bluuebunny/crossref_metadata_2025 huggingface dataset:
Combinatorial Synthesis and Multidimensional NMR Spectroscopy: An Approach to Understanding Protein–Ligand Interactions
Authors: Chen James K. , Schreiber Stuart L.
Published: January 2004

Combinatorial chemistry is a laboratory emulation of natural recombination and selection processes. Strategies in this developing discipline involve the generation of diverse, molecular libraries through combinatorial synthesis and the selection of compounds that possess a desired property. Such approaches can facilitate the identification of ligands that bind to biological receptors, promoting our chemical understanding of cellular processes. This article illustrates that the coupling of combinatorial synthesis, multidimensional NMR spectroscopy, and biochemical methods has enhanced our understanding of a protein receptor used commonly in signal transduction, the Src Homology 3 (SH3) domain. This novel approach to studying molecular recognition has revealed a set of rules that govern SH3–ligand interactions, allowing models of receptor–ligand complexes to be constructed with only a knowledge of the polypeptide sequences. Combining combinatorial synthesis with structural methods provides a powerful new approach to understanding how proteins bind their ligands in general.
The Catalytic Oxidative Coupling of Methane
Authors: Lunsford Jack H.
Published: January 2004

One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH4 is first oxidatively converted into C2H6, and then into C2H4. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH4 on a metal oxide surface, and a homogeneous gas‐phase component, which includes free‐radical chemistry. Ethane is produced mainly by the coupling of the surface‐generated CH radicals in the gas phase. The yield of C2H4 and C2H6 is limited by secondary reactions of CH radicals with the surface and by the further oxidation of C2H4, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH4 conversion with 80% combined C2H4 and C2H6 selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH4 on certain catalysts. There is spectroscopic evidence for surface O− or O ions. In addition to the oxidative coupling of CH4, cross‐coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross‐coupling reaction also appears to involve surface‐generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.
Filled Buckyballs: Recent Developments from the Endohedral Metallofullerenes of Lanthanides
Authors: Edelmann Frank T.
Published: January 2004

Metal‐containing fullerenes—an intensively studied area of research. Recent developments in the field of endohedral metallofullerenes of lanthanides and the perspectives these results provide for the future are discussed herein. An additional theme is the specific incorporation of metal salts in carbon nanotubes, relatives of fullerenes, which Green et al. reported recently.
Formal Oxidation State versus Partial Charge—A Comment
Authors: Kaupp Martin , von Schnering Hans Georg
Published: January 2004

Copper(III) or copper(I)? The interpretation of the copper center in the anionic complex 1 as Cu1 by Snyder in the first issue of this year stimulated a spontaneous reply by Kaupp et al., which is published here with Snyder's response. The question whether the oxidation state in general is a purely formal concept has apparently not yet been answered to the satisfaction of all.
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Al,Ti Avoidance in the Microporous Titanoaluminosilicate ETAS‐10
Authors: Anderson M. W. , Philippou A. , Lin Z. , Ferreira A. , Rocha J.
Published: January 2004

Al atoms are incorporated into tetrahedral Si sites in the microporous titanosilicate ETS‐10 in such a way that the product, ETAS‐10, does not have Al‐O‐Ti units. Put simply, the Al and Ti centers avoid each other. The scope of this phenomenon and its ramifications are discussed.
Ligand‐Accelerated Catalysis
Authors: Berrisford David J. , Bolm Carsten , Sharpless K. Barry
Published: January 2004

The search for new metal‐catalyzed asymmetric reactions has provided some fascinating insights into the effects imposed on the metal catalysts by chiral ligands. A practical consequence is the discovery of ligand‐accelerated catalysis (LAC). Thus, an existing catalyzed process is improved by the addition of a specific ligand, which leads to a faster, “ligand‐accelerated” reaction. Both homogeneous and heterogeneous catalysts are known to exhibit this behavior. The concept is especially valuable in reactions catalyzed by early transition metals, where dynamic ligand exchange processes require an efficient in situ self‐selection of a highly reactive catalyst from a variety of thermodynamically dictated assemblies. Results of detailed mechanistic studies will be presented, and the significance of LAC phenomena in transformations catalyzed by early and late transition metals will be discussed.
On the Synthesis of the FeMo Cofactor of Nitrogenase: Gene‐Controlled in Nature versus Laboratory‐Produced by Man
Authors: Müller Achim , Krahn Eugen
Published: January 2004

One of the primary challenges of chemistry is the controlled synthesis of compounds with tailor‐made structures and properties. Natural products serve as inspiration in this quest, ranging from biocatalysts with optimal selectivity and activity to “inorganic materials” with exceptional properties, whose generation can be described by the term biomineralization. It is of fundamental importance to comprehend the courses of events at the interface between gene expression and the subsequent processes of epigenesis that are no longer under gene control. Chemistry has been able to achieve many goals; however, in the area of controlled syntheses of highly complex, tailor‐made metal clusters, there is a lack of fundamental theories and principles. This is especially true for the fascinating metal–sulfur cluster of nitrogenase, which, in this enzyme, functions as the active center for the N2 reduction and, so far, has eluded all attempts to be synthesized in the laboratory. To understand the biosynthesis of this cluster, information from genetics and chemistry must be combined.
Self‐Assembly of Molecular‐Sized Boxes
Authors: Hunter C. A.
Published: January 2004

A simple recipe for tailored cavities for the recognition of neutral organic guests: just mix difunctional chelate ligands with metal ions having a preferred coordination geometry. Boxes, cylinders, or tetrahedra form in high yields by self‐assembly. Of course it's not quite this easy, but the studies discussed here indicate that this strategy for the construction of macrocyclic hosts has a bright future.
Does the Death Knell Toll for the Metallic Bond?
Authors: Schön J. Christian
Published: January 2004

“The metallic bond still lives!” maintains the author of this highlight. In two recent papers theoreticians argue against the concept of the metallic bond as a distinct type of bond, and favor treating it as a limiting case of the covalent bond based on ab initio descriptions of bonding in solids. Still, J. C. Schön is convinced that the metallic bond, correctly understood as a qualitative concept, will continue to be used by solid‐state chemists.
Novel Genetically Engineered Tetracenomycins
Authors: Decker Heinrich , Haag Sabine , Udvarnoki Györgyi , Rohr Jürgen
Published: January 2004

Genetically engineered hybrid strains, in which the genes of the urdamycin‐producer Streptomyce fradiae are combined with some of the biosynthetic genes of the elloramycin‐producer Streptomyces olivaceus or the genes of the tetracenomycin‐producer Streptomyces glaucescens, assemble novel hybrid antibiotics of the tetracenomycin type. The results prove that this combinatorial biosynthesis method may be a useful alternative for the production of new natural products which can then be screened.
Basic Principles of Thermoanalytical Techniques and Their Applications in Preparative Chemistry
Authors: Cammenga Heiko Karl , Epple Matthias
Published: January 2004

The investigation of thermodynamic properties and reactivity yields interesting insights into the chemistry of newly synthesized substances. With thermal analysis extensive information can be gained from small samples (often only a few milligrams). In addition, the data obtained by thermal analysis can be used to plan and optimize a synthesis. Among the most important applications are identification and purity analysis, and the determination of characteristic temperatures and enthalpies of phase transitions (melting, vaporization), phase transformations, and reactions. Investigations into the kinetics of consecutive reactions and decomposition reactions are also possible. With the instruments available today such analyses can usually be performed quickly and easily. In this review the fundamentals of thermoanalytical methods are described and illustrated with selected examples of applications to low and high molecular weight compounds.
Synthetic Ribozymes and the First Deoxyribozyme
Authors: Burgstaller Petra , Famulok Michael
Published: January 2004

Artifical enzymes based on nucleic acids can be more versatile than their natural analogues, the ribozymes. Recently Joyce et al. produced a ribozyme that can hydrolyze amide bonds and a deoxyribozyme (a DNA enzyme!) that specifically cleaves the phosphodiester bond of a ribonucleotide with Pb2+ as a cofactor. For the synthesis of RNA and DNA enzymes the method of choice is in vitro selection combined with in vitro evolution.
The Structure of the D. gigas [NiFe] Hydrogenase and the Nature of the Hydrogenase Nickel Complex
Authors: Halcrow Malcolm A.
Published: January 2004

Several surprises came to light in the recent single‐crystal X‐ray structural determination of a [NiFe] hydrogenase by Volbeda et al. For example, the Ni fraction of the enzyme clearly forms part of an unprecedented cysteine‐bridged heterometallic binuclear system. In retrospect, the feasibility of such a structural motif is evident from the known coordination chemistry of nickel thiolate complexes. But the mechanism of H2 oxidation by this NiFe(?) complex remains tantalizingly opaque and continues to challenge bioinorganic chemists.
Galloxane and Alumoxane Hydroxides: [Ga12tBu12(μ3‐O)8(μ‐O)2(μ‐OH)4] and [Al6tBu6(μ3‐O)4(μ‐OH)4]
Authors: Landry Christopher C. , Harlan C. Jeff , Bott Simon G. , Barron Andrew R.
Published: January 2004

An unusual dodecagallium cage is isolated from the oxidation and hydrolysis of [tBu2Ga(Spy)]. The structure of 1 consists of a [tBuGa(μ‐X)GatBu]‐capped oxocube (X = O, OH). In addition 2 has been isolated. Both structures show crystallographic disorder of oxo and hydroxo ligands. H‐Spy = 2‐mercaptopyridine.
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Hyperbolic Lone Pair Structure in RhBi4
Authors: Grin Yuri , Wedig Ulrich , von Schnering Hans Georg̀
Published: January 2004

Two catena‐like interpenetrating enantiomorphic frameworks of condensed tetragonal antiprisms, RhBi8/2, form the basis of the crystal structure of RhBi4. The electron localization function (ELF) shows high localization of the valence electrons only on the periphery of the frameworks (lone pairs of Bi atoms), whereas the electrons inside the nets are delocalized. The periodic nodal surface (PNS) I2‐Y** describes perfectly the hyperbolic organization of the entire RhBi4 structure. It separates the regions with low electron localization inside the polyhedral nets from the hyperbolic layer of the lone pairs.
Optical Activity of Chiral Dendritic Surfaces
Authors: Jansen Johan F. G. A. , Peerlings H. W. I. , de Brabander‐Van den Berg Ellen M. M. , Meijer E. W.
Published: January 2004

The optical rotation of poly(propylene imine) dendrimers with 64 identical enantiomerically pure amino acid end groups tends to zero. This remarkable behavior results from the packing of the end groups in the highly dense, multiply hydrogenbonded surface, which gives rise to different frozen‐in conformations, leading to an internal compensation of optical activity.
A Stable Aquairon(III) Complex with S = 1: Structure and Spectroscopic Properties
Authors: Bartos Michael J. , Kidwell Christopher , Kauffmann Karl E. , Gordon‐Wylie Scott W. , Collins Terrence J. , Clark George C. , Münck Eckard , Weintraub Susan T.
Published: January 2004

For corrigendum see DOI:10.1002/anie.199516491
Phosphorane Iminato Complexes of Manganese and Cobalt with Heterocubane Structure
Authors: Mai Hans‐Joachim , zu Köcker Rolf Meyer , Wocadlo Sigrid , Massa Werner , Dehnicke Kurt
Published: January 2004

Mn4N4 and Co4N4 cubes with almost square faces are found in the title compounds 1 and 2. They were obtained by the melt reaction of the anhydrous dihalides MnI2 and CoCl2, and Me3SiNPEt3. Complexes 1 and 2 are the first phosphorane iminato complexes of manganese(II) and cobalt(II) with heterocubane structure.
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The Synthesis of Tris(perfluoroalkyl)‐phosphanes
Authors: Kampa Joel J. , Nail John W. , Lagow Richard J.
Published: January 2004

Symmetric, unsymmetric, and bridged perfluorinated trialkylphosphanes and diphosphanes are accessible for the first time by the general method outlined in this communication. In the synthesis, trialkylphosphanes are treated with elemental fluorine, and the resulting difluorotris(perfluoroalkyl)phosphoranes reduced to the desired perfluorinated phosphanes with tris(trimethylsilyl)phosphane.
Recent Trends in Photoaffinity Labeling
Authors: Kotzyba‐Hibert Florence , Kapfer Isabelle , Goeldner Maurice
Published: January 2004

Investigation of receptor—ligand interactions remains an inexhaustible challenge for chemists and biologists. Structural exploration of biological receptors is the starting point for a better understanding of how they function. Photoaffinity labeling is a biochemical approach to identify and characterize receptors targeting further structural investigations. The primary structure of a receptor protein was typically obtained by reverse genetics after exhaustive purification and sequencing of the N‐terminal peptide, which allowed the design of the corresponding oligonucleotide probes. Synthesis of these oligonucleotide probes then led to identification of cDNA clones by hybridization. Following this strategy, several membrane neurotransmitter receptors and constituent polypeptides, present in very small quantities in the central nervous system, were identified and their sequence deduced from the cDNA sequence. Since photoaffinity labeling implies the formation of a covalent bond between a radiolabeled ligand analogue and a receptor binding site, it becomes theoretically possible to isolate and sequence radiolabeled peptides and then synthesize the corresponding oligonucleotide probes. Photoaffinity labeling might avoid the critical solubilization and purification steps of the classical approach. To our knowledge, no such example of primary structure determination based on photoaffinity labeling experiments has been reported. However, the extraordinary developments in gene cloning technologies, in particular homology cloning and expression cloning, have made this approach obsolete and raised the question of new perspectives for photoaffinity labeling technology. In this article we present an update on selected original developments, as well as new challenges for this method. Photoaffinity labeling not only gives access to structural elements but is also a potential tool for the investigation of functional aspects of biological receptors, for example their role in signal transduction mechanisms.
Sulfuric Acid from Sulfur Trioxide and Water—A Surprisingly Complex Reaction
Authors: Steudel Ralf
Published: January 2004

On paper very simple, but in reality far more complicated—this applies to the formation of sulfuric acid in the atmosphere, which leads to the environmental problem of acid rain. In this reaction sulfur trioxide is transformed into sulfur dioxide, which reacts with water dimers to give sulfuric acid. Ab initio calculations as well as spectroscopic and kinetic studies have yielded new information on the reaction mechanism.
My Search for Carbocations and Their Role in Chemistry (Nobel Lecture)
Authors: Olah George A.
Published: January 2004

Every generation of scientific men (i.e., scientists) starts where the previous generation left off, and the most advanced discoveries of one age constitute elementary axioms of the next.

Aldous Huxley
G Proteins and Regulation of Adenylate Cyclase (Nobel Lecture)
Authors: Gilman Alfred G.
Published: January 2004

How is the hormone‐activated synthesis of the second messenger cyclic AMP regulated? Or, in more general terms, how does the interaction of an extracellular agonist with a receptor lead to intracellular enzyme activity when receptor and enzyme are not distinct macromolecules? The mediators are membrane‐bound, guanine nucleotide binding regulatory proteins (G proteins). When the receptor is activated, G proteins dissociate into their subunits. These in turn activate or inhibit enzymes such as adenylate cyclases, which catalyze the synthesis of cyclic AMP. In these signal transmission processes G proteins act as molecular switches and amplifiers.
Rapid Water Diffusion in a Cage‐Type Crystal Lattice: β‐Cyclodextrin Dodecahydrate
Authors: Steiner Thomas , da Silva Aida M. Moreira , Teixeira‐Dias José J. C. , Müller Jörn , Saenger Wolfram
Published: January 2004

Despite the absence of water channels, H/D and HO/HO exchange experiments show that water molecules diffuse rapidly through the crystal lattice of β‐cyclodextrin · 12H2O (CD). Exchange takes place in minutes and follows first‐order kinetics. This is explained by the structural fluctuations of the β‐CD molecules, which became apparent in crystallographic studies and in computer simulations.
Lyotropic Phases Formed by “Molecular Bottlebrushes”
Authors: Wintermantel Matthias , Fischer Karl , Gerle Markus , Ries Roland , Schmidt Manfred , Kajiwara Kanji , Urakawa Hiroshi , Wataoka Isao
Published: January 2004

Extremely stiff macromolecules in the form of “hairy rods” or more precisely “molecular bottlebrushes” (without a handle) are easily accessible by the polymerization of polymacromonomers. As anticipated from their high degree of stiffness, these molecules form lyotropic, liquid‐crystalline phases, although they consist of “commodity” monomers like styrene and methylacrylate.
Burial of Hydrocarbon Causes Cooperative Enhancement of Electrostatic Binding
Authors: Sharman Gary J. , Searle Mark S. , Benhamu Bellinda , Groves Patrick , Williams Dudley H.
Published: January 2004

The introduction of alanine methyl groups in adjacent positions on a peptide ligand results in a cooperative enhancement of electrostatic binding from the burial of hydrocarbon. This observation may explain why larger changes in free energy occur when hydrocarbon substituents are buried in a molecular recognition site than can be accounted for by the solvent transfer model (classical hydrophobic effect).
Structure Organization of Aluminosilicate Polyanions with Surfactants: Optimization of Al Incorporation in Aluminosilicate Mesostructural Materials
Authors: Fu Guoyi , Fyfe Colin A. , Schwieger Wilhelm , Kokotailo George T.
Published: January 2004

Hexagonal and lamellar aluminosilicate mesostructures with a Si/Al ratio close to 1 have been obtained by a new two‐step synthetic route. The precursor aluminosilicate Al4Si4O12(OH) and the surfactants organize into mesophasic precipitates when the corresponding solutions are mixed together. Further condensation and structure ordering of the precipitated phases can then be achieved by vapor‐phase treatment or refluxing isopropyl alcohol treatment leading to mesostructural materials.
Single‐Crystal X‐Ray Structure of the Metastable Aryne Precursor Tetrafluorophenyllithium and of 1,2,3,4‐Tetrafluorobenzene
Authors: Kottke Thomas , Sung Kuangsen , Lagow Richad J.
Published: January 2004

Do intramolecular LiF interactions affect the reactivityof aryne precursors? This was the key question for structural studies of lithiated and nonlithiated tetrafluorobenzene. No correlation between LiF distances and the ease of LiF elimination exists, and the lengths of the CarylF bonds in the lithiated compound are less affected by LiF interactions than by the “anionic” character of the benzene ring.
Patterns in Hydrogen Bonding: Functionality and Graph Set Analysis in Crystals
Authors: Bernstein Joel , Davis Raymond E. , Shimoni Liat , Chang Ning‐Leh
Published: January 2004

Whereas much of organic chemistry has classically dealt with the preparation and study of the properties of individual molecules, an increasingly significant portion of the activity in chemical research involves understanding and utilizing the nature of the interactions between molecules. Two representative areas of this evolution are supramolecular chemistry and molecular recognition. The interactions between molecules are governed by intermolecular forces whose energetic and geometric properties are much less well understood than those of classical chemical bonds between atoms. Among the strongest of these interactions, however, are hydrogen bonds, whose directional properties are better understood on the local level (that is, for a single hydrogen bond) than many other types of non‐bonded interactions. Nevertheless, the means by which to characterize, understand, and predict the consequences of many hydrogen bonds among molecules, and the resulting formation of molecular aggregates (on the microscopic scale) or crystals (on the macroscopic scale) has remained largely enigmatic. One of the most promising systematic approaches to resolving this enigma was initially developed by the late M. C. Etter, who applied graph theory to recognize, and then utilize, patterns of hydrogen bonding for the understanding and design of molecular crystals. In working with Etter's original ideas the power and potential utility of this approach on one hand, and on the other, the need to develop and extend the initial Etter formalism was generally recognized. It with that latter purpose that we originally undertook the present review.
Exciting Results from the Field of Homogeneous Two‐Phase Catalysis
Authors: Cornils Boy
Published: January 2004

The advantages of aqueous two‐phase catalysis–‐easy separation of the catalyst, resource‐saving procedures, etc.—can, according to very recent research, even be used when the solubility of the starting materials in water is low and the transfer of the organic substrate into the aqueous phase or at least to the phase boundary is hindered. The application of “promotor ligands” or of catalysts with partly perfluorinated ligands enables homogeneous two‐phase catalysis, which ideally combines the advantages of homogeneous and heterogeneous catalysis.
Novel Biological Copper Proteins through Anion Addition to the Mutant Met121Gly of Pseudomonas aeruginosa Azurin
Authors: Vidakovic Momcilo , Germanas Juris P.
Published: January 2004

Azide, cyanide, and thiocyanate are the anions that, on addition to a variant produced by site‐directed mutagenesis, Met121Gly, of Pseudomonas aeruginosa azurin, produce unprecedented copper proteins with markedly different spectroscopic properties. This ability to generate ligand‐accessible metal centers in coordinatively saturated metalloproteins by directed mutagenesis might possibly bring the synthesis of metalloproteins that can be used as ion sensors a step nearer.
Photochemically Initiated Protein Splicing
Authors: Cook Sandra N. , Jack William E. , Xiong Xiaofeng , Danley Lora E. , Ellman Jonathan A. , Schultz Peter G. , Noren Christopher J.
Published: January 2004

More details on the mechanism of protein splicing were revealed from the substitution of O‐(2‐nitrobenzyl)serine for the conserved Ser1082 at the upstream splice junction of the self‐splicing DNA polymerase of Thermococcus litoralis. Full length precursor protein was produced, which underwent protein splicing only upon photolysis.
Scalar Coupling Constants—Their Analysis and Their Application for the Elucidation of Structures
Authors: Eberstadt Matthias , Gemmecker Gerd , Mierke Dale F. , Kessler Horst
Published: January 2004

Since their discovery in the early fifties, scalar/coupling constants have been of great interest to the NMR spectroscopist. Their impact on structure determination by NMR spectroscopy is founded on the fact that the size of the coupling constant is directly related to molecular conformation. Today, for most chemical substances the parameters for the Karplus relationship, which relates the vicinial (3‐bond) coupling constant to the dihedral angle, have been determined. In addition to proton–proton distances, the application of coupling constants in modern conformational analysis is indispensable. In the study of larger molecules which are of current interest, more and more involved experiments are necessary in order to overcome signal overlap and increasing line widths. A large number of experimental techniques for the determination of coupling constants has been developed; however, for this reason the choice of the most appropriate experiment to use has become more difficult. This decision must be made carefully to maximize instrument usage and obtain the largest number of couplings with the greatest accuracy possible. Many of the computer programs used in structure calculations can directly apply coupling constant restraints, similar to proton–proton distances developed from NOEs. Therefore, not only is the quality of the structure improved, but the molecular motions (internal dynamics) are better described. In this article, we review the techniques that exist today with particular attention paid to helping the non‐expert to choose the appropriate experiment for the problem at hand. In addition, the use of coupling constants in computer simulations are discussed.
Thermotropic Liquid Crystals Formed by Intermolecular Hydrogen Bonding Interactions
Authors: Paleos Constantinos M. , Tsiourvas Dimitris
Published: January 2004

The role of hydrogen bonding in the formation or stabilization of liquid crystalline phases has only recently been appreciated. Following the first, wellestablished examples of liquid crystal formation from the dimerization of aromatic carboxylic acids, through hydrogen bonding, several classes of compounds have recently been synthesized, the liquid crystalline behavior of which is also dependent on intermolecular hydrogen bonds between similar or dissimilar molecules. In this review the main classes of compounds exhibiting liquid crystallinity due to hydrogen bonding are presented to show the diversity of organic compounds that can be used as building elements in liquid crystals. The molecules are either of the rigid‐rod anisotropic or amphiphilic types such as molecules appropriately functionalized with pyridyl and carboxyl groups, whose interaction leads to the formation of liquid crystals; amphiphilic carbohydrates and amphiphilic and bolaamphiphilic compounds with multiple hydroxyl groups whose dimerization or association is indispensable for the formation of liquid crystals; and certain amphiphilic carboxylic acids with monomeric or polymeric mesogens and amphiphilic‐type compounds bearing different moieties, whose interaction may lead to the formation of mesomorphic compounds. Associated with the macroscopic display of liquid crystalline phases is the supramolecular structure, and therefore rather extended discussion of these structures are included in this review.
Calculation of the Thermodynamic Properties of Polycyclic Aromatic Hydrocarbons by an Incremental Procedure
Authors: Nass Karen , Lenoir Dieter , Kettrup Antonius
Published: January 2004

The relationship between molecular structure and vapor pressure was studied for polycyclic aromatic hydrocarbons. The recognition of defined structural features enables the thermodynamic properties to be calculated, and the results agree well with experimental data.
σ Metathesis as a Critical Step for the Transition Metal Catalyzed Formation of Formic Acid from CO2 and H2? An Ab Initio Investigation
Authors: Hutschka François , Dedieu Alain , Leitner Walter
Published: January 2004

The hydrogenation of CO2 to formic acid catalyzed by three‐coordinate d8 hydridorhodium complexes has been studied by ab initio calculations on [HRh(PH3)2] as the model catalyst. A catalytic cycle involving a 2σ + 2σ metathesis reaction between H2 and [Rh(PH3)2(O2CH)], in which rhodium remains in the oxidation state I, is proposed as an alternative to the traditional pathway involving RhI/RhIII oxidation/reduction sequences.
Novel Porphyrinoids for Chemistry and Medicine by Biomimetic Syntheses
Authors: Franck Burchard , Nonn Ansgar
Published: January 2004

In 1926 Hans Fischer and Bruno Walach synthesized the first porphyrins.

H. Fischer, B. Walach, Justus Liebigs Ann. Chem. 1926, 450, 164–181.

Currently more than 1400 new articles concerning the synthesis and uses of porphyrins are published every year.

CAS Online search for 1994.

However, the strong interest in these compounds indicated by this is in sharp contrast to their restricted availability. This is reflected in the current price of up to 500 DM for 5 mg of the most important porphyrins used in research and other applications (see Scheme 3).

Biomimetic syntheses offer possibilities for an improved approach to porphyrins. By following the example set by nature it is also possible to obtain novel porphyrinoids which are different from naturally occurring porphyrins. This is exemplified by N,N′‐bridged porphyrinogens, which have cage structures, inverted porphyrinoids (N atoms in the outer periphery) and, in particular, porphyrins with expanded systems. Among the family of expanded porphyrins are superarenes with up to 34 π electrons. Their pronounced aromaticity is indicated by 1H NMR spectra, bond length equivalence, planar structures, and electrophilic substitution. With their strong absorption bands, the strongest of which have ϵ values of > 1000000, a value which exceeds the absorption intensity of all other organic pigments observed until now, and their ability to act as efficient photosensitizers, the expanded porphyrins open interesting perspectives in the fields of photochemistry and photomedicine.
Fluorocarbonate, [FCO2]−: Preparation and Structure
Authors: Zhang Xiongzhi , Gross Udo , Seppelt Konrad
Published: January 2004

By using “naked” fluoride ions in the form of tetramethylammonium fluoride or hexamethylpiperidinium fluoride, the fluorocarbonate (fluoroformate) [FCO2]− can be synthesized. This anion has an extraordinarily weak CF bond. It was characterized by 13C solid‐state NMR, IR, and Raman spectroscopy, as well as by ab initio calculations.
Chemistry of Inositol Lipid Mediated Cellular Signaling
Authors: Potter Barry V. L. , Lampe Dethard
Published: January 2004

It is now slightly more than a decade since Michael Berridge and his collaborators reported in Nature “…micromolar concentrations of Ins(1,4,5)P3 (1D‐myo‐inositol 1,4,5‐trisphosphate) release Ca2+ from a non‐mitochondrial intracellular Ca2+ store in pancreatic acinar cells. Our results strongly suggest that this is the same Ca2+ store that is released by acetylcholine”. This observation ushered in an a new era in the field of signal transduction with the discovery of a small‐molecule second messenger liniking the spatially separated events of cell surface receptor activation and intracellular Ca2+ mobilization. This event, which has spawned what is now one of the most active fields of current biology, also stimulated a renaissance in inositol and inositol phosphate chemistry. The synthesis of inositol polyphosphates presents a number of problems: the regiospecific protection of inositol and the optical resolution of the resulting precursors, the phosphorylation of the polyol, removal of all phosphate protecting groups without phosphate migration, and finally the purification of the water‐soluble target polyanion. With the solution of these problems over the last few years it is now possible to look beyond the synthesis of naturally occurring inositol polyphosphates, whose number has been steadily increasing, to the design of chemically modified inositol phosphate analogues with the prospect of developing enzyme inhibitors, rationally modified receptor ligands and antagonists, and perhaps, through pharmacological intervention in signal transduction pathways, even the therapeutical agents of the future.
CH and CC Bond Activation by Bare Transition‐Metal Oxide Cations in the Gas Phase
Authors: Schröder Detlef , Schwarz Helmut
Published: January 2004

Over the last decade the gas‐phase chemistry of bare transition‐metal oxide cations MO+ has received considerable attention. This interest is primarily due to the particular role of metal oxides in the oxidation of organic compounds in a variety of chemical and biochemical transformations. At a molecular level the simplest model system for these processes deals with reactions of bare metal‐oxide ions in the gas phase. Due to the high oxophilicities of the early transition metals, their monoxide cations MO+ do not mediate O‐atom transfer to any organic compounds at all. In contrast, monoxide cations of late transition metal can oxygenate a variety of hydro‐carbons, and the most reactive ions, MnO+, FeO+, NiO+, OsO+, and PtO+, even activate methane. Insight into the reaction mechanisms of these oxidation processes can be obtained by analysis of reaction kinetics, isotope effects, product distributions etc., and for the reactions of MO+ with alkanes the initial CH bond activation by MO+ is often rate‐determining. Interestingly, the high reactivity of some MO+ ions is not always associated with a decrease in regioselectivity; for example, FeO+ ions induce regiospecific γ‐CH bond activation of dialkylketones in the gas phase. The situation for the epoxidation of olefins in the gas phase turns out to be even more complex than for condensedphase analogues. This is primarily because the metal ion that mediates O‐atom transfer to the olefin also catalyzes the isomerization of the epoxides formed, to afford the energetically more stable aldehydes or ketones. Aromatic compounds can also be hydroxylated by MO+ ions, and particularly the oxidation of benzene by bare FeO+ ions in the gas phase reveals striking parallels to the metabolism of arenes. Furthermore, the storing capabilities of ion cyclotron resonance mass spectrometers even permit the design of catalytic processes in which a single metal ion converts more than one substrate molecule into an oxygenated product in a sequence of strictly bimolecular reactions. The most outstanding examples are the Pt+‐mediated oxidation of methane by molecular oxygen and the Co+‐mediated hydroxylation of benzene by N2O as oxidant. Finally, the key features of the gas‐phase reactions are compared with observations in condensed‐phase systems in which metal oxides are anticipated as central intermediates. The result of this comparison is promising in the sense that, in general, the understanding of transition‐metal‐mediated oxidations in the gas phase may lead to a more uniform description of these processes at a molecular level. Ultimately, it is hoped that gas‐phase studies will serve as one of the building blocks in the evolution of tailor‐made catalysts.
Terminal E1 Ligands from Elements of Group 15
Authors: Scheer Manfred
Published: January 2004

From objects of speculation to manipulable compounds: Two research groups have achieved the synthesis of the long sought metal complexes with terminal phosphido (P3−) ligands by independent routes, and both papers can be found in this issue of Angewandle Chemie. Cummins et al. report on the synthesis of 1 on p. 2042ff, Schrock et al. on the synthesis of 2 on p. 2044ff. Together with phosphaalkynes RCP these are the first compounds with a singly coordinated phosphorus and will be of interest to theoreticians and preparative chemists alike. R = C(CD3)2CH3; Ar = 3,5‐C6H3Me2; N3N = (Me3SiNCH2CH2)3N; M = Mo, W.
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The Metallic Bond—Dead or Alive? A Comment and a Reply
Authors: Allen Leland C. , Burdett Jeremy K.
Published: January 2004

What's in a name?! The letters discuss the usefulness of assigning qualitative concepts to the results of rigorous mathematical calculations. Exact definitions of the terms do not solve the dilemma, but pitfalls can be avoided with a knowledge of the assumption behind the calculations.
The Metallic Bond—Dead or Alive? A Comment and a Reply
Authors: Schön J. Christian
Published: January 2004

What's in a name?! The letters discuss the usefulness of assigning qualitative concepts to the results of rigorous mathematical calculations. Exact definitions of the terms do not solve the dilemma, but pitfalls can be avoided with a knowledge of the assumption behind the calculations.
Tantalum‐Mediated Coupling of Methane and Carbon Dioxide in the Gas Phase
Authors: Wesendrup Ralf , Schwarz Helmut
Published: January 2004

The simultaneous activation of CO2 and CH4 by Ta+ ions in the gas phase was studied by means of FT‐ICR mass spectrometry. The cationic oxides TaO+ and TaO can be obtained by reduction of CO2, but they are inert toward CH4. In contrast, the coupling of CO2 and CH4 is achieved by the complex [TaCH2O]+ and yields TaO and C2H2O, which is probably a ketene.
The Conquest of Taxol
Authors: Nicolaou Kyriacos C. , Guy Rodney K.
Published: January 2004

Much has been written about taxol, one of the newest weapons against cancer, and its producer, the Pacific Yew tree (Taxus brevifolia).

K. C. Nicolaou, W.‐M. Dai, R. K. Guy, Angew. Chem. 1994, 106, 38–69; Angew. Chem. Int. Ed. Engl. 1994, 33, 15–44.

In this article, the authors give a frank and behind‐the‐scenes account of their encounter with this well‐known molecule, which they and their collaborators faced as a synthetic target.

K. C. Nicolaou, Z. Yang, J.‐J. Liu, H. Ueno, P. G. Nantermet, R. K. Guy, C. F. Claiborne, J. Renaud, E. A. Couladouros, K. Paulvannan, E. J. Sorensen, Nature (London) 1994, 367, 630–634.

K. C. Nicolaou, J.‐J. Liu, Z. Yang, H. Ueno, R. K. Guy, E. A. Couladouros, E. J. Sorensen, J. Am. Chem. Soc. 1995, 117, 624–633.

K. C. Nicolaou, J.‐J. Liu, Z. Yang, H. Ueno, E. J. Sorensen, C. F. Claiborne, R. K. Guy, C.‐K. Hwang, M. Nakada, P. G. Nantermet, J. Am. Chem. Soc. 1995, 117, 634–644.

K. C. Nicolaou, Z. Yang, J.‐J. Liu, P. G. Nantermet, C. F. Claiborne, J. Renaud, R. K. Guy, K. Shibayama, J. Am. Chem. Soc. 1995, 117, 645–652.

K. C. Nicolaou, H. Ueno, J.‐J. Liu, P. G. Nantermet, Z. Yang, J. Renaud, K. Paulvannan, R. Chadha, J. Am. Chem. Soc. 1995, 117, 653–659.

Once again total synthesis is found to offer excellent opportunities for developing new synthetic strategies and novel reactions. The team of chemists who took up this challenge emerged with valuable experience and confidence in their skills.
Cytomimetic Organic Chemistry: Early Developments
Authors: Menger Fredric M. , Gabrielson Kurt D.
Published: January 2004

This article describes how chemical and physical stimuli cause a simple system, the giant vesicle, to undergo a variety of “cytomimetic” transformations such as fusion, fission, endocytosis, budding, aggregation, birthing, and foraging. For example, when a giant vesicle, which happens to have a smaller vesicle inside it, is exposed to octyl glucoside, the smaller vesicle can pass through the outer membrane into the external medium (“birthing”). The resulting injury to the membrane of the host vesicle heals immediately. Addition of cholic acid, on the other hand, induces a feeding frenzy in which a vesicle grows rapidly as it consumes its smaller neighbors. After the food is gone, the giant vesicle then self‐destructs (a case of “birth, growth, and death”). Such lifelike morphological changes were obtained by using commercially available chemicals; thus these processes should be assigned to organic chemistry, and not to biology or even biochemistry, and not to biology or even biochemistry. Experimental details (e.g. the preparation and observation of the vesicles) are included in this review in hopes of helping others enter this undeveloped field.
Gold in Chains: Self‐Assembly of a Gold(I) Catenane
Authors: Grohmann Andreas
Published: January 2004

Aurophilic interactions play a pivotal role in the preparation of interlocking “rings of gold”, as described recently [Angew. Chem. Int. Ed. Engl. 1995, 34, 1894]. Secondary Au…Au bonding thus complements the more traditional tools of supramolecular synthesis, such as van der Waals interactions, charge‐transfer interactions, and hydrogen bonding.
Condensed RuSn6 Octahedra in Ru3Sn15O14
Authors: Reichelt Werner , Söhnel Tilo , Rademacher Otto , Oppermann Heinrich , Simon Arndt , Köhler Jürgen , Mattausch Hansjürgen
Published: January 2004

The metal‐rich composition of the red‐translucent title compound is in itself indicative of unusual bonding. Condensed RuSn6 octahedra are present in the structure, in which in some cases the unusual oxidation state +I can be assigned to the tin. The localization of all the bonding electrons is verified by band structure calculations and interpreted with the 18‐electron rule. Accordingly, Ru3Sn15O14 is a solid‐state analogue of molecular carbonyl–nitrosyl complexes.
A Hexadecanuclear Polyolatometalate of Copper(II) Ions and Multideprotonated D‐Sorbitol
Authors: Klüfers Peter , Schuhmacher Jörg
Published: January 2004

Unsymmetrical, multiply bridging ligands are suitable building blocks for the construction of large ligand–metal frameworks. The nonpolymeric copper(II) complex 1 containing the most copper centers to date was obtained with D‐sorbitol. It contains sixfold and fourfold deprotonated sorbitol. The copper atoms form four groups, within which the atoms are antiferromagnetically coupled even at room temperature.
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Temperature and Solvent Effect on the Nucleophilic Addition of n‐Butyllithium to the N‐(Trimethylsilyl)imine of (2S)‐Lactal
Authors: Cainelli Gianfranco , Giacomini Daria , Walzl Martin
Published: January 2004

Are the Felkin–Ahn and Cram chelate models reliable? These models predict asymmetric induction without consideration of solvent and temperature. However, the addition of n‐butyllithium to the N‐(trimethylsily)imine of (2S)‐lactal leads to syn/anti ratios between 98:2 and 28:72 when performed in different solvents and at different temperatures. The reaction appears to be the first example of nucleophilic addition that follows the isoinversion principle (isoinversion temperature Ti = −21 °C). This indicates a stepwise process with at least two diastereoselective steps that are affected differently by temperature and solvent.
Volatile Metal Alkoxides according to the Concept of Donor Functionalization
Authors: Herrmann Wolfgang A. , Huber Norbert W. , Runte Oliver
Published: January 2004

Since the fundamental works of D. C. Bradley and R. C. Mehrotra, metal alkoxides have attracted attention because of the diversity of their low‐ and high‐molecular‐weight structures; they are also generating increasing interest as precursor compounds for solving technical materials problems. The understanding of the hydrolytic nucleation behavior is a prerequisite for the optimization of materials from sol–gel processes. For metal alkoxides to be precursors in chemical vapor deposition (CVD) processes in the preparation of inorganic oxidic materials, they should be sufficiently volatile, and sublimation should occur without decomposition at as low a temperature as possible (< 150 °C). Only recently, using the “donor functionalization” concept, was a ligand type systematically developed that unifies the advantages of both steric demands and σ‐donor stabilization and so stabilizes low‐molecularweight metal alkoxides. Even large metal ions of low charge (for example Ba2+) can thus form volatile alkoxides. O‐ and N‐donor functions in bidentate and multidentate alkoxo ligands are particularly advantageous; hence, for example, the vanadium derivative [V(OCMe2CH2OMe)3] is one of the most volatile metal alkoxides known to date. The first alkoxides of the alkaline earth metals calcium, strontium, and barium, which sublime without decomposition, have the formula [M2{OC(CH2OiPr)2tBu}4]. This article presents a critical inventory of the metal alkoxides with particular regard to the aspect of volatility. It also describes successes of the donor functionalization concept and shows—in prespective—how alkoxo ligands can be “tailor made” for metals according to their charge‐to‐radius ratio by further development of the concept.
Carbon Dioxide as a Raw Material: The Synthesis of Formic Acid and Its Derivatives from CO2
Authors: Leitner Walter
Published: January 2004

The use of carbon dioxide as a raw material for chemical syntheses is an ecologically and economically valuable extension to the carbon sources used at the present time. In order to convert the thermodynamically stable and comparatively unreactive CO2 molecule into the desired product in an efficient manner, suitable reaction conditions and activation mechanisms must be found. The catalytic reduction of CO2 to formic acid and its derivatives has been intensively studied in recent years. A number of new approaches to the synthesis of formic acid from CO2 have reached such a state of knowledge that continuing development may well lead to industrial‐scale operation in the near future. This can to a large extent be attributed to the fruitful interaction between investigative work into reaction mechanisms and the development of new catalytic systems.
Detection and Reactions of Radical Cations Generated by Photolysis of Aromatic Compounds with Tetranitromethane in 1,1,1,3,3,3‐Hexafluoro‐2‐propanol at Room Temperature
Authors: Eberson Lennart , Persson Ola , Hartshorn Michael P.
Published: January 2004

Extremely persistent even at room temperature are the radical cations of aromatic compounds such as dimethoxybenzene and dimethyl‐ and methoxynaphthalene, when generated photolytically in 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFP). They can be detected by EPR spectroscopy even in the presence of nucleophiles such as trinitromethanide ion. Their half‐lives in HFP often exceed those found in CF3COOH by a factor of more than 100!
Mechanism of the Fe+ Mediated CC and CH Bond Activations in Ethane from a Theoretical Viewpoint
Authors: Holthausen Max C. , Fiedler Andreas , Schwarz Helmut , Koch Wolfram
Published: January 2004

An explanation for the absence of H2 and CH4 elimination in the gas phase reaction of Fe+ with ethane is provided by the in‐depth analysis of computed stationary points in the system [Fe, C2, H6]+ presented herein. The results of the calculations verify the experimentally observed direct fragmentation processes.
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