Palladium is one of the most useful catalysts in organic syntheses. Many types of palladium complex catalysts have been developed and utilized in various reactions. Furthermore, "Environmentally-friendly organic syntheses" are desired these days, so there have been many developmental studies of palladium complex catalysts with respect to green chemistry in recent years.
Palladium complex catalyst 1 developed recently by Ikegami et al. is one of those catalysts with excellent properties. Catalyst 1 is solid-phase catalyst which consists of amphiphilic copolymer ligands and palladium. It uptakes substrates and reagents of both hydrophilic and hydrophobic nature in the vicinity of palladium to smoothly promote the reaction. The palladium complex catalyst forms a networked supermolecular complex, which is insoluble in water and organic solvents, and is easily recovered from reaction system. Since the palladium is firmly held in place by the phosphino group of copolymer ligand, it is not lost during the reaction or upon work up; therefore, the catalyst can be recovered and reused multiple times.

Ikegami, et al. have demonstrated an application of 1 for the Suzuki-Miyaura coupling reaction in water, as this reaction does not require any organic solvents.¹⁾ This method requires as little as 5×10^–4 ~ 5×10^–5 mol equiv. of 1 to effectively catalyze the reaction. After the reaction, 1 can be recovered by simple filtration. The recovered catalyst 1 can be reused, and it maintains its catalytic activity even after being reused ten times.

As shown in the figure, in the presence of sodium carbonate and catalyst 1, halobenzene and 1.1 mol equiv.
of phenylboronic acid react to form product in water as the solvent. The reaction vessel is fitted with glass-filter, and after the reaction is completed, the mixture is filtered while it is still hot and then washed with hot water. The catalyst 1 remains in the reaction vessel, while the filtrate contains the biphenyls, as well as sodium carbonate and residual phenylboronic acid. When the filtrate is cooled, the biphenyls crystallize out of the aqueous phase. Upon filtration, the biphenyls are easily obtained. 1 that remaines in the reaction vessel can be reused right away. An application of this catalyst for the Heck reaction has been reported as one example of a reaction in an organic solvent.²⁾

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