Highly double selective nitration of nitrostilbenes over zeolite

Article abstract of DOI:10.1039/c4cc04688a

A feasible zeolite-assisted ortho C-H nitration of nitrostilbenes has been developed for the first time, which can be used in situ. It uses acetyl nitrate as a mild, easily handled and commercially available nitrating reagent, leading to the synthesis of polynitrostilbenes with double selectivities and in good yields. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04688a

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Highly double selective nitration of nitrostilbenes
over zeolite†
Cite this: Chem. Commun., 2014,
50, 10710
Jun-Hui Xu, Jian-Ping Wei, Zheng Hao, Qing-Guo Ma and Xin-Hua Peng*
Received 20th June 2014,
Accepted 14th July 2014
DOI: 10.1039/c4cc04688a
www.rsc.org/chemcomm
A feasible zeolite-assisted ortho C–H nitration of nitrostilbenes has
been developed for the first time, which can be used in situ. It uses
acetyl nitrate as a mild, easily handled and commercially available
nitrating reagent, leading to the synthesis of polynitrostilbenes with
double selectivities and in good yields.
Nitrostilbenes, such as 2,20,4,4,6,60-hexanitrostilbene (HNS), are
well-known, high-energy materials with excellent thermal stabi-
lity and detonation properties.1 Furthermore, nitrostilbene-
based derivatives can be found in various natural products,2
and are playing an increasingly prominent role in the area of
non-linear optical (NLO) application.3 In energetic materials
chemistry, the presence of nitro groups tends to decrease the
heat of formation but contributes markedly to the overall
energetic performance.4 There is a lot of interest and challenges
in the syntheses and chemistry of energetic materials, contain-
ing CQC unsaturated double bonds together with more nitro
groups attached in the molecule. However, traditional nitration
Scheme 1 Nitration with acetyl nitrates.
conditions are limited in scope due to poor regioselectivity and issues, which include metal reagent employed, absence of
poor chemoselectivity arising from overnitration and the lack of recovery and reuse of the catalyst.
compatibility with easily oxidized functionalities, which result in
The use of solid catalysts is potentially attractive because of
complexity of the mixture of products formed. Acetyl nitrate in the ease of recovery and reuse of the catalyst, in addition to
acetic anhydride solution as typical electrophilic reagent reacts possibilities that the solid might enhance the selectivity, parti-
rapidly with many alkenes to give mixtures of nitro-alkenes, cularly if it is a zeolite.10 Our initial successes in regioselective
b-nitro nitrates, and b-nitro acetates.5 For example, nitration of nitration of simple aromatics or naphthalene over zeolite by
styrenes and stilbenes with acetyl nitrate gave 40–70% yields Kyodai nitration make us prefer this direction.11 Improved
of b-nitro acetates6 [eqn (1), Scheme 1]. Recently, Maiti and para-regioselection for single ring aromatics has also been
co-workers reported the preparation of (E)-nitroolefins from achieved with zeolites as the catalyst,12 particularly when nitric
nitration of olefins using AgNO2/TEMPO,7 t-BuONO/TEMPO,8 acid is used in combination with acid anhydride [eqn (2),
and Fe(NO3)3/TEMPO.9 These methods while offering significant Scheme 1]. Although this method is effective, a situation may
improvements in direct nitration of olefins have problematic arise where one needs to introduce a nitro group through
nitration of substrates with –CQC– unsaturated double bonds
as well as aromatic rings. Furthermore, nitration of nitro-
stilbenes exhibits a high degree of selectivity, including regio-
selective and chemoselectivity towards aryl rings and –CQC– double
bond. Herein, we report a zeolite-assisted double selective nitration
School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing 210094, China. E-mail: xhpeng@mail.njust.edu.cn
† Electronic supplementary information (ESI) available: Detailed experimental
procedures, spectral data, electrostatic potential calculations and crystallographic
method by which nitrostilbenes can be converted to their ortho-
nitrated derivatives in high yield using in situ acetyl nitrate as a mild,
data. CCDC 965295 and 999375. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc04688a
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easily handled and commercially available nitrating reagent [eqn (3), substrates bearing a para chloro (2b) and bromo (product 2i) or
Scheme 1]. meta chloro group (2c and 2k). In contrast, when another chloro
Our initial nitration of trans,trans-1,3-distyryl-2,4,6-trinitro- group was introduced in ortho position (2d), the major product
benzene using HNO3/H2SO4, and a complex mixture of pro- isolated with the same regioselectivity but with low yields. How-
ducts was observed, which are mainly consisted of oxidation ever, if the sterically challenging ortho-substituted nitrostilbenes
products (Table S1, entry 1, ESI†). However, when treated with derivatives as substrates often provided complex mixtures under
HNO3/Ac2O, an isolated ortho-nitrated product 1 along with a usual conditions.13 Para-substituents with –CH3 or –OCH3 reacted
wide product distributions were observed (Table S1, entries 2–4, smoothly to afford the expected nitro products (2e and 2f).
ESI†). With the presence of zeolites (SiO2:Al2O3 = 25) in HNO3/ However, when (E)-2-(4-methylstyryl)-1,3,5-trinitrobenzene was
Ac2O, an exciting result was obtained. By further optimizing the selected as substrate, small amounts of Z-isomer was detected
amount of nitration reagent, catalyst species and loading (2n). Notably, if the –OCH3 group appeared in meta-position,
amount, we discovered that 10 equiv. of HNO3, 0.20 g of zeolite di-nitration products were isolated in good yields (2g and 2m).
in excess acetic anhydride at room temperature in 5 min could Strong electron-withdrawing substituent with 4-nitro also gave a
provide 95% isolated yield of 1 (Table S1, entry 4, ESI†). nitro product (2h), albeit in lower yield. Unfortunately, 3-nitro
Remarkably, running the nitration with NO2/O2 or NO2/O3 had nitrostilbenes (e.g. (E)-1,3,5-trinitro-2-(3-nitrostyryl) benzene) did
no positive effect on the outcome of the reaction (Table S1, not form the desired nitration product under the reaction condi-
entries 12 and 13, ESI†). The –CQC– double bonds in nitration tions. Attempting the reaction on other types of nitrostilbenes,
products disappeared, which might be attributed to oxidation in which were found to be problematic under traditional conditions,
the double bonds.
resulting in successful nitration in excellent yields (2o and 2p).
With the fully optimized reaction conditions, we explored the The regiochemical designation of the products (2c and 2e) was
scope and limitations of the method. Generally, all the nitration unequivocally proven by single crystal X-ray diffraction.14
products were obtained in a good isolated yield (Table 1). (E)-1,3,5-
Next, we investigated the nitration of nitrostilbenes analogues
Trinitro-2-styrylbenzene gave excellent yields of the desired nitro by decreasing the number of nitro groups under identical condi-
product (2j, 93%) under the optimized conditions. High regio- tions (Table 2). The presence of (E)-2,4-dinitro-1-styrylbenzene did
selectivity for the less hindered ortho C–H bond was observed for not alter the expected outcome of the reaction (3a, 85%). However,
the principal (E)-a,2,4-trinitrostilbenes, as well as ortho-nitrated
products, were found, when the hydrogen atom in para-position
was substituted by –CH3 or –OCH3 group (3b–e). With the presence
Table 1 Substrate scope of nitrostilbenes nitration
of another methoxy group, two nitro groups were introduced in aryl
ring and one nitro group in –CQC– double bond (3f). If strong
electron-withdrawing nitro group was introduced in meta position,
3g was afforded in acceptable yields. Moreover, isomerization of the
trans-geometry of the reactant 2,4-trinitrostilbenes to cis-geometry
in the products occurred, which was similar to the report by
Srinivasan’s nitration with fuming nitric acid.15 This isomerization
may be due to the favorable p–p interaction between the two
aryl rings during nitration. Further investigation on nitrating
Table 2 Nitration of other nitrostilbenes
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trans-stilbene and trans–para-nitrostilbene under usual conditions
revealed that this protocol was ortho C–H nitration enhanced (3h–j).
In order to check the possibility of reuse of the zeolite, it was
recovered following extraction of the products and was regenerated
by heating overnight in air at 450 1C. Nitration reactions were then
conducted under identical conditions using the recovered zeolite.
The results showed that under the standard conditions, there was
only a slight decrease (to 93%) in the yield of 2a even after using the
same zeolite six times under identical conditions. To demonstrate
the scalability of the selective nitration method, 3,30-((1E,10E)-(2,4,6-
trinitro-1,3-phenylene)bis-(ethene-2,1-diyl))bis(chlorobenzene) was
reacted on a 15 mmol scale. In this experiment, the desired
nitrostilbene product (2c) was separated in excellent yield (90%).
Nitration by acetyl nitrate with high ortho–para ratios is
caused by ‘‘ortho effect’’ on steric ground16 or coordination
mechanism.17 Prins et al. applied liquid and solid-state MAS
NMR to Ac2O/HNO3 liquid and in beta zeolite and identified
acetyl nitrate and acetic acid, which were coordinated to the
aluminium framework of the zeolite.18 The Brønsted acid site
in the zeolite wall is responsible for the generation of NO2+ ion
from acetyl nitrate during electrophilic attack on sub-
strates.5,6,19 It is further supported by the results that when
increasing the Si/Al ratio in beta zeolite, a decrease in ortho-
selectivity is observed (Table S1, entries 6, 8 and 9, ESI†).
Moreover, the higher selectivity for the zeolite may be ascribed
to the 3D structure of zeolite in comparison to the 2D structure
of K10 (Table S1, entries 6 and 11, ESI†).
Fig. 1 Electrostatic potential distribution (left) and average negative
values around corresponding atoms in kcal molꢀ1 (right).
Scheme 2 Possible mechanism for zeolite-assisted regioselective ortho-
nitration of nitrostilbenes.
+
Molecular electrostatic potential20 (ESP) was employed in the
explanation of chemoselectivity, which reveals the regions of the
molecule to which an electrophile would initially be attracted.
The structure of trans-stilbene was shown to be nearly planar,
while the structures of nitrostilbenes were twisted upon increas-
ing the number of nitro groups (Fig. S1, S2, S4, S6 and S8, ESI†).
Moreover, the ESP surface minima of trans-stilbene was about
ꢀ14.35 kcal molꢀ1 and the electrons were well dispersed over the
molecule (Fig. 1(a) and Table S7, ESI†). However, the ESP surface
minima of nitrostilbenes were concentrated on the nitro-group
for its lone pair electrons (Fig. 1(b) and (c)). The ESP quantitative
analysis of trans –CQC– double bond and aryl rings, trans-
stilbene (ꢀ14.23 kcal molꢀ1, ꢀ14.35 kcal molꢀ1), trans-1-styryl-
2,4-dinitrobenzene (ꢀ1.47 kcal molꢀ1, ꢀ6.35 kcal molꢀ1), and
gave NO2 as a nucleus complex of the attacking electrophile
with the –CQC– double bond, and migration to the nucleus
could conceivably take place by means of the delocalized elec-
tron cloud with the extended p–p conjugation. Since entrance to
the nucleus would occur nearest the ortho position, this site
should then be most susceptible to substitution (Scheme 2).
In conclusion, we have developed a novel heterogeneous
method by which nitrostilbenes can be nitrated with double
selectivities in high yield using acetyl nitrate as the nitrating
agent. This method uses very mild conditions and generates
little waste compared to alternative nitration protocols. Preli-
minary results, including experimental details and ESP analy-
sis, suggest that the reaction proceeds through an electrophile
and linear coordination mechanisms. Efforts in further expand-
ing the scope of the reaction and elucidating the mechanism
are ongoing and will be reported in due course.
trans,trans-1,3-distyryl-2,4,6-trinitrobenzene (ꢀ1.35 kcal molꢀ1
,
ꢀ7.26 kcal molꢀ1) well explained the decreases in chemical
activity of trans –CQC– double bonds (Tables S2, S4 and S6,
ESI†). Furthermore, the ESP surface minima values of nitrostil-
benes in aryl rings were higher than trans-stilbene’s but lower
than those of their corresponding trans –CQC– double bond’s.
The average negative values around corresponding atoms are
also presented in Fig. 1 (right), and the meta position of the aryl
ring owned the minimum values for nitrostilbenes. However, it
was found that the existing nitro group decreased the electro-
Notes and references
´ˇ
1 (a) K. G. Shipp, J. Org. Chem., 1964, 29, 2620–2623; (b) M. Rohac,
ˇ
S. Zeman and A. Ru˚ˇzicka, Chem. Mater., 2008, 20, 3105–3109;
(c) K. G. Shipp and L. A. Kaplan, J. Org. Chem., 1966, 31, 857–861;
¨
(d) T. Rieckmann, S. Volker, L. Lichtblau and R. Schirra, J. Anal. Appl.
Pyrolysis, 2001, 58–59, 569–587.
2 A. C. Grimsdale, K. Leok Chan, R. E. Martin, P. G. Jokisz and
A. B. Holmes, Chem. Rev., 2009, 109, 897–1091.
3 (a) G. Likhtenshtein, Stilbenes: Applications in Chemistry, Life
Sciences and Materials Science, Wiley-VCH, 2009; (b) K. Janowska,
R. Matczak, J. Zakrzewski and H. Krawczyk, Tetrahedron Lett., 2012,
53, 6504–6507; (c) S. Mochida, K. Hirano, T. Satoh and M. Miura,
Org. Lett., 2010, 12, 5776–5779; (d) B. R. Cho, S. J. Lee, S. H. Lee,
K. H. Son, Y. H. Kim, J. Y. Doo, G. J. Lee, T. I. Kang, Y. K. Lee, M. Cho
+
static interaction of trans –CQC– double bond with NO2
sharply, which would favor substitution at the ortho position of
the benzene ring. The high regioselectivity for nitration of
nitrostilbenes by acetyl nitrate attributed to a linear coordination
mechanism.21 Initially, acetyl nitrate was activated by zeolite and
10712 | Chem. Commun., 2014, 50, 10710--10713
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and S. J. Jeon, Chem. Mater., 2001, 13, 1438–1440; (e) S. R. Marder,
Chem. Commun., 2006, 131–134.
J. Catal., 2013, 297, 244–247; (g) S. Bernasconi, G. D. Pirngruber and
R. Prins, J. Catal., 2004, 224, 297–303.
4 V. Thottempudi and J. M. Shreeve, J. Am. Chem. Soc., 2011, 133, 11 (a) X. Peng and H. Suzuki, Org. Lett., 2001, 3, 3431–3434; (b) X. Peng,
19982–19992.
H. Suzuki and C. Lu, Tetrahedron Lett., 2001, 42, 4357–4359;
(c) X. Peng, N. Fukui, M. Mizuta and H. Suzuki, Org. Biomol. Chem.,
2003, 1, 2326–2335.
5 (a) F. G. Bordwell and E. W. Garbisch, J. Am. Chem. Soc., 1960, 82,
3588–3598; (b) F. G. Bordwell and E. W. Garbisch, J. Org. Chem.,
1962, 27, 3049–3055; (c) F. G. Bordwell and E. W. Garbisch, J. Org. 12 (a) K. Smith, S. Almeer, S. J. Black and C. Peters, J. Mater. Chem.,
Chem., 1963, 28, 1765–1769.
2002, 12, 3285–3289; (b) K. Smith and G. A. El-Hiti, Green Chem.,
2011, 13, 1579–1608.
6 F. G. Bordwell and E. W. Garbisch, J. Org. Chem., 1962, 27, 2322–2325.
7 S. Maity, S. Manna, S. Rana, T. Naveen, A. Mallick and D. Maiti, 13 Nitration of (E)-2-(2-chlorostyryl)-1,3,5-trinitrobenzene and 2,20-
J. Am. Chem. Soc., 2013, 135, 3355–3358.
((1E,10E)-(2,4,6-trinitro-1,3-phenylene)bis(ethene-2,1 diyl))bis(chlor-
8 (a) S. Maity, T. Naveen, U. Sharma and D. Maiti, Org. Lett., 2013, 15,
obenzene) under the typical conditions.
3384–3387; (b) S. Manna, S. Jana, T. Saboo, A. Maji and D. Maiti, 14 Experimental details of the structure determination can be found in
Chem. Commun., 2013, 49, 5286–5288. the ESI.† CCDC 965295 and 999375.
9 T. Naveen, S. Maity, U. Sharma and D. Maiti, J. Org. Chem., 2013, 78, 15 S. Saravanan and P. C. Srinivasan, Synth. Commun., 2003, 33,
5949–5954. 1261–1266.
10 (a) B. M. Choudary, M. Sateesh, M. Lakshmi Kantam, K. Koteswara 16 M. A. Paul, J. Am. Chem. Soc., 1958, 80, 5332–5333.
Rao, K. V. Ram Prasad, K. V. Raghavan and J. A. R. P. Sarma, Chem. 17 P. Kovacic and J. J. Hiller, J. Org. Chem., 1965, 30, 2871–2873.
Commun., 2000, 25–26; (b) K. Smith, S. Almeer and S. J. Black, Chem. 18 M. Haouas, S. Bernasconi, A. Kogelbauer and R. Prins, Phys. Chem.
Commun., 2000, 1571–1572; (c) K. Smith, T. Gibbins, R. W. Millar
Chem. Phys., 2001, 3, 5067–5075.
and R. P. Claridge, J. Chem. Soc., Perkin Trans. 1, 2000, 2753–2758; 19 F. G. Bordwell and J. B. Biranowski, J. Org. Chem., 1967, 32, 629–634.
(d) K. Smith, A. Musson and G. A. DeBoos, Chem. Commun., 1996, 20 J. S. Murray and P. Politzer, Wiley Interdiscip. Rev.: Comput. Mol. Sci.,
469–470; (e) K. Smith, A. Musson and G. A. DeBoos, J. Org. Chem.,
2011, 1, 153–163.
1998, 63, 8448–8454; ( f ) K. Smith, M. H. Alotaibi and G. A. El-Hiti, 21 P. Kovacic and J. J. Hiller, J. Org. Chem., 1965, 30, 1581–1588.
This journal is ©The Royal Society of Chemistry 2014
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