Synthesis of (E)-nitroalkenes catalysed by ethanolamine supported on silica

Article abstract of DOI:10.1007/s10562-009-0223-5

A simple method for preparing (E)-nitroalkenes based on a Henry reaction in the presence of a heterogenized homogeneous catalyst consisting of silica-supported ethanolamine is proposed. With 4-substituted benzaldehydes, the reaction gives the corresponding (E)-nitrostyrenes in high yields and a short time. Heterocyclic carboxaldehydes also give good results, but the presence of an N or S atom in the ring has a slightly adverse effect on the reaction. The synthetic process is quite novel and interesting. The catalyst remains active and exhibits no substantial loss of activity or selectivity over up to three reaction cycles.

Full text of DOI:10.1007/s10562-009-0223-5

Catal Lett (2010) 134:131–137
DOI 10.1007/s10562-009-0223-5
Synthesis of (E)-nitroalkenes Catalysed by Ethanolamine
Supported on Silica
•
Manuel Mora Cesar Jimenez-Sanchidrian
•
´
Francisco Jose Urbano Jose Rafael Ruiz
´
´
•
´
´
Received: 23 July 2009 / Accepted: 9 November 2009 / Published online: 24 November 2009
Ó Springer Science+Business Media, LLC 2009
Abstract A simple method for preparing (E)-nitroalkenes
based on a Henry reaction in the presence of a heteroge-
nized homogeneous catalyst consisting of silica-supported
ethanolamine is proposed. With 4-substituted benzalde-
hydes, the reaction gives the corresponding (E)-nitrosty-
renes in high yields and a short time. Heterocyclic
carboxaldehydes also give good results, but the presence of
an N or S atom in the ring has a slightly adverse effect on
the reaction. The synthetic process is quite novel and
interesting. The catalyst remains active and exhibits no
substantial loss of activity or selectivity over up to three
reaction cycles.
via a nitroalcohol intermediate that undergoes anti-peri-
planar elimination of water to form the corresponding
nitrostyrene.
The Henry reaction was named after the French scientist
L. Henry, who developed it in the late XIX century [4, 5].
As stated above, it is a condensation reaction between a
primary or secondary nitroalkane and a carbonyl compound
(usually an aldehyde) that gives a mixture of 2-nitroalkanol
diastereomers. Because the process is essentially an aldol
condensation (Scheme 1), it requires the presence of a
base—usually in catalytic amounts.
The Henry reaction is catalysed by a variety of sub-
stances including organic bases (alkoxides mainly) and
inorganic bases (aqueous or alcoholic solutions of an
alkaline hydroxide) [6]. These are all strong bases and can
therefore affect other functional groups present in the
reactant molecules. Also, they can only act in a homoge-
neous medium, so they require isolation and purification of
the end products, and may even be rendered useless for
recycling. These shortcomings of homogeneous catalysts in
the Henry reaction have lately been circumvented by
using heterogeneous systems with results as good as or
even better than those obtained with homogeneous cata-
lysts [4, 7]. The greatest advantages of using a heteroge-
neous catalyst in this context are the ease with which it can
be removed from the reaction medium and the ability to re-
use it over several reaction cycles. Heterogeneous catalysts
consisting of alumina [8–10], anionic clays [11–14] and
metal oxides [7–15], among other substances, have so far
provided excellent activity and conversion results in many
applications.
Keywords Henry reaction Á Nitroaldol condensation Á
Ethanolamine Á Silica
1 Introduction
Nitroalkenes are highly useful organic chemicals. Thus,
nitrostyrenes possess medical properties [1, 2] and, even
more interesting, are the precursors of phenylalkylamines,
which constitute an extremely important family of bioac-
tive compounds in humans. Phenylalkylamines can be
obtained in various ways [3] one of the most commonly
used of which involves the formation of a C–C bond by
nitroaldol condensation between a benzaldehyde and a
nitroalkane (i.e., a Henry reaction). The process takes place
´
´
M. Mora Á C. Jimenez-Sanchidrian Á F. J. Urbano Á
J. R. Ruiz (&)
Heterogenized homogeneous catalysts are also being
widely used in various synthetic processes obviously
including the Henry reaction [16–18]. These catalysts
provide a number of advantages such as energy savings,
´
´
´
Departamento de Quımica Organica, Universidad de Cordoba,
Campus de Rabanales, Edificio Marie Curie, Carretera Nacional
IV-A, km. 396, 14014 Cordoba, Spain
e-mail: qo1ruarj@uco.es
123

132
M. Mora et al.
δ
O
δ
NO2
CH
R´
C
−
−
:BH
:B
H+
O
O
H
−
−
+
O
R
CH2 NO2
R
R
NO2
CH
R
CH
N
CH
R´
R
NO2
CH
NO2
C
OH
+
H2O
CH
R´
CH
R´
R
Scheme 1 General Henry reaction
minimal waste production and increased purity in the end
products [19].
liquid nitrogen temperature on a Micromeritics ASAP-
2010 instrument. Surface areas were calculated with the
Brunnauer–Emmett–Teller (BET) method [23], and pore
distributions with the Barrett–Joynet–Halenda (BJH)
method [24]. All samples were degassed to a pressure
below 3 mm Hg at 423 K prior to analysis.
13C-CP/MAS NMR spectra were recorded at
100.62 MHz on a Bruker ACP-400 spectrometer. Samples
were spun at the magic angle at 3.5 kHz on a zirconia rotor.
All measurements were made at room temperature. The
contact time for magnetization transfer between protons
and 13C was 5 ms (15,000 scans). Tetramethylsilane was
used as internal standard.
In this work, we successfully used a catalyst consisting
of silica-supported ethanolamine in the Henry reaction of
substituted benzaldehydes and heterocyclic carboxaldehy-
des with nitromethane. Supported alkylamines have pre-
viously been used in a number of synthetic procedures
[20–22] where alkylamine groups tether on silica via
covalent bonds. In our catalyst, ethanolamine groups are
physisorbed on the silica; this reduces its stability and
shelf-life, and may lead to leaching of the reagent from the
matrix. In any case, the catalyst remained stable and
reusable over 3 reaction cycles.
Thermogravimetric curves were obtained with a Seta-
ram Setsys instrument. Samples were heated from 25 to
800 °C at 10 °C/min in an argon atmosphere.
2 Experimental
2.1 Reagents and Products
2.2.2 General Procedure for the Henry Reaction
Nitromethane, ethanolamine and all aldehydes studied
were purchased from Aldrich or Fluka and used as
received. Silica was supplied by Merck (ref. 7734). The
products of the Henry reaction were all identified by mass
spectrometry.
The Henry reaction was performed in two-neck flasks
containing 6 mL of nitromethane, 5 mmol of aldehyde and
0.1 g of catalyst at 85 °C. One of the flask necks was fitted
with a reflux condenser and the other used to withdraw
samples for analysis at regular intervals. The system was
stirred throughout the process. Products were identified
from their retention times as measured by GC–MS on an
HP 5980 GC instrument furnished with a Supelcowax
30 m 9 0.32 mm column and interfaced to an HP 5971
MSD instrument.
2.2 General Procedure for Preparation of Catalyst
The silica support was obtained by calcining commercially
available silica at 650 °C in the air for 3 h. The catalyst
was prepared by mixing 5 g of calcined silica and 10 mL
of ethanolamine at 80 °C for 12 h, after which the solid
was filtered off, washed with methanol three times and
refluxed with 100 mL of methanol for 12 h in order to
remove all unsupported ethanolamine prior to filtering and
drying in a stove at 100 °C for 24 h.
3 Results and Discussion
3.1 Characterization of the Catalyst
The silica-supported ethanolamine catalyst used was des-
ignated ETAM/SiO2 and readily obtained as described in
the Sect. 2
2.2.1 Characterization of the Catalyst
The textural properties of the solids (specific surface area,
pore volume and average pore radius) were determined
from nitrogen adsorption–desorption isotherms obtained at
Nitrogen adsorption at –196 °C was used to determine
the textural properties of the pure silica support (SiO2) and
silica-supported ethanolamine catalyst (ETAM/SiO2).
123

Synthesis of (E)-nitroalkenes
133
Figure 1 shows the adsorption–desorption isotherms for
each solid. As can be seen, there were no substantial dif-
ferences between them; thus, the curves were of type IV in
the BDDT classification and exhibited hysteresis cycles of
type H1 (open-ended cylindrical pores) in the de Boer
classification [25]. Table 1 shows the specific surface areas
of the support and catalyst as established from the previous
isotherms. Based on them, ethanolamine slightly reduced
the surface area of the support.
Table 1 Specific surface area of the support (SiO2) and catalyst
(ETAM/SiO2), and amount of ETAM on the catalyst
a
Entry
Solid
SBET
Amount of ETAMb
1
2
a
SiO2
410
270
–
ETAM/SiO2
12
Specific surface area, in m2/g catalyst
b
lg ETAM/100 mg catalyst
The silica-supported ethanolamine phase was examined
by using various instrumental techniques. 13C-CP/MAS
NMR revealed the presence of ethanolamine on the catalyst
surface. Figure 2a shows the spectrum for the catalyst,
which exhibits the two typical signals for the two methy-
lene groups in ethanolamine. A comparison of the chemical
shifts with those of a spectrum for pure liquid ethanolamine
recorded under identical conditions (Fig. 2a) reveals that
the signals for the catalyst are shifted slightly upfield.
The amount of ethanolamine sorbed by the silica was
determined by comparing the thermogravimetric curves for
the support and pure catalyst (Table 1). Also, desorbed
products present on the catalyst surface were unambigu-
ously identified by thermal programmed desorption–mass
spectrometry (TPD–MS). Based on the electron-impact
mass spectrum for ethanolamine, the monitored peaks were
those at m/z 30 (base peak), 43 (7% abundance) and 61
(molecular peak, 6% abundance) obtained in the TPD–MS
tests. Fig. 3 shows their TPD-MS profiles. As can be seen,
the signals appeared within a narrow temperature range
(320–340 °C). Such high temperatures suggest that etha-
nolamine somehow binds to the support surface, from
which it can only be desorbed at temperatures well above
its boiling point. This result, together with the chemical
shifts of the 13C signals in the CP/MAS NMR spectrum,
suggests that ethanolamine does not simply deposit onto
the silica surface, but rather interacts with it—possibly via
the hydroxyl groups in both.
3.2 Henry Reaction
The primary aim of this initial work on the Henry reaction
was to check whether the proposed catalyst would be active
if nitromethane were used in overstoichiometric amounts.
Fig. 1 Nitrogen adsorption–
desorption isotherms for the
support (SiO2) and catalyst
(ETAM/SiO2)
600
500
400
300
200
100
Adsortion
Adsorption
Desortion
Desorption
ETAM/SiO
SET
2
0
600
500
400
300
200
100
0
Adsortion
Adsorption
Desortion
Desorption
SiO
Silica gel
2
0,0
0,2
0,4
0,6
0,8
1,0
Relative Pressure (P/P
)
0
123

134
M. Mora et al.
very low conversion and TOF values at fairly short reaction
times. At 85 °C, an amount of 100 mg of catalyst provided
near-quantitative conversion after 7 h. By contrast,
obtaining similar results at 65 °C required a few more
hours of reaction. Also, the reaction hardly developed
below 65 °C. An amount of catalyst of 100 mg (12.1 lg of
ETAM) was therefore chosen for further testing since
smaller amounts lengthened the reaction time considerably
(Table 2, entries 5 and 6). Also, no reaction occurred in the
absence of catalyst (entry 7 in Table 2) or the sole presence
of SiO2 (i.e. the absence of ethanolamine, entry 8 in
Table 2). A temperature of 85 °C and an amount of cata-
lyst (ETAM/SiO2) of 100 mg were thus adopted. Finally,
ethanolamine was found to provide much poorer results in
the absence of support than when supported on silica.
Table 3 shows the conversion to p-substituted (E)-ni-
trostyrenes and TOF values obtained by using p-substituted
benzaldehydes as substrates. As can be seen, the results
were moderate to good in all cases except with substrates
bearing strongly electron-withdrawing substituents. This
testifies to the efficiency of ETAM/SiO2 as a catalyst for
the Henry reaction, where it provides near-quantitative
conversion in most cases.
(b)
(a)
180 160 140 120 100
80
60
40
20
0
ppm
Fig. 2 13C-CP/MAS NMR spectra for the catalyst (a) and liquid
ethanolamine (b)
We also studied the Henry reaction with heterocyclic
aldehydes as substrates (Table 4). As can be seen, the
conversion and TOF values for furaldehydes (entries 7 and
8 in Table 4) were similar to those previously obtained
with benzaldehyde. However, the presence of a sulphur or
nitrogen atom in the ring had an adverse effect on con-
version. Thus, the presence of a carboxaldehyde group in
position 3 of the thiophene ring resulted in a substantially
increased TOF value. This suggests that the sulphur atom
takes part in the reaction, possibly through electronic
interactions. This effect was also observed in 2-furaldehyde
and 3-furaldehyde—the latter exhibited high conversion
and TOF values. With pyridines, conversion increased with
increasing distance of the carboxaldehyde group to the
pyridine nitrogen. Similar trends were observed in fural-
dehydes and thiophene carboxaldehydes.
10
m/z = 43
m/z = 61
1
m/z = 30
3.3 Catalyst Re-use and Leaching
Catalyst re-usability was assessed in the reaction of benz-
aldehyde with nitromethane. To this end, the reaction was
stopped after 7 h (i.e. at 98% conversion) and the catalyst
removed by filtration and washed with methanol several
times. Figure 4 shows the results obtained after four re-use
cycles. As can be seen, the catalytic retained virtually its
whole activity, and also its selectivity. These results by
themselves suggest that the catalyst undergoes no leaching
during the reaction. However, in order to check that no
catalyst passed into the solution and acted in the homo-
geneous phase, a hot filtering test was conducted. Such a
150
300
450
600
T (°)
Fig. 3 TPD–MS profiles for the catalyst
The reaction of benzaldehyde with nitromethane over the
proposed catalyst was used as a model to optimize some
operating conditions. Table 2 illustrates the influence of the
temperature and amount of catalyst used on (E)-nitrosty-
rene conversion. Temperatures below 60 °C resulted in
123

Synthesis of (E)-nitroalkenes
135
Table 2 Influence of the reaction temperature and amount of catalyst on conversion in the reaction of benzaldehyde with nitromethane in the
presence of ETAM/SiO2 as catalyst
H
ETAM/SiO
O
H
NO2
2
+
CH
3NO2
H
1
2
Entry
T (°C)a
Catalyst (mg)b
Time (h)c
Conversion (%)d
TOFe
1
2
3
4
5
6
7f
R.T.
40
65
85
85
85
85
85
85
100
100
100
100
60
24
24
24
7
0
6
0
0.062
0.7541
3.514
0.982
0.037
0
72
98
76
14
0
12
24
24
24
7
25
–
g
8
100
–
0
0
h
9
55
1.971
Reaction conditions: 5 mmol benzaldehyde; 6 mL nitromethane
a
Reaction temperature
b
Amount of catalyst
c
Reaction time
d
Conversion to (E)-nitrostyrene
e
Turnover frequency (mmol nitrostyrene/lmol ETAM h)
f
Without catalyst
g
With SiO2 as catalyst
h
Amount of catalyst: 1.2 lL of ethanolamine
Table 3 Synthesis of 4-substituted (E)-nitrostyrenes in the presence of ETAM/SiO2 as catalyst
H
ETAM/SiO
O
H
NO2
2
R
R
+
CH3NO2
H
3
4
Entry
R
Product
Time (h)a
Conversion (%)b
TOFc
1
2
3a: H-
4a
4b
4c
4d
4e
4f
7
7
98
96
97
96
92
93
98
94
56
68
3.514
3.449
3.484
3.449
2.892
2.923
1.760
1.477
0.587
0.713
3b: CH3CH2O-
3c: CH3O-
3d: CH3COO-
3e: CH3CH2-
3f: CH3-
3
7
4
7
5
8
6
8
7
3 g: Cl-
4 g
4 h
4i
14
16
24
24
8
3 h: Br-
9
3i: O2N-
10
3j: NC-
4j
Reaction conditions: 5 mmol benzaldehyde; 6 mL nitromethane. Amount of catalyst: 100 mg
a
Reaction time
b
Conversion to (E)-nitrostyrene
c
Turnover frequency [mmol 4-substituted (E)-nitrostyrene/lmol ETAM h]
test, which was previously applied by our group to other
heterogenized homogeneous catalysts [26, 27], involves
stopping the reaction at a low conversion level to remove
the catalyst by hot filtering and then using the filtrate to
continue the process for a further 24 h. This test was per-
formed under the experimental conditions described in
123

136
M. Mora et al.
Table 4 Synthesis of heterocyclic (E)-nitroalkenes in the presence of ETAM/SiO2 as catalyst
H
ETAM/SiO
O
NO2
+
CH3NO2
R
R
H
H
5
6
Entry
R
Product
Time (h)a
Conversion (%)b
TOFc
1
5a:
6a
24
28
0.293
S
S
S
2
3
4
5b:
5c:
5d:
6b
6c
6d
12
24
24
96
25
66
2.012
0.262
0.692
H3C
N
H
5
5e:
6e
24
51
0.534
N
CH3
CH3
6
7
5f:
6f
25
8
57
93
0.597
2.923
H3C
N
H
5g:
6g
O
O
8
5h:
5i:
6h
6i
8
100
57
3.143
0.597
0.671
9
24
24
N
10
5j:
6j
64
N
N
11
12
5k:
5l:
6k
6l
24
24
72
59
0.754
0.618
CH3
N
N
13
5m:
6m
24
56
0.608
H3C
Reaction conditions: 5 mmol aldehyde: 6 mL nitromethane. Amount of catalyst: 100 mg
a
Reaction time
b
Conversion to (E)-nitroalkene
c
Turnover frequency (mmol aldehyde/lmol ETAM h)
entry 4 of Table 2. Thus, the reaction was stopped after 1 h
(viz. at 16.5% conversion to nitrostyrene) and the catalyst
removed. The process was then allowed to continue at
85 °C in the filtrate for a further 24 h but no increase in
conversion was observed. This allows one to exclude the
possibility that some ethanolamine may leach from the
123

Synthesis of (E)-nitroalkenes
137
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A basic catalyst consisting of silica-supported ethanolamine
proved highly efficient in the synthesis of (E)-nitrostyrenes.
In fact, only if the starting benzaldehyde bore an electron-
withdrawing substituent, was conversion to the corre-
sponding nitrostyrene incomplete. The proposed catalyst is
also active with heterocyclic aldehydes, which were con-
verted in a quantitative manner as well. Finally, the catalyst
can be re-used at least three times with no appreciable loss
of reactivity or selectivity following easy removal from the
liquid reaction mixture. Also, based on the results of a hot
filtering test, no leaching of the catalyst from the support
occurs at any time during the process.
123