A facile method for the dechlorination of mono- and dichlorobiphenyls using Raney Ni-Al alloy in dilute aqueous solutions of alkali hydroxides or alkali metal carbonates

Article abstract of DOI:10.1016/j.tet.2009.01.052

Raney Ni-Al alloy in a dilute aqueous alkaline solution of NaOH, KOH, CsOH, LiOH, or Ca(OH) ₂ or of alkali metal carbonates such as Na₂CO₃, K₂CO₃ becomes a very powerful reducing agent and dechlorinates the polychlorobiphenyl(PCB) congeners mono- and dichlorobiphenyls effectively to afford biphenyl and/or phenylcyclohexane, respectively. The reactions are run under mild conditions without the use of organic solvents. The reductive dechlorination of dichlorobiphenyls with a Raney Ni-Al alloy under ultrasonication was investigated, also.

Full text of DOI:10.1016/j.tet.2009.01.052

Tetrahedron 65 (2009) 2497–2505
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
A facile method for the dechlorination of mono- and dichlorobiphenyls using
Raney Ni–Al alloy in dilute aqueous solutions of alkali hydroxides
or alkali metal carbonates
Guo-Bin Liu a , Masashi Tashiro , Thies Thiemann
,
b
c,
*
*
a
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, PR China
National Institute of Advanced Industrial Science and Technology, AIST Central 5, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koh-en, Kasuga, Fukuoka 816-8580, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Raney Ni–Al alloy in a dilute aqueous alkaline solution of NaOH, KOH, CsOH, LiOH, or Ca(OH) 2 or of alkali
Received 4 December 2008
Received in revised form 14 January 2009
Accepted 14 January 2009
Available online 20 January 2009
2 3 2 3
metal carbonates such as Na CO , K CO becomes a very powerful reducing agent and dechlorinates the
polychlorobiphenyl(PCB) congeners mono- and dichlorobiphenyls effectively to afford biphenyl and/or
phenylcyclohexane, respectively. The reactions are run under mild conditions without the use of organic
solvents. The reductive dechlorination of dichlorobiphenyls with a Raney Ni–Al alloy under ultra-
sonication was investigated, also.
Ó 2009 Elsevier Ltd. All rights reserved.
3
1. Introduction
dechlorination. They are photochemical dechlorination, electro-
4
5
lytic reduction, sodium, potassium or calcium-based reduction,
Much attention has been focused on the remediation of envi-
ronmental pollution caused by chlorinated organic compounds
such as polychlorinated biphenyls (PCBs), which have been utilized
extensively in transformer oils, heat-exchanger fluids and carbon-
less copy paper for several decades. PCBs are hazardous substances
transition metal-promoted reductive dechlorination with sodium
borohydride or sodium alkoxyborohydride, Raney Ni-mediated
6
dechlorination using excess amounts of silyl hydride as an addi-
7
8
tional reductant, iron-promoted reduction and dechlorination
with an excess amount of sodium borohydride at elevated tem-
1
9
and are long-lived, once released into the biosphere. Although
peratures. Additionally, the Pd/C–Et
3
N mediated catalytic hydro-
10
nowadays the use and production of PCBs are severely restricted, it
has been estimated that the total world production from the 1930s
to the 1980s was between one and two million tons. Since PCBs
exhibit a high degree of chemical and biological stability, due to low
degradation by reductive and oxidative enzymatic pathways, and
also exhibit lipid solubility, they have accumulated in the envi-
ronment, especially along the food chains. Possible adverse effects
on wildlife and human health have been reported such as, in the
dechlorination has become of interest. Some disadvantages can
be noted for most of these documented processes. The combustion
of PCBs usually produces corrosive HCl gas and detectable amounts
2
of highly toxic chlorinated dioxins. Photochemical methods re-
quire UV irradiation. Thus, the efficiency of dechlorination is low
3
and the method is not of general practicability. Transition metal-
promoted reductive dechlorination needs stoichiometric or even an
6
,7
excess amount of transition metal. Also, in some cases, complete
1
5
gravest cases, induction of skin lesions and tumours. It has been
dechlorination cannot be achieved. Harsh reaction conditions such
7
–9
reported that PCBs have been detected in animal and human lipid-
as elevated temperaturesare necessary for many of the processes.
1
rich tissues and secretions, including mother-milk. Therefore, it is
Alkali metals as reductants, such as sodium and potassium, are
5
very important to develop effective methods, which could be used
to reductively dechlorinate these recalcitrant compounds. To the
best of our knowledge, the destruction of PCBs has usually been
troublesome to handle. All of the reactions using chemical re-
ductants have been carried out in organic solvents such as methanol
and N,N-dimethylformamide, some of which have considerable
toxicity. A generally practical dechlorination method for PCBs and
congeners has not yet appeared.
2
carried out by normal and plasma incineration. A few chemical
methods are known to transform PCBs and congeners through
Raney Ni–Al alloy, which is commercially available, cheap and
easy to handle, has been widely employed as a starting material for
preparing Raney Ni catalyst and as a reductant in 10–25% aq NaOH
solution in organic synthesis. In continuation of our work on the
dehalogenation of aromatic halides with Raney Ni–Al alloy in
*
Corresponding authors.
E-mail addresses: liuguobin@fudan.edu.cn (G.-B. Liu), thies@cm.kyushu-u.ac.jp
11
(
T. Thiemann).
0
040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.01.052

2498
G.-B. Liu et al. / Tetrahedron 65 (2009) 2497–2505
aqueous media,12 we turned our attention to the development of
a convenient and practical method for the disposal of PCBs. In
a preliminary communication, we have reported previously that
Raney Ni–Al becomes a very strong reducing agent in dilute
aqueous alkaline solutions and reduces monochlorobiphenyls
easily to give biphenyl and/or phenylcyclohexane. The method is
carried out under mild conditions and in the absence of organic
solvents.13 These successful results prompted us to develop a more
convenient and practical method for the disposal of PCBs. An ob-
vious advantage of this method is that water is employed as the
solvent and proton source. No organic solvent is used. The re-
duction is carried at atmospheric pressure. No special apparatus is
required.
Scheme 1.
(5 mmol, 942 mg) with 0.5 g of Raney Ni–Al alloy failed. Compound
1a (20%) remained unreacted, even though the reduction was found
to run to completion very quickly with 1.0 g of the Raney Ni–Al
alloy in 0.2 wt % aq NaOH solution (Table 1, run 12 vs 13).
Furthermore, the dechlorination of 1a at lower temperature
(60 C) under ultrasonication was examined. It should be noted that
ꢁ
In this paper, we wish to give a detailed report on the reductive
dechlorination of a series of pure PCB congeners, mono- and
dichlorobiphenyls with Raney Ni–Al alloy in dilute basic aqueous
solutions. The effects of the nature, the concentration and the
amount of the alkaline and alkali metal carbonate solution, the
amount of the Raney Ni–Al alloy used, the reaction temperature
and the order of addition of the reagents on the dechlorination are
investigated.
a mixture of 2 and 3 was obtained, when performing the reaction in
a 0.5 wt % aq NaOH solution according to method A or when car-
rying out the experiment according to method B by adding drop-
wise a 1 wt % aq NaOH solution to the reaction mixture (Table 1,
runs 3 and 10). Compound 2 was afforded as the sole product in 86%
yield, when adding a 0.2 wt % aq NaOH solution according to
method B (Table 1, run 16). Without ultrasonic irradiation, the de-
chlorination did not proceed until completion and some 1a
remained unreacted even after 6 h (Table 1, run 17). These results
indicate that ultrasonication accelerates the reductive de-
chlorination of 1a.
2
. Dechlorination of monochlorobiphenyls 1a–c (Scheme 1)
.1. Effects of the concentration of the aq NaOH solution, the
2
amount of the Raney Ni–Al alloy used, the reaction
temperature and the order of adding the reagents
2.2. Effect of the nature of the alkaline solution
Regarding the experimental handling of the reactions, method B
Two methods were employed. Method A: the Raney Ni–Al alloy
was added gradually to a mixture of 1 in an alkaline solution;
Method B: an alkaline solution was added dropwise to a mixture of
is more useful than method A, since adding dropwise an aq NaOH
solution to the reaction vessel is more convenient than adding the
Raney Ni–Al alloy to the reaction solution. Therefore, the de-
chlorination of 1a in alkaline solutions other than aq NaOH was
investigated using method B (Table 2). It is noteworthy that the use
of Raney Ni–Al alloy in 1 wt % aq KOH or CsOH solutions led to
strongly reducing agents, comparatively stronger than using the
alloy in other alkaline aqueous solutions under otherwise identical
conditions. Compound 3 was obtained selectively and in high yields
1
and Raney Ni–Al alloy in water. As a model reaction, the de-
chlorination of 4-chlorobiphenyl (1a), which among the PCB con-
geners is known to be one of the most difficult to be
dechlorinated,3a,d was examined under various reaction conditions,
as shown in Table 1. At first, the effect of the concentration of the aq
NaOH solution on the dechlorination of 1a was investigated. When
using method A with 1.0 g Raney Ni–Al alloy for 1 mmol of sub-
strate, the reaction time could be greatly shortened (8 h/3 h)
upon decreasing the concentration of the aq NaOH solution
(
89–91%) (Table 2, runs 1 and 2). In case of adding 1 wt % aqueous
solutions of Ca(OH) , LiOH and Ba(OH) , the ratio of 3 to 2 tended to
2
2
decrease in that order (Table 2, runs 1–6). That is to say, the re-
ducing ability of the Raney Ni–Al alloy exhibited upon dropwise
addition of 1 wt % alkaline aqueous solutions follows the order:
(
(
5 wt %/0.5 wt %). In a dilute NaOH solution, phenylcyclohexane
3) was afforded in addition to biphenyl (2). Here, dechlorination
and hydrogenation of one of aromatic rings occurred at the same
time (Table 1, runs 2–4 vs 1). In the case of a 10 wt % aq NaOH so-
lution, complete dechlorination was not achieved. Interestingly,
2 2
KOH>CsOH>NaOH>Ca(OH) >Ba(OH) >LiOH. When the reductive
dechlorination was conducted by adding more dilute solutions such
as 0.2 wt % KOH, CsOH and LiOH aqueous solutions, 1a was con-
verted to a mixture of 2 (main product) and 3 (minor product)
0
.2 g of Raney Ni–Al alloy per 1 mmol of substrate was enough for
a complete dechlorination of 1a when using a 0.1 wt % NaOH so-
lution. Dechlorination was incomplete under otherwise identical
conditions, when using a 0.5 wt % aq NaOH solution (Table 1, run 5
vs 6). In a more concentrated, aq NaOH solution, e.g., in 5 wt % aq
NaOH, the reduction was found to be slow (8 h), and only the
dechlorinated product, biphenyl (2), was afforded in good yield
(
Table 2, runs 7–10). The dechlorination of 1a was found to be in-
complete when adding 0.2 wt % aq Ca(OH) or 0.2 wt % aq Ba(OH)
2
2
solutions (Table 2, runs 11 and 12). From these results, it can be
concluded that the strength of the base influences the reducing
ability of the Raney Ni–Al alloy, and affects the dechlorination of 1a.
(Table 1, run 2).
On the other hand, in case of method B, Raney Ni–Al alloy was
found to be even more effective. More 3 was formed as compared to
2.3. Effect of the nature of the alloy; effect of using metal
powders
when using method A (Table 1, runs 7–12 vs 1–6). The reductive
ꢀ
dechlorination accelerated with a decrease of the OH concentra-
From the above, it could be deduced that Raney Ni–Al alloy
becomes a very strong reducing agent in 1 wt % aq NaOH solutions.
With this information in hand, the behaviour of other alloys such as
Co–Al, Cu–Al and Fe–Al alloys was examined in the dechlorination
of 1a (Table 3). The reaction was found to be slow with Co–Al and
with Cu–Al alloy as compared to Raney Ni–Al alloy. In both cases,
a complete dechlorination of 1a could not be realized (Table 3, runs
2 and 3). With Fe–Al alloy, the reductive dechlorination hardly
occurred at all. In the case of using Al (2.5 g for 5 mmol substrate)
and Ni (2.5 g for 5 mmol substrate) or a combination of Al (2.5 g)
tion of the aq NaOH solutions (Table 1, runs 7–10). The ratio of 3
tended to increase with an increase of the amount of Raney Ni–Al
alloy used (Table 1, run 4 vs 5). It is noteworthy that in method A,
only 2 was formed when the reaction was performed in a 5 wt %
NaOH solution. When using method B, however, a mixture of 2 and
3
was obtained upon adding dropwise a 10 wt % aq NaOH solution
to the reaction mixture (Table 1, run 2 vs 8). These results reveal
that the order of addition of the reagents affects the reductive de-
chlorination of 1a. However, an attempt to completely reduce 1a

G.-B. Liu et al. / Tetrahedron 65 (2009) 2497–2505
2499
Table 1
Effects of the concentration of the NaOH solution, the amount of Raney Ni–Al alloy used, the reaction temperature and the method of addition in the dechlorination of 1aa
NaOH solutionb (ml)
Method
Temp ( C)
ꢁ
Time (h)
Ratiod (%)
Run
Ni–Al (g)
1a
2
3
1
2
3
4
5
6
5
5
5
5
1
1
10% NaOH (80)
5% NaOH (100)
1% NaOH (400)
0.5% NaOH (800)
0.5% NaOH (800)
0.1% NaOH (800)
A
A
A
A
A
A
90
90
90
90
90
90
8
8
3
3
4
1
46
0
0
0
34
0
54
0
0
64
54
7
100 (80)e
36
46
59
83
17
7
8
9
5
5
5
5
1
1
0.5
20% NaOH (40)/H
10% NaOH (50)/H
2% NaOH (200)/H
1% NaOH (400)/H
1% NaOH (400)/H
2
O (40)
2
O (50)
B
B
B
B
B
B
B
90
90
90
90
90
90
90
8
7
4
2
4
1
4
0
0
0
0
24
0
89
72
31
13
60
82
75
11
28
69
87
16
18
5
2
O (200)
O (400)
O (400)
1
0
2
2
1
1
1
2
0.2% NaOH (400/) H
0.2% NaOH (400) H
2
O (400)
O (400)
13
2
20
c
1
4
5
5
1
1
0.5% NaOH (800)
1% NaOH (400)/H
0.2% NaOH (400)/H
0.2% NaOH (400)/H
A
B
B
B
60
60
60
60
3
2
2
6
0
0
0
67
47
43
53
0
c
1
5
2
O (400)
c
100 (86)e
47
1
6
2
O (400)
O (400)
17
2
48
5
a
Compound 1a (5 mmol, 942 mg).
Added dropwise within 0.5 h.
Under ultrasonic irradiation.
GC ratio.
b
c
d
e
Isolated yields in parentheses.
and Ni (2.5 g) for 5 mmol of substrate, the reduction did not take
place, and 1a remained unreacted. These results clearly show that
only Ni–Al alloy is highly effective for the reduction of 1a. In order
to develop a reducing system catalytic in Ni, the dechlorination of
further to 50 ml in order to accomplish the dechlorination of 1a.
Similarly, a decrease in the volume of a 0.2 wt % aq NaOH solution
(400 ml/100 ml) led to the reaction time being prolonged
(1 h/3 h). The ratio of 2 (82%/86%) to 3 (18%/14%) exhibited
a small change (Table 4, runs 6 and 7). The dechlorination of 1a
could no longer be accomplished when the volume of the
0.2 wt % NaOH solution was reduced even further (Table 4, run 8).
1
a using a combination of Raney Ni–Al alloy and Al metal was
studied. The total amount of Al was kept at 2.5 g and a 1 wt % aq
NaOH solution was added (Table 3, run 5). The Raney Ni–Al alloy
was used in different combinations with Al. However, no satisfac-
tory results were obtained and a catalytic system could not be
found.
Table 3
a
Dechlorination of 1a with different alloys (method B)
Run Alloy (g)
NaOH solutionb (ml)
Time (h) Ratioc (%)
1
a
2
3
2
.4. Effect of the amount of the aq NaOH solution used
1
2
3
4
5
Ni–Al (5)
Co–Al (5)
Cu–Al (5)
Fe–Al (5)
1% NaOH (400)/H
1% NaOH (400)/H
1% NaOH (400)/H
1% NaOH (400)/H
2
2
2
2
O (400)
O (400)
O (400)
O (400)
2
5
5
5
5
0
13 87
84 16
64 36
94
0
0
0
8
When the volume of the 1 wt % aq NaOH solution was de-
creased (400 ml/50 ml), reaction times became longer (2 h/5
h). However, the ratio of 2 (13%/20%) to 3 (87%/80%) only
changed slightly (Table 4, runs 1–3). It was found that the time of
adding the NaOH solution had to be adjusted from 0.5 h to 1.5 h,
when the volume of the 1 wt % NaOH solution was decreased
6
Ni–Al (0.6)/Al (2.2) 1% NaOH (400)/H O (400)
2
42 50
a
b
c
Compound 1a (5 mmol, 942 mg).
Added dropwise within 0.5 h.
GC ratio.
Table 2
Effects of the nature of the alkaline solution in the dechlorination of 1a (method B) at
ꢁ
a
Table 4
Effect of the volume of NaOH solution on the dechlorination of 1a (method B)a
90 C
Run Ni–Al (g) Alkaline solutionb (ml)
Time (h) Ratioc (%)
Run Ni–Al NaOH solutionb (ml)
(g)
Temp Time Ratioe (%)
ꢁ
1a
2
3
( C)
(h)
1a
2
3
1
2
3
4
5
6
5
5
5
5
5
5
1% KOH (400)/H
1% CsOH (400)/H
1% NaOH (400)/H
1% Ca(OH0) (400)/H
1% LiOH (400)/H
1% Ba(OH)
2
O (400)
O (400)
O (400)
2
4
2
4
4
4
0
0
0
0
0
0
0
0
100 (89)d
100 (91)d
1
2
3
4
5
6
7
8
9
5
5
5
5
5
1
1
1
1
1
1% NaOH (400)/H
1% NaOH (100)/H
2
O (400)
O (100)
90
90
90
90
90
2
3
5
5
7
1
3
6
2
4
0
0
13
17
87
83
53
80
48
18
14
4
2
2
2
13 87
15 85
41 59
71 29
1% NaOH (50)/H
1% NaOH (50)/H
1% NaOH (25)/H
2
O (50)
2
O (50)
2
O (25)
21 26
20
31 21
2
2
O (400)
c
c
0
2
O (400)
2
(400)/H
2
O (400)
0.2% NaOH (400)/H
0.2% NaOH (100)/H
0.2% NaOH (50)/H O (50)
0.2% NaOH (400)/H O (400) 60
0.2% NaOH (100)/H
O (100) 60d
2
O (400) 90
O (100) 90
90
0
0
82
86
7
8
9
1
1
1
1
1
1
0.2% NaOH (400)/H
0.2% KOH (400)/H O (400)
0.2% CsOH (400)/H O (400)
0.2% LiOH (400)/H O (400)
2
O (400)
1
2
5
5
5
5
0
0
0
0
82 18
80 20
88 12
2
c
2
2
39 57
0
0
d
100 (86)f
100 (82)f
2
2
0
0
1
0
2
91
9
0
0
10
2
1
1
0.2% Ca(OH)
0.2% Ba(OH)
2
2
(400)/H
(400)/H
2
O (400)
O (400)
46 54
41 59
a
Compound 1a (5 mmol, 942 mg).
12
2
b
c
d
e
f
Added dropwise within 0.5 h, unless stated otherwise.
Added dropwise within 1.5 h.
Under ultrasonication.
GC ratio.
Isolated yield in parentheses.
a
b
c
Compound 1a (5 mmol, 942 mg).
Added dropwise within 0.5 h.
GC ratio.
d
Isolated yields in parentheses.

2500
G.-B. Liu et al. / Tetrahedron 65 (2009) 2497–2505
Table 5
a
Dechlorination of 1b,c in an aq alkaline solution (method B)
Alkaline solutionb (ml)
Temp ( C)
ꢁ
Time (h)
Ratioe (%)
Run
Sub.
Ni–Al (g)
2
3
c
100 (91)f
67
1
2
3
4
5
1b
1b
1b
1b
1c
1c
1c
5
5
1
1
5
5
1
1% KOH (50)/H
1% NaOH (50)/H
2
O (50)
O (50)
90
90
90
60
90
90
4
5
3
4
4
5
4
0
33
84
100 (84)
0
c
2
0.2% NaOH (100)/H
0.2% NaOH (100)/H
1% KOH (50)/H
1% NaOH (50)/H
0.2% NaOH (100)/H
2
O (100)
O (100)
16
0
d
f
2
c
100 (88)f
2
O (50)
O (50)
O (100)
c
6
7
2
40
100 (80)
60
0
d
f
2
60
a
b
c
d
e
f
Compound 1a (5 mmol, 942 mg).
Added dropwise within 0.5 h, unless stated otherwise.
Added dropwise within 1.5 h.
Under ultrasonication.
GC ratio.
Isolated yield in parentheses.
Under ultrasonic irradiation, the reductive dechlorination also
became slower as the volume of the 0.2 wt % aq NaOH solution
was decreased. Here, 2 was formed as the sole product (Table 4,
runs 9 and 10). These results could be important for the disposal
of PCBs in larger-scale processes since the small volume of
aqueous alkaline solution is more acceptable from the viewpoint
of economic and environmental concerns.
2 3
of the volume of a 1 wt % aq Na CO solution (100 ml/50 ml)
resulted in the reaction time being lengthened (4 h/6 h) (Table 6,
runs 2 and 6). Rather surprisingly, when the volume of a 0.2 wt %
2 3
Na CO solution was reduced (400 ml/100 ml), a complete de-
chlorination could no longer be accomplished. This result differs
from that observed for the dechlorination of 1a with a 0.2 wt %
NaOH solution (method B). It is particularly noteworthy that in
2 3
a 1 wt % aq K CO solution, the reducing power of the Raney Ni–Al
alloy is stronger than in other aqueous metal carbonate solutions.
Here, 3 was formed selectively in 89% yield (Table 6, runs 5 and 6).
2
.5. Dechlorination of monochlorobiphenyls 1b and 1c
2 3 2 3
When performing the reaction in 1 wt % aq Cs CO or Li CO so-
Raney Ni–Al alloy is also effective in the reduction of 2-chloro
lutions, a mixture of 2 (main product) and 3 (minor product) was
obtained (Table 6, runs 7 and 8). Hence, the reducing power of the
Raney Ni–Al alloy in 1 wt % alkali metal carbonate solutions shows
and 3-chlorobiphenyls (1b,c) when using method B (Table 5).
Compound 3 was afforded selectively and in high yields, when
a 1 wt % aq KOH solution was added dropwise to a mixture of
chlorobiphenyl and Raney Ni–Al alloy. On the other hand, a mixture
of 2 (main product) and 3 (minor product) was produced upon
using a 1 wt % aq NaOH solution. When the reduction was per-
formed by adding a 0.2 wt % aq NaOH solution, 2 was obtained as
2 3 2 3 2 3 2 3
the trend: K CO >Na CO >Cs CO >Li CO . The ratio of 3 is seen to
increase with an increase of the amount of the Raney Ni–Al alloy.
These results clearly indicate that the nature of alkali metal car-
bonate plays an important role in the reductive dechlorination of
1a. Interestingly, dechlorination of 1a was readily accomplished
with Raney Ni–Al (1.0 g/5 mmol substrate) in the more dilute,
ꢁ
the major product, together with 3. Under ultrasonication at 60 C,
1
b,c were dechlorinated easily and 2 was obtained as the sole
0
.2 wt % aq Na
the more concentrated, 1 wt % Na
0). Under ultrasonic irradiation at 60 C, the dechlorination of 1a
was incomplete, when a 0.2 wt % aq Na CO solution was added
Table 6, runs 9 and 10). Here, the relatively low reaction temper-
2
CO
3
solution, while the reduction was sluggish in
product in 83–88% yield. An effect of the volume of the aq NaOH
solutions was observed similar to that with 1a: the reaction time
increased with decreasing volume, i.e., with decreasing amount of
the aq NaOH solution.
2
CO solution (Table 6, run 9 vs
3
ꢁ
1
2
3
(
ature might be the reason. In fact, 90% of 1a remained unreacted,
3
. Dechlorination of monochlorobiphenyls 1a–c with Raney
Ni–Al alloy in dilute aqueous solutions of alkali metal
carbonates (Scheme 2)
Table 6
Effects of the nature, concentration and volume of the alkali metal carbonate so-
lution and the amount of Raney Ni–Al alloy used for the dechlorination of 1a
a
(method B)
3
.1. Effects of the nature, the concentration and the amount
Run Ni–Al Metal carbonate
Temp Time Ratioe (%)
of the aqueous alkali metal carbonate solutions and the
amount of Raney Ni–Al alloy used
solutionb (ml)
( C)
ꢁ
(h)
(g)
1a
2
3
1
2
3
4
5
6
7
8
9
5
5
5
5
5
5
5
5
1
1
1
1
1
10% Na
2
CO
3
(100)/H
2
O (100)
90
90
90
90
90
90
90
90
90
5
2
4
6
2
5
4
5
4
3
6
5
5
0
0
0
0
0
0
0
0
79 21
28 72
30 70
29 71
0
0
Based on the findings described above, we decided to in-
vestigate the behaviour of the Raney Ni–Al alloy in much more
weakly basic solutions such as in dilute aqueous alkali metal car-
bonate solutions, with reductions carried out according to method
B (Table 6). Similar to what we observed for dilute aq alkaline so-
lutions, the reducing capability of the Raney Ni–Al alloy was en-
hanced greatly with a decrease in the concentration of the alkali
1% Na CO
2
3
(400)/H O (400)
2
1% Na CO3 (100)/H O (100)
2
2
c
c
2
1% Na CO
3
(50)/H
(400)/H
(50)/H
(400)/H
(400)/H
(400)/H
2
O (50)
1% K
1% K
2
CO
CO
3
2
O (400)
100 (93)f
100 (89)f
2
3
2
O (50)
O (400)
O (400)
O (400)
(400)/H
(100)/H
1% Cs
2
CO
CO
CO
3
2
75 25
78 22
1% Li
2
3
2
1% Na
2
3
2
23 65 12
80 20
1
0
0.2% Na
0.2% Na
0.2% Na
0.2% Na
2
CO
2
CO
2
CO
2
CO
3
3
3
3
2
O (400) 90
0
2 3
carbonate in aq Na CO solutions (10 wt %/1 wt %), giving a raise
in product yield of 3 (21%/71%) (Table 6, runs 1–4). The reduction
1
1
2
O (100) 90
59 41
66 30
90 10
0
4
0
1
2
(400)/H
O (400) 60d
2
(400)/H O (400) 60
2
13
a
Compound 1a (5 mmol, 942 mg).
b
c
d
e
f
Added dropwise within 0.5 h, unless stated otherwise.
Added dropwise within 1.5 h.
Under ultrasonic irradiation.
GC ratio.
Scheme 2.
Isolated yield in parentheses.

G.-B. Liu et al. / Tetrahedron 65 (2009) 2497–2505
2501
ꢁ
when using a 0.2 wt % aq Na
2
CO
3
solution at 60 C without ultra-
Table 8
Dechlorination of 1a in an alkali metal carbonate solution using method Ba
sonication (Table 6, run 13). It must be noted that a reductive de-
chlorination of 1b,c (5 mmol substrate) could no longer be
accomplished, when the volume of the 0.2 wt % aq Na
Run
Ni–Al (g)
Metal carbonate
solutionb (ml)
Time (h)
Ratioc (%)
2 3
CO solution
1
a
2
3
was decreased further than 400 ml.
1
2
3
4
5
5
5
5
1
1
1% (NH
1% KHCO
1% NaHCO3 (400)/H O (400)
4
)
2
CO
3
(400)/H
2
O (400)
5
5
5
6
5
58
52
36
90
91
42
46
64
10
9
0
2
0
0
0
As expected, treatment of 2-chloro and 3-chlorobiphenyls (1b,c)
with Raney Ni–Al alloy and using 1 wt % aq K CO solution (method
B) afforded 3 selectively and in high yields (90% and 91%) (Table 7,
runs 1 and 5). With a 1 wt % aq Na CO solution (method B),
a mixture of 2 (main product) and 3 (minor product) was obtained.
When the reaction was conducted with a 0.2 wt % aq Na CO so-
lution, 2 was afforded, albeit with 3 as a by-product. Similar effects
of the overall amount of carbonate salt used in the reaction as
regulated by the volume of the aq alkali carbonate solution used
could be observed. Thus, it must be noted that the dechlorinations
3
(400)/H
2
O (400)
2
3
2
0.2% KHCO
0.2% NaHCO
3
(400)/H
2
O (400)
O (400)
3
(400)/H
2
2
3
a
b
c
Compound 1a (5 mmol, 942 mg).
Added dropwise within 0.5 h.
GC ratio.
2
3
well for the reductive dechlorination of commercial PCBs, too. 2,3-
Dichlorobiphenyl (4a) was selected as a model substrate and
various reaction conditions were investigated. First, the effect of
the amount of aq NaOH solution per mmol of substrate was ex-
amined. The reaction time needed for complete dechlorination
increased with a decrease in amount of 1 wt % or 0.2 wt % aq NaOH
solutions used (Table 9, run 1 vs 2 and 4 vs 5). Complete de-
chlorination of 4a could not be accomplished when the volume of
the aq NaOH solution was reduced even further than 400 ml (runs
of 1b,c were not complete, when the volume of a 0.2 wt % aq Na
solution was reduced even further than 400 ml (Table 7, run 8).
2 3
CO
3
.2. Dechlorination of 1a in other basic salt solutions
The fact that Raney Ni–Al alloy is a very strong reducing agent in
dilute aq solutions of alkali metal carbonates (see above) encour-
aged us to investigate its reactivity in other aq salt solutions (Table
3
and 6). The ratios of the dechlorinated products formed were
8
). In 1 wt % aq solutions of (NH
reductive dechlorination of 1a was found to proceed very slowly
Table 8, runs 1–3). In 0.2 wt % aq solutions of NaHCO and KHCO
and in 1 wt % aq solutions of NaOAc and NH Cl, very little reductive
4 2 3 3 3
) CO , NaHCO and KHCO , the
found also to depend on the amount of the Raney Ni–Al alloy used.
The reducing ability of Raney Ni–Al alloy in a 1 wt % KOH solution
is strong enough to convert 4a to 3 selectively and in high yield
(
3
3
,
4
(
88%). Under ultrasonic irradiation, 4a was dechlorinated reduc-
dechlorination occurred. These results clearly show that Raney Ni–
Al alloy is a much stronger reducing agent in dilute aq solutions of
NaOH, KOH, Na CO and K CO than in other basic salt solutions.
2 3 2 3
tively to afford a mixture of 2 (main product) and 3 (minor
product). As expected, the amount of the Raney Ni–Al alloy had to
be doubled for the dechlorination of 4a in comparison to the
monochlorobiphenyls. The reaction conditions found to be best for
the dechlorination of 4a were applied to the dechlorination of 2,4-,
4
. Dechlorination of dichlorobiphenyls with the Raney Ni–Al
alloy in dilute aqueous alkaline solutions
0 0
,5-, 3,4-, 3,5-, 3,3 - and 4,4 -dichlorobiphenyls (4b–g). Com-
2
pounds 4b–g were reductively dechlorinated to give a mixture of 2
and 3 in the case of adding dropwise a 1 wt % aq NaOH solution to
a mixture of dichlorobiphenyl and Raney Ni–Al alloy in water. It
must be noted that 4c–f were dechlorinated to produce 3 as the
dominant product, albeit with a trace amount of 2 (2–4%). These
results clearly show that 4c–f are more easily reduced than 4b,g
and are even more easily reduced than the monochlorobiphenyls,
as determined by the product ratios found by GC. Treatment of
4
.1. Effects of the concentration of the aq NaOH solution, the
amount of the Raney Ni–Al alloy used and the reaction
temperature
Our success in the dechlorination of monochlorobiphenyls
prompted us to test this powerful reducing system towards the PCB
congeners dichlorobiphenyls (4a–g) (Tables 9 and 10 and Scheme
3
). The reason for selecting dichlorobiphenyls as model compounds
4a,b,d,f,g with Raney Ni–Al alloy (1.0 g/2.5 mmol substrate) gave
for PCBs in initial studies was based on their lower reactivity to-
wards nucleophilic aromatic substitution (relative to the more
highly chlorinated congeners).
Therefore, these compounds pose the tougher challenge to
dechlorination. Reaction conditions, which give good results in the
dechlorination of dichlorobiphenyls might be expected to work
a mixture of 2 (main product) and 3 (minor product) in 0.2 wt % aq
NaOH solution. Surprisingly, both 4c and 4e, which have a chloro
atom at the meta position of one phenyl group, were dechlori-
nated to afford 3 (78–80%) as the main product, together with
a small amount of 2 (20–22%) under identical reaction conditions
14
(Table 9, runs 15 and 23). These results clearly indicate that 4c and
4e may be activated by a meta-positioned chloro atom. However,
only small amounts of 3 are produced in the reductive de-
chlorination of 3,4-dichlorobiphenyl (4d), which has one meta-
positioned chloro atom, and of 3,3 -dichlorobiphenyl (4f), which
Table 7
a
Dechlorination of 1b,c in alkali metal carbonate solution using method B.
0
Time Ratiod (%)
(h)
Run Sub. Ni–Al (g) Metal carbonate
b
has one meta-positioned chloro atom on each aromatic ring, again
under identical reaction conditions. These findings clearly show
that the reductive dechlorination is strongly dependent upon the
structure of dichlorobiphenyls. All compounds 4b–g were con-
verted to 3 selectively and in high yields (88–95%) by adding
dropwise a 1 wt % aq KOH solution. Under ultrasonication, 4b–f
were dechlorinated to give a mixture of 2 and 3, when a 0.2 wt %
aq NaOH solution was added dropwise. However, the de-
chlorination of 4g was not complete under the identical reaction
conditions and 4g (65%) remained unreacted. Indeed, it has been
reported that 4g is one of the most difficult PCB congeners to be
solution (ml)
1a
2
3
c
100 (91)e
1
2
3
4
5
6
7
8
1b
1b
1b
1b
1c
1c
1c
1c
5
5
1
1
5
5
1
1
1% K
2
CO
3
(50)/H
2
O (50)
O (50)
5
6
5
6
5
6
5
6
0
0
0
0
c
1% Na
2
CO
3
(50)/H
2
29 71
86 14
0.2% Na
0.2% Na
2
CO
CO
3
(400)/H
(100)/H
2
O (400)
O (100)
2
3
2
65 35
0
c
c
100 (90)e
1% K
2
CO
3
(50)/H
2
O (50)
O (50)
0
0
0
0
1% Na
2
CO
3
(50)/H
2
31 69
84 16
0.2% Na
0.2% Na
2
CO
CO
3
(400)/H
(100)/H
2
O (400)
O (100)
2
3
2
70 30
0
a
Compounds 1b,c (5 mmol, 942 mg).
b
c
Added dropwise within 0.5 h, unless otherwise stated.
Added dropwise within 1.5 h.
GC ratio.
14
d
e
dechlorinated. Finally, it should be noted that two chloro atoms
seemed to be replaced rapidly one after the other in the
Isolated yield in parentheses.

2502
G.-B. Liu et al. / Tetrahedron 65 (2009) 2497–2505
Table 9
a
Dechlorination of 4a–g in MOH solutions using method B
Alkaline solutionb (ml)
Temp ( C)
ꢁ
Time (h)
Ratioe (%)
Run
Sub.
Ni–Al (g)
4
2
3
1
2
3
4a
4a
4a
4a
4a
4a
4a
4a
5
5
5
1
1
1
5
1
1% NaOH (100)/H
2
O (100)
90
90
90
90
90
90
90
4
5
5
2
6
6
4
6
0
0
21
0
0
38
0
23
25
23
80
87
56
0
77
75
56
20
13
6
1% NaOH (50)/H
1% NaOH (25)/H
2
2
O (50)
O (25)
c
4
5
0.2% NaOH (400)/H
0.2% NaOH (100)/H
0.2% NaOH (50)/H
1% KOH (50)/H O (50)
0.2% NaOH (100)/H O (100)
2
O (400)
c
2
O (100)
6
7
8
2
O (50)
100 (88)f
11
2
c
d
2
60
0
89
9
4b
4b
4b
4b
5
5
1
1
1% KOH (50)/H
1% NaOH (50)/H O (50)
0.2% NaOH (100)/H
0.2% NaOH (100)/H
2
O (50)
90
90
90
3
5
3
6
0
0
0
0
0
12
70
79
100 (89)f
88
30
21
1
0
2
c
1
1
1
2
2
O (100)
O (100)
2c
60
d
1
3
4c
4c
4c
4c
5
5
1
1
1% KOH (50)/H
1% NaOH (50)/H
0.2% NaOH (100)/H
0.2% NaOH (100)/H
2
O (50)
O (50)
90
90
90
3
5
3
6
0
0
0
0
0
4
20
69
100 (93)f
96
80
31
1
4
2
c
1
5
2
2
O (100)
O (100)
c
d
16
60
1
7
4d
4d
4d
4d
5
5
1
1
1% KOH (50)/H
1% NaOH (50)/H
0.2% NaOH (100)/H
0.2% NaOH (100)/H
2
O (50)
O (50)
90
90
90
3
3
3
6
0
0
0
0
0
4
75
83
100 (90)f
96
25
17
1
8
2
c
1
2
9
2
2
O (100)
O (100)
0c
60
d
2
1
4e
4e
4e
4e
5
5
1
1
1% KOH (50)/H
1% NaOH (50)/H
0.2% NaOH (100)/H
0.2% NaOH (100)/H
2
O (50)
O (50)
90
90
90
3
4
4
6
0
0
0
0
0
2
22
72
100 (95)c
98
78
28
22
23
24
2
c
c
2
2
O (100)
O (100)
d
60
2
2
2
2
5
4f
4f
4f
4f
5
5
1
1
1% KOH (50)/H
1% NaOH (50)/H
0.2% NaOH (100)/H
0.2% NaOH (100)/H
2
O (50)
O (50)
90
90
90
3
5
3
6
0
0
0
0
0
3
79
82
100 (94)f
97
21
18
6
2
c
c
7
8
2
2
O (100)
O (100)
d
60
2
3
9
0
4g
4g
4g
4g
5
5
1
1
1% KOH (50)/H
1% NaOH (50)/H
0.2% NaOH (100)/H
0.2% NaOH (100)/H
2
O (50)
O (50)
90
90
90
4
5
6
6
0
0
0
0
34
81
13
100 (89)f
66
19
22
2
c
3
1
2
2
O (100)
O (100)
c
d
3
2
60
65
a
b
c
d
e
f
Compounds 4a–g (2.5 mmol, 572 mg).
Added dropwise within 1.5 h, unless stated otherwise.
Added dropwise within 0.5 h.
Under ultrasonication.
GC ratio.
Isolated yield in parentheses.
dechlorination of 4a–g, since monochlorophenyls as possible in-
termediates could only be detected in small amounts (<3%), al-
though some amount of dichlorobiphenyls 4a–g remained
unreacted at the time. Thus, intramolecular selectivities between
two chloro atoms are low. It has been reported that a certain
intramolecular selectivity could be observed in the following
reaction time increased for
terestingly, the amount of the 0.2 wt % Na
a
complete dechlorination. In-
CO solution could be
2
3
reduced (400 ml/100 ml per 2.5 mmol of substrate) in the deha-
logenation of 4a, even though this was not possible in the case of
the complete dechlorination of the monochlorobiphenyls 1a–c
(method B), as described above. These findings can be attractive for
the disposal of commercial PCBs at a larger-scale since it is expected
that the higher chlorinated congeners can be dechlorinated more
easily than dichlorobiphenyls. An increase of 3 was observed in the
dehalogenation of 4a when the amount of the Raney Ni–Al alloy
was increased. Under ultrasonication, the reductive dechlorination
could not be achieved. This is most likely due to the low reaction
2 4
cases: 2-Cl>4-Cl in the dechlorination of 4b using NiCl /NaBH in
DMF at room temperature; and 3-Cl>2-Cl (4a), 4-Cl>2-Cl (4b), 5-
Cl>2-Cl (4c) and 3-Cl>4-Cl (4d) in the dechlorination of 4a–d with
6
f
(
Ph
3
P)
2 2 4
NiCl /NaBH in DMF at room temperature.
5
. Dechlorination of dichlorobiphenyls with Raney Ni–Al
alloy in dilute aqueous alkali metal carbonate solutions
2 3
temperature. It should be noted that in a 1 wt % aq Na CO solution,
dichlorobiphenyls 4a–g were reduced easily to give predominantly
3, together with a trace amount of 2, independently of the position
of the chloro atoms. These results clearly show that 4a–g are more
easily dechlorinated with Raney Ni–Al alloy in dilute alkali metal
carbonate solutions. Also, 4a–g were reduced to 3 selectively and in
5
.1. Effects of the nature of the alkali carbonate, the
concentration and the amount of the aqueous metal
carbonate solution, the amount of the Raney Ni–Al
alloy used and the reaction temperature
high yields by adding dropwise a 1 wt % aq K
reaction mixture. Compounds 4a–g were reduced to 2, albeit with 3
as a by-product, when a 0.2 wt % aq Na CO solution was employed.
2 3
CO solution to the
It was found that monochlorobiphenyls were dechlorinated
easily with Raney Ni–Al alloy in a dilute aq Na
2
CO
3
solution (see
2
3
above). With this information in mind, we decided to examine the
efficiency of this strongly reducing system towards dichloro-
biphenyls (4a–g) (Table 10). As a model reaction, the dechlorination
of 4a was tested under various reaction conditions. As expected,
Again, the intramolecular selectivity between chloro atoms was
low, and possible intermediates, monochlorobiphenyls, were
detected only in trace amounts (GC ratio <3%) in the dechlorination
of 4a–g with Raney Ni–Al alloy in both 1 wt % and 0.2 wt % aq
when a decrease in amount of 1 wt % aq Na
2
CO
3
solution, the
2 3
Na CO solutions.

G.-B. Liu et al. / Tetrahedron 65 (2009) 2497–2505
2503
Table 10
a
Dechlorination of 4a–g using method B
solutionb (ml)
Temp ( C)
ꢁ
Time (h)
Ratioe (%)
Run
Sub.
Ni–Al (g)
M
2
CO
3
4
2
3
1c
2
4a
4a
4a
4a
4a
4a
4a
5
5
5
1
1
1
1
1% Na
1% Na
CO
CO
(100)/H
O (100)
O (50)
O (50)
90
90
90
90
90
4
5
3
3
5
5
5
0
0
0
0
0
13
16
0
77
81
17
18
87
84
2
3
2
2
3
(50)/H
2
3
4
5
6
1% K
2
CO
3
(50)/H
2
100 (88)f
c
0.2% Na
0.2% Na
0.2% Na
0.2% Na
2
CO
2
CO
2
CO
2
CO
3
3
3
3
(400)/H
2
O (400)
O (100)
O (100)
O (100)
23
19
0
c
(100)/H
(100)/H
(100)/H
2
2
2
c
d
60
60
73
82
c
7
0
8
9
1
4b
4b
4b
5
5
1
1% K
1% Na
0.2% Na
2
CO
3
(50)/H
(50)/H
(100)/H
2
O (50)
O (50)
O (100)
90
90
90
3
5
4
0
0
0
0
7
64
100 (92)f
93
36
2
CO
3
2
0c
CO
2
3
2
1
1
4c
4c
4c
5
5
1
1% K CO
2
3
(50)/H
2
O (50)
90
90
90
3
5
5
0
0
0
0
5
66
100 (87)f
95
34
1
2
2
1% Na CO
3
(50)/H O (50)
(100)/H O (100)
2
c
13
0.2% Na
2
CO
3
2
1
4
4d
4d
4d
5
5
1
1% K
1% Na
0.2% Na
2
CO
3
(50)/H
(50)/H
(100)/H
2
O (50)
O (50)
O (100)
90
90
90
3
4
4
0
0
0
0
3
53
100 (90)f
97
47
1
5
2
CO
3
2
c
c
16
2
CO
3
2
1
7
4e
4e
4e
5
5
1
1% K CO
1% Na
0.2% Na
2
3
(50)/H
2
O (50)
90
90
90
3
4
4
0
0
0
0
4
16
100 (92)f
96
84
1
8
2
CO
3
(50)/H O (50)
(100)/H O (100)
2
19
2
CO
3
2
2
0
4f
4f
4f
5
5
1
1% K CO
2
3
(50)/H
2
O (50)
90
90
90
3
4
3
0
0
0
0
8
56
100 (90)f
92
2
1
2
1% Na CO
3
(50)/H O (50)
(100)/H O (100)
2
c
c
22
0.2% Na
2
CO
3
2
44
23
24
25
4g
4g
4g
5
5
1
1% K CO
2
3
(50)/H
2
O (50)
90
90
90
5
6
5
0
0
0
0
11
72
100 (93)f
89
28
2
1% Na CO
3
(50)/H O (50)
(100)/H O (100)
2
0.2% Na
2
CO
3
2
a
b
c
d
e
f
Compounds 4a–g (2.5 mmol, 572 mg).
Added dropwise within 1.5 h, unless stated otherwise.
Added dropwise within 0.5 h.
Under ultrasonic irradiation.
GC ratio.
Isolated yield in parentheses.
In the reductions using very dilute alkaline (0.2 wt % NaOH,
Table 9, run 24) or alkali carbonate solutions (0.2 wt % Na CO
2
3
,
Clm
Cln
H H
Table 10, run 19), 4e shows the largest proportion of phenyl-
cyclohexane (3). It may be due to the chloro functions positioned so
close to each other and to the remaining phenyl substituent that
they reinforce the polarization within the phenyl group through the
field effect and thus facilitate the approach of the molecule to the
catalyst and perhaps even the hydrogenation itself.
H
H
H
H
H
H
H H
Ni
Ni
Figure 1.
Mono- and dichlorobiphenyls, which are p-electron rich, are
adsorbed on the surface of the active nickel metal, and reaction
with the reactive form of hydrogen occurs at the catalyst’s surface.
6
. Reaction mechanism
Although the reaction mechanism of the dechlorination is not
2 3 2 3
We have noted that in dilute aq NaOH, KOH, Na CO and K CO
totally clarified yet, it can be proposed that aluminium metal of the
Raney Ni–Al alloy reacts with the basic solution, e.g., with the aq
NaOH solution, and produces a reactive form of hydrogen and
highly porous nickel metal on the catalyst’s surface. The actual
hydrodechlorination may well operate as a mixture of ionic and
radical pathways as has been suggested for the hydro-
dechlorination of chloroarenes over another supported nickel cat-
alyst. In the case of an ionic pathway the basicity of the solvent
would exert an effect that would overlie the effect of the base on
the production of the catalytic surface (see below) (Fig. 1).
solutions, Raney Ni–Al alloy becomes a stronger reductant than in
more concentrated alkaline solutions. The reasons for this may be
a solution-dependent particle size distribution of active nickel
metal, which plays an important role in the reductive de-
chlorination reaction, and a different surface porosity of the cata-
lyst, although both of these still need to be substantiated. In the
case of using a 10 wt % aq NaOH solution, the Raney Ni–Al alloy
reacts more uncontrolled with the NaOH solution and gives off
hydrogen gas, which escapes unreactedly from the reaction vessel.
In fact, an effervescence of hydrogen gas was observed in that case
15
(Scheme 4). On the other hand, it has been reported that 2 N NaOH
Raney Ni-Al alloy
+
aq M CO solution
2
3
Clm
Cln
4a-g
2
3
a: 2,3-Cl ; b: 2,4-Cl ; c: 2,5-Cl ; d: 3,4-Cl ; e: 3,5-Cl ; f: 3,3'-Cl ; g: 4,4'-Cl ;
2
2
2
2
2
2
2
Scheme 3.
Scheme 4.

2504
G.-B. Liu et al. / Tetrahedron 65 (2009) 2497–2505
can partly deactivate16 the catalytic surface of Raney Ni–Al so that
not all of the catalyst is used effectively for the dechlorination.
dimethylpolysiloxane), using a hydrogen flame ionization detector.
The relative ratio was calculated on the basis of the peak area of the
GC (assuming equal FID-sensitivity for all substrates).
7. Conclusion
8.2. Dechlorination of monochlorobiphenyls (1a–c)
In conclusion, it was found that Raney Ni–Al alloy in dilute so-
lutions of alkali hydroxides and alkali metal carbonates becomes
a powerful reducing agent. This is especially true in 0.5 wt % aq
A typical procedure is described as follows:
Method A. Within 0.5 h and at 90 C, Raney Ni–Al alloy (5.0 g)
ꢁ
NaOH, KOH, Na
2
CO
3
and K
2
CO
3
solutions. A series of PCB congeners,
was added in small portions to a mixture of 1a (5 mmol, 942 mg) in
a 0.5 wt % aq NaOH solution (400 ml). Afterwards, the reaction
mixture was stirred for 3 h (monitored by GC). Then, the mixture
was cooled to room temperature. The insoluble materials were
filtered off over Celite and the residue was washed with dichloro-
methane (3ꢂ15 ml). The filtrate was extracted with dichloro-
methane (3ꢂ25 ml), and the organic layer was dried over
anhydrous magnesium sulfate. The solvent was evaporated in
vacuo to afford a mixture of biphenyl (2) and phenylcyclohexane (3)
in a ratio of 46:54 (Table 1, run 4).
mono- and dichlorobiphenyls, were reduced facilely to biphenyl (2)
and/or phenylcyclohexane (3) under mild reaction conditions. No
organic solvents were used in the reactions. The dechlorinated
products strongly depended upon the concentration and the nature
of the aq alkaline and alkali metal carbonate solutions, and the
amount of the Raney Ni–Al alloy used. When adding dropwise
a 1 wt % aq alkaline solution to the chlorinated biphenyl in water,
a dependence of the dechlorination on the basicity of the metal
hydroxide was found: KOH>CsOH>NaOH>Ca(OH)
2
>LiOH>
ꢁ
Ba(OH) . In the case of adding a 1 wt % aq alkali metal carbonate
2
Method B. Within 0.5 h and at 90 C, a 1 wt % aq NaOH solution
solution to the chlorinated biphenyl in water, the ease of de-
chlorination was found to show the following trend:
(400 ml) was added dropwise to a mixture of 1a (5 mmol, 942 mg)
and Raney Ni–Al alloy (5.0 g) in water (400 ml). The reaction mix-
ꢁ
K
2
CO
3
>Na
2
CO
3
>Cs
2
CO
3
>Li
2
CO
3
. The effects of the amount of aq
ture was stirred for 2 h at 90 C. Thereafter, the reaction mixture
alkaline and alkali metal carbonate solutions used, reaction tem-
perature and the order of addition of reagents were discussed
above. Overall, this dechlorination is operationally simple. No harsh
reaction conditions such as elevated temperatures, high pressures,
a hydrogen atmosphere, an inert gas atmosphere or special appa-
rati are required. Moreover, this procedure offers the practical
convenience of obviating the need of handling inflammable and
toxic organic solvents. From the environmental protection point of
view, this process shows potential advantages in reducing the ad-
verse impact of solvent disposal on the environment. Hence, this
procedure is environmentally benign for the potential treatment of
toxic PCBs. We believe that the Raney Ni–Al alloy in dilute aqueous
alkaline and alkali metal carbonate solutions provides another very
useful technique for the reductive dehalogenation of aromatic
halides and for the hydrogenation of aromatic compounds, a tech-
nique that is characterized by the ease and quickness of the re-
action, simplicity of manipulation and mildness of reaction
conditions. Convenience and low costs are also advantages of this
method. Raney Ni–Al alloy is readily available commercially and is,
of course, cheaper than the Raney Ni catalyst made from it. It is
expected that this procedure would have potential application for
the disposal of toxic PCBs in certain cases in the industry such as
where relatively small volumes of material is contaminated by PCBs
in relatively high concentrations. Needless to say that the spent
was subjected to a work-up procedure described above to give 2
and 3 in a ratio of 13:87 (Table 1, run 10).
8.3. Dechlorination of monochlorobiphenyls (1a–c) under
ultrasonication
A typical procedure is as follows: according to method B,
a 0.2 wt % aq NaOH solution (400 ml) was added dropwise to
a mixture of 1a (5 mmol, 942 mg), and Raney Ni–Al alloy (1.0 g) in
ꢁ
water (200 ml) within 0.5 h at 60 C and under ultrasonication
(Ultrasonic Multi Cleaner W-115, HONDA). The reaction mixture was
stirred and irradiated simultaneously for 2 h and then subjected to
the work-up procedure described above to give 2 (670 mg, yield
19c
87%) and 3 in a ratio of 13:87 (Table 1, run 16).
8.4. Dechlorination of dichlorobiphenyls (4a–g)
A typical procedure is as follows: according to method B,
a 0.2 wt % aq Na CO solution (100 ml) was added dropwise to
2 3
a mixture of 4a (2.5 mmol, 572 mg) and Raney Ni–Al alloy (1.0 g) in
ꢁ
water (100 ml) within 0.5 h and at 90 C. The reaction mixture was
ꢁ
stirred for 5 h at 90 C (monitored by GC) and then subjected to the
work-up described above to give 2 and 3 in a ratio of 81:19 (Table
10, run 5).
17a
nickel needs to be recycled. Efficient nickel recycling from larger
amounts of aqueous medium such as by use of ion exchange resin
Compounds 2 and 3 were compared with commercially avail-
1
able samples and on the basis of their H NMR spectra and GC re-
17
has been forwarded by other authors, the technology of which is
also used in electroplating.
tention times.
References and notes
8
8
. Experimental section
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Al (Wako) and Fe–Al (Wako) (50:50, wt %) were readily available
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2
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