Selective partial hydrogenation of dinitrobenzenes to nitroanilines catalyzed by Ru/C

Article abstract of DOI:10.1246/cl.2008.974

Ru/C was found to be a highly effective catalyst for the selective partial hydrogenation of a range of dinitrobenzenes to their corresponding nitroanilines with excellent selectivity under mild conditions. Furthermore, the effect from other substitute groups of dinitrobenzenes on partial hydrogenation was also explored in this study. Copyright

Full text of DOI:10.1246/cl.2008.974

974
Chemistry Letters Vol.37, No.9 (2008)
Selective Partial Hydrogenation of Dinitrobenzenes
to Nitroanilines Catalyzed by Ru/C
Jie Hou, Yonghuan Ma, Yuhan Li, Fang Guo, and Lianhai Luꢀ
State Key Laboratory of Fine Chemicals, Dalian University of Technology (DUT),
Dalian 116012, P. R. China
(Received June 11, 2008; CL-080585; E-mail: lianhai@dlut.edu.cn)
Ru/C was found to be a highly effective catalyst for the
and Tween 20 as stabilizer, followed by reduction with sodium
borohydride under stirring and impregnation of active carbon
powder into the solution (See Supporting Information).14
Our initial investigation focused on the hydrogenation of
m-dinitrobenzene (1a) using Ru/C catalyst, and the reaction
process was monitored by gas chromatography (GC). In accord
with Figure 1, GC monitoring of the catalytic reduction of 1a
over Ru/C showed that hydrogenation of m-dinitrobenzene
was a stepwise reaction, in which the initial hydrogenation step
took place readily to afford m-nitroaniline (1b), which then
evolved into the final m-phenyldiamine (1c) with nearly constant
reaction rate. The excellent selectivity to 1b (>99%) was ob-
tained until 1a converted completely, while the other nitro group
was untouched. Thus, the highest yield to 1b could be achieved
through controlling hydrogen consumption or reaction time.
More importantly, Ru/C was also found to be very efficient
in the selective partial hydrogenation of a range of substituted
dinitrobenzenes in this study (Table 1), implying the synthetic
utility of this catalytic transformation.
As can be seen from Table 1, m-, o-, p-dinitrobenzenes gave
corresponding nitroanilines with excellent selectivity under the
optimal conditions, indicating the position of two nitro groups
had very little effect on the selectivity of partial hydrogenation.
Furthermore, for substituted dinitrobenzenes, both electron-
donating and electron-withdrawing substituted derivatives could
be partially hydrogenated to corresponding nitroanilines with
high selectivity (>95%).
selective partial hydrogenation of a range of dinitrobenzenes
to their corresponding nitroanilines with excellent selectivity
under mild conditions. Furthermore, the effect from other substi-
tute groups of dinitrobenzenes on partial hydrogenation was also
explored in this study.
Various nitroanilines are widely used as important inter-
mediates in many chemistry-related industries.1 Generally,
nitroanilines are obtained either from anilines by nitration after
acetylation, with subsequent removal of the acetyl group by hy-
drolysis,2 or from partial reduction of dinitrobenzenes using sul-
fide reagents or iron powder.3 Both two methods are hazardous
to the environment and tedious in further separation and waste
disposal. For reasons of economy and environment protection,
selective catalytical hydrogenation of dinitrobenzenes to obtain
nitroanilines has attracted much attention in recent years.4–8
During past thirty years, ruthenium-catalyzed processes
have become one of the most preferred methodologies in organic
synthesis because of their highly efficient performance and ver-
satile applications.9 For example, Ru-based catalysts are effec-
tive in selective hydrogenolysis of glycerol to 1,2-propanediol,10
ammonia synthesis,11 and partial selective hydrogenation of
benzene to cyclohexene.12 Very recently, PVP–Ru/Al2O3 was
discovered to be an effective catalyst for partial hydrogenation
of m-dinitrobenzene to m-nitroaniline in almost 100% yield;
however, the Sn4þ was added to the catalysis system as modifier,
and the study only investigated the hydrogenation of m-dinitro-
benzene.13
It was evident from Table 1 that the existence of electron-
withdrawing groups (such as –COOH and –F) could make partial
hydrogenation occur more easily when compared to electron-
donating substituted dinitrobenzenes (such as –OH, –NH2,
–CH3, and –OCH3) in expression of reaction conditions. The
facile reduction of electrophilic substituted dinitrobenzenes
Herein, we describe a simple and controllable scheme for
selective partial reduction of dinitrobenzenes to corresponding
nitroanilines based on a Ru/C-catalyzed process with excellent
selectivity. The process involves two steps including the initial
hydrogenation of dinitrobenzenes to nitroanilines with high
selectivity and subsequent hydrogenation of the nitroanilines
to phenylenediamines (Scheme 1).
1.0
0.8
In the first experiments, Ru/C catalyst was prepared by mix-
ing predetermined quantities of the appropriate homogeneous
stock solutions of metal chloride precursors containing Brij35
0.6
m-DNB
m-NA
m-PDA
0.4
O2N
O2N
H2N
Cat.
H2
Cat.
H2
NO2
NH2
NH2
0.2
0.0
R
a
R
b
R
c
0
20
40
60
80
100
120
140
Time /min
( R = H,CH3,OH,NH2,OCH3,F,COOH)
Figure 1. Product distribution as a function of time for the
hydrogenation of m-DNB catalyzed by Ru/C (90 ꢁC, 1.5 MPa,
m-DNB 17 mmol, 5% Ru/C 0.1 g, CH3OH 20 mL).
Scheme 1. Simplified reaction scheme for the hydrogenation of
dinitroaromatics.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.9 (2008)
975
Table 1. Selective partial hydrogenation of various dinitrobenzenes to corresponding nitroanilines catalyzed by 5% Ru/Ca
Entry
Substrate
Product
T/ꢁC P/MPa Conversion/%b Selectivity/%c
1
2
3
4
m-Dinitrobenzene
o-Dinitrobenzene
p-Dinitrobenzene
m-Nitroaniline
o-Nitroaniline
p-Nitroaniline
90
90
90
90
1.5
1.5
1.5
1.5
100
100
99
99
95
99
92
2-Methyl-1,3-dinitrobenzene
2-Methyl-3-nitroaniline
2-Methyl-5-nitroaniline
4-Methyl-3-nitroaniline
2,5-Dimethyl-4-nitroaniline
2-Hydroxy-5-nitroaniline
2-Amino-4-nitroaniline
4-Amino-2-nitroaniline
2-Methoxy-5-nitroaniline
3-Amino-5-nitrobenzoic acid
100
77e
21e
5
1-Methyl-2,4-dinitrobenzene
90
1.5
98
98d
99d
6
7
1,4-Dimethyl-2,5-dinitrobenzene
2,4-Dinitrophenol
90
90
1.5
1.5
95
100
100
100
81e
18e
8
2,4-Dinitrobenzenamine
90
1.5
100
9
10
1-Methoxy-2,4-dinitrobenzene
3,5-Dinitrobenzoic acid
90
50
1.5
1.0
1.0
98
100
100
96
97
95
11 2-Hydroxy-3,5-dinitrobenzoic acid 3-Amino-2-hydroxy-5-nitrobenzoic acid 50
2-Fluoro-5-nitroaniline
4-Fluoro-3-nitroaniline
46e
51e
12
1-Fluoro-2,4-dinitrobenzene
50
1.0
100
97d
aReaction conditions: substrate = 17 mmol, CH3OH = 20 mL, catalyst = 0.1 g. bConversion of dinitrobenzenes. cSelectivity of nitroani-
d
e
lines. Total selectivity of nitroanilines. Ratio of two partially hydrogenated isomers.
can be attributed to the relatively low electron density on the
benzene ring, which may facilitate the reduction of nitro groups
on these substrates.
Hunger, Industrial Organic Pigments: Production, Proper-
ties, Applications, 3rd rev. ed., Wiley-VCH, Weinheim,
2004, p. 185.
Furthermore, it is worthy to note that 2,4-dinitrobenzene
derivatives might produce two isomeric partially hydrogenated
products. But it was interesting that the nitro group at the
ortho position of most 2,4-dinitrobenzenes with electrophobic
substitutent preferred to be hydrogenated than the nitro group
at the para position. For example, the single partially hydrogen-
ated products of 7a and 9a were 2-hydroxy-5-nitroaniline and
2-methoxy-5-nitroaniline, respectively. Additionally, 5a and
8a also presented obvious regioselectivity (Table 1), the prod-
ucts of partially hydrogenation of 8a were 2-amino-4-nitroani-
line and 4-amino-2-nitroaniline, and the ratio was 81 to 18; for
5a, the ratio of two isomeric products was 77 to 21. It also should
be noted that there was no obvious selectivity to two isomeric
products (46:51) of 1-fluoro-2,4-dinitrobenzene (12a), which
might be attributed to the inductive effects of this special
deactivating group (–F).
In summary, an operationally simple and highly efficient
procedure for selective partial hydrogenation of various substi-
tuted dinitrobenzenes has been developed. This catalytic trans-
formation using Ru/C catalyst is based on a tandem process
involving the initial hydrogenation of one nitro group into an
amino group to obtain nitroanilines and subsequent transfer
reduction of nitroanilines. We are confident that this simple
methodology will be of interest to a wide range of synthetic
organic chemists since it greatly facilitates the route to acquire
a number of new substituted nitroanilines from dinitroaromatics.
In addition, efforts to explain the mechanism of this catalytic
reaction are now in progress.
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This work is partly supported by China National 863
Project, Grant No. 2007AA03Z345.
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1
14 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/.
Published on the web (Advance View) August 23, 2008; doi:10.1246/cl.2008.974