Melamine and melamine-formaldehyde polymers as ligands for palladium and application to Suzuki-Miyaura cross-coupling reactions in sustainable solvents.

The Suzuki-Miyaura cross-coupling reaction is a foundation stone of modern organic synthesis, as evidenced by its widespread use in the preparation of pharmaceuticals, agrochemicals, polymers, and other functional materials. With the prevalence of this venerable reaction in industrial synthesis, it is prudent to ensure its application adheres to the tenets of green chemistry. The introduction of cross-coupling catalysts that are active in sustainable solvents is therefore an important endeavor. In this report, a melamine-palladium complex is introduced as a versatile catalyst for the Suzuki-Miyaura cross-coupling reaction. This catalyst is soluble and active in both water and the renewable organic solvent ethyl lactate. The melamine-palladium catalyst can also be cross-linked by reaction with formaldehyde to generate an insoluble polymeric catalyst that can be recovered after the cross-coupling. The melamine-palladium system is inexpensive, easy to handle, bench-stable, and effective in catalysis in the presence of a variety of impurities (high cross-coupling yields were obtained in reactions run in unfiltered river water to illustrate this final point). Additionally, investigations reported herein revealed an intriguing relationship between catalytic efficiency and the base employed in the cross-coupling reaction. Implications for the mechanism of transmetalation in aqueous Suzuki-Miyaura cross-coupling reaction are discussed.