Synthesis of imidazo[1,5-a] pyridines via oxidative amination of the C(sp3)-H bond under air using metal-organic framework Cu-MOF-74 as an efficient heterogeneous catalyst

Article abstract of DOI:10.1039/c6ra00852f

A crystalline porous copper-based metal-organic framework Cu-MOF-74 was synthesized, and was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. The Cu-MOF-74 used was a recyclable catalyst for the condensation-cyclization reaction between 2-benzoyl pyridine and different benzylamines via oxidative amination of the C(sp³)-H bond to form 1,3-diarylated imidazo[1,5-a]pyridines using air as the oxidant. The Cu-MOF-74 exhibited better catalytic activity for the transformation than other MOFs including Cu(BDC), Cu₂(BDC)₂(DABCO), Cu₃(BTC)₂, Cu₂(BPDC)₂(BPY), Cu(OBA), Cu₂(OBA)₂(BPY), and Cu(INA)₂. The solid catalyst could be recovered and reused several times without a significant degradation in catalytic activity. To the best of our knowledge, the synthesis of 1,3-diarylated imidazo[1,5-a]pyridines via oxidative amination of the C(sp³)-H bond was not previously performed under heterogeneous catalysis conditions.

Full text of DOI:10.1039/c6ra00852f

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3
Synthesis of imidazo[1,5-a]pyridines via oxidative amination of C(sp )-H bond under air
using metal-organic framework Cu-MOF-74 as an efficient heterogeneous catalyst
*
*
Hanh T. H. Nguyen, Oanh T. K. Nguyen, Thanh Truong , Nam T. S. Phan
Department of Chemical Engineering, HCMC University of Technology, VNUꢀHCM,
2
68 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam
*
Email: tvthanh@hcmut.edu.vn, ptsnam@hcmut.edu.vn
Ph: (+84 8) 38647256 ext. 5681
Fx: (+84 8) 38637504
Abstract
A crystalline porous copperꢀbased metalꢀorganic framework CuꢀMOFꢀ74 was synthesized, and
was characterized by Xꢀray powder diffraction (XRD), scanning electron microscopy (SEM),
transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform
infrared (FTꢀIR), atomic absorption spectrophotometry (AAS), and nitrogen physisorption
measurements. The CuꢀMOFꢀ74 was used a recyclable catalyst for the condensationꢀcyclization
reaction between 2ꢀbenzoyl pyridine and different benzylamines via oxidative amination of
3
C(sp )ꢀH bond to form 1,3ꢀdiarylated imidazo[1,5ꢀa]pyridines using air as the oxidant. The Cuꢀ
MOFꢀ74 exhibited better catalytic activity for the transformation than other MOFs including
Cu(BDC), Cu (BDC) (DABCO), Cu (BTC) , Cu (BPDC) (BPY), Cu(OBA), Cu (OBA) (BPY),
2
2
3
2
2
2
2
2
and Cu(INA) . The solid catalyst could be recovered and reused several times without a
2
significant degradation in catalytic activity. To the best of our knowledge, the synthesis of 1,3ꢀ
3
diarylated imidazo[1,5ꢀa]pyridines via oxidative amination of C(sp )ꢀH bond was not previously
performed under heterogeneous catalysis conditions.
Keywords: Metalꢀorganic framework; copper; amination; imidazo[1,5ꢀa]pyridines;
heterogeneous catalyst.
1

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1. Introduction
Imidazo[1,5ꢀa]pyridines have emerged as valuable scaffolds in a variety of pharmaceutical
candidates and agrochemicals, as well as in numerous functional materials [1ꢀ5]. Traditionally,
these fused heterocycles could be achieved by the Vilsmeierꢀtype cyclizations of Nꢀ2ꢀ
pyridylmethylamides or Nꢀ2ꢀpyridylmethylthioamides with several reagents [6, 7]. Alternative
synthetic approaches have been explored to produce imidazo[1,5ꢀa]pyridines. Murai and coꢀ
workers reported the oxidative condensationꢀcyclization of aldehydes and arylꢀ2ꢀ
pyridylmethylamines in the presence of a stoichiometric amount of elemental sulfur as an
oxidant [8]. Paoletti and Crawforth previously demonstrated a oneꢀpot synthesis of imidazo[1,5ꢀ
a]pyridines starting from a carboxylic acid and 2ꢀmethylaminopyridines [9]. Shen and coꢀ
workers developed a baseꢀmediated protocol to synthesize these heterocycles from the reaction
of 1,1ꢀdibromoꢀ1ꢀalkenes with 2ꢀaminomethylpyridines [10]. Manivannan and Fulwa pointed out
that 3ꢀsubstituted imidazo[1,5ꢀa]pyridines could be synthesized from the threeꢀcomponent
cyclization of aldehydes, 2ꢀcyanopyridine, and ammonium acetate [11]. As described in scheme
1, Zeng and coꢀworkers recently reported the first example of a CuBrꢀcatalyzed direct
3
transannulation of Nꢀheteroaryl aldehydes or Nꢀheteroaryl ketones with alkylamines via Csp ꢀH
amination to produce imidazo[1,5ꢀa]pyridines using molecular oxygen as a sole oxidant [12]. Xu
and Adimurthy also performed the similar transformation using Cu(OAc) and CuI as catalysts,
2
respectively [13]. To achieve more environmentally benign approaches to imidazo[1,5ꢀ
a]pyridines in terms of the ease of handling, simple workup, recyclability and reusability,
heterogeneous catalysts should be explored to replace conventional copper salts.
2

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Scheme 1. Previous works using homogeneous copper salt catalysts
Metalꢀorganic frameworks (MOFs), also known as porous coordination polymers, are a new
class of hybrid network supramolecular solid materials, consisting of organic linkers and metalꢀ
connecting points [14ꢀ19]. Compared to conventional crystalline materials, MOFs would exhibit
many advantages in terms of wellꢀdefined structures, high surface areas, high porosity, structural
diversity, and the ability to tune the surface hydrophobicity/hydrophilicity [14ꢀ23]. Although
challenges in commercialization still remain to be solved, potential applications of MOFs in
several fields have been extensively investigated [14, 15, 24ꢀ29]. During the last few years,
MOFs as heterogeneous catalysts have been one of the hot topics [30ꢀ36]. The combination of
organic linkers and metal cations in the framework could maximize the opportunity for high
dispersion and high loading of catalytically active sites [31, 37ꢀ43]. Indeed, both carbonꢀcarbon
[
35, 44ꢀ53] and carbonꢀheteroatom forming organic transformations [54ꢀ62] using MOFs as
heterogeneous catalysts or catalyst supports have been mentioned in the literature, and this topic
would be extensively explored in the near future [30ꢀ32, 63, 64]. Among several popular MOFs,
copperꢀbased frameworks have attracted significant attention as catalysts for numerous organic
transformations due to their unsaturated open copper metal sites [53, 57, 65ꢀ71]. In this work, we
wish to report the synthesis of 1,3ꢀdiarylated imidazo[1,5ꢀa]pyridines via oxidative amination of
3
C(sp )ꢀH bond using a copperꢀbased metalꢀorganic framework CuꢀMOFꢀ74 as an efficient
heterogeneous catalyst with molecular oxygen in air as the stoichiometric oxidant. To the best of
our knowledge, this transformation was not previously carried out under heterogeneous catalysis
conditions.
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2. Experimental
2.1. Materials and instrumentation
All reagents and starting materials were obtained commercially from SigmaꢀAldrich and Merck,
and were used as received without any further purification unless otherwise noted. Nitrogen
physisorption measurements were conducted using a Micromeritics 2020 volumetric adsorption
o
analyzer system. Samples were pretreated by heating under vacuum at 150 C for 3 h. A Netzsch
Thermoanalyzer STA 409 was used for thermogravimetric analysis (TGA) with a heating rate of
o
1
0 C/min under a nitrogen atmosphere. Xꢀray powder diffraction (XRD) patterns were recorded
using a Cu Kα radiation source on a D8 Advance Bruker powder diffractometer. Scanning
electron microscopy studies were conducted on a S4800 Scanning Electron Microscope (SEM).
Transmission electron microscopy studies were performed using a JEOL JEM 1400
Transmission Electron Microscope (TEM) at 100 kV. The CuꢀMOFꢀ74 sample was dispersed on
holey carbon grids for TEM observation. Elemental analysis with atomic absorption
spectrophotometry (AAS) was performed on an AAꢀ6800 Shimadzu. Fourier transform infrared
(FTꢀIR) spectra were obtained on a Nicolet 6700 instrument, with samples being dispersed on
potassium bromide pallets.
Gas chromatographic (GC) analyses were performed using a Shimadzu GC 2010ꢀPlus equipped
with a flame ionization detector (FID) and an SPBꢀ5 column (length = 30 m, inner diameter =
0.25 mm, and film thickness = 0.25 ꢁm). The temperature program for GC analysis held samples
4

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o
o
o
o
at 100 C for 1 min; heated them from 100 to 280 C at 40 C/min; and held them at 280 C for
o
8
.5 min. Inlet and detector temperatures were set constant at 280 C. Diphenyl ether was used as
an internal standard to calculate GC yield. GCꢀMS analyses were performed using a Shimadzu
GCMSꢀQP2010Ultra with a ZBꢀ5MS column (length = 30 m, inner diameter = 0.25 mm, and
o
film thickness = 0.25 ꢁm). The temperature program for GCꢀMS analysis held samples at 50 C
o
o
o
for 2 min; heated samples from 50 to 280 C at 10 C/min and held them at 280 C for 10 min.
o
Inlet temperature was set constant at 280 C. MS spectra were compared with the spectra
1
13
gathered in the NIST library. The H NMR and C NMR were recorded on Bruker AV 500
spectrometers using residual solvent peak as a reference.
2.2. Synthesis of the metal-organic framework Cu-MOF-74
In a typical preparation, a solid mixture of H dhtp (H dhtp = 2,5ꢀdihydroxyterephthalic acid;
2
2
0
.186 g, 0.97mmol), and Cu(NO ) .3H O (0.500g, 2.07 mmol) was dissolved in a mixture of
3 2 2
DMF (DMF = N,N’ꢀdimethylformamide; 20 ml), and water (1 ml). The resulting solution was
o
then distributed to three 10 ml vials. The vials were then heated at 85 C in an isothermal oven
for 18 h. After cooling the vials to room temperature, the solid product was removed by
decanting with mother liquor and washed in DMF (3 x 20 ml) for 3 days. Solvent exchange was
carried out with methanol (3 x 20 ml) at room temperature for 3 days. The material was then
o
evacuated under vacuum at 150 C for 5 h, yielding 0.260 g of CuꢀMOFꢀ74 in the form of
reddish black crystals (60 % based on H dhtp).
2
2.3. Catalytic studies
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In a typical experiment, a mixture of 2ꢀbenzoyl pyridine (0.037 g, 0.2 mmol) and diphenyl ether
0.034g, 0.2 mmol) as an internal standard in DMF (1 ml) was added into a 10ꢀml screw tube
(
containing the preꢀdetermining amount of CuꢀMOFꢀ74 catalyst and benzylamine (0.066g, 0.6
mmol) at room temperature. The catalyst concentration was calculated with respect to the
o
copper/ 2ꢀbenzoyl pyridine molar ratio. The reaction mixture was stirred at 120 C for 8 h.
Reaction yield was monitored by withdrawing aliquots from the reaction mixture, quenching
with water (1 ml). The organic components were then extracted into ethyl acetate (2 ml), dried
over anhydrous Na SO , and analyzed by GC with reference diphenyl ether. The combined
2
4
organic layers were concentrated under reduced pressure. The resulting residue was purified by
column chromatography (ethyl acetate/hexane=1:8) to afford 1,3ꢀdiphenylimidazo[1,5ꢀ
1
13
a]pyridine. The product identity was further confirmed by GCꢀMS, H NMR and C NMR. To
investigate the recyclability of CuꢀMOFꢀ74, the catalyst was separated from the reaction mixture
o
by simple centrifugation, washed with copious amounts of DMF and methanol, dried 150 C
under vacuum in 6 h, and reused if necessary. For the leaching test, upon completion of the
reaction, the CuꢀMOFꢀ74 catalyst was separated from the reaction mixture by simple
centrifugation. The liquid phase was transferred to a new reactor vessel, and fresh reagents were
added to the solution. Reaction progress, if any, was monitored by GC as previously described.
3. Results and discussion
The metalꢀorganic framework CuꢀMOFꢀ74 was synthesized according to a slightly modified
literature procedure [72], and was characterized by many different techniques, including XRD,
SEM, TEM, TGA, FTꢀIR, AAS, and nitrogen physisorption measurements (Fig. S1 – Fig. S7).
6

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The CuꢀMOF was assessed for its catalytic activity in the condensationꢀcyclization reaction
between 2ꢀbenzoyl pyridine and benzylamine to form 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine as the
3
principal product via oxidative amination of C(sp )ꢀH bond (Scheme 2). Initial studies addressed
the effect of temperature on the yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine. In the first
example of the direct transannulation of Nꢀheteroaryl aldehydes or Nꢀheteroaryl ketones with
3
alkylamines via Csp ꢀH amination to produce imidazo[1,5ꢀa]pyridines under air, Zeng and coꢀ
o
workers carried out the reaction at 80 C [12]. However, low yields were observed for the case
of Nꢀheteroaryl ketones. Xu and coꢀworkers demonstrated that the yields of imidazo[1,5ꢀ
o
a]pyridines could be improved at 110 C [13]. The condensationꢀcyclization reaction was then
carried out under air in DMF for 8 h, using 3 equivalents of benzylamine, at 2ꢀbenzoyl pyridine
concentration of 0.2 M, in the presence of 10 mol% CuꢀMOFꢀ74 catalyst, at room temperature,
o
o
o
o
1
00 C, 110 C, 120 C, and 130 C, respectively. It was observed that the transformation could
not proceed at room temperature, with no trace amount of product being detected after 8 h.
o
Increasing the temperature to 100 C resulted in 23% yield of 1,3ꢀdiphenylimidazo[1,5ꢀ
o
a]pyridine after 8 h. This value could be improved to 71% for the reaction carried out at 120 C.
o
Increasing the temperature to higher than 120 C was found to be unnecessary as the formation
of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine was not favored (Fig. 1).
Cu-MOF-74
NH2
+
N
N
DMF, air
N
O
Scheme 2. The synthesis of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine via oxidative amination of
3
C(sp )ꢀH bond using CuꢀMOFꢀ74 catalyst.
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100
80
60
40
20
0
RT
100
110
120
130
Temperature (oC)
Fig. 1. Effect of temperature on the reaction yield.
Another factor that should be addressed for the condensationꢀcyclization reaction between 2ꢀ
benzoyl pyridine and benzylamine to form 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine via oxidative
3
amination of C(sp )ꢀH bond is the catalyst concentration. In the first example of the
3
imidazo[1,5ꢀa]pyridine synthesis via Csp ꢀH amination from Nꢀheteroaryl aldehydes or Nꢀ
heteroaryl ketones and alkylamines, up to 20 mol% CuBr was employed [12]. Xu and coꢀ
workers also performed the condensationꢀcyclization between 2ꢀbenzoyl pyridine and
benzylamine using 15 mol% Cu(OAc) as catalyst [13]. The reaction was then carried out under
2
o
air in DMF at 120 C for 8 h, using 3 equivalents of benzylamine, at 2ꢀbenzoyl pyridine
concentration of 0.2 M, in the presence of 2.5 mol%, 5 mol%, 7.5 mol%, 10 mol%, 12.5 mol%,
and 15 mol% CuꢀMOFꢀ74 catalyst, respectively. It should be noted that only 14% yield of 1,3ꢀ
diphenylimidazo[1,5ꢀa]pyridine was formed in the absence of the CuꢀMOFꢀ74, indicating the
8

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necessity of using the CuꢀMOF as catalyst for the condensationꢀcyclization between 2ꢀbenzoyl
pyridine and benzylamine. As expected, the presence of CuꢀMOFꢀ74 as catalyst in the reaction
mixture led to a significant enhancement in the yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine.
The reaction using 5 mol% CuꢀMOFꢀ74 catalyst afforded 40% yield after 8 h, while 56% yield
was observed for that using 7.5 mol% catalyst. It was found that the yield could be improved to
71% when 10 mol% CuꢀMOFꢀ74 catalyst was employed. However, using more than 10 mol%
CuꢀMOFꢀ74 catalyst was found to be unnecessary for the condensationꢀcyclization reaction as
the yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine was not increased significantly (Fig. 2).
100
80
60
40
20
0
0
2.5
5
7.5
10
12.5 15
Catalyst concentration (%)
Fig. 2. Effect of catalyst concentration on the reaction yield.
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100
80
60
40
20
0
0.4
0.2
0.13
0.1
0.08
2-benzoyl pyridine concentration (M)
Fig. 3. Effect of 2ꢀbenzoyl pyridine concentration on the reaction yield.
The rate of liquid phase organic transformations using solid catalysts might be significantly
affected by the reactant concentration due to the mass transfer phenomenon. It was therefore
decided to investigate the impact of reactant concentration on the formation of 1,3ꢀ
diphenylimidazo[1,5ꢀa]pyridine in the condensationꢀcyclization reaction between 2ꢀbenzoyl
pyridine and benzylamine using CuꢀMOFꢀ74 catalyst. The reaction was then carried out in DMF
o
under air at 120 C for 8 h, using 3 equivalents of benzylamine, in the presence of 10 mol% Cuꢀ
MOFꢀ74 catalyst, at 2ꢀbenzoyl pyridine concentration of 0.08 M, 0.1 M, 0.13 M, 0.2 M, and 0.4
M, respectively. It was observed that the reactant concentration exhibited a significant effect on
the yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine. The condensationꢀcyclization reaction using 2ꢀ
benzoyl pyridine concentration of 0.08 M proceeded with difficulty, affording only 41% yield
after 8 h. Increasing the concentration of 2ꢀbenzoyl pyridine led to an enhancement in the yield
of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine, and best yield was observed at 2ꢀbenzoyl pyridine
10

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concentration of 0.2 M. Lower yield was detected for the transformation using 2ꢀbenzoyl
pyridine concentration of 0.4 M (Fig. 3). Furthermore, it was also found that the reagent molar
ratio exhibited a significant impact on the yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine in the
condensationꢀcyclization reaction between 2ꢀbenzoyl pyridine and benzylamine using CuꢀMOFꢀ
74 catalyst. The condensationꢀcyclization reaction using 1 equivalent of benzylamine afforded
only 21% yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine after 8 h, while this value could be
improved to 71% for that using 3 equivalents of benzylamine. Using more than 3 equivalents of
benzylamine led to a drop in the reaction yield (Fig. 4).
100
80
60
40
20
0
1:1
1:2
1:3
1:4
Reactant molar ratio
Fig. 4. Effect of reagent molar ratio on the reaction yield.
The solvent could exhibit a significant effect on the reaction rate of many organic
transformations using solid catalysts, depending on the nature of the catalyst [73, 74]. In the first
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example of the CuBrꢀcatalyzed direct transannulation of Nꢀheteroaryl aldehydes or Nꢀheteroaryl
3
ketones with alkylamines via Csp ꢀH amination to produce imidazo[1,5ꢀa]pyridines, Zeng and
coꢀworkers pointed out that acetonitrile should be the solvent of choice [12]. Xu and coꢀworkers
performed the same transformation using Cu(OAc) as catalyst in different solvents, and
2
demonstrated that DMF would be the best solvent for the transformation [13]. It was therefore
decided to investigate the effect of different solvents on the condensationꢀcyclization reaction
between 2ꢀbenzoyl pyridine and benzylamine to form 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine via
3
oxidative amination of C(sp )ꢀH bond using CuꢀMOFꢀ74 catalyst. The reaction was then carried
o
out under air at 120 C for 8 h, using 3 equivalents of benzylamine, in the presence of 10 mol%
CuꢀMOFꢀ74 catalyst, at 2ꢀbenzoyl pyridine concentration of 0.2 M, in NMA, NMP, toluene,
chlorobenzene, DMF, DMSO, nꢀbutanol, pꢀxylene, benzonitrile, and tertꢀbutanol as solvent,
respectively. It was observed that the yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine was
significantly affected by the solvent. The condensationꢀcyclization reaction proceeded with
difficulty in chlorobenzene, pꢀxylene, and tertꢀbutanol, affording 45%, 37%, and 44% yields
after 8 h. The reaction carried out in NMA offered 68% yield of 1,3ꢀdiphenylimidazo[1,5ꢀ
a]pyridine after 8 h, while 69% yield was observed for that carried out in benzonitrile. Among
these solvents, DMF exhibited the best performance, with 71% yield being achieved after 8 h
(Fig. 5).
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100
80
60
40
20
0
Solvent
Fig. 5. Effect of solvent on the reaction yield.
The catalytic activity of CuꢀMOFꢀ74 in the condensationꢀcyclization reaction between 2ꢀ
benzoyl pyridine and benzylamine to form 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine via oxidative
3
amination of C(sp )ꢀH bond was also compared with that of other homogeneous catalysts,
including Cu(NO ) , CuCl , CuSO , Cu(OAc) , Cu(acac) , CuBr , CuBr, CuI, CuCl, Co(OAc) ,
3
2
2
4
2
2
2
2
Zn(OAc) , Mn(OAc) , Ni(OAc) , AgOAc, and 2,5ꢀdihydroxyterephthalic acid. The reaction was
2
2
2
o
then carried out in DMF under air at 120 C for 8 h, using 3 equivalents of benzylamine, in the
presence of 10 mol% catalyst, at 2ꢀbenzoyl pyridine concentration of 0.2 M. It was found that
copper species were more active for the transformation than other metals. Indeed, the
condensationꢀcyclization reaction afforded 22%, 15%, 13%, 19%, and 9% yields of 1,3ꢀ
diphenylimidazo[1,5ꢀa]pyridine after 8 h for the case of Co(OAc) , Zn(OAc) , Mn(OAc) ,
2
2
2
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Ni(OAc) , and AgOAc as catalyst, respectively. CuBr offered low activity for the reaction, with
2
2
9% yield being detected after 8 h. The reaction using Cu(NO ) as catalyst, the precursor of the
3 2
CuꢀMOFꢀ74, proceeded to 40% yield after 8 h. Cu(acac) were found to be more catalytically
2
active than Cu(NO ) , producing 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine in a yield of 48% after 8 h.
3
2
Among these copper salts, Cu(OAc) exhibited the best performance as catalyst for the
2
condensationꢀcyclization reaction, with 78% yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine being
3
achieved after 8 h (Fig. 6). In the first example of the imidazo[1,5ꢀa]pyridine synthesis via Csp ꢀ
H amination from Nꢀheteroaryl aldehydes or Nꢀheteroaryl ketones and alkylamines, Zeng and coꢀ
workers reported that CuBr was more active than other copper salts [12]. However, Xu and coꢀ
workers demonstrated that Cu(OAc) was the best catalyst for the condensationꢀcyclization
2
between 2ꢀbenzoyl pyridine and benzylamine [13].
100
80
60
40
20
0
Catalyst
Fig. 6. Different homogeneous catalysts for the condensationꢀcyclization reaction.
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100
80
60
40
20
0
Catalyst
Fig. 7. Different MOFs as catalyst for the condensationꢀcyclization reaction.
To highlight the advantage of using CuꢀMOFꢀ74 as catalyst for the condensationꢀcyclization
reaction between 2ꢀbenzoyl pyridine and benzylamine to form 1,3ꢀdiphenylimidazo[1,5ꢀ
3
a]pyridine via oxidative amination of C(sp )ꢀH bond, the catalytic activity of the CuꢀMOFꢀ74
was compared with that of other MOFs including Cu(BDC), Cu (BDC) (DABCO), Cu (BTC) ,
2
2
3
2
Cu (BPDC) (BPY), Cu(OBA), Cu (OBA) (BPY), and Cu(INA) . These MOFs were synthesized
2
2
2
2
2
by solvothermal method, and characterized according to literature procedures [46, 71, 72, 75ꢀ80].
o
The reaction was then carried out in DMF under air at 120 C for 8 h, using 3 equivalents of
benzylamine, in the presence of 10 mol% catalyst, at 2ꢀbenzoyl pyridine concentration of 0.2 M.
The reaction using Cu(BDC) catalyst could proceed to 37% yield of 1,3ꢀdiphenylimidazo[1,5ꢀ
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a]pyridine after 8 h. Cu (BDC) (DABCO), Cu (BTC) , and Cu (BPDC) (BPY) were found to be
2
2
3
2
2
2
less active for the condensationꢀcyclization reaction than Cu(BDC), affording 33%, 22%, and
3% yields of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine, respectively, after 8 h. The yield of the
desired product could be improved to 46% and 48%, using Cu (OBA) (BPY) and Cu(OBA),
3
2
2
respectively, as catalyst. Cu(INA) offered higher activity than Cu(OBA), producing 1,3ꢀ
2
diphenylimidazo[1,5ꢀa]pyridine in a yield of 56% after 8 h. Among these CuꢀMOFs, CuꢀMOFꢀ
74 should be the catalyst of choice, with 71% yield being achieved after 8 h. It should be noted
that the reaction using the mixture of 2,5ꢀdihydroxyterephthalic acid and Cu(NO ) , two
3
2
precursors of the CuꢀMOFꢀ74, afforded only 38% yield after 8 h (Fig. 7). These observations
would indicate that the framework structure should be important for the condensationꢀcyclization
transformation (See Table S1 and Table S2 in Supporting Information for detailed data).
To investigate the possibility that some of active copper sites on CuꢀMOFꢀ74 could dissolve into
the reaction solution during the course of the reaction, the leaching test was then carried out for
the condensationꢀcyclization between 2ꢀbenzoyl pyridine and benzylamine to form 1,3ꢀ
3
diphenylimidazo[1,5ꢀa]pyridine via oxidative amination of C(sp )ꢀH bond. Indeed, in several
cases, these leached species in the liquid phase could contribute significantly to the total yield of
the reaction, thus implying that the transformation would not proceed under real heterogeneous
catalysis conditions [74]. In order to check if any 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine could be
formed via homogeneous catalysis, a control experiment was carried out. The condensationꢀ
o
cyclization reaction was then carried out at 120 C under air in DMF for 10 h, using 3
equivalents of benzylamine, at 2ꢀbenzoyl pyridine concentration of 0.2 M, in the presence of 10
mol% CuꢀMOFꢀ74 catalyst. Upon completion of the reaction, the CuꢀMOFꢀ74 catalyst was
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separated from the reaction mixture by simple centrifugation. The liquid phase was transferred to
a new reactor vessel, and fresh reagents were added to the solution. The resulting mixture was
o
then stirred for 10 h at 120 C. Experimental results showed that no further yield of 1,3ꢀ
diphenylimidazo[1,5ꢀa]pyridine was detected in the absence of the solid CuꢀMOFꢀ74 catalyst
(Fig. 8). It should be noted that the transformation could proceed to 71% yield of 1,3ꢀ
diphenylimidazo[1,5ꢀa]pyridine in the presence of CuꢀMOFꢀ74 catalyst. These observations
confirmed that the condensationꢀcyclization between 2ꢀbenzoyl pyridine and benzylamine to
form 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine could only occur in the presence of the solid CuꢀMOFꢀ
74 catalyst, and no contribution from catalytically active species soluble in the solution, if any,
was detected.
100
Leaching test
80
60
40
20
0
10 mol% catalyst
0
2
4
6
8
10
Time (h)
Fig. 8. Leaching test indicated no contribution from homogeneous catalysis of active species
leaching into reaction solution.
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To gain insights into the reaction pathway of the condensationꢀcyclization between 2ꢀbenzoyl
pyridine and benzylamine to form 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine via oxidative amination of
3
C(sp )ꢀH bond using CuꢀMOFꢀ74 catalyst, some mechanistic studies were carried out. In the first
o
experiment, the condensationꢀcyclization reaction was then carried out at 120 C under argon in
DMF for 8 h, using 3 equivalents of benzylamine, at 2ꢀbenzoyl pyridine concentration of 0.2 M,
in the presence of 10 mol% CuꢀMOFꢀ74 catalyst. It was observed that performing the reaction
under air should be necessary, as the transformation carried out under argon afforded only 13%
yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine after 8 h. It should be noted that up to 71% yield of
the desired product was achieved for the condensationꢀcyclization reaction after 8 h under air.
However, in the presence of diꢀtertꢀbutyl peroxide as the oxidant, the transformation afforded
only 7% yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine after 8 h. Indeed, homocoupling byꢀproduct
of benzylamine was detected in the reaction mixture under this condition. In the second
experiment, the condensationꢀcyclization reaction was carried out under air in the presence of
water (5% by volume). It was observed that water exhibited a significant impact on the
performance of the CuꢀMOFꢀ74 catalyst, with only 31% yield of the desired product being
recorded after 8 h. This observation confirmed that the formation of 1,3ꢀdiphenylimidazo[1,5ꢀ
a]pyridine via the condensationꢀcyclization between 2ꢀbenzoyl pyridine and benzylamine using
CuꢀMOFꢀ74 catalyst was not favored by water residue in the solvent. In the third experiment, the
CuꢀMOFꢀ74 catalyst was soaked in pyridine as catalyst poison prior to use in the condensationꢀ
cyclization between 2ꢀbenzoyl pyridine and benzylamine. Under this condition, only 31% yield
of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine was detected after 8 h. This result indicated that the
strong adsorption of pyridine on the copper sites in the CuꢀMOFꢀ74 would deactivate the Cuꢀ
MOF catalyst. However, further mechanistic studies would be necessary to clarify the reaction
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pathway of the condensationꢀcyclization between 2ꢀbenzoyl pyridine and benzylamine to form
3
1
,3ꢀdiphenylimidazo[1,5ꢀa]pyridine via oxidative amination of C(sp )ꢀH bond using CuꢀMOFꢀ74
catalyst.
Although several copper salts exhibited high activity in the condensationꢀcyclization between 2ꢀ
benzoyl pyridine and benzylamine to form 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine via oxidative
3
amination of C(sp )ꢀH bond, these homogeneous catalysts could not be reused. Under the view
of green chemistry, issues that should be addressed when using solid catalysts for organic
transformations should be the ease of separation as well as the deactivation and reusability of the
catalysts. It would be targeted that the solid catalyst can be recovered and reused several times
before it eventually deactivates completely. The CuꢀMOFꢀ74 catalyst was therefore investigated
for recoverability and reusability in the condensationꢀcyclization reaction between 2ꢀbenzoyl
pyridine and benzylamine over 8 successive runs, by repeatedly separating the CuꢀMOFꢀ74 from
the reaction mixture, washing it and then reusing it. The condensationꢀcyclization reaction was
o
then carried out at 120 C under air in DMF for 8 h, using 3 equivalents of benzylamine, at 2ꢀ
benzoyl pyridine concentration of 0.2 M, in the presence of 10 mol% CuꢀMOFꢀ74 catalyst. After
the first run, the CuꢀMOF catalyst was separated from the reaction mixture by simple
centrifugation, washed with copious amounts of DMF and dichloromethane to remove any
o
physisorbed reagents, dried 150 C under vacuum in 2 h. The recovered CuꢀMOFꢀ74 catalyst
was then reused in further reaction under identical conditions to those of the first run. It was
observed that the CuꢀMOFꢀ74 catalyst could be recovered and reused many times in the
condensationꢀcyclization between 2ꢀbenzoyl pyridine and benzylamine to form 1,3ꢀ
3
diphenylimidazo[1,5ꢀa]pyridine via oxidative amination of C(sp )ꢀH bond without a significant
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degradation in catalytic activity. Indeed, 67% yield of 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine was
th
still observed in the 8 run (Fig. 9). Moreover, XRD (Fig. 10) and FTꢀIR (Fig. 11) results of the
reused CuꢀMOFꢀ74 also confirmed that the CuꢀMOF structure could be maintained during the
course of the transformation.
100
80
60
40
20
0
1
2
3
4
5
6
7
8
Run time
Fig. 9. Catalyst recycling studies.
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1400
1200
1000
800
600
400
200
0
b)
a)
5
10
15
20
25
30
2-Theta scale
Fig. 10. Xꢀray powder diffractograms of the fresh (a) and reused (b) CuꢀMOFꢀ74 catalyst.
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b)
a)
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig. 11. FTꢀIR spectra of the fresh (a) and reused (b) CuꢀMOFꢀ74 catalyst.
To investigate whether reactions occurred at outside or inside the pores, experiments using
grinded CuꢀMOFꢀ74 were performed under identical conditions. The results indicated that
similar reaction yields were obtained in both cases (Table 1. It is known that catalysts with
smaller size often provide better reaction efficiency if reaction occurs at the external catalyst
surface. Though it is possible that reactions took place inside the catalyst cavities, further
spectroscopic studies are still needed to gain more insights about this issue.
Table 1. Reaction with grinded CuꢀMOFꢀ74
MOFs
Median size (µm)
Mean size (µm)
41.09
Reaction yield
Nonꢀgrinded CuꢀMOFꢀ74
Grinded CuꢀMOFꢀ74
29.96
5.13
70
71
9.99
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3
Table 2. Synthesis of different imidazo[1,5ꢀa]pyridines via oxidative amination of C(sp )ꢀH
bond.
Entry
1
Benzylamines
Imidazo[1,5ꢀa]pyridines
Isolated yields
%)
(
70
53
54
NH2
N
N
2
Cl
NH2
N
N
Cl
3
Cl
NH2
N
N
Cl
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4
53
NH2
Cl
N
N
Cl
5
6
7
OMe
61
70
73
NH2
N
N
OMe
MeO
NH2
N
N
OMe
NH2
MeO
N
N
OMe
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8
67
NH2
H N
2
N
N
NH2
The study was then extended to the synthesis of different imidazo[1,5ꢀa]pyridines via oxidative
3
amination of C(sp )ꢀH bond using CuꢀMOFꢀ74 catalyst. The condensationꢀcyclization reaction
o
was then carried out at 120 C under air in DMF for 8 h, using 3 equivalents of benzylamine, at
2ꢀbenzoyl pyridine concentration of 0.2 M, in the presence of 10 mol% CuꢀMOFꢀ74 catalyst.
The desired imidazo[1,5ꢀa]pyridine was purified by column chromatography. It was observed
that 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine was produced in 70% isolated yield via the
condensationꢀcyclization reaction between 2ꢀbenzoyl pyridine and benzylamine using CuꢀMOFꢀ
7
4 catalyst. However, the presence of a chloro group on the benzene ring in benzylamine
resulted in a drop in the yield of the corresponding 1,3ꢀdiphenylimidazo[1,5ꢀa]pyridine. Indeed,
ꢀ(2ꢀchlorophenyl)ꢀ1ꢀphenylimidazo[1,5ꢀa]pyridine was obtained in 53% yield for the
3
condensationꢀcyclization reaction between 2ꢀbenzoyl pyridine and 2ꢀchlorobenzylamine using
CuꢀMOFꢀ74 catalyst. Similarly, 3ꢀ(3ꢀchlorophenyl)ꢀ1ꢀphenylimidazo[1,5ꢀa]pyridine and 3ꢀ(4ꢀ
chlorophenyl)ꢀ1ꢀphenylimidazo[1,5ꢀa]pyridine were produced in 54% and 53% yields for the
reaction of 2ꢀbenzoyl pyridine with 3ꢀchlorobenzylamine and 4ꢀchlorobenzylamine, respectively.
The presence of a methoxy group on the benzene ring in benzylamine was found to accelerate
the transformation slightly. In the presence of 10 mol% CuꢀMOFꢀ74 catalyst, the reaction of 2ꢀ
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benzoyl pyridine with 2ꢀmethoxybenzylamine could offer 61% yield of 3ꢀ(2ꢀmethoxyphenyl)ꢀ1ꢀ
phenylimidazo[1,5ꢀa]pyridine, while 70% yield of 3ꢀ(3ꢀmethoxyphenyl)ꢀ1ꢀphenylimidazo[1,5ꢀ
a]pyridine was obtained for the case of 3ꢀmethoxybenzylamine. 4ꢀMethoxybenzylamine was
more reactive towards the condensationꢀcyclization reaction with 2ꢀbenzoyl pyridine, producing
3ꢀ(4ꢀmethoxyphenyl)ꢀ1ꢀphenylimidazo[1,5ꢀa]pyridine in 73% yield. It was also observed that the
reaction of 2ꢀbenzoyl pyridine with 4ꢀaminobenzylamine could proceed to 68% yield of 4ꢀ(1ꢀ
phenylimidazo[1,5ꢀa]pyridinꢀ3ꢀyl)benzenamine (Table 2).
4. Conclusions
In summary, the metalꢀorganic framework CuꢀMOFꢀ74 was synthesized by a solvothermal
method, and was characterized by a variety of different techniques including XRD, SEM, TEM,
FTꢀIR, TGA, AAS, and nitrogen physisorption measurements. The CuꢀMOF could be used as an
efficient heterogeneous catalyst for the condensationꢀcyclization reaction between 2ꢀbenzoyl
3
pyridine and different benzylamines via oxidative amination of C(sp )ꢀH bond to form 1,3ꢀ
diarylated imidazo[1,5ꢀa]pyridines using air as the oxidant. The CuꢀMOFꢀ74 offered better
catalytic activity for the transformation than other MOFs including Cu(BDC),
Cu (BDC) (DABCO), Cu (BTC) , Cu (BPDC) (BPY), Cu(OBA), Cu (OBA) (BPY), and
2
2
3
2
2
2
2
2
Cu(INA) . The condensationꢀcyclization transformation could only proceed in the presence of
2
the solid CuꢀMOF catalyst, and the contribution of leached active copper species to the formation
of 1,3ꢀdiarylated imidazo[1,5ꢀa]pyridines, if any, was negligible. The solid catalyst could be
separated from the reaction mixture by centrifugation, and could be recovered and reused several
times for the synthesis of 1,3ꢀdiarylated imidazo[1,5ꢀa]pyridines without a significant
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degradation in catalytic activity. The fact that the condensationꢀcyclization reaction to produce
,3ꢀdiarylated imidazo[1,5ꢀa]pyridines could proceed under heterogeneous catalysis conditions
1
should be of advantages, and might be interested to the chemical industry.
Acknowledgements
The Vietnam National University – Ho Chi Minh City (VNUꢀHCM) is acknowledged for
financial support through project No. B2015ꢀ20ꢀ03.
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