Pyridine-derived palladium complexes immobilized on ordered mesoporous silica as catalysts for Heck-type reactions

Palladium complexes with pyridine-carboimine and quinoline-carboimine ligands immobilized on ordered mesoporous silica (FSM-16) were prepared by the following reactions: (1) surface modification of FSM-16 with 3-aminopropyl group via silylation; (2) preparation of pyridine-carboimine and quinoline-carboimine ligands on FSM-16; (3) formation of palladium complex on FSM-16 with PdCl2. The palladium complexes were characterized by X-ray diffraction (XRD), N-2-adsorption, FT-IR, Si-29 and C-13 MAS NMR, inductively coupled plasma (ICP) and TG analysis. These catalysts were found to be active in the Heck vinylation of aryl iodides and activated aryl bromides with methyl acrylate. Conversion of 100% in 1-5 h reaction and selectivity for trans-stereoisomer higher than 99% have been achieved. In the case of less reactive aryl bromides, addition of quaternary ammonium salt increased activity of catalysts, but significant leaching of palladium into solution has been observed. The palladium complexes showed high catalytic activity even for Suzuki coupling of aryl iodides and aryl bromides with the yield of coupling products 50-100% after 1.5 h reaction. Hindered substrates such as 9-bromoanthracene and 9,10-dibromoanthracene were also coupled in yield of 40-65%. Nevertheless, slightly higher concentration of Pd (1.5 times as high as for mono-coupling) is required for complete di-coupling in the case of 9,10-dibromoanthracene. These immobilized catalysts can be easily separated from the reaction mixture by filtering and re-used for further reaction. Catalysis by these palladium complexes can offer an alternative to homogeneous catalysts. (C) 2004 Elsevier B.V. All rights reserved.