Chiral metal complexes polymerization

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

Arylation, olefins, 187, 190 Arylketimines, iridium hydrogenation, 83 Arylpropanoic acid, Grignard coupling, 190 Aspartame, 8, 27 Asymmetric catalysis characteristics, 11 chiral metal complexes, 122 covalently bound intermediates, 323 electrochemistry, 342 hydrogen-bonded associates, 328 industrial applications, 8, 357 optically active compounds, 2 phase-transfer reactions, 333 photochemistry, 341 polymerization, 174, 332 purely organic compounds, 323 see also specific complexes Asymmetric induction, 71, 155 Attractive interaction, 196, 216 Autoinduction, 330 Axial chirality, 18 Aza-Diels-Alder reaction, 220 Azetidinone, 44, 80 Aziridination, olefins, 207... 

The induction of chirality in Cp- metal derivatives may also be studied. There are different ways that even achiral substituents on a cyclopentadienyl ring can give chiral metal complexes. The induction of chirality can proceed through their substitution pattern and/or a hindered ring or substituent rotation. The isotactic polymerization of propylene by means of metallocene catalysts is one example where such a metallocenic chirality has already been employed in an important stereoselective synthesis. 

Spectacular achievements in catalytic asymmetric epoxidation of olefins using chiral Mnm-salen complexes have stimulated a great deal of interest in designing polymeric analogs of these complexes and in their use as recyclable chiral catalysts. Techniques of copolymerization of appropriate functional monomers have been utilized to prepare these polymers, and both organic and inorganic polymers have been used as the carriers to immobilize these metal complexes.103... 

Sherrington et al. were the first to attempt the synthesis of chiral polymeric metal complexes by the chemical modification route,78,177,178 whereby the [Mn(salen)Cl] units are attached in a pendant fashion, by only one of the aromatic rings, to poly(styrene) or poly(methacrylate) resin beads of various morphology. Epoxidation of 1-phenylcyclohexene gave enantioselectivity values between 61% and 91%. 

A closer similarity exists between the C2-symmetric octahedral isospecific model sites, which have been proposed for the heterogeneous polymerization catalysts,13 15 and some slightly distorted octahedral metal complexes, including bidentate or tetradentate ligands, which have recently been described as active in isospecific olefin polymerization in the presence of MAO.128-130 In fact, all these catalytic systems can be described in terms of racemic mixtures of active species with A or A chiralities. 

Lee, K. Y. Kawthekar, R. B. Kim, G. J. (2007) Synthesis of chiral intermediates eatalyzed by new ehiral polymeric (salen) cobalt complexes bearing Lewis acidic metal halides., Korean Chem. Soc., 28 1553-1561. 

A limiting factor of complexation gas chromatography is the low temperature range (25-120°C). Therefore, improved thermostable polymeric stationary phases, e.g., Chirasil-Metal, in which the chiral metal chelates are chemically anchored to a polysiloxane backbone, have been prepared155 156. 

Considerable effort has been directed to the stereoselective synthesis of aimulated cyclopentadienyl complexes of the group 4 transition metals. Ci asymmetric and C2 synunetric aimulated cyclopentadienyl ligands have been used to prepare chiral organotitanium complexes, which are now being actively studied as catalysts for asymmetric synthesis and olefin polymerization see Asymmetric Synthesis by Homogeneous Catalysis). 

Towards the end of the second millennium, studies of the transition elements continued to make major contributions to chemical science and technology. The development of new catalysts and reagents represents one area of activity. Examples are provided by the activation of saturated hydrocarbons by rhodium or lutetium complexes, new syntheses of optically active products in reactions which employ chiral metal compounds, and transition metal compounds which catalyse the stereospecific polymerization of alkenes. The ability of transition metal centres to bind to several organic molecules has been exploited in the construction of new two- and three-dimensional molecular architectures (Figure 1.4). New materials containing transition elements are being developed, one... 

The molecular imprinting method can be used to synthesize enantioselective solid materials for asymmetric organic synthesis. The first attempt to use a metal complex with an attached chiral ligand as a template was attempted by Lemaire [52]. The Rh complex, ((15,25)-V,V -dimethyl-l,2-diphenylethane diamine)-[Rh(CgHj2)Cl]2 coordinated with optically pure l-(5)-phenylethoxide or phenylethoxide (Rh 1-phenylethanolate) (template) was polymerized in the presence of isocyanate, and the polyurea-supported Rh complex is reacted with isopropanol to extract the template from the polymer backbone. They reported the influence of molecular imprinting on catalytic performance (conversion and enantiomeric excess) for the asymmetric transfer hydrogenation (Table 22.2). The imprinted polymer exhibited higher enantioselectivity compared to a nonimprinted... 

Screening of an impressive series of polymers derived from different bulky methacrylate esters, e.g., 42 (Chart 8), and using a variety of chiral ligands has revealed the scope of the process of forming helical poly(methacrylate ester)s and their applicability in, for example, the separation of chiral compounds.151 These polymers were prepared not only by anionic polymerization, but also by cationic, free-radical, and Ziegler—Natta techniques. Recently, Nakano and Okamoto reported the use of a co-balt(II)—salophen complex (43) in the polymerization of methacrylate ester 41.155 The free-radical polymerization in the presence of this optically active metal complex resulted in the formation of an almost completely isotactic polymer with an excess of one helical sense. 

Stereoselective epoxidation of alkenes, desymmetrization of maso-TV-sulfonylaziri-dines, Baeyer-Villiger oxidation of cyclobutanones, Diels-Alder reactions of 1,2-dihydropyridines, and polymerization of lactides using metal complexes of chiral binaphthyl Schiff-base ligands 03CCR(242)97. 

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