Silica and polymer-tethered Pd-SCS-pincer complexes: evidence for precatalyst decomposition to form soluble catalytic species in Mizoroki-Heck chemistry

Palladium(II)-SCS-pincer complexes have been covalently immobilized on porous silica and polynorbornene supports and evaluated in the Mizoroki-Heck reaction of iodobenzene and n-butyl acrylate. Kinetic experiments and poisoning studies indicate that the pincer complexes decompose under reaction conditions via rupture of the palladium-ligand bonds to liberate active Pd(0) homogeneous species. There is no evidence for catalysis by the intact Pd(II) pincer complex under any conditions evaluated, ruling out a Pd(II)-Pd(IV) catalytic mechanism that has been postulated for palladacycle catalysts under the conditions used here. (C) 2004 Elsevier Inc. All rights reserved.