Silica-supported imine palladacycles - recyclable catalysts for the Suzukireaction?

Silica-supported, imine-based palladacyclic catalysts have been synthesised and the crystal structure of complex 9, the triphenylphosphine adduct of t he pre-supported precursor complex 8, has been determined. The solid-suppor ted catalysts show considerably lower activity in the Suzuki reaction than their homogeneous counterparts. Poor recyclability of the silica-immobilise d catalysts and the presence of active catalysts in solution indicate that imine-based palladacyclic catalysts are unstable with respect to liberation of zero-valent palladium species. Whilst the solid-supported complexes are not useful as catalysts, they do function as excellent mechanistic probes. Studies on model complexes give further information on the processes that cause the liberation of zero-valent species not only from the solid-support ed catalysts, but also from homogeneous systems. In all cases it appears th at a reductive-elimination event occurs to generate the active catalyst. (C ) 2001 Published by Elsevier Science B.V.