Download Activation of Carbon Chlorine Bonds Using Palladium (II) Catalysts

Activation of Carbon Chlorine Bonds Using Palladium (II) Catalysts
Briana Goddard and Derk A. Wierda
Saint Anselm College, Department of Chemistry
Abstract
Chlorophenylbis(tricyclohexylphosphine)palladium(II) showed catalytic activity
capable of activating carbon chlorine bonds in chlorobenzene, metadichlorobenzene, 2,4-dichlorotoluene and possibly chloropentane. Chloro(pchlorophenyl)bis(tricyclohexylphosphine)palladium(II) showed catalytic activity
with chlorobenzene, but not with para-dichlorobezene or with metadichlorobenzene.
Results and Discussion
GC/MS Chromatogram for Catalyst (2) in Chlorobenzene
The catalytic activity of both (1) and (2) were tested in the presence of various
monochlorinated and dichlorinated species. The base triethylamine was used to
trap the hydrochloric acid generated and precipitated out of solution. The
resulting solutions were analyzed using gas chromatography and mass
spectroscopy. As expected, (1) was active with chlorobenzene, and yielded
benzene.
Before the Reaction
Chlorophenylbis(tricyclohexyl
phosphine)palladium(II)
(1)
Chloro(p-chlorophenyl)bis
(tricyclohexylphosphine)palladium(II)
(2)
When in the presence of para-dichlorobenzene and ortho-dichlorobenzene, the
catalyst was inactive. However, when in solution with meta-dichlorobenzene the
reaction produced both benzene and chlorobenzene.
Toluene
Chlorobenzene
Triethylamine
Introduction
Polychlorinated Biphenyls (PCBs) are stable molecules with high boiling points
and electrical insulating properties. Due to these characteristics, they were used
in various industrial equipment, including transformers, capacitors, and hydraulic
systems. However, PCBs also have many adverse health affects, including
carcinogenic properties. In order to safely dispose of these chemicals, the inert
carbon chlorine bond must be activated.
(1) was also tested with 2,4-dichlorotoluene, generating chlorotoluene and
probably toluene, but the results were obscured by the solvent peak. In order to
test whether (1) could activate chlorinated molecules that are not aromatic,
chloropentane was used. The results showed evidence of a reaction, but were
inconclusive.
After the Reaction
(2) was active in chlorobenzene, producing benzene. In the presence of paradichlorobenzene and meta-dichlorobenzene, no benzene was produced. In both
reactions hydrogen gas was consumed, and a solid was produced, but no benzene
was detected. It is possible that the para-dichlorobenzene and metadichlorobenzene were causing the polymerization of the phenyl groups.
Toluene
Benzene
In the past there has been success activating carbon chlorine bonds with
transition metals complexes. Nickel and platinum have been used, but palladium
seems to be the most reactive. The reaction of Pd(DBA)3 and 2PR3 creates
phosphine complexes containing Pd(0). PR3 can be PCy3, PCy2Ph, PBz3, or
Pmtolyl3. Chlorobenzene and dichloromethane been activated using some of these
phosphine complexes in an oxidative addition reaction.
When pressurized under hydrogen the catalyst
chlorophenylbis(tricyclohexylphosphine)palladium(II) undergoes a reductive
elimination reaction, yielding the active complex
bis(tricyclohexylphosphine)palladium(0), as well as benzene and hydrochloric
acid. The active species can then react again with chlorobenzene in an oxidative
addition reaction, regenerating the catalyst. A base must be present in the reaction
to trap the hydrochloric acid and prevent it from reacting with the active catalyst.
References
Huser, M.; Youinou, M.; Osborn, J. A. Chlorocarbon Activaton: Catalytic
Carbonylation of Dichloromethane and Chlorobenzene. Angew. Chem. Int. Ed.
Engl. 1989, 28.1386-1388
Results using Catalyst (1)
Chlorobenzene
Results using Catalyst (2)
Acetone
Conclusion
Standard Conditions
Solvent
Toluene
Base
Pressure
Triethylamine
Began at 57 psi,
and dropped as
the reaction
proceeded
Time
At least 8 hours
Temperature
Room
temperature
Concentration
The reactant was
in a 50:1 mole
ratio with the
catalyst
Chlorophenylbis(tricyclohexylphosphine)palladium(II) underwent a reductive
elimination reaction to yield the fourteen valence electron reactive species
bis(tricyclohexylphosine)palladium(0), which readily participated in oxidative
addition reactions to activate carbon chlorine bonds. Catalyst (1) can activate
both monochlorinated and dichlorinated species. Chloro(pchlorophenyl)bis(tricyclohexylphosphine)palladium(II) also exhibited catalytic
activity, but was far less reactive than catalyst (1). Catalyst (2) has not yet
activated a dichlorinated molecule, but was successful in yielding benzene from
chlorobenzene.