Poly(4-vinylpyridine) and Quadrapure TU as selective poisons for soluble catalytic species in palladium-catalyzed coupling reactions - Application to leaching from polymer-entrapped palladium

Poly(4-vinylpyridine) and Quadrapure TU were successfully used as poisons for palladium-catalyzed Heck couplings of iodoarenes and n-butyl acrylate using a poly(urea) entrapped palladium precatalyst, Pd-EnCat 40, in a variety of reaction solvents and conditions. These results suggest that all of the Heck coupling takes place outside the poly(urea) matrix via a soluble palladium species leached from the solid. This observation is consistent with previous conclusions that leached species are also solely active when using other supported catalysts such as Pd/C or with supported palladacycles. Control experiments including poisoning by Hg(0), hot filtration, 3-phase tests, and monitoring of reaction kinetics after recycle support this conclusion. The quantity of PVPy or Quadrapure TU needed for quenching depends on the amount of palladium that is leached and thus applying these poisons to other palladium catalysts in which the amount of soluble palladium is unknown requires an initial trial and error approach.