Mesoporous nitrogen-doped carbon for copper-mediated Ullmann-type C-O/-N/-S cross-coupling reactions

Article abstract of DOI:10.1039/c2ra22559j

Copper-driven Ullmann-type cross-coupling reactions are regarded as one of the most important methods for the C-O, C-N, C-S and some other bonds formation. Herein, we report that mesoporous nitrogen-doped carbon materials with high nitrogen content function as novel, active and recyclable N-type heterogeneous promoters for copper-catalyzed cross-couplings. By using CuI as the catalyst and a mesoporous nitrogen-doped carbon material as the promoter, the Ullmann-type coupling reactions of various aryl halides with phenols, amines, imidazol, and thiophenols at 100 °C gave the corresponding products in good yields. Inspired by the efficiency of mesoporous nitrogen-doped carbon material as a promoter, nitrogen-doped carbon supported CuO nanoparticles are prepared and successfully employed as efficient heterogeneous catalysts in the C-O cross-coupling reactions. Furthermore, the nitrogen-doped carbon material supported CuO can be simply recycled by centrifugation and reused at least four times with trace copper leaching. The mesoporous nitrogen-doped carbon materials as heterogeneous promoters or copper-based catalyst supports may represent a promising catalyst system in modern Ullmann chemistry, which indeed could lead to improved processing steps.

Full text of DOI:10.1039/c2ra22559j

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Mesoporous nitrogen-doped carbon for copper-
mediated Ullmann-type C–O/–N/–S cross-coupling
Cite this: RSC Advances, 2013, 3,
1890
reactions3
Pengfei Zhang,ab Jiayin Yuan,b Haoran Li,a Xiaofeng Liu,b Xuan Xu,a
Markus Antoniettib and Yong Wang*a
Copper-driven Ullmann-type cross-coupling reactions are regarded as one of the most important methods
for the C–O, C–N, C–S and some other bonds formation. Herein, we report that mesoporous nitrogen-
doped carbon materials with high nitrogen content function as novel, active and recyclable N-type
heterogeneous promoters for copper-catalyzed cross-couplings. By using CuI as the catalyst and a
mesoporous nitrogen-doped carbon material as the promoter, the Ullmann-type coupling reactions of
various aryl halides with phenols, amines, imidazol, and thiophenols at 100 uC gave the corresponding
products in good yields. Inspired by the efficiency of mesoporous nitrogen-doped carbon material as a
promoter, nitrogen-doped carbon supported CuO nanoparticles are prepared and successfully employed
as efficient heterogeneous catalysts in the C–O cross-coupling reactions. Furthermore, the nitrogen-doped
carbon material supported CuO can be simply recycled by centrifugation and reused at least four times
with trace copper leaching. The mesoporous nitrogen-doped carbon materials as heterogeneous
promoters or copper-based catalyst supports may represent a promising catalyst system in modern
Ullmann chemistry, which indeed could lead to improved processing steps.
Received 18th October 2012,
Accepted 27th November 2012
DOI: 10.1039/c2ra22559j
www.rsc.org/advances
rich ligands with N–, O–, or a mixture of both types of binding
sites, such as 1,2-diamines,4a 1,10-phenanthroline,4b ethylene
Introduction
glycol,4c amino acids,4d b-diketones,4e imino-pyridines,4f,g
8-hydroxyquinoline,4h oximes or Schiff bases,4i have been
successfully introduced as promoters for the Ullmann-type
cross-coupling reactions. While the introduction of these
chelators has led to significant breakthrough, little progress
has been made in the heterogeneous ligand.5 Of the several
reports, most focused on the C–N cross-coupling reactions,
and very few have been assembled for the C–O cross-coupling
processes.6 The development of heterogeneous catalytic
systems7 is expected to reduce the time consumed for the
purification processes and avoid metal residue contamination
in the desired product, which is highly desirable in pharma-
ceutical chemistry.
The formation of C–heteroatom (O, N, S) bonds by cross-
coupling reactions represents a powerful tool for the prepara-
tion of numerous compounds in biological, pharmaceutical,
and material science on both laboratory and industrial scales.1
Among the various strategies developed to date, the copper-
catalyzed Ullmann-type reaction has proven to be a convenient
synthetic route due to its cost-effectiveness and environmen-
tally friendly nature. Nevertheless for
a long time the
unfavourable reaction conditions, such as high reaction
temperature, low tolerance towards functional groups and
the stoichiometric demand for copper reagents, prevented
further development of these methods.2 In the early 21st
century, the discovery of versatile and very efficient copper/
ligand systems for the Ullmann-type reactions, which
employed only catalytic amounts of metal under much milder
conditions (90–110 uC), has stimulated a renewed interest in
the copper-assisted cross-coupling reactions.3 Many electron-
Meanwhile, nitrogen-containing carbon materials have
attracted more and more interest due to their improved
performance in many fields such as CO2 sequestration,8
environmental protection,9 or in electrochemistry10 as oxygen
reduction reactions.11 It is believed that doping nitrogen into
carbon nanostructures adds another dimension to these
structures’ properties. The strong electron donor behavior of
nitrogen leads to enhanced p bonding ability and basic
aKey Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry,
Zhejiang University, Hangzhou, 310028, P. R. China. E-mail: chemwy@zju.edu.cn;
Fax: +86 571-87951895
bDepartment of Colloid Chemistry, Max-Planck Institute of Colloids and Interfaces,
property.12 The incorporated nitrogen atoms (Fig. S1, ESI )
3
Research Campus Golm, Potsdam, 14424, Germany
can be distinguished as chemical nitrogen (such as amine or
nitrosyl group) and structural nitrogen (such as pyridinic,
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c2ra22559j
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pyrollic or quarternary-graphitic nitrogen).13 The similar
chemical environment of the nitrogen in the N-type ligands
and the N-doped carbon materials offers an interesting
hypothesis: Can N-doped carbon materials function as
electron-rich promoters for the copper-mediated cross-cou-
plings? To the best of our knowledge, this question has been
so far not yet answered. Here, we wish to report our efforts in
the search for N-doped carbon materials with both high
nitrogen content and large specific surface areas for copper-
catalytic Ullmann-type C–O, C–N, and C–S cross-couplings.
Moreover, to reuse the copper species and eliminate wasted
copper in the product, we also present N-doped carbon
supported CuO nanoparticles as active and recyclable hetero-
geneous catalysts for this process.
Results and discussion
Fig. 2 a) TEM image, b) N2 adsorption–desorption isotherms and c) XRD
Synthesis and characterization of mesoporous nitrogen-doped
carbon
patterns of the as-synthesized mesoporous N-doped carbon materials. d) XPS
analysis of N 1s in Meso-N-C-1.
Here, we prepared mesoporous N-doped carbon materials by
carbonizing nonvolatile ionic liquids (ILs) or ILs with nitrogen-
rich additives (Fig. 1).14,15 Mesoporous N-doped carbon
materials with different nitrogen contents were obtained from
three precursors: the IL 1-ethyl-3-methylimidazolium dicyana-
mide (Emim-dca) with 2-guanidinobenzimidazol (10 mol%),
Emim-dca, and 3-cyanomethyl-1-vinylimidazolium bromide
(Cmvim-Br), by using silica nanoparticles (Ludox HS40) as a
hard template for the mesostructure. The transmission
electron microscopy (TEM) image of Meso-N-C-1 (Fig. 2a)
clearly visualized its pore morphology. The porosity was
investigated by N2 sorption measurements. Evaluating the
data according to Brunauer–Emmett–Teller (BET) method,
specific surface areas up to 1062, 831, and 427 m2 g21 for
Meso-N-C-1, Meso-N-C-2, and Meso-N-C-3, respectively, are
nitrogen species on the surface of Meso-N-C-1 was further
investigated by XPS. The N 1(s) envelope analysis indicated a
high pyridinic surface character (401.5 eV; 46.5%), contribu-
tions from pyrrolic motifs (400.3 eV; 29.2%) and amines (399.5
eV; 24.3%) (Fig. 2d).16 These incorporated nitrogens with lone
pairs (pyridinic character and amine type: 70.8%) are expected
to anchor to copper for the cross-coupling reactions.10b
Catalytic tests of mesoporous N-doped carbon in Ullmann-type
cross-couplings
We started the initial investigation on copper-mediated
Ullmann-type reactions with the cross-coupling of phenyl
iodide (PhI) and phenol as a model system. The experimental
results showed that the catalyst system containing 10 mol% of
CuCl2 or CuCl and 50 mg of Meso-N-C-1 could drive the
reaction, furnishing diphenyl ether (DPE) in 47% and 44%
yield, respectively (Table 1, entries 1–2). In the absence of
Meso-N-C-1 (Table 1, entries 3–4), relatively low yields of DPE
were obtained, illustrating that Meso-N-C-1 largely promoted
the O-arylation reaction. When Meso-N-C-1 alone was applied
in this reaction (Table 1, entry 5), the product DPE was not
formed at all. It seems that Meso-N-C-1 alone cannot induce
this cross-coupling and it plays the role of a promoter, together
with copper salts for the described reaction. Arylations
conducted in DMSO were found faster than those conducted
in DMF or 1,4-dioxane (Table 1, entries 6–7). Among various
copper sources at the same concentration (10 mol%), CuI was
the most effective, with a moderate yield (64%) (Table 1,
entries 8–12). At an elevated concentration of CuI (15 mol%), a
relatively satisfactory yield (85%) was reached (Table 1, entry
13). Therefore, CuI was chosen in the subsequent experiments
aiming at investigating the effect of bases. Among the four
different bases (KOH, K2CO3, K3PO4 and Cs2CO3), KOH
apparently rendered the maximum yield for the O-arylation
(Table 1, entries 9, 14–16). In addition, compared with Meso-N-
C-1, the mesoporous N-doped carbon materials Meso-N-C-2
reached (Fig. 2b, Table S1, ESI ). X-Ray diffraction (XRD)
3
patterns of the prepared materials afforded two broad
diffraction peaks for all samples, in accordance to the inter-
plane (002) (2h = 24.7u) and inter-plane (110) (2h = 43.7u)
reflections of graphite-like carbon (Fig. 2c). The chemical
composition of the prepared materials was analyzed by
elemental analysis. As expected, Meso-N-C-1 contains a large
amount of N (12.9 wt%) (Table S1, ESI ). The nature of the
3
Fig. 1 Chemical structures of the ionic liquids and the additive used as N-doped
carbon precursors.
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yields (Table 2, entries 5, 10, 11). Relative good reactivity was
observed with sterically hindred o-cresol (Table 2, entry 12). As
a heterogeneous promoter, the Meso-N-C-1 can be easily
separated by centrifugation and washed with water and
ethanol. The recovered Meso-N-C-1 can be reused at least four
times without losing activity and showed no difference in FT-
Table 1 Effect of catalyst, base, and solvent on the cross-coupling of phenol
with iodobenzenea
IR spectra (Table 2, entries 6–9; Fig. S2, ESI ). The reaction
3
conditions were also suitable for the cross-coupling of
phenylamine with PhI (Table 2, entry 13). N-arylation of
imidazole with PhI gave the coupled product in 88% yield
(Table 2, entry 14). Furthermore, thiophenol and 4-methylthio-
phenol were coupled with PhI under the generalized reaction
conditions to afford the corresponding products in good yields
(Table 2, entries 15–16).
Entry
Cu source
Carbon
Base
Solvent
Yield (%)b
1
2
3
4
CuCl2
CuCl
CuCl2
CuCl
—
CuCl
CuCl
CuBr
CuI
CuSO4
Cu(OAc)2
Cu(acac)2
CuI
Meso-N-C-1
Meso-N-C-1
—
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
K2CO3
K3PO4
Cs2CO3
KOH
KOH
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
Dioxane
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
47
44
4
9
0
—
5
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-1
Meso-N-C-2
Meso-N-C-3
6
14
30
43
64
42
25
55
85 (77)
16
15
27
36
59
7c
8
Synthesis and characterization of (CuO)x@Meso-N-C
9
Encouraged by the efficiency of N-doped carbon materials as
promoters for the copper-catalyzed Ullmann-type cross-cou-
plings, we tried to use N-doped carbon materials to support
copper species as heterogeneous catalysts for the same
transformation.18 In the subsequent work, we prepared CuO
nanoparticles supported on N-doped carbon (Meso-N-C-1) with
a high surface area by deposition-precipitation method.19
These samples were denoted as (CuO)x@Meso-N-C: x corre-
sponds to the used amount of CuCl2 in mmol while the
amount of Meso-N-C-1 remains the same (200 mg). The
appearance of diffraction lines at 2h = 35.4u, 2h = 38.6u
indicates the presence of the crystalline phase of CuO in the
(CuO)1.0@Meso-N-C (Fig. 3a).18c The average size of the CuO
crystallite was calculated to be y14.5 nm using Scherrer’s
formula, a little larger than the average particle size (12.6 ¡ 3
10
11
12
13d
14
15
16
17d
18d
CuI
CuI
CuI
CuI
CuI
a
Reaction conditions: PhI (1 mmol), phenol (1.5 mmol), copper salts
(0.1 mmol), Carbon 50 mg, base (3 mmol), solvent (4 mL), 100 uC,
24 h. GC yields with biphenyl as internal standard (isolated yield
in parenthesis). Solvent: 1,4-dioxane. CuI 0.15 mmol was used for
the test.
b
c
d
and Meso-N-C-3 with less N content afforded lower yields,
which hints that this coupling is really supported by the
structural nitrogen on the carbon materials (Table 1, entries 17–
18). That result agrees with observation in the oxidation of
benzyl alcohol by Pd supported on polymeric carbon–nitrogen
materials, in which the catalytic activity correlates with the
concentration of nitrogen on the carbon supports.17 It is
therefore assumed that the doped nitrogen atoms, in particular
those with lone pairs, can anchor the copper species, and then
promote cross-coupling reactions to a larger degree.
In the next step, our efforts were directed to identify the
scope of this method. The cross-couplings between several aryl
halides and phenol were tested under the optimized condi-
tions. It was found that the CuI/Meso-N-C-1 system can also
work with iodobenzene having an electron rich group, for
example: 1-methyl-4-iodobenzene, but with a moderate yield
(Table 2, entry 1). In the presence of iodobenzene substituted
by an electron-withdrawing group (–F), our catalytic system
gave an excellent yield for the corresponding biphenyl ether in
10 h (Table 2, entry 2). Phenyl bromide proved to be less
efficient than PhI, giving 79% yield in 48 h (Table 2, entry 3).
The coupling of phenol and chlorobenzene containing an
electron-withdrawing group (–NO2) proceeded smoothly
(Table 2, entry 4).
nm) determined by TEM image (Fig. S3, ESI ). The pore
3
structures were retained after the incorporation of CuO into
Meso-N-C-1, resulting in high specific surface areas from 373
to 488 m2
g
21, which in principle can be a benefit for
heterogeneous catalysis, as the pore structure is believed to
largely enhance the mass transfer properties and result in
good CuO dispersity (Fig. 3b, Table S2, ESI ). Based on X-ray
3
photoelectron spectroscopy (XPS) analysis, it is clear that
copper is in the divalent state with d9 character. XPS detected
the 2p3/2 and 2p1/2 main peaks at 934.8 and 955.2 eV,
respectively. Two shake-up satellite peaks, about y8 eV higher
than main peaks, were observed, which were diagnostic of an
open 3d9 shell, corresponding to the Cu2+ state (Fig. 3c).20
Catalytic tests of (CuO)x@Meso-N-C in Ullmann-type cross-
couplings
Indeed, the catalytic system (CuO)1.0@Meso-N-C gave an
excellent yield in the cross-coupling of PhI and 4-methylphe-
nol (Table 3, entry 1). The final CuO content of the
(CuO)1.0@Meso-N-C was determined as y30% by TGA
thermograms under O2 flow (Fig. 3d). Being a heterogeneous
catalyst, (CuO)1.0@Meso-N-C can be easily recovered by
centrifugation and reused four times without any loss of
In an endeavour to expand the scope of the above
methodology, various substituted phenols, amines, imidazol,
and thiophenols were examined with PhI for the respective O–,
N–, and S-arylated products. Phenols containing electron-
donating groups at the para site provided moderate to good
efficiency (Table 3, entries 2–5; Fig. S3, ESI ). The liquid phase
3
of the reaction mixture was collected by hot filtration after the
reaction and analyzed by ICP-MS. A very low amount of
dissolved copper (y0.5% of the total copper) was detected in
the solution at the end of the reaction. Moreover, after we
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Table 2 The CuI/Meso-N-C-1 catalyzed Ullmann-type O–, N–, and S-arylation with aryl halidesa
Entry
1
Aryl halide
Phenol/amine/imidazol/thiophenol
Product
Time (h)
24
Yield (%)b
33
2
10
48
24
94
79
95
3c
4
5
20
20
20
20
20
24
96 (87)
96
94
94
95
6d
7e
8f
9g
10
84
11
24
57
12
24
24
86
13h
73 (65)
14c
15h
16h
24
15
15
88
98 (90)
96
a
Reaction conditions: Aryl halide (1 mmol), phenol/amine/imidazol/thiophenol (1.5 mmol), CuI (0.15 mmol), Meso-N-C-1 (50 mg), KOH (3
b
c
mmol), DMSO (4 mL), 100 uC. GC yield with biphenyl as internal standard (isolated yield in parenthesis). Reaction temperature: 125 uC.
d
e
f
Second run to test the reusability of catalyst. Third run to test the reusability of the catalyst. Fourth run to test the reusability of the
g
h
catalyst. Fifth run to test the reusability of the catalyst. In Ar atmosphere.
removed the (CuO)1.0@Meso-N-C from the reaction solution
after 7 h (Yield: 47%), the supernatant did not show any
further reactivity in the next 10 h. These results indicated that
the copper catalyst indeed functioned in a heterogeneous
manner.
We also studied the (CuO)x@Meso-N-C catalysts with
exhibited the best activity for this cross-coupling reaction with
a 94% yield for DPE, while (CuO)x@Meso-N-C (x = 0.5; 0.2)
with less Cu content gave lower yields (Table 3, entries 6–8).
These results provide evidence that the activity is attributed to
the copper species loaded on the carbon material.
different Cu amounts using the cross-coupling between PhI
and phenol as a model reaction. The (CuO)1.0@Meso-N-C
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for the Ullmann-type cross-coupling of aryl halides with
phenols. The high catalytic performance is to be contributed
to the heterojunction between special nitrogen-containing
structure in the carbon materials and copper species.
Meanwhile, (CuO)1.0@Meso-N-C, a copper-based fully hetero-
geneous catalyst, presented outstanding performance with
high activity and stability in the Ullmann-type cross-coupling
reaction.
We believe that mesoporous nitrogen-doped carbons,
acting as nitrogen-rich promoters, can be combined with
many more transition metals (Pd, Au, Ni, Zn, Co etc.) in two
ways: (1) merging with metal salts in a physical manner; (2)
acting as support structures for metal nanoparticles, thereby
opening up new possibilities for discovering many attractive
heterogeneous catalytic methodologies in both organic
research and chemical industries.
Fig. 3 a) XRD pattern, b) N2 adsorption–desorption isotherms and c) XPS
spectrum of the copper-containing catalysts; d) TGA thermograms of
(CuO)1.0@Meso-N-C under flowing O2.
Experimental section
Synthesis of mesoporous N-doped carbon
After adding 2.5 g of 12 nm SiO2 nanoparticles (Ludox HS40) to
1 g ionic liquid (IL): 1-ethyl-3-methylimidazolium dicyanamide
Conclusions
In summary, mesoporous nitrogen-doped carbon materials with
high nitrogen contents were developed as simple, sustainable,
and efficient heterogeneous N-type ligands for the copper-
mediated Ullmann-type C–O, C–N, and C–S cross-couplings.
Under the optimized conditions, the CuI/Meso-N-C-1 system
successfully promoted the cross-couplings between various aryl
halides and phenols, amines, imidazol, and thiophenols with
moderate to high yields. This novel catalyst system offers an
excellent complement to the Cu-catalyzed methodology, at the
same time revealing an important application of nitrogen-doped
carbon materials in the field of catalysis.
(Emim-dca),
3-cyanomethyl-1-vinylimidazolium
bromide
(Cmvim-Br), or a prepared mixture of Emim-dca (1 g) with
2-guanidinobenzimidazol (0.1 g), the resulting mixtures were
heated to 80 uC overnight to remove the water. Then, the
obtained mixtures were transferred into a crucible and heated
to 1000 uC in 5 h under nitrogen flow. The final temperature
was kept for 1 h. After cooling, the dark solid was treated with
4 M NH4HF2 (twice) for one day to remove the Si template. The
dispersion was then filtered, and the precipitate was washed
with deionized (DI) water. The obtained solid was dried in an
air oven at 80 uC for 24 h.
Furthermore, CuO supported on nitrogen-doped carbon
was proven as an active and recyclable heterogeneous catalyst
Synthesis of (CuO)x@Meso-N-C
The deposition of CuO was achieved using the deposition-
precipitation (DP) method. In the DP process, CuCl2?2H2O (x =
1, 1 mmol; x = 0.5, 0.5 mmol; x = 0.2, 0.2 mmol) was added to
the Meso-N-C-1 solution (200 mg in 40 mL H2O) and the
mixture was ultrasonicated for 20 min. Then, 0.1 M NaOH was
added drop by drop to adjust the solution pH value to y9
under ultrasonication (y20 min). The precipitate was cen-
trifuged and washed with DI water until the solution pH value
was around 7. The obtained solid was dried at 80 uC for 24 h
and then calcined at 180 uC for 24 h to obtain the catalyst.
Table 3 The Ullmann-type cross-couplings between phenols and PhI catalyzed
by copper supported on N-doped carbon materialsa
Entry Aryl halide Phenols
1
Catalyst
(CuO)1.0
Time (h) Yield (%)b
@
17
93 (84)
Meso-N-C
2c
3d
4e
5f
6
17
17
91
95
96
94
94
(CuO)1.0
Meso-N-C 17
17
(CuO)1.0
Meso-N-C
(CuO)0.5
Meso-N-C
(CuO)0.2
Meso-N-C
@
Typical procedure for the Ullmann-type cross-coupling
reaction
@
24
24
24
Aryl halide (1 mmol), phenol/amine/imidazol/thiophenol (1.5
mmol), base (3 mmol), catalysts (used as described in the
manuscript) and 4 mL solvent were added into a 10 mL round
bottom glass-reactor. The reaction was performed at 100 uC in
an oil bath with magnetic stirring for the desired time. When
the reaction was carried out in an Ar atmosphere, the
atmosphere inside was replaced with Ar three times and then
the reaction started with an Ar balloon. After completion of the
reaction, biphenyl was added to the mixture as the internal
7
8
@
42
13
@
a
Reaction conditions: PhI (1 mmol), phenols (1.5 mmol),
b
(CuO)x@Meso-N-C (50 mg); 100 uC, DMSO 4 mL. GC yield with
biphenyl as internal standard (isolated yield in parenthesis).
c
d
Second run to test the reusability of catalyst. Third run to test the
e
reusability of the catalyst. Fourth run to test the reusability of the
catalyst. Fifth run to test the reusability of the catalyst.
f
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For the recycling study, the Ullmann-type reaction was
performed with phenyl iodide and 4-methylphenol maintain-
ing the same reaction conditions as described above, except
using the recovered catalyst. Each time, after the completion of
reaction, the catalyst was recovered by centrifugation and then
washed thoroughly with ethyl acetate followed by DI water to
remove the base present in the used catalyst. The recovered
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Acknowledgements
Financial support from the Joint Petroleum and Petrochemical
Funds of the National Natural Science Foundation of China
and China National Petroleum Corporation (U1162124), the
Fundamental Research Funds for the Central Universities, the
Program for Zhejiang Leading Team of S&T Innovation, the
Partner Group Program of Zhejiang University and the Max
Planck Society are greatly appreciated.
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