bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Photolysis of Iodoaromatic Compounds
Authors: Sharma R. K. , Kharasch N. 
Published: December 2003

AbstractUnder appropriate conditions, the photolysis of iodoaromatic compounds yields derivatives of biphenyl and phenanthrene, as well as organometallic compounds. Ring closures of o‐terphenyls to triphenylenes and of benzanilides to phenanthridones are described. The formation of dehydrobenzene (benzyne) on photolysis of 1,2‐diiodobenzene in solution has been established. The photolysis of various substituted iodoaromatic compounds or of substances such as CH3I, ICN, ICH3, COOH, or RCCI, provides a general method for the generation of a large variety of free radicals in solution. The potential uses of photochemically initiated exchanges of iodine in ArI for 131I, Li, D, Cl, or Br are noted.
Methods and Results of Optical Investigations on Semiconductor Interfaces
Authors: Beckmann K. H. 
Published: December 2003

AbstractMeasurement of the state of polarization of reflected light (ellipsometry) permits the determination of the thickness and refractive index of thin layers formed on a surface. The IR absorption spectra of such thin layers, which can be measured by means of internal reflection spectroscopy (IRS), provide information about their chemical composition. These methods have been used to study adsorption processes and the formation of reaction layers at semiconductor interfaces, and may also be used for measurements of free charge carriers in the space‐charge region and in surface states. Results of such investigations are given in this article.
Reactions with N‐Carbonylsulfamoyl Chloride
Authors: Graf R. 
Published: December 2003

AbstractN‐Carbonylsulfamoyl chloride (NCSA) is an intermediate that can be obtained from cyanogen chloride and sulfur trioxide, and which reacts with many compounds, via electrophilic substitution, much more readily than other isocyanates. Reactions with compounds containing hydroxyl, amino, or carboxyl groups lead to N‐chlorosulfonylurethanes, ‐ureas, and ‐amides, respectively. This group of products also includes the parent substance sulfamoyl chloride (amidosulfonyl chloride). The reaction of N‐carbonylsulfamoyl chloride with olefins provides an easy route, not only to derivatives of β‐amino acids and unsaturated carboxylic acids, but also to many β‐lactams. New, very reactive sulfonylisocyanates of several types can be obtained by further reactions of primary products obtained from N‐carbonylsulfamoyl chloride and by the reaction of N‐carbonylsulfamoyl chloride with olefins in the presence of free‐radical formers. When allowed to react with a number of compounds, NCSA undergoes loss of carbon dioxide to form products containing the grouping = NSO2Cl.
The Discovery, Isolation, Elucidation of Structure, and Synthesis of Antamanide
Authors: Wieland Th. , Lüben G. , Ottenheym H. , Faesel J. , De Vries J. X. , Prox A. , Schmid J. 
Published: December 2003

AbstractAntamanide is the name we have given to a constituent of the fungus Amanita phalloides. This substance counteracts the lethal action of the Amanita toxins phalloidine and α‐amanatine if administered to the white mouse before, or simultaneously with, the poisons. Its concentration in the fungus is, however, so low that the toxic action of the latter predominates. Antamanide is a cyclic decapeptide formed from the L‐amino acids alanine, phenylalanine, proline, and valine in the molar ratio 1:4:4:1. The amino‐acid sequence was determined by a combination of gas chromatography and mass spectrometry. The structural formula assigned was confirmed by synthesis according to a classic route of peptide chemistry.
Problems and Possibilities of Modern Trace Analysis
Authors: Specker H. 
Published: December 2003

AbstractAs a result of the development of measuring techniques, analytical chemists can now determine very small quantities as well as trace concentrations of the order of 10−8% with good accuracy. In exceptional cases it is even possible to detect 105 atoms of an element. In this review, definitions of important analytical terms are outlined and a critical appraisal of the principal methods of trace analysis is given.
Conductance of Electrolyte Solutions
Authors: Barthel J. 
Published: December 2003

AbstractThe present article deals with methods and results of conductance studies with special reference to nonaqueous solvents, e.g. ethanol, dioxane, dimethylformamide, acetonitrile, nitromethane, and acetone, in some cases mixed with water, as well as NH3, SO2, I2, etc. On the basis of a model in which the ions are represented as charged spheres and the solvent as a homogeneous isotropic medium, the conductance theory can (at least for low electrolyte concentrations) give a more or less complete representation of the problem. A new statistical treatment of the transport problem confirms important results of this representation. The present state of the investigation is finally outlined in a review of recent publications.
Cycloadditions — Definition, Classification, and Characterization
Authors: Huisgen R. 
Published: December 2003

AbstractThe growing volume of publications on cycloadditions indicates that this general type of reaction merits a place beside the better‐known substitutions, eliminations, and additions. The world of phenomena does not provide us with definition and classification principles. These are created by the scientist and are not free from arbitrariness. A terminology[1] is suggested for cycloadditions. These reactions do not all proceed by the same mechanism. Since ideas about reaction mechanism can change with the introduction of new criteria, it is shortsighted to base definitions and classifications on mechanism alone. Consequently, the definition of cycloadditions will not be restricted simply to multi‐center processes with a cyclic electron shift.
Carbonylation of Olefins under Mild Temperature Conditions in the Presence of Palladium Complexes
Authors: Bittler K. , Kutepow N. V. , Neubauer D. , Reis H. 
Published: December 2003

AbstractDuring a search for catalysts that allow carbonylation reactions on olefins to proceed below 100 °C, complex palladium(II) compounds having the formula LmPdXn were found to be catalytically active. L denotes a ligand such as a phosphine, nitrile, amine, or olefin, X is an acid residue, and m+n is 3 or 4. The catalysts permit the carbonylation of heat‐sensitive compounds, as well as selective carbonylation of polyunsaturated olefins. The new process can also be carried out on the industrial scale, as is shown by the carbonylation of cyclododecatriene
Nitrogen Oxide Fluorides
Authors: Schmutzler R. 
Published: December 2003

AbstractOxyfluorides of nitrogen, i.e. compounds containing the grouping FNO, have been known for many years in the form of simple compounds, such as NOF and NO2F. Detailed studies of physical, structural, and chemical aspects of this class of compounds have been conducted only in recent years, after the potential of NF compounds as rocket propellants had been recognized. Several novel types of oxyfluorides of nitrogen, such as difluorohydroxyl amines, RONF2, or trifluoroamine oxide, F3NO, have been discovered recently. A further major development in ONF chemistry is indicated by the discovery that compounds of great chemical potential are formed between HF and NOF or NO2F, respectively.The compound O2NOF, although not an oxyfluoride of nitrogen, will also be discussed in this review, because of its particular chemical relationship to NO2F.
Free Radical Studies at Low Temperatures
Authors: Mile B. 
Published: December 2003

AbstractFree radicals can be produced in the gas phase and then condensed together with the molecules of an inert gas; they can also be generated and trapped in isolated sites in rigid solids. IR and ESR spectra of trapped radicals provide information concerning their structure and chemical properties. The techniques employed for producing and studying radicals are reviewed and a number of recent IR and ESR investigations of trapped radicals are discussed. The last part of the article deals exclusively with the applications of the rotating cryostat to the study of the ESR spectra and reactions of trapped radicals at low temperatures.
Modification of the Orientation of Substitution Reactions on Thiophene and Furan Derivatives
Authors: Goldfarb Ja. L. , Volkenštein Ju. B. , Belenkij L. I. 
Published: December 2003

AbstractUnder normal conditions, thiophene and furan derivatives are substituted in the α position, and no convenient alternative methods for the preparation of β‐substitution products have been available until now. The present article describes a method that permits the synthesis of many β‐substituted thiophenes and furans. In this method, the carbonyl group in α‐aldehydes or ketones of the thiophene and furan series is blocked by complex formation with an excess of aluminum chloride, so that electrophilic substitution takes place in position 4. In another useful method, the carbonyl group is blocked by acetalization. The acetals can be metalated in the ring by organolithium compounds.
Syntheses of Cyclopropylcarbonyl Compounds
Authors: Conia J. M. 
Published: December 2003

AbstractThere are three important direct routes to cyclopropylcarbonyl compounds: 1. Cyclization of chains of three carbon atoms, the first or third of which is adjacent to a carbonyl or potential carbonyl carbon atom (this type includes syntheses by intramolecular alkylation of γ‐halogeno ketones or related compounds in alkaline media); 2. insertion of a methylene group or substituted methylene group into the olefinic double bond of an α,β‐unsaturated carbonyl compound; and 3. introduction of an acetonyl group into the double bond of an olefin. However, cyclopropylcarbonyl compounds can also be obtained from 1,2‐epoxycyclobutane and 2‐bromocyclobutanone derivatives by ring contraction. Another possibility is the dehalogenation of α,α‐bis(bromomethyl) cycloalkanones. This review concludes with a discussion of these little known routes and of a particularly suitable method which involves the reaction of methylene iodide, a ZnCu couple, and α,β‐unsaturated ketones.
Observations on the Crystal Structures of Electron Donor‐Acceptor Complexes
Authors: Prout C. K. , Wright J. D. 
Published: December 2003

AbstractThe theoretical interpretation of electron donor‐acceptor complex formation in terms of charge transfer interactions has stimulated many structure determinations for these complexes. These fall into three classes, depending on the type of orbitals involved in charge transfer. In σ‐σ complexes, intermolecular bonds become shorter and intramolecular bonds become longer as charge transfer increases. Relative orientations correspond to overlap of donor and acceptor molecules in directions of “preferred polarizability”. Intermolecular bond lengths in σ‐π complexes show similar trends, and the axial orientation in the benzene‐halogen complexes is probably the result of the best compromise between orbital overlap and energy factors. π‐π Complexes contain stacks of alternate plane‐to‐plane donor and acceptor molecules, arranged in three characteristic ways. There is little correlation between interplanar spacing in these stacks and charge transfer properties. The relative orientations of donor and acceptor molecules within the stacks are determined by a combination of charge transfer interactions (maximized when aromatic rings of donor and acceptor molecules are displaced by half a ring diameter) and dipole‐induced dipole interactions (maximized, for example, when a polar bond of one molecule overlaps a polarizable region of another). Crystal packing requirements and dispersion forces modify these effects, and no satisfactory theoretical treatment of this complex combination of interactions is yet available.
On the Chemistry of Clay
Authors: Hofmann U. 
Published: December 2003

AbstractKaolinite, illite, chlorite, montmorillonite, and vermiculite are among the most important of the clay minerals. Cations are embedded between the silicate layers and on the basal faces of the crystals. Whereas only the cations on the outer faces are exchangeable in the first three of the above minerals, those between the layers can also be replaced by others in montmorillonite and vermiculite. These characteristics and the ability of montmorillonite and vermiculite to undergo intracrystalline swelling and to form inclusion compounds are responsible for the industrial importance of kaolin and clay. The structure of halloysite is particularly interesting, since the silicate layers in this case are rolled up to form tabes. The possible role of the clay minerals as catalysts in the formation of petroleum and in the beginning of life is finally discussed.
Replacement of Plasma by High Molecular Weight Substances
Authors: Appel W. , Biekert E. 
Published: December 2003

AbstractIn the course of the development work, natural and synthetic polymers were examined to see whether they were suitable for use as plasma substitutes. The criteria of a plasma substitute are discussed in this connection. The most important of the base substances used at present (dextran, modified gelatin, polyvinylpyrrolidone, and hydroxyethyl starch) are described, and a typical example of proven therapeutic value is discussed for each type.
The Cytogenetics of Human Tumors
Authors: Zang K. D. , Singer H. 
Published: December 2003

AbstractChromosome changes can be observed in many tumors, suggesting that a connection exists between chromosome aberrations and tumor formation. A general introduction to the techniques and objectives of research in cytogenetics is followed by a discussion of the modern theories of the etiology of tumors, using human meningioma as an example. Meningiomas are relatively benign, slowly growing tumors of the meninges.
Preparation, Properties and Reactions of Polychloroamine Derivatives
Authors: Holtschmidt H. , Degener E. , Schmelzer H.‐G. , Tarnow H. , Zecher W. 
Published: December 2003

AbstractHighly chlorinated amine derivatives can be prepared by the high‐temperature chlorination of amines and amine derivatives. Essential for the success of this reaction is an adequate temperature control suited to the constitution of the amines. In many cases the chlorination products achieve stability by cleavage reactions with formation of the imidic acid chloride or isocyanide dichloride grouping. Cyclic compounds are formed under special conditions. The chlorination process described can be used to obtain e.g. polychloroaryl and polychloroalkyl isocyanide dichlorides, polychloroaryl and polychloroalkyl imidic acid chlorides, and polychloroheterocyclic compounds.
The Gabriel Synthesis of Primary Amines
Authors: Gibson M. S. , Bradshaw R. W. 
Published: December 2003

AbstractReaction of potassium phthalimide with halogenoalkanes and with a variety of other alkylating agents leads to the N‐alkylphthalimide. N‐Substituted phthalimides may be converted into the corresponding primary amine by hydrolysis or hydrazinolysis. The scope and limitations of these reactions, which together constitute the Gabriel Synthesis of primary amines, are reviewed.
Mechanistic Organic Photochemistry
Authors: Zimmerman H. E. 
Published: December 2003

AbstractOrganic photochemical reactions can be understood as transformations of the electronically excited states of the reactant molecules. By considering Lewis structure or molecular orbital representations of these excited states it is possible to outline the several possible reactions available in the case of a given reactant. A number of different types of photochemical transformations are now reasonably well understood. In these cases one finds the same common controlling feature, namely the tendency for an excited state species to follow mechanistic pathways of minimum energy and the requirement for continuous electron redistribution in following these pathways. These preferred transformations can often be selected by inspection of relative bond orders for different types of bonding, by comparison of the potential energy surfaces available to the excited state molecules, and by use of correlation diagrams. The reactions derive from both singlet and triplet states, and one of the more reliable methods now available for identifying excited states reacting is termed the “fingerprint method”. Examples of the author's mechanistic approach are given both for ketone and for hydrocarbon photochemistry.
Reactions of Isocyanide Dihalides and their Derivatives
Authors: Kühle E. , Anders B. , Klauke E. , Tarnow H. , Zumach G. 
Published: December 2003

AbstractThe readily obtainable isocyanide dichlorides can be used as starting materials for the preparation of many types of substances. Exchange reactions yield numerous derivatives of formic and carbonic acids, while heterocyclic products have been obtained by reaction with bifunctional partners. The present article deals not only with reactions that are known from the literature but also with the author's own work, some of which has not been published before.
The Chemistry of Hypervalent Molecules
Authors: Musher J. I. 
Published: December 2003

AbstractMolecules formed from atoms of Groups V–VIII of the periodic table in their higher valences are studied and a general theory for their electronic and geometric structure is presented. These molecules, which we call hypervalent molecules, are unique in that a donor atom can possess two identical monovalent ligands which are bonded by different types of chemical bonds. The present study provides a simple theoretical description of such molecules using doubly‐occupied lone‐pair orbitals to form hypervalent bonds, and these bonds are divided into two classes as to whether they involve p electrons alone or utilize s electrons as well. While the theoretical classification is of some interest in itself, there are even more interesting problems concerning the physico‐chemical properties of these molecules and the syntheses of new molecules and polymers. We thus present a brief description of the energetics, possibilities for intermolecular inversion and site‐exchange, and synthetic paths for the preparation of hypervalent molecules, as well as indicate a number of new molecules which can probably be synthesized using these ideas.For corrigendum see DOI:10.1002/anie.196901311
Reactions of Aromatic Nitro Compounds with Bases
Authors: Buck P. 
Published: December 2003

AbstractAromatic nitro compounds can react in many ways with bases. The reaction path followed depends mainly on the degree of nitration (with a sharp jump from mononitro to di‐ and trinitro compounds) and the base used. Redox reactions or nucleophilic aromatic substitutions predominate with mononitro compounds, whereas Meisenheimer additions together with nucleophilic aromatic substitutions are preferred by dinitro and trinitro compounds; deprotonations are less common. The bromine in nitrobromobenzenes can be replaced by metal at low temperatures. Very weak bases generally give electron donor‐acceptor complexes.
The Composition and Conformation of Sugars in Solution
Authors: Angyal S. J. 
Published: December 2003

AbstractThe conformations of the aldoses in solution, and the compositions of the equilibrium mixtures formed from aldoses after dissolution in water, have been determined by NMR spectroscopy. Calculations based on non‐bonded interactions and on the anomeric effect account for the observed conformations, and allow an approximate prediction of the α:β ratio of pyranoses in equilibrium and of the extent of anhydride formation from sugars in acid solution. In certain circumstances the furanose forms are more stable than the pyranose forms.
Steps in Precipitation Reactions
Authors: Lieser K. H. 
Published: December 2003

AbstractThe combined application of various methods of investigation (e.g. nephelometry, conductivity measurements, electron microscopy, isotope exchange, BET surface area determinations, paper chromatograhy, coprecipitation) lead to a refined insight into the course of precipitation reactions. The formation of a new solid phase within a solution can, in the case of ionic crystals, be formally described as proceeding via a number of steps – nucleation, growth, ripening, and recrystallization (aging) – which overlap in time. The precipitation of hydroxides is a more complex process since additional chemical reactions (e.g. hydrolysis, condensation reactions) take place within the newly formed solid phase.
Biosynthesis of Lichen Substances, Products of a Symbiotic Association
Authors: Mosbach K. 
Published: December 2003

AbstractLichens are composed of algae and fungi living together in symbiotic association and forming characteristic, often polycyclic lichen substances that are rarely encountered in other organisms. While the cultures of the unassociated individual constituents of the lichens can live for an unlimited time some doubt surrounds their viability in nature; their biosynthetic powers are not additive in the lichens. The results concerning the biosynthesis of the lichens that are discussed in the present article were obtained mainly with 14C‐labeled compounds.
Conjugated Additions in Hydrogen Fluoride
Authors: German L. S. , Knunyantz I. L. 
Published: December 2003

AbstractFluorine and another substituent can be added in a single step to compounds having CC double bonds in anhydrous hydrogen fluoride. It is possible in this way to obtain e.g. α‐fluorinated β‐halogeno, β‐alkyl, β‐nitro, and β‐amino compounds, and also (in some cases by secondary reactions) fluorinated ethers, alcohols, esters, and carboxylic acids, β‐fluoro alcohols can be obtained by conjugated “hypofluorination” in the presence of oxenium ions.
Divalent Silicon Intermediates
Authors: Atwell W. H. , Weyenberg D. R. 
Published: December 2003

AbstractLike carbenes, silylenes are assumed to occur as intermediates in many reactions. A detailed account of the reactivity of the silylenes is given and the reductive and thermolytic methods used for their preparation are discussed. Insertion and interception reactions emphasize the formal relationship between these species and the carbenes. Silacyclopropenes and silacyclopropanes have not, as yet, been isolated.
Topochemical Investigations on Peptide Systems
Authors: Shemyakin M. M. , Ovchinnikov Yu. A. , Ivanov V. T. 
Published: December 2003

AbstractThe basic principles of the topochemical approach to the investigation of the structure‐function relation in peptide systems are formulated. This approach makes use of the new possibility of transforming natural peptides, consisting in the modification of the molecule as a whole and utilization of the resultant analogs to elucidate the boundaries of the stereoelectronic complementarity of the biologically active peptide to the corresponding receptor. In particular, on the example of depsipeptide antibiotics and their topochemical analogs the fruitfulness of using such compounds as tools in elucidating the physicochemical basis of functioning of biological membranes is shown. The topochemical principle has also been applied in preparing specific competitive inhibitors of proteolytic enzymes, whose study may shed light on the nature of the forces binding the substrate to the contact site of the corresponding enzyme.
Non‐Enzymatic Activation of Molecular Nitrogen
Authors: Henrici‐Olivé G. , Olivé S. 
Published: December 2003

AbstractSeveral transition metal complexes that can absorb nitrogen from the gas phase are now known. Some of the N2‐metal complexes are stable enough to be isolated and their structure elucidated, the N2molecule remaining chemically inert. In other cases reduction to N3−is possible, but the structure of the reactive intermediate N2‐metal complex can be approached only by mechanistic studies. In the stable complexes the nitrogen is bound via a lone pair of electrons in the direction of the molecular axis (“end‐on”), in the reducible complexes possibly “edge‐on”.