Metal–organic framework of amine-MIL-53(Al) as active and reusable liquid-phase reaction inductor for multicomponent condensation of Ugi-type reactions

Article abstract of DOI:10.1002/aoc.3584

Metal–organic framework of NH₂-MIL-53(Al), with coordinative unsaturated aluminium sites, has been shown to be active in the Groebke–Blackburn–Bienaymé multicomponent coupling reaction based on Ugi-type amine and aldehyde condensation over isocyanide and then a cyclization process. Interestingly this reaction occurred under solvent-free conditions with high yield, in which the NH₂-MIL-53(Al) could be recovered and reused for five reaction cycles, giving a total turnover number of 455.

Full text of DOI:10.1002/aoc.3584

Received: 7 June 2016
DOI 10.1002/aoc.3584
Revised: 2 July 2016
Accepted: 19 July 2016
F U L L PA P E R
Metal–organic framework of amine‐MIL‐53(Al) as active and
reusable liquid‐phase reaction inductor for multicomponent
condensation of Ugi‐type reactions
*
Sadegh Rostamnia | Maryam Jafari
Organic and Nano Group (ONG), Department of
Metal–organic framework of NH2‐MIL‐53(Al), with coordinative unsaturated alu-
Chemistry, Faculty of Science, University of
Maragheh, PO Box 55181‐83111, Maragheh, Iran
minium sites, has been shown to be active in the Groebke–Blackburn–Bienaymé
multicomponent coupling reaction based on Ugi‐type amine and aldehyde conden-
sation over isocyanide and then a cyclization process. Interestingly this reaction
occurred under solvent‐free conditions with high yield, in which the NH2‐MIL‐
53(Al) could be recovered and reused for five reaction cycles, giving a total turnover
number of 455.
Correspondence
Sadegh Rostamnia, Organic and Nano Group
(ONG), Department of Chemistry, Faculty of
Science, University of Maragheh, PO Box 55181‐
83111, Maragheh, Iran.
Email: rostamnia@maragheh.ac.ir;
srostamnia@gmail.com
KEYWORDS
Groebke–Blackburn–Bienaymé (3‐GB2WB), isocyanide, metal–organic framework,
multicomponent reaction, NH2‐MIL‐53(Al)
1
|
INTRODUCTION
major challenge is the mass transfer of organic substrates into
the inorganic pores of the material.[5] We recently reported
Fused imidazo[1,2‐a] annulated nitrogen heterocycles of pyr-
idine constitute a class of biologically active compounds that
are potent antibacterial agents and calcium channel blockers
and which also exhibit cytoprotective properties (e.g.
zolpidem, Figure 1).[1] Due to the marked biological activity
of N‐fused 6,5‐membered small molecules,[1,2] a number of
different protocols for the Ugi‐type Groebke–Blackburn–
Bienaymé three‐component (3‐GBB) reaction have been
developed.[3] The synthesis of imidazo[1,2‐a]pyridines has
been achieved in the presence of Brønsted acids, as well as
with Lewis acids, via isocyanide‐based multicomponent con-
densation.[3,4] However, many of these methods have draw-
backs such as low yields of products, long reaction times,
harsh reaction conditions, tedious work‐ups leading to the
generation of large amounts of toxic metal‐containing waste,
the requirement for an inert atmosphere and the use of stoi-
chiometric or relatively expensive reagents.[4] As a result,
the development of new synthetic methods for this purpose
remains an attractive goal.
successful preparation of hybrid organic–inorganic materials
(Org@SBA‐15) with lipophilic pore walls, in which
organic–inorganic mesochannels in reaction conditions as a
catalyst provide a synergistic means of an efficient approach
of reactants to catalytic active sites and suitable
mesochannels to drive out the products for next cycles, in
the presence of ultrasonic irradiation as a mass transfer accel-
erator.[6] We then developed catalytic applications of un‐
functionalized SBA‐15 in the presence of fluorinated
alcohols in one‐pot multicomponent reactions in which we
revealed that RFOH and SBA‐15 nanoreactor can be a good
pair together in mass transfer and have catalytic properties
when they are used in organic reactions.[6]
Hybrid metal–organic frameworks (MOFs) which are
formed from inorganic building units and organic linkers
have attracted much attention in the past decade.[7] These
new hybrid organic–inorganic compounds have a crystalline
three‐ or two‐ and one‐dimensional open framework and high
specific surface area and pore volume and sometimes flexible
structures and also elevated level of structural diversity.[7]
Recently, thermally and chemically stable IRMOF‐3 (NH2‐
MOF) has been used as a heterogeneous catalyst by our
group and others.[8] In these investigations, IRMOF‐3 with
Porous nanostructures such as MCM‐41 and SBA‐15
with high surface area, adjustable pore sizes and tunable
properties have various advantages related to alternative
materials.[5] In applications of inorganic porous catalysts, a
Appl. Organometal. Chem. 2016; 1–6
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
ROSTAMNIA AND JAFARI
FIGURE
1
Zolpidem, a pharmacologically important imidazo[1,2‐a]
pyridine
non‐coordinated amino groups demonstrates that the basicity
of the aniline‐like amino group is enhanced when incorpo-
rated inside the pores of MOF channels.[8] Various authors
have prepared a NH2‐MIL‐53(Al) material, in which the
organic ligand is coordinated to unsaturated aluminium ions
(Al3+).[8b,8h,8i] These catalysts have been used for basic
catalysed Knoevenagel condensation of benzaldehyde and
ethyl cyanoacetate, and their activity was compared to that
of IRMOF‐3. In one report, the basic NH2‐MIL‐53(Al) was
less active for the condensation product (ethyl (E)‐α‐
cyanocinnamate) in the pores of the framework.[8a] On the
other hand, well‐dispersed Pd nanoparticles supported on
NH2‐MIL‐53(Al) were prepared and used as highly active
catalysts for the Suzuki reaction by Cao and co‐workers.[9]
In the work reported here, we synthesized and investigated
the action of NH2‐MIL‐53(Al) in a 3‐GBB reaction.
FIGURE 2 FT‐IR spectrum of NH2‐MIL‐53(Al) at 25°C
vibrations of NH2 groups, respectively. Carboxylate CÀO
asymmetric stretching gives rise to a band at 1624 cm−1
,
and the band at 1389 cm−1 is assigned to carboxylate CÀO
symmetric stretching. The two bands at 1495 and 1439 cm−1
correspond to aromatic CÀC stretching. The 1334 cm−1 band
can be assigned to CÀN stretching vibrations. Bands at 1255
and 776 cm−1 are assigned to aromatic CÀH in‐plane bending
and out‐of‐plane bending, respectively. The peak centred at
661 cm−1 was ascribed to HÀNÀH wagging.[8i] An additional
absorption peak is observed at around 1670 cm−1, which can
be attributed to molecules of free DMF trapped in MOF
pores.[8h] Based on investigations of Gascon and co‐
workers,[8h] the amino groups of NH2‐MIL‐53(Al) are free
of hydrogen bonding interactions with basic properties.
According to the FT‐IR wavelength analysis of the hydroxyl
groups (appearing at 3620 cm−1) the OH groups show a
stronger basicity in the NH2‐MIL‐53(Al), with carbon diox-
ide adsorptions, hydrogen carbonates may be formed due to
the interaction of CO2 with the hydroxyl groups of the
MOF.[8b,8g]
2
| EXPERIMENTAL
2.1 | Chemicals and characterization
All reagents were obtained from Sigma Aldrich (Germany)
and Fluka (Switzerland) and were used without further puri-
fication. Fourier transform infrared (FT‐IR) spectra were
recorded using a Bruker Vector 22 FT‐IR instrument. Melt-
ing points were measured with an Electrothermal 9100 appa-
ratus. 1H NMR and 13C NMR spectra were measured
(CDCl3) with a Bruker DRX‐300 AVANCE spectrometer at
300 and 75 MHz, respectively.
Main diffraction peaks at around 2θ = 9.1°, 11.6° and
18.3° are readily recognized in the X‐ray diffraction
(XRD) pattern (Figure 3b).[8h] The analysis of the XRD
pattern reveals that the peaks at 2θ = 9.1° and 11.6° corre-
spond to the (110) and (200) reflections, respectively. The
peak at 2θ = 18.3° corresponds to the (211) and (220)
reflections which are merged into a single peak.[8h] The
observed diffraction peaks agree with the structure of
NH2‐MIL‐53(Al).[8b,8g,8h,9] Results from elemental analysis
(dried MOF) are as follows for the dried material (wt%):
C, 44.6; H, 4.5; N, 9.1. The BET surface area for the syn-
thesized MOF is 712 m2 g−1 and its total pore volume is
0.96 cm3 g−1. The crystal structure of NH2‐MIL‐53(Al)
seems to change to amorphous after being reused five
times. Figure 3(a) shows a scanning electron microscopy
(SEM) image of the as‐synthesized NH2‐MIL‐53(Al) sam-
ple. SEM images of NH2‐MIL‐53(Al) were obtained to
investigate the morphology of the particles of this
organic–inorganic hybrid material. Figure 3(a) shows that
the NH2‐MIL‐53(Al) material is composed of small
nanosheets.
2.2 | Synthesis of NH2‐MIL‐53(Al)
NH2‐MIL‐53(Al) was synthesized based on a previously pub-
lished method with slight modifications.[8b,8h] In a typical
method, aluminium chloride hexahydrate (2.55 g) and 2‐
aminoterephthalic acid (2.8 g) were dissolved in 25 ml of
dimethylformamide (DMF). The produced mixture was then
moved to a Teflon‐lined autoclave and heated for 70 h at
423 K, then cooled slowly at a rate of 0.4°C min−1 to room
temperature. The resulting product was washed once with
methanol. Then, for removing organic species trapped within
the pores, the sample was washed with excess hot methanol
(70°C) for 5 h and then filtered and dried at 80°C overnight.
The MOF was then characterized using a variety of
techniques.
In the FT‐IR spectrum (Figure 2), a broad band centred at
around 3620 cm−1 could be assigned to the bridging
hydroxyl group of the MIL‐53.[8i] Two sharp bands at 3496
and 3386 cm−1 are due to symmetric and asymmetric

ROSTAMNIA AND JAFARI
3
The catalyst was washed with CH2Cl2 and diethyl ether to
remove residual product and dried under vacuum and reused
in a subsequent reaction. TON represents the average number
of substrate molecules converted into product per molecule of
catalyst.
3
| RESULTS AND DISCUSSION
A few air‐ and moisture‐stable MOFs for catalytic proposes
have been reported.[10] Among the new MOF catalytic sys-
tems, IRMOF‐3 and NH2‐MIL‐53 family as amine‐reach
catalysts have been employed.[7–10] The NH2‐MIL‐53(Al)
material was prepared with 2‐aminoterephthalic acid and alu-
minium chloride hexahydrate in DMF using a solvothermal
method in a Teflon‐lined autoclave. This MOF is a crystalline
material with one‐dimensional diamond‐shaped pores
(Figure 4).
Duo
to
the
pharmaceutical
prominence
of
imidazopyridines, several catalysts and precursors have been
reported for the synthesis of these components.[1] On order to
determine the conditions for Ugi‐like multicomponent syn-
thesis of imidazopyridines and the catalytic activity of NH2‐
MIL‐53(Al) we investigated the MOF‐catalysed 3‐GBB
condensation by a systematic study both for reactivity and
efficiency, and also for heterogeneity and reusability features
based on green chemistry principles. To our surprise, NH2‐
MIL‐53(Al) acts as a heterogeneous green catalyst for the
3‐GBB condensation reaction. For our study, cyclohexyl
isocyanide, benzaldehyde and 2‐aminopyridine were chosen
as the benchmark substrates in the model reaction
(Scheme 1).
FIGURE 3 (a) SEM micrograph of NH2‐MIL‐53(Al) and (b) XRD patterns
of the synthesized (black) and recycled (blue) NH2‐MIL‐53(Al)
2.3 | Synthesis of aminoimidazo[1,2‐a]pyridine
A mixture of aldehyde (1 mmol), 2‐aminopyridine (1 mmol)
and isocyanide (1.2 mmol) with a catalytic amount of NH2‐
MIL‐53(Al) (2 mol%, 0.0042 g) under solvent‐free condi-
tions was stirred at 50°C for 3 h. NH2‐MIL‐53(Al) (0.02 g)
was heated at 60°C in solvent‐free conditions. After comple-
tion of the reaction the solvent was removed under vacuum.
After complete disappearance of starting material as indi-
cated by TLC, the resulting mixture was diluted with CH2Cl2.
The catalyst was completely recovered from the residue. The
CH2Cl2 was evaporated under reduced pressure and the reac-
tion mixture was then subjected to column chromatography
on silica gel using ethyl acetate and n‐hexane (2:1) to afford
the aminoimidazopyridine product in high purity and all iso-
lated products gave satisfactory spectral and physical data
(see supporting information).[2b,2c] The isolated catalyst
could be reused on addition of new portions of substrate.
Due to the fact that the nature of the amine‐decorated
NH2‐MIL‐53 (Al(OH)[O2CÀC6H3NH2ÀCO2) or its syn-
thetic component may play an important role in the synthesis
of 4a, in comparison with the same structure of non‐amine
2.4 | Recovery and turnover number (TON) study
FIGURE 4 Preparation process of NH2‐MIL‐53(Al)
The recycling possibility and activity of the magnetic catalyst
were studied using model reactions in the presence of NH2‐
MIL‐53(Al). During this study (scale of the reaction selected
was 5 mmol), when the reaction was completed (TLC), the
catalyst was easily separated from the product (after adding
CH2Cl2), following decantation of the reaction solution.
SCHEME 1 Expected products of 3‐GBB condensation

4
ROSTAMNIA AND JAFARI
MOF (MIL‐53(Al)), the initial reaction was carried out using
4 mol% neutral aluminium triacetate (Al(CH3CO2)3) as non‐
coordination polymeric and non‐porous carboxylic acid alu-
minium complex, 2‐aminoterephthalic ester (NH2‐TE),
NH2‐MIL‐53 and MIL‐53 catalyst at 50°C in 4 ml of ethanol.
The NH2‐MIL‐53 was assessed for its catalytic activity in the
3‐GBB condensation by studying the model reaction to pro-
duce 4a as the principal product. When the model reaction
was run using NH2‐MIL‐53, the product 4a was obtained
87% yield during 4 h (Figure 5).
The model reaction was examined in polar and non‐polar
solvents such as toluene, dichloromethane, methanol, ethanol
and water using 2 mol% MOF (Table 1). When the reaction
was run under solvent‐free conditions, the product was
obtained in 46% yield at 25°C in 3 h. Under solvent‐free con-
ditions the yield increased sharply when the temperature was
raised from room temperature to 65°C. Experimental results
show that in various solvents the yield of reaction at 65°C
is better that that at room temperature. However, solvent‐free
conditions and 3 h at 65°C are the best conditions for this
reaction (Table 1, entry 12).
We examined several reactions to obtain the optimum
amount of catalyst in solvent‐free conditions (Table 1, entries
8–13). Results show that 2 mol% catalyst is the optimum
amount for the model reaction giving a yield of 95%. Of
course by increasing the temperature and amount of catalyst
the yield can be increased to 98%, but based on an energy
point of view this is not economic.
FIGURE 5 Reaction scale (2.2 mmol)
TABLE 1 Study of model reaction under various conditionsa
Time
(min)
Yield
(%)
Entry
Solventb
MOF (mol%)
Temp. (°C)
1
2
H2O
H2O
MeOH
EtOH
EtOH
CH2Cl2
PhMe
S‐Fc
4
4
4
4
4
4
4
4
2
3
2
2
1
90
180
90
r.t.
r.t.
65
50
65
r.t.
65
65
65
65
r.t.
65
65
15
25
65
87
70
10
80
84
80
95
35
95
76
3
4
240
90
5
6
90
7
90
8
90
9
S‐F
90
10
11
12
13
S‐F
180
180
200
180
S‐F
We then used NH2‐MIL‐53(Al) with the optimized condi-
tions for the synthesis of imidazo[1,2‐a]pyridine derivatives
4 from substrates 1, 2 and 3 (Table 2). It can be seen that
all the isocyanides and aminopyridines with aldehyde deriva-
tives gave good to excellent yields. There was no significant
difference in the yield of product between electron‐rich and
S‐F
S‐F
aReaction conditions: 2‐aminopyridine (1 mmol), benzaldehyde (1 mmol),
isocyanide (1.2 mmol), MOF (0.02 g).
b4 ml of solvent.
cS‐F, solvent‐free.
TABLE 2 NH2‐MIL‐53(Al)‐catalysed 3‐GBB condensation
Entry
4
R′
Ar
R
Time (min)
Yield (%)a
1
4a
4b
4c
H
C6H5
c‐Hexb
c‐Hex
c‐Hex
c‐Hex
t‐Bu
180
180
180
180
160
180
180
180
200
160
200
180
95
88
94
90
95
87
91
82
85
85
80
92
2
H
4‐NO2C6H4
4‐ClC6H4
3‐NO2C6H4
C6H5
3
H
4
4d
4e
H
5
H
6
4f
6‐Me
6‐Me
6‐Me
6‐Me
6‐Me
5‐Me
5‐Me
C6H5
c‐Hex
c‐Hex
c‐Hex
c‐Hex
t‐Bu
7
4 g
4i
4‐ClC6H4
3‐NO2C6H4
4‐NO2C6H4
4‐NO2C6H4
C6H5
8
9
4j
10
11
12
4 k
4 l
4 m
c‐Hex
c‐Hex
4‐ClC6H4
aIsolated yield.
bc‐Hex, cyclohexyl.

ROSTAMNIA AND JAFARI
5
unsubstituted aminopyridines. On the basis of these results,
to extend the scope and generality of this method, the 3‐
GBB reaction of substituted aldehydes was also investigated.
The results obtained indicated that NH2‐MIL‐53(Al) is a suit-
able reaction inductor for 3‐GBB condensation in solvent‐
free conditions.
When reactions involve the use of solid catalysts, the
reusability and recovery of the catalysts are important factors.
So, in the next step we investigated the recyclability of NH2‐
MIL‐53(Al). After completion of reaction, by increasing
dichloromethane and centrifugation the catalyst can be sepa-
rated from the reaction mixture. For the model reaction, it
was found that NH2‐MIL‐53(Al) could be recovered and
reused in five reaction cycles (Figure 3b), giving a total
TON of 455 (Table 3).
A proposed mechanism (pathways A and B) for this reac-
tion is shown in Scheme 2. Although this mechanism has not
been exactly demonstrated by experiment, but based on
literature reports of catalytic applications of NH2‐MIL‐
53(Al),[1–3,8–10] the most probable mechanisms for the syn-
thesis of 3‐aminoimidazo[1,2‐a]pyridines 4 are outlined in
Scheme 2 incorporating the roles of the Lewis acidic and
amine‐based organocatalytic pathways in activating the alde-
hydes and intermediate imines.
SCHEME 3 Test for chemoselectivity and size selectivity of the MOF
This indicates that there is some amount of reactivity occur-
ring outside of the framework or via defects present in the
crystal domains which facilitate guest diffusion.[8a,8c]
4
| CONCLUSIONS
We successfully synthesized NH2‐MIL‐53(Al) and utilized
it for the synthesis of 3‐aminoimidazo[1,2‐a]pyridines. The
advantage of this MOF‐catalysed reaction is that it can be
carried out under solvent‐free conditions and in a short
reaction time. Additionally, using this method, a small
amount of catalyst was used and facilely recycled, and espe-
cially the use of toxic organic solvents in the reaction was
avoided, which provides a green and effective method for
3‐GBB condensation.
To investigate the chemoselectivity and size selectivity of
the our MOF, the 3‐GBB reaction was performed using an
equimolecular mixture of benzaldehyde (1 mmol) and p‐
nitrobenzaldehyde (1 mmol) under the same reaction condi-
tions as described in Section 2. After 70 min of reaction,
the yield of 4a was 44%, whereas 4b was 11% (Scheme 3).
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SUPPORTING INFORMATION
Additional Supporting Information may be found online in
the supporting information tab for this article.
How to cite this article: Rostamnia, S., and Jafari, M.
(2016), Metal–organic framework of amine‐MIL‐
53(Al) as active and reusable liquid‐phase reaction
inductor for multicomponent condensation of Ugi‐type
reactions, Appl. Organometal. Chem., doi: 10.1002/
aoc.3584
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