Regioselective dechlorination of 2,3-dichloronitrobenzene into 3-chloronitrobenzene and regioselective dechlorination-hydrogenation into 3-chloroaniline

Article abstract of DOI:10.1016/S0040-4039(01)00103-4

Tetrakis(triphenylphosphine)palladium, Pd(PPh₃)₄, is an efficient catalyst for the selective dechlorination of 2,3-dichloronitrobenzene into 3-chloronitrobenzene. During the reaction the selectivity is over 90% in a very reproducible manner and the reaction can be stopped at the maximum selectivity. Pd(PPh₃)₄ is also a catalyst for the one-pot transformation of 2,3-dichloronitrobenzene into 3-chloroaniline (selectivity >60%).

Full text of DOI:10.1016/S0040-4039(01)00103-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2301–2303
Regioselective dechlorination of 2,3-dichloronitrobenzene into
3-chloronitrobenzene and regioselective
dechlorination–hydrogenation into 3-chloroaniline
Ariane Angeloff,a Jean-Jacques Brunet,a,* Pierre Legars,b Denis Neibeckera and Denis Souyrib
aLaboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France
bSocie´te´ Nationale des Poudres et Explosifs, Chemin de la Loge, 31078 Toulouse Cedex 04, France
Received 15 January 2001; accepted 18 January 2001
Abstract—Tetrakis(triphenylphosphine)palladium, Pd(PPh3)4, is an efficient catalyst for the selective dechlorination of 2,3-
dichloronitrobenzene into 3-chloronitrobenzene. During the reaction the selectivity is over 90% in a very reproducible manner and
the reaction can be stopped at the maximum selectivity. Pd(PPh3)4 is also a catalyst for the one-pot transformation of
2,3-dichloronitrobenzene into 3-chloroaniline (selectivity >60%). © 2001 Elsevier Science Ltd. All rights reserved.
Polychloroarenes are often by-products of industrial
reactions and are persistent environmental pollutants
which require effective means of disposal.1 In the last
few years many studies have been devoted to the prob-
lem of dechlorination of polychloroarenes,2 which
appears to be a necessary step before incineration.3
However, some chloroarenes are also important chemi-
cals for the manufacture of fertilizers, herbicides,
etc….4 Therefore, from both an economic and environ-
mental point of view, it would be challenging to trans-
form useless, toxic polychloroarenes into chloroarenes
that can be used for further chemical synthesis.
erogeneous platinum catalysts seem to be the most
efficient.6 However, the only catalytic systems for
reaction (1b) are based on Pd/C in highly acidic
media (e.g. aqueous HCl 4 M) in the presence of
iodides or Lewis acids (150°C, 20 bars H2).7,8 These
experimental conditions are prohibitive for an
industrial application.
3. For the (2a+2b) sequence, only a few catalytic sys-
tems have been tested for the selective dechlorina-
tion of chloronitrobenzenes into nitrobenzene.
These are palladium-based catalysts, either
heterogeneous9,10 or homogeneous.11,12
On the basis of the above data, it was decided to
investigate the possible transformation of 2,3-DCNB
according to the (2a+2b) sequence:
“ either in two steps: since reaction (2b) is possible
with platinum-based catalysts (vide supra) the prob-
2,3-Dichloronitrobenzene (2,3-DCNB) is a by-product
of the nitration of 1,2-dichlorobenzene. We have con-
sidered the transformation of 2,3-DCNB into 3-
chloroaniline (3-CA), which is used for the manufacture
of the herbicide chlorpropham. Formally, this transfor-
mation can be carried out according to two reaction
sequences (Scheme 1). Examination of the literature on
the reactions depicted in Scheme 1 led to the following
conclusions:
1. The particular case of 2,3-DCNB has never been
considered.
2. For what concerns the (1a+1b) sequence, several
catalytic systems allow the selective hydrogenation
of monochloronitrobenzenes into the corresponding
chloroanilines (reaction 1a).5,6 For this reaction, het-
Keywords: chloronitroarenes; dechlorination; hydrogenation; palla-
dium catalyst.
Scheme 1. The two possible reaction sequences for the trans-
* Corresponding author. E-mail: brunet@lcc-toulouse.fr
formation of 2,3-DCNB into 3-CA.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00103-4

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A. Angeloff et al. / Tetrahedron Letters 42 (2001) 2301–2303
Figure 1. Dechlorination of 2,3-DCNB with 15% Pd(PPh3)4 (curves A) and 8% Pd(PPh3)4 (curves B).
lem was to find a regioselective dechlorination of
2,3-DCNB into 3-chloronitrobenzene (3-CNB).
“ Or in one-step: in this case, no literature data could
direct the choice of a catalytic system.
maximum selectivity obtained with this catalyst. Later,
3-CNB is transformed into NB and 3-CA. AN forms
slowly only after 6 h reaction.
Curves B: up to total conversion of 2,3-DCNB, the
reaction occurs in the same manner as with 15% cata-
lyst (curves A) to give de 3-CNB (91%) and NB (9%),
with slightly lower selectivity than above. The differ-
ence only appears later: nitro groups are more rapidly
hydrogenated, 3-CNB into 3-CA and NB into AN.
2,3-DCA is never detected. The most interesting result
is that, after 7 h under these conditions, the percentage
of 3-CA reaches 62%, which is the best result for the
one-pot transformation of 2,3-DCNB into 3-CA.
Preparative runs on ca. 10 g of 2,3-DCNB indicated
that the same selectivities are observed.
In a preliminary study, six palladium-based catalytic
systems, either heterogeneous (Pd/C, Pd/Al2O3, PVP-
PdCl2) or homogeneous (Pd(OAc)2/PEt3, Pd(OAc)2/
PCy3), Pd(PPh3)4) were tested for the transformation of
2,3-DCNB under 1 atm hydrogen pressure. Monitoring
the reactions by GC analysis indicated that, among the
six palladium catalysts considered, Pd(PPh3)4 exhibits
the more promising selectivity for the dechlorination of
2,3-DCNB into 3-CNB.
When a solution of 2,3-DCNB (6.2 mmol) in DMF (15
mL) in the presence of AcOK (7.5 mmol) and
Pd(PPh3)4 is heated from 20 to 120°C (internal temper-
ature 116 2°C, reached in 0.3 h) under hydrogen (1
atm), GC and GC–MS analyses indicate the highly
selective formation of 3-CNB which is later trans-
formed into nitrobenzene (NB), 3-CA or aniline (AN),
depending on the catalyst concentration (vide infra).
No loss of material is observed (GC calibration with
anisole).
Unexpectedly, when the catalyst concentration is
decreased to 1%, the transformation of 2,3-DCNB is no
more selective. Large proportions of AN and 3-CA are
formed before total conversion of 2,3-DCNB. For
instance, after 3 h reaction, GC analysis indicates the
following product distribution: 2,3-DCNB (11%), 2,3-
DCA (6%), 3-CA (15%), 3-CNB (21%), NB (9%), AN
(30%). A similar observation has been mentioned by
Ohta et al. for the dechlorination of 4-chloronitroben-
zene with the same catalyst.12 At the beginning,
Pd(PPh3)4 selectively dechlorinates 2,3-DCNB to 3-
CNB, but after some turnovers hydrogenation occurs.
These observations suggest that there is a change in the
state of the palladium species, maybe from Pd(PPh3)n to
metallic palladium. The observed effect on the selectiv-
ity obviously depends on the catalyst concentration.
When the Pd(PPh3)4 concentration is 15% (mol/mol),
2,3-DCNB is consumed within 4 h. At that moment,
the selectivity in 3-CNB is 94%. When the catalyst
concentration is reduced to 8%, the selectivity in 3-
CNB after total consumption of 2,3-DCNB is 91%. The
above selectivities proved to be reproducible within
1–2% variations, depending on the exact time of sam-
pling. The corresponding curves are shown in Fig. 1.
In conclusion, tetrakis(triphenylphosphine)palladium,
Pd(PPh3)4, is an efficient catalyst for the selective
dechlorination of 2,3-dichloronitrobenzene into 3-
chloronitrobenzene. During the reaction the selectivity
is over 90% in a very reproducible manner and the
reaction can be stopped at the maximum selectivity.
Curves A: up to 50% conversion (2 h), the reaction is
fully selective. Small amounts of 2,3-DCA are formed,
but never exceed 1–2%. After 3 h, NB is slowly formed.
When 2,3-DCNB is totally consumed (4 h), the reaction
medium contains 3-CNB (94%), 2,3-dichloroaniline
(2,3-DCA) (1%) and NB (4%), corresponding to the

A. Angeloff et al. / Tetrahedron Letters 42 (2001) 2301–2303
2303
This is the first reported example of both chemoselec-
tive (versus the nitro group) and regioselective (versus
chlorine in the meta position) dechlorination of a poly-
chloronitrobenzene. Pd(PPh3)4 is also a catalyst for the
one-pot transformation of 2,3-dichloronitrobenzene
into 3-chloroaniline (selectivity >60%). This is the first
example of the regioselective dechlorination–hydro-
genation sequence of polychloronitrobenzenes.
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The authors gratefully thank the Centre National de la
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Poudres et Explosifs for financial support.
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