Showing papers in "Angewandte Chemie in 1969"

The Chemistry of Hypervalent Molecules

Abstract: Molecules formed from atoms of Groups V–VIII of the periodic table in their higher valences are studied and a general theory for their electronic and geometric structure is presented These molecules, which we call hypervalent molecules, are unique in that a donor atom can possess two identical monovalent ligands which are bonded by different types of chemical bonds The present study provides a simple theoretical description of such molecules using doubly-occupied lone-pair orbitals to form hypervalent bonds, and these bonds are divided into two classes as to whether they involve p electrons alone or utilize s electrons as well While the theoretical classification is of some interest in itself, there are even more interesting problems concerning the physico-chemical properties of these molecules and the syntheses of new molecules and polymers We thus present a brief description of the energetics, possibilities for intermolecular inversion and site-exchange, and synthetic paths for the preparation of hypervalent molecules, as well as indicate a number of new molecules which can probably be synthesized using these ideas For corrigendum see DOI:101002/anie196901311

The Ene Reaction

Abstract: The ene reaction is defined as the indirect substituting addition of a compound with a double bond (enophile) to an olefin with an allylic hydrogen (ene). For a long time the reaction has been neglected and has remained overshadowed by the related Diels-Alder addition. It is shown that the ene reaction possesses wide scope and applicability ranging from industrial to biosynthetic processes. Preparative aspects are summarized and current views on the mechanism are discussed.

Electrochromism and Solvatochromism

Abstract: The position and the intensity of electronic bands are influenced by an electric field Pronounced changes in the position of absorption bands are mainly due to the dipole moment of the molecule in the ground state and the change in the dipole moment during the excitation process, and pronounced changes in intensity are due to the field dependence of the transition moment, which can be described by the transition polarizability The effect of an external electric field on the optical absorption (electrochromism) of suitable molecules can be used to determine the dipole moment in the ground state, the change in dipole moment during the excitation process, the direction of the transition moment of the electronic band, and certain components of the transition polarizability tensor These data largely determine the strong solvatochromism (solvent-dependence of the position and intensity of electronic bands), which is observed in particular with molecules having large dipole moments Smaller contributions to solvatochromism result from dispersion interactions, which predominate in the case of nonpolar molecules The models developed have been experimentally checked and verified by a combination of electro-optical absorption measurements (influence of an external electric field on absorption) and investigation of the solvent-dependence of the electronic bands

The Composition and Conformation of Sugars in Solution

Abstract: The conformations of the aldoses in solution, and the compositions of the equilibrium mixtures formed from aldoses after dissolution in water, have been determined by NMR spectroscopy. Calculations based on non-bonded interactions and on the anomeric effect account for the observed conformations, and allow an approximate prediction of the α:β ratio of pyranoses in equilibrium and of the extent of anhydride formation from sugars in acid solution. In certain circumstances the furanose forms are more stable than the pyranose forms.

Planar Complexes Containing Metal‐Metal Bonds

Abstract: Planar complexes, and particularly those of platinum, can form structures containing linear chains of heavy metal atoms with metal-metal distances as short as 3.1 A. The bonding in these chains can be strengthened by partial oxidation, and the bond lengths can be reduced to 2.8 A in this way. Model structures for the novel nonstoichiometric products, such as K2[Pt(CN)4]Cl0·32 · 2.6 H2O, are considered. The bonding is also discussed on the basis of a one-dimensional band model.

Mechanisms and Stereochemistry of Heterolytic Fragmentation

Abstract: Heterolytic fragmentation can proceed by several mechanisms. A study of the solvolysis of γ-aminoalcohol derivatives NCCCX, where X equals halogen or OSO2R, shows that fragmentation may occur by a one-step mechanism or by several multi-step mechanisms depending on structural, electronic, and steric factors. The one-step synchronous mechanism has rigorous stereoelectronic requirements. It is associated with an increased reaction rate as evidenced by the “frangomeric” effect. Ketoxime derivatives RR′CNX may undergo Beckmann fragmentation by the synchronous mechanism or by prior rearrangement depending on their α-substituents. Fragmentation is frequently accompanied by conventional substitution, elimination, and rearrangement reactions. Under such circumstances knowledge of all possible mechanisms of a given substrate is essential to bring about a desired transformation.

The Meaning and Use of the Arrhenius Activation Energy

Abstract: The nature of the Arrhenius activation energy and frequency factor is reexamined in terms of information now becoming available on the microscopic aspects of collisional reactions. It is pointed out that the activation energy is not generally equal to the threshold for reaction, and its correct conceptual meaning is discussed. The temperature dependence of this quantity and its relation to the threshold energy is developed for a number of representative forms of the energy dependence of the reaction cross-section (excitation function). The uses and limitations of the activation energy as a means of evaluating thresholds, excitation functions, and the presence of tunneling processes are discussed.

Methods and Possibilities of Nucleophilic Acylation

Abstract: Nucleophilic acylation is the attachment of an group to an electrophilic center. A brief discussion of the advantages offered by acylations of this type in syntheses is followed by a description of the direct methods (reactions with acylmetal compounds) and the methods proceeding via “masked” acyl anions. The latter methods are based on the use of either metalated vinyl ether derivatives or carbanions derived from acetals. Organyllithiums of sulfur-stabilized carbanions are important reagents of this type.

Transfer Ribonucleic Acids

Abstract: Transfer ribonucleic acids (tRNAs)1 occur in all living organisms. In biological protein synthesis they accept activated amino acids which are then transferred to growing peptide chains. With molecular weights lying between 25000 and 30000, tRNAs are easily within the reach of today's physical, chemical, and biochemical methods. The primary structures of several tRNAs as well as some relationships between structure and function have been elucidated. Three-dimensional structure, specificity, and mechanism of action are the subjects of present research efforts.

Mechanismen und Stereochemie der heterolytischen Fragmentierung

Abstract: Heterolytische Fragmentierungen konnen nach mehreren Reaktionsmechanismen ablaufen. Am Beispiel von γ-Halogenalkylaminen und γ-Aminosulfonaten (NCCCX) kann gezeigt werden, das infolge struktureller und sterischer Faktoren ein- oder mehrstufige Mechanismen in Erscheinung treten. Die einstufige, synchrone Fragmentierung ist an strenge stereoelektronische Bedingungen gebunden und hat eine erhohte Reaktionsgeschwindigkeit zur Folge, die sich im „frangomeren Effekt” ausert. Ketoxim-Derivate RR′CNX reagieren je nach der Natur der α-Substituenten entweder unter synchroner Beckmann-Fragmentierung oder unter Beckmann-Umlagerung zu Nitrilium-Salzen, welche ebenfalls zu Fragmentierungsprodukten fuhren konnen. Da Fragmentierungen haufig von konventionellen Substitutionen, Eliminationen und Umlagerungen begleitet werden, setzt eine erfolgreiche Reaktionslenkung die Kenntnis aller moglichen Mechanismen voraus.

Die En‐Reaktion

Abstract: Unter „En-Reaktionen” versteht man die indirekte substituierende Addition einer Verbindung mit Doppelbindung (Enophil) an ein Olefin mit allylischem Wasserstoff (En). Lange Zeit stand die En-Reaktion im Schatten der verwandten Diels-Alder-Addition. In diesem Aufsatz wird gezeigt, das das Anwendungsgebiet der En-Reaktion von industriellen bis zu biosynthetischen Prozessen reicht. Einer Zusammenstellung der praparativen Aspekte folgte eine Diskussion des Reaktionsmechanismus.

Lysozymes: A Chapter of Molecular Biology

Abstract: Hen egg-white lysozyme was the first enzyme whose tertiary structure could be elucidated. The peptide chain of this enzyme is arranged in two sections, of approximately equal size, that are separated by a deep cleft. Substrates (and inhibitors) are bound in this cleft via hydrogen bonds and are hydrolyzed under the action of Glu 35 and Asp 52, which form the active site of the enzyme. Although the lysozymes, which occur in many species of animals and plants, exhibit differences in their chemical behavior, they have the same qualitative biological activity; quantitatively important differences have been noted which also concern the specificity. Infection of E. coli with bacteriophages gives rise to a lysozyme whose formation is controlled by the phage DNA. The fact that mutated lysozymes are produced when the phages are treated with mutagens opens new fields of research in molecular biology.

Methoden und Möglichkeiten der nucleophilen Acylierung

Abstract: Unter nucleophiler Acylierung versteht man die Einfuhrung einer an einem elektrophilen Zentrum. Nach einer kurzen Diskussion der Vorteile, die solche Acylierungsmittel bei Synthesen bringen konnen, werden die direkten Methoden (Reaktionen mit Metallacylverbindungen) und die uber „verkappte” Acylanionen verlaufenden Verfahren behandelt. Diese verwenden entweder metallierte Vinylatherderivate oder Carbanionen, die sich von Acetalen ableiten. Dabei spielen Lithiumorganyle von Schwefelstabilisierten Carbanionen eine wichtige Rolle.

Cycloadditionen mit polaren Zwischenstufen

Abstract: Eine Diskussion der Kriterien, die zur experimentellen Unterscheidung einstufiger von zweistufigen Cycloadditionen herangezogen worden sind, last erkennen, das brauchbare Aussagen meist die Isolierung oder das Abfangen von Zwischenprodukten voraussetzen; die Isolierung von Zwischenprodukten hat aber auch praparative Bedeutung.

Transport phenomena in membranes.

Abstract: The realization that many elementary biological processes occur at membrane-like structures has led to renewed interest in the theory of molecular transport through membranes. Coupling phenomena in membranes, some of which are very complex, can now be rationally described with the aid of the thermodynamics of irreversible processes. The investigation of theoretical membrane models has proved a valuable complement to the thermodynamic method.

Hydration Structure and Intermolecular Interaction in Polyelectrolytes

Abstract: In hydration, the intermolecular interaction is of decisive importance, since the ions modify the properties of the hydration water molecules, and the water molecules in turn have a strong influence on the interaction between the ions. These processes can be studied in detail with the aid of IR spectroscopy, which in addition provides a picture of the hydration structure. IR spectroscopy also shows that the excess protons are in energy bands and allows the detection of the proton dispersion forces acting between tunneling protons. The results of IR studies on polyelectrolytes contribute to our understanding of the many biological processes that depend on the behavior of ions.

Metal Complexes of Thiophosphinic and Selenophosphinic Acids

Abstract: The anions of thiophosphinic and selenophosphinic acids R2P(X) YH (X = S, Se; Y = O, S, Se) can act as bidentate ligands. They combine with many metals to form complexes containing a four-membered chelate ring, or to give coordination polymers in which they form ligand bridges. The preparation, properties, and reactions of these compounds, as well as the dielectric properties and analytical use of dithiophosphinato complexes, which are also of industrial interest, are described. Some thiophosphinato and selenophosphinato complexes exhibit concentration-dependent association via ligand bridges. Evidence of the chelate nature of the ligands R2P(X)Y⊖ was obtained from IR spectroscopic studies. The ligand field parameters of the anion (C2H5)2P(S)S⊖ were deduced from the electronic spectra of octahedral diethyldithiophosphinato complexes, and the position of the ligand in the spectrochemical and nephelauxetic series were determined from these parameters.

Planare Komplexe mit Metall‐Metall‐Bindungen

Abstract: Planare Komplexe, vor allem die des Platins, konnen Strukturen mit linearen Ketten von Schwermetallatomen mit Metall-Metall-Abstanden bis herab zu 3,1 A bilden. Die Bindung in derartigen Ketten last sich durch partielle Oxidation verstarken, wodurch die Abstande bis auf 2,8 A verringert werden konnen. Dabei bilden sich nicht-stochiometrische Verbindungen eines neuen Typs, von dem Modellstrukturen wie K2[Pt(CN)4]Cl0.32 · 2,6H2O behandelt werden. Die Bindung wird auf der Grundlage eines eindimensionalen Bandermodells diskutiert.

Zusammensetzung und Konformation von Zuckern in Lösung

Abstract: Die Konformation der Aldosen in Losung und die Zusammensetzung der Gleichgewichtsmischungen, die sich beim Losen dieser Zucker in Wasser bilden, sind NMR-spektroskopisch bestimmt worden. Berechnungen auf der Basis nichtbindender Wechselwirkungen und des anomeren Effekts gestatten eine Erklarung der beobachteten Konformationen und eine approximative Vorhersage des α/β-Verhaltnisses der Pyranosen im Gleichgewicht sowie des Ausmases der Anhydridbildung bei Zuckern in saurer Losung. In manchen Fallen sind die Furanose- bestandiger als die Pyranoseformen.

Topochemical Investigations on Peptide Systems

Abstract: The basic principles of the topochemical approach to the investigation of the structure-function relation in peptide systems are formulated. This approach makes use of the new possibility of transforming natural peptides, consisting in the modification of the molecule as a whole and utilization of the resultant analogs to elucidate the boundaries of the stereoelectronic complementarity of the biologically active peptide to the corresponding receptor. In particular, on the example of depsipeptide antibiotics and their topochemical analogs the fruitfulness of using such compounds as tools in elucidating the physicochemical basis of functioning of biological membranes is shown. The topochemical principle has also been applied in preparing specific competitive inhibitors of proteolytic enzymes, whose study may shed light on the nature of the forces binding the substrate to the contact site of the corresponding enzyme.