bluuebunny/crossref_metadata_2025_split huggingface dataset:
Direct Identification of the Carbon Skeleton of Organic Compounds using Double Quantum Coherence 13C‐NMR Spectroscopy. The INADEQUATE Pulse Sequence
Authors: Buddrus Joachim , Bauer Hans 
Published: December 2003

AbstractNMR spectroscopy plays an important part in the determination of the structures of organic compounds. The parameters of importance here are the chemical shifts of the 1H and 13C nuclei and the spin‐spin interactions both between 1H nuclei and between 1H and 13C nuclei. Couplings between 13C nuclei were almost completely neglected until a few years ago, since they were extremely difficult to observe because of the low natural abundance of 13C. However, it is these couplings which afford information directly on the carbon‐carbon connectivities in the molecule. It is now possible to use a special NMR pulse sequence to make these couplings more readily visible: the result of using this sequence is a 13C‐NMR spectrum from which the carbon skeleton concerned can be directly read off. Two‐dimensional spectra in particular are very easy to evaluate. The pulse sequence involved, which bears the somewhat puzzling name INADEQUATE, produces double‐quantum coherences from which the NMR signals of the coupled carbon nuclei can be obtained. In this article the principle of double‐quantum coherence is described and a number of examples for the application of the INADEQUATE pulse sequences to problems in synthetic organic chemistry, biosynthesis and natural products chemistry are presented; in addition, the possibility of applying the INADEQUATE method to other nuclei is considered.
Charge Transfer and Radical Ions in Photochemistry
Authors: Mattay Jochen 
Published: December 2003

AbstractBimolecular photoreactions usually occur via several steps involving “complex‐type” intermediates (excimers or exciplexes). Although physical chemists have investigated them for nearly thirty years (Förster, Weller), and postulated their existence in numerous publications, it is only in the last decade that evidence for an exciplex intermediate in cycloadditions (Caldwell) has been obtained, thereby establishing the link between spectroscopy and synthetic photochemistry. Since then, many investigations have confirmed the role of exciplexes as intermediates in bimolecular photoreactions. The photochemically induced charge transfer from a donor to an acceptor substrate determines not only the bond strengths in the primary exciplex but also influences—via the charge‐transfer nature of the complex—the structure of the exciplex and thereby the selectivity of subsequent reactions. Such chemical processes between the donor and the acceptor prevent the reverse transfer of electrons, which usually results in regeneration of the starting materials, thereby only causing the dissipation of energy and occasionally—under specific conditions—the generation of molecules of the starting material in the triplet state. Ionic photodissociation also reduces the probability of a reverse transfer of electrons. This process, which produces solvated radical ions, is promoted by polar solvents and salt effects, particularly special salt effects. By this means it is possible to generate specific radical cations from, for example, alkenes and dienes and to study their reactions. Although in some areas, for example physical and biophysical chemistry, discussions of single electron transfer (SET) processes have long been found in the basic textbooks, organic chemistry lags far behind, although—and this is shown by the numerous recent publications—organic chemists are also aware of the fundamental importance of these processes. In this connection photochemistry plays a central role, since its instruments make it possible to carry out specific SET processes, to analyze them, and to use them in chemical reactions.
Ether Cleavage with Organo‐Alkali‐Metal Compounds and Alkali Metals
Authors: Maercker Adalbert 
Published: December 2003

AbstractReactions with organo‐alkali‐metal compounds are mostly carried out in ether‐type solvents. It has long been known that ether cleavage takes place at the same time. The mechanism of the cleavage, in particular the fact that dialkyl ethers can be decomposed by a variety of mechanisms acting simultaneously, has only become clear in the most recent investigations. The observation that even with purely aliphatic ethers a considerable amount of Wittig ether rearrangement can occur is remarkable. Unusual secondary reactions can also occur by means of which alkyl‐ and aryllithium compounds in ether or tetrahydrofuran yield new organolithium compounds—sometimes with rearrangement. The reactions of alkyl aryl ethers with alkali metals are also varied and five different mechanisms for them have been discussed in the literature. It is interesting, for example, that the cleavage of anisole can be directed simply by changing the solvent so that in one case only cleavage of the aryl‐oxygen bond occurs and in the other almost 100% cleavage of the alkyl‐oxygen bond results. The formation of biphenyl as a by‐product upon cleavage of the alkyl‐oxygen bond in anisole was also puzzling. This was later shown to occur not through the dimerization of phenyl radicals but via 2‐methoxybiphenyl. The number of cleavage mechanisms reduces to two if one assumes two different σ*‐radical anions as being intermediates in the kinetically controlled reaction. Comparison of the reactivity of thioethers and ethers reveals not only gradual but also fundamental differences—the cleavage of the thioethers is thermodynamically controlled.
Fluorides of Copper, Silver, Gold, and Palladium
Authors: Müller Bernd G. 
Published: December 2003

AbstractFluorine, by far the most reactive of the non‐metals, is capable of forming a large number of compounds with nearly all other elements (exceptions (so far): He, Ne, and Ar), even under comparatively “mild” reaction conditions. These compounds usually differ markedly from those of the heavier halogens in composition, structure, and chemical and physical properties. Thus, for example, it is generally quite easy to prepare fluorides containing elements in high oxidation states (often their maximum), as in AgF2, CsAgF4, PdF3, CsAuF6 etc., whereas the corresponding chlorides, bromides or iodides are in many cases (still) unknown. Conversely, the synthesis of fluorides containing these elements in middle or low oxidation states often meets with considerable difficulty, even where it is possible at all, as, e.g., in the case of CuF, AuF, PtF2, SeF2. Finally, there are also some examples of compounds MXn, which with X = F are stable, but with X = Cl are unstable or decompose easily (e.g. CoF3/CoCl3, VF4/VCl4, PbF4/PbCl4, AsF5/AsCl5). Consequently, fluorine compounds are of great general interest.
Natural Fats and Oils—Renewable Raw Materials for the Chemical Industry
Authors: Baumann Horst , Bühler Matthias , Fochem Heinz , Hirsinger Frank , Zoebelein Hans , Falbe Jürgen 
Published: December 2003

AbstractSince last century, the supply of raw materials for the chemical industry has undergone a radical change. Whereas at the beginning of the nineteenth century the demand for basic chemicals was satisfied entirely by renewable raw materials, from about 1850 the chemical industry came to rely increasingly upon coal. In the 1940's, mineral oil started to become increasingly important, and during the past thirty years it has remained by far as the most important source of raw materials. Renewable raw materials are likely to become important again in the future, as the choice of raw materials is now of great significance not only for economic reasons by influencing competitiveness, but also because this choice largely determines the properties of the derivatives produced and their ecological effects. Following the two oil crises of the 1970's, there is also now a growing awareness of the limits of raw material resources, and against the continuing background of agricultural surpluses, chemists are again showing an increasing interest in renewable raw materials. Since the end of the seventies, the Bundesministerium für Forschung und Technologie (Ministry of Research and Technology of the Federal Republic of Germany) has supported research projects on renewable raw materials in universities and in industry. For the chemical industry the use of natural products as raw materials opens up a wider spectrum of synthetic methods and finished products, some of which are not accessible by petrochemical routes.
Flexibility and Rigidity of Proteins and Protein–Pigment Complexes
Authors: Huber Robert 
Published: December 2003

AbstractProteins may be rigid or flexible to various degrees as required for optimal function. Flexibility of large parts of a protein, which rearrange or move, is particularly interesting and will be discussed in this article. We differentiate between several categories, although the boundaries between them are diffuse: flexibility of peptide segments, order–disorder transitions of spatially contiguous regions, and domain motions. The domains may be flexibly linked to allow rather unrestricted motions or the motions may be constrained to certain modes. The various categories of large‐scale flexibility will be illustrated with the following examples: (1) Small protein proteinase inhibitors are rather rigid molecules which provide binding surfaces complementary to their cognate proteases but show also limited segmental flexibility and adaptation. (2) Large plasma proteinase inhibitors exhibit large conformational changes after interaction with proteases probably for regulatory purposes. (3) Pancreatic serine proteases employ a disorder–order transition of their activation domain as a means to regulate enzymic activity. (4) Immunoglobulins show rather unrestricted and also hinged domain motions in different parts of the molecule probably to allow binding to antigens in different arrangements. (5) Citrate synthase adopts open and closed forms by a hinged domain motion to bind substrates and release products and to perform the catalytic condensation reaction, respectively. (6) Riboflavin synthase, a bifunctional multienzyme complex, catalyzes two consecutive reactions by means of two subunits, α and β. The β‐subunits form a shell, in which the α‐subunits are enclosed. Diffusional motion of the catalytic intermediates is therefore restricted. In addition, rearrangement of the N‐terminal segment occurs during the assembly of the β‐subunit. In contrast, rigidity is dominant in the structures of the light‐harvesting complexes and the photosynthetic reaction centers involved in photosynthetic light reactions. These are large protein–pigment complexes in which the proteins serve as matrices to hold the pigments in the appropriate conformation and relative arrangement. Since motion would contribute to deactivation of the photoexcited states of the pigments and diminish the efficiency of light‐energy and electron transfer, the functional role of rigidity is easy to rationalize for these proteins.
Structure Elucidation and Artificial Intelligence
Authors: Bremser Wolfgang 
Published: December 2003

AbstractThe difference between the older and the more recent computer systems amounts, in the final analysis, to the difference between an insect and a human being. The insect comes “fully programmed” into the world–programmed by instincts which it follows without using any thought processes. On the other hand, the human being must first learn the correct behavior–however, such learning can develop an impetus all its own: becoming, indeed, proceeding from his own decision and his awareness, the human being can change his former action programs. The creation of a computer system of the X‐th generation will actually far more resemble educating a child than the traditional programming of a computer.
[{Hg3Co(C4H6NO)6}(NO3) 2]n: A Macrobicyclic Bimetallic Chain Polymer Incorporating Deprotonated 2‐Pyrrolidone Bridges
Authors: Goodgame David M. L. , Williams David J. , Winpenny Richard E. P. 
Published: December 2003

AbstractFor corrigendum see DOI:10.1002/anie.198805892For corrigendum see DOI:10.1002/anie.198805892For corrigendum see DOI:10.1002/anie.198805892For corrigendum see DOI:10.1002/anie.198805892For corrigendum see DOI:10.1002/anie.198805892
One or Several Pioneers? The Discovery of Noble‐Gas Compounds
Authors: Laszlo Pierre , Schrobilgen Gary J. 
Published: December 2003

AbstractFor the general public, science history and restaurant menus share a labeling style. Like Melba peach, Soubise sauce, or tournedos Rossini, many a scientific discovery is credited to a single thinker or tinkerer. Such ultrasimplification rubs into oblivion other important contributors or codiscoverers. The discovery of noble‐gas compounds conforms to such a stereotyped and mythic view: according to the standard account, Neil S. Bartlett, then at the University of British Columbia, made it a quarter of a century ago on March 23, 1962. We do not attempt here a full and definitive history; it would be premature and we do not have the space. Rather, we shall document the concept and the earlier attempts at synthesis–they antedated Bartlett's success by three decades. The motivation for this note may be summarized in the following questions: (1) What sort of insight and input were those of Linus Pauling? (2) What were the likely causes for failure of the attempt by Yost and Kaye to synthesize noble‐gas fluorides in the 1930s? (3) Were there other researchers who envisioned a noble‐gas reactivity?
Carbon Dioxide as an Alternative C1 Synthetic Unit: Activation by Transition‐Metal Complexes
Authors: Behr Arno 
Published: December 2003

AbstractThe carbon dioxide molecule has been of limited importance as a synthetic unit in organic chemistry. When it is coordinated to transition metals, however, completely new possibilities arise; CO2 can bond to metal complexes in a variety of ways and can enter into insertion and coupling reactions, or become catalytically attached to other substrates. The formation of CC bonds between carbon dioxide and unsaturated hydrocarbons under conditions of homogeneous catalysis makes available new synthetic routes to industrially interesting organic compounds.
Isoleucyl‐tRNA Synthetase: An Enzyme with Several Catalytic Cycles Displaying Variation in Specificity and Energy Consumption
Authors: Freist Wolfgang 
Published: December 2003

AbstractThe translation of DNA via the genetic code into protein is a fundamental process carried out by all living systems. The extreme accuracy with which it is performed requires that the enzymes involved operate with extremely high specificity. It is for this reason that the structure and function of these proteins have been of special interest to molecular biologists and biochemists for the last twenty years. The discrimination between the different amino acids present in the cytosol and their esterification with the correct tRNA is the special task of the aminoacyl‐tRNA synthetases. One of the most difficult discriminations that has to be performed is the specificity for isoleucine. There are many amino acids in the cell that have a structure similar to isoleucine, in particular valine. To solve this problem, the aminoacyl tRNA synthetases have evolved unique proofreading mechanisms. The most detailed investigations of these mechanisms have been on the isoleucyl‐tRNA synthetases from yeast and from Escherichia coli. Both enzymes are single‐chain polypeptides with molecular weights of about 110 000. Sequence homologies with other aminoacyl‐tRNA synthetases seem to occur only within narrow regions, for example in the ATP binding site. The identification of the correct amino acid is achieved in a four‐step process. In the first two steps the hydrophobic interaction of the amino acid side chain with the enzyme is utilized for discrimination. The two hydrolytic proofreading steps that follow make use of the steric hindrance associated with the amino acid side chain. The enzymes are characterized by the special property that slight variations in the reaction conditions cause conformational changes that affect the catalytic cycle. In this way the observed specificity of the enzymes can be changed to almost any value. In general, high specificity is associated with high energy cost, which is reflected in high ATP consumption. Under physiological conditions a medium level of specificity and energy consumption seems to be preferred.
Photochemical Reactions of Ligands in Transition‐Metal Complexes
Authors: Poznyak Anatoli L. , Pavlovski Valeri I. 
Published: December 2003

AbstractThree types of photochemical reaction of coordination compounds are well‐known and have often been discussed: redox reactions, substitutions, and isomerizations. Less well known is a fourth type of reaction, which occurs fairly frequently: reaction of the ligands themselves. It is the purpose of this article to fill the corresponding gap in the photochemical literature and to summarize our current knowledge of such reactions and of their mechanisms and possible applications.
“Proton Sponges” and the Geometry of Hydrogen Bonds: Aromatic Nitrogen Bases with Exceptional Basicities
Authors: Staab Heinz A. , Saupe Thomas 
Published: December 2003

AbstractCertain aromatic diamines (the “proton sponges”) are found to have exceptionally high basicity constants: this is due to spatial interaction of the basic centers, which are in close proximity. The two factors which are most important in causing this effect are, on the one hand, the extreme steric strain in these systems and the destabilizing effect of the overlap of the nitrogen lone pairs of the neutral diamines and, on the other, the strong NċHċN hydrogen bonds which are formed on monoprotonation and which lead to a considerable relaxation of the steric strain. By the systematic variation of the structures of such aromatic diamines we have been able to study these effects as a function of steric factors, in particular of the geometry and the bond length of the NċHċN hydrogen bonds, by means of X‐ray structural analysis. The hydrophobic shielding of the basic centers and the NċHċN hydrogen bonds, which was characteristic of the “proton sponge” compounds studied previously, is indeed responsible for the extremely low rate of protonation and deprotonation of these compounds; however, it apparently has no influence on their high thermodynamic basicity. The recent synthesis and basicity determination of a new type of “proton sponge” with no hydrophobic shielding whatever show that not only very strong but also kinetically active bases are accessible using the “proton sponge” concept. Their unusual properties, which are discussed here as the result of steric interactions between two basic centers, provide examples of the fact that cooperative steric interactions of reactive structural elements can lead to properties which cannot be derived from an isolated consideration of the various functional groups. Such “proximity effects” are certainly of general importance in chemistry and biochemistry; the study of their structure‐function relationships is worthy of closer consideration.
Atomic Motions in Molecular Crystals from Diffraction Measurements
Authors: Dunitz Jack D. , Maverick Emily F. , Trueblood Kenneth N. 
Published: December 2003

AbstractThe information provided by modern crystal structure analyses is not limited to the atomic arrangement. It also includes, for each atom, a set of quantities known as anisotropic Gaussian displacement parameters (ADP's), which provide information about averaged displacements of atoms from their mean positions. From analysis of these quantities, conclusions can be drawn about the rigid‐body motion of molecules, about large‐amplitude internal molecular motions, and about the identification of any disorder present in the crystals as being mainly dynamic or mainly static in nature. For some crystals, such analyses yield energy barriers to rotation of rigid molecules or molecular fragments that are in good agreement with values obtained by other physical methods.
Electron Transfer and Charge Transfer: Twin Themes in Unifying the Mechanisms of Organic and Organometallic Reactions
Authors: Kochi Jay K. 
Published: December 2003

AbstractThe broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer‐sphere and inner‐sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D⊕, A⊖], for which the free energy change can be evaluated from the charge‐transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge‐transfer ion pairs [D⊕, A⊖] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge‐transfer bands of various EDA complexes with a 25‐ps laser pulse. Such studies confirm the radical ion pair [Arene⊕, NO2] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron‐transfer mechanism between arenes and the nitryl cation (NO) electrophile.
New Transition Metal Clusters with Ligands from Main Groups Five and Six
Authors: Fenske Dieter , Ohmer Johannes , Hachgenei Johannes , Merzweiler Kurt 
Published: December 2003

AbstractIn both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three‐ or four‐membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe3)2 (E = S, Se, Te) and E′R(SiMe3)2 (R = Ph, Me, Et; E′ = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations; a variety of reactions in the ligand sphere are also possible.