Palladium Catalysts Supported on Mesoporous Molecular Sieves Bearing Nitrogen Donor Groups: Preparation and Use in Heck and Suzuki C-C Bond-Forming Reactions

A series of supported catalysts is prepared by treatment of SBA-15-type mesoporous molecular sieve bearing SiCH2CH2CH2NHCH2CH2CH2NEt2 groups with palladium(II) acetate. These catalysts are studied in Suzuki biaryl couplings and in Heck reactions to establish the influence of metal loading and innocent surface modifications (trimethylsilylation). The Suzuki reaction proceeded efficiently with model and practically relevant substrates; the catalyst performance increasing with an increasing degree of metalation (decreasing N/Pd ratio). Catalyst poisoning tests revealed that the reaction takes place in the liquid phase with the catalyst serving as a reservoir of active metal species and also as a stabilizing support once the reaction is performed. In the Heck reactions, on the other hand, the catalyst performance strongly changed with the reaction temperature and with N/Pd ratio. The material with the lowest metal loading (0.01 mmol palladium per gram of material, N/Pd ratio Ca. 100:1) proved particularly attractive in the Heck coupling, being highly active at elevated temperatures, recyclabel, and capable of acting as a bifunctional catalyst (i.e., functioning without any external base).