Dehalogenation of Chlorinated Benzenes and Aroclors
J . Org. Chem., Vol. 63, No. 11, 1998 3543
°C, the average chlorination numbers of Aroclor 1242,
1248, and 1254 decrease from 3.27, 3.90, and 4.96 to 2.46,
3.09 and 3.53, respectively. All possible isomeric prod-
ucts are formed, traces of biphenyl can be detected after
90 h or less. This performance is better than, for
example, that reported by Roth et al.30 for a system using
Ni(PPh3)4 and NaBH4.
Current efforts are directed along these lines, especially
toward the ultimate target of degradation of PCBs to
biphenyl.
Exp er im en ta l Section
Recover y of Ca ta lyst. We have also succeeded in
recovering a substantial portion of the metal from the
catalytic reaction. With test mixtures containing all the
usual ingredients but the less expensive PdCl2(PPh3)2 as
catalyst, we have reproducibly recovered 40% of the metal
from the reaction residue. A simple treatment includes
an aqueous washing of the vacuum-dried residue, fol-
lowed by digestion with aqua regia. Gradual introduction
of concentrated HCl gives essentially H2PdCl6 which
readily yields K2PdCl4 when treated with excess KCl.
PdCl2(dppf) can be easily obtained from K2PdCl4. Such
recycling of a precious metal would enhance the value of
this process in any potential industrial application.
All reactions were carried out under argon or oxygen-free
nitrogen except where otherwise stated. Standard Schlenk
techniques were used. Dppf31 and PdCl2(dppf)32 were synthe-
sized according to literature methods. Chlorinated benzenes
were obtained from Aldrich; PCB mixtures came from Ac-
cuStandard (Aroclor 1242 and 1254) and Chem Service (Aro-
clor 1248).
In a typical reaction, the substrate (1 mmol) was dissolved
in freshly degassed THF (20 mL) together with PdCl2(dppf)‚CH2-
Cl2 (0.037 g, 0.045 mmol), forming an orange solution. Upon
addition of of TMEDA (2 mL, 13.4 mmol, excess), the solution
turned deep red.33 Addition of NaBH4 (0.19-1.14 g, 0.19 g (5
mmol) per mmol of “Cl”) led to a further deepening in color,
followed by effervescence, during which the suspension (NaBH4
is only slightly soluble in THF) slowly turned brown-black.
Con clu sion
The suspension was stirred for several days (at room
temperature or 67 °C) under inert atmosphere (argon or
oxygen-free nitrogen) and atmospheric pressure. Samples of
ca. 0.5 mL were withdrawn periodically by filtering through
a Teflon delivery tube. After addition of a standard solution
(known concentration of anthracene in THF), the sample was
diluted to 10 mL with THF and analyzed with a HP 5890 series
II gas chromatograph (column HP1, cross-linked methyl
silicone gum, 25 m × 0.32 mm × 0.52 µm film thickness).
CG-program: 32 °C, keep for 5 min, heat at 10 °C/min to 260
°C, keep for 1 min. The injector was held at 200 °C and the
FID detector at 250 °C.
We have demonstrated that a catalyst system contain-
ing PdCl2(dppf) and NaBH4 can efficiently dechlorinate
higher chlorinated benzenes in THF at room temperature
when TMEDA is used as the supporting base. Within
200 h, complete degradation of substrate to a mixture of
lower chlorinated isomers (including monochlorobenzene,
but not benzene) is accomplished at room temperature
(for hexa- and pentachlorobenzene) and at 67 °C (for
hexa-, penta-, 1,2,4,5-tetra-, 1,2,3,4-tetra-, 1,2,4-tri-, and
1,2,3-trichlorobenzene). These conditions are mild com-
pared to many reported systems. The present catalytic
system also shows encouraging dechlorination activity
toward PCB mixtures. Again, the conditions used are
significantly milder than the incineration conditions
currently in practice. It is notable that a high level of
chlorination leads to high rates and yields. This catalytic
approach therefore complements well the biodegradation
approach which generally works better for the lower
chlorinated congeners. As toxicity of chlorobenzenes and
PCBs increases with higher chlorine content, a system
which works best for higher chlorinated aromatics is
clearly an advantage.
Sample filtering and diluting was shown to be sufficient to
quench the reaction. GC/MS (Shimadzu QP5000) was used
to confirm peak identity.
Concentration ratios obtained by the area ratio method
(assuming equal FID-sensitivity for all substrates) are almost
identical to those obtained by internal calibration. When
anthracene was used as the internal standard, (FID sensitivity
) 1), the sensitivities of chlorinated benzenes varied only from
0.51 (hexachlorobenzene) to 0.63 (benzene). No internal
standard was used for the analysis of Aroclors, which is
justified since relative molar response factors for PCB conge-
ners have been reported to range from 0.94 to 1.09.34
A pronounced selectivity can be observed for the
chlorinated benzenes with meta-substituted Cl atoms
being removed preferentially. This is probably linked to
electronic factors governing the ease of oxidative addition.
The observed site selectivity is unique and could be used
to prepare isomers that are not easily accessible by
common synthetic routes.
We propose a catalytic cycle involving oxidative addi-
tion, hydride transfer, and reductive elimination with the
first as the rate-determining step. A bidentate basic
ligand like TMEDA can enhance the performance of the
catalyst, through the formation of [Pd(dppf)(TMEDA)]
and TMEDA‚2BH3, for example.
Ack n ow led gm en t. This work was supported by the
NUS research grant RP 960655. We thank B. Wei for
important preliminary work on debromination and
valuable discussions and Z. Wang for experimental
assistance. L.L. acknowledges a scholarship from NUS.
The authors thank Shimadzu Corporation for the loan
of the QP5000 GC/MS.
J O971112K
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(33) In some cases, an orange-red solution is formed. In other cases
(especially after prolonged standing), the deep-red solution reverts to
its original orange color.
Although the conversion rates for the lower chlorinated
species are still unsatisfactory, this catalytic system can
serve as a starting point for further investigations.
Preliminary results show that a change in solvent can
further improve the dechlorination rates and yields.
(30) Roth, J . A.; Dakoji, S. R.; Hughes, R. C.; Camody, R. E. Environ.
Sci. Technol. 1994, 28, 80.
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