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...

A novel, soluble Ziegler‐Natta catalyst enables the preparation of isotactic polypropylene, which exhibits a higher degree of isotacticity than polymers that can be produced industrially with heterogeneous catalysts. Furthermore, isotactic polybutene can also be obtained with such a catalyst. The chiral, rigid zirconocene derivative 1 and methylalumoxane 2 are used for the catalysis. The polymerization activity is remarkably high. equation image

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.

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A synthesis for racemic ethylene-bis(4,5,6, 7-tetrahydro-l-indenyl)titanium dichloride is described. The molecular structures of this compound, of its mesoisomer and of a binaphtholate complex of the (S,S)-enantiomer have been determined. Cleavage of this binaphtholate complex gives the pure (S,8)enantiomer. * For part III. see l'cf. 1.

23chromyl acetate oxidation of alkenes and styrenes.63•64As stated above, 6 could also represent the activated complex in the rate-determining step for the chromyl chloride oxidation of alkenes and cycloalkenes. It is also of interest to note that epoxides have been isolated in the chromic acid oxidation of allylic alcohols,65 cyclohexene,66 and styrenes,67 and in the chromyl acetate oxidation of alkenes and styrenes.63•64 The carbonyl products obtained in the chromyl chloride oxidation could arise from isomerization of epoxide intermediates which could be formed from 5, 6, or 7.We conclude from the relative rates, the solvent effects, the thermodynamic data, the magnitude of the p+ value, and the secondary deuterium kinetic isotope effects that the electrophilic addition of chromyl chloride to styrene probably involves an activated complex resembling 5 or 6 in the rate-determining step.

Ion pairs of the type Cp x 2 ZrMe + ‚‚‚Acontaining various ansa-zirconocene methyl cations in contact with Me-B(C 6 F 5 ) 3or B(C 6 F 5 ) 4anions have been studied with regard to their anion exchange kinetics by 2D-NMR methods in benzene or toluene solutions. The resultssacceleration of anion exchange by added Li + ‚‚‚Me-B(C 6 F 5 ) 3 -, substantial nonproductive exchange between added and Zr-bound Me-B(C 6 F 5 ) 3anions, an increase of exchange rates at increased zirconocene concentrations, and the exclusively entropic origin of this rate increasesall indicate that anion exchange occurs by way of ion quadruples or higher ionic aggregates, rather than via dissociation to solvent-separated ions. These findings imply that solvent-separated (i.e. anionfree) alkyl zirconocene cations are unlikely to be relevant intermediates in reaction systems containing Cp x 2 ZrMe + ‚‚‚Aion pairs and, hence, also in zirconocene-based catalyst systems for the polymerization of R-olefins.

In a study of the reaction system MAO/(C5H5)2ZrMe2, the size of the ion pair [(C5H5)2Zr(mu-Me)2AlMe2]+ [Me-MAO]- was determined by pulsed field-gradient NMR of its cationic moiety. A mean effective hydrodynamic radius of 12.2-12.5 A, determined from diffusion rates in benzene solution at different zirconocene and MAO concentrations, indicates that the ion pair remains associated even at the lowest concentrations studied. At elevated concentrations, aggregation to ion quadruples or higher aggregates is indicated by an apparent size increase and by shifts of the C5H5 and Me 1H NMR signals. The equilibrium constant for the reaction [(C5H5)2ZrMe+...Me-MAO-] + 1/2Al2Me6 right harpoon over left harpoon [(C5H5)2Zr(mu-Me)2AlMe2]+ [Me-MAO]- changes at different Al/Zr ratios; this indicates that MAO contains various species that produce Me-MAO- anions with different Lewis basicities. The volume of the Me-MAO- anion suggests that it contains 150-200 Al atoms.