Microencapsulated palladium

Microencapsulated palladium (MC[Pd(PPh3)j) was found to catalyze the allylic alkylation of 1,3-diphenylpropenyl ester with good stereoselectivity (up to 83%) in the presence of an external chiral ligand 107 (Scheme 36) [122]. 

Chen FR, Huang MM, Li YQ (2014) Synthesis of a novel cellulose microencapsulated palladium nanoparticle and its catalytic activities in Suzuki-Miyaura and Mizoroki-Heck reactions. Ind Eng Chem Res 53 8339-8345... 

The second general method, IMPR, for the preparation of polymer supported metal catalysts is much less popular. In spite of this, microencapsulation of palladium in a polyurea matrix, generated by interfacial polymerization of isocyanate oligomers in the presence of palladium acetate [128], proved to be very effective in the production of the EnCat catalysts (Scheme 3). In this case, the formation of the polymer matrix implies only hydrolysis-condensation processes, and is therefore much more compatible with the presence of a transition metal compound. That is why palladium(II) survives the microencapsulation reaction... 

Figure 11. Pore size distribution of palladium(II) EnCat 30 and palladium(II) EnCat 40 swollen in THF the materials differ in the isocyanate/solvent ratio in the microencapsulation mixture (see text), which was 30/70 and 40/60 (w/w), respectively. (Reprinted from Ref. [38], 2005, with permission from Reaxa Ltd.)...

Some of the most remarkable achievements include microencapsulation in polystyrenes such as entrapped 0s04 for olefin hydroxylation (exploiting the interaction between n-electrons of benzene rings of the polystyrenes used as polymer backbones and the vacant orbitals of the catalysts) 5 polyurea-entrapped palladium (PdEnCat)6 for a multiplicity of C C forming reactions and the use of carboxylic acid-functionalized polymer (FibreCat).7 In general, however, metal leaching cannot be avoided. The PdEnCat catalyst, for instance, leaches some 4% of palladium per catalytic reaction run. 

An approach to immobilization which has recently become popular is microencapsulation in polymers, such as polystyrene and polyurea, developed by the groups of Kobayashi [34] and Ley [35], respectively. For example, microencapsulation of palladium salts or palladium nanoparticles in polyurea microcapsules... 

Several groups have described the fabrication of microcapsules loaded with a catalyst. Catalysts encapsulated include palladium(II) acetate (86) and osmium tetraoxide (87). Microencapsulated catalysts are described as effective, easily isolated from a reaction system by filtration, and reusable. Shchukin and co-workers (88) recognized that chemical processes can be performed within microcapsules to produce imique products that are retained within the microcapsules. They produced crystalline WO3 nanoparticles inside microcapsules with a polyelectrolyte shell. 

Kobayashi and coworkers further developed a new immobilizing technique for metal catalysts, a PI method [58-61]. They originally used the technique for palladium catalysts, and then applied it to Lewis acids. The PI method was successfully used for the preparation of immobilized Sc(OTf)3. When copolymer (122) was used for the microencapsulation of Sc(OTf)3, remarkable solvent effects were observed. Random aggregation of copolymer (122)-Sc(OTf)3 was obtained in toluene, which was named as polymer incarcerated (PI) Sc(OTf)3. On the other hand, spherical micelles were formed in THF-cyclohexane, which was named polymer-micelle incarcerated (PMI) Sc(OTf)3.. PMI Sc(OTf)3 worked well in the Mukaiyama-aldol reaction of benzaldehyde with (123) and showed higher catalytic activity compared to that of PI Sc(OTf)3 mainly due to its larger surface area of PMI Sc(OTf)3. This catalyst was also used in other reactions such as Mannich-type (123) and (125) and Michael (127) and (128) reactions. For Michael reactions, inorganic support such as montmorilonite-enwrapped Scandium is also an efficient catalyst [62]. 

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