Palladium-catalyzed hydrodechlorination of ary chlorides and its mechanism

Article abstract of DOI:10.1016/j.ica.2010.08.031

Successful hydrodechlorinations of aryl chlorides were carried out in the presence of palladium catalyst supported by dppf (1,1′- bis(diphenylphosphino)ferrocene) and sodium formate in DMA (N,N- dimethylacetamide). A series of substituted aryl chlorides as substrates were studied to investigate the influence of electronic effects on the reaction. It was found that the substrates with electron-donating groups are more active than those with electron-withdrawing groups. A proposed mechanism of hydrodechlorination via decarboxylation and reductive elimination was discussed with the supported of in situ IR data. It is suggested that the decarboxylation is the key step of the reaction. This inference of the mechanism is consistent with the results from the in situ IR experiments.

Full text of DOI:10.1016/j.ica.2010.08.031

Inorganica Chimica Acta 365 (2011) 133–136
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Palladium-catalyzed hydrodechlorination of ary chlorides and its mechanism
⇑
Chen Zhang, Xiaoyan Li, Hongjian Sun
School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27, 250100 Jinan, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Successful hydrodechlorinations of aryl chlorides were carried out in the presence of palladium catalyst
supported by dppf (1,1 -bis(diphenylphosphino)ferrocene) and sodium formate in DMA (N,N-dimethyl-
Received 25 February 2010
Received in revised form 16 August 2010
Accepted 21 August 2010
Available online 26 August 2010
0
acetamide). A series of substituted aryl chlorides as substrates were studied to investigate the influence
of electronic effects on the reaction. It was found that the substrates with electron-donating groups are
more active than those with electron-withdrawing groups. A proposed mechanism of hydrodechlorina-
tion via decarboxylation and reductive elimination was discussed with the supported of in situ IR data.
It is suggested that the decarboxylation is the key step of the reaction. This inference of the mechanism
is consistent with the results from the in situ IR experiments.
Keywords:
Palladium
Dechlorination
Aryl chloride
In situ IR
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Organic halides are very important and versatile compounds
ent a simple and green catalytic hydrodechlorination of aryl
chloride with sodium formate in DMA in the presence of the palla-
dium catalyst supported by dppf. A proposed mechanism of this
with many applications as starting materials, solvents, reagents,
and intermediates in synthetic organic chemistry [1–3]. Especially,
aryl chlorides are much more economical than aryl bromides and
iodides, but the main problem of application aryl chlorides is
how to degrade them. According to ‘‘Stockholm Convention on Per-
sistent Organic Pollutants (POPs)”, some aryl chlorides are treated
as POPs. Because of the stable C–Cl bond, the complete degradation
of these compounds always needs many years.
Generally, there are four methods to solve these problems, het-
erogeneous catalysis [4,5], electrocatalysis degradation [6,7], bio-
degradation [8–10] and homogeneous catalysis [11,12]. Despite
the large variety of reducing agents available for the removal of
chlorine substituents from organic chlorides, new non-toxic, eco-
nomical, simple, and selective reducing systems are in constant
demand [13–16]. Several homogeneous catalytic systems for hyd-
rodechlorination of aryl chlorides have been reported. However
these catalytic systems have their own disadvantages: at high
reaction temperature or with long reaction time [17], under high
process was discussed according to the result of in situ IR study.
2
. Results
Chlorobenzene, 4-chlorophenetole, 2-chlorophenetole, and 4-
chloronitrobenzene were used as substrate to study the ligand ef-
fects under the conditions of 2.5 mol% PdCl (PhCN) , 2.5 mol%
2
2
bidentate phosphine ligands, 1.5 equiv sodium formate in 3 mL
DMA under nitrogen at 75 °C for 1 h (Eq. (1) and Table 1).
Cl
1.5 eq HCOONa, 2.5mol% Cat.
ð1Þ
3mL DMA, 75 ºC, 1h
R
R
From Table 1 it can be concluded that dppf is the best support-
ing ligand among the three chelate ligands to this catalytic system.
Under the same reaction conditions dppf was selected as the
supporting ligand and used in hydrodechlorination of more sub-
strates (Table 2). It can be seen that dppf is a good supporting li-
gand for not only the electron-rich but also electron-deficient
aromatic compounds. We consider that the steric hindrance of eth-
oxy group on ortho-position of 2-chlorophenole makes this reduc-
tion not perfect by comparing the result of 2-chlorophenetole to
that from 4-chlorophenetole.
2
pressure of H or with strong base [18], and in the volatile and
toxic solvents [19]. In addition, some bidentate phosphine ligands,
0
such as dppf (1,1 -bis(diphenylphosphino)ferrocene), dppe (1,2-
bis(diphenylphosphino)ethane), dppp (1,3-bis(diphenylphosphino)-
propane), were used in palladium-catalyzed C–C and C–N coupling
reaction [20–22]. This demonstrates that these ligands have poten-
tial ability in dechlorination of aryl chloride. In this paper we pres-
With reduced amount of the catalyst (0.5 mol%) and prolonged
reaction time (5 h), this catalytic system was further studied (Table
3). The results indicate that the reaction temperature is the key
⇑
Corresponding author. Tel.: +86 531 88361350; fax: +86 531 88564464.
E-mail address: hjsun@sdu.edu.cn (H. Sun).
0
020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.031

1
34
C. Zhang et al. / Inorganica Chimica Acta 365 (2011) 133–136
Table 1
Degree of conversion for dechlorination of aryl chlorides with different ligands.
Table 3
a
a
Degree of conversion for dechlorination with 0.5 mol% catalyst.
Substrates
Product
dppf
dppp
dppe
Temperature (°C)
b
b
>99b
>99b
3
Chlorobenzene
benzene
>99
>99
Substrates
Products
60 65
70
75
80
85
b
b
4
2
4
-Chlorophenetole
-Chlorophenetole
-Chloronitrobenzene
phenetole
phenetole
nitrobenzene
>99
>99
Chlorobenzene
benzene
benzene
benzene
benzene
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
>99
5
>99
>99
78
0
97
12
55
50
0
91
63
77
o-Dichlorobenzene
m-Dichlorobenzene
p-Dichlorobenzene
>99
>99
>99
78
b
>99
50
a
1
mmol substrate, 2.5 mol% cat. [PdCl
2 2
(PhCN) + bidentate phosphine ligand],
1,2,4-Trichlorobenzene benzene
>99
1
.5 eq HCOONa, 3 mL DMA, 75 °C, 1 h.
b
4-Chloroanisole
anisole
The residual amount of the starting chloroaromatics is <1% and no other by-
2
4
2
4
4
4
-Chlorophenetole
-Chlorophenetole
-Chlorotoluene
-Chlorotoluene
-Chlorobiphenyl
phenetole
phenetole
toluene
toluene
biphenyl
56
>99
products were detected.
>99
0
0
0
0
factor in this catalytic system. The chloroarenes with electron-
withdrawing groups need higher reaction temperature than those
with electron-donating groups. It can be also said that the
substrates with electron-donating groups are more active than
those with electron-withdrawing groups. This conclusion is oppo-
site to that by Milstein [17,23].
0
>99
9
>99
>99
8
>99
44
-Chloroacetophenone acetophenone
0
0
0
0
2-Chloronitrobenzene
3-Chloronitrobenzene
nitrobenzene
nitrobenzene
nitrobenzene
24
4-Chloronitrobenzene
0
a
1 mmol substrate, 0.5 mol% cat. [PdCl (PhCN)2 + dppf], 1.5 eq HCOONa, 3 mL
2
In order to explain this difference, in situ IR experiments were
carried out under the same catalytic reactions [24,25]. According
to the experimental results a different hydrodechlorination mech-
anism from that of Milstein and co-workers [17] is proposed in
Scheme 1. The oxidative addition of aryl chloride on palladium
centre is the first step. Ligand substitution of intermediate 1 by for-
mate anion affords intermediate 2 with the release of NaCl. Decar-
boxylation of intermediate 2 delivers the hydrido palladium(II)
intermediate 3. The end dechlorinated product is formed via reduc-
tive elimination through the combination of the hydrido-H and the
phenyl groups with the regeneration of palladium(0) species.
Because the chloroarenes with electron-donating group, such as
DMA, 1 h.
P
Pd(II)
Pd(0)
P
P
P
H
R
R
Pd
P
P
Cl
3
4
-chloroanisole, need lower reaction temperature (Table 3), it is
CO2
suggested that the decarboxylation is the key step of the reaction.
This inference of the mechanism is consistent with the results from
the in situ IR experiments.
The hydrodechlorination of 3-chloronitrobenzene was taken as
model reaction to track this process with in situ IR technology. In a
R
O
O
H
P
P
P
Cl
HCOONa
Pd
Pd
typical example, PdCl
2
(PhCN)2, dppf, 3-chloronitrobenzene and so-
P
NaCl
dium formate were added into DMA in the proper order. Then the
mixture was heated from 14 to 75 °C. When the characteristic peak
of 3-chloronitrobenzene disappeared, we stopped the reaction.
In the IR spectra of the substrate 3-chloronitrobenzene, at
R
R
1
2
ꢀ1
1
603 cm the C@C vibration of the phenyl ring was registered.
Scheme 1. Proposed mechanism of dechlorination.
The N@O vibration of the nitro group was found at 1538 and
ꢀ
1
ꢀ1
1
364 cm . The peak at 1066 cm is assigned to the C–Cl bond.
With heating the reaction mixture the reaction started. The charac-
teristic peak of (C@O) in coordinated carboxyl group of the
ꢀ1
of the reaction temperature the peak at 1655 cm becomes high-
er, so the concentration of intermediate 2 containing coordinated
formate anion increased. When the concentration and temperature
increased to certain extent, the hydrodechlorination started and
the concentration of the intermediate 2 reached a dynamic bal-
ance. A Pd–H band absorption (nearby 1800 cm ) was not spec-
troscopically detected in the in situ IR. It might be that the
equilibrium concentration of the intermediate 3 is below the limit
m
ꢀ1
intermediate 2 appeared at 1655 cm (Scheme 2) [26]. The wave-
number of this band is higher than that in free formate
ꢀ1
ꢀ1
(
1595 cm ). The peak of product (1052 cm , C–H bond vibration)
ꢀ1
appeared gradually with the slow decreasing of the peak intensity
ꢀ1
2
of the substrate (1364 cm , NO -group). In Scheme 3 the develop-
ing trends of the peak intensity are illustrated. With the increasing
Table 2
a
Dechlorination with 2.5 mol% catalyst.
Substrates
Products
Degree of conversionb Substrates
Products
Degree of conversionb
Chlorobenzene
benzene
benzene
benzene
benzene
>99
>99
>99
>99
>99
>99
2-chlorotoluene
4-chlorotoluene
4-chlorobiphenyl
toluene
toluene
biphenyl
>99
>99
>99
2-Dichlorobenzene
m-Dichlorobenzene
p-Dichlorobenzene
4-chloroacetophenone acetophenone >99
1
4
2
4
,2,4-Trichlorobenzene benzene
-Chloroanisole
-Chlorophenetole
-Chlorophenetole
2-Chloronitrobenzene
3-Chloronitrobenzene
4-Chloronitrobenzene
nitrobenzene
nitrobenzene
nitrobenzene
>99
>99
>99
anisole
phenetole 91
phenetole >99
a
1
mmol substrate, 2.5 mol% cat. [PdCl
2 2
(PhCN) + dppf], 1.5 eq HCOONa, 3 mL DMA, 75 °C, 1 h.
b
The residual amount of the starting chloroaromatics is <1%.

C. Zhang et al. / Inorganica Chimica Acta 365 (2011) 133–136
135
Scheme 2. The in situ IR spectra of the dechlorination of 3-chloronitrobenzene (all the IR spectra of the reaction with DMA as background).
Scheme 3. Peak intensity trend of characteristic peaks.
of the IR detection. In addition, it is also possible that the reductive
elimination is too fast to detect the hydride species or that the Pd–
H vibration is too weak to check up. This result accords with our
proposed mechanism.
the reaction temperature. The fluctuation of the peak intensity of
intermediate 2 could be thought as a result of the maldistribution
of its concentration in the solution, probably caused by the tem-
perature gradient or/and the slow stirring.
We consider that the dissolving of the substrate, 3-chloronitro-
benzene in DMA is slow, therefore its peak intensity (Scheme 3) in-
creases at the beginning time until it dissolves completely. It is not
clear why there is a drop between 20 and 22 min after the starting.
After the reaction the peak intensity of intermediate 2 disappeared
completely from the spectrum. It is sufficient to subtract the DMA
background though the reaction mixture at the beginning is a sus-
pension because it becomes a clear solution with the increasing of
3. Conclusions
Successful hydrodechlorinations of aryl chlorides were carried
out in the presence of palladium catalyst supported by dppf and
sodium formate in DMA. A series of substituted aryl chlorides as
substrates were studied to investigate the influence of electronic

1
36
C. Zhang et al. / Inorganica Chimica Acta 365 (2011) 133–136
effects on the reaction. It was found that the substrates with
electron-donating groups are more active than those with elec-
tron-withdrawing groups. A proposed mechanism of hydrodechlo-
rination via decarboxylation and reductive elimination was
discussed. It is suggested that the decarboxylation is the key step
of the reaction. This inference of the mechanism is consistent with
the results from the in situ IR experiments.
Acknowledgements
This work was supported by NSFC No. 20872080 and the
Science Foundation of Shandong Y2006B18.
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