Would you please explain for our readers what exactly are
palladacycles, and what makes them important?
Dupont: In general terms, they are heterocyclic compounds
in which one heteroatom is palladium. However, the vast majority
of these compounds are composed of a Pd-carbon bond stabilized
intramolecularly by coordination of a two-electron donor group
such as those containing nitrogen, sulphur, phosphorus atoms,
etc. Most of the interest centers on their facile synthesis and
easily handling. Moreover, the possibility of modulating their
electronic and steric properties renders them an interesting and
varied family of organometallic compounds.
Palladacycle chemistry is a fascinating area and this
interesting class of compounds represents a challenge to
chemists not only in terms of their synthesis but also in terms
of their structures, design, and types of ligands metalated.
They have applications in domains as diverse as biological
chemistry, material science, synthesis, ligand resolution, or
catalysis.
|
John Spencer
(left) and Jairton Dupont (right),
with their colleague, Michel Pfeffer
(center) |
 |
|
"Palladacycle chemistry is a fascinating
area and this interesting class of compounds
represents a challenge to chemists not only
in terms of their synthesis but also in
terms of their structures, design, and types
of ligands metalated."
Jairton Dupont
"The most important application of
palladacycles is no doubt in catalysis."
John Spencer |
|
Spencer: The most important application of palladacycles
is no doubt in catalysis. Industry is looking for efficient
synthetic routes to high-value end products such as
pharmaceuticals using very small amounts of (expensive)
catalysts. Palladacycles have been shown to be very active in a
number of important C-C and C-X (heteroatom) bond-forming
reactions, although we need to be able to show greater substrate
compatibility, as very high turnover reactions proceed with a
variety of simple "archetypal" substrates yet these are not
transposable to more challenging coupling partners where much
higher palladacycle loadings are needed.
What were the major findings and implications from your work on
palladacycles?
Dupont: There were two major findings in two different
periods. The first period was during my thesis under the
supervision of Michel Pfeffer in Strasbourg in which we
developed methods for the synthesis of sulphur-containing
palladacycles via C-H bond activation. The second period was in
Brazil in which we used palladacycles as catalyst precursors for
C-C coupling reactions. In particular we demonstrated that these
palladacycles are simple reservoirs of catalytically active
Pd(0) species.
Spencer: I actually followed on from Jairton’s Ph.D. in
Michel Pfeffer’s laboratory in Strasbourg! I synthesized a
series of sulphur-containing heterocycles using many of the
complexes that had been developed by Jairton. We were also able
to synthesize Aporphine-related heterocycles using catalytic
quantities of a palladacycle as well as look into mechanistic
implications of palladacycle chemistry including the stereo and
regiochemistry of alkyne insertions in the Pd-C bond. Since I
spent almost 10 years in industry, I was removed from the
palladacycle area, although I was delighted to participate in a
palladacycle review with Jairton and Michel—the 2001 European
Journal of Inorganic Chemistry paper.
What initially sparked your interest in this line of research?
Dupont: Initially, my main interest was in organometallic
chemistry and I went to Strasbourg to do my thesis under the
supervision of Michel Pfeffer, and the synthesis and reactivity
of palladacycles (called cyclopalladated compounds at that time)
was the major area of research of his group.
Spencer: I did my (Chemistry with French) undergraduate
studies at Sussex University, where there is a strong research
activity in organometallic chemistry and organic synthesis. I
wished to combine the two and study in a good French university,
and the project that I was given in Michel’s laboratory in
Strasbourg met all of the criteria.
Would you talk a little about one of your more recent papers, "The
potential of palladacycles: more than just precatalysts" (Chem.
Rev. 105[6]: 2527-71, June 2005), and perhaps speculate on why
it is highly cited?
Dupont: This paper is a review covering all major aspects
of the synthesis and applications of palladacycles. Although at
that time there were already excellent specialized reviews on
palladacycles, they were mainly focused on their specific
applications. Our review is not comprehensive but it covers the
palladacycle chemistry that was not already covered in the
specialized reviews and, most importantly, we tried to
critically present the main achievements, potential, and
limitations of palladacycles. It is highly cited probably
because it "resumes" the most recent achievements of
palladacycle chemistry (which remains a very active and
important area) and it is more convenient for some people to
cite a single paper than a list of original references.
Spencer: This paper took well in excess of two years to
complete as I was working on other projects in industry. Let us
not forget the massive contributions of our coauthor, Crestina [Consorti],
to this project as well.
Another problem with an area such as this is that as soon as
you are satisfied with an initial draft for submission, a
groundbreaking paper is published and you have to add this to
your review! I feel that we were probably the first to get the
right balance of a number of important palladacycle-related
subjects and we were able to have an initial snapshot of the
burgeoning area of palladacycle-mediated catalytic C-H
activation chemistry, which is a very important area of
catalysis.
Where have you taken this work since this paper? Where do you see
this research going in the next 10 years?
Dupont:
We are now mainly applying these compounds in
various fields such as for the generation of palladium
nanoparticles or catalyst precursors for the synthesis of
important biologically molecules (DNA detectors for instance). I
have no doubt that palladacycles will still be used as catalyst
precursors but also in important and "hot" areas, such as in
photoluminescent devices and medicinal and biological chemistry.
Spencer: We are working on synthetic, medicinal, and
catalytic applications of palladacycles, but this is at a very
early stage and in need of significant funding to be able to
flourish. Many groups are working on palladacycle-mediated
double C-H activations, e.g. Melanie Sanford’s, and these could
prove to be very atom-economical and industrially relevant.
Jairton Dupont
Professor of Organic Chemistry
Instituto de Química
Universidade Federal do Rio Grande do Sul – UFRGS
Porto Alegre RS Brazil
John Spencer
Reader in Medicinal Chemistry
School of Science
University of Greenwich at Medway
Kent, United Kingdom
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