Ligand-free copper-catalyzed coupling of phenols with nitroarenes by using a metal-organic framework as a robust and recoverable catalyst

Article abstract of DOI:10.1002/cctc.201300032

A highly porous metal-organic framework Cu₂(BDC)₂(DABCO) (H₂BDC=1,4-benzenedicarboxylic acid, DABCO=1,4-diazabicyclo[2.2.2]octane) was synthesized and used as an efficient recyclable heterogeneous catalyst for the coupling reaction of phenols with nitroarenes to form diaryl ethers without using a ligand. Physical characterization of the MOF was obtained by using XRD, SEM, TEM, thermogravimetric analysis (TGA), FTIR spectroscopy, atomic absorption spectrophotometry (AAS), H₂ temperature-programmed reduction (H₂-TPR), and N₂ physisorption measurements. The Cu₂(BDC)₂(DABCO)-catalyzed coupling reaction offers several advantages compared to the conventional Ullmann reaction for the synthesis of unsymmetrical diaryl ethers. To the best of our knowledge, the ligand-free Cu-catalyzed O-arylation reaction of phenols with nitroarenes that uses a heterogeneous catalyst has not been mentioned previously in the literature.

Full text of DOI:10.1002/cctc.201300032

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DOI: 10.1002/cctc.201300032
Ligand-Free Copper-Catalyzed Coupling of Phenols with
Nitroarenes by using a Metal–Organic Framework as
a Robust and Recoverable Catalyst
Nam T. S. Phan,* Tung T. Nguyen, Vu T. Nguyen, and Khoa D. Nguyen[a]
A highly porous metal–organic framework Cu2(BDC)2(DABCO)
(H2BDC=1,4-benzenedicarboxylic acid, DABCO=1,4-
try (AAS), H2 temperature-programmed reduction (H2-TPR), and
N2 physisorption measurements. The Cu2(BDC)2(DABCO)-cata-
lyzed coupling reaction offers several advantages compared to
the conventional Ullmann reaction for the synthesis of unsym-
metrical diaryl ethers. To the best of our knowledge, the
ligand-free Cu-catalyzed O-arylation reaction of phenols with
nitroarenes that uses a heterogeneous catalyst has not been
mentioned previously in the literature.
diazabicyclo[2.2.2]octane) was synthesized and used as an effi-
cient recyclable heterogeneous catalyst for the coupling reac-
tion of phenols with nitroarenes to form diaryl ethers without
using a ligand. Physical characterization of the MOF was ob-
tained by using XRD, SEM, TEM, thermogravimetric analysis
(TGA), FTIR spectroscopy, atomic absorption spectrophotome-
Introduction
Diaryl ethers have attracted significant attention as important
intermediates employed commonly in the synthesis of several
biologically active natural products and pharmaceutical candi-
dates as well as functional polymers.[1,2] These structures are
conventionally synthesized by the Ullmann coupling reaction
of phenols with aryl halides in the presence of Cu salts as cata-
lysts.[3] However, a variety of drawbacks that include the large
amount of catalyst required,[4,5] long reaction time,[6,7] high
temperature,[8,9] and significant quantities of hazardous Cu-
based waste generated[8] present undesirable economic, envi-
Moreover, catalyst recovery also decreases the contamination
of the desired products with hazardous or harmful metals.[17,18]
Metal–organic frameworks (MOFs) have recently emerged as
promising materials for potential applications in several fields
that include gas separation and storage, nonlinear optics, tem-
plated low-dimensional material preparation, sensors and lumi-
nescence, drug storage and delivery, biomedical imaging, and
catalysis.[19–24] By combining the special properties of both or-
ganic and inorganic porous materials, MOFs possess several
advantages such as high surface areas, well-defined structures,
tunable pore sizes, and structural diversity.[19–28] Although the
application of MOFs in catalysis is a young research area, MOFs
have been employed as solid catalysts or catalyst supports for
a variety of organic transformations,[29–31] which range from CÀ
C[32–37] to CÀheteroatom[38–41] bond formation. As several MOF
structures contain metal sites with potential coordinative unsa-
turation, their application in the field of heterogeneous cataly-
sis is expected to be an area that will attract further research
in the near future.[28–30] In this work, we report the coupling re-
action of phenols with nitroarenes in the presence of the MOF
Cu2(BDC)2(DABCO) (H2BDC=1,4-benzenedicarboxylic acid,
DABCO=1,4-diazabicyclo[2.2.2]octane) as an efficiently recov-
erable heterogeneous catalyst without that does not use any
ligands. This Cu2(BDC)2(DABCO)-catalyzed transformation offers
several advantages compared to the conventional Ullmann re-
action and avoids the formation of halide byproducts. To the
best of our knowledge, the ligand-free Cu-catalyzed O-aryla-
tion of phenols with nitroarenes to form diaryl ethers that uses
a heterogeneous catalyst has not been mentioned previously
in the literature.
ronmental, and health and safety issues. Recently, several Pd-
[10,11]
,
Rh-,[12] and Fe-catalyzed[13,14] coupling reactions of phe-
nols with aryl halides that use suitable additives or ligands
have been considered as alternatives to the traditional Cu-cata-
lyzed Ullmann transformation. One of the main disadvantages
of the Ullmann-type reaction is the use of hazardous aryl hal-
ides as well as the formation of halide byproducts, which raise
significant environmental concerns. Therefore, alternative reac-
tants to aryl halides for the Ullmann-type transformation
should be investigated.[15] Recently, Chen and co-workers re-
ported the first example of homogeneous ligand-free Cu-cata-
lyzed O-arylation of phenols with nitroarenes to achieve diaryl
ethers.[16] To target more environmentally benign processes,
a heterogeneous Cu-based catalyst is favored in terms of the
ease of handling, simple work-up, recyclability, and reusability.
[a] Dr. N. T. S. Phan, T. T. Nguyen, V. T. Nguyen, K. D. Nguyen
Department of Chemical Engineering
HCMC University of Technology, VNU-HCM
268 Ly Thuong Kiet, District 10, Ho Chi Minh City (Vietnam)
Fax: (+84)8-38637504
E-mail: ptsnam@hcmut.edu.vn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300032.
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Results and Discussion
Catalyst synthesis and characterization
Cu2(BDC)2(DABCO) was synthesized in a yield of 66% by using
a solvothermal method according to a slightly modified litera-
ture procedure.[42–45] The DABCO-based MOF was characterized
by using a variety of techniques. The XRD pattern of Cu2-
(BDC)2(DABCO) (Figure S1) shows the presence of very sharp
peaks, which are consistent with the theoretical patterns and
with those previously reported.[44] Elemental analysis by using
atomic absorption spectrophotometry (AAS) indicated a Cu
loading of 3.62 mmolgÀ1. SEM of Cu2(BDC)2(DABCO) revealed
that well-shaped, high-quality cubic crystals were formed (Fig-
ure S2). The SEM images, in conjunction with the XRD patterns,
confirmed that a crystalline material was achieved. As expect-
ed, TEM showed that Cu2(BDC)2(DABCO) possesses a porous
structure (Figure S3). However, the pore structure of the MOF
was more complex than that of conventionally used micro-
and mesoporous materials. N2 physisorption measurements re-
vealed that the material contains mainly micropores (diame-
ter<20 ꢁ) with a median pore width of 6.3 ꢁ (Figure S4). A
Langmuir surface area of 1487 m2 gÀ1 was calculated from N2
adsorption–desorption isotherms (Figure S5). Indeed, Gumma
and co-workers[43] and Achmann and co-workers[45] previously
synthesized Cu2(BDC)2(DABCO), and surface areas of approxi-
mately 1400 m2 gÀ1 were observed. Thermogravimetric analysis
(TGA) indicated that the material was stable up to over 2008C
(Figure S6). The FTIR spectrum of Cu2(BDC)2(DABCO) was signif-
icantly different to those of H2BDC and DABCO and showed
the deprotonation of the ÀCOOH groups of H2BDC upon reac-
tion with Cu cations (Figure S7).[42] A H2 temperature-pro-
grammed desorption (H2-TPR) experiment revealed the nature
of the Cu species within the Cu2(BDC)2(DABCO) structure. A
peak at 2958C was a result of the reduction of Cu2+ to Cu+,
and two sharp peaks observed at 396 and 4158C were attribut-
ed to the reduction of CuO structures formed during the total
collapse of the Cu2(BDC)2(DABCO) structure (Figure S8). The
XRD pattern of the solid product confirmed the formation of
Cu (Figure S9). Although these assignments were previously re-
ported in several Cu-based catalytic systems,[46–48] further inves-
tigation is needed to elucidate the nature of the metal sites in
the MOF structure.
Scheme 1. The coupling of phenols with nitroarenes to form diaryl ethers in
the presence of Cu2(BDC)2(DABCO) as catalyst.
Figure 1. Effect of temperature on the reaction conversion.
was significantly accelerated if the temperature was increased
to 808C with 84% conversion obtained after 2 h. Quantitative
conversion was achieved for the reaction performed at 1008C
within 2 h. Indeed, in the first example of the coupling reaction
of phenol with 4-nitrobenzaldehyde, Chen and co-workers also
performed the reaction at 1008C in DMF for 4 h in the pres-
ence of 5 mol% Cu(OAc)2·H2O catalyst.[16] Moreover, this O-ary-
lation occurred effectively at a lower temperature than the
conventional Ullmann reaction that uses heterogeneous cata-
lysts, in which temperatures of 120,[49] 130,[50] 140,[8] and
1508C[51] are required.
One important factor that should be addressed in an investi-
gation of the coupling reaction of phenol with 4-nitrobenzal-
dehyde with Cu2(BDC)2(DABCO) as catalyst is the Cu concentra-
tion. The O-arylation reaction was performed at 1008C in DMF
with a 4-nitrobenzaldehyde/phenol molar ratio of 1:2 and two
equivalents of K2CO3 as the base in the presence of 1, 2, 3, and
5 mol% of Cu2(BDC)2(DABCO) as catalyst (Figure 2). The cata-
lyst concentration was calculated based on the Cu/4-nitroben-
Catalytic studies
Cu2(BDC)2(DABCO) was assessed for its catalytic activity in the
O-arylation of phenol with 4-nitrobenzaldehyde to form 4-for-
myldiphenyl ether as the principal product (Scheme 1). Initial
studies addressed the effect of temperature on the conversion
to 4-formyldiphenyl ether. The O-arylation reaction was per-
formed in DMF with a 4-nitrobenzaldehyde/phenol molar ratio
of 1:2 and two equivalents of K2CO3 as the base in the pres-
ence of 5 mol% Cu2(BDC)2(DABCO) as the catalyst at 30, 60, 80,
and 1008C (Figure 1). Almost no reaction was observed at
308C, and the reaction proceeded with difficulty at 608C to
afford 22% conversion after 2 h. As expected, the reaction rate
Figure 2. Effect of catalyst concentration on the reaction conversion.
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zaldehyde molar ratio. The coupling reaction afforded more
than 99% conversion within 2 h with 3 mol% catalyst. Interest-
ingly, increasing the catalyst concentration beyond 3 mol%
was unnecessary as the reaction rate was not enhanced signifi-
cantly. As expected, decreasing the catalyst concentration to
2 mol% resulted in a decrease in the reaction rate, although
a quantitative conversion was still obtained after 2 h. The reac-
tion that used 1 mol% catalyst proceeded to 95% conversion
after 2 h. Almost no reaction occurred in the absence of Cu2-
(BDC)2(DABCO), which indicates the importance of the MOF-
based catalyst. These results are comparable to those reported
by Chen and co-workers in the first example of the coupling
reaction of phenol with 4-nitrobenzaldehyde, in which 5 mol%
of Cu(OAc)2·H2O catalyst was required for the same reaction
performed at 1008C for 4 h.[16] Moreover, this coupling reaction
proceeded readily in the presence of a significantly lower
amount of Cu than the conventional Ullmann reaction. Indeed,
several Ullmann transformations employed Cu concentrations
of up to 10 mol% or more, for example, CuI and tris-(2-amino-
ethyl)amine as a ligand (10 mol%),[52] CuI and tetrabutylammo-
nium bromide as a phase-transfer catalyst (10 mol%),[53] CuI
and a tetradentate N-based ligand (10 mol%),[54] sol–gel-immo-
bilized Cu catalyst (10 mol%),[6] CuO nanoparticles (10 mol%),[1]
Cu nanoparticles (10 mol%),[55] Cu(OTf)2 and 1,1’-binaphthyl-
2,2’-diamine as a ligand (20 mol%).[56]
With this result in mind, we investigated the effect of the
molar ratio of the reagents on the reaction conversion. The
coupling reaction was performed at 1008C in DMF with two
equivalents of K2CO3 as the base in the presence of 3 mol%
Cu2(BDC)2(DABCO) catalyst with 4-nitrobenzaldehyde/phenol
molar ratios of 1:2, 1:1.5, and 1:1. The O-arylation reaction be-
tween phenol and 4-nitrobenzaldehyde should be performed
at a reagent molar ratio of 1:1.5, for which quantitative conver-
sion was achieved within 2 h. The use of two equivalents of
phenol did not enhance the reaction rate significantly, whereas
the use of one equivalent of phenol slowed the reaction with
67% conversion observed after 2 h (Figure 3).
Figure 4. Effect of solvent on the reaction conversion.
and toluene (Figure 4). The reaction was performed at 1008C
with the reagent molar ratio of 1:1.5 in the presence of
3 mol% catalyst and two equivalents of K2CO3 as the base. Tol-
uene was completely ineffective for the reaction, and not even
a trace amount of the product was detected. NMP was also un-
suitable for the reaction, which afforded 62% conversion after
2 h. In addition, Cu2(BDC)2(DABCO) was partially decomposed
during the reaction in NMP. Both DMF and DMA were effective
for the coupling reaction, in which more than 99% conversion
was achieved after 2 h. However, DMF exhibited the best per-
formance, and should be the solvent of choice for the coupling
reaction with Cu2(BDC)2(DABCO). Chen and co-workers previ-
ously investigated the coupling reaction of phenol with 4-ni-
trobenzaldehyde with Cu(OAc)2·H2O catalyst in different sol-
vents and also found that DMF was the best solvent for this re-
action.[16] This result is comparable with that previously report-
ed for the Ullmann reaction, in which DMF exhibited high per-
formance in several homogeneous[13,53] and heterogeneous[8,14]
catalyst systems.
For liquid-phase reactions that use solid metal-based cata-
lysts, the possibility that some of the active sites on the solid
phase could dissolve into the solution during the course of the
reaction is a serious problem. Therefore, the transformation
could proceed under either homogeneous or partially homo-
geneous catalysis conditions.[57] To confirm if Cu species leach-
ed from solid Cu2(BDC)2(DABCO) could be significantly active
in the coupling reaction of phenol with 4-nitrobenzaldehyde,
an experiment was performed that used filtration during the
course of the reaction. If the catalytic transformation continued
after the solid catalyst was separated from the liquid phase,
this would prove that the O-arylation occurred either under
homogeneous or partially homogeneous catalysis conditions.
The O-arylation reaction was performed at 1008C in DMF with
a 4-nitrobenzaldehyde/phenol molar ratio of 1: 1.5 and two
equivalents of K2CO3 as the base in the presence of 3 mol%
Cu2(BDC)2(DABCO) catalyst. After 20 min reaction time, the
DMF phase was separated from the solid Cu2(BDC)2(DABCO) by
filtration and transferred to a new reactor vessel. The reaction
mixture without the solid catalyst was then stirred for an addi-
tional 100 min at 1008C, during which aliquots were sampled
at different intervals and analyzed by using GC. No further con-
version to form more 4-formyldiphenyl ether was observed for
the O-arylation transformation after the solid Cu2(BDC)2-
For transition-metal-catalyzed transformations, the effect of
different solvents on the reaction rate is normally an important
issue that should be considered. Therefore, we studied the re-
action conversion in different solvents, which included DMF,
N,N-dimethylacetamide (DMA), N-methyl-2-pyrrolidone (NMP),
Figure 3. Effect of 4-nitrobenzaldehyde/phenol molar ratio on the reaction
conversion.
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the coupling reaction of phenol with 4-nitrobenzaldehyde, it is
clear that the use of Cu2(BDC)2(DABCO) is advantageous for
the reaction in terms of catalyst separation, recovery, and
recycling.
As with several other transition-metal-catalyzed transforma-
tions, the presence of two equivalents of a base is required to
speed up the transmetalation step in the catalytic cycle.[58]
Chen and co-workers previously employed a variety of differ-
ent bases for the coupling reaction between phenol with 4-ni-
trobenzaldehyde, which included Na2CO3, K2CO3, Cs2CO3,
NaHCO3, KF, CsF, tBuONa, tBuOK, and DABCO, and found that
Cs2CO3 exhibited the best performance for the reaction, al-
though reasonable yields were observed with Na2CO3 and
K2CO3.[16] Indeed, Cs2CO3 is more efficient than other bases and,
therefore, it was the base of choice for several Ullmann trans-
formations.[1,4,13,52,56] However, because of the relatively high
cost of Cs2CO3, more readily available and inexpensive bases
could be used for the coupling reactions. Therefore, we investi-
gated the effect of different bases on the reaction conversion
by using Na2CO3, K2CO3, K3PO4, CH3COONa, tBuOK, and DABCO
(Figure 7). The O-arylation reaction was performed at 1008C in
Figure 5. Leaching test indicated no contribution from the homogeneous
catalysis of active Cu species leached into reaction solution.
(DABCO) catalyst was separated from the reaction mixture
(Figure 5). These results confirm that the coupling reaction of
phenol with 4-nitrobenzaldehyde could only proceed in the
presence of solid Cu2(BDC)2(DABCO) and the contribution from
leached active Cu species in the solution phase, if any, was
negligible.
The catalytic activity of Cu2(BDC)2(DABCO) in the coupling
reaction of phenol with 4-nitrobenzaldehyde to form 4-formyl-
diphenyl ether was also compared to that of the three compo-
nents of Cu2(BDC)2(DABCO) (i.e., Cu(NO3)2·3H2O, H2BDC, and
DABCO) and other Cu salts (e.g., CuCl2, Cu(OAc)2, and CuI). The
O-arylation reaction was performed at 1008C in DMF with a 4-
nitrobenzaldehyde/phenol molar ratio of 1:1.5 and two equiva-
lents of K2CO3 as the base in the presence of 3 mol% Cu-based
catalyst. The Cu salts exhibited high catalytic activities for the
reaction. The reaction that used CuCl2 as catalyst afforded
more than 99% conversion after 2 h, which is comparable to
that with Cu2(BDC)2(DABCO). CuI and Cu(NO3)2 were found to
be slightly less active than Cu2(BDC)2(DABCO), although con-
versions of 96% were still obtained after 2 h for the reaction
with these Cu salts. Among these catalysts, Cu(OAc)2 had the
lowest catalytic activity with a conversion of 83% observed
after 2 h (Figure 6). This observation differs from that reported
by Chen and co-workers, in which Cu(OAc)2 was more active
than CuCl2 for the same reaction.[16] As expected, H2BDC and
DABCO exhibited no activity for the coupling reaction, and not
even a trace amount of product was detected. Although sever-
al homogeneous Cu salts exhibited reasonable activities for
Figure 7. Effect of different bases on the reaction conversion.
DMF with a 4-nitrobenzaldehyde/phenol molar ratio of 1:1.5
and two equivalents of the base in the presence of 3 mol%
Cu2(BDC)2(DABCO). Cs2CO3 and K3PO4 should not be used for
the reaction as the competitive Cannizzaro reaction of 4-nitro-
benzaldehyde occurred significantly. Na2CO3, CH3COONa,
tBuOK, and DABCO were unsuitable for the coupling reaction
as low conversions were observed after 2 h. K2CO3 was the
base of choice for the coupling reaction that uses the Cu2-
(BDC)2(DABCO) catalyst, and more than 99% conversion was
achieved after 2 h. As quantitative conversion was achieved in
the presence of a base more available and less expensive than
Cs2CO3, our protocol is advantageous.
For the development of more environmentally benign pro-
cesses, one important issue for organic reactions that use solid
catalysts is the ease of separation as well as the deactivation
and reusability of the catalysts. It is expected that the solid cat-
alyst could be separated easily from the reaction mixture and
could be recovered and reused several times before it eventu-
ally deactivates completely. Therefore, we investigated the re-
coverability and reusability of Cu2(BDC)2(DABCO) in the cou-
Figure 6. Effect of different Cu catalysts on the reaction conversion.
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pling reaction between phenol with 4-nitrobenzaldehyde by
repeatedly separating the catalyst from the reaction mixture,
washing it, and then reusing it. The O-arylation reaction was
performed at 1008C in DMF with a 4-nitrobenzaldehyde/
phenol molar ratio of 1:1.5 and two equivalents of K2CO3 as
the base in the presence of 3 mol% Cu2(BDC)2(DABCO) cata-
lyst. After each run, the catalyst was separated from the reac-
tion mixture by filtration and washed with copious DMF to
remove any physisorbed reagents. The recovered Cu2(BDC)2-
(DABCO) was then reused in further reactions under identical
conditions to those of the first run. Cu2(BDC)2(DABCO) could
be recovered and reused several times without a significant
degradation in catalytic activity, and a conversion of more than
99% was still achieved in the 6th run (Figure 8). To confirm the
Figure 10. XRD pattern of a) fresh Cu2(BDC)2(DABCO) and b) Cu2(BDC)2-
(DABCO) after the 6th reuse.
between phenol with 4-nitrobenzaldehyde to form 4-formyldi-
phenyl ether, which offers a green protocol for this transforma-
tion.
We extended our study to the coupling reaction between 4-
nitrobenzaldehyde with different phenols, which included 4-
methoxyphenol, 4-methylphenol, phenol, 4-chlorophenol, and
4-cyanophenol (Figure 11). The O-arylation reaction was per-
Figure 8. Catalyst recycling studies.
recoverability and reusability of Cu2(BDC)2(DABCO) in the cou-
pling reaction between phenol with 4-nitrobenzaldehyde, the
recovered catalyst was characterized by FTIR spectroscopy and
XRD. The FTIR spectrum of the reused Cu2(BDC)2(DABCO) was
similar to that of the fresh catalyst (Figure 9). Moreover, the
XRD pattern of the reused catalyst indicated that the crystallin-
ity of Cu2(BDC)2(DABCO) was maintained during the course of
the reaction (Figure 10). These results indicate that Cu2(BDC)2-
(DABCO) has an excellent reusability in the coupling reaction
Figure 11. Effect of different substituents on the phenol on the reaction
conversion.
formed at 1008C in DMF with a 4-nitrobenzaldehyde/phenol
molar ratio of 1:1.5 and two equivalents of K2CO3 as the base
in the presence of 3 mol% Cu2(BDC)2(DABCO) catalyst. The for-
mation of diaryl ethers in the coupling reaction was accelerat-
ed for phenol structures that contain electron-donating
groups. The O-arylation of 4-methoxyphenol with 4-nitroben-
zaldehyde afforded 85% conversion after 20 min and more
than 99% conversion after 40 min. 4-Methylphenol was slightly
less active than 4-methoxyphenol, although the coupling reac-
tion also gave more than 99% conversion after 40 min. As ex-
pected, lower conversions were obtained for phenol structures
that contain electron-withdrawing groups. However, more
than 99% conversion was still achieved after 2 h for the reac-
tion of 4-chlorophenol with 4-nitrobenzaldehyde, whereas the
coupling reaction of 4-cyanophenol proceeded slowly with
Figure 9. FTIR spectra of a) fresh Cu2(BDC)2(DABCO) and b) Cu2(BDC)2-
(DABCO) after the 6th reuse.
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54% conversion observed after 2 h. This is consistent with the
study of Chen and co-workers, in which phenols that con-
tained electron-donating groups were good substrates for this
transformation, although high yields were still obtained for
electron-deficient phenols.[16] Zhang and co-workers previously
reported that no reaction occurred for 4-chlorophenol in the
conventional Ullmann reaction.[1] Several other groups have
also reported that the Ullmann transformation of electron-defi-
cient phenols proceeds with difficulty.[13,54,59] The use of our
protocol for the O-arylation of nitroarenes with phenols, there-
fore, offers more advantages than the conventional Ullmann
reaction.
and 81%, respectively. This result is consistent with the study
of Chen and co-workers, in which 1-nitro-4-phenoxybenzene
was the only product for the same reaction.[16]
Conclusions
A highly crystalline, porous metal–organic framework, Cu2-
(BDC)2(DABCO), was synthesized by the reaction of H2BDC,
DABCO, and Cu(NO3)2·3H2O by using a solvothermal method
and was characterized by using XRD, TEM, SEM, FTIR spectros-
copy, TGA, AAS, H2-TPR, and N2 physisorption measurements.
This MOF was used as an efficient heterogeneous catalyst for
the coupling reaction of phenols with nitroarenes to form
diaryl ethers in the absence of ligands. The O-arylation reaction
could only proceed in the presence of the solid Cu2(BDC)2-
(DABCO) catalyst, and no contribution from leached active spe-
cies in the liquid phase was detected. The catalyst could be
reused several times in the coupling reaction without a signifi-
cant degradation in catalytic activity. The Cu2(BDC)2(DABCO)-
catalyzed coupling reaction offers several advantages com-
pared to the conventional Ullmann reaction as it avoids the
formation of halide byproducts. To the best of our knowledge,
the Cu-catalyzed O-arylation of phenols with nitroarenes to
form diaryl ethers that uses a heterogeneous catalyst has not
been reported in the literature. This work contributes to the
development of applications of MOFs in the field of catalysis,
which is of interest to the chemical industry.
To further investigate the scope of the protocol, several ni-
troarenes were also employed for the coupling reaction with
phenol using Cu2(BDC)2(DABCO), which included 4-nitrobenzal-
dehyde, 4-nitrobenzonitrile, 4-nitroacetophenone, 1-fluoro-4-ni-
trobenzene, 1-bromo-4-nitrobenzene, nitrobenzene, and 4-ni-
trotoluene (Figure 12). The O-arylation reaction was performed
Experimental Section
Materials and instrumentation
All reagents and starting materials were obtained commercially
from Sigma–Aldrich or Merck and were used as received without
further purification unless otherwise noted. N2 physisorption meas-
urements were conducted by using a Micromeritics 2020 volumet-
ric adsorption analyzer system. Samples were pretreated by heat-
ing under vacuum at 1508C for 3 h. A Netzsch Thermoanalyzer
STA 409 was used for TGA with a heating rate of 108CminÀ1 under
a N2 atmosphere. XRD patterns were recorded by using a Cu-Ka ra-
diation source in combination with a D8 Advance Bruker powder
diffractometer. SEM was conducted by using a S4800 scanning
electron microscope. TEM was performed by using a JEOL JEM
1400 transmission electron microscope at 100 kV. The Cu2(BDC)2-
(DABCO) sample was dispersed onto holey carbon grids for TEM
observation. Elemental analysis with AAS was performed by using
an AA-6800 Shimadzu. FTIR spectra were obtained by using
a Bruker TENSOR37 instrument, and the samples were dispersed in
KBr pellets. H2-TPR experiments were performed by using a Micro-
meritics Chemisorb 2720 instrument equipped with a thermal con-
ductivity detector (TCD). The samples were outgassed at 1008C for
30 min with He, then cooled to r.t. and exposed to 50 mLminÀ1 of
10% H2/Ar as the temperature was ramped at 58CminÀ1 to 6008C.
The amount of H2 consumed was determined from the TCD signal
intensities, which were calibrated by using a Ag2O reference
sample.
Figure 12. Effect of different substituents on the nitroarenes on the reaction
conversion.
at 1008C in DMF with a nitroarene/phenol molar ratio of 1:1.5
and two equivalents of K2CO3 as the base in the presence of
3 mol% Cu2(BDC)2(DABCO) catalyst. Nitrobenzene and 4-nitro-
toluene were totally unreactive in the coupling reaction, and
not even trace amounts of the product were detected. Nitroar-
enes that contain electron-withdrawing groups could be used
for this transformation. More than 99, 94, and 81% conversions
to the corresponding diaryl ethers were achieved for 4-nitro-
benzaldehyde, 4-nitrobenzonitrile, and 4-nitroacetophenone,
respectively. These results confirm that the electron-donating
groups should be present in the phenol and the electron-with-
drawing substituents should be in the nitroarene in the syn-
thesis of unsymmetrical diaryl ethers by using this protocol.
Indeed, the conventional Ullmann reaction requires the same
situation for the formation of unsymmetrical diaryl ethers, al-
though a longer reaction time[6,7] and higher temperature[8,9]
should be employed. For 1-fluoro-4-nitrobenzene and 1-
bromo-4-nitrobenzene, aromatic nucleophilic substitution oc-
curred instead of the O-arylation reaction to produce 1-nitro-4-
phenoxybenzene as the only product in conversions of 100
GC was performed by using a Shimadzu GC 2010-Plus equipped
with a flame ionization detector (FID) and an SPB-5 column
(length=30 m, inner diameter=0.25 mm, film thickness=
0.25 mm). The temperature program for GC analysis heated the
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samples from 120 to 2808C at 408CminÀ1 and held them at 2808C
for 3 min. The inlet and detector temperatures were 2808C. n-Hex-
adecane was used as an internal standard to calculate the reaction
conversions. GC–MS analyses were performed by using a Hewlett
Packard GC–MS 5972 with a RTX-5MS column (length=30 m, inner
diameter=0.25 mm, film thickness=0. 5 mm). The temperature
program for GC–MS heated the samples from 60 to 2808C at
108CminÀ1 and held them at 2808C for 2 min. The inlet tempera-
ture was 2808C. MS spectra were compared with the spectra gath-
ered in the National Institute of Standards and Technology (NIST)
library.
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Keywords: synthesis design
catalysis · metal–organic frameworks · sustainable chemistry
·
copper
·
heterogeneous
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Received: March 15, 2013
Published online on && &&, 0000
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N. T. S. Phan,* T. T. Nguyen, V. T. Nguyen,
K. D. Nguyen
&& – &&
Ligand-Free Copper-Catalyzed
Coupling of Phenols with Nitroarenes
by using a Metal–Organic Framework
as a Robust and Recoverable Catalyst
Catalytic MOF: A highly porous metal–
organic framework Cu2(BDC)2(DABCO)
(H2BDC=1,4-benzenedicarboxylic acid,
DABCO=1,4-diazabicyclo[2.2.2]octane)
was synthesized and used as an efficient
recyclable heterogeneous catalyst for
the coupling reaction of phenols with
nitroarenes to form diaryl ethers with-
out using a ligand.
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