Highly efficient catalytic hydrodehalogenation of polychlorinated biphenyls (PCBs)

Article abstract of DOI:10.1039/b910415a

The hydrodehalogenation of polychlorinated phenyls and biphenyls (PCBs), catalysed by palladium N-heterocyclic carbene complexes, proceeds with excellent yields, at very low catalyst loadings and at room temperature, using isopropanol as the hydrogen source and NaOH as the base.

Full text of DOI:10.1039/b910415a

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Highly efficient catalytic hydrodehalogenation of polychlorinated
biphenyls (PCBs)w
a
b
ab
Saı
¨
d Akzinnay,z Fabrice Bisaro and Catherine S. J. Cazin*
Received (in Cambridge, UK) 27th May 2009, Accepted 20th August 2009
First published as an Advance Article on the web 7th September 2009
DOI: 10.1039/b910415a
The hydrodehalogenation of polychlorinated phenyls and
biphenyls (PCBs), catalysed by palladium N-heterocyclic
carbene complexes, proceeds with excellent yields, at very low
catalyst loadings and at room temperature, using isopropanol as
the hydrogen source and NaOH as the base.
promoted by a well-defined palladium complex bearing a
N-heterocyclic carbene (NHC).
1,2-Dichlorobenzene was chosen as a model substrate for
the initial assessment of [Pd(m-Cl)Cl(IPr)] , 1, in hydro-
2
dehalogenation, with isopropanol as hydrogen source and
t
solvent, and KO Bu as base (Table 1). This preliminary
Polychlorinated biphenyls (PCBs) belong to the list of
persistent organic pollutants (POPs) included in the Stockholm
investigation showed that the reaction proceeds at room
temperature, and that within 10 minutes, quantitative
dehalogenation occurred, using 2 mol% Pd (Table 1, entry 1).
Attempts to decrease the catalyst loading were then under-
taken and it was found that, at room temperature, 1 mol% Pd
was at least needed for the reaction to reach completion, but
required 24 h (Table 1, entries 2–4). The use of higher
temperatures was then envisioned and permitted to decrease
the catalyst loading to 0.04 mol% Pd at 60 1C (Table 1,
entries 5–7) and 0.008 mol% Pd at 80 1C (Table 1,
entries 8 and 9). The use of even lower catalyst loadings was
also attempted, but led to poorer conversion.
1
Convention. These substances were heavily used from the
1
930s to the 1970s by industry, mainly as insulation in
electrical transformers. During this period, there was often
no effective control on waste disposal so these substances have
spread throughout the environment, at such an extent that
almost all living creatures, including humans, have been
exposed to them. Awareness of the presence of PCBs in the
environment and their toxicity to humans and wildlife grew in
the 1960s and 1970s, leading to a ban in 1979 on their sale
2
and production. The destruction of PCBs is often performed
by incineration, which raises environmental concerns and
prompted researchers to develop greener remediation
methodologies. Amongst those, hydrodehalogenation has been
shown to be a viable alternative by means of biodegradation
Investigation of the catalyst sensitivity was then undertaken.
While the reaction could not be conducted under air (Table 1,
i
entry 10), the presence of water (5% in PrOH) did not
significantly alter catalytic activity (at rt, with 0.2 mol% Pd
43% of benzene formed).
3
4
strategies and transition metal catalysed processes. Among
the various transition metals, palladium is the most commonly
used, both for heterogeneous and homogeneous reactions.
The use of EtOH and MeOH, as solvents and hydrogen
sources, was also attempted. For these experiments, sealed
tubes were used; in both cases, the reaction did not take place
(Table 1, entries 11 and 12). In order to further assess the
2
The dimeric species of type [Pd(m-Cl)Cl(NHC)] are efficient
5
systems for cross coupling reactions. [Pd(m-Cl)Cl(IPr)]
(
2
, 1,
0
IPr = N,N -bis-(2,6-(diisopropyl)phenyl)imidazol-2-ylidene)
a
Table 1 Hydrodehalogenation of 1,2-dichlorobenzene
was recently shown to be an excellent precatalyst for the
Suzuki–Miyaura coupling involving challenging aryl chlorides
at low catalyst loading and under mild and economical
5b
reaction conditions. Since it has been suggested that the
Suzuki–Miyaura reaction and the catalytic hydrodehalogenation
are intertwined, sharing the oxidative addition step and leading
b
Conv. (%)
Cat. loading/
Entry mol% (Pd)
6
Solvent T/1C Time
a
b
in both instances to the (IPr)–Pd(0) species after one turnover,
i
we believed that in the absence of boronic acid, our system
might effectively perform the hydrodehalogenation reactions
of polychlorinated phenyls and biphenyls (PCBs). Herein
we disclose the hydrodehalogenation of PCBs efficiently
1
2
3
4
5
6
7
8
9
1
1
1
2
1
0.5
0.2
0.1
0.04
0.02
0.008
0.004
0.2
PrOH
PrOH
PrOH
PrOH
PrOH
PrOH
PrOH
PrOH
PrOH
PrOH
rt
rt
rt
rt
60
60
60
80
80
rt
10 min
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
24 h
—
—
—
37
—
—
15
—
23
—
8
100
100
95
i
i
i
i
i
i
i
i
i
63
100
100
85
a
Institute of Chemical Research of Catalonia (ICIQ) Av. Paı¨sos
Catalans 16, 43007 Tarragona, Spain
School of Chemistry, University of St Andrews, St Andrews, Fife,
UK KY16 9ST. E-mail: cc111@st-andrews.ac.uk;
c
100
76
b
d
0
1
2
0.004
0.004
EtOH
MeOH
80
80
—
—
Fax: +44 (0)1334 463 808; Tel: +44 (0)1334 463 698
w Electronic supplementary information (ESI) available: Procedures
for hydrodehalogenation, synthesis of chlorinated biphenyl substrates
1
a
t
Reaction conditions: 1,2-dichlorobenzene (1 mmol), KO Bu
13
1
b
(2.2 mmol), solvent (2 mL). Conversion to C
and related C{ H} NMR spectra. See DOI: 10.1039/b910415a
6 6 6 5
H and/or C H Cl,
z Visiting student: IUP Chimie Applique
Universite d’Orleans, rue de Chartres, BP6759, Orle
France.
´
e, Faculte
´
des Sciences,
ans F-45069,
based on 1,2-dichlorobenzene, determined by GC, minimum average
´
´
´
c
of two runs. TON per Cl = 25 000. Reaction performed under air.
d
5
752 | Chem. Commun., 2009, 5752–5753
This journal is ꢀc The Royal Society of Chemistry 2009

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a
a
Table 2 Hydrodehalogenation of 1,2,4,5-tetrachlorobenzene
Table 3 Hydrodehalogenation of polychlorinated biphenyls
b
c
Cat loading Conv.
(mol% Pd) (%)
Yield
(%)
TON
(per Cl)
Cat.
Entry Substrate
1
b
Conv. (%)
loading/
mol%
(Pd)
0.04
0.04
499
499
92
91
2 500
7 500
c
Entry Catalyst
Base
a
b
c
d
t
KO Bu — — — 100
d
1
2
3
4
5
6
7
8
9
[Pd(m-Cl)Cl(IPr)]
[Pd(m-Cl)Cl(IPr)]
[Pd(m-Cl)Cl(SIPr)]
[Pd(m-Cl)Cl(IMes)]
2
2
1
1
0.04
0.02
0.02
0.02
0.02
2
3
t
KO Bu
—
1
4
95
KO Bu — — 26 64
t
2
2
3
t
KO Bu — 16 73 11
2
0.04
1
499
499
95
84
10 000
1 000
t
KO Bu — 62 34
[Pd(m-Cl)Cl(SIMes)]
2
4
4
3
t
[Pd(Z -cinnamyl)Cl(IPr)] 5 0.02
KO Bu — — 55 45
t
3
[Pd(Z -allyl)Cl(IPr)] 6
0.02
0.04
0.04
KO Bu
—
9 51 40
4
[Pd(m-Cl)Cl(IPr)]
[Pd(m-Cl)Cl(IPr)]
2
1
1
NaOMe 4 14 48 23
NaOH — — — 100
d
2
a
i
Reaction conditions: substrate (0.04–1 mmol), PrOH (2–3 mL),
b
NaOH (10% excess with respect to Cl), 24 h. Conversion to
a
i
Reaction conditions: 1,2,4,5-tetrachlorobenzene (0.5 mmol), PrOH
b
2 mL), base (2.2 mmol), 24 h. Conversion to hydrodehalogenation
(
products, based on 1,2,4,5-tetrachlorobenzene, determined by GC,
biphenyl, based on chlorinated substrate, determined by GC.
c
Isolated yield, minimum average of two reactions.
c
minimum average of two runs. Ratio of dichlorobenzene isomers:
d
ortho–meta–para = 1 : 0.1 : 0.3. TON per Cl = 10 000.
Application of the methodology to mono- and poly-
chlorinated biphenyl derivatives was successful, as all reactions
proceeded quantitatively. For mono-, tri- and tetra-chlorobiphenyl
substrates, very low catalyst loadings were used (0.04 mol%
Pd) to achieve complete conversion to biphenyl (Table 3,
entries 1–3). Remarkably, our system was able to fully
generality of the reaction, hydrodehalogenation of 1,2,4,5-
tetrachlorobenzene,
a more challenging polychlorinated
phenyl derivative, was studied (Table 2).
The reaction proceeded smoothly towards the formation of
t
benzene, when using KO Bu (10% excess with respect to Cl) as
the base, and 0.04 mol% Pd of catalyst 1, in isopropanol.
hydrodehalogenate decachlorobiphenyl (Table 3, entry 4).
8
In conclusion, the commercially available [Pd(m-Cl)Cl(IPr)]
(
Table 2, entry 1). The catalyst loading was further
decreased, and a satisfactory 95% conversion was obtained
Table 2, entry 2). These reaction conditions were used to
compare catalytic efficiency of congeners of complex 1, the
dimers [Pd(m-Cl)Cl(SIPr)] , 2, [Pd(m-Cl)Cl(IMes)] , 3 and
Pd(m-Cl)Cl(SIMes)] , 4 (Fig. 1). The nature of the NHC
ligand leads to drastic changes in catalyst performance, with
conversions to benzene ranging from to 95%, the
2
,
1, was shown to hydrodehalogenate polychlorinated phenyls
(
and PCBs under very mild reaction conditions, using inexpensive
reagents. While strong bases, strong reducing agents and harsh
reaction conditions are generally used for such transformations,
NaOH as base and isopropanol as reducing agent were found
sufficient in the present system. The methodology developed
offers a very powerful, low-cost and safe technology for the
destruction of PCBs, whose proliferation in the environment
continues to be a serious health issue. It also offers a system
capable of promoting the room temperature dechlorination of
chloroarenes at low Pd loading, a reaction of significant
2
2
[
2
4
IPr-bearing system being the most efficient (Table 2, entries 2–5).
Interestingly, the monomeric complexes bearing an IPr ligand
3
3
[
Pd(Z -cinnamyl)Cl(IPr)], 5, and [Pd(Z -allyl)Cl(IPr)], 6
(
Fig. 1), performed poorly compared to [Pd(m-Cl)Cl(IPr)] , 1
2
7
(
Table 2, entries 2, 6 and 7). A drawback of the system
9
interest in organic synthesis.
t
depicted so far is the cost of the base used (KO Bu). Screening
of alkoxide bases showed that while unsatisfactory results
were obtained with NaOMe (complete formation of benzene
was achieved only when using 150% excess with respect to Cl),
complete dehalogenation was achieved with the inexpensive
NaOH (Table 2, entries 8 and 9).
We thank the ICIQ Foundation and the University of
St Andrews for funding.
Notes and references
1
2
UNEP, UNEP 2007 Annual Report, 2008, 90.
P. G. Shields, Cancer Epidemiol. Biomarkers Prev., 2006, 15, 830.
Finally, the best system found ([Pd(m-Cl)Cl(IPr)]
–NaOH– PrOH) was applied to the dehalogenation of a
2
,
3 For a recent review, see: K. Furukawa and H. Fujihara, J. Biosci.
Bioeng., 2008, 105, 433.
i
1
4
For a review, see: F. Alonso, I. P. Beletskaya and M. Yus, Chem.
Rev., 2002, 102, 4009.
range of polychlorinated biphenyls (Table 3).
5
(a) M. S. Viciu, R. M. Kissling, E. D. Stevens and S. P. Nolan, Org.
Lett., 2002, 4, 2229; (b) O. Diebolt, P. Braunstein, S. P. Nolan and
C. S. J. Cazin, Chem. Commun., 2008, 3190; (c) C. E. Hartmann,
S. P. Nolan and C. S. J. Cazin, Organometallics, 2009, 28, 2915.
O. Navarro, N. Marion, Y. Oonishi, R. A. Kelly and S. P. Nolan,
J. Org. Chem., 2006, 71, 685.
N. Marion, O. Navarro, J. Mei, E. D. Stevens, N. M. Scott and
S. P. Nolan, J. Am. Chem. Soc., 2006, 128, 4101.
2
[Pd(m-Cl)Cl(IPr)] , 1, is commercially available in research and large
quantities from Aldrich, Strem and Umicore AG.
6
7
8
Fig. 1 Well-defined systems studied.
9 V. V. Grushin and H. Alper, Chem. Rev., 1994, 94, 1047.
Chem. Commun., 2009, 5752–5753 | 5753
This journal is ꢀc The Royal Society of Chemistry 2009