Dehydrogenation of nitro derivatives of bibenzyl to corresponding nitro stilbene with dioxygen catalyzed by 2,2,6,6-tetramethylpiperidine-1-oxyl

Article abstract of DOI:10.1007/s13738-012-0114-3

A simple and effective oxidative dehydrogenation of nitro derivatives of bibenzyl was developed using a mixture of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and metal salts as catalyst under an atmospheric pressure of oxygen. The oxidation of several nitro derivatives of bibenzyl led to corresponding nitro stilbene in good yields, and the yields increased with the increase in the substituted nitro group on the bibenzyl. For example, the aerobic dehydrogenation of 2,2,4,4,6,6-hexanitrobibenzyl in the presence of a catalytic amount of TEMPO with metal salts gave 2,2,4,4,6,6-hexanitrostilbene in 81 % yield and 4,4-dinitrostilbene-2,2-disulfonic acid (75 %) was obtained from 4,4-dinitrobibenzyl-2,2-disulfonic acid. Iranian Chemical Society 2012.

Full text of DOI:10.1007/s13738-012-0114-3

J IRAN CHEM SOC (2012) 9:971–975
DOI 10.1007/s13738-012-0114-3
ORIGINAL PAPER
Dehydrogenation of nitro derivatives of bibenzyl
to corresponding nitro stilbene with dioxygen catalyzed
by 2,2,6,6-tetramethylpiperidine-1-oxyl
Tingting Lu Ming Lu
•
Received: 20 December 2011 / Accepted: 24 May 2012 / Published online: 6 June 2012
Ó Iranian Chemical Society 2012
Abstract A simple and effective oxidative dehydroge-
nation of nitro derivatives of bibenzyl was developed
using a mixture of 2,2,6,6-tetramethylpiperidine-1-oxyl
their corresponding nitrotoluene by means of base-cata-
lysed auto-oxidation [1] (HNS was produced from 2,4,6-
trinitrostibene by treatment with NaClO in THF–alcohol
which was called Shipp’s method [5]), but this method
suffered from the problem of low yields and a lot of side
products (the yield of HNS was only 40 %).
(
TEMPO) and metal salts as catalyst under an atmospheric
pressure of oxygen. The oxidation of several nitro deriva-
tives of bibenzyl led to corresponding nitro stilbene in good
yields, and the yields increased with the increase in the
substituted nitro group on the bibenzyl. For example, the
The stable tetraalkylnitroxyl radical 2,2,6,6-tetra-
methylpiperidine-1-oxyl (TEMPO) is a well-known oxi-
dation catalyst, whose numerous applications in the
oxidation of alcohols, sulfides, and organometallic com-
pounds have been recently reviewed. For example,
TEMPO together with transitional metals (copper [6], iron
[7]) or metal-free catalyst [8] (nitrite [9]) provided efficient
catalysis for aerobic oxidation of alcohol [10]. Herein, we
report that TEMPO in combination with metal salts, serves
as a novel and effective catalytic system for the oxidation
of nitro derivatives of bibenzyl under oxygen to their
corresponding nitro stilbene in moderate conditions.
0
0
0
aerobic dehydrogenation of 2,2 ,4,4 ,6,6 -hexanitrobibenzyl
in the presence of a catalytic amount of TEMPO with metal
0
0
0
salts gave 2,2 ,4,4 ,6,6 -hexanitrostilbene in 81 % yield
0
0
and 4,4 -dinitrostilbene-2,2 -disulfonic acid (75 %) was
0
0
obtained from 4,4 -dinitrobibenzyl-2,2 -disulfonic acid.
Keywords Dehydrogenation ꢀ TEMPO ꢀ
Nitro derivatives of bibenzyl ꢀ Nitro stilbene
Introduction
The nitro derivatives of stilbene have great importance as
fine chemicals in organic chemistry [1]. For example,
Experimental
0
0
0
2
,2 ,4,4 ,6,6 -hexanitrostilbene (HNS) is a thermally stable
Materials and methods
explosive which is used in military ordnance systems [2].
0
0
4
,4 -Dinitrostilbene-2,2 -disulfonic acid (DNS) is a key
TEMPO purchased from Acros Organics was chemically
pure and used without further purification. Other reagents
were all commercial chemicals with analytical pure grade
(ACS grade). Distilled water was used for all the reactions.
intermediate for the synthesis of triazinyl fluorescent
0
brighteners [3] and 2,2 -dinitrostilbene can be employed to
produce carbamazepine which is an anticonvulsant drug
used primarily in the treatment of epilepsy [4]. The nitro
stilbene derivatives were usually obtained directly from
1
H NMR spectra were recorded on a Bruker 500-MHz
spectrometer with tetramethylsilane (TMS) as an internal
standard. IR spectra were recorded on a Bruker Vector 22
infrared spectrometer, as KBr pellets with absorption in
T. Lu ꢀ M. Lu (&)
-1
cm . High-performance liquid chromatography (HPLC)
School of Chemical Engineering, Nanjing University of Science
and Technology, Nanjing 210094, People’s Republic of China
e-mail: luming302@126.com; tt252525@gmail.com
experiments were performed on a liquid chromatograph
(Dionex Softron GmbH, America), consisting of a pump
123

9
72
J IRAN CHEM SOC (2012) 9:971–975
0
(
(
P680) and ultraviolet–visible light detector (UVD) system
170U). Diacovery C18 column was used for the HPLC
4,4 -Dinitrostilbene
analysis with a mobile phase consisting of acetonitrile and
methanol [11].
Yellow needles, mp 292–293 °C (lit [15]. mp
1
294–295 °C); H NMR (CDCl
.72 (s, 4H); 7.33 (m, 2H). IR (KBr): 3,109, 3,097, 3,087,
3,040, 1,606, 1,604, 1,594, 1,376, 1,342, 1,318, 1,274,
,259, 1,179, 1,110, 1,093, 976, 967, 862, 852, 821, 816,
-d6): d (ppm) 8.28 (s, 4H);
3
7
Experimental procedure
1
0
0
0
-1
2
,2 ,4,4 ,6,6 -Hexanitrobibenzyl (HNBB) was prepared by
767, 742, 696, 636, 516, 509, 476 [cm ].
1
the procedure given in the literature [12]. H NMR
0 0
4,4 -Dinitrostilbene-2,2 -disulfonic acid
(
DMSO-d6): d (ppm) 3.344 (s, 4H, CH ); 9.098 (s, 4H, Ar–
2
H). IR (KBr): 3,103, 2,888, 1,821, 1,641, 1,545, 1,468,
-
,356, 1,226, 1166, 1089, 916, 804, 710 [cm ].
1
1
1
mp 264–266 °C (lit [16] mp 266 °C); H NMR (CDCl
-
3
d6): d (ppm) 7.9 (d; 1H); 8.3 (d; 1H); 8.4 (s; 2H); 8.6 (s;
1H). IR (KBr): 3,400, 2,800, 1,664, 1,590, 1,576, 1,520,
1,484, 1,475, 1,350, 1,296, 1,140, 1,080, 976, 968, 876,
Representative procedure for the of dehydrogenation
HNBB
-
1
848, 752, 628 [cm ].
-
A mixture of HNBB (2.2 9 10 mol), TEMPO (1.3 9
3
-
4
-4
mol), and transition metal salts (1.4 9 10 mol) in
1
0
DMSO (15 mL) were added in a 50 mL four-neck round
flask equipped with magnetic stirring apparatus and water-
Results and discussion
bath. O was bubbled into the flask at the flow rate of
2
Direct dehydrogenation of HNBB to HNS
2
0 mL/h. The reaction mixture was stirred and then poured
into excess water. After standing for about 2 h to allow
formation of the precipitate, the solid formed was filtered
off. The crude product was washed with hot acetone and
dried (80 % yield). Then the product was analyzed by HPLC
and identified through comparison of the authentic sample.
The dehydrogenation of HNBB was chosen as a model
reaction (Scheme 1) and carried out in the presence of
catalytic amounts of TEMPO and metal salts under various
conditions (Table 1).
HNBB was allowed to react under O (1 atm) catalyzed
2
by TEMPO (6 mol %) and metal salts (6.5 mol %) in
DMSO (15 mL), leading to HNS in 80 % yield (Run 1).
When the reaction was carried out using lower amount of
TEMPO, the yield decreased (Runs 2 and 3). Removing of
TEMPO from the oxidation system resulted in 20 % yield
(Run 4) and the yield of HNS was only 10 % in the absence
of metal salts under these conditions (Run 5). The base
basically did not influence the yield (Run 18). Substitution
of TEMPO by 4-hydroxy-TEMPO led to a slightly lower
yield (Run 19). The oxidation of HNBB under air was also
examined and found to afford HNS in moderate yield
HNS
1
Yellow needles, mp 316–317 °C (lit [13] 318 °C).
NMR (DMSO-d ): d (ppm) 7.142 (s, 2H); 9.111 (s, 4H); IR
H
6
(
KBr): 3,102, 2,894, 1,610, 1,600, 1,534, 1,477, 1,341,
-
1
1
,268, 1,176, 1,075, 960, 917, 794, 722, 556, 514 [cm ].
In a similar way, the following nitro stilbenes were
obtained and characterized.
0 0
,2 ,4,4 -Tetranitrostilbene
2
(
Run 20).
1
Yellow needles, mp 268–269 °C (lit [14] 260 °C);
H
To optimize the yield of HNS, several metal salts co-
NMR (DMSO-d ): d (ppm) 8.81 (d, 2H); 8.62 (m, 2H);
catalysts were screened in combination with TEMPO. In
contrast to the relative catalytic activity of metal ion, the
decreasing order observed was: Fe(II) [ Cu(I) [ Zn(II) [
Co(II) [ Mn(II) [ Ni(II) [ Fe(III) [ Cu(II) (Run 1, Runs
6
8
1
7
.21 (m, 2H); 7.79 (s, 2H). IR (KBr): 3,201, 2,857, 1,606,
,596, 1,520, 1,450, 1,345, 1,224, 1,180, 1,056, 950, 920,
-
80, 710, 558, 510 [cm ].
1
7
–13). The catalytic activity of the transition metal was
0
2
,2 -Dinitrostilbene
mainly due to the fact that it could undergo an oxygen-
rebound reaction in which the transition metal served as a
relay for the oxygen atom transfer from the terminal oxi-
dant to the HNBB via an oxotransition metal reactive
intermediate. The transition metal in the reaction was
capable of readily undergoing one-electron changes [e.g.,
Fe(II) ? Fe(III) and Cu(I) ? Cu(II)] [17], however,
Ni(II), Fe(III) and Cu(II) were difficult to accept any
1
Yellow needles, mp 195–195.5 °C (lit [1] 196 °C); H
NMR (CDCl -d6): d (ppm) 7.50 (t, 2H); 7.58 (s, 2H); 7.70
3
(
t, 2H, H meta); 7.83 (dt, 2H); 8.06 (dt, 2H). IR (KBr):
3
1
6
,050, 2,840, 1,784, 1,636, 1,533, 1,443, 1,347, 1,328,
,290, 1,020, 996, 920, 892, 786, 775, 736, 708, 692, 636,
-
00, 472 [cm ].
1
1
23

J IRAN CHEM SOC (2012) 9:971–975
973
Scheme 1 The equation of the
dehydrogenation of HNBB
Table 1 Oxidation of HNBB
Run
TEMPO
mol %)
Metal salts (mol %)
Solvent
Yield of
HNS (%)
2
with O by TEMPO under
(
various conditions
1
2
3
4
5
6
7
8
9
6
1
0.1
0
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
FeCl
FeCl
FeCl
FeCl
–
2
2
2
2
(6.5)
(6.5)
(6.5)
(6.5)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
81.0
55.8
45.2
20.6
10.0
79.4
62.7
41.8
39.8
39.8
32.9
7.9
FeSO
CuCl (6.5)
ZnCl (6.5)
Co(acac)
4
ꢀ4H
2
O (6.5)
2
a
2
(6.5)
-3
1
0
MnSO
NiCl2
4
2
ꢀH O (6.5)
HNBB (2.21 9 10 mol) was
allowed to react with O (1 atm)
2
11
in the presence of TEMPO and
metal salts in DMSO (15 mL) at
1
1
2
3
FeCl
CuSO
FeCl (6.5)
3
4
(6.5)
Trace
15.9
12.3
–
5
a
5 °C for 8 h
Co (acac) is the abbreviation
2
14
15
16
2
CH CN
3
of cobalt(II) acetylacetonate
b
NaOH (10 mol %) was
FeCl
FeCl
FeCl
FeCl
FeCl
FeCl
2
2
2
2
2
2
(6.5)
(6.5)
(6.5)
(6.5)
(6.5)
(6.5)
Acetone
DMF
added
c
1
1
1
2
7
8
9
0
Tetramethylene sulfone
DMSO
–
b
c
d
4
instead of TEMPO
-Hydroxy-TEMPO was
77.7
62.2
41.6
DMSO
d
The reaction is performed
under air instead of O
DMSO
2
electron, therefore, their catalytic activities were relatively
low. The anions basically did not have the influence as the
instead due to its similar structure as DMSO, but the
reaction did not happen (Run 17).
yield of HNS used FeSO was similar to that of FeCl2
4
(
Run 6).
We next examined the effect of several solvents in the
Influences of the reaction temperature and time
on the dehydrogenation of HNBB
present dehydrogenation of HNBB (Runs 14–17). In
contrast to the TEMPO-catalyzed aerobic oxidation of
alcohol to aldehyde where acetonitrile was the best sol-
vent, the employment of acetonitrile in the present oxi-
dation [7] under these conditions led to a considerable
decrease of the yield of HNS. The boiling point of
acetone was very low to be chosen. Using DMF as the
solvent brought to lots of unknown side products. On the
basis of these results, DMSO was thought to be the best
solvent for the dehydrogenation. A likely explanation
was that DMSO not only has ability to dissolve the
substrate and the catalyzer but also it can bond to metal
ion (iron or copper) through sulfur and oxygen which
increased the activity of the metal catalyst [18]. To
confirm this point, tetramethylene sulfone was used
To obtain information on the reaction course of the present
oxidation, the influence of the reaction time on the dehy-
drogenation of HNBB was investigated (Fig. 1). It can be
seen as the reaction time was longer, the yield was higher
and it reached maximum at 8 h. Prolonging the reaction
time even to 12h resulted no change in the yield. This
phenomenon indicated that the reaction fully completed
within 8 h.
From Fig. 1, it was also shown the yield of HNS
increased with the increase in temperature and it reached
maximum at 55 °C, but then decreased when the temper-
ature further increased. The reason may be the side-reac-
tions which competed with the dehydrogenation could be
accelerated by high temperature [12].
123

9
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J IRAN CHEM SOC (2012) 9:971–975
1
6.4 % yield (Run 4 and Run 6), respectively. However,
0
when 10 mol % of KOH was added, the yield of 4,4 -di-
0
nitrobibenzyl and 2,2 -dinitrobibenzyl increased to 54.2 %
(Run 3) and 45.5 % (Run 5), respectively. In addition,
oxidations of non-nitro bibenzyl derivatives were also
performed to extend the present method with our catalytic
system, but the results were unsatisfied. The dehydrogena-
tion of bibenzyl to stilbene catalyzed by 30 mol % TEMPO
and 5 mol % FeCl with 40 mol % KOH was difficult and
2
0
the yield was only 6.8 % (Run 7). And neither 4,4 -diami-
0
nobibenzyl nor 2,2 -diaminobibenzyl can be oxidized to
their corresponding diaminostilbene under the same con-
dition (Runs 8 and 9).
The comparison between new method
and the traditional method
Fig. 1 Influences of reaction time and temperature on the dehydro-
-3
genation of HNBB: HNBB (2.21 9 10 mol) was allowed to react
with O (1 atm) in the presence of TEMPO and metal salts in DMSO
15 mL)
2
(
The classical process for the production of nitro stilbene is
based on the oxidation of corresponding nitrotoluene cat-
alyzed by base, but this method suffered from inherent
problem. For example, the yield of HNS from Shipp’s
method was only 42 % in theory while this process gen-
erates a large number of undesirable byproducts (HNBB)
[5]. And the air oxidation of p-nitrotoluene-o-sulfonic acid
(NTS) under aqueous alkaline conditions gave DNS
resulting from the formation of byproducts mainly
The dehydrogenation of different nitro derivatives
of bibenzyl
On the basis of these results, the oxidation of various nitro
derivatives of bibenzyl with O in the presence of a catalytic
2
amount of TEMPO and metal salts was examined (Table 2).
0
0
0
The oxidation of 2,2 ,4,4 -tetranitrobibenzyl (Run 1) and
0
including 4-nitrobenzaldehyde-2-sulfonic acid and 4,4 -di-
0
0
4
,4 -dinitrobibenzyl-2,2 -disulfonic acid (Run 2) was very
nitrobibenzyl-2,2 -disulfonic acid [3].
0
0
efficient: both 2,2 ,4,4 -tetranitrostilbene and DNS were
obtained with 65.8 and 75.8 %, respectively, the latter
needing better metal salt (Run 2) (Mn(CH COO) ꢀ4H O).
Compared to the traditional method, the new method
had many advantages. First, the yields of products were
significantly enhanced. We took the production of HNS as
an example. In the new process, the reaction occurred in
essentially two stages. In stage one, TNT reacted exo-
thermically with NaOCl to form HNBB and the yield was
steady at 80 % in our previous work [20]. In stage two,
3
2
2
0
0
The reaction of 4,4 -dinitrobibenzyl and 2,2 -dinitrobiben-
zyl occurred with some difficulty, probably because of their
0
0
lower basicity than 2,2 ,4,4 -tetranitrobibenzyl [19], to give
0
0
4
,4 -dinitrostilbene and 2,2 -dinitrostilbene in 22.6 and
Table 2 Dehydrogenation of different bibenzyl derivatives catalyzed by TEMPO
a
b
Run
Substrate
Product
Catalyst
Method
Yield (%)
0
0
0
0
1
2
3
4
5
6
7
8
9
2,2 ,4,4 -Tetranitrobibenzyl
0
2,2 ,4,4 -Hetranitrostilbene
0
A
C
B
A
B
A
D
D
D
E
E
F
65.8
75.8
54.2
22.6
45.5
16.4
6.8
0
0
4,4 -Dinitrobibenzyl-2,2 -disulfonic acid
0
4,4 -Dinitrostilbene-2, 2 -disulfonic acid
0
4,4 -Dinitrobibenzyl
0
4,4 -Dinitrostilbene
0
4,4 -Dinitrobibenzyl
0
4,4 -Dinitrostilbene
0
F
2,2 -Dinitrobibenzyl
0
2,2 -Dinitrostilbene
0
F
2,2 -Dinitrobibenzyl
2,2 -Dinitrostilbene
F
Bibenzyl
0
Stilbene
0
G
G
G
4,4 -Diaminobibenzyl
0
4,4 -Diaminostilbene
0
–
2,2 -Diaminobibenzyl
2,2 -Diaminostilbene
–
Substrate (2 mmol) was allowed to react with O
2
in the presence of catalyst in DMSO (15 mL)
a
A: TEMPO (10 mol %), FeCl
CH COO) O (5 mol %); D: TEMPO (30 mol %), FeCl
ꢀ4H
E: O (1 atm), 100 °C, 12 h; G: O
(1 atm), 70 °C, 7 h; F: O
2
(5 mol %); B: TEMPO (20 mol %), FeCl
(5 mol %), KOH (40 mol %)
(1 atm), 120 °C, 24 h
2
(5 mol %), KOH (20 %); C: TEMPO (20 mol %), Mn
(
3
2
2
2
b
2
2
2
1
23

J IRAN CHEM SOC (2012) 9:971–975
975
HNBB was allowed to be dehydrogenated with O cata-
2
References
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2
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(
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In summary, we have developed a procedure for the
dehydrogenation of nitro derivatives of bibenzyl under
oxygen to the corresponding nitro stilbene by the use of a
combined catalyst system of TEMPO with metal salts, and
therefore, constitute a promising strategy in the explosive
and organic industry. Further studies will focus on the use
of this novel catalytic system in the dehydrogenation of
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123