Preparation and Reactivation of Heterogeneous Palladium Catalysts and Applications in Sonogashira, Suzuki, and Heck Reactions in Aqueous Media.

A new type of heterogeneous palladium catalyst, PdMgAl-LDH, was facilely prepared by the immobilization of Pd2+ species in the layers of a Mg-Al layered double hydroxide (LDH) with co-precipitation, and then fully characterized by using powder XRD, thermogravimetric differential thermal analysis, TEM, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy techniques. These catalysts can efficiently catalyze copper-free Sonogashira, Suzuki and Heck coupling reactions of various aryl iodides, bromides, and chlorides in aqueous media under phosphine-ligand- and organic-base-free conditions. These catalysts feature easy recovery through simple filtration and could be reused at least six times without a marked loss in activity. Notably, they can be facilely reactivated by a combination of nitrolysis with co-precipitation. The basic LDH skeletons could effectively stabilize the Pd0 species created in situ and donate electron density to the Pd0 center to facilitate the oxidative addition of aryl halides, thus the PdMgAl-LDH catalysts are stable during catalysis.