Most associate liquid crystals with their everyday use in laptop computers, mobile phones, digital cameras, and other electronic devices. However, in contrast to their rodlike (calamitic) counterparts, first described in 1907 by Vorländer, disklike (discotic, columnar) liquid crystals, which were discovered in 1977 by Chandrasekhar et al., offer further applications as a result of their orientation in the columnar mesophase, making them ideal candidates for molecular wires in various optical and electronic devices such as photocopiers, laser printers, photovoltaic cells, light-emitting diodes, field-effect transistors, and holographic data storage. Beginning with an overview of the various mesophases and characterization methods, this Review will focus on the major classes of columnar mesogens rather than presenting a library of columnar liquid crystals. Emphasis will be given to efficient synthetic procedures, and relevant mesomorphic and physical properties. Finally, some applications and perspectives in materials science and molecular electronics will be discussed.

No abstract

Die meisten Menschen verbinden mit dem Begriff Flüssigkristalle sofort die Displays von Laptop‐Computern, Mobiltelefonen, Digitalkameras und anderen elektronischen Geräten. Im Unterschied zu ihren stäbchenförmigen (calamitischen) Verwandten, die erstmals 1907 von Vorländer beschrieben wurden, bieten aber die 1977 von Chandrasekhar entdeckten scheibenförmigen (diskotischen) Flüssigkristalle andere vielversprechende Verwendungsmöglichkeiten. Ihre einzigartige Anordnung in columnaren Mesophasen macht sie zu idealen Kandidaten für molekulare Drähte in vielen optischen und elektronischen Geräten, beispielsweise in Photokopierern, Laserdruckern, Solarzellen, organischen Leuchtdioden und Feldeffekttransistoren oder zur holographischen Datenspeicherung. Einer Übersicht über die verschiedenen Mesophasentypen und Charakterisierungsmethoden columnarer Flüssigkristalle folgen ausgewählte Beispiele aus den Hauptklassen columnarer Mesogene unter besonderer Berücksichtigung effizienter Syntheseverfahren, mesomorpher Eigenschaften und der für Anwendungen relevanten physikalischen Eigenschaften der Flüssigkristalle. Abschließend werden Anwendungsbeispiele und Perspektiven für einen Einsatz in den Materialwissenschaften und der molekularen Elektronik vorgestellt.

Quaternary stereocenters are a particular challenge for stereoselective synthesis. With a central view on this specific structural issue, selected examples from the recent literature are highlighted in order to evaluate the state of the art of asymmetric C À C bond formation. The review is divided into sections on addition and substitution reactions, rearrangements and cycloaddition reactions.

Stereoselective C À C bond-forming reactions are of particular importance for the preparation of enantiopure natural compounds and pharmaceuticals. The wide variety of available chiral auxiliaries, reagents, and catalysts nowadays enables the generation of tertiary stereocenters in most cases without any difficulty. However, the approach to complex compounds with quaternary stereocenters is still a challenge for synthetic organic chemists, and every enantioselective procedure in which a fully substituted carbon center is constructed is of value.[1]The Michael reaction, the conjugated or 1,4-addition of enolates to acceptor-substituted olefins, is a fundamental CÀC-coupling reaction, which is catalyzed not only by Brønsted bases, [2] but also by a number of metal compounds. [3] An important breakthrough in the field of extensively investigated metal-catalyzed, enantioselective Michael reactions [4] was recently reported by Sodeoka and co-workers.[5] For the first time, quaternary stereocenters could be generated with > 90 % ee at a relatively high temperature of À20 8C in a Michael reaction of b-dicarbonyl compounds 1 with enones 2 using palladium(ii) as the catalytically active transition metal. The Pd II -diaquadiphosphane complexes 3 a and 3 b are formed with chiral ligands (R)-tol-binap and (R)-binap, respectively. Scheme 1 shows four typical reaction products 4 a-d as examples.Both cyclic (1 a) and acyclic (1 b) bketoesters react with methyl vinyl ketone (2 a) at À20 8C in a Michael reaction promoted by 3 a to give the corresponding products 4 a and 4 b in moderate to good yields with > 90 % ee. Noteworthy is the use of the sterically demanding tert-butyl and phenyl b-ketoesters. The conversion of b-diketone 1 c (X = CH 2 Ar) in the presence of catalyst 3 b (10 mol %) at À10 8C is an outstanding example of an asymmetric Michael addition to acyclic triketones such as 4 c with very high stereoselectivity, an achievement thus far not reported in the literature. Furthermore, the reaction of 1 d with substituted enone 2 b catalyzed by 3 b at À20 8C affords a mixture of two diastereoisomers (d.r. = 8:1), and an enantiomeric excess of 99 % for the major diastereomer of 4 d.The high efficiency of the method developed by Sodeoka and co-workers is clearly demonstrated in view of the work published so far: In 1975 Wynberg and co-workers applied cinchona alkaloids in base-catalyzed, enantioselective Michael reactions.[6] About 10 years later, the first of these reactions catalyzed by a transition metal, cobalt, was published by Brunner and Hammer.[7] Both methods result in quaternary stereocenters with up to 68 % ee, which was improved in the following years by Desimoni et al. to 75 % ee.