Helical Metallocenes

The first helicene synthesized, which was bridged by a ferrocene unit, is based on the pentahelicene skeleton At its terminal phenylene rings, cyclopentadienyl units were condensed. The di-lithium salt of this compound with iron(III)chloride yielded the helical ferrocene 142 with an uninterrupted conjugation between the two cyclopentadienyl rings (Fig. 55). The cyclopentadienyl moieties are clamped by the iron atom in an angle deviating 19.4° from parallelity. 

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Recently, Katz employed oxazaborolidine (2) to catalyze the enantioselective reduction of ketone (7) (eq 6). The resultant carbinol was used for the synthesis of optically active helical metallocene oligimers. ... 

Olefin Polymers Isotactic polymers of propylene and 1-butene obtained by optically active metallocene catalyst (145) have been reported to show large specific rotation in suspension ([a]D-123°, -250° for polypropylene [a]D+130° for polybutene), which was lost when the polymers were completely dissolved or heated [176,177]. The optical activity was ascribed to a helical conformation of the polymer chain with preferential screw sense. 

In catalysts obtained from achiral non-bridged metallocenes of class I with C2v molecular symmetry (double helical), such as Cp2MtX2, the positions of the coordinated monomer and of the alkyl ligand are not chirotopic and, therefore, the catalyst control is completely lacking (Table 3.1) [68],... 

In catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry (helical), such as racemic isomers of ansa-metallocenes, e.g. rac.-(IndCH2)2MtX2 (Table 3.1) and rac.-(ThindCH2)2MtX2, the two coordination positions available for the incoming monomer and the growing... 

Lummus Novolen Technology Polypropylene homopolymer, random copolymers, impact copolymers, including Metallocene PP Propylene (and ethylene for production of copolymers) Polymerization of propylene in one or two gas-phase reactors stirred by helical agitators to produce a wide range of products 27 2010... 

Recently, metallocene catalysts were used to incorporate randomly distributed stereo- and regioirregularities into the iPP chain. Decreasing isotactic segment length promotes formation of the 7-modification [30, 31]. Opfical clarity of 7-iPP is significantly improved with respect to a-iPP. Special nucleating agents have been introduced to favor formation of the 3-modification [32]. Crystallization of sPP, which forms 2j helices. 

Note the P-helical conformation at the metallocene and the intramolecular hydrogen bond. (Reprinted from [39], with permission from The Royal Society of Chemistry). 

Stereoisomerism and Connectivity 300 Total Synthesis of an Antibiotic with a Staggering Number of Stereocenters 303 The Descriptors for the Amino Acids Can Lead to Confusion 307 Chiral Shift Reagents 308 C2 Ligands in Asymmetric Synthesis 313 Enzymatic Reactions, Molecular Imprints, and Enantiotopic Discrimination 320 Biological Knots—DNA and Proteins 325 Polypropylene Structure and the Mass of the Universe 331 Controlling Polymer Tacticity—The Metallocenes 332 CD Used to Distinguish a-Helices from [3-Sheets 335 Creating Chiral Phosphates for Use as Mechanistic Probes 335... 

Previous polypropylenes found to favor the y-modification, which are thermally unstable with respect to the a-modification, were very low in MW the values were 2600 for thermally degraded polymers and 740 to 3900 by special synthesis.This is compared with s of 9,000 to 90,000 for the present anisotactic polypropylenes. Therefore, the ani-PP formed by the chiral metallocene catalyst, which crystallizes preferentially in the thermally stable y-form, probable are comprised of chains containing frequent with discontinuity in their helical configurations. 

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