One-pot synthesis of nitroalkenes via the Henry reaction over amino-functionalized MIL-101 catalysts

Article abstract of DOI:10.1016/j.catcom.2012.09.032

Ethylenediamine moieties with different amounts were incorporated into the metal-organic framework MIL-101 to prepare the amino-functionalized MIL-101 materials, which were used as heterogeneous catalysts in the one-pot synthesis of nitroalkenes via the Henry reaction. Various reaction conditions were optimized. Under the optimized conditions, the selectivity towards nitroalkenes is extremely high in combination with nearly complete conversion of the reactant nitroalkanes. The amino-functionalized MIL-101 behaves as a true heterogeneous catalyst, can be easily recovered by filtration, and can be reused at least twice without significant loss of its catalytic performance.

Full text of DOI:10.1016/j.catcom.2012.09.032

Catalysis Communications 29 (2012) 101–104
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
One-pot synthesis of nitroalkenes via the Henry reaction over amino-functionalized
MIL-101 catalysts
⁎
⁎⁎
Huixian Yu, Jianwu Xie , Yijun Zhong, Fumin Zhang, Weidong Zhu
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry and Department of Chemistry and Life Sciences, Zhejiang Normal University,
321004 Jinhua, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 June 2012
Received in revised form 12 September 2012
Accepted 27 September 2012
Available online 4 October 2012
Ethylenediamine moieties with different amounts were incorporated into the metal–organic framework
MIL-101 to prepare the amino-functionalized MIL-101 materials, which were used as heterogeneous cata-
lysts in the one-pot synthesis of nitroalkenes via the Henry reaction. Various reaction conditions were opti-
mized. Under the optimized conditions, the selectivity towards nitroalkenes is extremely high in combination
with nearly complete conversion of the reactant nitroalkanes. The amino-functionalized MIL-101 behaves as
a true heterogeneous catalyst, can be easily recovered by filtration, and can be reused at least twice without
significant loss of its catalytic performance.
Keywords:
Nitroalkanes
C\C bond formation
Henry reaction
© 2012 Elsevier B.V. All rights reserved.
Metal–organic frameworks
1. Introduction
inorganic subunits in fully crystalline porous materials has given
rise to thousands of MOF structures with a vast topological richness
Nitroalkenes are of great synthetic potential in organic chemistry
[1,2]. As the nitro group is a strong electron-withdrawing group,
nitroalkenes are widespread used as versatile synthetic reagents in
Michael addition [3], Friedel–Crafts alkylation [4,5], domino reac-
tion [6,7], 1,3-dipolar cycloaddition [8,9], and Morita–Baylis–Hillman
reaction [10,11]. In addition, the nitro group can be converted into vari-
ous functional groups, such as carbonyl or amino derivatives [12–14].
Therefore, nitroalkenes have become important intermediates in organic
synthesis. The classic method for the synthesis of nitroalkenes is to use
the Henry reaction, i.e., the condensation of carbonyl compounds with
nitroalkanes, followed by β-elimination of the resulting 2-nitro alcohols
catalyzed by homogeneous catalysts [15–17]. In recent years, various
heterogeneous catalysts such as amino-functionalized silicas (APS)
[18,19], zeolite [20], and MCM-41 [21] have also been employed in the
Henry reaction. However, most of the reported methods have one or
more of the following drawbacks: use of expensive reagents, poor selec-
tivity, complicated reaction assembly, and low yield of the desired
products, etc.
[22,23]. Impressive progress has been made during the past decade,
yielding promising results in the field of catalysis [24,25]. Although
some MOF materials like MIL-101 have shown unusual thermal
and chemical stability, the use of MOFs in applications like catalysis
is still mainly limited by the lack of functional and selective sites
in most ultra-stable MOFs [26]. Therefore, one of the current
challenges is the development of stable MOFs including functional
organic sites that could be used either directly or after post-synthetic
modification.
Chromium(III) terephthalate MIL-101 is built up from super-
tetrahedral (ST) building units, which are formed by rigid terephthalate
ligands and Cr3+ octahedral clusters [26]. The resulting solid possesses
two types of quasi-spherical mesoporous cages formed by 12 pentago-
nal and 16 faces, respectively. The so-called medium cavities are acces-
sible through 1.2 nm pentagonal windows, while the large cavities are
communicated through the same pentagonal windows and 1.6 nm hex-
agonal windows [27]. The presence of coordinatively unsaturated-metal
sites (CUS) in MIL-101 allows its use as a mild Lewis acid and, more im-
portant, allows its post-functionalization via grafting of active species
such as amines [28]. The zeotype cavities of two different sizes (extended
MTN topology), the fully accessible porosity, together with a high ther-
mal and chemical stability make MIL-101 an excellent candidate for cat-
alytic purposes [28].
Metal organic frameworks (MOFs) are nowadays at the front of
materials research. In recent years, the combination of organic and
⁎
Corresponding author. Tel./fax: +86 579 82282610.
In this work an attempt was to incorporate amine moieties with
different amounts into MIL-101 and then to use these amino-
⁎⁎ Corresponding author. Tel./fax: +86 579 82282932.
E-mail addresses: xiejw@zjnu.cn (J. Xie), weidongzhu@zjnu.cn (W. Zhu).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.09.032

102
H. Yu et al. / Catalysis Communications 29 (2012) 101–104
functionalized MIL-101 materials as heterogeneous catalysts for the
one-pot synthesis of various nitroalkenes, for the first time, to the
best of our knowledge.
In an initial study, a series of organic solvents were screened for the
reaction over GNM-3. It was found that GNM-3 exhibited a good
catalytic activity for the condensation of benzaldehyde 1a and
nitromethane 2 in all the solvents and 2-nitrostyrene 4a was obtained
as a main product in almost all cases. As shown in Table 1, a type of
solvent was found to have a significant effect on the conversion and
selectivity under the same conditions. An excellent selectivity was
obtained while the conversion was only moderate, when the reaction
was carried out in cyclohexane (Table 1, entry 1), and using n-heptane
as the solvent, the conversion was increased while the selectivity was
almost the same as that using cyclohexane as the solvent (Table 1,
entry 2). Using n-butanol as the solvent, differently, an excellent
conversion was obtained but the selectivity was lowered, compared to
those using cyclohexane and n-heptane as the solvents (Table 1, entry
4). In terms of the selectivity, toluene was revealed as the best medium,
in which a moderate conversion was still obtained (Table 1, entry 3).
In order to get a higher conversion, various parameters, including
the amount of EDA incorporated in MIL-101 and reaction temperature
and time, were optimized using toluene as the solvent. By increasing
the reaction temperature from 80 °C to 110 °C, the excellent conversion
(100%) and selectivity (100%) were obtained in the presence of GNM-3
(Table 1, entry 5). However, the conversion was decreased, when the
reaction time was shortened (Table 1, entries 6–8). The effects of the
amount of EDA incorporated in MIL-101 on the catalytic activity and
selectivity showed that under the same reaction conditions the selectiv-
ity was the same (100%) while the conversion was increased with
increasing the amount of EDA incorporated in MIL-101 in the order of
GNM-1, GNM-2, and GNM-3 (Table 1, entries 9, 10, and 5). However,
GNM-4 having the highest amino loading in the MIL-101 supported
catalysts investigated was less active than GNM-3 (Table 1, entry 11),
probably ascribed to the overloaded EDA congregating near the entries
of the cavities and the exterior surfaces of MIL-101, which would cause
a strong diffusion resistance to the reactant molecules. In addition, the
dehydrated MIL-101 as catalyst revealed a rather low activity (Table 1,
entry 12), indicating that the support MIL-101 itself hardly catalyzed
the Henry reaction. A homogeneous solution containing EDA in equiva-
lent amount to that in GNM-3 was less active and selective than GNM-3
(Table 1, entry 13), which should be further clarified in our future work.
In summary, it was found that GNM-3 was the best catalyst for the
2. Experimental
MIL-101(Cr) was initially prepared from the hydrothermal reaction
of terephthalic acid with Cr(NO3)3·9H2O, HF, and H2O at 220 °C for 8 h.
The as-synthesized MIL-101 was further purified by a three-step
process using filtration, hot ethanol, and aqueous NH4F solutions (see
Supplementary data). The nomenclature used for the prepared
amino-functionalized MIL-101 samples is GNM-1, GNM-2, GNM-3,
and GNM-4 in abbreviation, corresponding to the different amounts of
ethylenediamine (EDA in abbreviation) used during the preparation.
For example, in a typical procedure for preparing GNM-3, the MIL-101
(0.5 g) dehydrated at 150 °C for 12 h was suspended in anhydrous
toluene (30 mL). To this suspension, 2.25 mmol of EDA was added,
and the mixture was stirred with heating to reflux for 12 h. For
GNM-1, GNM-2, and GNM-4, 0.75, 1.5, and 3.0 mmol of EDA, respec-
tively, were used, but the other preparation conditions were the same.
The detailed methods for the characterization and the textural and
physicochemical properties of the synthesized samples are presented
in Supplementary data.
All catalytic measurements for the condensation of various
aldehydes and nitromethane were carried out in a glass tube (20 mL)
or flask (50 mL or 250 mL), depending on the total volume of the reac-
tion medium, equipped with a reflux condenser and a magnetic stirrer.
Before the reaction, the amino-functionalized MIL-101 catalysts were
treated at 120 °C for 12 h in an oven to remove residual water in the
samples. The reaction products were analyzed using a gas chromato-
graph (Agilent 6820) equipped with a capillary column (HP-5) and an
FID detector. The detailed methods for catalytic measurements and
analysis are described in Supplementary data.
3. Results and discussion
We first decided to examine the reaction between benzaldehyde
1a and nitromethane 2 in the presence of different catalysts, as
shown in Table 1.
Table 2
Table 1
Condensation of aldehyde
1 and nitromethane 2 over the amino-functionalized
Condensation of benzaldehyde 1a and nitromethane 2 over the amino-functionalized
MIL-101 catalyst (GNM-3).a
MIL-101 catalysts.a
Entry Ar
Conv. (%)b Sel. (4) (%)c Sel. (3) (%)d Sel. (5) (%)e
Entry
Solvent
Catalystb
t (h)
Conv. (%)c
Sel. (4a) (%)d
1
C6H5 (1a)
p-MeC6H4 (1b)
p-MeOC6H4 (1c) 100
100
100
100 4a
100 4b
100 4c
92 4d
92 4e
81 4f
–
–
1
2
3
4
Cyclohexane
n-Heptane
Toluene
n-Butanol
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
GNM-3
GNM-3
GNM-3
GNM-3
GNM-3
GNM-3
GNM-3
GNM-3
GNM-1
GNM-2
GNM-4
MIL-101
EDA
8.0
8.0
8.0
8.0
8.0
6.0
7.0
7.5
8.0
8.0
8.0
8.0
8.0
63
82
80
98
100
95
97
99
94
96
98
6
98 (2)f
97 (3)f
100
2f
3f
4
–
–
–
–
p-NO2C6H4 (1d)
m-NO2C6H4 (1e)
p-ClC6H4 (1f)
m-ClC6H4 (1g)
o-ClC6H4 (1h)
p-FC6H4 (1i)
2-Naphthyl (1j)
2-Furanyl (1k)
2-Thienyl (1l)
C6H5 (1a)
81
85
100
83
8 3d
8 3e
19 3f
3 3g
10 3h
10 3i
12 3j
3 3k
7 3l
1 3a
–
60 (25)f
100
5
6
7
8
–
5e
6e
7e
8e
9e
10e
11e
12e
13e
–
100
100
100
100
93 4g
90 4h
90 4i
4 5g
–
97
93
9
–
10f
11
12
13g
100
100
92
88 4j
93 4k
93 4l
–
100
4 5k
–
100
95 (5)f
94 (6)f
98
99 4a
–
98
a
Unless otherwise noted, the reactions were performed with 1 mmol of 1, 10 mmol
of 2, and 30 mg of catalyst GNM-3 in 2 mL of toluene at 110 °C for 8 h.
a
Unless otherwise noted, the reactions were performed with 1 mmol of 1a,
b
10 mmol of 2, and 30 mg of catalyst in 2 mL of solvent at 80 °C.
Conversion of reactant 1.
Selectivity for 4.
Selectivity for 3.
Selectivity for 5.
b
c
Amounts of EDA grafted in MIL-101, GNM-1: 1.67 mmol g−1, GNM-2: 1.77 mmol g−1
,
d
GNM-3: 1.97 mmol g−1, and GNM-4: 2.06 mmol g−1
.
c
d
e
f
e
Conversion of reactant 1a.
Selectivity for 4a.
At 110 °C.
f
6 h.
g
The reaction was performed with 45 mmol of 1, 450 mmol of 2, and 1.35 g of
Selectivity for 3a.
catalyst GNM-3 in 90 mL of toluene at 110 °C.

H. Yu et al. / Catalysis Communications 29 (2012) 101–104
103
Table 3
Henry reaction and the excellent conversion (100%) and selectivity
(100%) were obtained after the reaction at 110 °C in toluene for 8.0 h
(Table 1, entry 5).
Reusability of catalyst GNM-3 for the condensation of benzaldehyde and
nitromethane.a
GNM-3
Having defined an efficiently catalytic system (GNM-3, toluene,
110 °C, and 8.0 h), the scope of the Henry reaction of aromatic aldehyde
1 and nitromethane 2 was explored. These results are summarized in
Table 2. To assess the impact of the structural and functional motifs on
the reaction, we tested a range of aromatic aldehydes. For all cases,
aromatic aldehyde 1 reacted with nitromethane 2 led to the corre-
sponding nitroalkene 4 in good to excellent conversion with high selec-
tivity. Electron-donating groups in the para-position of benzaldehyde
enhanced the reactivity of the aldehyde with nitromethane, and the
corresponding nitroalkenes were formed in excellent conversion with
high selectivity (Table 2, entries 2 and 3). Conversely, the slightly
lower product selectivities were detected, when the reactant benzalde-
hydes with electron-withdrawing groups on the ortho, meta or para
positions were used (Table 2, entries 4–9). The high conversion and
selectivity were also obtained in the reactions of heteroaryl substituted
aldehydes with nitromethane (Table 2, entries 10–12).
A reaction mechanism for the nitroalkanes catalyzed by the
amino-functionalized MIL-101 catalysts is proposed, as illustrated in
Scheme 1: (a) aldehyde 1 reacts with an amino group to form the
intermediate of imine I and (b) the α-proton of nitromethane 2 is
abstracted by another amino group, accompanied by the nucleophilic
attack of the deprotonated nitromethane III on imine I, resulting in
the product nitroalkane 4.
In order to investigate the scalability of the synthesis of
nitroalkenes via the Henry reaction over GNM-3, as an example, the
condensation reaction of benzaldehyde and nitromethane was scaled
up by 45 times in comparison with the reaction shown in Table 2,
entry 1. The reaction was also proven effectively and conveniently
to prepare the nitroalkene on a gram-scale (>5 g) with slightly
inferior conversion and selectivity (Table 2, entry 13).
Being a heterogeneous catalyst in nature, the catalyst should be
reusable. Therefore, the reusability of catalyst GNM-3 was investigat-
ed. The catalyst was indeed easily isolated from the reaction suspen-
sion after the reaction and was readily used in the next run. As shown
in Table 3, in the second and third runs, the same selectivity as that in
the first run for the desired product was reserved but the conversion
was only slightly decreased in the second run and started to decrease
significantly in the third run. The elemental analysis of the reused
catalysts showed that the grafted EDA in GNM-3 extracted into the
organic phase was the main cause for the reduced catalytic activity.
Ph
O2N CH3
+
Ph
O
110 oC, 8.0 h
NO2
1a
2
4a
Run
N content in the catalyst (mmol g−1
)
Conv. (%)b
Select. (4a)c
1
2
3
3.94 (1.97)d
3.64 (1.82)d
2.57 (1.29)d
100
98
76
100
100
100
a
The reactions were performed with 15 mmol of 1, 150 mmol of 2, and 450 mg of
catalyst GNM-3 in 30 mL of toluene at 110 °C and the catalyst was collected by filtra-
tion and used in the next runs.
b
Conversion of reactant 1a.
Selectivity for 4a.
Numbers in parentheses denote the content of free amino groups available for the
c
d
reaction.
The development of a stable amino-functionalized MIL-101 catalyst,
therefore, remains a main challenge.
4. Conclusions
The amino-functionalized MIL-101 catalyst worked well in the
Henry reaction of various aldehydes and nitromethane with excellent
selectivity and conversion for the synthesis of the corresponding
nitroalkenes. There was an optimized amount of the amine grafted in
MIL-101 for the catalytic performance. The solvent of the reaction
medium that strongly affected the yield of nitroalkenes and toluene
seemed to be the best solvent among the solvents screened. This
heterogenized catalyst combined the advantages of both homogeneous
and heterogeneous systems and therefore provided an easily recyclable
and reusable solid catalyst that has uniform and precisely engineered
active sites similar to those of its homogeneous counterpart.
Acknowledgments
The financial support provided by the National Natural Science
Foundation of China (20971109 and 21036006) and the Program
for Changjiang Scholars and Innovative Research Team in Chinese
Universities (IRT0980) is gratefully acknowledged.
Scheme 1. Possible reaction mechanism.

104
H. Yu et al. / Catalysis Communications 29 (2012) 101–104
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