bluuebunny/crossref_metadata_2025_split huggingface dataset:
Eliminations from Olefins
Authors: Köbrich G. 
Published: December 2003

AbstractEliminations from olefins are very often initiated by bases, and usually lead to formation of acetylene derivatives. In view of the numerous side‐reactions and subsequent additions or rearrangments observed, the nature of the base used is of considerable significance. Organometallic bases are very versatile. In addition to dehydrohalogenation, dehalogenation by metals, and also thermolytic and photolytic eliminations. (e.g. the retrodiene reaction with exchange of substituents) are discussed. Elimination from low‐membered cyclic olefins yields cycloalkynes having strained ring‐systems, and the existence of these can be demonstrated by trapping them. The mechanisms known for β‐elimination (E2, E1 and E1cB) occur also in the case of olefins; however – due to the sp2‐hybridization of the carbon atoms taking part – they are realized with different rates of reaction relative to saturated compounds. α‐Eliminations from olefines having aryl residues in the β‐position lead to formation of arylacetylenes by rearrangement of the carbon skeleton. The mechanism of this reaction, which is known as the Fritsch‐Buttenberg‐Wiechell rearrangement, is discussed in considerable detail, and several variations of the reaction are considered. According to present‐day knowledge carbenes are involved in α‐eliminations only when both β‐positions of the olefin are occupied by aliphatic substituents, or when they are occupied by aromatic residues where rearrangement to acetylenes is impossible for steric reasons (as for example with 9‐chloromethylenefluorene). With organolithium compounds a number of dehydrohalogenations, which are formally β‐eliminations, actually proceed via deprotonation on the halogenbearing carbon atom (α‐metallation). α‐Metallations are the rate‐determining steps when ether is used as solvent, but proceed quickly in tetrahydrofuran even at low temperatures. Compounds of the type \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$\raise1pt\hbox{$>$}{\rm C}\raise1pt\hbox{=\kern-3.45pt=}{\rm C(Li)Cl}$\end{document} may be prepared by this method and are recognized as intermediates in α‐eliminations.
Recent Development in the Chemistry of Heterosiloxanes
Authors: Schmidbaur H. 
Published: December 2003

AbstractProgress in the chemistry of organoheterosiloxanes is reviewed in the light of the latest preparative and spectroscopic results. Attention is focussed on the heterosiloxane structural unit SiOX, the bonding in which – depending on the hetero‐atom – differs characteristically from that in the siloxane bridge SiOSi.
Exchange Rates of Ligands in Complex Ions
Authors: Pearson R. G. , Anderson Mary M. 
Published: December 2003

AbstractIt is possible to study the very rapid exchange rates between molecules or ions, L, coordinated to paramagnetic ions and the free molecules or ions, L*, in the body of the solution by measuring the line width of the NMR signal of L*.

Lifetimes of coordinated ligands that can be measured fall into the range of milliseconds to microseconds. The lifetime, or an upper limit to the lifetime, can be measured directly.
Recent Results in the Catalytic Oligomerization and Co‐oligomerization of Butadiene
Authors: Müller Herbert , Wittenberg D. , Seibt H. , Scharf E. 
Published: December 2003

AbstractA number of new catalyst systems based on transition‐metal compounds have been developed for the oligomerization and co‐oligomerization of butadiene. These catalysis promote dienylations and the formation of other CC linkages, and make possible the synthesis of new open‐chain and cyclic compounds from butadiene.
N‐Substituted Tetrahydro‐1,4‐oxazines – A New Class of Fungicidal Compounds
Authors: König K.‐H. , Pommer E.‐H. , Sanne W. 
Published: December 2003

AbstractTetrahydro‐1,4‐oxazines with large alicyclic or long alkyl groupsCompounds of this type are the subject of German and other patent applications.
attached to the nitrogen exhibit good or very good fungicidal activities against fungus diseases of crops, particularly against the true mildew varieties. ‐ By examining several hundred compounds, the relations between chemical constitution and fungicidal action were investigated in an attempt to find the most effective compounds. The factors varied were the nature, chain length, and ring size of the N‐substituent, and the nature, number, and positions of the C‐substituents on the heterocyclic system. Furthermore, the lone electron pair at the nitrogen atom was incorporated by alkylation, formation of amine oxides or salts, or by complexing with heavy metal salts, and the resulting changes in the fungicidal activity were determined.
Mechanization of Chemical Documentation
Authors: Meyer Ernst 
Published: December 2003

AbstractThe principal reasons for the failure of mechanized documentation to gain wide acceptance in chemistry are the inflexibility fo the systems used and the difficulties encountered in precisely formulating all search questions. The present paper describes documentation methods for structural formulae, reaction types, and other data; these methods are more flexible than those commonly used. To avoid the use of electronic computers for simple enquireies, it is suggested that small card files should be supplied ready‐encoded from a central store, and mechanically sorted in accordance with the code which best suits the particular case.
The Hetarynes
Authors: Kauffmann Th. 
Published: December 2003

AbstractInvestigations carried out during the last six years have shown that, as in the case of benzene, an extra bond can be introduced into heteroaromatic rings. Since this possibility does not appear to be confined to exceptional cases, the heterocyclic arynes (hetarynes) promise a much wider variety than the carbocyclic arynes. Hetarynes occur mainly as intermediates in nucleophilic substitutions on halogenated heterocyclic compounds. Since the replacement of a CH group in dehydrobenzene by a nitrogen atom and the condensation of benzene rings have stabilizing effects, the stability of hetarynes is expected to increase with the number of N atoms and with the number of condensed aromatic rings.
Anaplerotic Sequences in Microbial Metabolism
Authors: Kornberg H. L. 
Published: December 2003

AbstractIn addition to catabolic and anabolic pathways, which enable nutrients to enter and to be withdrawn from the central routes of metabolism, enzymic sequences operate to ensure that intermediates, removed from these central routes in the course of biosynthesis, are replenished. These obligatory pathways of replenishment are generically designated “anaplerotic sequences”. The nature of these sequences, operative in micro‐organisms growing on C3‐ and C2‐compounds, is discussed in outline, and the mode of regulation of one of them (the glyoxylate cycle) is examined in detail.
New Reactions of Alkylidenephosphoranes and their Preparative Uses. Part II. Alkylidenephosphoranes and Halogen Compounds
Authors: Bestmann H. J. 
Published: December 2003

AbstractThe reaction of alkylidenephosphoranes with compounds containing halogens can be used to prepare ylides, which can be converted in to useful products, e.g. by hydrolysis, by thermal decomposition, or by other reactions. Examples of such products are ketones including cyclic, unsaturated, and branched‐chain ketones (including cyclic, unsaturated, and branched‐chain ketones), carboxlic esters (including those of unsaturated, branched‐ chain, polyenecarboxylic, acetylenecarboxylic, and allenecarboxylic acids), and aldehydes.
New Reactions of Alkylidenephosphoranes and their Preparative Uses III. Alkylidenephosphoranes and Reagents Containing Multiple Bonds
Authors: Bestmann H. J. 
Published: December 2003

AbstractA few examples of preparatively useful syntheses using alkylidenephosphoranes and substances containing a CN or CC double bond are the preparations of olefins, allenes, pyran derivatives, and cyclopropane derivatives. Olefins, ketones, and monocyclic compounds can be obtained by autoxidation of alkylidenephosphoranes; α, β‐dioxo compounds (including cyclicones) can be prepared by oxidation with peroxy‐acids. Alkylidenephosphoranes can also be used for the synthesis of azines and Schiff bases. Of the many possibilities offered by the Wittig reaction, only the preparation of tritiated aldehydes and olefins will be considered.
The Use of Electron Paramagnetic Resonance in Organic Chemistry
Authors: Schneider F. , Mobius K. , Plato M. 
Published: December 2003

AbstractThe more recent applications of electron paramagnetic resonance (EPR) to organic chemistry include the investigation of triplet states, donor‐acceptor complexes, short‐lived free‐radical intermediates, and reaction kinetics. The principles of the theoretical calculations required to obtain the maximum information from EPR spectra are outlined for free radicals containing π‐electron systems. EPR intensities, line widths, hyperfine splitting, and g factors permit the determination of free‐radical yields, ionic charges, rotation frequencies in rotamers, electron density distributions, electronegativities, intramolecular torsion angles, solvent effects (resulting from hydrogen bonding), and π‐orbital energies. – The production of free radicals with the aid of initiation reaction in flow systems, by irradiation, and by electrolysis seems particularly promising.
Synthesis of Transcurium Elements by Nuclear Reactions with Heavy Ions
Authors: Keller C. 
Published: December 2003

AbstractExperimental techniques are described for the preparation of transcurium elements. Nuclear reactions with heavy ions appear to be most promising for the preparation of new transcurium elements and isotopes. Since these nuclear reactions have extremely small cross‐sections, only few atoms will be produced. Owing to their short half‐lives, the heaviest nuclei can presumably be identified only indirectly through the probabilities of their formation and distintegration.
Phosphorescence Spectroscopy as an Analytical Method for Aromatic and Heterocyclic Compounds
Authors: Zander M. 
Published: December 2003

AbstractThe phosphorescence of aromatic and heterocyclic compounds at low temperatures can be utilized for their identification and determination. The theoretical and experimental basis of phosphorimetry, which represents a valuable addition to the better‐known analytical methods, is described, and examples of its application to coal‐tar chemistry are discussed.
New Products of Reaction of Organometallic Compounds with Sulfur, Selenium, Tellurium, and Phosphorus
Authors: Schumann Herbert , Schmidt Max. 
Published: December 2003

AbstractBond fission of molecular sulfur, selenium, and tellurium by lithiotriphenylmetal compounds (of Ge, Sn, Pb) is summarized. The products are suitable as starting materials for synthesis of new “ether analogues”. Transphenylation with tetraphenylstannane is interpreted as a high‐temperature variant of the usual fission of chalcogen molecules by nucleophilic reagents. In principle, transphenylation can be applied also to other elements, as is illustrated for phosphorus. In the syntheses achieved, many of the tin‐phosphorus compounds arising as intermediates can be isolated.
The Chemistry of Silicon‐Sulfur Compounds
Authors: Haas A. 
Published: December 2003

AbstractThe preparations, properties, structures, and chemical reactions of silicon‐sulfur compounds are reported. The chain structure of the polymeric silicon disulfide is used as a basis from which the various classes of compounds can be derived: cyclothiasilanes, disilyl sulfides, silanethiols, and organothiosilanes. Infrared data and force constants of the Si‐S bond are discussed.
Electronic Structure and Properties of Non‐Alternant Hydrocarbons
Authors: Zahradník R. 
Published: December 2003

AbstractThe present paper is concerned mainly with studies on the electronic structure of nonalternant hydrocarbons by quantum‐chemical methods. The references to original publications are by no means exhaustive, but an attempt has been made to cover the entire field. (Analogues and derivatives of non‐alternant hydrocarbons are only touched.) The intention is to outline the present situation with regard to quantum‐mechanical studies on non‐alternant hydrocarbons, and to indicate to the chemist the use of the theoretical characteristics obtained by various approximations of the MOLCAO method.The simplest modification of the MO–LCAO method is the Hückel approximation (HMO), in which the molecular orbitals ψi are expressed as linear combinations of the atomic orbitals ϕj; in the π‐electron approximation only the 2pzatomic orbitals are considered. For the interaction of n orbitals we have:The π electron densities and the bond ordes can be calculated from the coefficients cij of the molecular orbitals. The energies Ei of the molecular orbitals are found by solving the determinantal equation' the n permitted levels are given by:where α is the Columb integral of the 2Pzorbital of carbon and β is the resonance intergral of the CC π‐bond (and also the unit of energy in the HMO theory). The occupied molecular orbital with the highest energy is denoted by k1 (normal state), and the unoccupied level with the lowest energy by k−1(valence state). The transfer of an electron from k1 to k−1is called N → V1 excitation.The main difference between the HMO method and the more accurate MOLCAO methods is that electronic interactions are explicitly taken into account in the latter; these methods arePople'sSCF (self‐consistent field) method andPariserandParr'sLCI (limited configuration interaction) method.
Structure Formation and Molecular Mobility in Water and in Aqueous Solutions
Authors: Wicke E. 
Published: December 2003

AbstractThe study of the structure of water and of aqueous solutions has recently received new impetus from the efforts at commercial desalineation of sea water and from developments in molecular biology. The current view that, apart from single molecules, water contains only one type of structural element, namely “flickering” network structures with tetrahedrally hydrogen‐bonded water molecules (two‐states model) is proving inadequate in the interpretation of new experimental data and in the calculation of thermodynamic functions. After a critical discussion of the basis of this model and of the concept of hydrogen bonds, a second kind of structural element, i.e. a third state, is suggested: small aggregates of molecules containing mainly non‐tetrahedral hydrogen bonds as well as some tetrahedral ones, and packed more densely than allowed by the lattice‐like structure. These aggregates – dimers to hexamers – can be regarded as the primary products of disruption of the network structures, and displace the latter as structural components in water with increasing temperature or concentration of solutes. This “combined” model allows a consistent interpretation of the properties of water and of the various effects of dissolved substances.
The Biosynthesis of Cyclitols
Authors: Kindl H. , Scholda R. , Hoffmann‐Ostenhof O. 
Published: December 2003

AbstractInformation has recently been obtained, mainly with the aid of radioactive compounds, about the biosynthesis of various cyclitols. It was found that the most widely occurring cyclitol, meso‐inositol, is biosynthesized by a pathway probably common to all organisms, in which the cyclohexane ring is formed by ring closure between the two terminal carbon atoms of D‐glucose. Cell‐free extracts or enzyme systems that catalyse the conversion of D‐glucose into meso‐inositol can be prepared from various biological materials. The biosynthesis of the other hexahydroxycyclohexanes (inositols) involves meso‐inositol as an intermediate. A few steps in the conversion of meso‐inositol into other inositols have been studied.
The Passivity of Metals
Authors: Schwabe K. 
Published: December 2003

AbstractAfter a survey of the various factors, depending both on the metal and on the attacking substance, that can influence the anodic dissolution of metals, the nature of the passive layer is discussed on the basis of electrochemical and optical studies. Passivation is characterized by a rapid decrease in the anodic current density as the Galvani tension is increased; this is due to the presence of a monomolecular layer, which prevents the metal ions from passing across the phase boundary. The growth of this layer is strongly influenced by transport processes, particularly with in the layer itself. The kinetics of the passivation and activation processes can be explained by the simultaneous occurrence of competing reactions which depend on the potential; exchange adsorption appears to play some part in the passive behavior of metals towards various acids.
Automation of Structure Analysis. Part II. Automation of the Evaluation
Authors: Hoppe W. 
Published: December 2003

AbstractThe extent to which it is possible to automate the determination of a structure from the data collected with automatic measuring equipment is discussed. The development of the automatic evaluation is synchronous with that of digital computer program.—Even the simple calculation of structure factors from the intensities (i.e. the analysis of the geometrical and physical intensity factors) takes several weeks when the many thousand reflections are processed “manually”, i.e. with a desk calculator. A large electronic computer, on the other hand, executes these calculations in a few seconds. Nevertheless, even the largest computers available at present are too small for many of the complex steps in the calculation and for the quantity of data involved. Evaluation methods which, owing to long computing times, can at present only be tested on relatively simple structures will become more attractive with the advent of machines with computing times in the nanosecond range.For corrigendum see DOI:10.1002/anie.196605112
The Structure of Glasses
Authors: Krebs H. 
Published: December 2003

AbstractA melt can readily solidify to a glass with a three‐dimensional or two‐dimensional network, or a chain structure, provided that an irregular bonding system can be formed by virtue of free rotation about the bonds between a central atom and the ligands which function as bridging atoms. Such an irregular structure can arise when the system contains a sufficient amount of bridging atoms such as O, F, and S, or bridging groups such as CH2, with bond angles less than 180°. When the network is formed predominantly by trivalent and tetravalent elements, such as As and Ge, the glasses – though they cannot be prepared by cooling of melts – can be obtained by other processes, e.g. by condensing the vaporized substances onto a surface (glasses in the wider sense of the word). As a result of extensive network formation, the bonding systems and, therefore, the short‐range order of the atomic arrangement in the melt differ from those in the glass or the crystal. A liquid mixture of substances having unlike molecular size and shape also can form a glass on solidification. Moreover, glasses can be formed even when the systems contains only one component opposing regular packing into a crystal lattice.
Problems of Iron Metabolism with Special Reference to Biochemical, Physiological, and Clinical Aspects
Authors: Keiderling W. , Wetzel H. P. 
Published: December 2003

AbstractA brief account of the functions and chemical structures of the principal iron compounds found in the human body is followed by a discussion of the physiology of iron metabolism, starting with its resorption, continuing with its transport and assimilation by the body cells, and concluding with its elimination. Special attention is paid to the problems of intracellular and extracellular transport of iron. A number of diseases with characteristic disturbances of the iron metabolism are also briefly described.
Electron Pyrolysis, a New Method of Organic Microanalysis
Authors: Schildknecht H. , Enzmann F. , Gessner K. , Penzien K. , Römer F. , Volkert O. 
Published: December 2003

AbstractOrganic compounds incubated with tritiated water of specific activity 5 Ci/ml are specifically fragmented. The β‐active fragments can be detected in very small quantities by chromatographic separation. Chemical identification of these fragments provides information about the original compounds, as has been shown by experiments with amino acids, esters, lactones, amides, quinones, terpenes, nitrogen‐sulfur heterocycles, and natural products.
Methods for the Synthesis of Uniform Polymers
Authors: Winter J. H. 
Published: December 2003

AbstractThe difficulties encountered in the preparation of polymers that are uniform chemically, structurally, and in their molecular weight are due mainly to the wide variety of possible polymers. This arcticle gives a review of attempts to synthesize homogeneous polymers by chain reactions and by stepwise synthesis. Syntheses on templates are extremely important in living cells; methods have recently been found for syntheses of this type that are independent of natural processes. Uniform polymers can also be obtained by reactions on existing polymers. In replicating polymers, this in principle requires the modification of only one polymer molecule.
Syntheses of Carboxylic Acids from 1, 1‐Dichloroehylene
Authors: Bott K. , Hellmann H. 
Published: December 2003

AbstractMany β‐alkyl‐ and β‐arylpropionic acids are readily obtainable by the reaction of secondary or tertiary alcohols, esters of these, or olefins with the inexpensive 1, 1‐dichloroethylene in sulfuric acid. The success of the synthesis depends on the bulk and energy of the carbonium ion intermediate formed from the alcohols or olefins. With carbonium ions having one H atom attached to the carbonium C atom, electrophilic substitution of the 1,1‐dichloroethylene takes place to a small extent. Dicarboxylic acids and carboxylic acids with higher molecular weights are sometimes formed as a result of side reactions.
Reactions of the Carbonyl Group in Fluorinated Ketones
Authors: Gambaryan N. P. , Rokhlin E. M. , Zeifman Yu. V. , Ching‐yun Chen , Knunyants I. L. 
Published: December 2003

AbstractThe chemistry of polyfluorinated ketones has recently received a great deal of attention. A. Ya. Yakoubovich, Usp. Chim. 25, 3 (1956).
H. P. Braendlin and E. T. McBee in: Advances in Fluorine Chemistry. Butterworths, London 1963, Vol. 3, p. 1.
The review by Braendlin and McBee [2] does not deal only with perfluorinated ketones; moreover, though published in 1963, it is already outdated. The papers discussed in [1,2] will not be reviewed in the present article.
. The carbonyl and imino groups of the fluorinated ketones and their imines react in many different ways. In particular, the electron‐attracting perfluoroalkyl groups intensify the electrophilic properties and weaken the nucleophilic properties of the carbonyl group. The perfluoroalkyl group also hinders the heterolytic removal of the hydroxyl group from adducts by reducing the stability of the carbonium ion . The increased electrophilicity of the carbonyl group and the increased stability of the addition products leads not only to a change in reactivity in the characteristic ketone reactions, but also to numerous new reactions which are not observed with non‐fluorinated ketones. Thus it is possible to synthesize a wide variety of fluoroorganic compounds from perfluorinated ketones.We have been concerned mainly with hexafluoroacetone, since in addition to being the most readily available and simplest perfluorinated ketone, this is also a typical representative of this class of compounds.
.
Conformational Aspects of Many‐Membered Rings
Authors: Dale J. 
Published: December 2003

AbstractThe stereochemistry of the various chemical grouping which constitute the structural units of ring systems is first reviewed. It is then shown that the strain‐free possibilities even of large rings are restricted, and that the ring conformation theoretically derived generally correspond to those found by physical methods. Variations of physical properties and of cyclization yield in homologous series are explained on a conformational basis.