POLYMERS Vol.63 No.3
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Catalysts in Polymer Science: A Key to New Catalysis
COVER STORY: Highlight Reviews
Next-Genetation Chiral Catalysts on Basis of Helical Polymers Michinori SUGINOME
<Abstract> Considerable effort has been devoted to the study of
non-racemic helical polymers in expectation of finding new molecular and supramolecular functions on the basis of their characteristic chiral backbone structures.
We recently established single-handed helical poly(quinoxaline-2,3-diyl)s bearing both chiral side chains and diarylphosphino pendants as a highly effective macromolecular scaffold for catalytic asymmetric reactions. Enantioselectivities up to 98% ee were obtained in hydrosilylation of styrenes. The polymer-based chiral catalyst could be reused several times with no drop of enantioselectivity at all. The polymer scaffold underwent solvent-dependent helix inversion to form right-(P) and left-handed (M) helical structures in chloroform and 1,1,2-trichloroethane, respectively. Use of the P- and M-helical polymers as chiral ligands in hydrosilylation afforded highly enantioenriched S- and R-products, respectively.3 The chiral polymer catalyst also served as a highly enantioselective catalyst in some other Pd- catalyzed asymmetric reactions such as Suzuki-Miyaura coupling and silaboration of meso-methylenecyclopropanes.
Keywords: Helical Polymer / Catalytic Asymmetric Synthesis / Transition-Metal-Catalyzed Reactions / Helix Inversion / Invresion of Catalyst Chirality / Palladium Catalyst / Hydrosilylation / Suzuki-Miyaura Cross-Coupling
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Metal Nanoparticles Stabilized by Functionalized Hyperbranched Polymers for Catalysis Hideo NAGASHIMA
<Abstract> Catalytic activity of metal nanoparticles has received attention in homogeneous catalysis. The rate and selectivity of homogeneous catalysis using organometallic complexes are controlled by appropriate design of auxiliary ligands bound to the metal center. For nano sized metal particles, they could be accomplished by appropriate design of the polymer supports with unique structures and properties. Hyperbranched polymers are one of the dendritic polymers, having unique features they received by facile one-pot preparation of monomers and existence of large numbers of functional groups. We found that hyperbranched polystyrenes bearing organoammonium groups are found to be a new polymer support for nano metal particle catalysts of gold, palladium, and platinum. By judicious choice of the R group, both water- and organo-soluble HPS-NR3+Cl- can be prepared. The resulting M@HPS-NR3+Cl- behaves as biphasic catalysts for several organic reactions. Since thin-films of Pd@HPS-NR3+Cl- can be made with ease, electroless plating of nickel and copper is possible on the surface of solid materials including plastic and ceramics.
Keywords: Metal Nanoparticles / Polymer Support / Hyperbranched Polymers / Ammonium Salts / Biphasic Catalysis / Electroless Plating
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Recent Developments in Rare Earth-Catalyzed Precision Copolymerization Masayoshi NISHIURA, Zhaomin HOU
<Abstract> This article describes recent studies on precision copolymerization by rare earth catalysts. The reaction of the rare earth dialkyl complexes with an equivalent of a borate compound such as [Ph3C][B(C6F54] or
[PhNMe2H][B(C6F54] generates easily the corresponding cationic monoalkyl species, which serve as excellent catalysts for the polymerization and copolymerization of a variety of olefins to yield a new family of polymer materials that show novel properties but are difficult to prepare by other catalysts.
Keywords: Rare Earth / Polymerization / Styrene / Isoprene / Copolymerization / Lanthanide / Hexadiene / Chain Shuttling Copolymerization
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COVER STORY: Topics and Products
Polymer-Immobilized Asymmetric Catalyst Shinichi ITSUNO
<Abstract> Quaternary ammonium salts of cinchona alkaloid show excellent catalytic activity in various asymmetric organic reactions. In order to immobilize these chiral catalysts in the main-chain of polymers, cinchona alkaloid dimers were polymerized with achiral comonomers. The cinchona alkaloid dimers were prepared by using the quinuclidine nitrogen atom (quaternization), the OH group (ether formation), and the vinyl group (thiol-ene reaction). These dimers were readily polymeried by the following methods. (1) Quaternization polymerization: the cinchona alkaloid dimer prepared through ether linkage was allowed to react with dihalide to result in the chiral polymer containing cinchona alkaloid ammonium salt. (2) Etherification polymerization: the quaternized dimer was allowed to react with dihalide under basic condition to yield the chiral polymer. (3) Ino-exchange polymerization: the polymerization reaction between the quaternized dimer and disodium disulfonate occurred to produce the chiral polymers. This polymerization was applied to the synthesis of another chiral polymeric catalyst containing MacMillan catalyst in the main-chain. These chiral polymers were successfully used as catalysts in asymmetric reactions including alkylation and Diels-Alder reactions.
Keywords: Cinchona Alkaloid / Polymer-Immobilized Catalyst / Chiral Polymer / Asymmetric Catalyst /
Quaternization Polymerization / Etherification Polymerization / Ion-Exchange Polymerization
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Artificial Metalloenzymes Takafumi UENO
<Abstract> Design of artificial metalloenzymes has become an important topic in the fields of biopolymers, biological chemistry, and inorganic chemistry due to the potential applications of artificial metalloenzymes in nanoscience and biotechnology. One of the general methods used to produce artificial metalloenzymes involves non-natural metal cofactors within protein scaffolds. This method has been used in the construction of small artificial metalloproteins with high activity and selectivity. In recent years, the above-mentioned general strategy has been applied to functionalize protein assemblies. The development of artificial metalloenzymes with hierarchically-assembled proteins would enable us to provide powerful tools for industrial and biological applications. In this review, we discuss the most significant recent research in this field as well as future directions.
Keywords: Artificial Metalloenzyme / Protein / Ferritin / Virus / Protein Crystal / Organometallic / Supramolecular Protein / Catalysis
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Preparation of Polymers by Using Cyclodextrins as Catalysts Akira HARADA, Yoshinori TAKASHIMA
<Abstract> Cyclodextrins(CDs)were found to initiate ring-opening polymerization of lactones selectively to produce polyesters in high yields, although lactones did not give any polymers under the same conditions without CDs. The lactones were activated in CD rings by inclusion. The products were found to be a polymer chain attached to the C2 hydroxyl group of a single glucopyranose unit of CD via an ester bond. The other free CDs include a growing polymer chain to support the polymerization. CD dimers were found to be much better catalysts for the polymerization of lactones, because one of the CD ring functions as a clamp as shown in DNA polymerases in the biological systems. This procedure provides a new environmentally benign approach to produce polyesters without using organic solvents and/or metal catalysts. The process might be used for the production of many kinds of polymers.
Keywords: Cyclodextrin / Polymerization / Lactone Molecular Clamp / Cyclodextrin Dimer
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Z -Selective Olefin Metathesis Catalysts and Their Applications Koji ENDO, Robert H. Grubbs
<Abstract> Olefin metathesis is a convenient and powerful method for the construction of carbon–carbon double bonds. This useful methodology has developed at a fast pace due to the discovery of well-defined transition metal catalysts in the 1980s. Now various highly efficient catalysts are available and widely used in both academic laboratories and industrial processes. However, stereo-control of the product olefin has been an earnest wish of the chemists working on this field. Because of the thermodynamic nature of metathesis, most catalysts produce a higher proportion of the thermodynamically favored E olefin isomer. This fundamental aspect of olefin metathesis has limited its applications in some areas of chemistry. Herein we report a new class of ruthenium-based metathesis catalysts in which an N-heterocyclic carbene ligand is chelated to the metal center. These catalysts show remarkable selectivity for the formation of Z olefins during a wide variety of metathesis reactions. In addition, applications of these catalysts to organic synthesis and polymer synthesis are also presented.
Keywords: Olefin Metathesis / Catalyst / Z-selective / Organic Synthesis / Polymer Synthesis
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Polymer Science and I: A Personal Account
Reason for Melittin Yu HOSHINO
<Abstract> The reason I chose Melittin as a model target peptide for plastic antibodies was not only a scientific one. I was looking for the most inexpensive target that would be captured by polymer nanoparticles via strong “non-specific” interactions.
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Front-Line Polymer Science
Recent Trends of Gelators Kenji HANABUSA
<Abstract> Recent trends in studies on gelators are described. Gelators are low molecular weight compounds that can form physical gels in liquids. The formed gels show thermally reversible sol-to-gel phase transition. Driving forces of gelation are co-operating noncovalent interactions; for example hydrogen bonding, van der Waals interaction, π-π interaction, and electrostatic interaction. The hydrogelators containing Olsalazine and Taxol are reported as drug delivery agents. Hydrogelators consisting of D-amino acids are stable in vivo and show sustained release behavior. Hydrogelators as cell culture scaffold materials are mentioned. The cast films derived from gels are useful for sensing applications. Gelators that can detect TNT, TATP, and amines are introduced. Gelators that can form thixotropic gels are synthesized. Gelators prepared from oligothiophene and crown ether segments form thixotropic gels in the presence of bis-ammonium salts. Cyclic(dipeptide) derivatives are reported as hydrogelator forming thixotropic gels. New triggers that induce physical gelation are mentioned.
Keywords: Gelators / Gels / Hydrogels / Gelation / Sensors / Drug Delivery / Thixotropy
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