bluuebunny/crossref_metadata_2025 huggingface dataset:
Calixarenes, Macrocycles with (Almost) Unlimited Possibilities
Authors: Böhmer Volker
Published: January 2004

The condensation reaction betweenp‐tert‐butylphenol and formaldehyde leads in a single step to good yields of cyclic oligomers in which, depending on the reaction conditions, either four, six, or eight phenol units are joined by methylene bridges. The beakerlike shape of the most stable conformation of the tetramer has led to their being given the name “calixarenes” (calix = chalice). Resorcinol can undergo condensation in a similar manner with a variety of aldehydes to afford cyclic tetramers with the same basic structure (the resorcarenes). In both cases the reaction does not require the use of dilution techniques, so that large quantities of product can be readily obtained. In addition, the parent compounds can be modified in various ways, in particular at the phenolic hydroxy groups or the phenyl residues; these approaches can be used separately or in combination. Calixarenes are thus ideal starting materials for the synthesis of various types of host molecules and can also act as building blocks for the construction of larger molecular systems with defined structures and functions. Their potential applications range from use as highly specific ligands for analytical chemistry, sensor techniques and medical diagnostics to their use in the decontamination of waste water and the construction of artificial enzymes and the synthesis of new materials for non‐linear optics or for ultrathin layers and sieve membranes with molecular pores.
Octadecanuclearity in Manganese Carboxylate Chemistry: K4[Mn18O16(O2CPh)22(phth)2(H2O)4] (phthH2 = phthalic acid)
Authors: Hendrickson David N. , Aubin Sheila M. J. , Squire Rachel C. , Folting Kirsten , Streib William E. , Christou George
Published: January 2004

Small change—large effect: If some of the carboxylato ligands in the coordination sphere of the anion in 1, which contains the butterfly‐like [Mn4(μ3‐O)2] unit, are replaced by dicarboxylato ligands by treatment with potassium hydrogen phthalate, the title compound 2 containing an Mn18O16 framework in its anion is formed. It is the largest Mn/O aggregate discovered to date.
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Synthesis and Structure of Organometallic Compounds with (Al2Si)2 and Al3Sb2 Frameworks
Authors: Schulz Stephan , Schoop Torsten , Roesky Herbert W. , Häming Ludger , Steiner Alexander , Herbst‐Irmer Regine
Published: January 2004

A complex with an eight‐membered Al4F4 ring in which pairs of Al atoms are bridged by Ph2Si units is the unexpected product of the reaction of [(Cp*Al)4] with Ph2SiF2[Eq. (a)]. The formation of 1 can be explained in terms of the marked fluorophilicity of [(Cp*Al)4]. Reaction of [(Cp*Al)4] with (tBuSb)4 yields [(Cp*Al)3Sb2], the first organometallic compound to contain an aluminium‐antimony bond.
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Stereospecific Olefin Polymerization with Chiral Metallocene Catalysts
Authors: Brintzinger Hans H. , Fischer David , Mülhaupt Rolf , Rieger Bernhard , Waymouth Robert M.
Published: January 2004

Current studies on novel, metallocenebased catalysts for the polymerization of α‐olefins have far‐reaching implications for the development of new materials as well as for the understanding of basic reaction mechanisms responsible for the growth of a polymer chain at a catalyst center and the control of its stereoregularity. In contrast to heterogeneous Ziegler–Natta catalysts, polymerization by a homogeneous, metallocene‐based catalyst occurs principally at a single type of metal center with a defined coordination environment. This makes it possible to correlate metallocene structures with polymer properties such as molecular weight, stereochemical microstructure, crystallization behavior, and mechanical properties. Homogeneous catalyst systems now afford efficient control of regio‐ and stereoregularities, molecular weights and molecular weight distributions, and comonomer incorporation. By providing a means for the homo‐ and copolymerization of cyclic olefins, the cyclopolymerization of dienes, and access even to functionalized polyolefins, these catalysts greatly expand the range and versatility of technically feasible types of polyolefin materials.

For corrigendum see DOI:10.1002/anie.199513681
Vibrational Spectroscopy in Supercritical Fluids: From Analysis and Hydrogen Bonding to Polymers and Synthesis
Authors: Poliakoff Martyn , Howdle Steven M. , Kazarian Sergei G.
Published: January 2004

Supercritical fluids are beginning to be used widely in chemistry. Applications range from extraction and chromatography in analytical chemistry to solvents for reaction chemistry and preparation of new materials. Spectroscopic monitoring is important in much of supercritical chemistry, and vibrational spectroscopy is particularly useful in this context because the vibrational spectrum of a given molecule is usually quite sensitive to the environment of that molecule. Thus, vibrational spectra are excellent probes of conditions within the fluid. In this review, we describe a variety of techniques and cells for IR and Raman spectroscopy in supercritical fluids and illustrate the breadth of applications in supercritical fluids. The examples include: the use of supercritical Xe as a spectroscopically transparent solvent for chemistry and for supercritical fluid chromatography with FTIR detection of analytes; Raman spectroscopy as a monitor for gases dissolved in supercritical CO2; the effect of solvent density on hydrogen bonding in supercritical fluids and the formation of reverse micelles; IR as a monitor for the supercritical impregnation/extraction of polymers and the reactions of organometallic compounds impreganated into polymers; reactions of organometallic compounds in supercritical fluids; and finally, the use of miniature flow reactors for laboratory‐scale preparative chemistry. Overall, our aim is to provide a starting point from which individual readers can judge whether such measurements might usefully be applied to their own particular problems.
Palladium‐Catalyzed Amination of Aryl Halides—Catalysts on New Routes to Known Targets
Authors: Beller Matthias
Published: January 2004

It's the base that matters! Sodium tert‐butoxide can be used to facilitate Pd‐catalyzed CN couplings directly with the amine. This was shown by research by Hartwig et al. and Buchwald et al. Thus, the use of ecologically unsound tin amides in the hetero‐Heck reaction may be avoided in future. Further improvement of the catalysts to make them industrially more interesting, and the extension of the method to CO couplings is eagerly awaited.
Signal Transduction: Evolution of an Idea (Nobel Lecture)
Authors: Rodbell Martin
Published: January 2004

In general there is no set of observations conceivable which can give enough information about the past of a system to give complete information as to its future.

Norbert Wiener
Molecular Imprinting in Cross‐Linked Materials with the Aid of Molecular Templates— A Way towards Artificial Antibodies
Authors: Wulff Günter
Published: January 2004

Can binding sites be produced in organic or inorganic polymers—similar to those in antibodies—which are able to recognize molecules and which may have catalytic action? In this article we review a method, analogous to a mechanism of antibody formation proposed earlier, by which in the presence of interacting monomers a cross‐linked polymer is formed around a molecule that acts as a template. After removal of the template, an imprint containing functional groups capable of chemical interaction remains in the polymer. The shape of the imprint and the arrangement of the functional groups are complementary to the structure of the template. If chiral templates are used, the success of the imprinting process can be assessed by the ability of the polymer to resolve the racemate of the template molecule. Through optimization of the process has led to chromatographic separation factors of α = 4–8, and to base line separations. There is also great interest in the surface imprinting of solid materials and monolayers. In all cases, the structure of the polymeric matrix in the imprinted materials and the function of the binding groups are of crucial importance. The mechanisms of imprinting and molecular recognition of substrates are by now well understood. A large number of potential applications for this class materials are being intensively developed, for example in the chromatogrphic resolution of recemates, and as artificial antibodies, chemosensors, and selective catalysts. The use of similarly produced materials as enzyme models is also of great interest.
From cyclo‐Te8 to Te Sheets: Are Nonclassical Polytellurides More Classical than We Thought?
Authors: Kanatzidis Mercouri G.
Published: January 2004

The importance of exploratory synthesis in advancing inorganic chemistry is exemplified by the interesting results that have been obtained in polychalcogenide chemistry. Thus, for example, Te8 rings and planar [Te6] layers have been found in the solid‐state structure of Cs3Te22 [Angew. Chem. 1995, 107, 450]. The counterions, which appear to function as templates, strongly influence the formation and structure of the products. Is this a route to polychalcogenides with defined structural features and material properties?
[Mo154(NO)14O420(OH)28(H2O)70](25 ± 5)−: A Water‐Soluble Big Wheel with More than 700 Atoms and a Relative Molecular Mass of About 24000
Authors: Müller Achim , Krickemeyer Erich , Meyer Jochen , Bögge Hartmut , Peters Frank , Plass Winfried , Diemann Ekkehard , Dillinger Stephan , Nonnenbruch Fritz , Randerath Markus , Menke Carsten
Published: January 2004

Complex clusters the size of small proteins can be generated in reduced molybdate solutions by linking defined fragments. An example of this is the giant cluster (NH4)25±5[Mo154(NO)14O420(OH)28(H2O)70]·ca. 350 H2O with a molar mass of more than 30000 gmol−1, the anion of which is shaped like a tire with a nanodimensional cavity more than 20 Å in diameter and has an extremely large inner and outer surface as well as a large number of mobile electrons.
Porous Solids by Design: [Zn(4,4′‐bpy)2(SiF6)]n·xDMF, a Single Framework Octahedral Coordination Polymer with Large Square Channels
Authors: Subramanian S. , Zaworotko Michael J.
Published: January 2004

For corrigendum see DOI:10.1002/anie.199525611
Rational Design of Orthogonal Receptor–Ligand Combinations
Authors: Belshaw Peter J. , Schoepfer Joseph G. , Liu Karen‐Qianye , Morrison Kim L. , Schreiber Stuart L.
Published: January 2004

A novel approach to understanding protein–ligand interactions is illustrated using the immunophilin receptor human cyclophilin A and its immunosuppressive ligand cyclosporin A. Synthetic chemistry is used to place “bumps” on the ligand, while site‐directed mutagenesis is used to create compensatory “holes” in the receptor. These novel receptor–ligand combinations can have an affinity even greater than that of the natural system, and are expected to facilitate the inducible dimerization of target proteins, and thus the activation of signaling proteins, in transgenic animals.
Influence of Lewis Base Complexation and the Degree of Aggregation of Alkyllithium Compounds on the Ration in Which They Form Addition Products and Enolates with Ketones
Authors: Luitjes Hendrikus , Schakel Marius , Schmitz Robert F. , Klumpp Gerhard W.
Published: January 2004

The difference between 1 and 2 lies only in the number of nitrogen atoms and not in the carbanionic unit. Since 1, which is a tetramer, reacts with ketones to form adducts almost exclusively, but 2 virtually only enolates, the type of product formed is linked to the degree of aggregation, which in turn depends on the Lewis base.
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Diastereo‐ and Enantioselective Synthesis of Polyfunctional Ketones with Adjacent Quaternary and Tertiary Centers by Asymmetric Carroll Rearrangement
Authors: Enders Dieter , Knopp Monika , Runsink Jan , Raabe Gerhard
Published: January 2004

Na Abstract.
Aligning and Trapping Molecules with Light
Authors: Kador Lothar
Published: January 2004

An alternative to matrix isolation, cold molecules can be trapped and aligned in vacuum by an intense infrared laser field according to theoretical considerations expounded recently. The method is based on the polarization of the molecules by the electric field of the light, which creates an attractive potential. If the polarizability of the molecules is anisotropic, the axis of highest polarizability will be aligned along the polarization direction of the laser.
A Molecular Composite Containing Organic and Inorganic Components—A Complex from β‐Cyclodextrin and Hydrated Magnesium Chloride
Authors: Nicolis Ioannis , Coleman Anthony W. , Charpin Pierrette , de Rango Colette
Published: January 2004

Two interlocking substructures–infinite chains of β‐cyclodextrin monomers and [Mg(H2O)6]Cl2building blocks–form the basis of a novel composite. The chains of inorganic components, comprising Mg(H2O)6octahedra linked through Cl anions and water of crystallization, are threaded through the cavities of the cyclodextrins. The complete structure is stabilized by hydrogen bonds.
Synthetic Aspects of Metal‐Catalyzed Oxidations of Amines and Related Reactions
Authors: Murahashi Shun‐Ichi
Published: January 2004

The metabolism of amines is governed by a variety of enzymes such as amine oxidase, flavoenzyme, and cytochrome P‐450. A wide variety of compounds are produced such as ammonia and alkaloids in selective and clean oxidation reactions that proceed under mild reaction conditions. Simulation of the functions of these enzymes with simple transition metal complex catalysts may lead to the discovery of biomimetic, catalytic oxidations of amines and related compounds. Indeed, metal complex catalyzed oxidations have been found to proceed with high efficiency. The first section of this review discusses the dehydrogenative oxidations of amines with transition metal catalysts by transition metal catalysts that simulate amine oxidase. The second section highlights the catalytic oxidation of secondary amines to nitrones by simulation of flavoenzymes. The third section describes the simulation of the function of cytochrome P‐450 with lowvalent ruthenium complexes and peroxides. Biomimetic ruthenium‐catalyzed oxidations of tertiary amines, secondary amines, and other substrates such as amides, β‐lactams, nitriles, alcohols, alkenes, ketones, and even nonactivated hydrocarbons can be performed selectively under mild conditions. These three general approaches provide highly useful strategies for synthesis of fine chemicals and biologically active compounds such as alkaloids, amino acids, and β‐lactams.
New Surprises in Metal Carbonyl Chemistry
Authors: Ellis John E. , Beck Wolfgang
Published: January 2004

Carbonyl complexes of calcium, silicon, and uranium have been prepared and exactly characterized within the last two years—sensational discoveries if one considers that previously, binary carbonyls of main group elements and actinides were only detected at very low temperatures as matrix‐isolated species. These exciting developments have again placed the spotlight on the rich and vital area of metal carbonyl chemistry.
Reactions of Neopentyl 2,2,2,‐Trifluoroethanesulfonate (Tresylate) with Nucleophiles: A Model Study for the Coupling of Nucleophiles with Tresyl Agarose
Authors: Jennissen Herbert P.
Published: January 2004

Is it prudent to compare homogeneous and heterogeneous phase reactions? One side of this discussion on the reactivity of certain tresylates to nucleophiles such as butylamine and butanethiol says “Yes”, but the other says “Not always”.
Enzymatic Synthesis of Peptides and Ras Lipopeptides Employing Choline Ester as a Solubilizing, Protecting, and Activating Group
Authors: Schelhaas Michael , Glomsda Simone , Hänsler Marion , Jakubke Hans‐Dieter , Waldmann Herbert
Published: January 2004

Since it can be removed under extremely mild conditions and with complete selectivity by choline esterase, the solubilizing choline ester function is advantageous for high‐yielding peptide syntheses. This enzyme‐labile protecting group was employed in the synthesis of the S‐palmitoylated and S‐farnesylated C‐terminal lipohexapeptide of the human N‐Ras protein.
Electronegativity and Molecular Properties
Authors: Bergmann D. , Hinze Juergen
Published: January 2004

The paradigm that the properties of the atoms determine the properties of the molecules that they form is systematically presented. To this end, three types of atomic properties are differentiated: (a) those that can be determined directly by spectroscopy, (b) those based on theoretical concepts, and (c) those that can be assigned to the atoms interacting in molecules. On the basis of the electronegativity values of the atoms, which can be determined from spectroscopic data together with the assumption that the electronegativities are equalized in bonds, partial charges of the atoms in molecules are determined. These partial charges are correlated with ESCA data and proton affinities. In addition, simple expressions are given for the reliable estimation of bond lengths, bond energies, and force constants.

For corrigendum see DOI:10.1002/anie.199607811
Stereocontrol in Organic Synthesis Using the Diphenylphosphoryl Group
Authors: Clayden Jonathan , Warren Stuart
Published: January 2004

In 1959, Horner showed that metalated alkyldiphenylphosphane oxides react with aldehydes or ketones to give alkenes. With this reaction, the diphenylphosphoryl (Ph2PO) group made its entrance into synthetic organic chemistry. In the thirty‐six years since that date, extensive research has shown that this olefination, the Horner–Wittig reaction, has unique properties that make it much more than simply the phosphane oxide cousin of the more famous phosphorus‐based olefinations—the Wittig reaction (based on phosphonium salts) and the Wadsworth–Emmons reaction (based on phosphonate esters). Early work on the Horner–Wittig reaction concentrated on the reactivity of phosphane oxides and the regioselectivity of their reactions, but more recently the power of the Ph2PO group to control the stereochemistry of alkenes, and to produce “on demand” either stereoisomer in high stereochemical purity, has emerged. From the study of these stereocontrolled Horner–Wittig reactions arose the realization that the Ph2PO group is useful not only for the control of the two‐dimensional stereochemistry of alkenes, but also of three‐dimensional stereochemistry in general.

After a brief introduction to phosphane oxide chemistry, this review will examine the Horner–Wittig reaction, in both its original and “stereocontrolled” varieties. From there, we will move on to an account of the stereoselective construction of molecules containing the Ph2PO group, concentrating on the stereochemical directing effects of the Ph2PO group and on the role of its unique combination of attributes—steric bulk, electronegativity, and Lewis basicity—in controlling these reactions. Finally, we will present what is intended as a practical guide to this chemistry, covering the type of functionalized alkenes that have been made with the help of the Ph2PO group and giving guidelines that we hope will help the organic chemist to make the most of the chemistry the Ph2PO group has to offer.
Boron‐Substituted Heterocycles as π‐Donors in Organometallic Chemistry—A Sandwich Complex with 1‐H‐Boratabenzene and Lithium
Authors: Nöth H. , Schmidt Martin
Published: January 2004

New impetus both for the organometallic chemistry of alkali metals as well as for organoboron chemistry should be stimulated by the synthesis of 1 by Fu et al. Though this compound is the second complex containing a hydrogen atom on the boron atom of boratabenzene, it is the first that has been characterized by X‐ray structural analysis.
Electron Domains and the VSEPR Model of Molecular Geometry
Authors: Gillespie Ronald J. , Robinson Edward A.
Published: January 2004

The valence shell electron pair repulsion (VSEPR) model—also known as the Gillespie–Nyholm rules—has for many years provided a useful basis for understanding and rationalizing molecular geometry, and because of its simplicity it has gained widespread acceptance as a pedagogical tool. In its original formulation the model was based on the concept that the valence shell electron pairs behave as if they repel each other and thus keep as far apart as possible. But in recent years more emphasis has been placed on the space occupied by a valence shell electron pair, called the domain of the electron pair, and on the relative sizes and shapes of these domains. This reformulated version of the model is simpler to apply, and it shows more clearly that the Pauli principle provides the physical basis of the model. Moreover, Bader and his co‐workers' analysis of the electron density distribution of many covalent molecules have shown that the local concentrations of electron density (charge concentrations) in the valence shells of the atoms in a molecule have the same relative locations and sizes as have been assumed for the electron pair domains in the VSEPR model, thus providing further support for the model. This increased understanding of the model has inspired efforts to examine the electron density distribution in molecules that have long been regarded as exceptions to the VSEPR model to try to understand these exceptions better. This work has shown that it is often important to consider not only the relative locations and sizes, but also the shapes, of both bonding and lone pair domains in accounting for the details of molecular geometry. It has also been shown that a basic assumption of the VSEPR model, namely that the core of an atom underlying its valence shell is spherical and has no influence on the geometry of a molecule, is normally valid for the nonmetals but often not valid for the metals, including the transition metals. The cores of polarizable metal atoms may be nonspherical because they include nonbonding electrons or because they are distorted by the ligands, and these nonspherical cores may have an important influence on the geometry of a molecule.
Polymerization of Ethylene Oxide with Alkyllithium Compounds and the Phosphazene Base “tBuP4”
Authors: Esswein Beate , Möller Martin
Published: January 2004

Well‐defined poly(ethylene oxide)s with low polydispersities are formed in the anionic polymerization of ethylene oxide with an alkyllithium/phosphazene base initiator system. The base can be regarded as a cryptand for Li+ ions. The equilibrium between complexed alkoxide ion pairs and reactive free anions is thus shifted.
Electrospray Mass Spectrometry of Biomacromolecular Complexes with Noncovalent Interactions—New Analytical Perspectives for Supramolecular Chemistry and Molecular Recognition Processes
Authors: Przybylski Michael , Glocker Michael O.
Published: January 2004

The development of “soft” ionization methods in recent years has enabled substantial progress in the mass spectrometric characterization of macromolecules, in particular important biopolymers such as proteins and nucleic acids. In contrast to the still existing limitations for the determination of molecular weights by other ionization methods such as fast atom bombardment and plasma desorption, electrospray ionization (ESI) and matrix‐assisted laser desorption have provided a breakthrough to macromolecules larger than 100 kDa. Whereas these methods have been successfully applied to determine the molecular weight and primary structure of biopolymers, the recently discovered direct characterization by ESI‐MS of complexes containing noncovalent interactions (“noncovalent complexes”) opens new perspectives for supramolecular chemistry and analytical biochemistry. Unlike other ionization methods ESI‐MS can be performed in homogeneous solution and under nearly physiological conditions of pH, concentration, and temperature. ESI mass spectra of biopolymers, particularly proteins, exhibit series of multiply charged macromolecular ions with charge states and distributions (“charge structures”) characteristic of structural states in solution, which enable a differentiation between native and denatured tertiary structures. In the first part of this article, fundamental principles, the present knowledge about ion formation mechanism(s) of ESI‐MS, the relations between tertiary structures in solution and charge structures of macro‐ions in the gas phase, and experimental preconditions for the identification of noncovalent complexes are described. The hitherto successful applications to the identification of enzyme–substrate and –inhibitor complexes, supramolecular protein–and protein–nucleotide complexes, double‐stranded polynucleotides, as well as synthetic self‐assembled complexes demonstrate broad potential for the direct analysis of specific noncovalent interactions. The present results suggest new applications for the characterization of supramolecular structures and molecular recognition processes that previously have not been amenable to mass spectrometry; for example, the sequence‐specific oligomerization of polypeptides, antigen–antibody complexes, enzyme–and receptor–ligand interactions, and the evaluation of molecular specificity in combinatorial syntheses and self‐assembled systems.
Main Group Element Analogues of Carbenes, Olefins, and Small Rings
Authors: Driess Matthias , Grützmacher Hansjörg
Published: January 2004

For a long time methylene (:CH2), ethylene (H2CCH2), and cyclopropane [(CH2)3] were considered as compounds whose higher main group homologues or analogues could not be synthesized. One consequence of the failure of earlier experiments was, for example, the “double bond rule” which was introduced over twenty years ago and is still often quoted. Although it does not dispute the existence of multiple bonding between elements, it states that it leads to very kinetically unstable species. This interpretation of the double bond rule has long since been surpassed due to improved experimental and quantum chemical methods which have allowed the synthesis of main group analogues of methylene, ethylene, and cyclopropane that are kinetically stable in solution and the solid state at room temperature. In the following review recent advances in the synthesis of these compounds will be referenced, and the concepts, which were developed by a systematic study, will be laid out. In doing so, a detailed description of the complex reactivity of the individual classes of compounds will be avoided in order to highlight the basic principles. These can then be used to approximately predict reactivity and characteristics of new compounds. There are significant differences between formally and even structurally analogous compounds with elements from the higher rows as compared to their “lighter” homologues. For example, while every chemist is familiar with the planar D2h symmetrical structure of H2CCH2 as the only “significant” isomer, it must be accepted that Sn2H4 has four isomers that are separated by very small energy differences. For cis/trans isomerism of olefins only the rotation of the CC double bond needs to be considered, but there are three possible mechanisms for the disilene derivatives R1RSiSiR1R. In the last century cyclopropane was considered to be not producible and this provided the impetus for Baeyer's strain theory. Investigations into small silicon cycles, which were discovered much later, show that this theory has more validity for these rings. As a result of the collected data, it is slowly becoming clearer that precisely the carbon compounds once seen as mere models are in reality the “exotic” compounds. Rules with broader applications can be developed by studying the compounds made up of main group elements.
Separation, Characterization, and Fraction Collection in the Nanoliter Domain with Capillary Electrophoresis
Authors: Paulus Aran
Published: January 2004

The smaller the better! Capillary electrophoresis aims at ever smaller amounts of analyte, as demonstrated by the latest developments described here. Recent results fuel the ultimate dream of the chemist: to find the needle in the haystack and play with it, or to separate and identify by spectroscopy a single component in a complex mixture and subsequently manipulate it, even if it is a single molecule.
A Trinuclear Organoantimony Cation: Structure of [Me2SbSb(Me2)SbMe2][Me2SbBr2]
Authors: Breunig Hans Joachim , Denker Michael , Lork Enno
Published: January 2004

Only stable in the crystal, the title compound 1 contains the first example of a polynuclear organoantimony cation. Interionic Sb …︁ Sb and Sb …︁ Br contacts are observed in 1, and reversible dissociation of 1 in solution gives the starting materials Me2SbBr and Me4Sb2. These components exchange Me2Sb groups in the formation of 1.
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Structures of XeF and Xe2F
Authors: Ellern Arkady , Mahjoub Ali‐Reza , Seppelt Konrad
Published: January 2004

The lattice symmetry chiefly determines the structure of the XeF ion in CsXeF7. The influence of the lone pair of electrons is here very weak, and the anion has a capped octahedral structure (C3v symmetry). The dinuclear anion Xe2F in NO2Xe2F13 can be structurally divided into an XeF6 molecule and an XeF ion. In this case the anion XeF is a capped trigonal prism (C2v symmetry).
Glycals in Organic Synthesis: The Evolution of Comprehensive Strategies for the Assembly of Oligosaccharides and Glycoconjugates of Biological Consequence
Authors: Danishefsky Samuel J. , Bilodeau Mark T.
Published: January 2004

This review provides a personal account of the explorations of a research group in oligosaccharide and glycoconjugate construction. The journey began twenty years ago with the study of Diels–Alder reactions of complex dienes. By extending this methodology to aldehydo‐type heterodienophile equivalents, access to unnatural glycals was gained (LACDAC reaction). From this point a broad‐ranging investigation of the use of glycals in the synthesis of oligosaccharides and other glycoconjugates was begun. Mobilization of glycals both as glycosyl donors and glycosyl acceptors led to the strategy of glycal assembly. Several new glycosylation techniques were developed to provide practical underpinning for this logic of glycal assembly. Glycal‐based paradigms have been shown to be nicely adaptable to solid phase supported synthesis. Moreover, glycal assembly—both in solution and on solid phases—has been used to gain relatively concise and efficient entry to a variety of biologically interesting and potentially valuable constructs. Some of these syntheses, particularly in the field of tumor antigens, have led to novel compounds which are in the final stages of preclinical assessment. This review presents an account of the chemical reasoning at the center of the program.
Polar Ozone Depletion (Nobel Lecture)
Authors: Molina Mario J.
Published: January 2004

Cause and result can be geographically widely separated. This fact is corroborated by the finding that the annually recurring ozone hole over Antarctica, which has been steadily increasing in size since 1985, is predominantly due to anthropogenic emissions from the Northern Hemisphere. How the realization dawned that it is primarily the chlorine atoms released by photochemical reactions of CFCs in the upper stratosphere that destroy the ozone shield is described by M. J. Molina in his Nobel Lecture.
Structural Aspects of Nucleic Acid Analogs and Antisense Oligonucleotides
Authors: Egli Martin
Published: January 2004

Hybridization of complementary oligonucleotides is essential to highly valuable research tools in many fields including genetics, molecular biology, and cell biology. For example, an antisense molecule for a particular segment of sense messenger RNA allows gene expression to be selectively turned off, and the polymerase chain reaction requires complementary primers in order to proceed. It is hoped that the antisense approach may lead to therapeutics for treatment of various diseases including cancer. Areas of active research in the antisense field focus on the mechanisms of cellular uptake of antisense molecules and their delivery to specific cell sites, an improved understanding of how these molecules inhibit the production of proteins, as well as the optimization of the chemical stability of antisense molecules and the thermodynamic stability of the duplexes they form with the mRNA targets. The last two issues in particular have prompted chemists to launch an extensive search for oligonucleotide analogs with improved binding properties for hybridization with RNA and higher resistance toward nuclease degradation. During the last years this research has resulted in a flurry of new chemical analogs of DNA and RNA with modifications in the sugar–phosphate backbone as well as in the nucleobase sites. However, to date little effort has been directed toward uncovering the exact origins of the gain or loss in stability when nucleic acid analogs bind to RNA. Although large amounts of thermodynamic data have been collected, the structural perturbations induced by the modifications in hybrid duplexes are only poorly understood. For many modified oligonucleotides the compatibility of protection, coupling, and deprotection chemistry with standard DNA and RNA synthesis protocols makes it now possible to generate modified nucleic acid fragments or mixed oligonucleotides containing modifications at selected sites in quantities suitable for three‐dimensional structure investigations. Such studies should reveal the structural origins of the observed changes in affinity and specificity of binding for particular modifications and may guide the development of second‐and third‐generation antisense molecules. In addition, the availability of a previously unimaginable variety of modified building blocks and the investigation of their structures provides the basis for a deeper understanding of the native DNA and RNA structures. This contribution will summarize the results of X‐ray crystallographic structure determinations of modified nucleic acid fragments conducted in our laboratory during the last three years and the insights gained from them.
A Stable Methyl Phosphane Oxide/Lithium Amide Complex: a Structural and MO Calculational Investigation of the Mechanism of Proton Abstraction by Alkali Metal Reagents
Authors: Davidson Matthew G. , Davies Robert P. , Mitchell Helen J. , Oakley Rachel M. , Raithby Paul R. , Snaith Ronald , Warren Stuart , Armstrongh David R.
Published: January 2004

The initiation step in a deprotonation of CH‐acidic compounds could be the formation of a complex between the organic precursor and the metalation reagent. This is the conclusion drawn from the reaction of Ph2P(O)CH3 with (Me3Si)2NLi, which yielded a stable dimeric adduct [(Me3Si)2NLi·OP(CH3)Ph2]2 with a central Li2N2 ring. MO calculations on the model system H2LiNLi·OP(CH3)2 support these experimental findings.
Achieving Stable, Reverse Water‐in‐Fluorocarbon Emulsions
Authors: Sadtler Vérpnique M. , Krafft Marie Pierre , Riess Jean G.
Published: January 2004

Dispersing water in a fluorocarbon seems against nature, since two more antipathetic types of liquids, in terms of intermolecular interactions, just do not exist. Every single characteristic relevant to water‐in‐fluorocarbon reverse emulsion stability (solubility and diffusivity parameters, interfacial tension and densities) is unfavorable. Yet stable emulsions of this type can be prepared with an appropriate fluorinated surfactant. Such reverse emulsions have potential as a new class of pulmonary drug‐delivery system.
Enantioselective Hydrogenations on Platinum Colloids
Authors: Bönnemann Helmut , Braun Gerhard A.
Published: January 2004

The chemical reduction of aqueous solutions of platinum salts in the presence of the alkaloid dihydrocinchonidine yields chirally coated platinum colloids. The particle sizes can be varied in a controlled fashion from 1.5 to 4 nm by changing the alkaloid concentration. These chirally coated platinum colloids function in quasi‐homogenous phases as enantioselective hydrogenation catalysts for ethylpyruvate (76% ee), and no structure sensitivity was observed.
Two‐Metal Ion Catalysis in Enzymatic Acyl‐ and Phosphoryl‐Transfer Reactions
Authors: Sträter Norbert , Lipscomb William N. , Klabunde Thomas , Krebs Bernt
Published: January 2004

Numerous studies, both in enzymatic and nonenzymatic catalysis, have been undertaken to understand the way by which metal ions, especially zinc ions, promote the hydrolysis of phosphate ester and amide bonds. Hydrolases containing one metal ion in the active site, termed mononuclear metallohydrolases, such as carboxypeptidase. A and thermolysin were among the first enzymes to have their structures unraveled by X‐ray crystallography. In recent years an increasing number of metalloenzymes have been identified that use two or more adjacent metal ions in the catalysis of phosphoryl‐transfer reactions (R‐OPO3 + R′‐OH → R′‐OPO3 + R‐OH; in the case of the phosphatase reaction R′‐OH is a water molecule) and carbonyl‐transfer reactions, for example, in peptidases or other amidases. These dinuclear metalloenzymes catalyze a great variety of these reactions, including hydrolytic cleavage of phosphomono‐, ‐di‐ and ‐triester bonds, phosphoanhydride bonds as well as of peptide bonds or urea. In addition, the formation of the phosphodiester bond of RNA and DNA by polymerases is catalyzed by a two‐metal ion mechanism. A remarkable diversity is also seen in the structures of the active sites of these di‐ and trinuclear metalloenzymes, even for enzymes that catalyze very similar reactions. The determination of the structure of a substrate, product, stable intermediate, or a reaction coordinate analogue compound bound to an active or inactivated enzyme is a powerful approach to investigate mechanistic details of enzyme action. Such studies have been applied to several of the metalloenzymes reviewed in this article; together with many other biochemical studies they provide a growing body of information on how the two (or more) metal ions cooperate to achieve efficient catalysis.
Cyclopeptide Libraries as New Chiral Selectors in Capillary Electrophoresis
Authors: Jung Günther , Hofstetter Heike , Feiertag Susanne , Stoll Dieter , Hofstetter Oliver , Wiesmüller Karl‐Heinz , Schurig Volker
Published: January 2004

Complex libraries composed of more than five thousand cyclopeptides can act as chiral selectors when added to the mobile phase in capillary electrophoresis. Cyclo‐hexapeptide sublibraries (such as those listed below) were used to achieve baseline separations of enantiomers of Nα‐dinitrophenyl (DNP) and 9‐fluorenylmethoxycarbonyl (FMOC) amino acids and Tröger's base. This approach can be used to identify selectors for difficult‐to‐separate racemates.
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The Tricks of Ribosomal Elongation Factors
Authors: Nierhaus Knud H.
Published: January 2004

The two universally existing ribosomal elongation factors ET‐Tu and EF‐G are archetypes of the large family of G‐proteins, which regulate a plethora of processes in the cell according to a basic scheme. In the past few years the crystal structures elucidated for various conformations of the elongation factors have provided exciting insights into the functions of G‐proteins and into the mechanism of protein biosynthesis.
Catalytic Activation of CH Bonds in Aromatic Hydrocarbons, Ethene, and Methane by the Naphthalene/Sodium System in Tetrahydrofuran
Authors: Rummel Siegbert , Ilatovskaya Margarita A. , Mysov Evgeny I. , Lenenko Vladimir S. , Langguth Helmuth , Shur Vladimir B.
Published: January 2004

Remarkable news concerning the long‐known and well‐investigated reaction system naphthalene/sodium in THF is reported: At room temperature a reversible intermolecular transfer of hydrogen takes place both between naphthalene rings and also with THF molecules. In contrast, the corresponding lithium and potassium systems show no such behavior. In addition, alkanes and cycloalkanes as solvent additives effect a significant increase in the H/D exchange rate in the sodium system. Benzene, toluene, ethene, and even methane (!) also take part in the H/D exchange reactions in the naphthalene/sodium system.
Defluorination of Perfluoroalkanes and Chlorofluorocarbons
Authors: Saunders Graham C.
Published: January 2004

Electron transfer from a reducing agent to a fluorocarbon leads to cleavage of the strong and supposedly inert CF bond. This process can be used to defluorinate perfluoroalkanes to produce perfluoroarenes or ‐alkenes catalytically under mild conditions. The transfer of two electrons from alkali metal oxalates to chlorofluoro‐carbons leads to complete degradation at relatively low temperatures [Eq. (a)].
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The Titanium(IV)‐Catalyzed Epoxidation of Alkenes by tert‐Alkyl Hydroperoxides
Authors: Oldroyd Richard D. , Thomas John Meurig , Maschmeyer Thomas , MacFaul Philip A. , Snelgrove Darren W. , Ingold Keith U. , Wayner Danial D. M.
Published: January 2004

A high catalytic activity towards alkene epoxidation is shown by titanium‐modified MCM41 mesoporous silicas if 2‐methyl‐1‐phenyl‐2‐propyl hydroperoxide (MPPH) is used as oxidant. This first example in which MPPH has been used as an efficient two‐electron oxidizing agent demonstrates that it is a competent replacement for tert‐butyl hydroperoxide (TBHP) in metal‐catalyzed, nonradical hydrocarbon oxidations and that the Ti‐MCM41 catalyzed epoxidation of alkenes proceeds by a nonradical mechanism.
Monitoring the Reaction Progress in Combinatorial Chemistry: 1H MAS NMR Investigations on Single Macro Beads in the Suspended State
Authors: Pursch Matthias , Schlotterbeck Götz , Tseng Li‐Hong , Albert Klaus , Rapp Wolfgang
Published: January 2004

Laborious cleavage and isolation steps are avoided! The method of 1H magic angle spinning (MAS) NMR spectroscopy can be used to monitor the course of the synthesis on a single resin particle in a suspended sample in the nanoliter detection range. This extremely useful technique for combinatorial chemistry was tested on a hydantoin reaction sequence.
TlI[C(SiMe3)3]—An Alkylthallium(I) Compound with a Distorted Tetrahedron of Tl Atoms in the Solid State
Authors: Uhl Werner , Keimling Sven Uwe , Klinkhammer Karl Wilhelm , Schwarz Wolfgang
Published: January 2004

Weak thallium–thallium interactions are present in the thallium tetrahedron of the crystalline alkylthallium(I) compound Tl[C(SiMe3)3], which is easily accessible from cyclopentadienylthallium(I) and tris(trimethylsilyl)methyllithium. In solution, however, only the monomer of the extremely thermally sensitive compound can be detected.
A Stable Hetero‐trans‐cyclopentene
Authors: Krebs Adolf , Pforr Karl‐Ingo , Raffay Walter , Thölke Bernd , König W. A. , Hardt Ingo , Boese Roland
Published: January 2004

For corrigendum see DOI:10.1002/anie.199704121
Laser Chemistry in Suspensions: New Products and Unique Reaction Conditions for the Carbon–Steam Reaction
Authors: Chen Huxiong , McGrath Thomas , Diebold Gerald J.
Published: January 2004

Irradiation of aqueous suspensions of carbon black with a pulsed laser heats the carbon particles to high temperatures, initiating chemical reactions. In addition to H2, CO, and CO2, the usual products in the carbon–steam and shift reactions, CH4, C2H2, C2H4, and C2H6 are also produced as the carbon is consumed.
The Application of Analytical Methods to the Detection of Hidden Explosives and Explosive Devices
Authors: Kolla Peter
Published: January 2004

The detection of hidden explosives has undergone an enormous development due to an increased desire for safety and the increased terrorist attacks in the last few years. This development was made possible in particular by the rapid advances in the development of powerful analytical techniques in general. These technologies, however, must be specially adapted for the problems of explosives detection. These problems encompass, for example, the large variety of different explosives, the camouflage of explosive devices, and the complexity of the composition of suspicious objects. Frequent air‐travelers have most certainly already been confronted with a so‐called explosives detection apparatus. Baggage controls at airports are a very important and well‐known example of the application of detection technologies. This example also serves to demonstrate the high technological requirements, such as the variability of the object to be examined and a control procedure for a sealed object that must be completed with high reliability in a short period of time. The search for explosive devices or weapons cannot, however, be limited to the recognition of an external appearance with the help of X‐ray imaging. These days, explosive devices, in particular, can readily be installed and hidden in objects of daily life by the use of tiny electric and electronic elements. Therefore, in addition to the application of X‐ray imaging, the use of other technologies becomes necessary. The following article describes and discusses methods and scientific limits of explosives detection under the precondition of possible use at security checkpoints.

For corrigendum see DOI:10.1002/anie.199713711
Compounds Containing Planar‐Tetracoordinate Carbon
Authors: Röttger Dirk , Erker Gerhard
Published: January 2004

It is a great challenge to synthesize compounds containing planar‐tetracoordinate carbon that are stable enough to be isolated, investigated, and handled under ambient conditions. There have therefore been many attempts to construct hydrocarbon frameworks that can incorporate and stabilize tetravalent carbon in this “unnatural” planar coordination geometry, for example, by steric constraints. However, success is apparently more readily achieved if the “square Planar” carbon atom is stabilized electronically. Planar‐tetracoordinate carbon is sp2‐hybridized and has an electron deficient σ system. The p orbital orthogonal to the bonding plane is occupied by two electrons. Therefore, σ‐donor/π‐accptor substituents—that is, many metals—are in principle able to stabilize this “unnatural” geometry of tetravalent carbon. Meanwhile, well‐established procedures for synthesizing very stable “anti‐van't Hoff/LeBel compounds” have been devised, in which the planar‐tetracoordinate carbon atom is stabilized, usually by the combined action of two directly bonded metals. In most cases, the square‐planar carbon is part of a double bond system. Typical stabilizing metal combinations are Zr/Al or Zr/B, but there are also examples containing two transition metals from the right‐hand side of the periodic table. Reliable estimates for the energy of stabilization of square planar carbon in such compounds are now available. Both theoretical and experimental investigations show that square planar carbon atoms that are stabilized by Zr/Zr+ and Zr/Al in dimetallic compounds are favored by about 12 and 40 kcalmol−1, respectively, over the trigonal planar alternatives. Square planar coordinated carbon is thus no longer an unusual feature in organometallic chemistry and must now be more frequently considered as an alternative structural possibility.