was carried out to further investigate the surface compositions and the chemical state of CoOx@NC (Fig. 4). The Co 2p3/2 XPS spectrum was fitted into four peaks at 778.9, 780.8, 782.9, and 786.5 eV (Fig. 4A), corresponding to the metallic Co, Co3O4, and CoO, and the satellite peak of Co2+.17 Apparently, Co3O4 (Co3+) and CoO (Co2+) are the major constituents on the surface of the catalysts (Table S9 in the ESI), which match well with the HRTEM observation. Three types of N atoms, including graphitic N (401.2 eV), pyrrolic N or N coordinated to Co (Co–Nx centers) (399.6 eV), and pyridinic N (398.2 eV),18,19 could be distinguished from the N 1s spectrum (Fig. 4B), indicating that the N atoms were successfully incorporated into the carbon framework. Clearly, pyridinic N and graphitic N are the dominant peaks in the N 1s XPS spectrum, which can serve as anchoring sites for Co atoms.17,19 Higher pyrolysis temperatures resulted in a decreased content of pyridinic N on the surface, while the catalyst CoOx@NC-800 possessed a relatively higher content of pyrrolic N or Co–Nx (Table S8 in the ESI). In addition, the +O 1s XPS spectrum showed various O functionalities such as C[double bond, length as m-dash]O (531.9 eV), O–C–O (532.4 eV), and O[double bond, length as m-dash]C–O (533.5 eV) in the lattice of the carbon material and Co–O (529.2 eV) species20 (Fig. 4C). The C 1s spectrum could be deconvoluted into four peaks, corresponding to C[double bond, length as m-dash]C (284.6 eV), C[double bond, length as m-dash]N (285.7 eV), C–N/C–O (287.0 eV) and O–C[double bond, length as m-dash]O (288.8 eV), respectively (Fig. 4D).19


image file: c8gc01374h-f4.tif
Fig. 4 (A) Co 2p3/2, (B) N 1s, (C) O 1s, and (D) C 1s XPS spectra for the catalysts CoOx@NC-700, 800, and 900.

To explore the general applicability of this cobalt nanocomposite catalyst, a variety of nitroarenes were subjected to the optimized reaction conditions for the synthesis of amines and the results are shown in Table 3. In general, various functionalized nitroarenes could be efficiently reduced, consistently providing the corresponding anilines in high yields with exclusive selectivity. Electron-donating and electron-withdrawing substituents at the ortho-, meta-, and para-positions on the phenyl ring of nitroarenes all gave high yields of their corresponding anilines (entries 2–15). However, the reaction times for accomplishing complete conversion are highly dependent on the nature and position of the substituents. Both electron-withdrawing and ortho-substituted nitroarenes generally required considerably longer reaction times, suggesting that the electronic and/or steric effect of the substituents on the aromatic rings is not negligible (entries 6, 9, 12 and 15). Among halogenated nitrobenzenes, chloro-substituted nitrobenzene (ortho-, meta-, and para-) underwent reduction efficiently to afford its corresponding anilines with high activity and exclusive selectivity (entries 8–10), while a certain amount of the dehalogenated side product, that is aniline, was obtained simultaneously for the hydrogenation of bromo-substituted benzenes (entries 11–13). Notably, only 50% selectivity to para-iodo-aniline was achieved accompanied by equal selectivity to dehalogenated side product aniline (entry 14). Such results clearly indicate that the dehalogenation process indeed occurs in the current catalysis system, yet strongly depends on the types of halogens. Gratifyingly, nitroarenes bearing amine, hydroxyl, cyano, or ester, and amide substituents were all tolerated by this protocol, giving the corresponding anilines in >99% yield without any signs of side products (entries 16–18, 22 and 23). Remarkably, the nitroarenes decorated with the most challenging reducible functional groups, such as aldehyde (entry 19), ketone (entry 17), and nitrile (entry 20), were successfully reduced to their aniline products with exclusive selectivity and maintained the reducible groups untouched, highlighting the excellent chemoselectivity of the cobalt nanocomposites on N-doped carbon and the obvious advantage compared to that of noble metal-based catalysts. It is noteworthy that, as an exception, both the C[triple bond, length as m-dash]C bond and the nitro group in para-nitrophenylacetylene as a substrate were reduced simultaneously to give para-ethylaniline as the sole product (entry 21). In addition, heteroatom-containing nitroarenes can also be efficiently reduced to their corresponding anilines (entries 24 & 25), which are in particular important intermediates in the pharmaceutical and agrochemical industries.

Table 3 Substrate scope for the hydrogenation of nitroarenesa

image file: c8gc01374h-u2.tif

Entry Substrate Time (h) Conv.b (%) Select.b (%)
a Reaction conditions: Nitroarene (0.5 mmol), CoOx@NC-800 (10 mg, 10 mol% of Co), H2O (2 mL), H2 (5 MPa), 110 °C. b Determined by GC and GC-MS using dodecane as an internal standard sample and confirmed with their corresponding authentic samples. c Selectivity ratio of para-iodoaniline to aniline equals 50/50. d Exclusive selectivity to para-ethylaniline.
1 image file: c8gc01374h-u3.tif 5 100 >99
2 image file: c8gc01374h-u4.tif 9 100 >99
3 image file: c8gc01374h-u5.tif 6 100 >99
4 image file: c8gc01374h-u6.tif 6 100 >99
5 image file: c8gc01374h-u7.tif 4 100 >99
6 image file: c8gc01374h-u8.tif 8 100 >99
7 image file: c8gc01374h-u9.tif 7 100 >99
8 image file: c8gc01374h-u10.tif 7 100 >99
9 image file: c8gc01374h-u11.tif 9 100 >99
10 image file: c8gc01374h-u12.tif 9 100 >99
11 image file: c8gc01374h-u13.tif 6 100 >99
12 image file: c8gc01374h-u14.tif 9 100 90
13 image file: c8gc01374h-u15.tif 6 100 92
14 image file: c8gc01374h-u16.tif 6 100 50c
15 image file: c8gc01374h-u17.tif 8 100 93
16 image file: c8gc01374h-u18.tif 8 100 >99
17 image file: c8gc01374h-u19.tif 6 100 >99
18 image file: c8gc01374h-u20.tif 5 100 >99
19 image file: c8gc01374h-u21.tif 5 100 >99
20 image file: c8gc01374h-u22.tif 5 100 >99
21 image file: c8gc01374h-u23.tif 5 100 >99d
22 image file: c8gc01374h-u24.tif 5 100 >99
23 image file: c8gc01374h-u25.tif 5 100 >99
24 image file: c8gc01374h-u26.tif 12 100 98
25 image file: c8gc01374h-u27.tif 12 100 95


The stability and reusability of the catalyst CoOx@NC-800 was also investigated. Upon completion of the reduction of nitrobenzene, the CoOx@NC-800 catalyst was recollected by using an external magnet, washed, and dried for subsequent cycles. As shown in Table S5 in the ESI, the catalyst exhibited good stability and can be easily separated and reused at least 6 times without a significant loss in both activity and selectivity, strongly indicating its robust stability.

Inspired by such impressive results for the direct hydrogenation of nitroarenes catalysed by the catalyst CoOx@NC-800, we became more interested in the catalytic transformation of nitroarenes with aldehydes to synthesize structurally complex imines. Imines are an exceptionally versatile functional group and are ubiquitous in pharmaceuticals, biologically active heterocycles, and natural products.21 Traditionally, imine synthesis is prevalently achieved by the condensation reaction of amines with highly reactive carbonyl compounds in the presence of Lewis acid catalysts. They can also be prepared by self-condensation of primary amines upon oxidation,22 the oxidative dehydrogenation of secondary amines,23 or by the oxidative coupling of alcohols with amines.24 However, these methods generally suffer from significant practical drawbacks, such as the high cost of catalysts, low catalytic activity, poor selectivity, harsh reaction conditions, difficulty in catalyst reuse, and/or concomitant waste production. Furthermore, aryl amines are most frequently used as starting materials for the synthesis of imines among these well-developed methods, which are generally prepared from the reduction of nitroarenes catalyzed by transition metals under high pressure of hydrogen or using stoichiometric metals. As such, from both economic and environmental viewpoints, there is a strong incentive to develop an alternative inexpensive base-metal approach for highly efficient and selective synthesis of imines from readily available and inexpensive nitroarenes.

The reductive amination of nitrobenzene with benzaldehyde was chosen as a benchmark reaction to optimize the reaction conditions (Tables S6 & 7 in the ESI). Nitrobenzene and benzaldehyde were first subjected to the optimal conditions for the hydrogenation of nitroarenes. To our delight, complete conversion of nitrobenzene was achieved with the formation of a mixture of N-diphenylmethanimine (73%) and N-benzylaniline (27%) simultaneously. Further investigations showed that an excellent selectivity (>99%) to N-diphenylmethanimine could be readily achieved by employing THF/H2O (4 mL, 4/1, v/v) as a solvent within 22 h under otherwise identical conditions.

With the optimized conditions in hand, the reductive amination of nitroarenes with various aromatic aldehydes was subsequently investigated to demonstrate the general applicability of the method (Table 4). A variety of halogenated, electron-donating, and electron-withdrawing substituted nitroarenes (1a–j) could efficiently couple with benzaldehyde to give their corresponding imines in 71–91% isolated yields. Apparently, ortho-substituted nitroarenes irrespective of electron-donating or withdrawing groups all gave relatively lower yields compared with their counterparts, suggestive of the steric effect on the reaction efficiency. Similarly, various substituted benzaldehydes decorated with hydroxyl, cyano, chloro, and methoxy groups could be effectively coupled with nitrobenzene to afford the corresponding imines in 67–90% isolated yields. In addition, the free combination of both substituted benzaldehyde and nitrobenzene could facilitate the reductive coupling smoothly, even in the case of an heteroatom containing aldehyde, under the present conditions, highlighting the great practical utility.

Table 4 Substrate scope for the reductive coupling of nitroarenes with benzyl aldehydesa
a Reaction conditions: Nitroarene (0.5 mmol), benzaldehyde (2 mmol), CoOx@NC-800 (20 mg, 20 mol% of Co), 110 °C, 5.0 MPa H2, THF (3.2 mL), H2O (0.8 mL), 24 h. Yields of the isolated product are reported.
image file: c8gc01374h-u28.tif


To our surprise, when nitrobenzene bears an amino group at the ortho position as a substrate to undergo the coupling reaction with benzaldehyde (Scheme 1), one-pot cascade successive reactions of the reduction of the nitro group, condensation of aldehyde with amine followed by cyclization took place smoothly under the optimized conditions, affording benzimidazole (2t) in 70% yield, which is an important skeleton and is frequently found in pharmaceuticals and biologically active compounds such as Pimobendan25 and N-(2-phenyl-1H-benzo[d]imidazol-6-yl)quinolin-4-amine.26 Further study shows that the isolated yield could be improved to 90% when H2O is used as the solvent instead of THF/H2O (4/1, v/v) under otherwise identical conditions. The procedure demonstrated here is considerably different from the previous case reported by Kempe,9 in which highly flammable and toxic triethylamine was employed as a solvent. Therefore, this transformation provides a new straightforward synthesis of benzimidazoles in a green and sustainable manner, further highlighting the utility of the catalyst.


image file: c8gc01374h-s1.tif
Scheme 1 One-pot cascade synthesis of benzimidazole and representative examples containing a benzimidazole skeleton.

To understand why excellent selectivity to imines rather than amines was achieved in this procedure, additional experiments were performed. Control experiments of the hydrogenation of benzyl aldehyde (eqn (1)) and N-diphenylmethanimine, respectively, reveal that the cobalt nanocomposite CoOx@NC-800 was not active for the reduction of the –CHO group but was active for the C[double bond, length as m-dash]N bond. When N-diphenylmethanimine as a substrate was subjected to the standard conditions, complete conversion with 63% selectivity to N-benzylaniline was achieved along with 37% selectivity to benzyl aldehyde and aniline due to the hydrolysis of imine in the presence of water (eqn (2)). Compared with the hydrogenation of nitrobenzene, the reduction rate of the C[double bond, length as m-dash]N bond is considerably slower than that of the nitro group. In sharp contrast, the reduction of C[double bond, length as m-dash]N was significantly suppressed in the presence of an equal amount of nitrobenzene for the hydrogenation reaction of N-diphenylmethanimine under other identical conditions. In this case, only 7% of N-diphenylmethanimine was converted to N-benzylaniline by retaining 80% of the starting material accompanied by 13% of hydrolytic products, while nitrobenzene was completely reduced to aniline (eqn (3)). These results further confirm the difficulty in the reduction of the C[double bond, length as m-dash]N bond and also indicate that the presence of nitroarene might effectively prevent the C[double bond, length as m-dash]N bond from further reduction.

To gain insight into the pathway of the reductive amination, we recorded the product distribution during the reaction process of the reductive amination of benzyl aldehyde with nitrobenzene under the standard conditions (Fig. 5). It can be easily found that N-diphenylmethanimine was exclusively generated before nitrobenzene was consumed completely, while N-benzylaniline started to form gradually due to further reduction of the C[double bond, length as m-dash]N bond once nitrobenzene completely dissappeared from the reaction mixture. Particularly, 61% yield of N-benzylaniline along with 31% yield of N-diphenylmethanimine was achieved when the reductive amination was prolonged to 36 h. Meanwhile, 8% of aniline was also observed due to the hydrolysis of N-diphenylmethanimine, which is in line with the observation of the control experiment (Scheme 2, eqn (2)). Further prolonging the reaction time results in the complete conversion of N-diphenylmethanimine to N-benzylaniline together with the formation of aniline in 83% and 17% yields, respectively. Based on these results, the switchable access to imines or amines is expected to be realized from the reductive amination of benzaldehydes with nitroarenes catalysed by a heterogeneous non-noble cobalt nanocomposite catalyst through a simple modification of reaction conditions, e.g., prolonging the reaction times or elevating the reaction temperature, in the present catalysis system (Fig. 5 and Table S6). Therefore, these observations clearly reveal that the reaction pathway, as illustrated in Scheme 3, involves sequential reduction of the nitro group to amine (i), condensation of aldehyde with in-generated amine to afford an imine with or without the aid of the catalyst (ii), followed by further reduction to form a secondary amine (iii), in which harsh conditions, such as high temperature or relatively longer reaction times, are required due to the difficulty of C[double bond, length as m-dash]N bond reduction.11a,b Note that, based on previously reported results7,11,12,19 and in combination with our control experiments, we postulate that a polar reaction mechanism is operative in the first step of the reduction of the nitro group to amine in the present reaction system, in which a heterolytic activation of the hydrogen molecule occurs through a synergistic effect between Co and N atoms on the surface of the catalyst, thereby considerably boosting the reaction efficiency compared with the catalyst CoOx@C-800 without any dopants.


image file: c8gc01374h-f5.tif
Fig. 5 The product distribution during the reductive amination of benzaldehyde with nitrobenzene. Reaction conditions: nitrobenzene (0.5 mmol), benzyl aldehyde (2 mmol), CoOx@NC-800 (20 mg, 20 mol% of Co), THF/H2O (4 mL, 4/1, v/v), H2 (5 MPa), 110 °C. The data were obtained based on nitrobenzene.

image file: c8gc01374h-s2.tif
Scheme 2 Control experiments under standard conditions.

image file: c8gc01374h-s3.tif
Scheme 3 Proposed reaction pathway for nitroarene hydrogenation and the reductive amination of nitroarenes with benzaldehydes.

Conclusions

In summary, we developed a facile fabrication of non-noble cobalt nanocomposites on hierarchically porous N-doped carbon derived from naturally renewable biomass together with naturally abundant and low-cost cobalt salts in an operationally simple and straightforward, cost-effective, and environmentally friendly process. The cobalt nanocomposites with mixed phases of metallic Co, CoO and Co3O4 were uniformly dispersed on N-doped carbon, which possess large surface areas, high pore volumes and micro-, meso- and macropore structures. The most active catalyst CoOx@NC-800 exhibited outstanding activity for the hydrogenation of a variety of nitroarenes with good stability with molecular hydrogen in water as a green medium. More importantly, the expedient and selective synthesis of imines from the reductive amination of nitroarenes with aldehydes in high isolated yields was carried out. This work provides not only a new strategy for the preparation of hybrid multi-component materials derived from biomass, but also a new heterogeneous, active and inexpensive catalyst for the synthesis of various aniline derivatives and structurally and biologically useful imines and even benzimidazoles.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the start-up financial support from Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT), Chinese Academy of Sciences (Grant No. Y6710619KL) and 13th-Five Key Project of the Chinese Academy of Sciences (Grant No. Y7720519KL).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Details of the preparation of the catalyst and catalytic reactions including reaction condition optimization and control experiments, NMR spectroscopy data. See DOI: 10.1039/c8gc01374h

This journal is © The Royal Society of Chemistry 2018