Mizoroki-Heck coupling using immobilized molecular precatalysts: Leaching active species from Pd pincers, entrapped Pd salts, and PdNHC complexes

The Mizoroki-Heck reaction is a palladium-catalyzed reaction of both practical importance and scientific significance. Two challenges currently faced by practitioners of the Heck and other palladium-catalyzed coupling reactions are (i) minimizing the costs associated with this reaction by developing high TON catalysts or facilitating catalyst recovery and (ii) elucidating the nature of the active species when using various different precatalysts. These two challenges, one practical and one fundamental, served as our motivation in our studies of immobilized molecular palladium(II) complexes as precatalysts in Mizoroki-Heck reactions. This Forum Article describes our investigations in this area, highlighting our approach to the elucidation of the active catalyst by combining kinetic and poisoning studies of several different related precatalysts, our development of new, selective catalyst poisons, and our quest for stable, recyclable supported Heck, Suzuki, and Sonogashira coupling catalysts. Under high-temperature conditions (120 degrees C), all precatalysts studied are conclusively shown to decompose, liberating soluble, active palladium(0) species that are the true catalysts. Techniques for the elucidation of the nature of the truly active Pd species are enumerated.