Jairton Dupont and John
Featured Paper Interview
This month, in-cites talks with Drs. Jairton
Dupont & John Spencer about their work on
palladacycles. According to
IndicatorsSM, two of their
papers on this topic are Highly Cited Papers in the
field of Chemistry: "Palladacycles – an old
organometallic family revisited: new, simple, and
efficient catalyst precursors for homogeneous
catalysis" (Dupont J, Pfeffer M, Spencer J, Eur.
J. Inorg. Chem. 8: 1917-27, August 2001) has 209
cites to date, and "The potential
of palladacycles: more than just precatalysts" (Dupont
J, Consorti CS, Spencer J, Chem. Rev. 105:
2527-71, June 2005) has 119 cites to date. Dr. Dupont
is Professor of Chemistry at the
Instituto de Química –
Universidade Federal do Rio Grande do Sul – UFRGS
in Brazil. Dr. Spencer wrote these articles whilst in
industry at the Thrombosis Research Institute and at
the James Black Foundation, and has recently joined the
University of Greenwich.
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
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
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
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
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: 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
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
Professor of Organic Chemistry
Instituto de Química
Universidade Federal do Rio Grande do Sul – UFRGS
Porto Alegre RS Brazil
Reader in Medicinal Chemistry
School of Science
University of Greenwich at Medway
Kent, United Kingdom