(377d) Heterogeneous Vs. Homogeneous Pd Coupling Pathways: Heck and Suzuki Couplings Using Palladium Immobilized on Propylthiol-Modified Mesoporous Silica

The use of palladium metal is ubiquitous as a catalyst in the development and synthesis of active pharmaceutical ingredients. Palladium is an expensive metal and has no known biological function. Therefore there is a need to recover spent palladium, which is driven both by cost and by government regulations requiring residual palladium in products to be very low. Thus, much research has been conducted with the aim of heterogenizing active palladium that can then be removed via simple filtration and reused without significant loss of activity.

A large number of supports and immobilization strategies have been employed. In most cases, these solid precatalysts are assumed to act as active, heterogeneous catalysts. That is, a heterogeneous catalytic reaction pathway is assumed. Rarely are specific tests to elucidate the nature of the active species undertaken and in cases when they are used, relatively ambiguous methods, such as hot filtration tests and comparisons of reaction yields upon catalyst recycle are most often employed. There is a growing consensus that the majority of catalysis from immobilized palladium is actually from leached metal in Heck catalysis, which is usually determined after more rigorous testing has been employed.1-3 Nonetheless, new heterogeneous catalysts continue to be claimed every year.

Herein we present a re-examination on recently reported ?heterogeneous? palladium pre-catalysts immobilized on alkylthiol-modified supports. Reinterpreting previous reports4,5 we conclude that catalysis is solely from leached metal and no direct evidence was found for catalysis by palladium species immobilized on a surface6,7. We also demonstrate how organic modified supports can be used as selective metal poisons, which can be used to test for presence of heterogeneous palladium catalysis8.

References:

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8. Richardson, J.M. and Jones, C.W. Adv. Synth Catal. 348, 1207 (2006)