Claus Krieger

Electrochemical oxidation and structural changes of 5,6-dihydrobenzo[c]cinnolines

The location of energy minima on the conformational energy surface of molecules by computational methods (conformational searching) continues to play a key role in computer-assisted molecular modeling. Although a number of conformational search procedures have been devised over the past several years, new more efficient methods are urgently needed if molecules with increased complexity are to be treated in a quantitative manner. In this paper we describe a method, termed low-mode search (LMOD), which is based on eigenvector following (or mode following), for the exhaustive exploration of the potential energy hypersurface of molecules. It is particularly efficient at searching the conformational space of both cyclic and acyclic molecules, and we describe its effectiveness for a number of conformational search problems including acyclic, monocyclic, and bicyclic hydrocarbons and cyclic pentapeptides. No special treatment of rings in cyclic molecules is necessary, nor is it necessary to define rotatable bonds. LMOD generates structures “automatically” with minimum input from the user. We demonstrate that LMOD is one of the most efficient procedures yet devised for conformational searching of smallto medium-sized molecules.

Magnetic-properties of 1,4-dihydro-1,2,4-benzotriazin -4-yl radicals

Abstract Magnetic susceptibilities of 1,4-dihydro-1,3-diphenyl-1,2,4-benzotriazin-4-yl (DDB 1 ), 3-tert-butyl-1,4-dihydro-1-phenyl-1,2,4- benzotriazin-4-yl (BDPB 2 ) and 1-(4-chlorophenyl)-1,4-dihydro-3-phenyl-1,2,4-benzotriazin-4-yl (CDPB 3 ) radicals have been measured in the temperature range of 4.2-300 K. The susceptibility of DDB 1 and BDPB 2 follows the Curie-Weiss law with a negative Weiss constant of −2.2 and −0.3 K, respectively. The magnetic susceptibility of CDPB 3 , on the other hand, exhibits a broad maximum at 138 K. This susceptibility can be interpreted in terms of an alternating antiferromagnetic Heisenberg linear chain model with an exchange interaction of 2 J 1 / k =−220 K and an alternation parameter of α= J 2 / J 1 =0.3. The results of the X-ray structure analysis of CDPB 3 support the above explanation.

Isomeric Tetrakis(dimethylamino)naphthalenes: Syntheses, Structure-Dependence of Basicities, Crystal Structures, and Physical Properties

For comparison to the recently described 2,3,6,7-tetrakis(dimethylamino)naphthalene (1) the three isomers 2,3, and 4 were synthesized. The basicities of this group of isomers are strongly dependent upon the different mutual orientations of the pairs of dimethylamino substituents: only the isomers 3 and, partially, 4, both with dimethylamino groups in adjacent peri-positions of the naphthalene, are strong “proton sponges”. For the isomers 1 and 2 with the same number and kind of twofold dimethylamino substituents in neighbouring ortho-positions, however, no significant basicity increase is observed. To explain this difference between the two groups of isomers it is suggested that in the ortho-pairs of 1 and 2 the C-N bonds diverge considerably, leading to an increased N···N distance and consequently to less stable [N···H···N]+ hydrogen bonds in contrast to the parallel C-N bonds in the peri-substituted isomers 3 and 4. X-ray crystal structure analyses of the bases and of some of the salts derived therefrom were solved and are discussed. Cyclic voltammetry indicates that 1 to 4 are strong electron donors, reacting easily to radical cations or dications which with suitable acids have been obtained as salts.

Synthesis, structure and basicity of 1,9-bis(dimethylamino)-dibenzothiophene and 1,9-bis(dimethylamino)-dibenzoselenophene1,2) ☆

Abstract In the context of studies on bis(dialkylamino)arenes with neighbouring amino functions the title compounds 5 and 6 were synthesized. Their structures and those of the monoprotonated cations 5a and 6a were determined by X-ray analysis. The strong basicities of 5 and 6 (pKa = 11.9 and 11.8 ± 0.1) are discussed in terms of lone pair interactions in 5 and 6 and the geometry of the N⋯H⋯N hydrogen bonds in 5a and 6a.

Sterically crowded tetracyano-para-quinodimethanes: synthesis and structure of 2,3,5,6-tetramethyl-7,7,8,8-tetracyano-para-quino=dimethane

Abstract The sterically crowded 2,3,5,6-tetramethyl-7,7,8,8-tetracyano= para -quinodimethane (=1) has been synthesized. The unique sterical structure of (=1) is discussed based on X-ray structure analysis.

Die kristall- und molekülstrukture von [6]Paracyclophan-8,9-dicarbonsäuredimethylester

Zusammenfassung The structure of the title compound 1b was determined by X-ray crystallography: The para carbon atoms C-7 and C-10 are lifted out of the plane of the other four benzene carbons by 19.4 and 19.5°.

Synthesis of C-glycosides from glycals or vinylogous lactones and trimethylsilyl ketene acetals

Abstract 2,3-Unsaturated C-glycosides were obtained in good to excellent yields from the trimethylsilyltriflate catalyzed reaction between carbohydrate derived cyclic enolethers (“glycals”) and trimethylsilyl ketene acetals; under similar conditions vinylogous lactones afforded the products of an 1,4-addition reaction.

Molecular structure and fluorescence behaviour of a benzoquinone/porphyrin/benzoquinone sandwich-molecule

Abstract For the benzoquinone/porphyrin/benzoquinone sandwich-molecule 1 a highly symmetrical structure with parallel ring planes and interplanar distances of 342 pm was derived from X-ray analysis. 1 shows, in comparison to 2, a very strong quenching of fluorescence indicating intramolecular electron transfer.

21,23-Dithia-3,13-diazaporphycenes − Novel Aromatic Porphycene Analogues Incorporating Thiazole

The syntheses of the first examples of aromatic thiazole-containing porphycene analogues 3 have been accomplished by oxidation of the corresponding 3,20:10,13-diepithio-6,9:16,19-diimino-1,11-diaza[20]annulenes 9 with DDQ. Compounds 9a−c have been synthesized by McMurry coupling reaction of the diformylated 2-(1H-pyrrol-2-yl)thiazoles 8a−c, readily available by a three-step reaction sequence using 2-pyrrolethiocarboxamide (4) as starting material. The aromaticity of the 21,23-dithia-3,13-diazaporphycenes 3 is clearly established by NMR and UV/Vis spectroscopy and verified by X-ray crystal structure analysis of 3b.

Syntheses, Structures and Basicities of 1,2,4,5-Tetrakis(dimethylamino)benzene and 2,3,6,7-Tetrakis(dimethylamino)naphthalene

1,2,4,5-Tetrakis(dimethylamino)benzene (4) and 2,3,6,7-tetrakis(dimethylamino)naphthalene (5) were prepared and structurally determined. Electron-donor functions, protonation, and the geometry of intramolecular hydrogen bonds are discussed. By oxidation of 4 to its dication the benzenoid aromaticity is cancelled in favour of two independent cyanine-type units as determined by X-ray structure analysis.