Synthesis of polynitrostilbenes using activated hydrotalcites as heterogeneous base catalysts

Article abstract of DOI:10.3184/174751914X14109571360770

An efficient and straightforward synthesis of polynitrostilbenes with the (E )-configuration by the condensation of polynitroarenes with aromatic aldehydes, catalysed by activated hydrotalcites as heterogeneous catalysts is described.

Full text of DOI:10.3184/174751914X14109571360770

580 RESEARCH PAPER
VOL. 38 OCTOBER, 580–582
JOURNAL OF CHEMICAL RESEARCH 2014
Synthesis of polynitrostilbenes using activated hydrotalcites as
heterogeneous base catalysts
Junhui Xu, Jianping Wei, Qingguo Ma and Xinhua Peng*
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P.R. China
Anefficientandstraightforwardsynthesisofpolynitrostilbeneswiththe(E)-configurationbythecondensationofpolynitroarenes
with aromatic aldehydes, catalysed by activated hydrotalcites as heterogeneous catalysts is described.
Keywords: polynitrostilbenes, Knoevenagel reaction, activated hydrotalcites, heterogeneous catalysis
Polynitrostilbenes compounds, such as hexanitrostilbene
(HNS), are well known heat‑resistant explosives that can
be used at a relatively high temperature.1,2 They are also
widely used as organic nonlinear optical materials for high‑
performance electro‑optic devices due to their entirely
π‑conjugated structures.3–5 Although other organic reactions
such as Heck and Wittig reactions yield similar structures, the
Knoevenagel reaction has the advantage of being a simple and
easy reaction to perform, since water is the only other product.
Furthermore, the products of this reaction are usually trans‑
alkenes, since it is a thermal condensation reaction and occurs
at a relatively high temperature.6–9 In contrast, the Wittig
reaction often produces a mixture of cis/trans‑alkenes.10 This
Knoevenagel reaction can be usually catalysed with either
organometallic or base or acid catalysts. However, the catalyst
needs to be neutralised or separated before purification of the
product, resulting in the generation of waste, loss of catalyst
and reduction of product yields.
Layered double hydroxides (LDHs) or hydrotalcite‑
like compounds (HTLCs) are synthetic or natural layered
materials made of positively charged two‑dimensional
sheets of mixed hydroxides with water and exchangeable
charge‑compensating anions.11–13 These materials have
been developed and applied as a heterogeneous catalysts or
metal support for aldol condensations,14,15 isomerisations,16,17
transesterification,18 Michael addition,19 dehydrogenation,20
hydrogenation,21,22 alkylation,23 oxidation,24–26 and Claisen–
Schmidt condensations.27 Their reactivity under relatively
benign conditions is useful for the synthesis of fine chemicals,
when ecofriendly and green chemistry is required. However, to
our knowledge, HTLCs which was employed in the synthesis
of nitrostilbenes has not been reported yet. In this work, the
activated HTLCs were first employed as efficient heterogeneous
catalysts for the synthesis of polynitrostilbenes via the
condensation of 2,4,6‑trinitrotoluene (TNT) or 2,4,6‑trinitro‑
m‑xylene (TNMX) with various aromatic aldehydes under mild
conditions.
Results and discussion
Table 1 Optimisation of reaction conditionsa
A condensation reaction of TNT and benzaldehyde, using
different catalysts under similar conditions, has been studied
to optimise the reaction conditions. As shown in Table 1, in
the absence of catalyst, no reaction took place even after 48 h
at reflux temperature (Table 1, entry 1). The parent HTc with
different Mg/Al ratio and Mg(OH)2 as well as MgO as typical
solid Lewis base did not show any catalytic activity (Table 1,
entries 2, 3, 12 and 13). In contrast, NaOH, as an ethanol‑
soluble strong base, catalysed the reaction to give the product
in poor yield (Table 1, entry 14). The best result was observed
when calcinated HTLCs with Mg/Al ratio of 4 (HTc‑4‑Cal) was
used as the catalyst (Table 1, entry 6). Lower yields (58–65%)
were observed for the reconstructed HTLcs catalysts (Table 1,
entries 9–11). When the reaction was conducted at room
temperature with HTc‑4‑Cal as catalyst, only 5% dehydrated
product was obtained while the expected aldol product was
not detected because of equilibrium issues (Table 1, entry 8).
To establish the optimal amount of catalyst, a model reaction
of TNT (2.0 mmol) and benzaldehyde (2.0 mmol) was carried
out using 0.03 g, 0.06 g, 0.09 g, 0.12 g and 0.16 g of activated
HTLCs (HTc‑4‑Cal) and rehydrated HTLCs (HTc‑4‑Rh) as the
catalyst at reflux. A catalyst loading of 0.12 g appeared to be
the optimal quantity. (Fig. S3 of the ESI).
CH3
O2N
NO2
NO2
CHO
Catalyst
+
Toluene,
Dean-Stark trap
NO2
O2N
NO2
Entry
Catalyst
None
HTc-3
Time/h
Yield/% b
1
2
3
4
5
6
7
8c
9
48
24
24
15
15
15
15
24
15
15
15
24
24
24
0
0
0
HTc-4
HTc-2-Cal
HTc-3-Cal
HTc-4-Cal
HTc-5-Cal
HTc-4-Cal
HTc-3-Rh
HTc-4-Rh
HTc-5-Rh
Mg(OH)2
MgO
66
68
70
61
5
63
65
58
0
10
11
12
13
14d
0
5
NaOH
aReaction conditions: TNT (2.0 mmol), benzaldehyde (2.0 mmol), dry
Using the optimised reaction conditions, some new
polynitrostilbenes were synthesised using TNT or TNMX with
aromatic aldehydes in the presence of activated HTLCs (HTc‑
4‑Cal). All the products with (E)‑configuration were separated
in acceptable yields (Table 2). However, the yield with TNMX
was lower than TNT as substrate, which may attribute to
the less acidity of methyl group in TNMX than that in TNT.
Interestingly, two kinds of mono‑condensation products were
toluene (40 mL), catalyst (0.10 g), reflux and equipped with a Dean–Stark
trap.
bIsolated yield.
cRoom temperature.
dEthanol as solvent.
* Correspondent. E‑mail: xhpeng@mail.njust.edu.cn
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JOURNAL OF CHEMICAL RESEARCH 2014 581
Table 2 Knoevenagel reaction between TNT or TNMX with aromatic aldehyesa
Entry
Substrate, Ar
Product
Yield/%
Entry
Substrate, Ar
Product
Yield/%
1
2
3
4
5
6
7
TNT, 2-ClC6H4-
TNT, 3-ClC6H4-
TNT, 4-ClC6H4-
TNT, 2-NO2C6H4-
TNT, 3-NO2C6H4-
TNT, 4-NO2C6H4-
TNT, 4-OCH3C6H4-
2a
2b
2c
2d
2e
2f
70
71
70
63
70
67
83
9
10
11
12
13
14
15
TNMX, 2-ClC6H4-
TNMX, 3-ClC6H4-
TNMX, 3-NO2C6H4-
TNMX, 3-OCH3C6H4-
TNMX, 4-OCH3C6H4-
TNMX, 4-BrC6H4-
TNMX, 3-OHC6H4-
3a
3b
3c
3d
3e
3f
61
60
43
73
71
65
70b,c
2g
3g
8
TNT, C6H5-
2h
56
16
TNMX, 4-N(CH3)2C6H4-
3h
64b,d
aReaction conditions: TNT or TNMX (2.0 mmol), aromatic aldehyde (2.0 mmol) for TNT substrate and aromatic aldehyde (4.0 mmol) for TNMX, HTc-4-Cal
(0.12 g), toluene as a solvent (40 mL), 24 h, the reflux rate was at about 100 drops/min; isolated yield after recrystallisation.
bIsolated yield via silica gel column.
cMono-condensation produce (5%) was formed.
dMono-condensation produce (8%) was formed.
separated in low yields for para-dimethylamino and meta-
hydroxyl benzaldehyde with TNMX. Moreover, to explore
the versatility of the catalyst in 2,4‑dinitrotoluene (DNT), we
studied the reaction between DNT and benzaldehyde under the
same conditions. No reaction occurred, which may be due to
the catalyst possessing basic sites with pKa values below the
pKa of the reactant.28
Conclusion
The environmentally benign, inexpensive and easily prepared
MgAl‑HTLCs were shown to act as heterogeneous catalysts
in the synthesis of polynitrostilbenes after calcination. All the
products with (E)‑configuration were generated in acceptable
yields. Efforts at elucidating the mechanism are ongoing and
will be reported in due course.
An important norm for heterogeneous catalysis is the
reusability of the catalyst. To study the catalyst reusability, the
reaction of TNT and benzaldehyde was carried out on HTc‑4‑
Cal catalyst several times. After the reaction was completed,
the catalyst was filtered, washed with toluene, dried at 373 K
and then activated by calcining to 773 K in a flow of nitrogen.
The recovered catalyst could be reused for more than five
reaction cycles without significant loss in catalytic activity
(Fig. S4 of the ESI), for the first cycle 75% and for the fifth
cycle 70% yield). To confirm the typical features of Mg(Al)
Ox mixed oxide and periclase‑like structure still existed, the
recovered catalyst after five cycles was also characterised with
XRD (Fig. S1c HTc‑4‑Cal‑Reused).
Surprisingly, the colour of solution changed to pink when
the catalyst was added to TNT solution. This provided a better
understanding of this type of reaction. In order to investigate
the mechanism of the condensation, UV‑Vis absorption
spectra were used for detecting the minor changes. As seen
in Fig. S5 (ESI), a new absorption peak appeared around
350 nm (Fig. S5a of the ESI), when mixing TNT with catalyst
(HTc‑4‑Cal) in acetonitrile solution. However, there was little
change in absorption when benzaldehyde and catalyst were
mixed (Fig. S5b of the ESI). It might ascribed to formation
of a methylene group with negative charge firstly with the
base catalysed. The possible mechanism for the formation of
(E)‑1,3,5‑trinitro‑2‑styrylbenzene is presented in Fig. 1. The
basic sites of activated HTLCs catalyst probably abstracts the
proton from TNT to give the intermediate I, which reacted
with aromatic aldehyde in an aldol reaction to form the product
II. Subsequently, removal of another hydrogen atom from II
gave III by active base sites catalysis which then furnished the
elimination product (E)‑1,3,5‑trinitro‑2‑styrylbenzene.
Experimental
CAUTION: Although no problems were encountered during the
synthesis, all polynitrocompounds are potential explosives and great
care must be taken throughout the processes. Safety equipment such
as Kevlar gloves, bar shields, and ear plugs are highly recommended
especially when collecting the energetic materials.
All chemical reagents and solvents (analytical grade) were used as
supplied unless otherwise stated. All experiments were monitored by
TLC, TLC plates were visualised by exposure to ultraviolet light
1
(254 nm). H and 13C spectra were recorded on a Bruker Avance III
500 MHz and 300 MHz digital NMR spectrometer, using CDCl3 or
DMSO‑d6 as a solvent and the chemical shifts were reported in δ
(ppm) values. Coupling constants were reported Hz. IR spectra were
recorded using Bruker Tensor 27 with the sample dispersed in a KBr
pellet and were reported in terms of frequency of absorption (cm–1).
The melting points were determined on an electrical melting point
apparatus in open capillary tubes and were uncorrected. The following
abbreviations were used to designate the multiplicities: s=singlet,
d=doublet, t=triplet, q=quartet, m=multiplet. Mass spectra were
obtained using Electrospray (ESI‑TOF) mass spectrometer 6500
Series and Bruker Daltonics flexAnalysis. The UV‑Vis absorption was
carried out on Shimadzu UV‑1800 Series, wavelength range (nm.)
from 200.00 to 700.00. Slit width was 1.0 nm and light source change
wavelength is 340.0 nm.
Synthesis of Mg–Al HTLCs
Mg–Al HTLCs were prepared using
a
standard aqueous
co‑precipitation method at constant pH and temperature as described
in the literature.29 HTLCs with different Mg/Al molar ratios of 2, 3,
4 and 5 were synthesised and labelled as HTc‑2, HTc‑3, HTc‑4, and
HTc‑5 respectively. The HTLCs precursors were then activated
by calcining to 773 K in a flow of nitrogen and named HTc‑Cal.
The temperature was raised at the rate of 10 K min–1 to reach 773 K
and maintained for 10 h. The calcined solid was then cooled in dry
nitrogen and rehydrated at room temperature under a flow of nitrogen
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582 JOURNAL OF CHEMICAL RESEARCH 2014
Fig. 1 Plausible mechanism for the formation of (E)-1,3,5-trinitro-2-styrylbenzene.
gas saturated with water vapour. The flow of wet nitrogen of 6 L h–1
was maintained for a specified period, depending on the amount of
catalyst to be rehydrated. The resultant rehydrated HTLCs was named
as HTc‑Rh. The powder X‑ray diffraction (P‑XRD) pattern (Fig. S1
in the ESI) and IR (Fig. S2 in the ESI) for samples of as‑synthesised
HTLCs and calcined forms as well as rehydrated HTLCs were
presented for confirming the structures.
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Synthesis of polynitrostilbenes
The reaction was carried out in a round bottom glass flask equipped
with a Dean‑Stark trap under vigorous stirring. Typical reactions were
performed with TNT (2.0 mmol), aromatic aldehyde (2.0 mmol) and
in dry toluene (40 mL) using prepared catalyst (0.12 g). The whole
process was refluxed and monitored by TLC. After the reaction was
completed (about 24 h), the catalyst was filtered and washed with
some toluene for reuse, then the filtrate was evaporated of toluene
for reuse (25–30 mL), ethanol was added dropwise to the residue
until a precipitate appeared. Then, the mixture was cooled to room
temperature and placed in a refrigerator overnight. The precipitated
product was collected, washed and dried. The solid was crystallised
with appropriate solvents. Otherwise, if there was no precipitate when
ethanol added, the product was achieved via silica gel column.
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Electronic Supplementary Information
Figures S1 to S5 are given in the ESI available through:
stl.publisher.ingentaconnect.com/content/stl/jcr/supp‑data
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Received 6 August 2014; accepted 21 August 2014
Paper 1402811 doi: 10.3184/174751914X14109571360770
Published online: 17 October 2014
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