bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ionic Hydrogenations and Dehydrogenations
Authors: Meerwein H. , Wunderlich Kl. , Zenner K. Fr. 
Published: December 2003

AbstractIn acetonitrile, 4‐chlorobenzenediazonium fluoroborates readily add onto trimethylamine to give 4‐trimethylammoniumbenzenediazonium chloride fluroborates. These addition products have a pronounced tendency to accept two hydrogen atoms in the form of a hydride ion and a proton from donors such as 1,4‐dihydrobenzene, 1,3‐dioxolans, 1,4‐dioxan etc. The diazonium group is thereby reduced, and the trimethylammonium group is split off as trimethylammonium fluroborate and replaced by the chloride ion. In the presence of a trace of copper, the reaction mechanism is changed from an ioinic to a free‐radieal one. In this case, just as in the absence of copper, the diazonium group is reduced, but the trimethylammonium group is retained. Alcohols behave in the same way as the donors of ionic hydrogen cited.
Isocyanates of Esters of Some Acids of Phosphorus and Silicion
Authors: Holtschmidt H. , Oertel G. 
Published: December 2003

AbstractSeveral aminoaryl esters of phosphoric and thiophosphoric acids were prepared by the reaction of phosphorus pentahalides with nitrophenols, followed by catalytic hydrogenation, or by treating aminophenols with phosphorus trihalides and oxidation to pentavalent phosphorus. These amino esters were then converted into isocyanato esters by the action of phosgene. Isocyanates of phosphonates have been synthetized on the same principle, as well as via the Arbusov reaction of halogen‐substituted isocyanates with trialkyl phosphites. The reaction of silicon halides or alkylhalogenosilanes with aminophenols yielded aminoaryl esters of silicic acid or its derivatives, which could also be treated with phosgene to convert them into isocyanato esters.
Stereochemistry of Displacement Reactions at Phosphorus Atoms
Authors: Hudson R. F. , Green M. 
Published: December 2003

AbstractRecent interest in the stereochemistry of displacement reactions involving second row elements has led to rapid advances in the preparation and resolution of asymmetric phosphorus compounds. The changes in optical activity accompanying substitution at an optically active phosphorus atom, the reactivity of cyclic phosphates, and the mechanism of intramolecular decomposition reactions involving a phosphorus centre are discussed in terms of the electronic structure of possible transition states.
Some Illustrative Applications of NMR Spectroscopy to Organic Chemistry
Authors: Roberts J. D. 
Published: December 2003

AbstractIn recent years, a revolution in the practice of organic chemistry has been brought about by the application of physical methods. Nuclear magnetic resonance spectroscopy has been especially important in this regard, but so many papers have been published dealing with its uses in organic chemistry that any brief review such as this one can only cover illustrative examples. After a general introduction, attention will be focussed here on several interesting structural and dynamic problems involving or related to research being carried on in our laboratories.
Metal π‐Complexes with Di‐ and Oligo‐olefinic Ligands
Authors: Fischer E. O. , Werner H. 
Published: December 2003

AbstractTo the somewhat casual observer, the concept “metal π‐complex” is primarily associated with the names “ferrocene” and “dibenzenechromium”. The preparation of these two compounds was in fact the starting point of a development which has had fruitful effects on theoretical as well as preparative chemistry and the limits of which are not yet in sight. In recent years, many straight‐chain and cyclic olefins with varying numbers of double bonds have been added to the list of ligands in metal π‐complexes, besides such “aromatic” systems as the cyclopentadienyl anion, benzene, and the tropylium cation. The literature in this area is already so copious that it seems advisable to adopt a division into compounds with mono‐olefins and those with di‐ and oligo‐olefins. The present review is concerned exclusively with the latter group.
Macromolecules with Non‐uniform Solubilities – Heterogels and Heteropolymers
Authors: Sadron Ch. 
Published: December 2003

AbstractIt is possible to produce chain‐like macromolecules A–B, with A and B parts which have different solubilities in a given solvent. In solvents, depending on the concentration, the insoluble parts of these non‐uniformly soluble macromolecules form micelles, aggregates, cylinders, or layers, while the soluble parts and the solvent fill up the gaps. The structure of these heterogels is destroyed by stirring or heating, but it can be fixed by polymerizing the solvent.
Recent Developments in the Chemistry of Metal Silyls of the Type M(SiR3)n
Authors: Wiberg E. , Stecher O. , Andrascheck H. J. , Kreuzbichler L. , Staude E. 
Published: December 2003

AbstractWhereas rapid strides have recently been made in the chemistry of the silyls of nonmetals, our knowledge of metal silyls is still scanty. Silyl compounds with every nonmetal are now known, but the literature on metal silyls is almost entirely restricted to silyl compounds of the alkali metals. This is all the more surprising, since the closely related field of elementalhomologous organometallic compounds of the type M(CR3)n (Where M = metal, R = hydrogen and/or organic residue, and n = valence of the metal) is among the most interesting and extensively studied areas of chemistry. This report will therefore deal with the results of recent investigations on metal silyls which led beyond the alkali silyls to silyl compounds of the alkaline earth metals, zinc, mercury, aluminum, and tin.
Further Contributions to the Knowledge of Semimetals
Authors: Klemm W. , Niermann H. 
Published: December 2003

AbstractMeasurements on semimetals have been continued. Studies of the dependence on temperature of various properties have provided details regarding the transition from the nonmetallic to the more metallic structure. A second‐order transition has been found for tin at 170 °C, in which the c‐axis remains unchanged, while the a‐axis undergoes a small, discontinuous elongation.
Preparation of Carbon Tetrachloride from Phosgene
Authors: Glemser O. , Schröder J. , Kleine‐Weischede K. , Meyer Boris , Peuschel G. , Flügel R. , Velde D. 
Published: December 2003

AbstractCarbon tetrachloride can be prepared from phosgene at 300–450°C. The catalysts for the reaction are transition metals of Groups V to VIII deposited on activated charcoal as carrier. Carbon monoxide and chlorine may be used as starting materials instead of phosgene.
Theories of the Glass Electrode
Authors: Schwabe K. , Suschke H. D. 
Published: December 2003

AbstractA review of the various theories of potential formation on the glass‐electrode is followed by a discussion of the deviations from the ideal hydrogen (H) electrode function in the acidic and alkaline ranges from the viewpoint of the phase‐boundary‐potential theory. The effect of the composition and solubility of the glasses on the electromotive behavior is interpreted as a “medium effect” in the gel layer.
The Biochemistry of Yeast Threonine Deaminase
Authors: Holzer H. , Boll M. , Cennamo C. 
Published: December 2003

AbstractAn optical assay, according to Warburg, for measuring the activity of the enzyme threonine deaminase (threonine → α‐ketobutyrate + NH) is described. Some of the properties of the enzyme from yeast have been studied using this assay. Pyridoxal phosphate and pyridoxamine phosphate stabilize the enzyme in the crude extract. Valine, leucine, and isoleucine, and to a lesser extent threonine, NH, and K+ protect the enzyme from inactivation when diluted with buffer. The enzyme is inhibited by isoleucine. The extend of the inhibition is dependent on the pH. Thus, in yeast, just as described for Escherichia coli by Umbarger, isoleucine exerts a control on its biosynthesis from threonine by means of a feed‐back mechanism, the extend of the control being related to the consumption of isoleucine in protein synthesis. NH represses the biosynthesis of threonine deaminase. This mechanism regulates the threonine deaminase‐catalysed formation of NH from threonine in accordance with the NH‐requirements of the growing cell.
Coordination Chemistry and Catalysis. Investigations on the Synthesis of Cyclooctatetraene by the Method of W. Reppe
Authors: Schrauzer G. N. , Glockner P. , Eichler S. 
Published: December 2003

AbstractThe relationship between the structure and the catalytic activity of nickel(II) complexes in the synthesis of cyclooctatetraene by the method of W. Reppe is discussed. The cyclotetramerization of acetylene takes place within labile Ni(II)‐acetylene π‐complexes. Inhibition tests have made it probable that four molecules of acetylene are grouped around the nickel ion in the transition state, in a configuration which favors the formation of the eight‐membered ring.
Polar Addition of Hydrogen Halides onto Olefins
Authors: Dewar J. S. , Fahey R. C. 
Published: December 2003

AbstractVarious structures have been proposed for the intermediates of electrophilic additions onto olefins; these include halonium ions, classical carbonium ions, π‐complexes (i.e. nonclassical carbonium ions), and π‐complexes with back‐coordination. It is shown here that it is impossible to use any one of these entities alone to explain all such electrophilic additions; the electrophile itself determines the nature of the transition state formed. Polar addition of hydrogen halides onto olefins appears to proceed via a classical carbonium ion which does not occur as the free ion but as an undissociated ion pair. Various other mechanisms have been excluded by studies reported here of the stereochemical course of such additions.
The Peroxyester Reaction
Authors: Sosnovsky G. , Lawesson S.‐O. 
Published: December 2003

AbstractThe reactions of t‐butyl peroxyesters with various classes of compounds in the presence of a catalytic amount of copper ions produce well‐defined acyloxy derivatives. This peroxyester reaction, discovered by Kharasch and Sosnovsky, can be used to effect one‐step oxidation of saturated and unsaturated hydrocarbons, alcohols, aldehydes, esters, ethers, sulfides, organosilicon compounds, etc. The reaction can be highly selective: in most cases only one specific carbon atom is attacked. However, with some ethers and, sulfides the reaction can take a more complex course, other products being formed in addition to the acyloxy derivatives. The mechanism of the reaction has been extensively investigated, and several concerted and chain mechanisms have been proposed.
Structure and Specificity of Organic Chelating Agents
Authors: Bayer Ernst , Fiedler H. , Hock K. L. , Otterbach D. , Schenk G. , Voelter W. 
Published: December 2003

AbstractMacromolecular chelating agents possessing high selectivity for rare metal ions have been synthetized. One of these, with the complexing group of glyoxal bis‐2‐mercaptoanil, is suitable for selective enrichment of gold from sea‐water, while another is suitable for copper and uranium. – The enrichment of metallic ions in nature is discussed, taking hemocyanin as an example. – Both stereochemistry and isomerism, as well as cyclic conjugation during chelate formation, may influence the selectivity of complexing agents. The quasi‐aromatic properties of chelates are discussed.
Organosodium and Organopotassium Compounds. Part II: Preparation and Synthetic Applications
Authors: Schlosser M. 
Published: December 2003

AbstractA selection of well established procedures for the preparation of the most common organosodium and organopotassium compounds is given. The wide synthetic applicability of organoalkali‐metal compounds is shown by means of examples. Unlike Part I, Part II deals not only with “true”organoalkali compounds, but also with salt‐like compounds such as sodium acetylides and sodiocarboxylic esters, since these posses considerable preparative value.
The Chemical Composition of the Stars
Authors: Unsöld A. 
Published: December 2003

AbstractThe abundances of the elements in the atmospheres, i.e. the directly observable layers, of the stars are related to the energy production by nuclear processes in the interior of the stars and to their evolutionary states. For purposes of quantitative analysis of a given stellar spectrum, several “model atmospheres” are first calculated, assuming tentative numerical values for the effective temperature Te, for the gravitational acceleration g at the surface of the star, and for the relative abundances of the more important elements. The appropriate temperature distribution is obtained from the theory of radiative equilibrium in conjunction with the quantum theory of the continuous absorption coefficient; the pressure stratification is obtained from the hydrostatic equation. Evaluation of the Fraunhofer lines requires a theory of line absorption coefficients and line widths. In conclusion, the final numerical values for Te, g, and the abundances of the elements in the stellar atmosphere are determined by comparison of the calculated spectra with the measured spectrum of the star. The main sequence stars and the hot supergiants, i.e. the star population I of the spiral arms and the disk population of the Milky Way, all have the same chemical composition, which coincides with that of the interstellar medium. In the old population II stars of the galactic halo (high velocity stars, subdwarfs), on the other hand, the abundance of all the heavy elements relative to that of hydrogen is reduced by a factor of as high as 200, possibly even 500; hydrogen seems to be the original material of our Milky way. In helium stars, almost all the hydrogen has been transformed into helium even in the atmosphere; it is likely that these are well mixed stars.
Free Radicals and Free‐Radical Reactions of Monovalent and Divalent Sulfur
Authors: Schmidt Ulrich 
Published: December 2003

AbstractOrganic free radicals of monovalent sulfur have not been hitherto observed in solution or in melts. Steric hindrance and resonance stabilization, which are responsible for the stability of the triarylmethyl, diphenylnitrogen, and phenoxyl radicals, are apparently insufficient to stabilize the organic free radicals of monovalent sulfur in such concentrations that they can be detected by current physical methods. – It was only in 1963 that aminopolysulfur radicals (R2NSnS·) were detected in solution, and arylsulfur radicals (Ar–S·) and phenylselenium radicals were isolated at ca. −180°C. – Organically bound sulfur can be stabilized in the free‐radical state if association of the radicals is prevented by fixing in a crystal lattice (“cystine radical”), by repulsion between radical ions (sulfinium salts), or by freezing‐in (arylsulfur radicals).
Structure and Acidity of Organic Compounds
Authors: Simon W. 
Published: December 2003

AbstractElucidation of the structures of organic acids and bases with the aid of their acidity constants is greatly hindered by their low solubilities in water and by the small quantities of material normally available. Apparent acidity constants K* can be determined potentiometrically on small samples (about 0.5 mg) for many classes of compounds of medium acidity. The apparent acidity constants K determined in the solvent system methyl cellosolvel water can be correlated, in a similar manner to thermodynamic quantities, with structural parameters (e.g. Hammett's σ* and Taft's σ* values). K values can yield information on the axial or equatorial position of carboxyl groups in cyclohexanecarboxylic acids.
Gradient and Low‐Temperature Thin‐Layer Chromatography
Authors: Stahl E. 
Published: December 2003

AbstractThe preparation and the use of gradient layers for thin‐layer chromatography are described. On a gradient layer there are three flow surfaces, all differing in their separation behavior. It is therefore possible to determine, for example, the basicity and/or activity which gives the optimum separation for a mixture of substances. Gradient thin‐layer chromatography was also carried out at low temperatures, after the effect of temperature on the separation of various mixtures had been studied.
Syntheses of Oxide Halides
Authors: Dehnicke K. 
Published: December 2003

AbstractKnowledge of the oxide halides has made significant advances in recent years as a result of new methods of preparation. Moreover, the systematic study of older methods has led to versatile syntheses which permit the preparation of many new oxide halides. A brief outline of the scope, limitations, and difficulties of the various processes is given together with a number of examples.
Strained Polycyclic Systems Consisting of Three‐ and Four‐Membered Rings
Authors: Seebach D. 
Published: December 2003

AbstractPolyclic compounds consisting of small rings have been known for several years. Their chemical and physical properties, which are of both theortical and practical interest, are discussed, and methods of preparation of a few examples are illustrated. The carbon skeletons of tetrahedrane, Dewar benzene, prismane, and cubane are discussed in greater detail, since the preparation of these compounds has now been achieved, or has at least been made possible.
Recent Advances in the Total Synthesis of Steroids
Authors: Velluz L. , Valls J. , Nominé G. 
Published: December 2003

AbstractIn the total synthesis of steroids on the industrial scale, three factors are important: stereoselectivity of the reactions used, resolution of racemic intermediates at the earliest possible stage, and convergence of the synthesis. Theoretical advances and wider practical experience in the chemistry of polycyclic compounds have recently led to the development of the first syntheses which are suitable for the industrial production of steroid hormones.
Light‐Induced Formation of Acids from Cyclic Ketones
Authors: Quinkert G. 
Published: December 2003

AbstractUltraviolet irradiation of aqueous solutions of cyclic ketones having a tetrahedral carbon atom in at least one of the two positions adjacent to the light‐absorbing carbonyl group results in ring opening accompanied by the formation of a carboxylic acid. Cyclohexadienones which are blocked in the ortho‐position are converted to hexadienoc acids. Cycloalkanones give saturated carboxylic acids and, in the presence of oxygen, also some unsaturated ones. The scope of the reactions is illustrated, and favorable conditions are described. The structure of the products and the route by which they are presumably formed are discussed.
Fused Salts and Their Use as Reaction Media
Authors: Sundermeyer W. 
Published: December 2003

AbstractApart from being of interest for physicochemical investigations, ionic liquids are a very important supplement to the non‐aqueous and water‐like solvents. The present discussion of the physical properties and current ideas on the structure of fused salts is followed by a report on the solubilities of gases, salts and metals. Our knowledge of fused salt baths and their use in electrochemical and electrometallurgical processes has recently been considerably expanded. Special attention is drawn to chemical reactions in fused electrolytes. Fused salts can also act as catalysts, so that they may often be advantageous reaction media for synthesis, if not the only media which can be used.
Formation and Reactivity of Phosphonium Salts in the Vitamin A Series
Authors: Freyschlag H. , Grassner H. , Nürrenbach A. , Pommer H. , Reif W. , Sarnecki W. 
Published: December 2003

AbstractFor syntheses in the carotenoid field using the Wittig reaction, particularly on the technical scale, it was necessary to carry out extensive studies on the preparation and properties of the intermediates. Hear the part of this work is discussed which dealt with quaternary phosphonium compounds of trimethylcyclohexene derivatives.
Metal Chelates as Polymerization Initiators
Authors: Kastning E.‐G. , Naarmann H. , Reis H. , Berding C. 
Published: December 2003

AbstractMetal chelates can be used as initiators in the polymerization of vinyl compounds. Their action, as shown here for the polymerization of styrene, is determined by both the central atom and the ligand. Chelates of manganese (III) and cobalt (III) with 1,3‐dicarbonyl compounds are most effective. The metal chelate initiators can be activated by γ‐ or ultraviolet irradiation, or else by combination with cocatalysts such as organic halogen compounds or unsaturated hydrocarbons which are typical π‐complex formers. There is some evidence indicating that the polymerization takes place via a free‐radical mechanism. The central metal atom affects the steric configuration of the polymer chain.
New Reactions of Alkylidenephosphoranes and their Preparative Uses. Part I: The Acid‐Base Character of Phosphonium Salts and Alkylidenephosphoranes
Authors: Bestmann H. J. 
Published: December 2003

AbstractPhosphonium salts may be regarded as Bronsted acids, and alkylidenephosphoranes as the conjugate bases. Compounds of the two classes exist in equilibrium with each other. Phosphonium salts and alkylidenephosphoranes can be obtained by this “transylidation”. Other methods are also given for the preparation of members of both classes.
The Chemistry of Nitrous Acid and of its Derivatives in Liquid Hydrogen Fluoride
Authors: Seel F. 
Published: December 2003

AbstractThe reaction of liquid hydrogen fluoride with nitrous acid, dinitrogen trioxide, dinitrogen tetroxide, and nitrosyl compounds leads to unexpectedly high‐boiling liquids which contain nitrosyl fluoride in the form of remarkably stable, electrolytically dissociated hydrogen fluoride solvates. Nitrosyl fluoride solvated by pure hydrogen fluoride is a new reagent, valuable in fluorine chemistry since it reacts with many elements and compounds to form fluorine derivatives of preparative importance.
Modern Theoretical Views on the Chemical Bond
Authors: Preuss H. 
Published: December 2003

AbstractStarting from classical concepts, semi‐empirical approaches to an understanding of the chemical bond are critically discussed, particularly with respect to their limitations. Rules such as the additivity of the bond constants and the octet rule are reviewed in relation to new chemical compounds. It is shown that these rules are not sufficient to explain chemical bonding; this is also true of the simple concept of hybridization, the basis of which is discussed. Some generally valid statements regarding the chemical bond can be propounded on the bais of wave‐mechanical considerations. It is shown that the hybridization concept and a number of other rules are too primitive to provide generally valid information or a basis for predictions about the chemical bond, and even that they are partly based on false premises. A better foundation for an understanding of the chemical bond can only be laid with the aid of wave mechanics.
Molecules Undergoing Fast, Reversible Valence‐Bond Isomerization. (Molecules with Fluctuating Bonds)
Authors: Schröder G. , Oth J. F. M. , Merényi R. 
Published: December 2003

AbstractCompounds which undergo fast and reversible valence‐bond isomerization (i.e. which have fluctuating cyclopropyl and/or double bonds) are taken to be those in which the average life‐time of the valence‐bond isomers at 0 °C is of the order of 100 seconds at most, and for which the maximum activation energy of the bond migration is 20 kcal/mole. The fast, reversible bond shift can scarcely be detected by chemical means, but can be recognized by NMR spectroscopy, since the time for which a proton occupies a position with a given magnetic environment has a pronounced influence on the NMR spectrum. Some examples of molecules with fluctuating bonds are cyclooctatetraene and its derivatives, unsaturated seven‐ membered ring systems, homotropilidene, bridged homotropilidene systems, bullvalene, and substituted bullvalenes. The last class is particularly interesting, since here the carbon atoms are continually changing their relative positions and neighbors.