Easy access to amides through aldehydic C-H bond functionalization catalyzed by heterogeneous Co-based catalysts

Article abstract of DOI:10.1021/cs501822r

A novel synthesis strategy for amides by oxidative amidation of aldehydes is developed using a heterogeneous Co-based catalyst. The Co composite was prepared by simple pyrolysis of a Co-containing MOF, to obtain well-dispersed Co nanoparticles enclosed by carbonized organic ligands. The catalysts were characterized by powder X-ray diffraction (PXRD), N₂ physical adsorption, atomic absorption spectroscopy (AAS), transmission electron microscopy (TEM), scanning electronic microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The small Co nanoparticles embedded in the N-doped carbons were highly dispersed with an average size of ca. 7 nm. The Co@C-N materials exhibited significantly enhanced catalytic activity in the oxidative amidation of aldehydes in comparison to those of commercial sources. A series of amides can be easily obtained in good to excellent yields. It was found that the reaction proceeded via radicals under mild conditions, and the carbonyl group in the amide product was from the aldehyde. Moreover, the catalyst could be easily separated by using an external magnetic field and reused several times without significant loss in catalytic efficiency under the investigated conditions. (Chemical Equation Presented).

Full text of DOI:10.1021/cs501822r

Research Article
pubs.acs.org/acscatalysis
Easy Access to Amides through Aldehydic C−H Bond
Functionalization Catalyzed by Heterogeneous Co-Based Catalysts
Cuihua Bai, Xianfang Yao, and Yingwei Li*
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of
China
S
* Supporting Information
ABSTRACT: A novel synthesis strategy for amides by oxidative
amidation of aldehydes is developed using a heterogeneous Co-based
catalyst. The Co composite was prepared by simple pyrolysis of a Co-
containing MOF, to obtain well-dispersed Co nanoparticles enclosed by
carbonized organic ligands. The catalysts were characterized by powder
X-ray diffraction (PXRD), N2 physical adsorption, atomic absorption
spectroscopy (AAS), transmission electron microscopy (TEM),
scanning electronic microscopy (SEM), and X-ray photoelectron
spectroscopy (XPS). The small Co nanoparticles embedded in the N-doped carbons were highly dispersed with an average
size of ca. 7 nm. The Co@C-N materials exhibited significantly enhanced catalytic activity in the oxidative amidation of aldehydes
in comparison to those of commercial sources. A series of amides can be easily obtained in good to excellent yields. It was found
that the reaction proceeded via radicals under mild conditions, and the carbonyl group in the amide product was from the
aldehyde. Moreover, the catalyst could be easily separated by using an external magnetic field and reused several times without
significant loss in catalytic efficiency under the investigated conditions.
KEYWORDS: amides, aldehydes, metal−organic frameworks, cobalt, nanoparticles, heterogeneous catalysis
Li et al. described a copper-catalyzed procedure that allowed
oxidative amidation of aldehydes with amine hydrochloride
salts in the presence of silver iodate.11 Wan et al. reported the
preparation of amides from oxidation of aldehydes with N,N-
disubstituted formamides in the presence of TBHP and a
catalytic amount of tetrabutylammonium iodide.12 Wu et al.
have developed a Zn(II)-catalyzed oxidative amidation of
aldehydes with alkylamines for amides using TBHP as
oxidant.13 High yields of amides were achieved over these
homogeneous systems. Considering the easy separation and
reusability of a heterogeneous system, it would be desirable to
develop highly efficient heterogeneous catalysts for these
transformations.
Herein, we report, for the first time, a cobalt-based catalyst
system for the oxidative amidation of aldehydes. Interestingly,
the reaction proceeds via acyl radicals under mild conditions to
give the corresponding amides in high yields. Moreover, the
cobalt-based material can be easily separated from the solution
after reaction by using an external magnetic field. Furthermore,
the catalyst is reusable and retains high catalytic activity even
after recycling for a number of times.
1. INTRODUCTION
Amides are inarguably among the most abundant motifs that
are found in biological activities, natural products, pharmaceut-
icals, and synthetic intermediates.1,2 N,N-Dimethylformamide
(DMF), for example, a widely utilized member of this class of
chemicals, is known to be useful as a solvent and also as an
important source for O, CO, NMe2, CONMe2, Me, and CHO
groups in organic reactions.3 The importance of the amides
necessitates the development of efficient methodologies for
their synthesis.2,4−9 The most widely used methods rely on
activation of a carboxylic acid and subsequent coupling of the
activated species with an amine.4,5 More recently, the
aminocarbonylation of aryl halides has been developed as a
powerful tool for amide preparation.6 Other catalytic
approaches, such as rearrangement of oximes, carbonylation
of alkenes or alkynes, and amidation of nitriles, have also been
reported.7,8 Despite the considerable achievements that have
been made in amide formation, these reaction systems are
unfortunately associated with some limitations, such as less
readily available starting materials, harsh reaction conditions,
and massive production of wastes.9 Therefore, the development
of a highly efficient and environmentally sound methodology
for the synthesis of amides from inexpensive and abundant
feedstocks is highly desirable in both academic research and
industrial applications.
The Co@C-N (carbon−nitrogen embedded cobalt nano-
particle) materials were prepared by simple pyrolysis of a
Co(II)-containing metal−organic framework (MOF). MOFs
have emerged as a new class of porous materials that are
In view of these findings, oxidative amidation of aldehydes
has emerged as an attractive approach for the chemoselective
synthesis of amides, due to the economy and formation of
environmentally acceptable byproducts.9a,b,10−13 In this regard,
Received: August 20, 2014
Revised: December 19, 2014
© XXXX American Chemical Society
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assembled with metal ions and organic ligands.14 Owing to
their ordered structures, high surface areas, and large pore
volumes, MOFs have been considered as alternative precursors
for the preparation of new metal oxides or carbon nanoma-
terials.15 These MOF-derived materials have been widely
utilized in a variety of fields such as heterogeneous catalysis,16
electrochemistry,17 gas adsorption,18 and sensors.19 To the best
of our knowledge, reports on the use of such MOF-derived
materials as catalysts are rare in liquid-phase organic synthesis.
The tube was sealed, and the mixture was stirred at 100 °C for
24 h. Yields were determined by GC-MS analysis. The pure
product was obtained by flash column chromatography (1/1−
4/1 petroleum ether/ethyl acetate).
2.6. Recycling of Catalyst. The recyclability of the Co@C-
N600 catalyst was investigated for the oxidative amidation of 4-
methylbenzaldehyde with DMF under the same reaction
conditions as described above, except using the recycled
catalyst. Each time, the catalyst was isolated from the solution
by magnetic separation after reaction, washed several times with
ethanol, dried under vacuum to remove the residual solvent,
and then reused as the catalyst in the next run.
2. EXPERIMENTAL SECTION
2.1. General Information. All chemicals were purchased
from commercial sources and used without further purification.
1H NMR and 13C NMR spectra were obtained on a Bruker
Avance III 400 spectrometer using CDCl3 as solvent and
tetrmethylsilane (TMS) as internal standard. The identification
and quantitation of products were performed on a GC-MS
spectrometer (Shimadzu GCMS-QP5050A equipped with a
0.25 mm × 30 m DB-WAX capillary column). TGA curves
were obtained on a Netzsch STA449C instrument under an
argon atmosphere. Powder X-ray diffraction patterns of the
samples were obtained on a Rigaku diffractometer (D/MAX-
IIIA, 3 kW) using Cu Kα radiation (40 kV, 30 mA, λ = 0.1543
nm). XPS spectra were recorded on Axis Ultra DLD using
Mono Al Kα (1486.6 eV, 10 mA × 15 kV) as the X-ray source.
Atomic absorption spectroscopy (AAS) was obtained on a
Hitachi Z-2300 instrument. The size and morphology of the
materials were determined by a scanning electronic microscope
(SEM, 1530 VP from LEO) equipped with an energy dispersive
X-ray detector (EDX, Inca 300 from Oxford) and high-
resolution transmission electron microscopy (HR-TEM, C/
M300 from Philips). BET surface area and pore size
measurements were performed with N2 adsorption/desorption
isotherms at 77 K on a Micromeritics ASAP 2020 M
instrument.
3. RESULTS AND DISCUSSION
CoIIMOF (Co9(btc)6(tpt)2(H2O)15; btc = 1,3,5-benzenetricar-
boxylate, tpt = 2,4,6-tris(4-pyridyl)-1,3,5-triazine) was prepared
Table 1. Surface Areas, Pore Volumes and Sizes, and
Chemical Compositions of the Co@C-N Samples
content (wt %)
pore
volume
(cm3 g−1
pore
size
(Å)
SBET
a
a
a
b
sample
(m2 g−1
)
)
C
N
H
Co
CoIIMOF
12
0.01
0.08
8.9
5.0
42.3
64.9
7.0
1.8
2.3
0.7
19.6
30.7
Co@C-
N500
237
Co@C-
N600
251
169
183
179
0.11
0.13
0.18
0.12
4.9
6.2
6.5
6.2
61.0
54.8
53.8
52.9
1.5
1.2
1.0
1.0
0.7
0.6
0.6
0.6
35.8
42.8
44.1
45.3
Co@C-
N700
Co@C-
N800
Co@C-
N900
a
b
Measured by elemental analysis. Measured by AAS.
2.2. Synthesis of CoIIMOF. In a typical synthesis,
Co(NO3)2·6H2O (464 mg) was added to a 48 mL 1/1/1 (v/
v/v) mixture of N,N-dimethylmethanamide (DMF), ethanol,
and water containing tpt (124 mg) and H3btc (168 mg) with
stirring in a vial. The vial was sealed and heated to 100 °C for
24 h. Red cubic-shaped crystals were formed. The powders
were collected by filtration, washed with DMF and ethanol, and
then dried in air.
2.3. Synthesis of Co@C-N Materials by Thermolysis of
CoIIMOF. Typically, 0.5 g of CoIIMOF was heated at a high
temperature for 8 h with a heating rate of 1 °C/min from room
temperature under an argon atmosphere. The as-synthesized
material is denoted as Co@C-Nx, where x indicates the MOF
pyrolysis temperature.
2.4. Procedures for the Oxidative Amidation of
Aldehydes with DMF. A mixture of aldehyde (0.5 mmol),
TBHP (5 equiv), DMF (2 mL), and catalyst (Co 10 mol %)
was placed in a Schlenk tube. The reaction solution was stirred
at 80 °C under atmospheric conditions. At the end of the
reaction, 100 μL of n-hexadecane as internal standard was
added. Then, the catalyst was isolated and a sample of the
liquid mixture was subjected to GC-MS analysis. The pure
product was obtained by flash column chromatography (1/1
petroleum ether/ethyl acetate).
2.5. Procedures for the Oxidative Amidation of
Benzaldehyde with Formamides. A mixture of formamides
(2.5 mmol), aldehyde (0.5 mmol), TBHP (5 equiv), toluene (1
mL), and catalyst (Co 10 mol %) was placed in a Schlenk tube.
Figure 1. Powder XRD patterns of (a) Co@C-N500, (b) Co@C-
N600, (c) Co@C-N700, (d) Co@C-N800, and (e) Co@C-N900.
according to the reported procedures.20 The powder X-ray
diffraction (XRD) patterns of the as-synthesized CoIIMOF
(Figure S1, Supporting Information) were similar to the
published XRD data, confirming the formation of pure
CoIIMOF crystals.20 Co@C-N materials were synthesized by
pyrolysis of CoIIMOF under a continuous flow of argon. TGA
of CoIIMOF indicated that the MOF structure began to
decompose when the temperature was increased to ca. 400 °C
under argon (Figure S2, Supporting Information). Therefore,
the calcination temperatures for carbonization of CoIIMOF
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Figure 2. SEM images of (a) the parent Co-MOF, (b) Co@C-N600, and the corresponding (c) C, (d) Co, and (e) N elemental maps.
Figure 3. TEM images of (a) Co@C-N500, (b) Co@C-N600, (c) Co@C-N700, (d) Co@C-N800, (e) Co@C-N900, and (f) Co@C-N600 after
reaction.
were varied from 500 to 900 °C. The prepared material is
denoted as Co@C-Nx, where x indicates the MOF thermolysis
temperature.
The chemical compositions of the resulting Co@C-N
materials were characterized by elemental analysis and AAS.
Co, C, N, and H elements were mainly detected, and their
weight contents are summarized in Table 1. The quantities of
Co in the samples were about 30−45 wt %. The N contents
decreased with an increase in the pyrolysis temperature from
500 to 900 °C.
The BET surface areas and porosities of the Co@C-N
materials were determined by N2 adsorption−desorption at 77
K, and the results are shown in Table 1. A very low surface area
and pore volume were observed for parent CoIIMOF, implying
a nonporous structure of the MOF. Notably, the BET surface
area and total pore volume showed a remarkable enhancement
after decomposition and carbonization of the MOF framework
at elevated temperatures, which ranged from 169 to 251 m2 g−1
and from 0.08 to 0.18 cm3 g−1, respectively, for the Co@C-N
materials.
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Table 2. Oxidative Amidation of 4-Methylbenzaldehyde with
DMF
Table 3. Scope of the Oxidative Amidation of Various
e
a
Aldehydes with DMF
b
entry
catalyst
oxidant
yield (%)
1
Co@C-N500
Co@C-N600
Co@C-N700
Co@C-N800
Co@C-N900
−
Co(NO3)2
CoIIMOF
CoO
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
−
72
90
56
75
60
3
2
3
4
5
6
7
9
8
32
35
34
14
3
9
10
11
12
Co3O4
Co
carbon
c
13
Co + carbon
Co@C-N600
Co@C-N600
Co@C-N600
Co@C-N600
18
−
−
−
67
14
15
16
H2O2
O2
d
17
TBHP
a
Reaction conditions (unless specified otherwise): 1a (0.5 mmol),
catalyst (10 mol % Co), oxidant (5 equiv), DMF (2 mL), 80 °C, 24 h.
b
c
Yield was determined by GC-MS analysis. 30 wt % Co was
d
mechanically mixed with activated carbon. TBHP (10 equiv).
The XRD patterns of the Co@C-N composites prepared at
temperatures below 600 °C showed only one weak and broad
peak at ca. 44° (Figure 1). The absence of the diffractions for
CoIIMOF indicated that the MOF structure was decomposed.
Five diffraction peaks at around 44.2, 51.5, 75.8, 92.2, and 97.6°
could be observed for the materials calcined at higher
temperatures, characteristic of metallic Co (JCPDS No. 15-
0806).21 The improved intensities of the Co diffraction peaks
with an increase in temperature suggested the production of a
Co phase with a higher crystallization degree. The XRD
diffraction observed at ca. 25° may be assigned to graphite-type
carbon sheets (Figure 1c−e).22
The surface morphology of Co@C-N was determined by
SEM (Figure 2), which clearly showed the original shape of
CoIIMOF crystals. However, the composite surface was
distorted with a rough surface, indicating the decomposition
and carbonization of the MOF frameworks. Element mapping
(Figure 2c−e) revealed a uniform distribution of C, N, and Co
in the material. The high dispersion nature of cobalt in the
porous N-doped carbons was further demonstrated by high-
resolution transmission electron microscopy (HRTEM)
(Figure 3). The average size of Co particles in the Co@C-
N600 material was around 7 nm (Figure S3, Supporting
Information). No significant aggregation of Co nanoparticles
was observed, which could be attributed to the isolation effect
of carbons formed from carbonization of the tpt and btc linkers
in CoIIMOF. As the pyrolysis temperature increased, the Co
nanoparticles tended to aggregate gradually, as shown in Figure
S3.
a
b
c
d
Yield was determined by GC-MS. Isolated yield. 100 °C. Reaction
conditions: 1r (0.5 mmol), 2 (2.5 mmol), Co@C-N600 (10 mol %
Co), TBHP (5 equiv relative to 1r), toluene (1 mL), 100 °C.
e
Reaction conditions (unless specified otherwise): 1 (0.5 mmol), Co@
C-N600 (10 mol % Co), TBHP (5 equiv), DMF (2 mL), 80 °C.
optimize the reaction parameters. A series of solvents, including
water, toluene, acetonitrile, and N,N-dimethylformamide
(DMF), were examined for the reaction. N,N,4-trimethylben-
zamide (3a) was obtained in the highest yield in DMF among
the investigated solvents (Table S1, Supporting Information).
With DMF as the solvent, the prepared Co@C-N materials
were all active for the oxidative amidation reaction, and Co@C-
N600 exhibited the best catalytic performance to give 3a in
90% yield at 80 °C (Table 2, entry 2). An increase of
temperature to 100 °C led to a remarkable decrease of
The catalytic activities of the Co@C-N materials were tested
in the oxidative amidation of aldehydes with DMF. First, 4-
methylbenzaldehyde (1a) was chosen as a model substrate to
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Table 4. Scope of the Oxidative Amidation with Various
Formamides
Table 5. Reusability of Catalyst in the Oxidative Amidation
of 4-Methylbenzaldehyde with DMF
d
a
use
first
second
88
third
89
fourth
85
fifth
yield (%)
90
87
a
Reaction conditions: 1a (0.5 mmol), Co@C-N600 (10 mol % Co),
TBHP (5 equiv), DMF (2 mL), 80 °C, 24 h.
activity (Table 2, entry 7). Similarly, the parent CoIIMOF did
not show a high activity, and notably, after reaction the MOF
structure was observed to be dissolved in the solution (Table 2,
entry 8). The use of CoO or Co3O4 nanoparticles as catalysts
afforded a similarly low yield of 3a under the investigated
conditions (Table 2, entries 9 and 10). In addition, metallic Co
nanoparticles (20−30 nm) showed some conversion while
activated carbon gave no activity, implying the requirement of a
metal to perform the oxidative amidation of aldehydes (Table
2, entries 11 and 12). It was noteworthy that only an 18% yield
of 3a was obtained when using the physical mixture of Co and
carbon as catalyst (Table 2, entry 13). These control
experiments demonstrated the importance of synergic inter-
actions between the C−N composite and Co nanoparticles in
determining the activity of the Co@C-N materials in the
oxidative amidation reaction.
Using Co@C-N600 as the catalyst, we further investigated
the influence of oxidants on the oxidative amidation of 4-
methylbenzaldehyde with DMF. It was observed that, under an
inert atmosphere without the addition of an oxidant, no desired
product 3a was obtained (Table 2, entry 14), indicating that
oxidant was necessary for the oxidative amidation reaction to
proceed. It is worth noting that no amides were detected when
other oxidants, such as H2O2 and O2, were employed in the
reaction (Table 2, entries 15 and 16). Increasing the amount of
oxidant led to a reduced yield due to the formation of p-toluic
acid as side product (Table 2, entry 17).
With the optimized reaction conditions in hand, we next
investigated the oxidative amidation of a wide range of
substituted aldehydes with DMF. As shown in Table 3,
aromatic aldehydes bearing an electron-donating group in the
para, meta, and ortho positions afforded the corresponding
substituted N,N-dimethyl amides in good to excellent yields
within 24 h (entries 1−5). In comparison to the parent
molecule (1b), the presence of a methyl group in the para and
meta positions did not change the yield (entries 1, 2, and 4)
remarkably but, when the methyl group was in the ortho
position, the yield dropped to 80% due to steric hindrance
(entry 3). The higher yield achieved for the substrate 1e could
be attributed to the stronger inductive effect of the methoxy
group in the para position (entry 5). 1-Naphthaldehyde also
underwent oxidative amidation smoothly and gave 3f in 95%
yield even with less reaction time (Table 3, entry 6). When the
reaction time was prolonged to 36 h, phenyl-substituted
benzaldehyde was also a compatible substrate for this
transformation and furnished the desired amide 3g in 89%
yield (Table 3, entry 7). Under these conditions, 4-fluoro-, 4-
chloro-, and 4-bromo-substituted phenyl aldehydes were also
selectively amidated at the aldehyde moiety to afford the
corresponding amides in 80−87% yields (Table 3, entries 8−
10). In general, electron-deficient benzaldehydes exhibited
slightly lower activities than electron-rich benzaldehydes (Table
3, entries 11−14). Heteroaryl aldehydes, such as furan-2-
carbaldehyde and isonicotinaldehyde, were also amidated with
a
b
c
Yield was determined by GC-MS. 36 h. Reaction conditions: 1b (2
d
mmol), 2n (0.5 mmol), TBHP (7.5 equiv), 120 °C, 48 h. Reaction
conditions (unless specified otherwise): 1b (0.5 mmol), 2b−n (2.5
mmol), Co@C-N600 (10 mol % Co), TBHP (5 equiv relative to 1),
toluene (1 mL), 100 °C, 24 h.
Figure 4. Magnetic separation of the Co@C-N600 catalyst after
reaction.
selectivity (Table S1). Blank runs (without a catalyst) gave
essentially no activity under identical conditions (Table 2, entry
6).
To verify the unique catalytic characters of the Co@C-N
composites, a series of Co-based nanomaterials were also
examined in the oxidative amidation reaction for comparison
purposes. Homogeneous Co(NO3)2 gave a very low catalytic
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Figure 5. XPS spectra of Co@C-N600 and CoIIMOF: (a) Co 2p of Co@C-N600; (b) N 1s of Co@C-N600 and CoIIMOF; and (c) survey spectrum
of Co@C-N600.
sources (Table S3, Supporting Information). Under the
optimized conditions, a range of alkyl-substituted formamides
were suitable for the formation of the corresponding amides
8b−g in moderate to high yields (Table 4, entries 1−6). Cyclic
and aryl formamides gave 61−92% yields (Table 4, entries 7−
12). It was interesting to note that piperazine-1,4-dicarbalde-
hyde (2n) was selectively amidated with benzaldehyde at one of
the formamide groups on the ring to furnish 8n in 77% yield
under the optimized conditions (Table 4, entry 13). However,
when the reaction time was prolonged and the temperature
enhanced with the presence of more benzaldehyde, the double-
amidation product 8o was formed in 52% yield (Table 4, entry
14).
The stability and reusability of the Co@C-N600 catalyst
were investigated. After the oxidative amidation reaction, the
catalyst could be easily separated by placing a magnet near the
reactor wall (Figure 4). The recycled catalyst was washed
several times with ethanol and then dried under vacuum to
remove the residual solvent. As shown in Table 5, the catalyst
could be reused at least five times with only a slight loss of
activity for the oxidative amidation of 4-methylbenzaldehyde
with DMF. The results were in accordance with AAS
experiments, in which only traces of Co (less than 0.1% of
the total cobalt) were detected in the solution collected by hot
filtration after reaction. TEM images (Figure 3f) also showed
that no apparent Co aggregation could be observed on the
recycled catalyst (Figure S3).
To elucidate the origin of the remarkable activity and stability
of the Co@C-N600 catalyst, we further characterized the
materials by X-ray photoelectron spectroscopy (XPS). The XPS
survey spectrum of Co@C-N600 (Figure 5c) mainly showed
the peaks of four elements present in the composite (i.e., C, N,
Figure 6. Oxidative amidation of (a) p-toluic acid (4), (b) tert-butyl
perester 5, and (c) tert-butyl ester 6 with DMF. N.D.: not detected.
DMF smoothly to provide the desired amides in good yields
(Table 3, entries 15 and 16). When aliphatic aldehydes, such as
pivalaldehyde, were subjected to the reaction, the correspond-
ing amide products were obtained in good yields using toluene
as solvent (Table 3, entries 17 and 18). Notably, the yields of
amides shown in Table 3 were comparable to the best results
reported in the literature for the oxidative amidation of the
same aldehydes with DMF under similar reaction conditions
(Table S2, Supporting Information).12
The scope of the formamides that can be used in this
reaction was studied. First, the reaction parameters were further
screened so that the reaction could be applied to various amide
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Figure 7. Oxidative amidation of 4-methylbenzaldehyde: (a) with DMF in the presence of 1 equiv of TEMPO and (b) with 13C-labeled DMF.
not detected in the experiments). Thus, we added these three
compounds in the reaction instead of an aldehyde (Figure 6).
No 3a product was detectable under identical conditions,
excluding the involvement of these compounds as intermedi-
ates in this transformation. When 1 equiv of TEMPO (2,2,6,6-
tetramethylpiperidine-N-oxyl, a typical radical inhibitor) was
added to the reaction of 4-methylbenzaldehyde with DMF, the
TEMPO adduct 7 was predominantly formed instead of the
amide 3a in 73% yield (Figure 7a), suggesting that the
amidation step might involve a radical pathway. Notably, when
DMF was labeled by 13C isotope, the reaction result proved
that the carbonyl group in the amide was from the aldehyde,
not from the DMF (Figure 7b).
On the basis of the control experiments, it can be concluded
that this transformation proceeds via C−H activation of
aldehydes involving a radical process.9−12 Thus, a possible
Figure 8. Proposed reaction mechanism for the oxidative amidation of
reaction mechanism is proposed as shown in Figure 8. At the
start of the reaction, the tert-butoxyl and tert-butylperoxyl
radicals are generated with the assistance of the cobalt-based
catalyst (step I). Then, these radicals abstract hydrogen from
the aldehyde and DMF to form acyl radical A and aminyl
radical B, respectively (step II). Finally, cross-coupling of the
radicals A with B leads to the production of the corresponding
amide 3 (step III).
aldehydes with DMF.
O, and Co). Two strong peaks at 793.5 and 778.7 eV, assigned
to Co 2p1/2 and Co 2p3/2 of metallic Co,23 respectively, were
observed in the XPS spectrum (Figure 5a). The N 1s spectra of
CoIIMOF showed two binding energies at around 399 and
399.7 eV, which were related to pyridine-type and triazine-type
nitrogens, respectively (Figure 5b). However, the N 1s spectra
of Co@C-N600 were quite different from those of the pristine
CoIIMOF material. Three distinct peaks were observed in the N
1s spectra of Co@C-N600 with electron binding energies of
398.8, 400.5, and 402.8 eV, suggesting the presence of three
kinds of coordination environments for N atoms. The peak
with the lowest binding energy could be attributed to pyridine-
type nitrogen that was bonded to a metal.24 Such a
coordination interaction between the nitrogen and cobalt was
advantageous to prevent a serious aggregation of Co during
pyrolysis of the MOF. The N 1s peak at 400.5 eV was assigned
to pyrrole-type nitrogen which usually may be found in the
carbonized nitrogen-containing organic materials.24 The porous
N-doped carbons could exhibit a remarkably enhanced
chemical reactivity due to their extended electronic structures.25
The small peak at 402.8 eV was typical for ammonium
species.24
4. CONCLUSIONS
In summary, we have developed a novel, highly efficient, and
reusable heterogeneous cobalt-based catalyst for oxidative
amidation of aldehydes. The Co@C-N materials are prepared
by simple thermolysis of a Co-containing MOF, in which small
Co nanoparticles are highly dispersed and enclosed in nitrogen-
doped carbons. The materials are highly active in oxidative
amidation of a wide range of aldehydes with formamides,
affording the corresponding amides in good to excellent yields
under mild reaction conditions. A series of control experiments
suggest that the reaction proceeds via a novel radical pathway
and the carbonyl group in the amide product is from the
aldehyde. Moreover, the cobalt-based catalyst is easily recycled
and can be reused a number of times without a significant loss
of activity. This work would provide an alternative and
environmentally benign methodology for the synthesis of
amides.
To gain more insights into the reaction mechanism of the
Co@C-N catalyst system, a series of control experiments were
carried out under various conditions. In the oxidative amidation
of 4-methylbenzaldehyde with DMF, we observed the
formation of a trace amount of p-toluic acid (4) in addition
to the desired product 3a. In addition, there was a possibility
that tert-butyl perester 5 or tert-butyl ester 6 could also be
produced under the reaction conditions (although they were
ASSOCIATED CONTENT
■
S
* Supporting Information
The following file is available free of charge on the ACS
Publications website at DOI: 10.1021/cs501822r.
890
DOI: 10.1021/cs501822r
ACS Catal. 2015, 5, 884−891

ACS Catalysis
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1
data for the products, and H NMR and 13C NMR
spectra (PDF)
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Corresponding Author
*E-mail for Y.L.: liyw@scut.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21322606 and 21436005), the Doctoral
Fund of Ministry of Education of China (20120172110012),
the Fundamental Research Funds for the Central Universities
(2013ZG0001), and the Guangdong Natural Science Founda-
tion (S2011020002397 and 10351064101000000).
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891
DOI: 10.1021/cs501822r
ACS Catal. 2015, 5, 884−891