An azobenzene-containing metal-organic framework as an efficient heterogeneous catalyst for direct amidation of benzoic acids: Synthesis of bioactive compounds

Article abstract of DOI:10.1039/c5cc05985b

An azobenzene-containing zirconium metal-organic framework was demonstrated to be an effective heterogeneous catalyst for the direct amidation of benzoic acids in tetrahydrofuran at 70°C. This finding was applied to the synthesis of several important, representative bioactive compounds.

Full text of DOI:10.1039/c5cc05985b

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DOI: 10.1039/C5CC05985B
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COMMUNICATION
Azobenzene-Containing Metal-Organic Framework as an Efficient
Heterogeneous Catalyst for Direct Amidation of Benzoic Acids:
Synthesis of Bioactive Compounds
Received 00th January 20xx,
Accepted 00th January 20xx
Hanh T. H. Nguyen,a Linh T. M. Hoang,a Long H. Ngo,b Ha L. Nguyen,b Chung K. Nguyen,a Binh T.
DOI: 10.1039/x0xx00000x
Nguyen,b Quang T. Ton,c Hong K. D. Nguyen,d Kyle E. Cordova,b and Thanh Truonga,*
www.rsc.org/
An azobenzene-containing zirconium metal-organic framework heterogeneous catalysts remains an underdeveloped field.
was demonstrated to be an effective heterogeneous catalyst for Metal-organic frameworks (MOFs) are a class of crystalline,
the direct amidation of benzoic acids in tetrahydrofuran at 70 °C. porous materials whose architectures can be designed and
This finding was applied to the synthesis of several important, characterized at the atomic level.11 MOFs are constructed by
representative bioactive compounds.
linking inorganic secondary building units (SBUs) and organic
linkers, in which both components can be tailored to suit
particular applications.11 As a result of the modular approach
to MOF synthesis, these materials have emerged as a class of
The formation of amide bonds has attracted much interest
due to its pervasiveness in biologically active compounds,
industrially-relevant polymers, and pharmaceutical products.1-3
Methodologies based on reactions of amines with alcohols,
aldehydes, nitriles, and aryl halides/CO have been reported,
often with poor atom economy and toxic/corrosive by-
products.1,4,5 However, the direct condensation of a carboxylic
acid with an amine remains the most desirable pathway as the
only side product is water. Due to the high activation barrier of
this route, protocols have been developed to employ
homogeneous biocatalysts, Lewis acid catalysts based on
boron reagents, or metal complexes to successfully form
amides from carboxylic acids.4-6 The success of these
promising
heterogeneous
catalysts
for
organic
transformations.12 It is worth noting that low stability towards
moisture as well as various harsh chemical environments often
limits MOFs in catalysis.11,12 However, MOFs based on
Zr6O4(OH)4(CO2)12 clusters are advantageous for such
applications as the resulting structures are typically moisture-
and chemically-stable and integrate Lewis acidic zirconium
sites within the backbone of the architecture.13,14
By taking advantage of the chemically stable SBUs as well
as modifying the linker, we were able to change the stability as
well as the reactivity of a MOF toward cross coupling
reactions.15 Herein, we report the synthesis and
characterization of a zirconium-based MOF, termed Zr-AzoBDC
(where AzoBDC = azobenzene-4,4ʹ-dicarboxylate), and its
application as a heterogeneous catalyst for direct amidation
reactions. The design strategy was based on the hypothesis
that a zirconium-based MOF with an azobenzene backbone
would provide large enough pore space for the substrates to
react, better affinity of reagents, and potentially promote
direct coupling through a cooperative effect between the
Lewis acidic SBUs and azo functionalities.14 Indeed, Zr-AzoBDC
demonstrated superior catalytic activity for amide formation
under significantly milder conditions when compared to other
zirconium-based MOFs, common Lewis and Brønsted acids,
and other catalytically-active MOFs.
homogeneous catalysts has led
a
push to develop
heterogeneous catalytic systems, including metal-based
catalysts,7 Lewis acidic zeolites,8 solid-supported boronic
acids,9 and immobilized enzymes.10 However, there remain
significant drawbacks as a result of harsh conditions employed
during the reaction, unavoidable by-products, complex
synthesis of catalysts, or limitations of heterogeneous systems
with respect to substrate scope.4-10 Therefore, the direct
amidation of carboxylic acids, especially with less active
substrates such as benzoic acids, catalyzed by more efficient
aFaculty of Chemical Engineering, HCMC University of Technology, VNU-HCM, 268
Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam. Email:
tvthanh@hcmut.edu.vn
bCenter for Molecular and NanoArchitecture, VNU-HCM, Ho Chi Minh City,
Vietnam.
Zr-AzoBDC was solvothermally synthesized with slight
modifications to previous reports.16 Specifically, H2-AzoBDC
and zirconium oxychloride octahydrate were reacted in N,N-
dimethylformamide (DMF) with acetic acid added as a
modulator (Figure 1a).16 The resulting solution was heated at
cDepartment of Chemistry, HCMC University of Science, VNU-HCM, 05 Nguyen Van
Cu, District 5, Ho Chi Minh City, Vietnam.
dSchool of Chemical Engineering, C4-306, Hanoi University of Science and
Technology, Hanoi, Vietnam
†Electronic Supplementary Information (ESI) available: Synthesis and
characterization of Zr-AzoBDC, additional catalytic data, and recycling studies. See
DOI: 10.1039/b000000x/
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Zr-AzoBDC for amidation reactions was investigVaietwedArtuiclseiOnnglinae
model coupling reaction of benzylamineD(BOnI:-1N0H.102)39w/Cit5hCCb0e5n9z8o5iBc
acid. Specifically, optimization of the catalytic activity with
respect to solvent, temperature, and catalyst loading was
undertaken (see Table S3, ESI).† During this process, molecular
sieves were added to the reaction with the realized
expectation that lower reaction temperatures would be
obtained. Optimal results were obtained in tetrahydrofuran
(THF) at 70 °C, with 10 mol% Zr-AzoBDC catalyst loading, when
82% gas chromatography (GC) yield was remarkably achieved
(see Table S3, entry 1, ESI).† In particular, the role of solvent
was found to play an important role in reaction efficiency.
Notably, in 1,4-dioxane, acetonitrile, and toluene, the model
coupling reaction was inefficient with <20% yield (see Table S3,
entries 2-4, ESI).† It was observed that by decreasing Zr-
AzoBDC catalyst loading, the reaction efficiency significantly
dropped (see Table S3, entries 1,5,6, ESI).† Catalytic reactions
taken place at 60 °C afforded only 50% yield while reactions at
increased temperatures (80 °C) produced similar results as the
optimal reaction conditions (see Table S3, entries 1,7,8, ESI).†
Additionally, formation of an ammonium carboxylate salt,
which retarded the nucleophilic attack by the amine nitrogen
atom are likely to result in poor yields when benzoic acid was
used in excess (see Table S3, entries 1,10, ESI). As expected,
reactions without molecular sieve offered < 2% and 42 % at 70
°C and 110 °C, respectively (see Table S3, entries 11, 12, ESI).
Previous reports for the amidation of benzoic acid suffered
either forcing conditions, low reaction yields, or stoichiometric
amounts of waste by-product derived from coupling
reagents.18-21 Moreover, the lowest temperature reported for
the amidation of benzoic acid, using metal- or borane-based
heterogeneous catalysts, is 110 °C.7,9 Clearly, Zr-AzoBDC
considerably improves upon all of these drawbacks. As
expected, only trace amount of desired product was observed
when reactions were carried out without added catalyst (see
Table S3, entry 9, ESI).†
The suitability of Zr-AzoBDC as an effective heterogeneous
catalyst was evaluated by comparative studies on the catalytic
activity of two isoreticular Zr-MOFs, UiO-66 and UiO-67, which
are built from the same Zr SBU and have the same underlying
topology as Zr-AzoBDC.13,16 As is shown in Table 1, UiO-66
exhibited no appreciable catalytic (entry 2). This observation is
attributed to the triangular pore windows (ca. 6 Å) and
octahedral pore cavity (ca. 11 Å) being too small to
accommodate the substrates within the UiO-66 structure.13,14
This points to the likelihood that catalytic amide formation
does not occur solely on the surface of Zr-AzoBDC, but rather
within the pores. When using UiO-67 as a catalyst, the yield
noticeably increases (41% yield), as the adverse pore size
effect is minimized (entry 3). However, the activity of UiO-67
remains much lower than Zr-AzoBDC. Interestingly, Zr-AzoBDC
also exhibited significantly higher activity than previously used
amidation catalysts, ZrCl4 and ZrOCl2 (entries 4,5).18 Other acid
catalysts and catalytically active MOFs were also evaluated as
shown in Table 1 (entries 6-15).22
Fig. 1. (a) Building units used to generate Zr-AzoBDC with a fcu net. Zr-AzoBDC (right)
results from Zr6O4(OH)4(CO2)12 secondary building units (left) joined by azobenzene-
4,4ʹ-dicarboxylic acid (H2-AzoBDC) linkers (left). Orange and yellow spheres represent
the tetrahedral and octahedral pore cavities, respectively. Atom colors: Zr, blue
polyhedra; C, black; N, green; O, red. H atoms are omitted for clarity. (b) PXRD analysis
showing the experimental pattern (red circles), simulated pattern (blue), and the
difference plot (green). The Bragg positions are shown in pink. Inset: Satisfactory
agreement extends to diffraction at higher angles.
85 °C for three days, yielding an orange microcrystalline solid,
which was subsequently activated and characterized (see ESI).†
Scanning electron microscopy analysis revealed homogeneity
with respect to the shape of the microcrystalline particles,
which provides support to a singular phase synthesized (see
Fig. S1, ESI).†
To elucidate the structural features of Zr-AzoBDC, powder
X-ray diffraction (PXRD) analysis on an activated sample, in
conjunction with structural modelling, was carried out (see
ESI).† Specifically, from the unit cell parameters of the indexed
PXRD pattern, a structural model was generated by linking the
primarily observed cuboctahedral-shaped, 12-connected (12-c)
Zr6O4(OH)4(CO2)12 SBU with the linear AzoBDC linkers into a
12,2-c fcu net (see ESI).† A full profile pattern refinement was
performed against the experimental PXRD pattern leading to
refined unit cell parameters (Fm-3, a = 29.3994 Å) and
satisfactory residual values (Rp = 8.48%, Rwp = 11.24%) (Figure
1b, see Tables S1 and S2, ESI).† The three-dimensional
structure of Zr-AzoBDC is described as face-centered cubic
composed of both tetrahedral and octahedral cavities with
internal pore diameters of ca. 14.8 Å and 16.8 Å, respectively,
which are in turn connected by ca. 9.2 Å triangular windows. It
is noted that these diameters are in line with those measured
for the isoreticular (having the same topology) MOFs, UiO-66, -
67, -68 as well as MOF-806.13 The potential solvent accessible
void, as calculated by PLATON,17 is 74%. This was supported by
N2 isotherm measurements at 77 K, in which Zr-AzoBDC
exhibited significant uptake in the low-pressure region,
resulting in calculated Brunauer-Emmett-Teller and Langmuir
surface areas of 3200 and 3500 m2 g-1, respectively (see Fig. S4,
ESI).†
In order to highlight the role of the azo functionality in
promoting the catalytic activity of Zr-AzoBDC, an amidation
After structural characterization, the catalytic activity of
2 | J. Name., 2012, 00, 1-3
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Table 1. Comparative study of catalystsa
Table 2. Reaction scope with respect to coupling partnersa
View Article Online
DOI: 10.1039/C5CC05985B
Entry
1
Acids
Amines
Product
Yield (%)
78
Entry
1
2
3
4
5
6
7
8
Type
Zr-MOFs
Catalyst
Zr-AzoBDC
UiO-66
UiO-67
ZrCl4
GC Yield (%)
82 (76)
trace
41 (33)
31
28
6
2
3
37
39
Zr salts
ZrOCl2
Other acids
H2-AzoBDC
Triflic acid
TFA
4
7
9
pTSA
ZSM-5
HY Zeolite
Co-ZIF-67
Ni2(BDC)2(DABCO)
Zn-ZIF-8
12
8
<2
<2
<2
<2
<2
4
55
97
81
10
11
12
13
14
15
5b
6c
Other MOFs
Cu2(BDC)2(DABCO)
aVolume of solvent, 2 mL; 0.2 mmol scale. Bn-NH2, benzylamine; MS, molecular
sieves; TFA, trifluoroacetic acid; pTSA, p-toluenesulfonic acid; ZIF, zeolitic
imidazolate framework; BDC, benzene-1,4-dicarboxylate; DABCO, 1,4-
diazabicyclo[2.2.2]octane. Numbers in parenthesis indicate isolated yields.
7
8
75
80
control reaction was performed, in which the resulting product
is sufficiently small in molecular size (Scheme 1).
9
68
71
48
As is shown, the varying pore sizes of UiO-66 and UiO-67 did
not significantly affect the reactivity. Thus, the high yield
obtained with Zr-AzoBDC confirms the positive impact of the
azo group on the catalyst activity. This finding was further
supported by reactions using AzoBDC linker and ZrOCl2 metal
cluster.
10
11
aReaction conditions: carboxylic acid derivatives (1 mmol), amine derivatives (1.5
mmol), catalyst (10 mol%), and activated 4 Å molecular sieves (0.5 g) in dry
solvent at 70 °C in a sealed tube under Ar atmosphere. bcatalyst (5 mol %).
ccatalyst (2 mol %).
Catalyst UiO-66 UiO-67
Yield 17 28
Zr-AzoBDC
66
H2-AzoBDC
<2
ZrOCl2
32
Reasonable yields were achieved for acid derivatives with both
activating and deactivating substituents at different
substituted positions (entries 1-4). The amidation of more
reactive acids (i.e. aliphatic carboxylic acids) achieved excellent
yields even at 2-5 mol% catalyst loading (entries 5, 6).
Additionally, cross-coupling reactions of benzoic acid with
substituted benzylamine derivatives were also investigated. As
is shown, desired products were also obtained in relatively
high yields (entries 7-10). Finally, the optimized catalytic
conditions were extended to the direct amidation of benzoic
acid with piperidine, a secondary amine source. The desired
product was realized in 48% yield (entry 11), thus, effectively
demonstrating the exceptional catalytic activity of Zr-AzoBDC
over a wide range of substrates.
The use of homogeneous catalysts in pharmaceutical
synthesis often represents a major problem regarding the
removal of contaminated metals.23 To further expand on the
potential of Zr-AzoBDC to be used in practical applications, we
utilized this heterogeneous catalyst in the synthesis of
pharmaceutically relevant amides possessing bioactivity
(Scheme 2). Specifically, Procainamide, an antiarrhythmic
agent, was efficiently synthesized from the respective
carboxylic acid. Previously, either the protection of the
aromatic amine group or a 2-step synthetic process, including
Scheme 1. Reactions of benzoic acid with methylamine.
Control experiments, using the model amidation reaction
with the corresponding optimized conditions, were
subsequently performed to ensure that the catalytic activity
did not originate from any leaching of Zr4+ ions from Zr-
AzoBDC into the reaction mixture. As expected, there was no
conversion detected in the catalyst-free reaction mixture after
ZrAzoBDC was removed (see ESI).† In addition, inductively
coupled plasma mass spectrometry revealed the concentration
of Zr4+ to be <10 ppm in the filtrate (see ESI).† Encouraged by
these results, recycling studies were undertaken. At the end of
each reaction period (24 h), Zr-AzoBDC was recovered from
the reaction mixture, washed with solvent, and re-used. This
process was performed 5 times (see Fig. S13, ESI).† PXRD
analysis of Zr-AzoBDC revealed that the crystallinity of this
material was retained (see Figs. S14 and S15, ESI).† Clearly, the
exceptional catalytic activity of Zr-AzoBDC was maintained
without significant degradation.
To assess the substrate scope of the Zr-AzoBDC
heterogeneous catalyst, we applied the optimized conditions
to a variety of carboxylic acid and amine coupling derivatives.
The isolated product yields are presented in Table 2.
This journal is © The Royal Society of Chemistry 20xx
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Scheme 2. Synthesis of bioactive amide-containing compounds by the heterogeneous
Zr-AzoBDC catalyst.
amidation of nitroarenes followed by hydrogenation, was
required.24 Similarly, Paracetamol was obtained directly from
acetic acid and 4-aminophenol in 73% yield through utilization
of Zr-AzoBDC. Finally, a reasonable yield was achieved in the
synthesis of Flutamide, an oral and non-steroidal antiandrogen
drug mainly used for prostate cancer treatment. It is noted
that these drugs were previously synthesized by reacting
amines with acid anhydride or acyl halides.25 One can argue
that increased efficiency in such reactions makes the chemical
processes more "green" by reducing the amount of steps in
the synthetic sequences and the resulting purification.
In conclusion, we have reported the synthesis of Zr-AzoBDC
constructed from an azobenzene-4,4ʹ-dicarboxylate (4-Az)
linker and Zr6O4(OH)4(CO2)12 cluster. This structure exhibited
exceptional catalytic activity toward direct amidation of
benzoic acids under mild conditions (10 mol% catalyst loading,
THF, 70 °C). The heterogeneous nature of Zr-AzoBDC enabled
it to be recycled and re-applied to new reactions (5 times)
without degradation in catalytic activity. Furthermore, the
substrate scope of Zr-AzoBDC was demonstrated to be widely
applicable to various substituted carboxylic acid and amine
derivatives for the synthesis of bioactive amide compounds.
Vietnam National University – Ho Chi Minh City (VNU-HCM)
is acknowledged for financial support via grant No. B2015-20-
04. Catalyst synthesis and characterization were supported by
B2011-50-01TĐ. We thank Dr. H. Furukawa, Prof. H. T. Nguyen,
Mr. T. T. Vu, Ms. A. M. Osborn, and Mr. H. Q. Pham for their
valuable discussions and assistance. Prof. O. M. Yaghi is
gratefully acknowledged for supporting MANAR.
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