The cover picture shows an autostereogram representation of the X-ray crystal structure of the smallest [2]catenane yet isolated. The catenane is formed in one step in an eight-molecule condensation from commercially available reagents and is isolated by simple washing and filtration of the reaction mixture. More about this catenane and analogously accessible [2]catenanes with differing dynamic properties is reported by D. A. Leigh et al. in two consecutive communications on p. 1209 ff. To see the 3-D image you must stare through the picture so that your focal point is behind the page. Placing the picture behind a piece of glass and focusing on your reflection helps if you cannot see the picture straight away. The autostereogram image is 2 x 108 times larger than the molecule itself. Did you see the molecule depicted here? REVIEWS Con tentsStereo-and regioregularity, molecular weight and molecular weight distribution of polyolefins, and incorporation of comonomers can be controlled efficiently with ansa-metallocene catalysts. Catalysts of this type can also be used for the polymerization of cyclic olefins and the production of functionalized polyolefins. In contrast to classical heterogeneous Ziegler-Natta catalysis, metallocene-catalyzed olefin polymerization occurs on uniform metal centers having a defined coordination environment. Recent research in this area has provided insights into fundamental mechanisms as well as advances in practical applications of this class of catalysts. ContentsTwo catena-like interpenetrating enantiomorphic frameworks of condensed tetragonal antiprisms. RhBi,,,, form the basis of the crystal structure of RhBi,. The electron localization function (ELF) shows high localization of the valence electrons only on the periphery of the frameworks (lone pairs of Bi atoms), whereas the electrons inside the nets are delocalized. The periodic nodal surface (PNS) I,-Y** describes perfectly the hyperbolic organization of the entire RhBi, structure. It separates the regions with low electron localization inside the polyhedral nets from the hyperbolic layer of the lone pairs.The optical rotation of poly(propy1ene imine) dendrimers with 64 identical enantiomerically pure amino acid end groups tends to zero. This remarkable behavior results from the packing of the end groups in the highly dense, multiply hydrogenbonded surface, which gives rise to different frozen-in conformations, leading to an internal compensation of optical activity.The condensation of eight molecules in one step provides 1, the smallest [2]catenane to date, in 20 YO yield. The cavity in one of the identical macrocycles of the catenane has dimensions of 4 x 6 A. Electrophilic a-alkylation and nucleophilic 1,2-addition to glycol aldehyde 1 (P = protecting group) affords syn-l,2-amino alcohols 2 in good yields and with high selectivities (89 to > 99% de, 94 to 2 9 9 % ee). This method, in which 1 is first converted into a chiral hydrazone, provides a flexible, direct route to y-amino-P-hydroxycarboxylic acids such as statin 3, an...

Untersuchungen an neuen Metallocenkatalysatoren zur Polymerisation von α‐Olefinen haben gegenwärtig weitreichende Auswirkungen auf die Entwicklung neuer Materialien und auch auf unser Verständnis der grundlegenden Reaktionsmechanismen, die für das Wachstum von Polymerketten an einem Katalysatorzentrum und für deren Stereoregularität verantwortlich sind. Im Gegensatz zu heterogenen Ziegler‐Natta‐Katalysatoren erfolgt die Polymerisation mit einem homogenen Metallocenkatalysator im Prinzip an einheitlichen Metallzentren mit definierter Koordinationssphäre. Dies macht es möglich, die Struktur der Metallocenkomplexe mit den Eigenschaften des Polymers, beispielsweise Molekulargewicht, stereochemischer Mikrostruktur, Kristallisationsverhalten und mechanischen Eigenschaften, zu korrelieren. Mit homogenen Katalysatorsystemen können Regio‐ und Stereoregularitäten, Molekulargewichte, Molekulargewichte, Molekulargewichtsverteilungen und Einbauverhältnisse von Comonomeren wirksam kontrolliert werden. Diese Katalysatoren eröffnen neue Zugänge zur Homo‐ und Copolymerisation cyclischer Olefine, zur Cyclopolymerisation von Dienen und sogar zu funktionalisierten Polyolefinen und erweitern somit das Spektrum und die Vielseitigkeit technisch verfügbarer Polyolefin‐Materialien.

The new C 1-symmetric complexes rac-[1-(9-η5-fluorenyl)-2-(2-methylbenz[e]-1-η5-indenyl)ethane]zirconium dichloride (14a), rac-[1-(9-η5-fluorenyl)-2-(4,5-cyclohexa-2-methyl-1-η5-indenyl)ethane]zirconium dichloride (14b), and rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]zirconium dichloride (15) were prepared in up to 93% yield. These compounds, activated with methyl aluminoxane, exhibit high active propene polymerization rates which remain constant over hours, even at elevated polymerization temperatures of 50 and 70 °C. The two different coordination sites of these “dual-side” catalysts lead to isotactic polypropenes with variable amounts of stereoerrors, depending on the monomer concentration. The 2-methyl substituent of the indenyl ligands results, at the same time, in significantly increased molecular weights of the polymer products (up to 2.3 × 105 g mol-1), the bulk properties of which can be adjusted from flexible, semicrystalline thermoplastic to excellent thermoplastic elastic.

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