Application of iron-based metal-organic frameworks in catalysis: Oxidant-promoted formation of coumarins using Fe3O(BPDC)3 as an efficient heterogeneous catalyst

Article abstract of DOI:10.1039/c5cy02191j

An iron-based metal-organic framework, Fe₃O(BPDC)₃, 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) spectroscopy, atomic absorption spectrophotometry (AAS), and nitrogen physisorption measurements. The Fe-MOF could be used as an efficient heterogeneous catalyst for the synthesis of coumarins from salicylaldehydes and activated methylene compounds in the presence of tert-butyl hydroperoxide as the oxidant under base-free conditions. The Fe₃O(BPDC)₃ was more catalytically active for the synthesis of coumarins than other MOFs such as Cu₂(BDC)₂(BPY), Cu₃(BTC)₂, Cu₂(BDC)₂(DABCO), Ni₂(BDC)₂(DABCO), and Fe₃O(BDC)₃. This Fe-MOF also offered significantly higher catalytic activity for the transformation than that of common metal salts such as FeCl₃, Fe(NO₃)₃, FeSO₄, CuCl₂, CuCl, Cu(NO₃)₂, and Ni(NO₃)₂. The Fe₃O(BPDC)₃ catalyst could be recovered and reused several times without significant degradation in catalytic activity. To the best of our knowledge, the oxidant-promoted formation of coumarins under heterogeneous catalysis conditions was not previously mentioned in the literature.

Full text of DOI:10.1039/c5cy02191j

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Application of iron-based metal-organic frameworks in catalysis: Oxidant-
promoted formation of coumarins using Fe3O(BPDC)3 as an efficient heterogeneous
catalyst
Thien N. Lieu, Khoa D. Nguyen, Dung T. Le, Thanh Truong*, Nam T. S. Phan*
Department of Chemical Engineering, HCMC University of Technology, VNUꢀHCM,
268 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
An ironꢀbased metalꢀorganic framework Fe3O(BPDC)3 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 FeꢀMOF could be used as an efficient heterogeneous
catalyst for the synthesis of coumarins from salicylaldehydes and activated methylene
compounds in the presence of tertꢀbutyl hydroperoxide as the oxidant under baseꢀfree
condition. The Fe3O(BPDC)3 was more catalytically active for the synthesis of coumarins
than other MOFs such as Cu2(BDC)2(BPY), Cu3(BTC)2, Cu2(BDC)2(DABCO),
Ni2(BDC)2(DABCO), and Fe3O(BDC)3. This FeꢀMOF also offered significantly higher
catalytic activity for the transformation than that of common metal salts such as FeCl3,
Fe(NO3)3, FeSO4, CuCl2, CuCl, Cu(NO3)2, and Ni(NO3)2. The Fe3O(BPDC)3 catalyst
could be recovered and reused several times without a significant degradation in catalytic
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activity. To the best of our knowledge, the oxidantꢀpromoted formation of coumarins
under heterogeneous catalysis conditions was not previously mentioned in the literature.
Keywords: Metalꢀorganic framework; coumarins; Fe3O(BPDC)3; heterogeneous catalyst;
oxidantꢀpromoted.
1. Introduction
Coumarins and their derivatives have emerged as important structural motifs in many
pharmaceutical candidates, fragrances, cosmetics, food additives, agrochemicals, and
molecular photonic devices [1, 2]. Classically, these structures could be synthesized by
numerous methods, including the Pechmann condensation [3, 4], the Knoevenagel
condensation [5, 6], the Perkin condensation [7], the Wittig reaction [8], and the Claisen
rearrangement [9]. These approaches still suffer from some drawbacks such as drastic
acidic or basic conditions, tedious manipulation, and lengthy workꢀup procedures [10].
Transition metalꢀcatalyzed formation of coumarins and their derivatives was also
reported. Kirai and Yamamoto previously demonstrated the synthesis of coumarins via
the Cu(OAc)2ꢀcatalyzed hydroarylation reaction between arylpropionic acid methyl esters
having a MOMꢀprotected hydroxy group at the ortho position and arylboronic acids [11].
Alper and coꢀworkers performed the Pd(OAc)2ꢀcatalyzed oxidative cyclocarbonylation of
2ꢀvinylphenols to produce various coumarins in the presence of low pressures of CO, and
air or 1,4ꢀ benzoquinone as the oxidant [12]. Iwasawa and coꢀworkers pointed out that
coumarins could be formed by the Pd(OAc)2ꢀcatalyzed direct carboxylation of alkenyl Cꢀ
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H bonds of 2ꢀhydroxystyrenes under atmospheric pressure of CO2 [13]. Recently, Shang
and coꢀworkers reported the synthesis of coumarins via FeCl3ꢀcatalyzed cascade reactions
of salicylaldehydes and activated methylene compounds with or without using a base
[14]. To develop greener protocols for the synthesis of coumarins in terms of the ease of
handling, simple workup, recyclability and reusability, heterogeneous catalysts should be
investigated.
Crystalline porous metalꢀorganic frameworks (MOFs) have attracted significant attention
in the last decade due to their potential applications in many fields, including energy
technologies, gas storage, gas separation, sensors, optics, biomedicals, and catalysis [15ꢀ
20]. Containing both organic linkers and metal ions in the frameworks, MOFs possess
several interesting properties, such as wellꢀdefined structures, high surface areas, high
porosity, structural diversity, the ability to tune the pore size, and the possibility to
modify the surface hydrophobicity/hydrophilicity [15, 16, 21ꢀ28]. Although the
application of MOFs in catalysis is relatively lagging behind the application in gas
capture and storage, it is expected that organic transformations using MOFs as
heterogeneous catalysts or catalyst supports would be extensively investigated in the near
future [29ꢀ31]. Either the catalytically active sites could be present on the framework or
they could be achieved via postꢀfunctionalization [30, 32ꢀ38]. During the last few years, a
variety of MOFs have been explored for catalysis applications, including both carbonꢀ
carbon [39ꢀ50] and carbonꢀheteroatom forming transformations [51ꢀ59]. The application
of ironꢀbased metalꢀorganic frameworks in catalysis was previously mentioned in the
literature, focusing on oxidation transformations [60ꢀ65]. In this work, we wish to report
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the synthesis of coumarins from salicylaldehydes and activated methylene compounds
using the ironꢀbased metalꢀorganic framework Fe3O(BPDC)3 as an efficient
heterogeneous catalyst in the presence of an oxidant under baseꢀfree condition. To the
best of our knowledge, the oxidantꢀpromoted formation of coumarins under
heterogeneous catalysis conditions was not previously mentioned in the literature.
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 analyzer system. Samples were pretreated by heating under
o
vacuum at 150 C for 3 h. A Netzsch Thermoanalyzer STA 409 was used for
o
thermogravimetric analysis (TGA) with a heating rate of 10 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 Hitachi Sꢀ4800 Scanning Electron Microscope
(SEM). Transmission electron microscopy studies were performed using a JEOL JEM
1400 Transmission Electron Microscope (TEM) at 100 kV. The Fe3O(BPDC)3 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.
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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
o
o
GC analysis held samples at 100 C for 1 min; heated them from 100 to 280 C at 40
o
oC/min; held them at 280 C for 2 min. Inlet and detector temperatures were set constant
o
at 280 C. Diphenyl ether was used as an internal standard to calculate reaction
conversions. GCꢀMS analyses were performed using a Shimadzu GCMSꢀQP2010Ultra
with a ZBꢀ5MS column (length = 30 m, inner diameter = 0.25 mm, and film thickness =
0.25 ꢁm). The temperature program for GCꢀMS analysis held samples at 50 oC for 2 min;
o
heated samples from 50 to 280oC at 10 oC/min and held them at 280 C for 10 min. Inlet
o
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 Fe3O(BPDC)3
In a typical preparation, a solid mixture of H2BPDC (H2BPDC = biphenylꢀ4,4’ꢀ
dicarboxylic acid; 0.08g, 0.33mmol), acetic acid (CH3COOH; 0.4ml, 7mmol), and
FeCl3·6H2O (0.1665g, 0.62mmol) was dissolved in DMF (DMF = N,N’ꢀ
dimethylformamide; 39 ml). The mixture was magnetically stirred for 1 h to achieve a
clear solution. The resulting solution was then distributed in ten 8ꢀml vials. The vials
o
were heated at 120 C in an isothermal oven for 48 h. After cooling the vials to room
temperature, the solid product was removed by decanting with mother liquor and washed
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with DMF (3× 10 ml) for three days. Solvent exchange was carried out with
dichloromethane (3× 10 ml) at room temperature for three days. The material was then
o
o
dried at 120 C for 24 h, and then at 150 C for 2 h under vacuum, yielding 0.0755g of
the Fe3O(BPDC)3 as orange crystals (76 % based on H2BPDC).
2.3. Catalytic studies
In a typical experiment, a mixture of salicylaldehyde (0.122 g, 1 mmol), and
diphenylether (0.1 ml, 0.8 mmol) as an internal standard in DMF (4 ml) was added into a
25 ml round bottom flask containing the Fe3O(BPDC)3 catalyst (0.015 g, 5 mol%). The
catalyst loading was calculated with respect to the iron/salicylaldehyde molar ratio. The
mixture was magnetically stirred for 3 min to disperse the FeꢀMOF catalyst in the liquid
phase. Methyl acetoacetate (0.348 g, 3 mmol) and tertꢀbutyl hydroperoxide in water
(0.032 g, 0.25 mmol) were then added. The resulting mixture was magnetically stirred at
o
60 C for 3 h. Reaction yield was monitored by withdrawing aliquots from the reaction
mixture at different time intervals, quenching with pure water (1 ml). The organic
components were then extracted into ethyl acetate (2 ml), dried over anhydrous Na2SO4,
analyzed by GC with reference to diphenylether. The product identity was further
1
confirmed by GCꢀMS, H NMR, and 13C NMR. To investigate the recyclability of the
Fe3O(BPDC)3, the catalyst was separated from the reaction mixture by simple
centrifugation, washed with copious amounts of dichloromethane, activated under
o
vacuum at 150 C for 3 h, and reused if necessary. For the leaching test, a catalytic
reaction was stopped after 60 min, analyzed by GC, and centrifuged to remove the solid
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catalyst. The reaction solution was then stirred for a further 120 min. Reaction progress,
if any, was monitored by GC as previously described.
3. Results and discussion
O
CHO
OH
Fe3O(BPDC)3
O
O
+
tert-butyl hydroperoxide
O
O
O
DMF
Scheme 1. The synthesis of 3ꢀacetylcoumarin from salicylaldehyde and methyl
acetoacetate using Fe3O(BPDC)3 catalyst.
The ironꢀbased metalꢀorganic framework Fe3O(BPDC)3 was synthesized according to a
slightly modified literature procedure [66], and was characterized by several techniques,
including XRD, SEM, TEM, TGA, FTꢀIR, AAS, and nitrogen physisorption
measurements (Fig. S1 – Fig. S7). The FeꢀMOF was assessed for its catalytic activity in
the reaction of salicylaldehyde with methyl acetoacetate to form 3ꢀacetylcoumarin as the
principal product (Scheme 1). Initial studies addressed the effect of temperature on the
yield of 3ꢀacetylcoumarin. Shang and coꢀworkers previously carried out the synthesis of
coumarins via FeCl3ꢀcatalyzed cascade reactions of salicylaldehydes and activated
o
methylene compounds at 80 C [14]. Kittakoop and coꢀworkers demonstrated that the
o
same reaction using a tendon hydrolysate as an organocatalyst could occur at 50 C,
though the reaction time was extended to more than 24 h [67]. The reaction was then
carried out at 5 mol% Fe3O(BPDC)3 catalyst in DMF for 180 min, using 3 equivalents of
methyl acetoacetate and 0.75 equivalent of tertꢀbutyl hydroperoxide as the oxidant, at
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o
room temperature, 50, 60, 70, 80, 90, and 100 C, respectively. It was observed that the
reaction could not occur at room temperature, with no trace amount of 3ꢀacetylcoumarin
o
being detected while reactions at 50 C afforded 45 % yields. Increasing the reaction
o
temperature to 60 C led to a significant enhancement in the reaction rate, with 75% GC
yield being achieved after 180 min. Performing the reaction at higher temperature was
found to be unnecessary as the yield of 3ꢀacetylcoumarin was not improved any further
(Fig. 1 and Fig. S8). It is noted that less than 2 % of other products in the reaction
mixture according to GC and LC analysis, indicating the excellent reaction selectivity at
tested conditions.
100 oC
50 oC
80 oC
70 oC
60 oC
90 oC
RT
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 1. Effect of temperature on the reaction yield.
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Effect of the catalyst concentration on the reaction yield was then kinetically
investigated. The reaction was then carried out at 1 mol%, 3 mol%, and 5 mol%
Fe3O(BPDC)3 catalyst, respectively. It should be noted that only 6% yield was detected
after 180 min in the absence of the Fe3O(BPDC)3 catalyst, confirming the necessity of the
FeꢀMOF for the reaction of salicylaldehyde with methyl acetoacetate. The reaction using
1 mol% Fe3O(BPDC)3 catalyst proceeded with difficulty, affording 48% yield after 180
min. The reaction yield could be improved to 64% in the presence of 3 mol%
Fe3O(BPDC)3 catalyst. Increasing the catalyst concentration to 5 mol% led to 75% yield
3ꢀacetylcoumarin (Fig. 2). Previously, the FeCl3ꢀcatalyzed cascade reactions of
salicylaldehydes and activated methylene compounds to form coumarins also required 5
mol% catalyst [14]. It was also observed that the amount of methyl acetoacetate exhibited
a dramatic effect on the reaction yield. The Fe3O(BPDC)3ꢀcatalyzed reaction using 2
equivalents of methyl acetoacetate proceeded to only 60% yield, while only 34% yield
was observed for the case of 1 equivalent of methyl acetoacetate (Fig. 3).
5% mol
100
3% mol
1% mol
0% mol
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
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Fig. 2. Effect of catalyst concentration on the reaction yield.
1:3
1:2
1:1
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 3. Effect of reagent molar ratio on the reaction yield.
As mentioned earlier, coumarins were classically synthesized under basic or acidic
conditions. In this work, it was found that the Fe3O(BPDC)3ꢀcatalyzed reaction of
salicylaldehyde with methyl acetoacetate to form 3ꢀacetylcoumarin could be promoted by
an oxidant under baseꢀfree condition. In order to investigate the impact of an oxidant on
the transformation, the reaction was carried out using 0.125, 0.25, 0.75, and 1.5
equivalents of tertꢀbutyl hydroperoxide as the oxidant, respectively. It should be noted
that no reaction occurred in the absence of the oxidant with no trace amount of product
being detected. Interestingly, experimental data indicated that the amount of the tertꢀbutyl
hydroperoxide exhibited a dramatic effect on the reaction. The reaction using 0.75
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equivalent of tertꢀbutyl hydroperoxide afforded 75% yield. Unexpectedly, increasing the
amount of tertꢀbutyl hydroperoxide led to a significant drop in the reaction rate, with only
62% yield being observed. It was found that the Fe3O(BPDC)3ꢀcatalyzed reaction of
salicylaldehyde with methyl acetoacetate could proceed readily to 91% yield in the
presence of 0.25 equivalent of tertꢀbutyl hydroperoxide. However, using less than 0.25
equivalent of tertꢀbutyl hydroperoxide resulted in only 64% yield (Fig. 4). These
observations indicated that the oxidant would play an important role in the catalytic cycle
for the formation of coumarins.
1.5 equiv.
0.25 equiv.
0 equiv.
0.75 equiv.
0.125 equiv.
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 4. Effect of oxidant concentration on the reaction yield.
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TBHP (in water)
Cumyl hydroperoxide
K2S2O8
TBHP (in decane)
H2O2
DABCO
Pyridine
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 5. Effect of oxidant on the reaction yield.
Moreover, it was also found that the nature of the oxidant also exhibited a significant
impact on the formation of coumarins. The reactions using various types of oxidants
including hydrogen peroxide, tertꢀbutyl hydroperoxide in water, tertꢀbutyl hydroperoxide
in decane, cumyl hydroperoxide, and K2S2O8 as the oxidant were carried out. It was
found that hydrogen peroxide was not suitable for the Fe3O(BPDC)3ꢀcatalyzed reaction
of salicylaldehyde with methyl acetoacetate, with only 11% yield being detected after
180 min. Tertꢀbutyl hydroperoxide in decane was found to be significantly less active
than tertꢀbutyl hydroperoxide in water, affording only 64% yield for the reaction. It
should be noted that the reaction using tertꢀbutyl hydroperoxide in water as the oxidant
could proceed to 91% yield. This could be rationalized based on the fact that using tertꢀ
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butyl hydroperoxide in decane would decrease the polarity of the reaction solvent. The
reaction using cumyl hydroperoxide as the oxidant also proceeded with slower yield than
that using tertꢀbutyl hydroperoxide in water, though 72% yield was still observed after
180 min. K2S2O8 was found to be totally ineffective for the reaction, with no trace amount
of 3ꢀacetylcoumarin being detected (Fig. 5). Indeed, the oxidation of salicylaldehyde to
its corresponding carboxylic acid as the byꢀreaction would be one of the reasons leading
to these observations.
5 mol% catalyst
adding acid ascorbic
adding TEMPO
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 6. Effect of ascorbic acid and TEMPO on the reaction yield.
To gain insights into the pathway of the Fe3O(BPDC)3ꢀcatalyzed reaction of
salicylaldehyde with methyl acetoacetate, further mechanistic studies were carried out.
The reaction was carried out at 60 oC in DMF for 180 min, using 3 equivalents of methyl
acetoacetate, in the presence of 5 mol% Fe3O(BPDC)3 catalyst, with 0.25 equivalent of
tertꢀbutyl hydroperoxide as the oxidant. After the first 60 min reaction time with 49%
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yield, 15 mol% of ascorbic acid as the antioxidant was added to the reaction solution. The
o
resulting mixture was stirred for an additional 120 min at 60 C with aliquots being
sampled at different time intervals, and analyzed by GC. The reaction was stopped in the
presence of the antioxidant, with no further 3ꢀacetylcoumarin being produced. In a
second experiment, 15 mol% of (2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀyl)oxy (TEMPO), was
introduced to the reaction mixture after the first 60 min. It was also observed that the
transformation was significantly affected by the radical trapping reagent (Fig. 6). It could
be proposed that the interaction of the radicals in the catalytic cycle with ascorbic acid or
TEMPO would stop the transformation. In a third experiment, the reaction was carried
out in the presence of pyridine as the catalyst poison. No reaction occurred in the
presence of pyridine, with no trace amount of product being detected after 180 min. It
should be noted that the reaction could not proceed in the absence of the Fe3O(BPDC)3
catalyst, even when tertꢀbutyl hydroperoxide was present in the reaction mixture. These
observations would confirm the function of the iron sites in the catalytic cycle. With
these mechanistic results, the tentative reaction route was proposed (Scheme 2).
Specifically, Knoevenagel condensation of acidic carbon in 1,3ꢀdiketones with aldehyde
resulted in α, βꢀunsaturated diketone [68]. Formation of tertꢀbutoxide radical from TBHP
under iron catalyst is wellꢀknown [69]. It was also reported that phenol derivative in the
presence of alkoxide radical and iron catalyst was converted into phenoxide radical
through hydrogen abstraction [70]. Subsequently, transesterification of phenoxide radical
with ester functional group in starting material via sequential single electron transfer led
to the formation of desired product [71].
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Scheme 2. Proposed reaction mechanism.
To further explore where the catalytic transformation occurred for the Fe3O(BPDC)3ꢀ
catalyzed reaction of salicylaldehyde with methyl acetoacetate, experiments using
grinded Fe3O(BPDC)3 were performed under identical conditions with aliquots being
sampled at different time intervals, and analyzed by GC. Indeed, smaller size crystals
normally exhibit higher catalytic activity due to the increased external surface of smaller
crystals [72]. DLS analysis showed the particle size distribution of the catalyst, with a
mean size of 35.8 µm and 25.6 µm being observed for the asꢀsynthesized and the grinded
Fe3O(BPDC)3, respectively (Fig. S9, Table S1). It was observed that slightly higher
yields of 3ꢀacetylcoumarin were obtained during the course of the reaction in the
presence of the grinded Fe3O(BPDC)3 as catalyst, though the difference was not
significant. Serri and coꢀworkers previously reported that Fe3O(BPDC)3 possessed 1D
channels of 10 × 11 Å2 along c axis [66]. In addition, based on MM2 calculation, the 3ꢀ
acetylcoumarin has the size of 4 × 9 Å2. Furthermore, pore flexibility in MOFs upon
temperature and guest molecules has been previously mentioned in the literature [73].
These observations would suggest that the Fe3O(BPDC)3ꢀcatalyzed reaction of
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salicylaldehyde with methyl acetoacetate to form 3ꢀacetylcoumarin would occur inside
the cavities as well as on the external surface of the FeꢀMOF. However, further studies
would be necessary to elucidate the issue.
non-grinded Fe3O(BPDC)3
grinded Fe3O(BPDC)3
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 7. Reactions with grinded Fe3O(BPDC)3.
Reaction solvents could have a profound impact on the rate of many organic
transformations using solid catalysts, depending on the nature of the catalyst [74, 75].
Shang and coꢀworkers previously carried out the synthesis of coumarins via FeCl3ꢀ
catalyzed cascade reactions of salicylaldehydes and activated methylene compounds in
several solvents, and pointed out that ethanol was the solvent of choice for the
transformation [14]. We therefore decided to investigate the effect of different solvents
on the Fe3O(BPDC)3ꢀcatalyzed reaction of salicylaldehyde with methyl acetoacetate to
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form 3ꢀacetylcoumarin. In particular, DMF, DEF, DMAc, DMSO, 1,4ꢀdioxane,
acetonitrile, dichloromethane, toluene, and ethanol were subsequently employed.
Interestingly, it was found that all DMAc, DMSO, 1,4ꢀdioxane, acetonitrile,
dichloromethane, toluene, and ethanol were totally ineffective for the reaction, with no
trace amount of 3ꢀacetylcoumarin being detected after 180 min. DEF was also found to
be unsuitable for the reaction, affording only 19% yield after 180 min. Among these
solvents, DMF exhibited the best performance, with 91% yield being achieved after 180
min. We also tried to increase the polarity of the solvent by adding water to the system.
However, the reaction carried out in the mixture of DMF/water proceeded with lower
yield, producing 3ꢀacetylcoumarin in 58% and 49% yields for the case of 95/5 and 90/10
(v/v) DMF/water system (Fig. 8).
DMF
DMF (10% water)
DMSO
DMF (5% water)
DEF
DMAc
Dioxane
Acetonitrile
Dichloromethane
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 8. Effect of solvent on the reaction yield.
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To emphasize the significant benefit of using the Fe3O(BPDC)3 as catalyst, the catalytic
activity of the Fe3O(BPDC)3 was compared with that of other MOFs, including MILꢀ
100(Fe), Fe3O(BDC)3, Cu2(BDC)2(BPY), Cu3(BTC)2, Cu2(BDC)2(DABCO), and
Ni2(BDC)2(DABCO). The reaction was carried out in DMF at 60 oC for 180 min, using 3
equivalents of methyl acetoacetate, in the presence of 5 mol% MOFs catalyst, with 0.25
equivalent of tertꢀbutyl hydroperoxide as the oxidant. It was found that the reaction using
Fe3O(BDC)3 catalyst proceeded to 56% yield after 180 min, while 91% yield was
achieved for the case of Fe3O(BPDC)3. MILꢀ100(Fe) was found to be less active for the
transformation than Fe3O(BPDC)3, though 79% yield of 3ꢀacetylcoumarin was still
obtained. Other MOFs offered lower cataytic activity for the reaction than FeꢀMOFs.
Indeed, 51%, 45%, and 42% yields were obtained after 180 min for the reaction using
Cu2(BDC)2(BPY),
Cu3(BTC)2,
and
Cu2(BDC)2(DABCO),
respectively.
Ni2(BDC)2(DABCO) exhibited no catalytic activity for the transformation, with no trace
amount of product being detected after 180 min (Table 1). To further highlight the
advantages of the approach, the catalytic activity of the Fe3O(BPDC)3 was also compared
with that of several metal salts, including FeCl3, Fe(NO3)3, FeSO4, CuCl2, CuCl,
Cu(NO3)2, and Ni(NO3)2. The reaction using Cu(NO3)2 as catalyst afforded 22% yield
after 180 min, while less than 5% yields were detected for that using FeCl3, Fe(NO3)3,
FeSO4, CuCl2, CuCl, and Ni(NO3)2 (Fig. 9). Indeed, Shang and coꢀworkers previously
carried out the synthesis of coumarins via FeCl3ꢀcatalyzed cascade reactions of
salicylaldehydes and ethyl acetoacetate, and reported that the presence of Na2CO3 as a
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base was necessary for the transformation, though the base was not employed for the case
of other activated methylene compounds [14].
Table 1. Reactivity of other MOFs for 3ꢀacetylcoumarin synthesis under optimized
conditions a.
Metal content
(%)
BET
Pore volume
(cm3/g)
MOFs
GC Yield
(m2/g)
Fe3O(BPDC)3
(MILꢀ126)
Fe3O(BDC)3
(MOFꢀ235)
FeBTC
91
56
79
16.2
25.3 [76]
21.29
1641
540 [76]
876
0.655
0.359[76]
0.41
(MILꢀ100)
Cu2(BDC)2(DABCO)
Cu2(BDC)2(BPY)
42
51
22.53
20.83
1085 [77]
0.529 [77]
0.232 [78]
447.82 [78]
Cu3(BTC)2
(MOFꢀ199)
45
<2
29.5 [79]
21.06
1507 [80, 81]
1474 [83]
0.41 [82]
Ni2(BDC)2(DABCO)
a Kinetic studies of reactions using these catalysts were placed in Supporting Information
(Fig. S8)
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Fe3O(BPDC)3
FeCl3
CuCl2
Fe(NO3)3
FeSO4
CuCl
Cu(NO3)2
Ni(NO3)2
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 9. Different homogeneous catalysts for the synthesis of 3ꢀacetylcoumarin.
With these results in mind, we then carried out the leaching test to confirm if the catalysis
of active iron species leached from the solid Fe3O(BPDC)3 catalyst, if any, could
contribute to the total yield of the reaction between salicylaldehyde and methyl
acetoacetate to form 3ꢀacetylcoumarin. In many cases, the leached species from the solid
catalyst could contribute significantly to the reaction, and therefore the reaction would
not proceed under real heterogeneous catalysis conditions [75]. A control experiment was
carried out using a simple centrifugation during the course of the reaction. Apparently, if
more product was formed after the solid catalyst was separated from the reaction mixture,
this observation would indicate that there should be contribution from homogeneous
catalysis. The reaction was carried out under optimized condition. After the first 60 min
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reaction time, with 49% yield being detected, the Fe3O(BPDC)3 catalyst was removed
from the reaction mixture by simple centrifugation. The liquid phase was then transferred
o
to a new reactor vessel, and magnetically stirred for an additional 120 min at 60 C with
aliquots being sampled at different time intervals, and analyzed by GC. Experimental
results showed that the formation of 3ꢀacetylcoumarin could not proceed after the
Fe3O(BPDC)3 catalyst was separated from the reaction mixture, and 91% yield of 3ꢀ
acetylcoumarin was owing to the heterogeneous catalysis (Fig. 10).
5 mol%
Leaching
No catalyst
100
80
60
40
20
0
0
30
60
90
120
150
180
Time (min)
Fig. 10. Leaching test indicated no contribution from homogeneous catalysis of active
species leaching into reaction solution.
One of the issues that should be seriously addressed for organic reactions carried out
under heterogeneous catalysis conditions is the ability to recover and reuse the catalyst. It
would be expected that the solid catalyst could be reused several times for the
transformation before it lost activity completely. The Fe3O(BPDC)3 catalyst was therefore
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investigated for recoverability and reusability in the reaction between salicylaldehyde and
methyl acetoacetate to form 3ꢀacetylcoumarin over six successive runs, by repeatedly
separating the FeꢀMOF catalyst from the reaction mixture, washing it and then reusing it.
The reaction was carried out in DMF at 60 oC for 180 min, using 3 equivalents of methyl
acetoacetate, in the presence of 5 mol% Fe3O(BPDC)3 catalyst, with 0.25 equivalent of
tertꢀbutyl hydroperoxide as the oxidant. After 180 min reaction time with 91% yield of 3ꢀ
acetylcoumarin being achieved, the FeꢀMOF catalyst was separated from the reaction
mixture by simple centrifugation, washed with copious amounts of dichloromethane to
o
remove any physisorbed reagents, activated under vacuum at 150 C for 3 h, and reused
in further reactions under identical conditions to those of the first run. It was found that
the Fe3O(BPDC)3 catalyst could be recovered and reused several times for the reaction
between salicylaldehyde and methyl acetoacetate to form 3ꢀacetylcoumarin without a
significant degradation in catalytic activity. Indeed, 94% yield of 3ꢀacetylcoumarin was
still achieved in the 6th run (Fig. 11). As compared to the FTꢀIR spectra of the fresh
Fe3O(BPDC)3, similar absorption was observed for the 4th used FeꢀMOF catalyst (Fig.
S11). The XRD result of the the 2nd and the 4th used Fe3O(BPDC)3 revealed that the
crystallinity of the FeꢀMOF catalyst could be maintained, though slight difference in the
diffractogram was detected (Fig. S12).
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100
80
60
40
20
0
1
2
3
4
5
6
Runs
Fig. 11. Catalyst recycling studies.
Table 2. The formation of several coumarins containing different substituents in the
presence of Fe3O(BPDC)3 catalyst.
Entry Salicylaldehydes
1,3ꢀDiketones
Products
Yields (%)a
89
CHO
OH
O
O
O
O
1
O
O
O
Cl
CHO
OH
O
O
O
O
2
Cl
94b
O
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Br
CHO
OH
O
O
O
O
O
O
O
O
O
O
3
4
5
6
7
8
78b
Br
O2N
O
O
O
O
O
O
O2N
CHO
O
96
95
65
80
44
OH
O
O
O
O
CHO
OH
O
O
O
O
CHO
OH
O
O
O
CHO
O
O
O
O
O
OH
O
CHO
OH
O
O
O
O
O
O
28b
CHO
OH
O
O
O
9
O
O
O
O
O
a
Reaction conditions: aldehyde (1 mmol), methyl acetoacetate (3 mmol), tertꢀbutyl
hydroperoxide (70% in water) (0.25 eqv.), Fe3O(BPDC)3 (5 mol%), DMF (4 ml), 60 ꢀ80
b
°C, 3 ꢀ 9 h in open air, isolated yield. See Supporting Information for details. GC yield
due to poor separation.
The scope of this work was also expanded to the formation of several coumarins
containing different substituents in the presence of Fe3O(BPDC)3 catalyst. The reaction
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was carried out in DMF at 60 oC for 180 min, using the reagent molar ratio of 1:3, in the
presence of 5 mol% Fe3O(BPDC)3 catalyst, with 0.25 equivalent of tertꢀbutyl
hydroperoxide as the oxidant. In the first series of experiments, we carried out the
reaction between methyl acetoacetate and different salicylaldehyde derivatives, including
salicylaldehyde,
5ꢀchlorosalicylaldehyde,
5ꢀmethoxysalicylaldehyde,
5ꢀbromosalicylaldehyde,
5ꢀ
nitrosalicylaldehyde,
and 4ꢀmethoxysalicylaldehyde,
respectively. The product was purified by column chromatography using ethyl acetate/nꢀ
hexane (2/1, v/v) mixture. The Fe3O(BPDC)3ꢀcatalyzed reaction of salicylaldehyde with
methyl acetoacetate could afford 3ꢀacetylcoumarin in 89% isolated yield after 180 min. It
was observed that the presence of a substituent in salicylaldehyde decreased the yield of
the transformation. However, by increasing the temperature and the reaction time, high
yields of the desired products were achieved (See Supporting Information for details).
Furthermore, the reaction of salicylaldehyde with ethyl acetoacetate using Fe3O(BPDC)3
catalyst was also studied under the same reaction condition. Ethyl acetoacetate was found
to be less reactive in this transformation, with 80% isolated yield of 3ꢀacetylcoumarin
being achieved after 180 min. Dimethyl malonate was also tested in the Fe3O(BPDC)3ꢀ
catalyzed transformation, affording methyl 2ꢀoxoꢀ2Hꢀchromeneꢀ3ꢀcarboxylate in a yield
of 28% after 180 min. It was found that the reaction between salicylaldehyde and methyl
3ꢀoxoꢀ3ꢀphenylpropanoate using Fe3O(BPDC)3 catalyst could proceed to 44% yield of 3ꢀ
benzoylcoumarin after 180 min (Table 2).
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4. Conclusions
In summary, the ironꢀbased metalꢀorganic framework Fe3O(BPDC)3 was synthesized by
a solvothermal method, and was characterized by several techniques including XRD,
SEM, TEM, FTꢀIR, TGA, AAS, and nitrogen physisorption measurements. The FeꢀMOF
could be used as an efficient heterogeneous catalyst for the synthesis of coumarins from
salicylaldehydes and activated methylene compounds in the presence of tertꢀbutyl
hydroperoxide as the oxidant. The transformation could proceed readily in the presence
of a catalytic amount of the FeꢀMOF without using a base. The Fe3O(BPDC)3 was more
catalytically active for the synthesis of coumarins than other MOFs such as
Cu2(BDC)2(BPY), Cu3(BTC)2, Cu2(BDC)2(DABCO),
Ni2(BDC)2(DABCO), and
Fe3O(BDC)3. This FeꢀMOF also exhibited significantly higher activity for the
transformation than that of common metal salts such as FeCl3, Fe(NO3)3, FeSO4, CuCl2,
CuCl, Cu(NO3)2, and Ni(NO3)2. The coumarin formation could only be detected in the
presence of the solid Fe3O(BPDC)3 catalyst, and the contribution of leached active iron
species, if any, was negligible. The FeꢀMOF catalyst could be recovered and reused
several times for the synthesis of coumarins from salicylaldehydes and activated
methylene compounds without a significant degradation in catalytic activity. The fact that
this transformation could proceed under heterogeneous and baseꢀfree conditions should
be of advantages, and might be interested to the chemical industry.
Acknowledgements
The Viet Nam National Foundation for Science and Technology Development ꢀ
NAFOSTED is acknowledged for financial support through project code 104.05ꢀ2014.77
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