Tridentate SCS palladium(II) complexes: New, highly stable, recyclable catalysts for the heck reaction

A new pincer-type SCS ligand containing Pd(II) is a simple, robust catalyst for Heck chemistry using a variety of alkene accepters and aryl iodides. It is less active with aryl bromides. While certain palladium(II) species insert slowly into the aryl C-H bond of an unsubstituted version of this ligand, the introduction of activating groups into the 5 position of the aromatic ring readily allows quantitative metal insertion. These ligands were synthesized and attached to soluble polymers by simple modification of inexpensive starting materials. For example, both 5-oxy and 5-amido SCS ligands were successfully appended to 5000 M-n poly(ethylene glycol) via ether or amide linkages, respectively. Both the 5-ore and 5-amido complexes are active as Heck catalysts in DMF solution in air. The PEG-bound 5-amido-SCS-Pd complex was recycled via solvent precipitation three times with no observed catalyst deactivation. While the 5-amido-SCS-Pd complexes are very robust, their 5-ore counterparts decompose slowly under certain conditions. These SCS catalysts are analogous to PCP-type catalysts previously reported in the literature but avoid the requirement of an air-sensitive phosphine synthesis.