A magnetic metal-organic framework as a highly active heterogeneous catalyst for one-pot synthesis of 2-substituted alkyl and aryl(indolyl)kojic acid derivatives

Article abstract of DOI:10.1039/c7nj01592e

A novel magnetic metal-organic framework, NiFe₂O₄@MOF-5, has been prepared and characterized using spectroscopic, microscopic, and magnetic techniques (FT-IR, XRD, TEM, SEM, EDX, and VSM). This magnetically separable catalyst exhibit

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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ARTICLE TYPE
Magnetic metal-organic framework as a highly active heterogeneous
catalyst for one-pot synthesis of 2-substituted alkyl and
aryl(indolyl)kojic acid derivatives
Hong-Yan Zhang, Xiao-Peng Hao, Li-Ping Mo, Sha-Sha Liu, Wen-Bo Zhang and Zhan-Hui Zhang*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
A novel magnetic metalꢀorganic framework NiFe2O4@MOFꢀ5 has been prepared and characterized by
spectroscopic, microscopic and magnetic techniques (FTꢀIR, XRD, TEM, SEM, EDX and VSM). This
magnetically separable catalyst exhibited high catalytic activity for synthesis of a variety of 2ꢀsubstituted alkyl and
10 aryl(indolyl)kojic acid derivatives via oneꢀpot, threeꢀcomponent reaction of aldehyde, indole and kojic acid under
solventꢀfree conditions. The catalyst can be easily magnetically separated and exhibits significant recyclability,
with no significant loss of activity after six consecutive runs.
Introduction
filtration and centrifugation.15 Among the various magnetic
45 materials, NiFe2O4 with the general formula (AB2O4) is one of
the most versatile magnetic materials because it has high
saturation magnetization, strong chemical stability, low toxic and
relatively high permeability.16 If the MOF is bound to the
magnetic component, the resulting magnetic MOF will has a
50 magnetic susceptibility and allow the catalyst to facilitate
separation and recycle from the reaction medium by applying an
external magnet after the reaction. As far as we know, there are
few examples on synthesis and application of magnetic MOFs in
organic synthesis.17
Moreover, multicomponent reactions (MCRs) have been
shown to be able to efficiently obtain a complex target compound
with attractive biological properties in a pot mode to ensure
atomic economy, high total yield and high selectivity, minimize
waste, labor and manpower, reduce timeꢀconsuming, and avoid
From the perspective of green and economic chemistry, the
15 design of efficient and recoverable heterogeneous catalysts has
become of paramount important topics of research in synthetic
organic chemistry, material science and engineering. Metalꢀ
organic frameworks (MOFs) as a class of porous materials from
metal ions or metallic clusters and multidirectional organic
20 linkers have summoned substantial amount of attention in the
past decade with promising performance in gas adsorption and
separations, chemical sensors, thin film devices, drug carriers,
and biomedical imaging owing to their incredibly large specific
surface area, tunable pore distribution, abundant aromatic ligands,
25 outstanding mechanical and solvent stability, high acidꢀbase
catalytic activities and ease of functionalization.1 Some MOFs
based on Cu, Fe, Co, Zr, Zn and Al have also emerged as
effective heterogeneous catalysts for several organic reactions
55
60 complex purification processes.18 The combination the merits of
multicomponent reactions with the advantage of easily recovered
heterogeneous catalyst can meet the requirements of green
chemistry.
such
as
oxidation,2
epoxidation,3
cyanosilylation,4
30 hydroxylation,5 acetalization,6 cyclization,7 cycloaddition,8
condensation reaction,9 coupling reactions,10 ring opening
reaction,11 Friedel–Crafts reaction,12 Friedländer reaction,13 as
well as multiꢀcomponent reaction.14
Heterocyclic compounds containing indole or kojic acid
65 nucleus have been found numerous applications in medical,
agrochemicals and functional materials. The indole nucleus is
another important structural motif and occurs in many
biologically active natural products, alkaloids, pharmaceuticals
and other functional molecules.19 Kojic acid is a naturally
70 occurring fungal metabolite produced from carbohydrate sources
in an aerobic process by various fungi and bacteria and has
shown antimicrobial, antibacterial, and antifungal activities. In
addition, kojic acid and its derivatives have been used in the food
industry as an additive to prevent the browning of food materials
In particular, the magnetic supported catalyst can be
35 separated from the reaction medium by applying an external
magnetic field due to the paramagnetic properties of the support.
It can make the catalyst removed and recycled more easily than
____________________________________________________
College of Chemistry and Material Science, Hebei Normal University,
40 Shijiazhuang 050024, China.
Eꢀmail: zhanhui@mail.nankai.edu.cn; Fax: +86ꢀ311ꢀ80787431
†Electronic Supplementary Information (ESI) available: 1H NMR and 13
C
NMR spectra of all compounds. See DOI: 10.1039/b000000x/
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and utilized as a skinꢀlightener and antibrowning agent in
cosmetic products owing to their potent tyrosinase inhibition
activity.20 In this regard, we sought to explore the establishment
of a single structural framework by combining indole and kojic
acid motifs which could enhance biological and pharmacological
activity.
5
Bearing these facts in mind and continuing our efforts toward
the design of magnetic nanocatalysts21 and sustainable synthesis
development,22 herein, we report on the preparation of a new type
10 of magnetic metalꢀorganic framework NiFe2O4@MOFꢀ5 and its
application for the synthesis of 2ꢀsubstituted alkyl and
aryl(indolyl) kojic acid derivatives via oneꢀpot, threeꢀcomponent
reaction of aldehyde, indole and kojic acid under solventꢀfree
Fig. 1 XRD pattern of MOFꢀ5, NiFe2O4 and NiFe2O4@MOFꢀ5
50
The composition of NiFe2O4@MOFꢀ5 was further investiged
by fourier transform infrared (FTꢀIR) (Fig. 2). Compared to that
of NiFe2O4, the spectrum of NiFe2O4@MOFꢀ5 displayed
additional adsorption bands which are associated with the MOFꢀ5
structure. For example, characteristics peaks at 1685, 1482, 1420
conditions (Scheme 1).
R1
O
O
O
NiFe2O4@MOF-5
solvent-free, 110 o
OH
OH
R1CHO
+
R
+
R
C
N
HO
N
HO
H
H
O
1
4
3
2
15
55 cmꢀ1, and 1284, 732cmꢀ1 are assigned to the C=O and CꢀH in the
BDC ring, which clearly indicates the presence of MOFꢀ5. The
presence of NiꢀO and FeꢀO bonds in the magnetic particles was
confirmed by the characteristic peak appeared at 590 cmꢀ1.
Elemental mapping by energyꢀdispersive Xꢀray spectroscopy
60 (EDX) confirmed the presence of C, O, Fe, Ni and Zn elements in
the structure of the material (Figure 3). Further examination by
atomic absorption spectroscopy (AAS) for NiFe2O4@MOFꢀ5 was
also showed that the contents of Fe, Ni and Zn were 16.18%, 8.22
and 9.06%, respectively. The result indicated that Ni/Fe molar
65 ratio is about 1:2 and the amount of MOFꢀ5 supported on
NiFe2O4 nanoparticles is approximately 27.19% based on
Zn4O(BDC)3.24
Scheme 1 NiFe2O4@MOFꢀ5 catalyzed one pot, threeꢀcomponent reaction
of aldehyde, indole and kojic acid
Results and discussion
As shown in Scheme 2, magnetic MOFꢀ5 (NiFe2O4@MOFꢀ5)
20 was prepared in a threeꢀstep process by using NiFe2O4 as a
magnetic core, the zinc ion as a connector and terephthalic acid
(H2BDC) as a linker. Firstly, the NiFe2O4 particles were
synthesized by a chemical coꢀprecipitation method using FeCl3
and NiCl2. Next, citrateꢀfunctionalized NiFe2O4 was obtained by
25 stirring of NiFe2O4 suspension in water solution of trisodium
citrate. Finally, MOFꢀ5 was deposited on citrateꢀfunctionalized
NiFe2O4 by dispersing NiFe2O4 in N,Nꢀdimethylformamide
(DMF) of zinc nitrate hexahydrate and H2BDC and heating at
130 °C in a teflonꢀlined steel autoclave. The morphology,
30 structure, and composition of asꢀprepared NiFe2O4@MOFꢀ5
nanospheress are investigated by different characterizing
techniques.
Zn(NO3)2• 6H2O
FeCl3
NaOH
H2BDC
sodium citrate
+
DMF
NiCl2
citrate-functionalized
NiFe2O4
NiFe2O4
NiFe2O4@MOF-5
35
Scheme 2 Synthesis of NiFe2O4@MOFꢀ5
The crystalline nature of NiFe2O4@MOFꢀ5 was determined
by using Xꢀray diffraction analysis (Fig. 1). The diffraction peaks
40 at 8.70o, 17.33o, 27.91o provide evidences to support the existence
of MOFꢀ5. The diffraction peaks of NiFe2O4@MOFꢀ5 2θ =
30.29o, 35.70o, 37.31o, 43.36o, 53.80o, 57.36o and 62.92o are
associated to the corresponding indices of the (220), (311), (222),
(400), (422), (511) and (440), which can be well indexed to the
45 cubic spinel phase of NiFe2O4 (JCPDS 10ꢀ0325).23 The results
demonstrated that the two crystalline phases of NiFe2O4 and
MOFꢀ5 were presented in NiFe2O4@MOFꢀ5.
Fig. 2 IR spectra of NiFe2O4, MOFꢀ5, NiFe2O4@MOFꢀ5 (fresh) and
70 NiFe2O4@MOFꢀ5 (recovered)
2
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Fig. 3 EDS spectra of MOFꢀ5, NiFe2O4 and NiFe2O4@MOFꢀ5
The scanning electron microscopy (SEM) observation showed
that NiFe2O4@MOFꢀ5 was composed of relatively uniform quasiꢀ
spherical particles (Fig. 4). The estimated mean diameter of these
particles is around 40ꢀ60 nm, which shows a quite agreement
with the values calculated by XRD data. The transmission
electron microscopy (TEM) showed formation of MOF shells on
the surfaces of the magnetic nanoparticles (Fig. 5).
Fig.
6
N2 adsorption/desorption curves of MOFꢀ5, NiFe2O4 and
5
NiFe2O4@MOFꢀ5
The thermal stability of NiFe2O4@MOFꢀ5 was examined by
25 carrying out TGA experiments. As shown in Fig. 7, two main
steps of weight loss were observed. The first step displayed a
weight loss of 2.11% that attributable to the loss of adsorbed
solvent or water molecules before 140 oC. The second step
o
o
showed a weight loss about 12.47 wt% from 140 C to 450 C,
30 resulting from thermal decomposition of MOFꢀ5.
10
Fig. 4 SEM profile of MOFꢀ5 (a) NiFe2O4 (b) and NiFe2O4@MOFꢀ5 (c)
Fig. 5 TEM profile of MOFꢀ5 (a) NiFe2O4 (b) and NiFe2O4@MOFꢀ5 (c)
The BET surface area of the MOFꢀ5 and NiFe2O4@MOFꢀ5
15 was confirmed by N2 adsorption experiments at 77 K (Figure 6).
The surface areas of MOFꢀ5 and NiFe2O4@MOFꢀ5 were
calculated by the BrunauerꢀEmmettꢀTeller method to be 240 and
123 m2gꢀ1, respectively. The appreciable decreases in surface area
indicated that the cavities of MOFꢀ5 were partially occupied by
20 dispersed NPs or/and blocked by the NPs located on the surface.
Fig. 7 TGA profile of MOFꢀ5, NiFe2O4 and NiFe2O4@MOFꢀ5
The magnetic properties of the NiFe2O4 and NiFe2O4@MOFꢀ5
were evaluated by vibrating sample magnetometery (VSM) at
35 room temperature. As illustrated in Fig. 8, the magnetic saturation
(MS) values of the NiFe2O4 MNPs is 51.2 emugꢀ1, and the
NiFe2O4@MOFꢀ5 composite is 19.6 emugꢀ1. The slightly lower
MS value of NiFe2O4@MOFꢀ5 mainly results from the
introduction of MOFꢀ5. The presence of suitable magnetic
40 properties allows NiFe2O4@MOFꢀ5 to be completely, efficiently
and rapidly separated from the reaction mixture by an external
magnet.
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Table 1. Reaction of benzaldehyde, indole and kojic acid under different
reaction conditionsa
Entry catalyst
Solvent Temperature (oC) Time/h Yield/% c
1
2
3
4
5
6
7
8
9
no
no
no
no
no
no
110
5.0
2.0
2.0
2.0
2.0
2.5
3.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
trace
10
Nano Fe3O4
Nano CuFe2O4
Nano CoFe2O4
Nano NiFe2O4
110
110
21
110
23
110
30
NiFe2O4@CuꢀBTC no
NiFe2O4@FeꢀMIL no
NiFe2O4@MOFꢀ5 no
NiFe2O4@MOFꢀ5 H2O
110
75
110
48
110
92
reflux
reflux
reflux
trace
10
10 NiFe2O4@MOFꢀ5 THF
11 NiFe2O4@MOFꢀ5 EtOH
trace
trace
trace
trace
trace
trace
trace
40
Fig. 8 Magnetization curve of NiFe2O4 and NiFe2O4@MOFꢀ5
12 NiFe2O4@MOFꢀ5 nꢀHexanereflux
13 NiFe2O4@MOFꢀ5 Toluene reflux
The activity of the prepared catalyst was tested for oneꢀpot
threeꢀcomponent reaction of benzaldehyde, indole and kojic acid
under solventꢀfree conditions. As shown in Table 1, almost no
reaction occurred in the absence of catalyst and the starting
materials were recovered when the reaction mixture was heated at
14 NiFe2O4@MOFꢀ5 DMF
15 NiFe2O4@MOFꢀ5 no
16 NiFe2O4@MOFꢀ5 no
17 NiFe2O4@MOFꢀ5 no
18 NiFe2O4@MOFꢀ5 no
19 NiFe2O4@MOFꢀ5 no
20 NiFe2O4@MOFꢀ5 no
21c NiFe2O4@MOFꢀ5 no
22d NiFe2O4@MOFꢀ5 no
23e NiFe2O4@MOFꢀ5 no
24f NiFe2O4@MOFꢀ5 no
110
25
5
40
60
o
110 C under solventꢀfree conditions (Table 1, entry 1). We next
80
investigated this reaction by using some magnetic nano catalysts
100
120
110
110
110
110
84
10 such as Fe3O4, CuFe2O4, CoFe2O4 and NiFe2O4, the desired
product 4a can be isolated in very low yields (Table 1, entries 2ꢀ
5). This result encouraged us to optimize the reaction paramters
to enhance the yield of 4a on this model reaction. Pleasingly, we
found that this reaction proceeded smoothly to afford the target
15 product 4a in 92% yield in the presence of our prepared
NiFe2O4@MOFꢀ5 (Table 1, entry 8). In comparison, low
catalytic activity was observed with other magnetic MOF such as
NiFe2O4@CuꢀBTC or NiFe2O4@FeꢀMIL as catalyst (entries 6
and 7). Some solvents such as water, tetrahydrofuran, ethanol, nꢀ
20 hexane, toluene, N,Nꢀdimethylformamide were examined, but the
results were disappointing. The yield was greatly affected by the
reaction temperature. A higher reaction temperature (120 oC)
does not make an obvious difference in the yield of product
(entry 20), but a lower temperature resulted in no product or gave
25 poor yield (Table 1, entries 15ꢀ19). The catalytic amount of
NiFe2O4@MOFꢀ5 was found to be crucial. Decreasing the
amount of catalyst from 20 to 15 and 10 mg led to a reduction in
yield to 86 and 77%, respectively (entries 21 and 22). Increasing
the amount of catalyst to 25 mg did not affect the reaction.
30 Consequently, performing the reaction in the presence of 20 mg
NiFe2O4@MOFꢀ5 at 110 oC under solventꢀfree was chosen as the
optimal conditions.
92
77
86
92
93
a Reaction condition: benzaldehyde (1 mmol), indole (1 mmol), kojic acid
(1 mmol), catalyst ( 20 mg) in solvent (2 ml) otherwise specified in the
c
d
table. b Isolated yields. 10 mg catalyst was used. 15 mg catalyst was
e
f
used. 25 mg catalyst was used. The reaction was carried out in 50
mmol scale.
Under the optimal reaction conditions, the scope of substrates
45 for the synthesis of 2ꢀsubstituted alkyl and aryl(indolyl) kojic
acid derivatives 4 was investigated and the selected results are
outlined in Table 2. It was generally observed that high to
excellent yields of the products were obtained for aromatic
aldehydes. Electronꢀdonating and
electronꢀwithdrawing
50 substituents on the aromatic ring of benzaldehyde were wellꢀ
tolerated and show no significant influence on the reaction times
and the yields of the products. In addition, 1ꢀnaphthaldehyde was
also a suitable substrate and gave the desired product 4n in 92%
yield (Table 2, entry 14). This reaction was also successful for
55 heteroaromatic carbaldehydes such as 5ꢀmethylꢀ2ꢀfuraldehyde, 2ꢀ
thiopheneꢀcarbaldehyde and 1Hꢀindoleꢀ3ꢀcarbaldehyde (Table 2,
entries 15ꢀ17). To our satisfaction, aliphatic aldehydes such as
cyclohexanecarbaldehyde and lilestralis are also approprite
staring materials under these reaction conditions, but give
60 relatively low yields (Table 2, entries 18 and 19).
To demonstrate the practical usefulness of this method, we
also enlarged the reaction scale to 50ꢀmmol for the model
35 reaction. An isolated yield of 93% of brown solid of 4a was
obtained after workup and recrystallization.
40
4
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Table 2 NiFe2O4@MOFꢀ5 catalyzed oneꢀpot synthesis of 2ꢀsubstituted alkyl and aryl(indolyl)kojjc acid derivativesa
Entry
Aldehyde
R
Product
Time/min
Yield/%b
mp/oC
PhCHO
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
4a
4b
4c
4d
4e
4f
60
70
75
65
85
80
60
60
60
65
70
65
60
90
75
65
80
90
92
92
92
90
90
86
90
92
95
90
90
92
90
92
80
85
90
73
81ꢀ82(79ꢀ81)25
115ꢀ117
1
2ꢀOCH3C6H4CHO
3ꢀOCH3C6H4CHO
4ꢀOCH3C6H4CHO
4ꢀCH3C6H4CHO
3ꢀOHꢀC6H4CHO
2ꢀFC6H4CHO
2
93ꢀ95
98ꢀ99 (97ꢀ99)25
3
4
142ꢀ144
5
100ꢀ102
6
4g
4h
4i
140ꢀ142
7
4ꢀClC6H4CHO
4ꢀBrꢀC6H4CHO
2ꢀNO2C6H4CHO
3ꢀNO2C6H4CHO
4ꢀNO2ꢀC6H4CHO
4ꢀCNꢀC6H4CHO
1ꢀNaphthaldehyde
5ꢀMethylꢀ2ꢀfuraldehyde
2ꢀThiopheneꢀcarbaldehyde
1Hꢀindoleꢀ3ꢀcarbaldehyde
Cyclohexanecarbaldehyde
3ꢀ(4ꢀ(tertꢀButyl)phenyl)ꢀ2ꢀ
methylpropanal
PhCHO
93ꢀ95
8
90ꢀ91 (87ꢀ89)26
168ꢀ170 (169ꢀ171)26
170ꢀ171 (172ꢀ174)26
169ꢀ170 (167ꢀ169)26
98ꢀ100
9
4j
10
11
12
13
14
15
16
17
18
4k
4l
4m
4n
4o
4p
4q
4r
193ꢀ195
94ꢀ96
190ꢀ192
116ꢀ118
99ꢀ101
H
4s
180
70
98ꢀ100
19
4ꢀOCH3
4ꢀCH3
6ꢀF
4t
75
85
65
70
70
75
75
80
85
90
90
92
90
91
91
92
89
82
128ꢀ130
112ꢀ114
84ꢀ86
20
21
22
23
24
25
26
27
28
PhCHO
4u
4v
PhCHO
PhCHO
5ꢀCl
4w
4x
94ꢀ96
PhCHO
6ꢀCl
93ꢀ95
PhCHO
5ꢀBr
4y
86ꢀ88
PhCHO
6ꢀBr
4z
92ꢀ94
4ꢀOCH3C6H4CHO
PhCHO
5ꢀBr
4aa
4ab
107ꢀ109
116ꢀ118
1ꢀCH3
a Reaction conditions: aldehyde (1 mmol), indole (1 mmol), kojjc acid (1 mmol) and NiFe2O4@MOFꢀ5 (20 mg) under solventꢀfree condition at 110 oC.
b Isolated yield.
Scheme 3 Terephthalaldehyde was involved in the reactions
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intermediate II. The resulting enone intermediate II may undergo
conjugate addition with indole to give the intermediate III, which
further enolization resulted in final product 4a.
Subsequently, a variety of indoles were evaluated in this
threeꢀcomponent reaction. The results indicated that the position
and nature of the substituents on the phenyl ring of indoles had
little influence on the efficiency of the reaction. Notably,
halogenꢀsubstituted indoles were wellꢀtolerated under this
reaction to afford the corresponding 4wꢀ4aa, possessing chloro or
bromo groups amenable to further functionalization. In addition,
the scope of the reaction was further extended to Nꢀprotected
indole such as 1ꢀmethyindole and delivered product 4ab in 82%
O
HO
5
OH
O
O
Ph
H
H
O
H
HO
O
10 yield (Table 2, entry 28).
O
Stimulated by these results, we then extended the scope of this
reaction to a substrate containing two aldehyde groups such as
terephthalaldehyde. As shown in Scheme 3, treatment of
terephthalaldehyde (1 mmol) with indole (1 mmol) and kojic
15 acid (1 mmol) under above optimal conditions led to formation
of the corresponding product 4ac in 89% yield. However, the
reaction of terephthalaldehyde (1 mmol) with indole (2 mmol)
and kojic acid (2 mmol) furnished 5 as a major product.
Finally, the catalyst recyclability has been evaluated in the
20 model reaction of benzaldehyde, indole and kojic acid. After the
completion of reaction, ethyl acetate was added and the catalyst
was separated magnetically from the reaction mixtures and
washed with ethyl acetate. The recovered catalyst was dried, and
then reused directly for the next round under same reaction
25 conditions. As can be seen from Fig. 9, the reaction was repeated
for up to six consecutive runs without significant loss of catalytic
O
O
I
H2O
H
Ph
O
HO
O
Ph
O
II
O
O
HO
NH
Ph
H
O
O
OH
N
HO
NH
H
4a
O
III
50
Scheme 4 The plausible mechanism for the synthesis of 2ꢀsubstituted
alkyl and aryl(indolyl)kojic acid derivatives
activity
.
Conclusions
55
In conclusion, a novel type of magnetically recoverable
metalꢀorganic framework NiFe2O4@MOFꢀ5 was preparated and
characterized by XRD, FTꢀIR, TEM, SEM, TGA, EDX, and
VSM techniques. It was proved to be an efficient heterogenerous
catalyst for oneꢀpot threꢀcomponent reaction of aldehyde, indole
60 and kojic acid under solventꢀfree conditions. A wide of aldehydes
and indoles can be tolerated, giving the structural diversity of 2ꢀ
substituted alkyl and aryl(indolyl)kojic acid derivatives in good
to excellent yields. Morever, this novel heterogenerous catalyst
could be easily recovered by a magnet and showed no significant
65 loss of its activity even after six consective runs.
Fig. 9 Recyling of the catalyst
30
The leaching of metal from NiFe2O4@MOFꢀ5 was also
determined by inductively coupled plasma (ICP). It was found
that the contents of the Ni, Fe and Zn after sixth cycles were
0.60, 0.32 and 0.36 ppm in ethyl acetate (5 ml), respectively.
The result confirmed that only a slight loss of the catalyst
Experimental Section
General Methods: All reagents and solvents are commercially
available and were used as received without further purification.
Melting points were determined on an Xꢀ5 digital melting point
70 apparatus and are uncorrected. The FTꢀIR spectra were recorded
on a Bruker Tensor 27 fourier transform infrared spectroscope
using KBr pellets. Power Xꢀray diffraction data were obtained
from a PANalytical X’Pert Pro Xꢀray diffractometer using CuꢀKα
(λ = 1.5406 Å) radiation as the Xꢀray source in the range of 2θ
75 value between 20 and 80o at room temperature. Transmission
electron microscope (TEM) and energy dispersive Xꢀray
spectroscopy (EDX) analyses were carried out on a Hitachi Hꢀ
7650 instrument operating at 80 KV. Scanning electron
microscope (SEM) images of samples were obtained from a
80 Hitachi Sꢀ4800 instrument. Nitrogen adsorption–desorption
35 occurred during the course of workꢀup process. In addition, the
recovered catalyst was subjected to FTIR analysis. A result of
FTIR spectrum indicates that the catalyst was intact even after
sixth cycles of reactions (Fig. 2).
Based on the experimental results described above and
40 previous literature report,25 a plausible reaction mechanism for
this threeꢀcomponent reaction is proposed (Scheme 4). It is
assumed that catalyst plays an important role in promoting
reaction between benzaldehyde and kojic acid by activation of
carbonyl carbon of benzaldehyde through interaction between the
45 catalyst and the oxygen of the aldehyde carbonyl group to give
intermediate I, followed by dehydration generated an enone
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DOI: 10.1039/C7NJ01592E
measurements of the materials were detected on a Quantachrome
NOVA 4000e surface area analyzer and surface area was
calculated by the BET equation. Thermogravimetric analysis
(TGA) was performed under nitrogen using a NETZSCH STA
449 F3 thermogravimetric analyzer. The magnetic measurements
were carried out using a Physical Property Measurement System
55 crude product. The crude product was further purified by column
chromatography on silica gel using ethyl acetate/hexane as the
eluent.
5
Acknowledgements
This research was financially supported by the National
60 Natural Science Foundation of China (No. 21272053) and the
Natural Science Foundation of Hebei Province (No.
B2015205182).
1
(PPMSꢀ6700). All H and 13C NMR spectra of the products were
obtained from DMSO solution with TMS as an internal standard
using a Bruker DRXꢀ500 spectrometer at 500 and 125 MHz,
10 respectively. Mass spectra were performed on a 3200 Qtrap
instrument with an ESI source.
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DOI: 10.1039/C7NJ01592E
Graphical Abstract:
Magnetic metal-organic framework as a highly active heterogeneous
catalyst for one-pot synthesis of 2-substituted alkyl and aryl(indolyl)kojic
acid derivatives
Hong-Yan Zhang , Xiao-Peng Hao, Li-Ping Mo, Sha-Sha Liu, Wen-Bo Zhang and
Zhan-Hui Zhang*
A novel magnetic metal-organic framework NiFe2O4@MOF-5 was prepared and
demonstrated to be a high efficient and magnetic separable catalyst for one-pot
three-component reaction of aldehyde, indole and kojic acid.