Are you using too much Palladium?

16 April 2004

In a Scientific Update conference last year*, David Ager (DSM, USA) described some work from his Dutch DSM colleagues demonstrating that lower palladium loadings could improve the performance of ligand-free Heck and Suzuki coupling reactions. This work was also published in the primary literature (A.H.M. de Vries et al, Organic Letters , 2003 , 5 , 3285).

Previously, ligand-free palladium catalysis had been restricted to iodide substrates, bromides tending to give significant amounts of palladium black, bringing the reaction to a halt. The DSM group found that provided the amount of palladium acetate is kept between 0.01 and 0.1 mol%, aryl bromides too can successfully be reacted in Heck and Suzuki fashion.

The rationale is that the Heck reaction is probably first order in palladium, whereas aggregation to form palladium black is likely to be higher order in palladium. By reducing the concentration of palladium, the rate of palladium black formation will therefore be more significantly slowed than the rate of Heck reaction. The difference between iodides and bromides probably reflects a difference in rate determining step, and the ability of molecular iodine to oxidise palladium black back up to Pd(II).

So in your next lab scale Heck or Suzuki reaction, before arbritrarily throwing in 5 or 10 mol% palladium catalyst, it might be worth trying a lower palladium loading, and potentially eliminating the need for any ligand....

* D.J. Ager (DSM, USA), "Homogeneous Catalysis: From Coors to Heineken", Presentation given at the 8 th International Conference on Organic Process Research and Development, Barcelona, 7-10 September 2003.