Studies On Polystyrene-supported NHC-Pd Complex As Catalyst For The Suzuki Reaction

Biaryl motif is found in a range of pharmaceuticals, herbicides, and natural products as well as in conducting polymers and liquid crystalline materials. The Suzuki cross-coupling reaction which produces biaryls has proven to be one of the most versatile methods, although several other methods are available for this purpose. The key advantages of the Suzuki coupling are the mild reaction conditions and the commercial availability of the diverse boronic acids that are environmentally safer than other organometallic reagents. Heterogeneous catalysts have many advantages for industial applications. The NHC ligands bind to both Pd0 and PdII centers substantially stronger than those widely used phosphine ligands, and are highly suitable for attachment to solid supports owing to its anticipated low level of metal leaching. However, only few examples of heterogeneous catalysts by anchoring NHC-Pd complexes for Suzuki reaction have been reported. So it is meaningful to do research on polymer-supported NHC-Pd complex as catalyst for Suzuki coupling reaction. Our work focused on the use of an ether linkage to attach some novel imidazolium salts to the Merrifield resin. The polymer-supported NHC-Pd complex were prepared from Pd(OAC)₂ and the polymer-supported imidazolium salts. Herein, five aspects were considered: (1) study on the swelling properties of the the polymer-supported imidazolium salts and the polymer-supported NHC-Pd in different solvents; (2) effects of solvent; (3) effects of base and reaction time; (4) study on the recyclability of the catalyst ; (5) the generality of this strategy. In conclusion, the Suzuki reaction of aryl bromides with arylboronic acid using the catalyst afforded the desired products in high yields in aqueous DMF solution under room temperature, and without the need of inert gas protection. Using only very low catalyst loading, the catalyst could be reused up to 6 times still retaining its activity.