Hitoshi Takada

Amphipathic monolith-supported palladium catalysts for chemoselective hydrogenation and cross-coupling reactions

A palladium catalyst immobilized on an amphipathic and monolithic polystyrene–divinylbenzene polymer bearing strongly acidic cation exchange functions (sulfonic acid moieties) (Pd/CM) was developed. It was used as a catalyst for hydrogenation and ligand-free cross-coupling reactions, such as the Suzuki–Miyaura, Mizoroki–Heck, and copper- and amine-free Sonogashira-type reactions, together with a palladium catalyst supported on monolithic polymer (Pd/AM) bearing basic anion exchange functions (ammonium salt moieties), which has been in practical use for the decomposition of hydrogen peroxide produced as a byproduct during the manufacture of ultrapure water. While the Pd/CM was highly active as a catalyst for the hydrogenation and a variety of reducible functional groups could be reduced, the use of Pd/AM led to a unique chemoselective hydrogenation. Aromatic carbonyl groups were tolerant under the Pd/AM-catalyzed hydrogenation conditions, although benzyl esters, benzyl ethers, and N-Cbz groups could be smoothly hydrocracked. The cross-coupling reactions readily proceeded using either catalyst. The palladium leaching from the Pd/CM into the reaction media was never observed during the Sonogashira-type reaction, which was hardly achieved by other palladium-supported heterogeneous catalysts due to the good affinity of the palladium species with alkynes.

Synthesis and Applications of Monolithic Ion Exchange Resin

We have synthesized a new type monolithic porous ion exchange resin which has a co-continuous porous structure. The pore structure is obtained by a two-step polymerization process. In the first step, an open-celled porous styrene-divinylbenzene (St-DVB) copolymer is synthesized by preparation of waterin-oil (W/O) emulsion and polymerization. In the second step, the obtained copolymer is soaked and polymerized in a solution containing a polymeric initiator, styrene and DVB. In order to obtain the monolithic ion exchange resin, functional groups, such as sulfonic acid and trimethylammonium, were introduced into the copolymer. The ion exchange capacities of the monolithic cation and anion exchange resins that have co-continuous porous structures were both over 4 meq/g. The pressure drop of the new monolith was about 5 times lower than that of previously reported open-celled monoliths. The ion exchange band length of the new monolith was about 10 times shorter than that of conventional ion exchange resins. As one application of the new monolith, we have developed a palladium-loaded monolithic anion exchange resin (Pd-AEMR) to decompose hydrogen peroxide (H2O2) in ultrapure water. TEM and EPMA data indicated that Pd particles of about 50 nm were loaded uniformly on the surface layer of Pd-AEMR. H2O2 decomposition rate when using Pd-AEMR was about 10 times faster than that of Pd-loaded bead-type resin.