Dehalogenation of polychloroarenes with sodium formate in propan-2-ol catalyzed by RhCl(PPh3)3

Article abstract of DOI:10.1039/b102191p

Polychloroarenes can be dehalogenated with sodium formate in propan-2-ol using RhCl(PPh₃)₃ as the catalyst. The hydrogenolysis of 1,2,4-trichlorobenzene is selective to 1,2-dichlorobenzene, whereas the hydrogenolysis of 1,3- and 1,4-dichlorobenzene is sequential and selective according to the sequence: chlorobenzene > benzene.

Full text of DOI:10.1039/b102191p

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Dehalogenation of polychloroarenes with sodium formate in
propan-2-ol catalyzed by RhCl(PPh3)3
Mar•

a A. Atienza,a Miguel A. Esteruelas,*b Marta Ferna
nb

ndez,a Juana Herreroa and
Montserrat Oliva

L e t t e r
a Departamento de Qu•
b Departamento de Qu•


mica, Universidad de Cantabria, 39005 Santander, Spain
mica Inorganica, Instituto de Ciencia de Materiales de Aragon,


Universidad de Zaragoza-C.S.I.C., 50009 Zaragoza, Spain. E-mail: maester=posta.unizar.es
Received (in Strasbourg, France) 5th March 2001, Accepted 20th April 2001
First published as an Advance Article on the web 16th May 2001
Polychloroarenes can be dehalogenated with sodium formate in
propan-2-ol using RhCl(PPh ) as the catalyst. The hydro-
3
3
genolysis of 1,2,4-trichlorobenzene is selective to 1,2-
dichlorobenzene, whereas the hydrogenolysis of 1,3- and
(
1)
1
,4-dichlorobenzene is sequential and selective according to the
system catalyzes the selective dechlorination of 1,2,4-trichloro-
benzene to 1,2-dichlorobenzene and the dechlorination of 1,3-
and 1,4-dichlorobenzene to chlorobenzene, which then yields
benzene at much slower rates.
The formation of acetone is not observed during the hydro-
genolysis of these substrates. This suggests that the solvent
does not act as a hydrogen source for the process and, in
accord with eqn. (1), sodium formate is the only active
reducing reagent in the reaction medium.
sequence: chlorobenzene > benzene.
Chloroarenes can be used in organic synthesis as precursors
for a wide variety of valuable products.1 However, their accu-
mulation in the environment implies a serious health hazard.2
Therefore, from the environmental point of view, the develop-
ment of simple and economical methods for the reductive
dehalogenation of aryl chlorides should be a target with high
priority.
The yields of the reactions, after 85 min, are given in Table
A limited number of methods for the hydrogenolysis of
chloroarenes containing homogeneous metal catalysts have
been reported as a result of the e†ort in this Ðeld.3 They
involve the use of molecular hydrogen,4 alcohols,5 cyclic
amines,6 magnesium dihydride,7 sodium hydride8 and alkyl
Grignard reagents9 as the hydrogen source.
1
. The preferred formation of 1,2-dichlorobenzene during the
dehalogenation of 1,2,4-trichlorobenzene suggests that the
extraction of the chlorine in position 4 is favored. This agrees
well with the fact that the dehalogenation of 1,2-dichloro-
benzene is slower than the dehalogenation of the related 1,4-
Table 1 Dehalogenation of polychloroarenes with sodium formate
catalyzed by RhCl(PPh ) a
Among the potential hydrogen sources, formic acid and its
salts should occupy a special place since the ease of hydrogen
donation is higher than with most donors. This is due to the
3 3
Substrate
Product
Time/min Yield (%)
fact that the stable CO molecule should be formed during the
2
hydrogenolysis. However, its use has been limited to a few
systems, mainly heterogeneous. Chlorotoluene has been
hydrodehalogenated with formic acid and its salts in the pres-
ence of palladiumÈcarbon (10% Pd/C).10 Milstein and co-
workers11 have reported that p-substituted chlorobenzenes
undergo efficient homogeneously catalyzed reductive dechlori-
8
5
88
nation to the corresponding arenes, using Pd(dippp) as cata-
lyst [dippp \ 1,3-bis(diisopropylphosphino)propane] and
sodium formate in alcohol or DMF as the hydrogen source.

Joo and co-workers12 have observed that when an aqueous
solution of sodium formate is the hydrogen donor and
8
5
5
3b
2
8
70
RuCl(TPPMS) (TPPMS \ m-sulfophenyl diphenylphosphine
2
anion) the catalyst, CÈX bonds of carbon tetrachloride, chlo-
roform and 1-hexylhalogenides are transformed into CÈH
bonds. However, under the same conditions, chlorobenzene is
inert.
8
5
40
In two recent papers, we have shown that coordination
complexes of metals of Groups
8
and 9, chieÑy
85
5c
RhCl(PPh ) ,13 catalyze the dehalogenation of poly-
3
3
chloroarenes with HSiEt .14 Now, we report that, in the pres-
a Conditions: T \ 85 ¡C; propan-2-ol (3 ml) as solvent; 2.4 mmol of
polycloroarene, 0.07 mmol of RhCl(PPh ) , 2.4(x ] 1) mmol
3
ence of RhCl(PPh ) , polychloroarenes can be dehalogenated
3
3
3
3
(
x \ number of chlorine atoms of the polychloroarene) of sodium
not only with HSiEt but also with sodium formate in
3
formate. b After 5000 min the yield of the reaction is ca. 6%. c Under
the conditions used for the polychloroarenes, an analyzable quantity
of benzene was not observed. The reported yield was obtained when
propan-2-ol [eqn. (1)].
The catalytic reactions were carried out under reÑux using
Rh : C H Cl and NaHCO : C H Cl molar ratios of
[
PhCl] was 4.9 M. After 5000 min the yield of the reaction is ca. 8%.
6
6~x
x
2
6 6~x
x
1
: 34 and x ] 1 : 1, respectively. Under these conditions, the
DOI: 10.1039/b102191p
New J. Chem., 2001, 25, 775È776
775
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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isomer. However, the preferred formation of 1,2-dichloro-
benzene is in contrast with previous studies on the oxidative
addition of 1,2,4-trichlorobenzene to phosphineÈnickel(0) com-
plexes, which show that the addition of the 4-ClÈC bond is
less favored than the oxidative addition of the 2-ClÈC bond.15
The dehalogenation of 1,2-dichlorobenzene is much slower
than the dehalogenation of the 1,3-isomer. This, together with
the preferred formation of 1,2-dichlorobenzene from 1,2,4-tri-
chlorobenzene, indicates that the dechlorination of chloro-
arenes containing chlorines in adjacent positions is more
difficult than the dehalogenation of those containing chlorines
in non-adjacent positions. This fact is characteristic for cata-
lysts of Groups 8 and 9 and appears to be independent of the
hydrogen source,14 it can be related to the capacity for coor-
dinating to the metal of the chlorine situated in the position
adjacent to the activated chlorine.16
conditions: 2.4(x ] 1) mmol (x \ number of chlorine atoms of
the polychloroarene used) of sodium formate was placed in
the Ñask. 0.07 mmol of RhCl(PPh ) was dissolved in 3 ml of
a propan-2-ol solution containing 2.4 mmol of the poly-
chloroarene and 200 lL of p-xylene, and added to the Ñask.
The Ñask was then immersed in an oil bath at 85 ¡C, and the
reaction solution magnetically stirred.
3
3
Acknowledgements
We acknowledge Ðnancial support from the DGES of Spain

(Project PB98-1591) and from the Comision Mixta Caja
Cantabria-Universidad de Cantabria.
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sorb GHP 80/100-mesh column and p-xylene as the internal
standard. The oven conditions were 35 ¡C (hold 4 min) to
1
999, 845.
1
0
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80 ¡C at 20 ¡C min~1 (hold 5 min). The reaction products
1
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
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General procedure
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1
776
New J. Chem., 2001, 25, 775È776