Showing papers by "Burkhard König published in 1994"

Synthesis and Photoinduced cis-trans Isomerization of Diaryl Enediyne Chromophores

Abstract: The photochemical and thermal isomerization of diarylethenes, such as stilbenes, is a well-known process that has been used to design photochromic materials' and molecular switches.2 By the formal insertion of two acetylenic units into the olefin-arene bond, diarylethenes are expanded to l,&diaryl 3-ene-1,5-diynes7 a chromophore with altered properties. Only one example of this interesting class of compounds has hitherto been de~cribed.~ We report here a new and general synthetic route to cis-diary1 enediynes and the first investigations of their photophysical properties. The palladium-catalyzed two-fold coupline of various iodoarenes (2a-g) with the parent enediyne l5 at room temperature gave the diaryl enediynes cis-3a-g in one step. The isolated yields are summarized in Table 1. In all cases complete retention of the cis-configuration of the enediyne was observed. However, the corresponding bromoarenes react sluggishly and do not give the desired products. The UV spectra of all diaryl enediynes show the expected strong absorption bands between 300 and 400 nm and the compounds cis-3a-g isomerize rapidly to a cis-trans mixture when solutions are exposed to sunlight or irradiated with low intensity UV light at 366 nm.6 In the photostationary state, a nearly equal mixture of both isomers is present as a consequence of the similar absorption spectra of the cisand trans-compounds. The isomers of 3a, 3c, and 3e were separated by column chromatography and irradiated individualy, leading to the original equilibrium mixtures. Assignment of the stereochemistry was based on the respective 3Jcis (3c: 10.7 Hz) and 3Jtrans (3c: 15.9 Hz) coupling constants obtained from the 13C satellite proton NMR spectra. If the irradiation is interrupted before the photostationary equilibrium is reached, the ratio of both isomers remains constant, even when the solutions are heated to 80 \"C for several hours. The clean isomerization process was

Palladium-Catalyzed Coupling of Vinylferrocene with Aromatic Halides - A Highly Efficient Route to (Ferrocenylvinyl)arenes

Abstract: (Ferrocenylvinyl)arenes 3, 5, and 7 are obtained from vinylferrocene (1) and substituted aromatic and heteroaromatic halides by palladium-catalyzed Heck-type reactions. Up to three ferrocene units are introduced in one step by the multi-fold reaction of 1,2-dibromo- (4) or 1,3,5-tribromobenzene (6) with 1. The first crystal structure of a bis(ferrocenylvinyl)benzene chromophore (5) is reported.

[2.2](4,7)Isobenzofuranophanes ‐ Synthesis, Characterisation, and Reactivity

Abstract: The isomeric Diels-Alder adducts 3, obtained by cycloaddition of tetraphenylcyclopentadienone to the 4,5:12,13-bis-(oxanorbornadieno)[2.2]paracyclophanes syn,syn- and anti,-syn-2[Note ][The stereochemical descriptors syn and anti refer to the orientation of the oxygen bridge in the oxabicyclo[2.2.1]heptadiene subunits with respect to the [2.2]paracyclophaneskeleton.], yield the unstable isobenzofuranophane 4 by consecutive extrusion of carbon monoxide and tetraphenylbenzene when heated to 180°C. The molecular ion of 4 was observed in the EI mass spectrum. The stable tetraphenyl-substituted analogue 10 was synthesized independently from the previously unknown 4,5,12,13-tetrabenzoyl[2.2]paracyclophane (9). UV/Vis as well as fluorescence spectra and an X-ray crystal structure analysis of 9 are reported.

Zirconocene complexes of [2.2]paracyclophanes with formal triple bonds in the bridges

Abstract: Zirconocene complexes of [2.2]paracylophanes with formal triple bonds in the bridges, corresponding to zirconacyclo-propenes, were synthesized from [2.2]paracylophan-1-ene (1) and 1,9(10)-dibromo[2.2]paracyclophan-1,9-diene (5) via their vinyl lithium derivatives and characterized by spectroscopic methods. The zirconocene-alkyne complexes 7, 9 react with bromine or iodine with loss of the bis(cyclopentadienyl)zirconium moiety to yield the expected stable halogenated products 8 and 10, respectively. Insertion of 2-butyne into the initially formed 16e complexes gave zirconacyclopentadiene complexes 3, 9.

100 Jahre „Schloß‐Schlüssel‐Prinzip”︁

Abstract: Die molekulare Erkennung verbindet wie kaum ein anderer Aspekt verschiedene wissenschaftliche Disziplinen. Genau 100 Jahre, nachdem Emil Fischer erstmals das „Schlos-Schlussel-Prinzip” zur Erklarung der Substratspezifitat bei Hefen vorschlug, ist die Frage nach der „molecular recognition” in Biologie, Chemie und Materialwissenschaften aktueller denn je.