bluuebunny/crossref_metadata_2025_split huggingface dataset:
Methods of Asymmetric Synthesis—Enantioselective Catalytic Hydrogenation
Authors: Izumi Yoshiharu 
Published: December 2003

AbstractIt is proposed that asymmetric syntheses be divided into enantioselective and diastereoselective syntheses. The enantioselective hydrogenations discussed in the present progress report were catalyzed by Raney nickel that had previously been treated with solutions of optically active compounds. Relationships exist between the enantioselectivity of the catalyst and the structure of the chiral compound used to modify it.
Signal Transducers: Terra Incognita of Molecular Biology
Authors: Delbrück Max 
Published: December 2003

AbstractIn this article four cases of signal transduction in biology are discussed, ranging from a case which we do understand in molecular detail (genetic induction) through two cases where the fog may be beginning to clear (vertebrate vision and bacterial chemotaxis) to a case where we are still struggling to define the participants of the play (the avoidance response of the fungus Phycomyces).—Although most of the facts presented are not brand new, the juxtaposition of them is perhaps unconventional and may be of interest at least by way of outlining an area of research of some promise. No doubt modern concepts and technologies of chemical and physical research will play a great role in this borderline area between a variety of fields, presenting great challenges to the ingenuity and versatility of the experimenter.
Stereochemistry of [2.2]Metacyclophanes
Authors: Vögtle Fritz , Neumann Peter 
Published: December 2003

AbstractIncreasing numbers of investigations have been carried out on bridged aromatic compounds, the independence of which was very recently acknowledged in the class name “phanes”. Special attention is being attracted by the [2.2] metacyclophanes, whose molecular architecture raises many interesting theoretical questions, mainly with regard to intramolecular steric and electronic interactions. In the present progress report, a short description of the classification and nomenclature of the phanes is followed by a discussion of new physical and chemical aspects of the stereochemistry of [2.2] metacyclophanes and a number of related [2.2] phanes with other aromatic nuclei.
The syn and anti Steric Course in Bimolecular Olefin‐Forming Eliminations
Authors: Sicher Jiři 
Published: December 2003

AbstractEarlier evidence led to the view that in bimolecular olefin‐forming eliminations the anti steric course is generally preferred over the syn. Recent experiments, however, show that in some common bimolecular elimination reactions, notably those involving onium bases, syn‐ and anti‐eliminations proceed side by side, the former preferentially leading to the trans‐ and the latter to the cis‐olefins.
Chemical Plant Protection — Past, Present, and Future
Authors: Sasse Klaus 
Published: December 2003

AbstractChemical plant protection greatly contributes to ensuring good harvests and is therefore of considerable agricultural and economic importance. After thirty years of systematic research we are now in possession of powerful chemical agents active against most of the significant enemies of cultivated plants, but the constant change in nature always calls for more and better plant protection agents. These agents play a mostly well‐known part in environmental pollution and in the overall toxic situation, and one of the aims of future development is to reduce further the dangers to man and the environment.
The Chemistry of Carbenoids and Other Thermolabile Organolithium Compounds
Authors: Köbrich Gert 
Published: December 2003

AbstractOrganolithium compounds are thermally quite stable, despite their extremely high reactivity. Thermolability, i.e. the property of undergoing irreversible changes at low temperatures without the participation of molecules of a different kind, is associated with additional functional groups, which may be readily eliminated together with the metal cation, may cause intramolecular rearrangements, or may lead to intermolecular additions and substitutions involving several molecules of the same type. Thermolabile organolithium compounds have frequently been postulated or detected as intermediates of important reactions, and are therefore of theoretical and practical interest. If certain precautions are taken, it is occasionally possible (mainly at low temperatures) to prepare them intact, and hence to release them from the constraint of the fate assigned to them as short‐lived intermediates. Investigations with this aim have for some time occupied a place of considerable importance in the chemistry of organometallic compounds.
Intermolecular Interactions: van der Waals Molecules
Authors: Ewing George E. 
Published: December 2003

AbstractRecent experiments using electron diffraction techniques, optical and infrared spectroscopy and mass spectroscopy have provided convincing evidence for the existence of bound states of gaseous Ar2, (H2)2, (O2)2, (NO)2 and other simple dimers. The bond strength of these van der Waals molecules is hundreds of times weaker than ordinary chemical bonds. These experimental studies yield information on the structure and stability of van der Waals molecules and can provide new insight into weak intermolecular interactions.
Organic Syntheses in the Plasma of Glow Discharges and Their Preparative Application
Authors: Suhr Harald 
Published: December 2003

AbstractIt is only during the last few years that reactions of organic substances as a result of electron collisions in the cold plasma of glow and corona discharges have been developed into a preparatively useful method with a wide range of possibilities. Its basic principles and development prospects are discussed in the present progress report on the basis of the research results that are known at present.
Energy Metabolism in Mitochondria
Authors: Heldt Hans Walter 
Published: December 2003

AbstractMitochondria are separate metabolic compartments within the cell. The functional boundary of the mitochondrial compartment is the inner membrane. This membrane contains the enzymatic apparatus for the electron transport and oxidative phosphorylation. The substrate breakdown cycles are localized in the mitochondrial matrix space. Specific carriers are responsible for the exchange of ADP, ATP, phosphate, and intermediates of the citric acid cycle between the matrix space and the extramitochondrial space. The particular importance of the adenine nucleotide transport to the regulation of the energy metabolism of the cell is discussed in detail.
Transcription of Genetic Information and Its Regulation by Protein Factors
Authors: Rüger Wolfgang 
Published: December 2003

AbstractDuring transcription, genetic information is transferred from DNA to mRNA, the process being catalyzed by DNA‐dependent RNA polymerase. Transcription comprises binding of the RNA polymerase to DNA, initiation, elongation, and termination. The interplay of the positive and negative control elements of these processes is illustrated for bacteria and bacteriophages.
Reactions of Electron‐Rich Olefins with Proton‐Active Compounds
Authors: Hocker Jürgen , Merten Rudolf 
Published: December 2003

AbstractProton‐active substances react with certain electron‐rich olefins with cleavage of the central CC double bond to give compounds that can be formally regarded as insertion products of nucleophilic carbenes. If they satisfy certain structural conditions, they isomerize with β elimination to give open‐chain compounds. Both CH‐acidic compounds and compounds containing NH or OH groups can undergo this reaction. The mechanisms are discussed, and the importance of the intermediate products to biochemistry (thiamine, tetrahydrofolic acid) is indicated.
The Stereoselective Acylation of Racemic Secondary Alcohols with Phenyltrifluoromethylketene as an Experimental Test of the Stereochemical Analogy Model
Authors: Anders Ernst , Ruch Ernst , Ugi Ivar 
Published: December 2003

AbstractValidity of the stereochemical analogy model[1,2] for stereoselective reactions is tested on acylations of racemic chiral secondary alcohols with phenyltrifluoromethylketene. The product ratios for the diastereomeric esters formed can be calculated using a formula derived from the general model for the special case, the results agreeing with the experimental data within the accuracy of the measuring method (integration of the 19F‐NMR signals).
Carbocations and Electrophilic Reactions
Authors: Olah George A. 
Published: December 2003

AbstractThe general concept of carbocations encompasses all cations of carbon containing compounds which can be differentiated into two distinct classes: trivalent (“classical”) carbenium ions and pentacoordinated (“nonclassical”) carbonium ions. In this paper the preparation and structural study (by NMR, IR, Raman and ESCA spectroscopy) of stable carbenium ions and carbonium ions are discussed. As is well known, trivalent carbenium ions play an important role in electrophilic reactions of π‐ and n‐donor systems. Similarly, pentacoordinated carbonium ions are the key to electrophilic reactions of σ‐donor systems (single bonds). The ability of single bonds to act as σ‐donor lies in their ability to form carbonium ions via triangular two electron, three‐center bond formation. Consequently there seems to be no principle difference between the electrophilic reactions of π‐ and σ‐bonds except that the former react more easily, even with weak electrophiles, whereas the latter necessitate more severe conditions. The role of carbocations in electrophilic reactions of π‐ and σ‐donor systems is discussed.
Polar Cycloadditions
Authors: Schmidt Richard R. 
Published: December 2003

AbstractCycloadditions with ionic components are known as “polar cycloadditions” to distinguish them from cycloadditions with dipolar and uncharged components. The structural requirements, the manner in which “polar” intermediates capable of addition are produced and their reactivity with activated and nonactivated multiple bonds, the many peculiarities of the course of the cycloaddition, and the favorable synthesis of heterocyclic systems by this reaction principle makes such a terminological distinction useful and necessary if the existing data are to be brought into order. The present review, which is the first on a field of chemistry that has not yet been very extensively investigated, deals initially with cationic and then with anionic polar cycloadditions.
Spectroscopic Properties of Organic Azo Compounds
Authors: Rau Hermann 
Published: December 2003

AbstractAzo compounds are characterized by a low‐energy1(n,π*) electronic state, which gives rise to a weak band at long wavelengths in the absorption spectrum. Coupling of this state with other low‐lying molecular states is responsible for the high intensity of the “azo band” compared to other forbidden transitions, for its lack of vibrational structure, and for the inability of nearly all organic azo compounds to fluoresce.
Mechanistic Analysis of Solution Reactions by Non‐Isothermal Calorimetry
Authors: Koch Erhard 
Published: December 2003

AbstractFor the rapid kinetic and energetic analysis of reactions in solution (τ20°C > 10‐4s) by differential thermal analysis (DTA), the following thermogram parameters are defined as characterizing the start of the reaction: 1. initial temperature, 2. activation energy of the initiation reaction. In conjunction with the shape index (asymmetry of the DTA curve) and the half width of the DTA curve referred to standard physical conditions (cell constant, heating rate, and temperature difference), these quantities allow a simple distinction between one‐step reactions of first and second order and composite reactions. It is possible to recognize whether a process involves parallel, successive, or equilibrium reactions, or combinations of these. The reaction mechanism can be clarified in many cases by measurements at various concentrations and heating rates and by discussion of the enthalpy values. The shortened method described in this report for the evaluation of the thermograms was derived with the aid of an analog computer and checked experimentally.
Chemistry and Applications of Liquid Crystals
Authors: Steinsträsser Ralf , Pohl Ludwig 
Published: December 2003

AbstractApproximately 5% of all organic compounds are transformed at their melting point into liquid crystals—thermodynamically stable, anisotropic liquids which in contrast to isotropic melts appear turbid and are also known as mesophases. Such melts are classed as smectic, nematic, and cholesteric liquid crystalline phases, depending upon the arrangement of the constituent molecules. The discovery of numerous potential applications during the past ten years has awakened the study of liquid crystals from its former slumber as a physical curiosity and placed it in the limelight of the scientific stage. Uses in display systems for measured values and for computer and process data, as well as for remote controlled timetables, for windows of variable light‐transmission, etc., appear particularly promising. Not only black‐and‐white contrasts are now possible but also color production.
Recent Results in Insect Pheromone Chemistry
Authors: MacConnell John G. , Silverstein Robert M. 
Published: December 2003

AbstractPheromones are substances which are secreted to the outside by an animal, especially an insect, and received by a second individual of the same species, in which they release a specific reaction, for example, a definite behavior or developmental process. The name is derived from the Greek pherein (to bear, transmit) and hormon (impel, excite). Pheromones include, inter alia, sex attractants, warning substances, and “aggregation pheromones”. Many of these substances are effective at very low concentration. The insect pheromones are predominantly derivatives of long‐chain, slightly branched hydrocarbons.
Photoelectron Spectra of Nonmetal Compounds and Their Interpretation by MO Models
Authors: Bock Hans , Ramsey Brian G. 
Published: December 2003

AbstractThe results of (low energy) photoelectron spectroscopy render possible a better appreciation of the “Nature of the Chemical Bond”. The application of this new experimental method is demonstrated utilizing representative compounds of the nonmetal elements, and a close symbiosis delineated with molecular orbital models. In particular, general consequences are discussed concerning electron deficiency, σ‐ and π‐interactions, electron pair delocalization, and substituent effects or geometric perturbations. Photoelectron spectroscopic ionization energies permit evaluation of parameters for specified molecular groups, allow correlation with numerous other experimental data, and are didactically valuable in the teaching of general chemistry.
The Allylcobalt System
Authors: Bönnemann Helmut , Grard Charles , Kopp Werner , Pump Wichard , Tanaka Kenjiro , Wilke Günther 
Published: December 2003

AbstractThe following report describes the preparation and reactions of organocobalt compounds, especially the thermally unstable tris(π‐allyl)cobalt and cyclooctenylcobalt compounds. The stoichiometric and catalytic transformations of these highly reactive organocobalt systems give an insight into the rôle of cobalt in homogeneous catalysis involving alkenes and alkynes.
Polymers as Reagents in Organic Synthesis
Authors: Overberger C. G. , Sannes K. N. 
Published: December 2003

AbstractPolymeric reagents offer a range of unique advantages. Intramolecular reactions, such as the cyclization of peptides or Dieckmann cyclizations, can be brought about by attaching the reactive molecules at remote points along a polymer chain. In contrast, if the reactive molecules are anchored in close proximity on a polymeric support then intermolecular reactions occur, as demonstrated for a ketone synthesis. Apart from such proximity effects, cooperative effects can also be exploited, e.g. in the formation of a 2:1 complex of a crown ether and an alkali metal ion. The hydrolysis of an ester with polyvinylimidazole provides a detailed illustration of hydrophobic interactions. The simple separation of polymer‐bound and low‐molecular compounds by filtration is another significant advantage.
Oxidative Coupling via Organocopper Compounds
Authors: Kauffmann Thomas 
Published: December 2003

AbstractThe type of reaction described here, which achieved preparative importance before the turn of the century in the form of the Glaser reaction, has found new applications in recent years: Syntheses of 1,3‐dienes, 2,3,6,7‐tetraaza‐1,3,5,7‐tetraenes, 2,3‐diaza‐1,3‐dienes, hydrazine derivatives, bis(heteroallyl) compounds, hetarenes, polyhetarenes, cyclopolyarenes, protophanes, phanes, and heteraprotophanes. The reaction, which consists in the metalation of the starting substance followed by oxidative coupling of the metal derivative, merits special preparative interest when several successive coupling processes are possible (e. g. polyyne, polyarene, cyclopolyarene, and protophane syntheses). The main factors limiting its applicability are the high thermal stability of some organic copper compounds and the disproportionation of the ligands that frequently occurs as a competing reaction. Selective asymmetric linkages are not generally possible by this method.
The Reactivity of the Porphyrin Ligand
Authors: Fuhrhop Jürgen‐Hinrich 
Published: December 2003

AbstractThe physical properties and the chemical reactivity of the porphyrin macrocycle, whose formation can be rationalized by a simple statistical argument, are related in many respects to those of aromatic hydrocarbons. The reactivity of the porphyrin ligand in its metal complexes varies within wide limits because of the difference in the inductive effects of different central metal ions on the conjugated π‐electron system. One‐electron reactions, phlorin formation by addition of hydrogen or oxygen to the methine bridges, and the formation of π complexes predominate. The biochemical reactivity of the porphyrin ligand largely corresponds to its in‐vitro chemistry.
The Synthesis of Unusual Organic Molecules Using Cyclic Peroxides. New synthetic methods (3)
Authors: Adam Waldemar 
Published: December 2003

AbstractThe photodecarboxylation of malonyl peroxides into α‐lactones[1] and the thermal conversion of the 1,4‐endo‐peroxide 4,5‐epoxy‐3,6‐epidioxy‐1‐cyclohexene into the novel benzene trioxide[2] are two recent examples of the potential of cyclic peroxides in the synthesis of unusual organic molecules. The former transformation entails a fragmentation, the latter a rearrangement process. Most reported examples fall into one of these two gross reaction types. Of the numerous examples that have been reported in the literature during the last two decades, only those shall be focused on that lead to unusual compounds or constitute efficient syntheses of known compounds, in order to stress the convenience of cyclic peroxides in the synthesis of organic compounds.
Dinitrogen‐Transition Metal Complexes: Synthesis, Properties, and Significance
Authors: Sellmann Dieter 
Published: December 2003

AbstractCompared to the large number of CO complexes, N2 complexes are still rare. In certain cases they may be formed from N2 gas and metal compounds under physiological conditions, and are therefore frequently considered as possible intermediates in N2 assimilation. However, despite numerous attempts it has not yet been possible to reduce the N2 ligand of a fully characterized N2 complex to NH3. This negative evidence recently prompted the discoverers of the first dinitrogen complex, the [Ru(NH3)5N2]2⊕ ion isolated in 1965, to express doubt whether such complexes really do play a part in the enzymatic reduction of N2. The latest findings nevertheless incline toward a somewhat more optimistic view. Thus, the mild partial reduction of the N2 ligand in metal complexes with inert gas configuration, hitherto considered nonreducible, justifies the hope that a suitable system permitting the catalytic reduction of molecular nitrogen under normal conditions will one day be found. Even apart from any technical potential, the N2 complexes constitute an interesting chapter of modern inorganic chemistry.
The Helicenes
Authors: Martin Richard Henri 
Published: December 2003

Abstract“Helicene” is the name introduced by Newman in 1955, to describe the benzologues of phenanthrene in which the extra ortho‐condensed rings give rise to a (regular) cylindrical helix. The pioneer work of Newman in this field cannot be overemphasized; his brillant synthesis and resolution of [6]helicene, achieved eighteen years ago, will remain as a landmark, for it opened the way to the study of a fascinating class of synthetic molecules. In the following review, an attempt is made to summarize the present state of our knowledge in this rapidly expanding field.
Fixation of Atmospheric Nitrogen by Microorganisms
Authors: Kleiner Diethelm 
Published: December 2003

AbstractResearch on the enzymatic assimilation of molecular nitrogen (N2 fixation) has made decisive advances in recent years. Both the purely scientific aspects of this process and its industrial implications are assuming ever‐increasing interest. Some of the results most important from the biochemical point of view have been collected in the present report.
Relationships between the Chemistry of Dispersed Metals and Chemical Syntheses with Metal Atoms
Authors: Kohlschütter Hans Wolfgang 
Published: December 2003

AbstractAn interesting and topical question in modern inorganic chemistry is: To what extent are the well‐known chemistry of dispersed metals and the new technique of chemical synthesis with free metal atoms interrelated? Or in other words: Where, do these two facets of metal chemistry meet? In an attempt to glean an answer to this question three important aspects are considered: metal sols, activated metals as powders, and deposits in vacuum processes (evaporation and cathodic sputtering of metals).
Prostaglandins
Authors: Bartmann Wilhelm 
Published: December 2003

AbstractThe prostaglandins are lipid‐soluble C20‐carboxylic acids containing a five‐membered ring, several oxygen function, and at least one double bond. They are found in animal organisms where they stimulate, inter alia, the smooth muscle. In the last decade more has been learned about the occurrence, biosynthesis, metabolism and pharmacological properties of the prostaglandins. Analytical methods for their determination, and syntheses for the naturally occurring prostglandins and their synthetic analogs have been developed. In this report, an attempt is made to show why new knowledge has dramatically stimulated pharmaceutical research in this field.
Microencapsulation
Authors: Sliwka Wolfgang 
Published: December 2003

AbstractMicroencapsulation is the enveloping of liquid droplets or fine solid particles to form microscopic capsules 1 to 5000 μm in diameter. For the fabrication of such capsules special techniques have been developed which take into account the substance to be encapsulated and the intended use of the capsules. The envelopes, consisting of natural or synthetic polymers, may be made permeable, semipermeable, or impermeable. Thus, there are several possibilities for controlled release of an encapsulated reactive substance, for example by destruction of the envelope or by permeation: alternatively, reactions can be allowed to take place within the microcapsules. Microencapsulation facilitates delivery of many substances to their site of utilization in a novel form of preparation and novel mode of dosage, the solids being dispersed very finely and having a very large surface area.