Functionalized hypercrosslinked polymers with knitted N-heterocyclic carbene-copper complexes as efficient and recyclable catalysts for organic transformations

Article abstract of DOI:10.1039/c5cy02260f

An N-heterocyclic carbene (NHC)-copper complex supported on hypercrosslinked polymers (HCPs) was successfully synthesized through a simple external cross-linking reaction. The structure and composition of the catalyst were characterized by scanning electr

Full text of DOI:10.1039/c5cy02260f

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Functionalized hypercrosslinked polymers with knitted N-
heterocyclic carbene–copper complexes as efficient and
recyclable catalysts for organic transformations
Received 00th January 20xx,
Accepted 00th January 20xx
Zhifang Jia,†ab Kewei Wang,†ab Tao Li,a Bien Tan,*a and Yanlong Gu*ac
DOI: 10.1039/x0xx00000x
An N-heterocyclic carbene (NHC)–copper complex supported on hypercrosslinked polymers (HCPs) was successfully
synthesized through a simple external cross-linking reaction. The structure and composition of the catalyst were
characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), nitrogen sorption, fourier
transform infrared spectrometer (FT-IR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and
atomic emission spectrometry (AES). The obtained HCP–NHC–Cu possesses a large BET surface area, large pore volume,
and good chemical and thermal stability. Hence, it was used as a solid catalyst for some organic transformations. An
oxidative condensation reaction of indoles, 1,3-dicarbonyl compounds, and phenylglyoxal monohydrate was developed
with the aid of the HCP–NHC–Cu catalyst, which produced various polysubstituted olefins in high yield. The three-
component click reaction of NaN3, phenylacetylene, and benzyl halide; the Ullmann C–N coupling; and the Glaser coupling
reaction proceeded satisfactorily under the action of the HCP–NHC–Cu catalyst. At the end of these reactions, the catalyst
was easily recovered, and reused several times without significant loss of activity in all the reactions.
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Microporous organic polymers (MOPs), as an important class
of microporous materials, are currently at the forefront of
materials research because of their unique properties,
Introduction
Catalysis can be characterized by the competition between
homogeneous and heterogeneous systems. Heterogeneous
catalysts are preferred for industrial processes because of their
easily separation and reuse in consecutive cycles.
Homogeneous catalysts, however, possess identical catalytic
sites and usually exhibit high selectivity. Consequently,
heterogenizing homogeneous catalysts by immobilization is a
trend toward the development of chemically homogeneous
but physically heterogeneous catalysts.1
including superior chemical, thermal, and hydrothermal
stabilities; synthetic diversity; low skeletal density; and high
surface area. One particular advantage of MOPs is the fine
control over the chemical nature of available surface areas,
which is critical for heterogeneous catalysis.10 However, the
application of MOPs as heterogeneous catalysts is rather
limited because of the high-cost of synthesizing MOPs. In some
cases, preparing MOPs entail the use of expensive noble metal
catalysts.11 This drawback can be overcome by using
hypercrosslinked polymers (HCPs), which have recently
emerged as low-cost MOPs with tunable pore sizes and surface
areas. The permanent porosity in HCPs is the result of
extensive chemical cross linkage, which prevents the collapse
of polymer chains to a dense nonporous state. HCPs are highly
desirable for the heterogenization of homogeneous catalysts
because some electron-donor ligands can be installed in the
skeletal framework without any additional chemical
modification.12 We recently reported a cost-effective method
M
icroporous materials, which have pores smaller than 2 nm,
have recently attracted much attention. Owing to their unique
properties, these materials have been utilized in various
technological areas, such as gas
separation and storage,
conduction, and heterogeneous catalysis.2 In the past few
years, substantial research has been conducted on
microporous materials, such as carbon materials,3 zeolites,4
silica,5 metal–organic frameworks,6 zeolitic imidazolate
frameworks,7 polysilsesquioxanes,8 and porous organic cages.9
for preparing HCPs, which involves “knitting”
a
a.Key laboratory of Material Chemistry for Energy Conversion and Storage, Ministry
of Education, School of Chemistry and Chemical Engineering, Huazhong University
of Science and Technology, 430074, Wuhan, China. Phone: (0)86-(0)27-87 54 37
32, Fax: (0)86-(0)27-87 54 45 32, E-mail: taoli@hust.edu.cn; klgyl@hust.edu.cn.
b.Department of Chemistry and Chemical Engineering, Shanxi Datong University,
Datong, 037009, People's Republic of China.
triphenylphosphine ligand with an external cross-linker.13 The
process involves a one-step Friedel–Crafts reaction of a low-
cost cross-linker and a less-functionalized aromatic compound.
Through this method, a microporous polymer with high
surface area can be produced. This “knitting” strategy is
compatible with the use of a wide range of building blocks,
thereby facilitating the heterogenization of catalytically active
components onto the porous channel.
c. State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Lanzhou, 730000 (P.R. China)e.
† Z. Jia and K. Wang contributed equally to this work.
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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N-heterocyclic carbenes (NHCs), also called “phosphine HCP–NHC with CuCl in the presence of sodium tert-butoxide in
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mimics” in catalysis, possess many favorable properties, such THF produced three HCP–NHC–Cu catalysts (
as good coordinating ability of the electron-rich site and FT-IR spectra of NHC, HCP–NHC, and HCP–NHC–Cu III
DOI: 10S.1c0h3e9/mC5eCY10).22T6h0eF
Fig. 1
(
)
structural tunability. In particular, its strong σ-electron- display a series of bands at around 1600–1450 and 900–650
donating ability highly strengthens the NHC–metal bonds. cm–1, which correspond respectively to the benzene skeleton
Therefore, their metal complexes exhibit good thermal stretching and the C–H out-of-plane bending vibrations of the
stability and can be utilized as catalysts at high temperatures benzene ring. Meanwhile, the bands at approximate 1600-
without decomposition. Previously, NHC–metal complexes 1650 cm–1 could be assigned to the –C=N– stretching
were often used as homogeneous catalysts.14 The high vibrations.
dissociation energies of metal–carbon bonds in NHC–metal
The HCP–NHC–Cu catalysts were also subjected to XPS
complexes also enable the NHCs to serve as high-performing analysis, and the results are shown in Fig. 2. The Cu 2p XPS
ligands in establishing heterogenized systems.15 To date, spectra of the HCP–NHC–Cu catalysts show peaks at 933.8–
several supporting materials, such as polymer, nanoparticle, 934.0 eV with strong shake-up satellites, indicating that the
and silica, have been used to immobilize NHC–metal immobilized Cu species is Cu2+ rather than metallic copper and
complexes.16 A polystyrene–supported bis-NHC–Pd catalyst Cu+.25 The oxidation of Cu+ clearly occurred during the
has been used in a Suzuki reaction.17 Iridium NHC complexes preparation of the HCP–NHC–Cu catalysts. Compared with the
immobilized on magnetic nanoparticles were proven to be homogeneous counterpart, CuCl2 (934.2 eV), the Cu2+ binding
quite effective for catalyzing ketone reduction.18 However, a energy in the Cu 2p XPS spectra of HCP–NHC–Cu
I, II, and III
small amount of iridium leaching was observed during its use, shifted negatively by 0.4, 0.3 and 0.2 eV, respectively. This
and the good catalytic activity was observed to only last for result may be attributed to the strong electron-donating effect
two runs. An SBA-15-supported NHC–Cu catalyst has been
utilized in the hydrothiolation of alkynes and electron-deficient
alkenes.19 Although various methods have been reported, the
diversity of immobilizing NHC metal complexes and their
downstream catalytic utilization is rather limited at the
moment.
We recently prepared a knitted NHC porous polymer that
could be utilized in the synthesis of a highly active palladium
catalyst for Suzuki–Miyaura coupling reactions.20 Our study
demonstrated that adopting the knitted NHC porous polymer
as dual ligand and solid support to immobilize homogeneous
metal complex catalysts is feasible. On the other hand, copper
complexes have been widely used as catalysts in organic
reactions.21 For example, copper catalysts have boosted the
development of click reactions, which were well adopted in
material, biology and medicine.22 In organic synthesis, the
recent developments of some C–C and C–N coupling reactions
should also be linked to the uses of copper-based catalysts.23
Particularly, copper NHC catalysts played pivotal roles in these
areas.24 Consequently, we noted that HCP–NHC–Cu, if
Scheme 1 HCP–NHC–Cu catalysts.
synthesized through a simple procedure, could sever as a
catalyst of choice for some important organic reactions. We
report herein, for the first time, the preparation and use of
HCP-NHC-Cu III
HCP–NHC–Cu
(Scheme 1), as catalyst in the oxidative
condensation reaction of indole, 1,3-dicarbonyl compound,
and phenylglyoxal monohydrate. The process generated high
yields of densely substituted olefins. The remarkable catalytic
ability of HCP–NHC–Cu was also demonstrated by some well-
known organic reactions, such as the three-component click
reactions of NaN3, phenylacetylenes, and alkyl halides, as well
as the Ullmann C–N and Glaser coupling reactions.
HCP-NHC
NHC
4000 3500 3000 2500 2000 1500 1000 500
Results and Discussion
The supporting material HCP–NHC was prepared from N,N-
dibenzylbenzimidazolium chloride and benzene as described
previously using FeCl3 as catalyst.20 The following treatment of
Wavenumber (cm-1)
Fig. 1 FT-IR spectra of NHC (a), HCP–NHC (b), and HCP–NHC–Cu
III (c).
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(b)
recycled after the fifth run
C 1s
recycled after the fifth run
*
(a)
thereby maximizing the accessibility of the catalytically active
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Cu 2p
sites toward the substrates. In addDitOioI:n1,0.1t0h3e9/Ca5CbYu0n2d2a60nFt
N 1s O 1s
*
micropore with
a suitable size distribution favors the
recycled after the first run
HCP-NHC-Cu III
recycled after the first run
HCP-NHC-Cu III
dispersion and anchoring of the metal catalyst.
*
TGA shows that the HCP–NHC support (Fig. 4) exhibits good
thermal stability in air (Tdec > 300 °C). HCP–NHC, fresh HCP–
NHC–Cu III, and recycled HCP–NHC–Cu III were also subjected
to SEM and TEM analyses. As shown in Fig. 5, no significant
change in terms of the morphology of the materials occurs
after loading the copper species.
*
HCP-NHC-Cu II
HCP-NHC-Cu I
HCP-NHC
*
HCP-NHC-Cu I
We then investigated the activities of HCP–NHC–Cu catalysts
to determine potential relationships between structure and
catalyst activity. Copper salts are extensively used as oxidation
0
200 400 600 800 1000 1200 930
Binding Energy (eV)
940
950
960
970
Binding Energy (eV)
Fig. 2 (a) XPS spectra of the HCPs in the regions of Cu 2p, C 1s, catalysts in organic reactions; hence, we first examined the
O 1s, and N 1s (left); (b) XPS spectrum of HCP–NHC–Cu in the HCP–NHC–Cu catalysts in such reactions. We recently studied
region of Cu 2p.
the three-component condensation reactions of aldehydes
with two nucleophiles.26 These reactions are normally
established by a tandem process involving (i) the generation of
an active intermediate through a reaction between an
aldehyde and the first nucleophile and the (ii) trapping of this
intermediate by the second nucleophile. Although numerous
multi-component reactions have been reported in the past
Table 1 Physical properties of the HCP–NHC and HCP–NHC–Cu
catalysts.
Samples
a
b
c
d
SBET
VMicro
Vtotal
Cu+
AES
(m2/g)
831
511
503
494
(m2/g)
155
(m2/g)
0.17
0.10
0.10
0.11
(mmol/g)
HCP–NHC
HCP–NHC–Cu I
HCP–NHC–Cu II
0
198
200
209
0.05
0.07
0.11
4.5
2000
(b)
HCP–NHC–Cu III
(a)
a
4.0
HCP-NHC-Cu III
Surface area calculated from the nitrogen adsorption
b
3.5
isotherm using the BET method. The micropore volume
1500
derived using a t-plot method based on the Halsey thickness
3.0
2.5
2.0
1.5
1.0
0.5
0.0
HCP-NHC-Cu II
c
d
equation. Total pore volume at P/P0 = 0.99. Data were
obtained by AES.
1000
500
0
HCP-NHC-Cu I
HCP-NHC
of the NHC ligand. As shown in Fig. 2b, the copper loadings in
HCP–NHC–Cu I, II, and III are 3.32, 4.44, and 7.78 wt%,
respectively. These findings are consistent with the results
obtained by AES (0.05, 0.07 and 0.11 mmol/g, respectively).
The porous properties of the samples were analyzed by
nitrogen sorption analysis at 77.3 K. The BET surface area of
HCP–NHC was found to be 831 m2/g. The adsorption of copper
salt resulted in a dramatic decrease of surface area. This
finding can be attributed to the partial pore filling. A slight
increase in material density is also partially responsible.
0.0
0.2
0.4
0.6
0.8
1.0
1
10
100
Relative Pressure (bar)
Pore Width (nm)
Fig. 3 (a) N2 sorption isotherms of the samples at 77.3 K (left);
(b) pore-size distributions calculated using the DFT methods
via the adsorption branch (slit pore models, differential pore
volumes, and pore width) of the samples.
Despite this tendency, the obtained HCP–NHC–Cu I, II, and III
exhibit large surface areas, with values of 511, 503, and 494
m2/g, respectively (Table 1).
The adsorption and desorption isotherms of HCP–NHC–Cu I–
100
90
III in Fig. 3a display a steep nitrogen gas uptake curve at low
relative pressure (P/P0 < 0.001), indicating the existence of
abundant micropore structures. At relative high–pressure
regions (P/P0 = 0.5–1.0), a slight hysteresis loop and a sharp
rise are clearly observed. This finding implies that a certain
amount of mesopores and a spot of macropore are present in
the material. These results coincide well with the pore-size
distribution of HCP–NHC–Cu shown in Fig. 3b. The presence of
80
HCP-NHC
70
60
HCP-NHC-Cu I
HCP-NHC-Cu II
HCP-NHC-Cu III
100 200 300 400 500 600 700 800
Temperature (oC)
a
spot of mesopore and macropore structure in the
heterogeneous support is essential to ensure the favorable
catalytic performance of the catalyst because these structures
enable the frameworks to be highly swollen in certain solvents,
Fig. 4 TGA spectra of HCP–NHC, HCP–NHC–Cu
II, and HCP–NHC–Cu III
I, HCP–NHC–Cu
.
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promote the modular three-component condensation and
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DOI: 10.1039/C5CY02260F
induce an oxidation step. To this end, the obtained HCP–NHC–
Cu materials were used in the three-component condensation
of phenylglyoxal monohydrate 1a, 2-methylindole 2a, and
dimedone 3a. The reaction was performed in ethanol (EtOH) at
80 °C. In the presence of a catalytic amount of HCP–NHC–Cu I,
two products were formed. The following spectroscopic
analysis reveals that the major product is 4a, and another one
is 5a (Table 2, entry 1). Under catalyst-free conditions, only 5a
was obtained (entry 2). This result indicated the necessity of
using a catalyst in the oxidation step. Some homogeneous
copper salts, such as Cu(OAc)2∙H2O, CuSO4∙5H2O, copper (II)
gluconate, and cupric acetylacetonate, were then used in this
Fig. 5 SEM images of HCP materials: a) HCP–NHC in 20 μm, b) reaction. Cupric acetate and cupric acetylacetonate clearly
became prominent among all the examined copper salts, and
the yields of 4a reached 80% and 78%, respectively (entries 5
and 6). By contrast, the performances of monovalent copper
salts, such as Cu2O, CuCl, CuBr, and CuI, as catalysts are not
successful because the formation of 5a is predominant in
these cases (entries 7–10). The yield of 4a reached only 67%
with HCP–NHC–Cu I. Even so, this result is highly promising
because the material is a solid, which can facilitate catalyst
recycling in such a liquid phase reaction. Encouraged by this
fresh HCP–NHC–Cu III in 5 μm, and c) recycled HCP–NHC–Cu III
in 50 μm. TEM images of HCP materials: d) HCP–NHC, e) fresh
HCP–NHC–Cu III, f) recycled HCP–NHC–Cu III
.
decade,27 the catalog of generated products is rather limited
because most of these compounds possess similar molecular
skeletons. To increase the diversity of the reaction products,
we attempted to integrate a foreign reaction step, for instance,
oxidation, in these condensation reactions. Therefore,
developing a catalyst is necessary, particularly, one that can
result,
we
then
investigated
other
Table 2 Oxidative condensation reactions of 1a, .
2a, and 3a a
Product Yield (%)
Entry
Catalyst
Solvent
4a
5a
29
95
75
74
17
15
87
60
71
79
trace
16
90
16
76
51
49
62
89
91
47
34
22
1
2
3
4
5
6
7
8
HCP–NHC–Cu I
—
CuSO4∙5H2O
Copper gluconate
Cu(OAc)2∙H2O
Cu(acac)2
Cu2O
CuCl
CuBr
CuI
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
1,4-dioxane
CH3CN
CH3NO2
H2O
67
—
24
20
80
78
trace
23
17
trace
99
82
—
82
21
35
32
trace
trace
trace
49
9
10
11
12
13
14
15
16
17
18
19
20
21
22b
23c
HCP–NHC–Cu III
HCP–NHC–Cu II
HCP–NHC
NHC–Cu
PS–NHC–Cu
HCP–NHC–Cu III
HCP–NHC–Cu III
HCP–NHC–Cu III
HCP–NHC–Cu III
HCP–NHC–Cu III
HCP–NHC–Cu III
HCP–NHC–Cu III
HCP–NHC–Cu III
HCP–NHC–Cu III
DCE
DMSO
EtOH
EtOH
EtOH
57
72
24
94d (94)e
trace
1a (1.0 mmol), 2a (1.0 mmol), 3a (1.0 mmol), catalyst (0.53 mol%), solvent (2 mL), 80 C, 1.5 h. 50 C. 0.5 h. d The reaction was
a
o
b
o
c
performed under 10 mmol scale. e The catalyst was used in the fifth run.
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with 2-methyl- or 2-phenyl-substituted indoles. The presence
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DOI: 10.1039/C5CY02260F
of an electron-withdrawing ethoxycarbonyl group in the indole
ring exerted no detrimental effect on the reaction yield (Fig. 6
4a 4i). C2-unsubstituted indoles also participated readily in
the condensation reaction, producing the corresponding
products in high good yields (4c and 4j ). Only moderate
yields were obtained when 6-nitro and 4-hydroxy indoles were
used (4o 4p). Remarkably, 5-methoxyl-4-azaindole could also
,
–
–n
,
be used as a nucleophilic reagent, and the corresponding
product 4q was obtained in 54% yield. Substrate scope with
respect to the alkylglyoxal component was then investigated.
With the aid of the HCP–NHC–Cu III catalyst, the alkylglyoxals
with cyclopropyl and methyl groups reacted readily with 2a
and 3a, producing the expected products 4r and 4s in 51% and
47% yield, respectively. When 1-(4-bromophenyl)glyoxal and 1,
3-benzodioxol-5-ylglyoxal hydrate were used as substrates, the
reactions proceeded favorably, with yields of 4t and 4u
reaching over 85%. HCP–NHC–Cu III was also been used in the
condensation reactions of 2a, 3a, and arylglyoxals with a
heterocyclic substituent, such benzofuranyl and thienyl. The
expected products 4v and 4w were obtained in 30% and 32%
yields respectively. The low yields were resulted mainly from
the easy formation of by-product through a reaction of 2-
methylindole and arylglyoxals. The scope of the reaction with
respect to cyclic-1,3-dicarbonyl component was next
investigated. Because of the low activity of cycloheptane-1,3-
dione in this reaction, the corresponding product 4x was
obtained only in 30% yield. Notably, the three-component
Fig. 6 Substrate scope of the oxidative condensation reaction.
HCP–NHC–Cu catalysts. A maximum yield of 99% was obtained
with HCP–NHC–Cu III (entry 11). The increase of copper
loading in HCP–NHC–Cu III catalyst seems beneficial for the
formation of 4a with excellent yield. Under identical conditions,
the reaction was also conducted under the HCP–NHC system.
In this case, 5a was readily formed and no 4a could be
detected (entry 13). These results clearly indicated that the
copper species is essential for ensuring the good catalytic
activity of the HCP–NHC–Cu III catalyst in the oxidation step.
Interestingly, with a homogeneous counterpart of this catalyst,
NHC–Cu, only 82% yield was obtained under the same
conditions (entry 14). When a referential copper catalyst (PS–
reaction of 1a, 2a, and 4,4-dimethylcyclohexane-1,3-dione also
proceeded readily under the same conditions, and the desired
product 4y was obtained in almost quantitative yield. However,
our attempts to use linear 1,3-dicarbonyl compounds as
substrates were unsuccessful. This result could be attributed
to the relatively lower activities of linear 1,3-dicarbonyl
compounds compared with those of the cyclic compound. The
generated products are a unique class of tetrasubstituted
alkenes, which are highly attractive synthetic targets for
efficient and selective synthetic methods because of the
compouds’ usefulness in organic synthesis.29 However, the
reported methods involve the use of either expensive reagents
or environmentally unfavorable conditions.30 The present
reaction offers a convenient and environmentally benign route
to access these densely substituted olefins.
NHC–Cu) prepared through
a
reported nucleophilic
substitution reaction28 was used, the yield of 4a reached only
21% (entry 15). This result demonstrated that the hierarchical
porous structure of HCP–NHC–Cu III is indeed beneficial for
this reaction. The effect of solvents on the model reaction was
also then investigated. 1,4-Dioxane and CH3CN were found to
be inappropriate as solvents for this reaction (entries 16 and
17). In dimethyl sulfoxide (DMSO), 4a was obtained in 49%
yield (entries 21). Finally, ethanol was proven to be the best
choice of solvent. The polar protic solvents were known to be
able to drive the three-component reactions of aldehydes and
two nucleophiles toward the formation of the target
product.26(a) This ability may ensure partially ethanol to be the
best choice of the solvent. Further investigation revealed that
the reaction was also affected by temperature and reaction
time, and the optimal conditions were as follows: 0.53 mol% of
HCP–NHC–Cu III, 80 °C, and 1.5 h (entries 22 and 23).
Notably, the reactions of 1a, 2a, and 3a can be effectively
scaled up with similar efficiency. For example, the reaction
Under the optimized conditions, we probed the scope of the
model reaction substrates. As shown in Fig.
6, many
substituted indoles reacted readily with 1a and 3a under the
optimal conditions. In particular, high yields were obtained
Fig. 7 Reuse of HCP–NHC–Cu III catalyst.
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5a (HCP-NHC-Cu III)
4a (HCP-NHC-Cu III)
5a (Catalyst-free)
4a (Catalyst-free)
polycarbonyl compound. However, we found by NMR analysis
that compound 5a exists exclusively in its enol form. In
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DOI: 10.1039/C5CY02260F
80
40
0
addition, such a clear enolization was not observed with 5b
(see ESI), which is a congener of 5a synthesized from linear a
1,3-diketone. This result coincides well with Sullivan’s report.33
Compound 5b was also subjected to the conditions of model
condensation. However, no dehydrogenative product was
obtained with the HCP–NHC–Cu III catalyst (Scheme 3). These
results led us to speculate that the inherent tendency of 5a to
present as its enol form probably conferred 5a a high reactivity
toward aerobic dehydrogenation.
0
5
10
15
20
25
30
Time (min-1)
On the basis of these observations, we proposed a plausible
mechanism. The formation of 5a through a selective assembly
Fig. 8 Comparison of kinetic profiles between the model
reactions with and without HCP–NHC–Cu III catalyst.
of 1a
, 2a, and 3a possibly corresponds to the initial event (Fig.
9
). The enol form of 5a allows the proton in the center carbon
under the 10 mmol scale generated 4a in 94% yield (3.62 g).
The recyclability of the HCP–NHC–Cu III catalyst was also
investigated in the 10 mmol scale. We found that the catalyst
was recycled for five runs without significant loss of its activity
to be highly active and hence easily absorbed by the metallic
species. Thus, it is easy to be oxidized to 4a. We believe that
the high tendency of 5a for oxidation is also related to the
extension of the conjugated system.
(Table 2, entry 24), as shown in Fig. 7. These results indicated
Give the high tendency of 5a toward oxidation, we explored
the possibility of using other aldehydes containing a hydrogen
bond acceptor to react with 2-methylindole and dimedone.
Under the use of HCP–NHC–Cu III as catalyst, 2-
chlorobenzaldehyde 6a reacted with 2a and 3a, producing
compound 7a in 41% yield (Scheme 4). The existence of C2-
chloro substituent in the phenyl ring of the aldehyde was
proven to be critical because our attempts to establish similar
that HCP–NHC–Cu III catalyst is a robust catalyst. An SEM
image of the recycled HCP–NHC–Cu III shows that its particle
size did not change significantly, and the integrity of the
catalyst remained intact (Fig. 5). XPS analysis of the recycled
HCP–NHC–Cu III demonstrated that all the elements were
retained (Fig. 2). ICP–MS showed that the Cu content of the
catalyst did not change appreciably after the reaction.
To shed light on the mechanism, some experiments were
then carried out. First, a mixture of 1a, 2a, and 3a was treated
in ethanol in the presence of HCP–NHC–Cu III. After 10 min of
stirring, compound 5a was clearly detected (Fig. 8). However,
the concentration of 5a decreased rapidly in the next 20 min,
while the concentration of 4a increased. In the absence of
three-component
oxidative
condensations
with
4-
chlorobenzaldehyde or benzaldehyde failed.
To extend the utility of HCP–NHC–Cu, we then examined
other organic reactions. The second reaction we investigated
was the three-component click reaction of NaN3,
phenylacetylenes, and alkyl halides. The 1,3-dipolar
cycloaddition reaction of organic azides and alkynes is an
important organic transformation that can produce, with its
mild conditions and high efficiency, triazole derivatives.34
Although this reaction has been extensively investigated under
various types of catalysts (homogeneous35 and
HCP–NHC–Cu III
generated. These results imply that
,
4a was hardly detected, though 5a could be
a
Cu2+ species is
responsible for the dehydrogenation of 5a. The three-
component reaction was also performed under a nitrogen
atmosphere. In this case, 5a was predominantly formed, and
4a was obtained only in 24% yield after 1.5 h of stirring at
80 °C (Scheme 2). A high yield of 4a in 99% was achieved by
performing the same reaction under an oxygen atmosphere.
Remarkably, the reaction time could be halved in this case
(Scheme 2). These results indicate that the presence of
molecular oxygen also plays
a
crucial role in the
dehydrogenation of 5a to form 4a
.
Literature survey revealed that an aerobic dehydrogenation
reaction of some linear polycarbonyl compounds can be
initiated by some metallic salts that exhibit certain oxidation
Scheme 2 Model three-component reactions under a N2 or O2
atmosphere.
31
capacity, such as Pd(OAc)2 or AgNO3.32 Notably, the use of
inorganic bases is necessary in these dehydrogenative
reactions. Sullivan et al33 have found that the linear
polycarbonyl compound diethyl diacetylsuccinate shows no
evidence of enolization by infrared and nuclear
magnetic resonance (NMR) spectra. We therefore deduced
that, in the reported homogeneous catalytic systems of
aerobic dehydrogenation of polycarbonyl compounds, adding
an inorganic base probably strengthens the enolization of the
Scheme 3 Aerobic dehydrogenation of 5b over an HCP–NHC–
Cu III catalyst.
6 | J. Name., 2012, 00, 1-3
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Catalysis Science & Technology
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Journal Name
ARTICLE
Table
3
Three-component click reaction ViewofArticleNOanNlin3e,
phenylacetylenes and benzyl bromide.a
DOI: 10.1039/C5CY02260F
Entry
Catalyst
Yield (%)b
1
2
3
4
5
6
7
8
HCP–NHC–Cu III
HCP–NHC–Cu II
99
89
77
0
0
22
HCP–NHC–Cu
—
I
HCP–NHC
NHC–Cu
PS–NHC–Cu
Fig. 9 Proposed mechanism.
24
HCP–NHC–Cu III
98c (96)d
a
Phenylacetylene (1.0 mmol), benzyl bromide (1.0 mmol),
NaN3 (1.1 mmol), catalyst (0.45 mol%), ethanol (2 mL), 80 °C. b
c
Isolated yield.
The reaction was performed under the
d
10 mmol scale. The yield in parentheses was generated after
the reuse of the catalyst in the sixth run.
Scheme 4 Oxidative condensation reaction of 6a, 2a, and 3a.
Table
phenylacetylenes and alkyl halides catalyzed by HCP-NHC-Cu
III a
4
Three-component click reactions of NaN3,
heterogeneous36), these catalyst systems are associated with
one or more disadvantages, such as the use of sodium
ascorbate as additive,37 tedious procedure for preparation,and
use of hazardous and carcinogenic organic solvents, including
nitrobenzene, acetonitrile, and dioxane for reactions. Hence, a
simple, green, and efficient procedure avoiding these
drawbacks would be valuable.
.
Entry
1
2
3
4
5
6
7
8
X
R1
Ph
R2
Product Yield (%)
Cl
Br
Br
Br
Br
Br
Br
Br
I
I
I
Br
Br
Br
I
Ph
Ph
Ph
Ph
Ph
8a
8b
8c
8d
8e
8f
91
99
92
99
99
93
99
98
93
99
90
92
61
72
76
p-F-Ph
p-MeO-Ph
p-tBu-Ph
p-Et-Ph
HCP–NHC–Cu III catalyst was proven to be highly active in
catalyzing this three-component click reaction in ethanol at
80 °C. A small amount of catalyst at 0.45 mol% was sufficient
to promote these three-component click reactions (Table 3
,
p-namyl-Ph Ph
p-nPr-Ph
Ph
Ph
Bn
Et
Et
8g
8h
8i
entries 1–3). No reaction occurred in the absence of catalyst or
with HCP–NHC (entries 4 and 5). With a homogeneous
counterpart of HCP–NHC–Cu catalysts, NHC–Cu, only 22% yield
was obtained (entry 6). When the referential catalyst, PS–
NHC–Cu, was used, the yield of 8a reached only 24% under
identical conditions (entry 7). Reactions scaled up to multigram
quantities provided uniform results (entry 8). In such reactions,
the HCP–NHC–Cu III catalyst was reused at least five times
without any significant loss of catalytic activity (entry 8).
The methodology established using the HCP–NHC–Cu III
catalyst also displayed good substrate scope. The cycloaddition
products 8a to 8l were obtained in yields ranging from 90% to
99% (Table 4, entry 1–12). The established spot-to-spot
reaction allowed us to isolate the product by using an
extremely simple procedure. After completion of the reaction,
the reaction solution was centrifuged, and the solid was
washed with ethanol three times. The product was in the
acquired organic phase. Interestingly, the product was
obtained directly by removing the volatile components.
Isolation by the chromatographic method was needed when
the products were formed in moderate yields (8m to 8o). In
this reaction, HCP–NHC–Cu III catalyst displayed some
advantages, such as simple operation for catalyst preparation,
avoidance of the use of reducing agent, minimal use of toxic
and volatile organic solvents, and clean and easy work-up
9
Ph
p-F-Ph
Ph
Ph
Ph
1-MeO-Bn
Ph
10
11
12
13
14
8j
8k
8l
ndecyl
p-CO2H-Ph
p-NO2-Ph
Ph
8m
8n
8o
15
nhexyl
a
Alkyne (1.0 mmol), halide (1.0 mmol), NaN3 (1.1 mmol), Cu
catalyst (0.45 mol%), EtOH (2 mL), 80 °C, 8 h.
procedure. All these properties render this HCP–NHC–Cu
catalyst highly desirable for practical use.
Two other reactions were also examined using HCP–NHC–Cu
III as the catalyst. Glaser oxidative coupling reaction plays an
important role in the synthesis of 1,3-diynes and is thus widely
utilized in materials chemistry.38 Some heterogeneous copper
species, for example, polymer-supported copper,39 Cu(II)-
TD@nSiO2,40 and MCM-41-immobilized copper(I) complex,41
have been used in this reaction, sometimes with remarkable
results. In this case, HCP–NHC–Cu III, with lower catalyst
loading and a cleaner catalyst system, was utilized to catalyze
this reaction. The desired product 9a was obtained in 99%
yield in the Glaser oxidative coupling reactions of
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ARTICLE
Journal Name
Analyzer (Elementar, Germany). TGA was perfVoierwmAertdicleoOnnlinae
DOI: 10.1039/C5CY02260F
Perkin-Elmer Instruments Pyris1 TGA thermobalance at a
heating rate of 10 °C min–1 under a nitrogen atmosphere. The
polymer morphologies were investigated using an FEI Sirion
200 field emission scanning electron microscope. TEM images
were obtained on a Tecnai G20 microscope (FEI Corp. USA)
instrument operated at an accelerating voltage of 200 kV. XPS
was performed on an ESCALAB.MK II Spectrometer (VG, UK).
Cu catalyst loading was obtained by atomic emission
Scheme 5 Glaser reaction of phenylacetylene catalyzed by
HCP–NHC–Cu III
.
1
spectroscopy on a Agilent 4100 MP-AES. H, 13C NMR spectra
were recorded on a Bruker AV-400. All the chemicals used
were of reagent grade and used as received without further
purification. 1,3-Dibenzyl-1H-benzo[d]imidazol-3-ium chloride
was synthesized in accordance with a reported procedure.42
All reactions were conducted in a 10 mL V-type flask equipped
with a triangular magnetic stirring bar.
Scheme 6 C–N coupling reaction of p-iodoanisole and indole
catalyzed by HCP–NHC–Cu III
.
General procedure for the synthesis of HCP–NHC
phenylacetylene (Scheme 5). In this reaction, the HCP–NHC–Cu
III catalyst was also recyclable. Meanwhile, the Ullmann C–N
coupling of p-iodoanisole and indole also proceeded readily in
the presence of the HCP–NHC–Cu III catalyst (Scheme 6). All
these results demonstrate that HCP–NHC–Cu is indeed a
versatile heterogeneous catalyst for organic transformations.
HCP–NHC was synthesized as described previously.20
Elemental analysis (%) showed the following components: C
(77.64%), H (5.32%), N (1.55%).
General procedure for the synthesis of HCP–NHC–Cu
HCP–NHC (0.50 g), CuCl (0.40 g, 4.0 mmol), tBuONa (0.39 g, 4.0
mmol) and 100 mL anhydrous tetrahydrofuran were placed in
a three-necked flask. Then, the reaction was stirred for 24 h
under N2 atmosphere at room temperature. Finally, the solid
was filtered and washed with methanol several times. With
this procedure, 0.62 g HCP–NHC–Cu III was obtained. HCP–
Conclusions
An NHC–copper complex supported on microporous and highly
stable HCPs was synthesized through a simple external cross-
linking reaction. The obtained HCP–NHC–Cu catalysts were
characterized by various spectroscopic methods, such as SEM,
TEM, N2 sorption, FT-IR, TGA, XPS, and AES. These catalysts
displayed outstanding catalytic performance in the oxidative
condensation reaction of indole, 1,3-cyclohexandione and
phenylglyoxal monohydrate. Various polysubstituted olefins
were prepared with high yield. This catalyst also displayed
good catalytic activity in many other organic reactions, such as
three-component click, Ullmann C–N coupling, and Glaser
coupling reactions. In all these reactions, the HCP–NHC–Cu
catalyst was easily recovered and reused without significant
loss of catalytic activity. The results obtained in this study
revealed that HCPs of NHC are indeed an excellent supporting
material for the immobilization of homogeneous copper
complex catalysts. This method may be applied during the
heterogenization of other metal complexes, a direction that
we are actively investigating.
NHC–Cu I and II were synthesized with the same procedure by
altering the amounts of CuCl and tBuONa.
General procedure for the synthesis of 4a–4y and 7a
In a typical reaction, phenylglyoxal monohydrate (0.15 g, 1.0
mmol), 2-methylindole (0.13 g, 1.0 mmol), dimedone (0.14 g,
1.0 mmol), and HCP–NHC–Cu III (48 mg, 0.0053 mmol) were
added to anhydrous ethanol (2 mL). The mixture was stirred
for 1.5 h at 80 °C. After reaction completion, the mixture was
cooled to room temperature and then centrifuged. The
supernatant organic phase was isolated, and the bottom solid
was washed with ethanol (2.0 mL × 3). All organic phases were
combined and then concentrated under reduced pressure.
Afterward, the desired product 4a was obtained in 99% yield
(0.38 g) by isolation with preparative thin-layer
chromatography (TLC) [eluting solvent: ethyl acetate/
petroleum ether = 1/2 (v/v)]. The procedures for synthesizing
4b–4y and 7a resemble this method, and the yields of 4b–4y
and 7a are listed in Fig. 6 and Scheme 4
.
Experimental section
The polymer surface areas, N2 adsorption isotherm (77.3 K),
and pore-size distributions were measured using Micromeritics
ASAP 2020 M surface area and porosity analyzer. Before
analysis, the samples were degassed at 110 °C for 8 h under
vacuum (10–5 bar). The FT-IR spectra were recorded under
ambient conditions in the wavenumber range of 4000–400 cm–
1 using an FT-IR Bruker (EQUINOX 55) spectrometer. Elemental
analysis was determined using a Vario Micro cube Elemental
General procedure for the synthesis of 8a–8o
In a typical reaction, phenylacetylene (0.10 g, 1.0 mmol),
benzyl bromide (0.17 g, 1.0 mmol), NaN3 (0.072 g, 1.1 mmol),
and HCP–NHC–Cu III (40 mg, 0.0045 mmol) were added in
anhydrous ethanol (2 mL). The resulting solution was stirred at
80 °C for about 8 h. After reaction completion, the mixture was
cooled to room temperature and then centrifuged. The
supernatant organic phase was isolated, and the bottom solid
8 | J. Name., 2012, 00, 1-3
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Catalysis Science & Technology
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Journal Name
ARTICLE
Chem. Int. Ed., 2010, 49, 8328–8344; (e) S. Ding and W.
Wang, Chem. Soc. Rev., 2013, 42, 548–D5O6I8:;10(f.1)0L3V.9iZe/whCu5ACr,tiYQcl0e.2OG2n6ali0onFe,
Y. Tan, W. Tian, J. Xu, K. Yang and C. Yang, Micropor.
Mesopor. Mater., 2015, 210, 1–9.
C. Zlotea, Y. Oumellal, S. Hwang, C. M. Ghimbeu, P. E. de
Jongh and M. Latroche, J. Phys. Chem. C, 2015, 119, 18091–
18098.
was washed with ethanol (2.0 mL × 3). All organic phases were
combined and then concentrated under reduced pressure.
Afterward, the desired product 8a was obtained in 99% yield
(0.23 g). The procedures for synthesizing 8b
this method. Compounds 8m 8o were isolated using
preparative TLC [eluting solvent: ethyl acetate/ petroleum
–8n are similar to
3
–
4
5
J. E. Schmidt, D. Xie and M. E. Davis, Chem. Sci., 2015,
5955–5963.
6,
ether = 1/5 (v/v)] and the yields are listed in Table 4
.
C. Knöel, J. Descarpentries, A. Benzaouia, V. Zeleňák, S.
Mornet, P. L. Llewellyn and V. Hornebecq, Micropor.
Mesopor. Mater., 2007, 99, 79–85.
J. Liu, L. Chen, H. cui, J. Zhang, L. Zhang and C. Su, Chem. Soc.
Rev., 2014, 43, 6011–6061.
W. Lee, H. Chien, Y. Lo, H. Chiu, T. Wang and D. Kang, ACS
Appl. Mater. Interfaces, 2015, 7, 18353–18361.
General procedure for the synthesis of 9a
Phenylacetylene (0.06 g, 0.6 mmol) was added to EtOH (1 mL),
HCP–NHC–Cu III (10 mg, 0.0011 mmol), and
6
7
8
9
tetramethylethylenediamine (0.035 g, 0.3 mmol). Then, the
mixture was stirred at 80 °C for 8 h under the precaution of
oxygen. After reaction completion, the mixture was
centrifuged, and all organic phases were combined and
concentrated under reduced pressure. The white product 9a
was obtained in 99% yield (0.60 g).
J. Rathore, Q. Dai, B. Davis, M. Sherwood, R. D. Miller, Q. Lin
and A. Nelson, J. Mater. Chem., 2011, 21, 14254–14258.
A. I. Cooper, Angew. Chem. Int. Ed., 2011, 50, 996–998.
10 A. I. Cooper, Adv. Mater., 2009, 21, 1291–1295.
11 (a) M. Jiang, Q. Wang, Q. Chen, X. Hub, X. Ren, Z. Li and B.
Han, Polymer, 2013, 54, 2952–2957; (b) V. Hanková, E.
Slováková, J. Zedník, J. Vohlídal, R. Sivkova, H. Balcar, A.
Zukal, J. Brus and J. Sedlček, Macromol. Rapid Commun.,
2012, 33, 158–163; (c) L. Sun, Y. Zou, Z. Liang, J. Yu and R. Xu,
General procedure for the synthesis of 10a
A
mixture of indole (0.06 g, 0.5 mmol), 1-iodo-4-
methoxybenzene (0.12 g, 0.5 mmol), K2CO3 (0.14 g, 1.0 mmol),
HCP–NHC–Cu III (24 mg, 0.0026 mmol), and DMSO (1.5 mL)
was stirred at 120 °C for 10 h. After reaction completion, the
mixture was cooled to room temperature, and the product
was obtained by isolation under preparative TLC [eluting
solution: petroleum ether/ethyl acetate = 5/1 (v/v)] with 93%
of yield (0.10 g).
Polym. Chem., 2014, 5, 471–478; (d) H. Li, Z. Li, Y. Zhang, X.
Luo, H. Xia, X. Liu and Y. Mua, RSC Adv., 2014, 4, 37767–
37772; (e) Z. Yang, Y. Zhao, H. Zhang, B. Yu, Z. Ma, G. Ji and Z.
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13 B. Li, Z. Guan, W. Wang, X. Yang, J. Hu, B. Tan and Tao Li, Adv.
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General procedure for the recycle the catalyst
14 (a) S. N. Sluijter, L. J. Jongkind and C. J. Elsevier, Eur. J. Inorg.
Chem., 2015, 2948–2955; (b) G. Lázaro, F. J. Fernández-
Alvarez, J. Munárriz, V. Polo, M. Iglesias, J. J. Pérez-Torrentea
To test the recyclability of HCP–NHC–Cu III catalyst, the
mixture of reaction system was filtrated at the end of the
reaction. The obtained solid catalyst was washed with ethanol
(2.0 mL × 4). After 12 h of drying under vacuum at 60 °C, the
catalyst was subjected to the next run. The procedure for the
reuse of the catalyst in the other reactions is similar to this
method.
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Acknowledgements
15 (a) K. V. S. Ranganath, S. Onitsuka, A. K. Kumarc and J.
The authors thank for National Natural Science Foundation of
China for the financial support (21373093, 21474033, 21473064,
and 51273074 ) and also for the Analytical and Testing Center
of HUST. Chutian Scholar Program of the Hubei Provincial
Government and the Cooperative Innovation Center of Hubei
Province are also acknowledged. This work is also supported
by the Fundamental Research Funds for the Central
Universities of China (2014ZZGH019).
Inanaga, Catal. Sci. Technol., 2013,
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Rose, A. Notzon, M. Heitbaum, G. Nickerl, S. Paasch, E.
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Catalysis Science & Technology
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DOI: 10.1039/C5CY02260F
For Table of Contents Use Only
Functionalized hypercrosslinked polymers with knitted
N-heterocyclic carbene–copper complexes as efficient and
recyclable catalysts for organic transformations
Zhifang Jia, Kewei Wang, Tao Li, Bien Tan, and Yanlong Gu
Graphic Abstract
An N-heterocyclic carbene –copper complex supported on hypercrosslinked polymers was
synthesized through a simple external cross-linking reaction and characterized by
spectroscopic methods, which displayed very good catalytic activity in many organic
reactions.