8636 Jagtap et al.
Asian J. Chem.
studies indicate that the catalyst is tolerant to water and can be
recycled with ease. No leaching of the Pd complex to the aque-
ous phase was observed in the course of the recycle study.
It is known that palladacycle complex may degenerate to
Pd metal under the reaction conditions used for Heck reaction28.
However, reports by Herrmann and coworkers indicate that
the palladacycle complex is stable in toluene under reflux even
for few days26,29. It was hence of interest to see whether any
decomposition of the catalyst was observed in the course of
the reaction. No precipitation of Pd was observed, even at the
end of four recycles. In the biphasic system proposed, the
organic phase comprises toluene, palladacycle catalyst and the
aryl halide. The sodium acrylate, product cinnamate and the
base/base-HX adduct are present in water. As a result, the
palladacycle has minimum exposure to these components. This
may be the reason why the palladacycle is stable under
reaction conditions in the biphasic medium used.
The idea of using such efficient catalyst in biphasic system
is more useful through industrial point of view. The results
produced here are of the reactions, those taken for very short
time (0.5 h) and are just to show the feasibility of the use of
catalyst in biphasic medium, however proper tailoring of
reaction may still improve the activity.
Conclusion
The palladacycle catalyst efficiently catalyzes Heck
coupling of acrylate salts with aromatic halides in a biphasic
[aqueous-organic] medium. Sodium hydroxide, carbonate and
bicarbonate were efficient although retained in the second,
non-catalyst phase. The activity observed for the vinylation
was highest for iodobenzene followed by bromobenzene. No
reaction was observed with chlorobenzene. The palladacycle
catalyst was stable in presence of water and could be recycled
for four times with no loss in the activity. Such a system
facilitates easy recycle of catalyst and also the removal of the
salts from the catalyst phase, ensuring high activity with respect
to cumulative turn over number and turn over frequency. The
products which are cinnamic acid derivatives were soluble in
water. The biphasic system developed has the following
advantages (i) efficient catalyst recovery and recycle, (ii) conve-
nient product separation and (iii) efficient removal of halide
in water. Strategies for total conversion of halide followed by
an efficient catalyst recycle, isolation of product and recovery
of the unreacted sodium acrylate have been outlined.
GCMS analysis of the isolated products:
Since the objective of this work was to develop a strategy
for the product separation and the catalyst recycle, a reaction
was conducted to complete conversion of the iodobenzene.
The concentration time profile for this experiment (organic
phase profile) is shown in Fig. 3. At the end of the reaction,
analysis of the organic phase showed negligible iodobenzene.
The acidification of the aqueous phase gave quantitative
formation of cinnamic acid. From this study it seems to be a
feasible strategy to recycle the catalyst after total conversion
of the halide. Residual sodium acrylate can also be recovered
from the aqueous phase for further recycle. The use of the
biphasic medium described above can thus provide an exce-
llent methodology for the recycle of catalyst, separation of
product and recycle of unreacted substrates, in the Heck
coupling of water-soluble substrates. A high cumulative turn
over number (TON) is achievable with the biphasic medium
proposed. (Up to 5100 has been achieved after 4 recycles in
this case).
1. m/z: 147 (M+ + 1), 131, 128, 91, 77, 51.
2. m/z: 178 (M+), 191, 133, 118, 89, 77, 63, 51.
3. m/z: 162 (M+ +1), 147, 115, 91, 44.
ACKNOWLEDGEMENTS
One of the authors (S.V. Jagtap) thank University Grant
Commission of India for providing the Ph.D. fellowship.
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Jagtap
