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AUTHOR COMMENTARIES - 2008

January 2008
(originally featured in in-cites.com)
 
 
Jairton Dupont and John Spencer
Featured Paper Interview

This month, in-cites talks with Drs. Jairton Dupont & John Spencer about their work on palladacycles. According to Essential Science 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[6]: 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 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.

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

                      



2008 : January 2008 - Author Commentaries : Jairton Dupont & John Spencer