On the chemistry of pyrrole pigments,XCI: Copper complexes of pyridinologous linear tri- and tetra-pyrroles as cyclopropanation catalysts

Summary In addition to two recently described pyridinologous linear tri- and tetrapyrroles a biisoquinologous system was prepared and its geometrical features derived by means of NMR measurements and force field calculations. The copper complexes of these three ligands were isolated and characterized, and then used as catalysts in the cyclopropanation of styrene. The results were found to be similar to those reported for a variety of catalysts in literature. Thus, it was demonstrated that these systems can be used in principle as catalysts.

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Zur Chemie von Pyrrolpigmenten, 91. Mitt.: Kupferkomplexe pyridinologer linearer Tri- und Tetrapyrrole als Katalysatoren für die Cyclopropanierung

Zusammenfassung Zusätzlich zu zwei jüngst beschriebenen linearen pyridinologen Tri- und Tetrapyrrolen wurde ein biisochinolylanaloges System dargestellt und seine Geometrie aus Kernresonanzmessungen und Kraftfeldrechnungen abgeleitet. Die Kupferkomplexe dieser drei Liganden wurden isoliert und charakterisiert und als Katalysatoren für die Cyclopropanierung von Styrol eingesetzt. Die Ergebnisse waren mit jenen aus der Literatur zahlreicher Komplexe vergleichbar. Damit konnte gezeigt werden, daß solche Systeme prinzipiell als Katalysatoren Verwendung finden können.

     

Tuning the Helicity of (2,2′-Bidipyrrinato)-nickel(II) Complexes

Summary.  The influence of the terminal substituents of 2,2′-bidipyrrin ligands on the geometrical and spectroscopic properties of the respective nickel(II) complexes was investigated. Helicity was found to be an intrinsic structural feature of all 2,2′-bidipyrrins studied to date, including a species with minimum helical overlap. Surprisingly, no changes became apparent upon increasing the size of the terminal substituent from methyl to 1,4-butadiyl. t-Butyl groups at the termini, however, were found to be sterically too demanding to allow the formation of stable nickel(II) complexes. Received October 5, 2001. Accepted (revised) November 14, 2001     

Sulfur‐Bridged BODIPY DYEmers

Reactions of BODIPY monomers with sulfur nucleophiles and electrophiles result in the formation of new BODIPY dimers. Mono‐ and disulfur bridges are established, and the new dyestuff molecules were studied with respect to their structural, optical, and electrochemical properties. X‐ray diffraction analyses reveal individual angulated orientations of the BODIPY subunits in all cases. DFT calculations provide solution conformers of the DYEmers which deviate in a specific manner from the crystallographic results. Clear exciton‐like splittings are observed in the absorption spectra, with maxima at up to 628 nm, in combination with the expected weak fluorescence in polar solvents. A strong communication between the BODIPY subunits was detected by cyclic voltammetry, where two separated one‐electron oxidation and reduction waves with peak‐to‐peak potential differences of 120–400 mV are observed. The qualitative applicability of the exciton model by Kasha for the interpretation of the absorption spectral shape with respect to the conformational state, subunit orientation and distance, and conjugation through the different sulfur bridges, is discussed in detail for the new BODIPY derivatives. This work is part of our concept of DYEmers, where the covalent oligomerisation of BODIPY‐type dye molecules with close distances is leading to new functional dyes with predictable properties.     

Crystal structure and conformation of 10-aryl-bilatrienes-abc

Summary Crystal structure determinations of etiobiliverdin-IV (1) and of twomeso-substituted arylbilatrienes-abc (the 2-imidazolylderivative2 and the 4-nitrophenylderivative3) are reported. All three molecules have aZ,Z,Z configuration and occur in the crystal in anall-syn conformation, which results in a helical overall molecular shape. As a consequence of the steric repulsion between the aryl substituent and the two methyl groups on positions 8 and 12, the pitch and the curvature of the helix are larger in the twomeso-substituted derivatives2 and3 compared to1. In each structure, the packing is dominated by the formation of centrosymmetric, S-shaped dimers via two hydrogen bonds between the pyrrolic H-atom and the carbonyl oxygen of ring A of two molecules related by the operation of an inversion center.

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Kristallstruktur und Konformation von 10-Aryl-bilatrienen-abc

Zusammenfassung Es wird über Kristallstrukturbestimmungen von Aetiobiliverdin-IV (1) sowie zweiermeso-substituierter Arylbilatriene-abc (dem 2-Imidazolylderivat2 und dem 4-Nitrophenylderivat3) berichtet. Alle drei Moleküle liegen im Kristall in einerZ,Z,Z Konfiguration vor und nehmen eineall-syn-Konformation ein, woraus sich eine helikale Molekülgestalt ergibt. Als Folge der sterischen Abstoßung zwischen dem Arylsubstituenten und den beiden Methylgruppen an den Positionen 8 und 12 sind die Ganghöhe und die Krümmung der Helix in den beidenmeso-substituierten Derivaten2 und3 gegenüber1 vergrößert. Die Kristallpackung wird in den drei Kristallstrukturen bestimmt durch die Ausbildung S-förmiger, zentrosymmetrischer Dimeren, die durch zwei intermolekulare Wasserstoffbrücken zwischen dem pyrrolischen Wasserstoffatom und dem Carbonylsauerstoff von Ring A zweier zentro-symmetrieäquivalenter Moleküle zusammengehalten werden.

     

An ab initio conformational study on 2,3-dihydrobilin-1,19(21H,24H)-dione, a model compound for open-chain tetrapyrroles

The molecule 2,3-dihydrobilin-1,19(21H,24H)-dione (DHB) was studied as a model of the fully conjugated linear open-chain tetrapyrroles phycocyanobilin (PCB), phycoerythrobilin (PEB) and phytochrome (PC) as well as biliverdin (BV) and bilirubin (BR). The rotations around the single bonds of the exocyclic methine bridges were investigated for all possible cis and trans, E and Z isomers of DHB. The geometries and energies of conformers were investigated with semiempirical and ab initio methods using AM1 and RHF/3-21G levels of theory. Results indicate that geometries with a central syn–cis configuration are preferred to other conformations around the central methine bridge. Four lowest energy conformations stabilized by hydrogen bonding and favorable geometric arrangements minimizing steric strain were predicted. This model elucidated the trends and identified variables associated with tetrapyrrole conformation and energy and thus may serve as a preliminary basis for studying other open-chain tetrapyrrole structures.     

Syntheses, Crystal Structures, and Conformations of 10,10-Spiro-Bilirubins

Summary. Crystal structure determinations of two novel bilirubin analogs with spirocyclohexyl and spirofluorenyl groups at C(10) are reported. Conformation-determining torsion angles and key hydrogen bond distances and angles are compared to those from molecular dynamics calculations, and to the corresponding data from X-ray determinations and molecular dynamics calculations of bilirubin. Like bilirubin, the component dipyrrinones of spirocyclohexyl and spirofluorenyl rubins are present in the bis-lactam form with (Z)-configuration double bonds at C(4) and C(15). Their crystal structures show considerable similarity to that of bilirubin: both pigments adopt a folded, intramolecularly hydrogen-bonded ridge-tile conformation stabilized by six hydrogen bonds. The interplanar angle of the spirocyclohexyl ridge-tile is nearly the same (94°) as that of bilirubin (95°), but the interplanar angle of the spirofluorenyl ridge-tile is noticeably smaller (84°). The hydrogen bond distances of both spiro-rubin crystal structures are generally longer by 0.1–0.2Å than those in bilirubin. Both new pigments exhibit excellent lipophilicity and, unlike bilirubin, are soluble in methanol.     

Precursors of biological cofactors from ultraviolet irradiation of circumstellar/interstellar ice analogues

Biological cofactors include functionalized derivatives of cyclic tetrapyrrole structures that incorporate different metal ions. They build up structural partnerships with proteins, which play a crucial role in biochemical reactions. Porphyrin, chlorin, bacteriochlorin, and corrin are the basic structures of cofactors (heme, chlorophyll, bacteriochlorophyll, siroheme, F 430, and vitamin B12). Laboratory and theoretical work suggest that the molecular building blocks of proteins (alpha-amino acids) and nucleic acids (carbohydrates, purines, and pyrimidines) were generated under prebiotic conditions. On the other hand, experimental data on the prebiotic chemistry of cofactors are rare. We propose to search directly for the pathways of the formation of cofactors in the laboratory. Herein we report on the detection of N-heterocycles and amines in the room-temperature residue obtained after photo- and thermal processing of an interstellar ice analogue under high vacuum at 12 K. Among them, hexahydro-1,3,5-triazine and its derivatives, together with monopyrrolic molecules, are precursors of porphinoid cofactors. Hexahydropyrimidine was also detected. This is the first detection of these compounds in experiments simulating circumstellar/interstellar conditions. Except for 2-aminopyrrole and 2,4-diaminofuran, which were only found in 13C-labeled experiments, all the reported species were detected in both 12C- and 13C-labeled experiments, excluding contamination. The molecules reported here might be present in circumstellar/interstellar grains and cometary dust and could be detected by the Stardust and Rosetta missions.