Greener Pd-catalyzed C-C coupling processes

Ernesto de Jesús

Inorganic Chemistry Department, Universidad de Alcalá, Alcalá de Henares (Madrid), Spain

ernesto.dejesus@uah.es

Catalysis plays a central role in the design of more sustainable processes in green chemistry. Homogeneous metal catalysts are especially useful in this context because they can afford highly selective organic transformations¹ in processes which are very efficient in terms of activity and atom efficiency, under relative mild reaction conditions. An ideal homogeneous metal catalyst should be highly active, long-lived and recyclable, therefore being able to afford elevated operational turnover numbers. The optimization of these properties by the modification of the ligands attached to the metal catalyst is sometimes limited by the fact that the requirements for a durable and active catalyst are often in opposition. Several examples from the recent work of our group in palladium-catalyzed carbon-carbon coupling reactions will serve to illustrate this point.

One of our areas of research deals with the design of more sustainable processes using water as the medium for metal-catalyzed organic syntheses. 1 For example, we have developed highly effective protocols for C–C coupling reactions of aryl and vinyl bromides with organosilanes in water that require very low loadings of palladium in “ligand-free” conditions.² These simple conditions are unsuccessful for the activation of chloroarenes but these are readily activated using water-soluble palladium(II) N-heterocyclic carbene (NHC) complexes instead.³

In the search of greener processes, we are also interested in new methodologies for the recycling of homogeneous catalysts. The commercial availability of solvent-resistant membranes tagged with nominal molecular weight cut-offs lower than 1000 Da has made nanofiltration a convenient and industrially-viable alternative for the separation of molecular-weight enlarged catalysts (MWE).⁴ One of the main problems for the recovery of MWE catalysts by nanofiltration is catalyst deactivation and leaching. We have found that the strong bonds formed by N-heterocyclic carbene complexes (NHC) are ideal to prepare highly recyclable MWE catalysts for the Pd catalyzed C-C Heck reaction in combination with ceramic membranes and poly(aryl ether) dendrons as molecular-weught enlargers.⁵

This research was supported by the Spanish Ministerio de Ciencia e Innovación (Project CTQ2008-02918/BQU).

1 K. H. Shaughnessy, Chem. Rev. 2009, 109, 643.

2 A. Gordillo, C. Lopez-Mardomingo, E. de Jesús, Org. Lett.2006, 8, 3517; A. Gordillo, C. Lopez-Mardomingo, E. de Jesús, Chem. Commun.2007, 4065; A. Gordillo, C. Lopez-Mardomingo, E. de Jesús, J. Am. Chem. Soc. 2009, 131, 4584.

3 R. Garrido-Lledó, A. Gordillo, C. Lopez-Mardomingo, E. de Jesús, submitted to Organometallics

4 H. P. Dijkstra, G. P. M. van Klink, G. van Koten, Acc. Chem. Res.2002, 25, 798; E. de Jesús, J. C. Flores, Ind. Eng. Chem. Res. , 2008, 47, 7968.