bluuebunny/crossref_metadata_2025_split huggingface dataset:
Methods of Reactivity Umpolung
Authors: Seebach Dieter 
Published: December 2003

AbstractThe past decade of organic chemistry may be charcterized as a period of violent development of new synthetic methods. This was accompained by a systematization of the analysis of synthetic problems (synthetic strategy). The planning of the synthesis of an organic target molecule is greatly facilitated by distinguishing between reagents X(C)n… with normal reactivity (acceptor properties at C1,3,5…, donor properties at X, C2,4…) and with reactivity umpolung (acceptor properties at X, C2,4…, donor properties at C1,3,5…). In this context, reactivity umpolung turned out to be useful as a heuristic principle, as a classification scheme, and as an aid for locating so‐called strategic bonds (synthon, transform, and antithesis according toE. J. Corey). There are six principal methods of umpolung: 1 2n‐oxidation, heteroatom exchange and modification, homologation and its reversal, the cyclopropane “trick”, use of acetylenes, and redox reactions; under certain circumstances none of these techniques is necessary in cases where direct umpolung is possible. Throughout the article, normal reactivity is indicated by green print; reactivity umpolung by red print.
Kinetics of Sorption, Desorption, and Diffusion in Zeolites
Authors: Doelle Hans‐Joachim , Riekert Lothar 
Published: December 2003

AbstractSorption and desorption in zeolites (molecular sieves) have to be considered as complex processes, involving simultaneous diffusion in zeolite crystals, mass transfer in the intercrystalline void of a pellet, and heat transfer between the zeolitic sorbent and its surroundings. The kinetics of sorption and desorption, respectively, of n‐C4H10 in zeolite X and of CO2 in zeolite A have been investigated: only the initial rates of uptake or release of the sorbet are controlled by mass transfer alone whereas ultimately they also depend on the rate of heat transfer from the sorbent to its surroundings or vice versa. Diffusivities of the sorbate in the zeolite crystals can be obtained from the kinetics of mass transfer, provided the resistance due to viscous or Knudsen flow between the crystals of the zeolitic sorbent can be eliminated. A sample consisting of a monolayer of single crystals had to be used for this purpose in the n‐butane/zeolite X system; the intracrystalline diffusivity obtained in this way is not in conflict with data obtained by NMR spectroscopy. The intracrystalline diffusivities obtained in this way—taking into account the coupling of several processes during sorption — are higher than values reported in the literature.
Circular Dichroism and Absolute Conformation: Application of Qualitative MO Theory to Chiroptical Phenomena
Authors: Snatzke Günther 
Published: December 2003

AbstractAlthough chiroptical methods have been used in chemistry for some 150 years, and the basic theory was developed byRosenfeld50 years ago, their success in determining absolute conformation rests mainly on empirical rules. Some of these can be proved theoretically; however, in the case of more complex molecules this is not yet always possible. In what follows, an attempt is made to bridge the wide gap between theory and empiricism: “qualitative MO‐theory”, which has already been successfully used to explain reaction mechanisms, the shape of molecules, or photoelectron spectra, can also be applied to circular dichroism (CD). In general we shall have to be content with a reasonable interpretation of the experimentally determined sign correlation, but even this will be of great value since it can yield general relations between the geometry of molecules and the sign of CD. Such relations become particularly important when only a few examples of a given chromophore are known. If first and second sphere of a molecule are achiral then sector rules can be put forward, and in other cases helicity or chirality rules.
Ureas as Solvents for Chemical Investigations
Authors: Barker Barbara J. , Rosenfarb Joseph , Caruso Joseph A. 
Published: December 2003

AbstractN‐Alkylated ureas, including cyclic derivatives, are readily obtained pure. These stable compounds are already used as industrial solvents owing to their considerable solvent power, their wide liquid range, and their favorable dielectric constants and dipole moments. They are being increasingly employed as reaction media for electrochemical and analytical studies.
The Chemistry of Iodine Azide
Authors: Dehnicke Kurt 
Published: December 2003

AbstractWhereas chlorine azide and, to a lesser extent, bromine azide have long been used as preparative reagents in spite of their explosive nature, little is known about the properties of iodine azide although it was actually discovered before the other two compounds. Hantzsch's warning to beware fo this material, uttered in 1900, still seems to be widely observed. Only recently has iodine azide been isolated as a pure substance, thus promoting its more detailed investigation. Proper use of this highly labile reagent opens up new vistas in synthetic chemistry.
Criteria for the Efficiency, Stability, and Capacity of Abiotic Photochemical Solar Energy Storage Systems
Authors: Scharf Hans‐Dieter , Fleischhauer Jörg , Leismann Hans , Ressler Ingrid , Schleker Wolf‐gang , Weitz Robert 
Published: December 2003

AbstractThe utilization of simple photochemical reactions for the storage of solar energy in the form of chemical energy in energy‐rich products has often been considered in the further development and improvement of e. g. simple thermosolar techniques. The hitherto proposed criteria for the qualification of an abiotic photochemical system are, however, mostly of a qualitative nature, so a mutal comparison of the systems is not precise enough. In this article it is shown how a useful correlation on the basis of time‐independent experimental data can be achieved and how, from the viewpoint of photochemistry, a comparative classification of known reactions is possible. The following reactions are compared: the [2 + 2]‐photocycloadditions of norbornadiene, dimethyl 2,3‐norbornadienedicarboxylate, and dicyclopentadienone, the photoisomerization of trans‐ to cis‐diacetylindigo, the photodissociation of nitrosyl chloride as well as a photocatalytic redox reaction. The quantity of material required and storage efficiency are by far the most favorable in the case of trans‐diacetylindigo. The main disadvantage of the latter however, is that the energy‐rich cis‐from rapidly reverts to the stable trans‐form at elevated temperatures.
Catalytic Reactions with Hydrosilane and Carbon Monoxide [New synthetic methods (30)]
Authors: Murai Shinji , Sonoda Noboru 
Published: December 2003

AbstractThe reagent hydrosilane/carbon monoxide opens up new possibilities for organic synthesis. Four cases will be discussed: 1. The reaction of olefins with hydrosilane (trialkylsilane) and carbon monoxide in the presence of Co, Ru, and Rh complexes leads to enol silyl ethers having one more carbon atom that the olefins. 2. Cyclic ethers underto carbonylative ring opening to ω‐siloxyaldehydes when reacted with hydrosilane and carbon monoxide in the presence of Co2(CO)8 as catalysts 3. Aldehydes are catalytically converted into the next higher α‐siloxyaldehydes or 1,2‐bis(siloxy)alkenes depending on the reaction conditions used. 4. The reaction of alkyl acetates proceeds in various ways depending on the nature of the alkyl group; enol silyl ethers or alkenes are optained.–Mechanisms of these Co2(CO)8 catalyzed reactions using hydrosilane and carbon monoxide are discussed in which HCo(CO)n or R3SiCo(CO)nL function as catalytically active agents. With these species there are four types of catalytic cycles.–The synthetic possibilities of these catalytic reactions have still not been fully explored.
Nodal Nomenclature–General Principles
Authors: Lozac'h Noël , Goodson Alan L. , Powell Warren H. 
Published: December 2003

AbstractA new, comprehensive graph‐based nomenclature system for chemical substances is proposed. Substances are named by using nodal nomenclature to name graphs derived from their structures by ignoring chemical details such as atom identities, bonding, charges, etc. After the fundamental graph is named, these chemical details are introduced by modifications and additions to the name of the graph to yield the name of the actual substance. Although development of nodal nomenclature originated with problems in naming some organic systems, it is not limited to organic substances. In this paper, only general principles for naming the graph are described; applications to specific substances will be discussed in future papers.
New Technologies for the Filmless Manufacture of Printing Forms
Authors: Vollmann Hansjörg W. 
Published: December 2003

AbstractThe economic reproduction of graphic information was first made possible in the western world by Gutenberg's invention of movable type. In the printing methods of that time, mechanical transfer processes played the major role, while chemical processes were of secondary importance. When Alois Senefelder invented planographic printing (lithoprinting) at the end of the 18th century he called the new process “chemical printing”. Since then, chemistry has attained great importance in the production of printing forms: Thus the manufacture of surface (litho‐) printing forms with light required temporary storage materials and image reproduction processes based on chemical reactions. Since the advent of electronic processing of information, however, several former temporary storage media have become obsolete; newer, more sensitive image reproduction processes are increasingly making less use of chemical principles than of the electrical properties of materials.
Heterocyclic Analogues of Methylenecyclopropanes
Authors: L'abbé Gerrit 
Published: December 2003

AbstractHeterocyclic three‐membered rings with an exocyclic double bond merit special interest in view of their ring strain and their fascinating chemistry. Their synthesis, thermal decomposition, ring opening, and cycloaddition reactions will be discussed in this review. α‐Lactams, alkylideneaziridines, diaziridinones, and diaziridinimines belong to the best known classes, but the chemistry of alkylideneoxiranes and ‐thiiranes, α‐lactones, α‐thiolactones, thiiranimines, and aziridinimines has also been studied to some extent. Two elusive three‐membered rings are oxiranimines and thiaziridinimines; the former were postulated as reactive intermediates in the thermal decomposition of α‐lactams, whereas the latter were trapped efficiently with double and triple bonds during thermolysis of N‐sulfonyliminothiatriazolines. In spite of many attempts to prepare thione derivatives of the title compounds, they are still unknown. Included in this review are also some open‐chain dipolar species which are isomeric with the heterocyclic three‐membered rings.
Cyclodextrin Inclusion Compounds in Research and Industry
Authors: Saenger Wolfram 
Published: December 2003

Abstractα‐, β‐, and γ‐Cyclodextrins are cyclic oligosaccharides consisting of six, seven, or eight glucose units, which can be obtained on a large scale from starch. They form inclusion compounds with smaller molecules which fit into their 5—8 Å cavity. These (crystalline) complexes are of interest for scientific research as, contrary to the classical clathrates, they exist in aqueous solution and can be used to study the hydrophobic interactions which are so important in biological systems. Cyclodextrins also serve as models both for polymeric starch and, in the form of their polyiodide complexes, for “blue iodine‐starch”. As cyclodextrins catalyze several chemical reactions they and their functionalized derivatives provide useful enzyme models. Cyclodextrins can be used to advantage in the production of pharmaceuticals, pesticides, foodstuffs, and toilet articles—the (micro‐encapsulated) active and aromatic substances enclosed within them are protected from the effects of light and atmosphere and can be easily handled and stored in powder from. Substances which are not very soluble in water become more soluble in the presence of cyclodextrins—creams and emulsions can be stabilized, and the growth and yield of grain harvests can be increased. Cyclodextrins can be chemically modified for many different purposes; polymerized cyclodextrin or cyclodextrin bound to a polymer carrier have already been employed in gel inclusion and affinity chromatography.
The Role of Pyridoxal Phosphate in the Catalysis of Glycogen Phosphorylases
Authors: Helmreich Ernst J. M. , Klein Helmut W. 
Published: December 2003

AbstractGlycogen phosphorylases catalyze the degradation of glycogen by phosphate (or arsenate) to glucose 1‐phosphate (or glucose + arsenate). All glycogen phosphorylases that have been studied so far contain pyridoxal 5′‐phosphate, a vitamin B6‐derivative, as cofactor. Removal of the cofactor results in an inactive apoenzyme. However, reduction of the azomethine bond linking pyridoxal phosphate to an ϵ‐aminolysyl side chain of the enzyme with NaBH4 does not inactivate glycogen phosphorylase. If therefore the cofactor should be involved in catalysis in glycogen phosphorylase it must function differently from all other classical pyridoxal phosphate dependent enzymes, for these are inactivated by reduction. 31P‐NMR spectroscopy has revealed that the 5′‐phosphate group of pyridoxal phosphate is present in catalytically active forms of glycogen phosphorylases as dianion in a hydrophobic environment shielded from aqueous solvent. Covalent and/or allosteric activation of muscle glycogen phosphorylases is accompanied by a transition of the monoprotonated form to the dianionic form of the phosphate group of the cofactor. We now report on such ionization changes in unregulated active potato‐ and E. coli maltodextrin phosphorylases on binding of glucose and oligosaccharides and following catalytic turnover, i.e. arsenolysis of α‐1,4‐glycosidic bonds. (Like glycogen phosphorylases, maltodextrin phosphorylases belong to the class of α‐glucan phosphorylases.) The results of experiments carried out by our group together with recent findings on the three dimensional structure of crystalline muscle glycogen phosphorylases indicate a participation of the dianionic phosphate group as proton acceptor for the glucosyl transfer to and from the glucosyl acceptor. Although other interpretations are not excluded, at present little doubt remains that in the case of glycogen phosphorylases the dianionic phosphate group of the cofactor functions in catalysis.
Photophysical and Photochemical Processes in Micellar Systems
Authors: Turro Nicholas J. , Grätzel Michael , Braun André M. 
Published: December 2003

AbstractA fascinating feature inherent to aqueous surfactant solutions is the phenomenon of self‐organization: above a certain critical concentration (the critical micelle concentration, CMC) detergent molecules associate spontaneously to build up structural entities of colloidal dimensions called micelles. The architecture of these agglomerates is such that the interior contains the hydrophobic alkyl chain of the amphiphile while the hydrophilic head groups are located at the surface and are in contact with bulk water. In the case of ionic micelles the interface is charged giving rise to an electrical double layer and a potential difference of up to several hundred millivolts between the micellar pseudophase and water. Thus micellar systems are microheterogeneous in character: the electrostatic potential and polarity prevailing in the interior of the aggregate differ from those of the bulk aqueous phase. A particularly attractive aspect of photochemical studies in micellar systems is the possibility of organizing the reactants at a molecular level: by comparison of the data in micelles with similar data in homogeneous solution one can learn about the molecular details of a given reaction and establish which conditions favor one pathway or another. In simple surfactant systems differences in rate and efficiency of a reaction will often be controlled by local electrostatic potentials and the compartmentalization of the reagents within the surfactant aggregates. Through the latter effect the statistics of probe distribution over the micelles becomes important in controlling fast photochemical events. Functional micelles are distinguished by the fact that the surfactant molecule contains a group which itself participates in the photoprocess. These units are unique in that self‐assembly often introduces striking cooperative effects.
Mechanistic Aspects of Diels‐Alder Reactions: A Critical Survey
Authors: Sauer Jürgen , Sustmann Reiner 
Published: December 2003

AbstractThe question of concerted or consecutive bond formation arises in all types of cycloaddition reactions. Stereochemical investigations and studies of regiospecificity as well as intensive kinetic investigations with regard to substituent and solvent effects, and the dependence of the reaction on temperature and pressure permit in many cases mechanistic conclusions concerning the Diels‐Alder reaction. Efforts towards a theoretical interpretation, ab initio and semiempirical calculations, the application of frontier molecular orbital theory (FMO) as well as thermochemical measurements permit a description of the energy hypersurface of these [4+2]‐cycloadditions. An attempt is made here to draw a line of distinction between the mechanistic alternatives—one‐step reaction versus two‐step reaction—considering all experimental and theoretical criteria.
Chemistry in Compressed Solutions
Authors: le Noble William J. , Kelm Hartwig 
Published: December 2003

AbstractContrary to “common sense” impression, modest pressures often have quite large effects on reactions in solution.—The volume profile of a chemical reaction in solution is easily measurable with considerable precision by means of the effect of pressure on the rate and equilibrium constant. The factors that govern the magnitude of these pressure effects are similar to those that affect the entropy changes and there is a rough correlation between them; however, the volume parameters are much less subject to apparently random fluctuations, have much greater appeal to the solution chemist's intuition, and most important, the activation volume is the only transition state property that can readily be determined in absolute terms (rather than as a difference value). In this paper we outline the experimental approach to the measurement of volume changes in wet chemical reactions, interpret the volume quantities, discuss a number of simple model equilibria and reactions, point out a number of contributions in both mechanistic and synthetic chemistry, and make an attempt to foresee future developments.
Reversible Topotactic Redox Reactions of Solids by Electron/Ion Transfer
Authors: Schöllhorn Robert 
Published: December 2003

AbstractSolids having suitable structural and electronic properties are able to form intercalation compounds by reversible redox reactions at room temperatureviatopotactic electron/ion transfer processes. The host lattices range from inorganic solids with different structural dimensionality to organic molecular solids. Similarly, depending on the host lattice type, the guest species may vary from protons and metal ions to large inorganic and organic molecular ions. The possibilities of a systematic “tailoring” of new stable or metastable compounds, the controlled modification of physical properties of solids, and the technical application of electronic/ionic conductors, provide a wide and attractive field for academic and applied research in an interdisciplinary area that involves solid state chemistry and physics, molecular chemistry, electrochemistry, and interface science.
Homoatomic Bonding of Main Group Elements
Authors: von Schnering Hans Georg 
Published: December 2003

AbstractClusters of main group elements are not rare. On the contrary, it is becoming difficult to avoid the discovery of new substances of this type. Clusters are the natural intermediate stages between an element and its isolated atoms or ions. In the form of polycations and polyanions they offer models for the stepwise oxidation and reduction of an element and represent a bridge between the elements. The great majority of homonuclear bonded structures are already present in the solid phases of simple systems. Mobilization of these clusters as molecules represents a great challenge.
Ternary Sulfides: Model Compounds for the Correlation of Crystal Structure and Magnetic Properties
Authors: Bronger Welf 
Published: December 2003

AbstractRelationships between crystal structure and magnetic properties enable an insight into the nature of the binding of atoms or ions in the solid state. Suitable as model substances are transition metal compounds in which the collective bonds that are generally present are directively degraded by incorporation of diamagnetic cations. This requirement of progressive degradation is met with in sulfides of the general composition AxMySz, where A ≙ alkali metal and M ≙ transition metal.
Toxication of Foreign Substances by Conjugation Reactions
Authors: Reichert Dieter 
Published: December 2003

AbstractElucidation of the metabolic pathways of foreign compounds in mammalian organisms contributes substantially to the understanding of toxic effects and is therefore a basic component of every risk analysis. The abundance of chemical reactions which take part in metabolic transformations allows one to speculate that we, with our present state of knowledge, are just at the beginning of a development which will help explain the interplay between chemical structure, biochemical transformation and toxic effects. This applies in particular to the conjugation of foreign compounds with structures and molecules endogenous to the body. Until recently it was thought that these conjugation reactions lead to chemically and biologically inert products, which could be easily eliminated by the organism. Using new biological testing procedures and sensitive methods of analysis, this assumption has been refuted. The fact is, that highly toxic, mutagenic and carcinogenic products can result from the chemical interactions of foreign substances with endogenous substrates.
Advances in Organophosphorus Chemistry Based on Dichloro(methyl)phosphane
Authors: Weissermel Klaus , Kleiner Hanns‐Jerg , Finke Manfred , Felcht Utz‐Hellmuth 
Published: December 2003

AbstractBesides phosphorus trichloride and phosphane, dichloro(methyl)phosphane is gaining importance as a starting material for the synthesis of organophosphorus compounds. It provides ready access to phosphonic, phosphinic and phosphonous acid derivatives, as well as their secondary products. The synthetic and application potential of organophosphorus compounds—based on industrially produced dichloro(methyl)phosphane—is illustrated by means of numerous examples.