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Polypalladated Arenes: Synthesis, Characterization and Reactivity. Applications in Organic Synthesis

Final Report Summary - POLYPALLADATEDARENES (Polypalladated arenes: synthesis, characterization and reactivity. Applications in organic synthesis)

An unknown to the general public for years, palladium has long been for the synthetic chemist a metal much more precious than its 'noble' neighbours in the periodic table: silver, gold or platinum. This low public profile has to some extent been corrected by the award of the 2010 Noble prize in chemistry to Professors Heck, Negishi and Suzuki, for their work in palladium-catalyzed cross coupling reactions. Organopalladium chemistry has grown in the last decades to become 'one of the most sophisticated tools available to chemists today' (1), with strong impact in such areas as the synthesis of medicines, food additives or advanced technological materials. And progress in this field is going on, with no end in sight (2).

Arylpalladium complexes are key intermediates in many palladium-catalysed cross-coupling reactions and a huge research effort has focused on the synthesis of these type of complexes, and on the investigation of their structure, properties and reactivity. In particular, it has been shown that the presence of a functional group on the aryl ring, next to the palladium atom, can result in new types of reactivity with respect to the unsubstituted analogues. The aim of this project has been to extend this chemistry to the synthesis of tripalladated complexes with functional groups next to each of the palladium atoms, a field which was almost unexplored (3).

The project has succeeded in the synthesis and full characterisation of several examples of functionalised tripalladated complexes of type II, specifically those with R = CHO or CH=CHR and [Pd] = [PdBr(N-N)], with N-N being a chelate N-donor ligand, such as 4,4'-di-tert-butyl-2,2'-bipyridine (tbbpy) or N,N,N',N'-tetramethylethylenediamine (tmeda). The investigation of the reactivity of these complexes has provided some interesting and promising results, such as CO insertion reactions in the C-Pd bonds, pincer-type complexes resulting from condensation reactions with amines, or the synthesis of novel Pd3/Tl and Pd6/Tl2 complexes with unprecedented structures (4). We hope that in the immediate future this chemistry will contribute to expand even more the applications of palladium-mediated synthesis.

(1) From the press release of the Royal Swedish Academy of Sciences, 6 October 2010.
(2) Palladium Reagents and Catalysts: New Perspectives for the 21st Century. J.Tsuji. Wiley, 2004.
(3) J. Vicente, M. Lyakhovych, D. Bautista, P. G. Jones, Organometallics 2001, 20, 4695-4699. C. J. Sumby, P. J. Stell, Organometallics 2003, 22, 2358.
(4) J. Vicente, R. V. Shenoy, E. Martínez-Viviente, P. G. Jones, Organometallics 2009, 28, 6101-6108, and unpublished results.