Activated Carbon Anchored Palladium Complexes For Heck Reaction

Heck reaction is one of the most powerful and widely used reactions for the formation of carbon-carbon bonds. It is generally catalyzed by palladium species such as palladium acetate, palladium chloride or tetrakis (triphenylphosphine) palladium. However, as with all homogeneous catalysts, there are separation and recycling problems that have so far precluded a more extensive industrial application of the Heck reaction. Heterogeneous palladium catalysts which can be more easily recovered from the reaction mixture can minimize these problems, and the development of such catalysts has attracted considerable interest.Activated carbons have some advantages as catalyst supports. They are relatively inexpensive, possess a high surface area, show chemical inertness both in acidic and basic media, and allow easy recovery of supported metal by simple combustion of the support. Palladium on carbon (Pd/C) proved to be active for Heck coupling under several different conditions and is one of the most frequently investigated catalysts. The common structural feature of the conventional Pd/C catalysts for the Heck reaction is the entrapment of palladium in the pores of carbons mainly by physical adsorption. Despite the many advantages associated with traditional Pd/C catalysts including their facile preparation and applicability to unactivated alkenes, their practical application in the Heck reaction is still not satisfactory. There is a need to design and develop new types of carbon-supported palladium nattalysts with high activities and stabilities.In the present work, three novel catalysts, activated carbon (AC) anchored amine palladium complexes (AC-NH2-Pd), AC anchored pyridine-carboimine palladium complexes (AC-NN-Pd) and AC anchored phosphine palladium complexs (AC-PPh2-Pd), were prepared by the following reactions:(1) surface modification of AC with nitric acid;(2) preparation of nitrogen and phosphine ligands on AC;(3) formation of palladium complexs an AC with palladium chloride. The prepared catalysts were characterized by FTIR, XPS, TEM. These immobilized catalysts exhibited high catalytic activity in the Heck reaction of aryl iodides with olefins. They can be recovered by simple filtration and reused up to4-6times without significant loss of yield.