(583bg) Investigation of the Leaching Behavior of a Palladium Catalyst Supported On 3-Mercaptopropyl-Functionalized Silica Gel

Catalytic C-C, C-O and C-N couplings using Pd-catalysts are widely employed both in academia and in industry, making Pd the most extensively used metal catalyst for the synthesis of a wide variety of organic compounds. There are numerous examples of papers and patents claiming new and highly active immobilized Pd-catalysts. However, detailed mechanistic studies have shown that many reactions using these catalysts likely occur in solution via leached Pd [1,2,3].  This leaching of the metal into the product implicates a time-consuming and costly cleaning step, which would make the whole process more expensive and it also leads to a decreasing turn over frequency (TOF) of the catalyst.

In this study we present the preparation and testing of a novel supported Pd-catalyst that shows virtually no metal leaching into the solution [4].

In particular, we report here the covalent immobilization of 2,2’-(1-methyl-11-dodecenylidene)bis(4,5-dihydrooxazole) on 3-mercaptopropyl-functionalized silica gel (MPSG). The metalation of the immobilized BOX ligand with Pd(OAc)₂ yielded the stable and active catalyst Pd(OAc)₂-BOX-MPSG. The performance of the novel catalytic system was tested using Suzuki-Miyaura reactions of different arylbromides with phenylboronic acid as a test system, indicating that > 95% yield can be obtained within 4h using the novel heterogeneous system with a catalyst loading of 2 mol%.

A special issue of this work, however, was the investigation of the behavior of the supported metal during the reaction and the amount of leached Pd into the solution. For this purpose we carried out “state-of the-art” tests including the hot filtration test, ICP/OES analysis, the three-phase test [5], and catalyst poisoning studies. Furthermore, the influence of different reaction solvents on the leaching behavior was studied as well as the reusability of the catalyst.

The results of these tests indicated that the metal leaching is minimal under the applied reaction conditions. The catalyst can be reused for at least 10 times without significant loss of activity, stability or structure. The ICP/OES analysis of samples taken during and after the Suzuki reactions showed that the Pd concentration in the reaction solution was less than the detection limit (i.e., 50 ppb) which corresponds to less than 0.11 % of the starting Pd-amount. Such stable and active heterogeneous catalysts would be definitely suitable to be implemented in continuous reaction processes for the preparation of active pharmaceutical intermediates and fine chemicals.

However, the addition of catalyst poisons led to a quenching of the catalytic activity, which indicates the presence of leached homogenous Pd. Additionally, TEM pictures of the used catalytic material indicated nanoparticles on the surface.

Therefore, further studies were carried out to fully understand the reaction mechanism of the presented catalytic system. The results of these studies will be presented in this contribution.

 References:

[1]   J.D. Webb, S. MacQuarrie, K. McEleney, C.M. Crudden, Journal of Catalysis, 252 (2007) 97

[2]   Y. Ji, S. Jain, R.J. Davis, J. Phys. Chem. B, 109 (2005) 17232

[3]   J.M. Richardson, C.W. Jones, Journal of Catalysis, 251 (2007) 80

[4]   H. Gruber-Woelfler, P. F. Radaschitz, P. W. Feenstra, W. Haas, J. Khinast, Journal of Catalysis, 286 (2012) 30

[5]   C.M. Crudden, M. Sateesh, R. Lewis, J. Am. Chem. Soc., 127 (2005) 10045