Bernt Krebs

Bio: Bernt Krebs is an academic researcher from University of Münster. The author has contributed to research in topics: Crystal structure & Ligand. The author has an hindex of 56, co-authored 729 publications receiving 16336 citations. Previous affiliations of Bernt Krebs include Leiden University & Clariant.

Regioselectivity of 5, 6, 7, 8‐Tetrafluoroquinoline and 6‐X‐Trifluoroquinoline (X = CF3, H) in Reactions with Nucleophiles

Abstract: A systematic study of the direction of nucleophilic attack of the nucleophiles Me3MEMe2 (M = Si, Sn; E = P, As), NaOMe, LiR (R = Me, nBu, Ph) and PhMgBr on 5, 6, 7, 8-tetrafluoroquinoline (1), 6-CF3-5, 7, 8-trifluoroquinoline (2) and 5, 7, 8-trifluoroquinoline (3) was performed with the aim to develop synthetic routes to specific functional derivatives and to gain a deeper insight into the mechanisms governing the regioselectivity. With the fairly “soft” nucleophiles Me3MEMe2 in general mixtures of 7-Me2EC-(main product) and 6-Me2EC-derivatives (side product) are formed (Schemes 1, 2, 4). Sulfuration of the isomer mixtures with E = P yields mixtures of the corresponding thiophosphano derivatives. The observed regioselectivity is explained by a concerted action of two factors: (i) The influence of the heteroatom N on the stabilization of the σ-complex type transition states and (ii) the collective effect of four fluorine substituents favouring 6- and 7-substitution. — The reaction of 1 with sodium methoxide (Scheme 3) was carried out to test the early conclusion on the exclusive formation of 7-methoxy-5, 6, 8-trifluoroquinoline (14) [2], made on the basis of a GC-analysis. For that purpose the molar ratio 1 : MeO— was varied from 1 : 1.25 over 1 : 1 to 1 : 0.5. — In the reactions of the quinoline precursors 1—3 with the organometallic reagents LiR (R = Me, nBu, Ph) and PhMgBr (Scheme 5) products of the nucleophile-addition at position 2 were obtained in high yields, which with hydrochloric acid led to 2-R-1, 2-dihydro-5, 6, 7, 8-tetrafluoroquinolines. In contact with air or by reaction with MnO2, oxidation to aromatic 2-R-5, 6, 7, 8-tetrafluoroquinolines occurred. To rationalize the observed differences between the “soft” Me3MEMe2 and the “hard” carbon-centred nucleophiles, two different hypothetic mechanisms are discussed. Since most of the compounds have been obtained in reaction mixtures, the assignment to structural formulae is mainly based on GCMS and CMS analyses together with 1H-, 19F- and 31P NMR data and comparison with literature information and with spectra registered for individual compounds. In addition, the molecular structures of the representatives 6, 20 and 29, determined by X-ray analyses, prove the structural formulae deduced from the spectra of products. Regioselektivitat von 5, 6, 7, 8-Tetrafluorchinolin und der 6-X-Trifluorchinoline mit X = CF3, H in Reaktionen mit Nucleophilen Die Nucleophile Me3MEMe2 (M = Si, Sn; E = P, As), NaOMe, LiR (R = Me, nBu, Ph) und PhMgBr wurden hinsichtlich der Richtung des nucleophilen Angriffs auf 5, 6, 7, 8-Tetrafluorchinolin (1), 6-CF3-5, 7, 8-trifluorchinolin (2) und 5, 7, 8-trifluorchinolin (3) untersucht mit dem Ziel, Synthesewege zu bestimmten funktionalisierten Derivaten zu entwickeln und einen tieferen Einblick in die Mechanismen der Regioselektivitat zu gewinnen. Bei Verwendung der relativ „weichen” Nucleophile Me3MEMe2 werden generell Gemische des 7-Me2EC- (Hauptprodukt) und 6-Me2EC-Derivats (Nebenprodukt) (Schemes 1, 2, 4) gebildet. Durch Sulfurierung der Isomerenmischungen mit E = P entstehen Gemische der entsprechenden Thiophosphan-Verbindungen. Die beobachtete Regioselektivitat wird mit einer konzertierten Aktion zweier Faktoren erklart: 1) Dem Einfluss des Heteroatoms N auf die Stabilitat der moglichen σ-Komplex-Intermediate und 2) der kollektiven Wirkung von vier Fluorsubstituenten, die 6- und 7-Substitution favorisieren. — Die Umsetzung von 1 mit Natriummethoxid (Scheme 3) wurde durchgefuhrt, um die fruher [2] aus einer GC-Analyse des Reaktionsgemisches gefolgerte ausschliesliche Bildung von 7-Methoxy-5, 6, 8, -trifluorchinolin (14) zu uberprufen. Dazu wurde das Molverhaltnis 1 : MeO— von 1 : 1, 25 uber 1 : 1 zu 1 : 0.5 variiert. — Bei den Umsetzungen der Chinolin-Derivate 1 — 3 mit den Organometall-Reagenzien LiR (R = Me, nBu, Ph) und PhMgBr wurden in hohen Ausbeuten Additionsprodukte der Nucleophile an C-2 erhalten, die mit Salzsaure 2-R-1, 2-dihydro-5, 6, 7, 8-tetrafluorchinoline lieferten. In Kontakt mit Luft oder durch Reaktion mit MnO2 erfolgte Oxidation zu den aromatischen 2-R-5, 6, 7, 8-tetrafluorchinolinen (s. Scheme 5). Zur Deutung der beobachteten Unterschiede zwischen den „weichen” Me3MEMe2- und den eher „harten” C-Nucleophilen werden zwei verschiedene hypothetische Mechanismen diskutiert. Da die meisten Verbindungen dieser Arbeit in Form von Gemischen anfielen, basiert die Zuordnung zu definierten Strukturen hauptsachlich auf GCMS- und CMS-Analysen sowie auf 1H-, 19F- und 31P-NMR-Daten im Vergleich mit Literaturangaben und mit Spektren der aus den Gemischen isolierten Vertreter. Zusatzlich beweisen die durch Rontgenstrukturanalyse ermittelten Molekulstrukturen von 6, 20 und 29 die aus den Spektren der Produktgemische abgeleiteten Formeln.

Trifluoromethyl phosphaalkenes of the type F3CPC(OR)NR2: Synthesis, spectroscopic investigations, and ligand properties [1]†

Abstract: The one-pot reaction of the alcohol adducts F3CP(H)CF2OR of perfluoro-2-phosphapropene with secondary amines in a 1:3 molar ratio affords the stable phosphaalkenes F3CPC(OR)NR2 (R = Me, Et) 1–4 in yields of 58%. NMR and He(I) PE spectroscopic investigations show that the lone pair electrons on nitrogen and oxygen participate in n/π conjugation. In contrast to typical low-coordinated double bonds the new derivatives do not react with alcohols, amines, and 1,3-dienes. The derivatives are more closely related to the phosphaalkenes F3C(F)NR2 than to perfluoro-2-phosphapropene. The reaction of F3C(OEt)NMe2 (3) with Cr(CO)5THF yields the η1(P) complex Cr(CO)5[F3CPC(OEt)NMe2] (7) with an unusually long sp2 PC bond (1.809 A).

Synthesis and Characterization of [Mn3(ppi)2(μ-OAc)4(H2O)2] · 2MeOH — Unusual Structural Properties of a Trinuclear Oxygen-Rich Manganese Complex

Abstract: The trinuclear manganese(II) complex [Mn3(ppi)2(μ-OAc)4(H2O)2]·2MeOH (1) (Hppi = 2-pyridylmethyl-2-hydroxyphenylimine) is prepared by dissolving two equivalents of Hppi (from the Schiff Base reaction of aminophenol and pyridine-2-carboxaldehyde) in acetonitrile and three equivalents of Mn(OAc)2·4H2O in methanol and combining both solutions. The resulting red precipitate was recrystallized to yield red crystals suitable for single crystal X-ray diffraction. Compound 1 crystallizes in the triclinic space group P1¯ (no. 2), with a = 9.691(2), b = 10.683(2), c = 11.541(2)A, α = 63.19(3)°, β = 67.47(3)°, γ = 69.11(3)°, V = 960.2(3)A3, and Z = 1. The binding mode of carboxylate in 1 represents a model for a transition state between symmetric syn, syn, anti-μ2-carboxolato-O- and syn, anti-μ2-carboxylato-O, O′-coordination. Therefore a rare binding mode for the phenomenon of the carboxylate shift is realized. Furthermore the complex is stabilized by a distinctive hydrogen bonding pathway. Synthese und Charakterisierung von [Mn3(ppi)2(μ-OAc)4(H2O)2] · 2MeOH — Ungewohnliche strukturelle Eigenschaften eines dreikernigen sauerstoffreichen Mangankomplexes Der dreikernige Mangan(II)-Komplex [Mn3(ppi)2(μ-OAc)4(H2O)2]·2MeOH (1) (Hppi = 2-Pyridylmethyl-2-hydroxyphenylimin) konnte durch die Umsetzung von zwei Aquivalenten Hppi (aus der Schiff Base Reaktion von Aminophenol and Pyridin-2-carboxaldehyd), gelost in Acetonitril und drei Aquivalenten Mn(OAc)2·4H2O, gelost in Methanol, dargestellt werden. Der entstehende Ruckstand wurde umkristallisiert, um rote Einkristalle mit einer ausreichenden Qualitat fur eine Rontgenstrukturanalyse zu erhalten. Verbindung 1 kristallisiert in der triklinen Raumgruppe P1¯ (Nr. 2), mit a = 9.691(2), b = 10.683(2), c = 11.541(2)A, α = 63.19(3)°, β = 67.47(3)°, γ = 69.11(3)°, V = 960.2(3)A3 und Z = 1. Der Bindungsmodus des Acetats in 1 ist ein Modell fur einen Ubergangszustand zwischen symmetrischer syn, syn, anti-μ2-carboxylato-O- und syn, anti-μ2-carboxylato-O, O′-Koordination. Daher kann Verbindung 1 als seltenes Beispiel fur das Phanomen des carboxylate shift angesehen werden. Weiterhin wird der Komplex durch ein ausgepragtes Wasserstoffbruckensystem stabilisiert.

Über die Reaktion von W3O mit lod. Darstellung, Kristallstruktur und Eigenschaften von WOI3

Abstract: Die Darstellung des bisher unbekannten WOI3 gelingt, wenn man das in der Literatur auch als β-Wolfram bezeichnete Wolframoxid der Zusammensetzung W3O mit lod in einer Glasampulle im Temperaturgefalle (440–400°C) umsetzt. In der kalten Zone schlagt sich WOI3 in mausgrauen verfilzten Nadeln nieder. WOI3 kristallisiert tetragonal im NbOCl3-Typ in der Raumgruppe P42/mnm mit a = 12,346(2), c = 3,765(1) A (20°C). Die Kristallstruktur bei–130°C wurde aus Einkristall-Diffraktometerdaten bestimmt (R = 0,023). Ergebnisse der Messungen der magnetischen Suszeptibilitat und des Schwingungsspektrums sowie das beim Erhitzen von WOI3 erhaltene Massenspektrum werden angegeben. On the Reaction of W3O with Iodine: Preparation, Crystal Structure, and Properties of WOI3 The novel tungsten oxide iodide WOI3 is obtained from the reaction of the metalrich tungsten oxide W3O (also known as ‘β-tungsten’) with iodine in a sealed glass tube placed in a temperature gradient (440–400°C). In the low-temperature region WOI3 is formed as mouse-grey intergrown needle-like crystals. WOI3 crystallizes in the tetragonal NbOCl3 structure type, space group P42/mnm, with a = 12.346(2), c = 3.765(1) A (20°C). The crystal structure at – 130°C was determined from single-crystal diffractometer data (R = 0.023). Results of magnetic susceptibility and vibrational spectroscopic measurements are given together with the mass spectrum of WOI3 obtained on heating.

The chemistry of graphene oxide

Abstract: The chemistry of graphene oxide is discussed in this critical review Particular emphasis is directed toward the synthesis of graphene oxide, as well as its structure Graphene oxide as a substrate for a variety of chemical transformations, including its reduction to graphene-like materials, is also discussed This review will be of value to synthetic chemists interested in this emerging field of materials science, as well as those investigating applications of graphene who would find a more thorough treatment of the chemistry of graphene oxide useful in understanding the scope and limitations of current approaches which utilize this material (91 references)

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

The hydrogen bond in the solid state.

Abstract: The hydrogen bond is the most important of all directional intermolecular interactions. It is operative in determining molecular conformation, molecular aggregation, and the function of a vast number of chemical systems ranging from inorganic to biological. Research into hydrogen bonds experienced a stagnant period in the 1980s, but re-opened around 1990, and has been in rapid development since then. In terms of modern concepts, the hydrogen bond is understood as a very broad phenomenon, and it is accepted that there are open borders to other effects. There are dozens of different types of X-H.A hydrogen bonds that occur commonly in the condensed phases, and in addition there are innumerable less common ones. Dissociation energies span more than two orders of magnitude (about 0.2-40 kcal mol(-1)). Within this range, the nature of the interaction is not constant, but its electrostatic, covalent, and dispersion contributions vary in their relative weights. The hydrogen bond has broad transition regions that merge continuously with the covalent bond, the van der Waals interaction, the ionic interaction, and also the cation-pi interaction. All hydrogen bonds can be considered as incipient proton transfer reactions, and for strong hydrogen bonds, this reaction can be in a very advanced state. In this review, a coherent survey is given on all these matters.

The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties

Abstract: The transition metal dichalcogenides are about 60 in number. Two-thirds of these assume layer structures. Crystals of such materials can be cleaved down to less than 1000 A and are then transparent in the region of direct band-to-band transitions. The transmission spectra of the family have been correlated group by group with the wide range of electrical and structural data available to yield useful working band models that are in accord with a molecular orbital approach. Several special topics have arisen; these include exciton screening, d-band formation, and the metal/insulator transition; also magnetism and superconductivity in such compounds. High pressure work seems to offer the possibility for testing the recent theory of excitonic insulators.

Multicopper Oxidases and Oxygenases

Abstract: Copper is an essential trace element in living systems, present in the parts per million concentration range. It is a key cofactor in a diverse array of biological oxidation-reduction reactions. These involve either outer-sphere electron transfer, as in the blue copper proteins and the Cu{sub A} site of cytochrome oxidase and nitrous oxide redutase, or inner-sphere electron transfer in the binding, activation, and reduction of dioxygen, superoxide, nitrite, and nitrous oxide. Copper sites have historically been divided into three classes based on their spectroscopic features, which reflect the geometric and electronic structure of the active site: type 1 (T1) or blue copper, type 2 (T2) or normal copper, and type 3 (T3) or coupled binuclear copper centers. 428 refs.