Palladacycle immobilization

Nowotny and co-workers prepared polystyrene-immobilized complex 309. The supported complex is labile under typical Heck conditions, the active catalyst being transferred to solution. When the reaction is run at 140 °C, the kinetics are sigmoidal, showing that the active species slowly builds up in solution, and then decomposes. In addition, on the second recycle, all activity is in solution, probably as colloidal Pd(0) and no activity is left within the polymer support. This is consistent with 308 and other supported palladacycles being simply a slow source of active colloidal palladium particles. 

Immobilized dicyclohexylphosphine ligand, 310, has been used as the starting point for the preparation of supported palladacycles 313 and 314. Simply stirring 310 and dimeric palladium complexes 311 and 312 for 1 h in dichloromethane gave 313 and 314, respectively (Scheme 104). They are active Suzuki coupling catalysts on the first use but cannot be recycled. 

Palladium species immobilized on various supports have also been applied as catalysts for Suzuki cross-coupling reactions of aryl bromides and chlorides with phenylboronic acids. Polymers, dendrimers, micro- and meso-porous materials, carbon and metal oxides have been used as carriers for Pd particles or complexes for these reactions. Polymers as supports were applied by Lee and Valiyaveettil et al. (using a particular capillary microreactor) [173] and by Bedford et al. (very efficient activation of aryl chlorides by polymer bound palladacycles) [174]. Buch-meiser et al. reported on the use of bispyrimidine-based Pd catalysts which were anchored onto a polymer support for Suzuki couplings of several aryl bromides [171]. Investigations of Corma et al. [130] and Plenio and coworkers [175] focused on the separation and reusability of Pd catalysts supported on soluble polymers. Astruc and Heuze et al. efficiently converted aryl chlorides using diphosphino Pd(II)-complexes on dendrimers [176]. 

---