bluuebunny/crossref_metadata_2025 huggingface dataset:
Vitamin B12: Experiments Concerning the Origin of Its Molecular Structure
Authors: Eschenmoser Albert
Published: December 2003

Following the chemical synthesis of vitamin B12, a search was begun for a potentially biomimetic “dark” variant of the photochemical A/D‐secocorrin → corrin cycloisomerization, the central ring‐closure step in one of the two cobyric acid syntheses. Significantly, not just one but a whole family of such variants was discovered. According to what is currently known, one of these variants can indeed be regarded as a chemical model for the reaction path followed by Nature in the biosynthetic construction of the corrin ring. These chemical studies of vitamin B12 biosynthesis had revealed that the A/D‐ring junction, regarded as the main obstacle to a chemical vitamin B12 synthesis at the outset, is in fact a structural element that is formed readily and in a variety of ways from structurally appropriate precursors. More recent investigations have shown that the same holds for other specific structural elements of the vitamin B12 molecule, including the characteristic arrangement of double bonds in the corrin chromophore, the special dimension of the macrocyclic ring of the corrin ligand, the specific attachment of the nucleotide loop to the propionic acid side chain of ring D, and the characteristic constitutional arrangement of the side chains around the ligand periphery (which vitamin B12 shares with all uroporphinoid cofactors). All these outwardly complex structural elements are found to “self‐assemble” with surprising ease under structurally appropriate preconditions; the amount of “external instruction” required for their formation turns out to be surprisingly small in view of the complexity and specificity of these structural elements. We view these findings as steps on the way toward a chemical rationalization of the vitamin B12 structure. The goal is to arrive experimentally at a perception of the biomolecule's intrinsic potential for structural self‐assembly. This potential, together with the specific type of reactivity related to the biological function, is considered to be responsible for the biomolecule having been chosen by natural selection. The chemical rationalization of the structure of biomolecules is an objective of organic natural product chemistry. The field of natural product synthesis provides appropriate conceptual and methodological tools to approach this objective experimentally.
Supramolecular Chemistry—Scope and Perspectives Molecules, Supermolecules, and Molecular Devices (Nobel Lecture)
Authors: Lehn Jean‐Marie
Published: December 2003

Supramolecular chemistry is the chemistry of the intermolecular bond, covering the structures and functions of the entities formed by association of two or more chemical species. Molecular recognition in the supermolecules formed by receptor‐substrate binding rests on the principles of molecular complementarity, as found in spherical and tetrahedral recognition, linear recognition by coreceptors, metalloreceptors, amphiphilic receptors, and anion coordination. Supramolecular catalysis by receptors bearing reactive groups effects bond cleavage reactions as well as synthetic bond formation via cocatalysis. Lipophilic receptor molecules act as selective carriers for various substrates and make it possible to set up coupled transport processes linked to electron and proton gradients or to light. Whereas endoreceptors bind substrates in molecular cavities by convergent interactions, exoreceptors rely on interactions between the surfaces of the receptor and the substrate; thus new types of receptors, such as the metallonucleates, may be designed. In combination with polymolecular assemblies, receptors, carriers, and catalysts may lead to molecular and supramolecular devices, defined as structurally organized and functionally integrated chemical systems built on supramolecular architectures. Their recognition, transfer, and transformation features are analyzed specifically from the point of view of molecular devices that would operate via photons, electrons, or ions, thus defining fields of molecular photonics, electronics, and ionics. Introduction of photosensitive groups yields photoactive receptors for the design of light‐conversion and charge‐separation centers. Redox‐active polyolefinic chains represent molecular wires for electron transfer through membranes. Tubular mesophases formed by stacking of suitable macrocyclic receptors may lead to ion channels. Molecular self‐assembling occurs with acyclic ligands that form complexes of double‐helical structure. Such developments in molecular and supramolecular design and engineering open perspectives towards the realization of molecular photonic, electronic, and ionic devices that would perform highly selective recognition, reaction, and transfer operations for signal and information processing at the molecular level.
Pharmaceutical Proteins
Authors: Blohm Dietmar , Bollschweiler Claus , Hillen Heinz
Published: December 2003

There is currently great interest in pharmaceutical proteins, and work in this area, from basic research to drug marketing, is expanding vigorously. The starting points of these developments are genetic engineering and biotechnology, which contribute to the discovery of new proteins and allow them to be produced on a worthwhile scale. Ever since the commercial introduction of insulin in 1923, of thyroid hormone in 1934, of factor VIII in 1948, and of calcitonin in 1970, hormones, serum proteins, and enzymes have been firmly established as therapeutic agents. Immunomodulators and various tissue proteins are in a turbulent phase of development as a new approach to the therapy of tumors. Increasingly, diagnostic aids now take the form of enzymes and monoclonal antibodies, and recently there has even been development of vaccines based on defined single proteins and oligopeptides. Of the more than 200 pharmaceutical proteins which have been investigated to date, more than half are undergoing research and development, about 100 are in clinical trials, and a dozen or so have already been marketed. The most important indications for them are cardiovascular disorders, tumors, autoimmune diseases, and infections. Genetically engineered proteins can be used as active substances, which are chemically and biologically exactly defined, for new drug products and in research to acquire new information on therapeutic use, based on the concept of therapy with endogenous proteins. Very recent developments have shown that proteins, either in their natural form or modified, are becoming of increasing importance for research into active substances.
Heterogeneous, Bimetallic Metathesis Catalysts by Reaction of Fischer‐type Carbene Complexes with Reduced Phillips Catalyst
Authors: Weiss Karin , Guthmann Wilfried , Maisuls Sergio
Published: December 2003

Catalytic metathesis reactions with carbene complexes are possible when these complexes are fixed on inorganic supports. Binding of Mo or W carbene complexes to the very efficient polymerization catalyst for the Phillips procedure, “SiO2/Cr”, resulted in their transformation to good metathesis catalysts. Cr carbene complexes, in contrast, gave rise to a system that is active neither for polymerization nor for metathesis.
Complexation of Neutral Molecules by Cyclophane Hosts
Authors: Diederich François
Published: December 2003

Since the discovery of the crown ethers by Pedersen twenty years ago, the chemistry of synthetic hosts for the selective complexation of organic and inorganic guests has seen an extraordinarily rapid development. This article discusses in particular the contributions provided by synthetic cyclophanes as hosts to the understanding of molecular complexation of neutral organic guest molecules in aqueous and organic solvents. In aqueous solution, cyclophanes form stoichiometric complexes with neutral aromatic guests which can approach enzyme‐substrate complexes in their stability. Efficient molecular complexation is also observed in organic environments. Here, as a result of large solvation effects, the strength of complexation is strongly dependent on the nature of the organic solvent. Electron donor‐acceptor interactions can contribute significantly to the stability of complexes formed between cyclophane hosts and aromatic guests. Force‐field calculations together with computer graphics are powerful tools in the design of water‐soluble, optically active hosts for chiral recognition of complexed racemic guests. Simple and selective functionalization of the cyclophane framework leads to stable, bioorganic catalysts. Like enzymes, these catalysts bind their substrates in a rapid equilibrium prior to the reaction steps. As a perspective, some fascinating research objectives in the field of molecular recognition and catalysis which can be targeted with designed cyclophane hosts are shown.
Microscopical Structuring of Solids by Molecular Beam Epitaxy—Spatially Resolved Materials Synthesis
Authors: Ploog Klaus
Published: December 2003

Interfaces and heterojunctions which are incorporated into a crystal in well‐defined geometrical and spatial arrangements can lead to a structuring or engineering of (semiconducting) solids down to atomic dimensions. The electrical and optical properties are then defined locally, and phenomena related to extremely small dimensions (“quantum size effects”) become more important than the actual chemical properties of the materials used. The technique of molecular beam epitaxy allows an atomic layer‐by‐layer deposition in a two‐dimensional growth process, and crystalline materials in alternating layers of arbitrary composition and only a few atomic layers thickness are formed. The synthesis of microscopically structured solids by molecular beam epitaxy affords access to a new class of materials with accurately tailored electrical, optical, magnetic, dielectric, mechanical etc. properties. The semiconductor and metal superlattices described in this article, which are made of alternating thin layers of two different materials, symbolize just the beginning of a new area of materials engineering on a molecular (or atomic) scale. This periodic modulation of the chemical composition normal to the surface imposes an artificial periodicity on the semiconductor or metal crystal, a periodicity of one or two orders of magnitude larger than its natural lattice spacing. The synthesis of other materials combinations, including semiconductor/metal, semiconductor/insulator, metal/insulator, polymers, and magnetic materials, with entirely different properties and for completely different applications will certainly follow. Finally, a large variety of desired combinations of elements can be selected, and even metastable compounds with novel exciting properties can be synthesized by molecular beam epitaxy.
Electron‐Transfer Chain Catalysis in Organotransition Metal Chemistry
Authors: Astruc Didier
Published: December 2003

Parallel to organic electrocatalysis, the field of organotransition metal electrocatalysis has developed explosively since 1980. The theoretical and experimental foundations established by Feldberg in 1971 (ECE mechanism) have been applied, using fast electrochemical techniques, to various organometallic reactions such as isomerization, ligand exchange, chelation, decomplexation, and CO insertion and extrusion. Most of the work performed to date concerns ligand exchange reactions of N‐donors and P‐donors in mononuclear compounds, initiated by oxidants (or anodes) and of carbonyls and P‐donors in clusters, initiated by reducing agents (or cathodes). The preparative aspects of electrocatalysis have already been impressively developed in cluster chemistry and indicate that the technique is extremely useful. This review first delineates the principles and characteristics of electrocatalysis applied in organotransition metal chemistry, and then, after outlining the choice of efficient initiating reagents, goes on to describe the systems up to August 1986.
Synthesis and Conformational Behavior of Dibenzo[2.2]metaparacyclophanediene
Authors: Wong Timothy , Cheung Siu Shing , Wong Henry N. C.
Published: December 2003

For corrigendum see DOI:10.1002/anie.198812001

For corrigendum see DOI:10.1002/anie.198812001

For corrigendum see DOI:10.1002/anie.198812001

For corrigendum see DOI:10.1002/anie.198812001

For corrigendum see DOI:10.1002/anie.198812001
Cyclobutanones and Cyclobutenones in Nature and in Synthesis [New Synthetic Methods(71)]
Authors: Belluš Daniel , Ernst Beat
Published: December 2003

The chemical reactivity of cyclobutanones and cyclobutenones is considerably different from that of cyclic ketones with larger rings; this is due to their ring strain of ca. 25 kcal/mol. Detailed knowledge regarding the influence of this ring strain on regio‐, chemo‐ and stereoselective transformations of four‐membered ring ketones is of particular importance. While several reactions, such as the Baeyer–Villiger reaction, the Beckmann and Favorskii rearrangements and cine‐substitution often proceed in a manner specific to four‐membered rings, other reactions such as the facile ring‐opening by nucleophiles, the rearrangement to tropolones, the thermal [2+2]‐cycloreversion, the isomerization to vinylketenes and the photochemical formation of oxacarbenes are rather specific to cyclobutanones and cyclobutenones. The remarkable selectivity and the excellent yields of such transformations, which are favored or caused by ring strain as the inherent driving force, offer the synthetic chemist fascinating possibilities for the development of new strategies for the synthesis of natural products and biologically active compounds.
Equipotential Surfaces in Periodic Charge Distributions
Authors: Mackay Alan Lindsay
Published: December 2003

A microcomputer is sufficient to calculate and display approximate equipotential surfaces in ionic crystals. It is necessary and, as could be shown, justified to consider only a few Fourier terms. Equipotential surfaces are helpful, for example, in understanding the principles of construction of ionic crystals and the role of electrostatic effects in protein structure.
Silica‐Supported FePd Bimetallic Particles: Formation from Mixed‐Metal Clusters and Catalytic Activity
Authors: Braunstein Pierre , Devenish Robert , Gallezot Pierre , Heaton Brian T. , Humphreys Colin J. , Kervennal Jacques , Mulley Suzanne , Ries Michel
Published: December 2003

Why are bi‐ and multimetallic heterogeneous catalysts more selective and stabler than their monometallic counterparts? Informative in this connection are AEM (analytical electron microscopy) investigations on catalysts which were prepared by thermal decomposition of the clusters 1 and 2 on SiO2 supports. The investigations indicate the presence of highly disperse bimetallic particles having the same metal ratio as the clusters. After the reaction the catalyst from the cluster 1 contains Pd particles (100–300 Å), while the catalyst from 2 contains both Pd as well as Fe particles (200–500 Å).
equation image
Synthesis of TiNF and Structure Determination by Powder Diffraction using Synchrotron Radiation
Authors: Wüstefeld Claus , Vogt Thomas , Löchner Ulrich , Strähle Joachim , Fuess Hartmut
Published: December 2003

An unproblematic determination of the structure of the pseudooxide TiNF, a light green to olive green, air stable and chemically substantially inert powder, has been achieved by synchroton‐powder diffraction. TiNF has an anatase structure and does not exhibit any superstructure, even according to electron‐microscopic findings. TiNF was synthesized by ammonolysis of activated (NH4)2TiF6 with a stream of dry, O2‐free NH3.
The Design of Molecular Hosts, Guests, and Their Complexes (Nobel Lecture)
Authors: Cram Donald J.
Published: December 2003

The founders of “supramolecular chemistry” include Charles J. Pedersen, Donald J. Cram, and Jean‐Marie Lehn, who shared the Nobel Prize for Chemistry in 1987 for their fundamental research in this area of organic chemistry. Their work focused the attention of many chemists on the cavities formed by certain types of molecules. Cations, anions, or neutral molecules can enter the cavities of specifically designed compounds and are held there by intermolecular forces. It is fully justified, therefore, to compare such compounds to biomolecules. How the development began, how it achieved its first successes, and what fascinating possibilities lie in store for future research are discussed by Pedersen and Cram in this issue and by Lehn in the January issue (page 89 ff.). The most recent results from Lehn's research are also reported, appropriately, in this issue.
The Discovery of Crown Ethers (Noble Lecture)
Authors: Pedersen Charles J.
Published: December 2003

The founders of “supramolecular chemistry” include Charles J. Pedersen, Donald J. Cram, and Jean‐Marie Lehn, who shared the Nobel Prize for Chemistry in 1987 for their fundamental research in this area of organic chemistry. Their work focused the attention of many chemists on the cavities formed by certain types of molecules. Cations, anions, or neutral molecules can enter the cavities of specifically designed compounds and are held there by inter‐molecular forces. It is fully justified, therefore, to compare such compounds to biomolecules. How the development began, how it achieved its first successes, and what fascinating possibilities lie in store for future research are discussed by Pedersen and Cram in this issue and by Lehn in the January issue (page 89 ff.). The most recent results from Lehn's research are also reported, appropriately, in this issue.
Say It in English, Please!
Authors: Schoenfeld† Robert
Published: December 2003

Robert Schoenfeld, managing editor of the Australian Journal of Chemistry until his retirement in 1985, was known to countless scientists as the author of The Chemist's English. He was widely admired for his outgoing manner, his enlivening humor, and his keen insight into the interplay of language and science, and he will be deeply missed. Shortly before his untimely death on November 24, 1987, he began writing a series of essays directed initially at German‐speaking writers of English. As in The Chemist's English, he hoped through “Say It in English, Please!” to foster an awareness of good English, this time by pointing out the pitfalls and finer problems awaiting the non‐native speaker. His intention was not to dictate usage but rather to entertain and to provoke discussion. By sharing his own joy in language, he wanted to make writing English more enjoyable for others. The nine completed essays are printed here as a fitting tribute to Bob, who might have begun the series as follows: “I submit these columns to the kind judgment of my readers.”

David Loewus

Angewandte Chemie
Transition Metals in the Synthesis and Functionalization of Indoles [New Synthesis Methods (72)]
Authors: Hegedus Louis S.
Published: December 2003

The use of transition‐metal complexes as reagents for the synthesis of complex organic compounds has been under development for at least several decades, and many extraordinary organic transformations of profound potential have been realized. However, adoption of this chemistry by the practicing synthetic organic chemist has been inordinately slow, and only now are transition‐metal reagents beginning to achieve their rightful place in the arsenal of organic synthesis. Several factors contributed to the initial reluctance of synthetic organic chemists to use organometallic reagents. Lacking education and experience in the ways of elements having d electrons, synthetic chemists viewed organometallic processes as something mysterious and unpredictable, and not to be discussed in polite society. Organometallic chemists did not help matters by advertising their latest advances as useful synthetic methodology, but restricting their studies to very simple organic systems lacking any serious functionality (e.g., the “methyl, ethyl, butyl, futile” syndrome). Happily, things have changed. Organometallic chemists have turned their attention to more complex systems, and more recently trained organic chemists have benefited from exposure to the application of transition metals. This combination has set the stage for major advances in the use of transition metals in the synthesis of complex organic compounds. This review deals with one aspect of this area, the use of transition metals in the synthesis of indoles.
Molecular Mechanisms of Biomineralization in the Formation of Calcified Shells
Authors: Krampitz Gottfried , Graser Gabriele
Published: December 2003

Biological calcification processes (calcification, biomineralization) occur in microorganisms, in plants, and in the animal kingdom. Under physiological conditions, the results of mineral deposition in biological systems can be seen in the formation of bones, teeth, mollusc shells, egg shells, pearls, and corals. There are, however, also pathological aspects of biomineralization, including the formation of kidney stones (renal calculus), gallstones (biliary calculus), intravasal depositions (atherosclerosis, calcinosis), gallstones (biliary calculus), intravasal depositions (atherosclerosis, calcinosis). Abnormal mineralization processes (demineralization) can be found, e.g., in bone resorption (osteoporosis) and caries. A detailed knowledge of the molecular mechanisms of biomineralization could help considerably toward solving some of the problems encountered in orthopedics, urology, cardiovascular science, dentistry and veterinary medicine. Research in biomineralization is always interdisciplinary. In this article the typical interaction between mineral phase and organic matrix will be demonstrated using two examples—mollusc shells and egg shells.
Tumor Lectinology: At the Intersection of Carbohydrate Chemistry, Biochemistry, Cell Biology, and Oncology
Authors: Gabius Hans‐Joachim
Published: December 2003

The scientific description of lectins, a class of sugar‐binding proteins distinct from both sugar‐specific enzymes and antibodies, began nearly 100 years ago. The natural ligands of lectins include the sugar moieties of cell glycoproteins, glycolipids, and proteoglycans. Lectins can selectively recognize these potential information‐storing or ‐transferring structural elements and so contribute to regulating biological processes. Plant lectins are now a well‐established means of characterizing and localizing glycoconjugate‐bound sugar determinants. However, work on endogenous lectins is still confronted by many open questions, whose answers require detailed experiments using a suitable model system. Tumors offer an obvious model, particularly since they allow the possible diagnostic and therapeutic uses of lectins to be explored directly. Tumor lectinology employs a broad range of methods, from the chemical synthesis of the sugar components of neoglycoproteins, and their use to seek out cellular sugar receptors with histochemical and cell‐biological techniques, to the biochemical and immunological characterization of the lectins expressed by tumor cells and the use of homology analysis to find further functional domains. The outlook for clinical applications depends on the harmonious integration of these disparate disciplines in tumor lectinology.
Phosphane‐Stabilized CuI and AgI Silanes
Authors: Cowley Alan H. , Elkins Tammy M. , Jones Richard A. , Nunn Christine M.
Published: December 2003

The CuSi bond in 1a is 2.34 Å long; 1a is the first neutral compound containing this structural element. Compound 1a is formed in 65% yield by reaction of CuCl with Ph3SiLi and Me3P (1:1:3) in THF. Compound 1b, the first compound with an AgSi bond, can be prepared in a similar way.
equation image
Solid State Structure of a Lithium Triorganostannate
Authors: Veith Michael , Ruloff Cornelius , Huch Volker , Töllner Fritz
Published: December 2003

The first details of the coordination sphere of lithium triorganostannates have been obtained with lithium tris(α‐furyl)stannate 1. The compound may be formulated as an ion pair. In the anion, the lithium ion exhibits an unusual, distorted octahedral oxygen coordination. The tin atoms are, as expected, coordinated in a pyramidal fashion.
equation image
Partially Reversible Intercalation of Ferrocene in a Zeolite‐like Host Lattice Consisting of Fe(CN)6 and Me3Sn Units
Authors: Brandt Peter , Brimah Abdul K. , Fischer R. Dieter
Published: December 2003

Only simply trituration at room temperature is required to cause the complete intercalation of ferrocene in the new 3D coordiantion polymer 1 [eq. (a)]. About 50% of the ferrocene can be sublimed out of the deep‐blue paramagnetic intercalate under high vacuum during three days at 80–160 °C.
equation image
Structure and Reactivity of Lithium Enolates. From Pinacolone to Selective C‐Alkylations of Peptides. Difficulties and Opportunities Afforded by Complex Structures
Authors: Seebach Dieter
Published: December 2003

The chemistry of lithium enolates is used to demonstrate that complex structures held together by noncovalent bonds (“supramolecules”) may dramatically influence the result of seemingly simple standard reactions of organic synthesis. Detailed structural data have been obtained by crystallographic investigations of numerous Li enolates and analogous derivatives. The most remarkable features of these structures are aggregation to give dimers, tetramers, and higher oligomers, complexation of the metal centers by solvent molecules and chelating ligands, and hydrogen‐bond formation of weak acids such as secondary amines with the anionoid part of the enolates. The presence in nonpolar solvents of the same supramolecules has been established by NMR‐spectroscopic, by osmometric, and by calorimetric measurements. The structures and the order of magnitude of the interactions have also been reproduced by ab‐initio calculations. Most importantly, supramolecules may be product‐forming species in synthetic reactions of Li enolates. A knowledge of the complex structures of Li enolates also improves our understanding of their reactivity. Thus, simple procedures have been developed to avoid complications caused by secondary amines, formed concomitantly with Li enolates by the common methods. Mixtures of achiral Li enolates and chiral Li amides can give rise to enantioselective reactions. Solubilization by LiX is observed, especially of multiply lithiated compounds. This effect is exploited for alkylations of N‐methylglycine (sarcosine) CH2 groups in open‐chain oligopeptides. Thus, the cyclic undecapeptide cyclosporine, a potent immunosuppressant, is converted into a THF‐soluble hexalithio derivative (without epimerization of stereogenic centers) and alkylated by a variety of electrophiles in the presence of either excess lithiumdiisopropyl amide or of up to 30 equivalents of lithium chloride. Depending on the nature of the LiX additive, a new stereogenic center of (R) or (S) configuration is created in the peptide chain by this process. A structure‐activity correlation in the series of cyclosporine derivatives thus available is discussed.
Chemical Aspects of Fusion Technology
Authors: Ache Hans J.
Published: December 2003

Managing thermally controlled nuclear fusion will certainly be regarded one day as one of the most successful accomplishments in nuclear physics. At the same time, however, it will represent a technical achievement unparalleled in the history of science and engineering. This in turn would mean, in retrospect, that decisive contributions had to come from a number of disciplines as diverse as materials and engineering sciences and classical chemistry, and that the same collaboration will have to continue in the future in order to reach the ultimate goal, to construct a reactor capable of producing energy from almost inexhaustible source materials (fuels), such as deuterium and lithium. What is the chemist's role in this development? Similarly as in the development of fission reactors, i.e., the nuclear power plants currently in operation, chemists will have to ensure the existence of a reliable fuel cycle–starting from the availability, storage and reprocessing of the fuel through to the provision for safe storage of the waste. In this review article an attempt will be made to outline the problems associated with these tasks and the approaches to be made by the chemist in solving them.
Intramolecular, Stoichiometric (Li, Mg, Zn) and Catalytic (Ni, Pd, Pt) Metallo‐Ene Reactions in Organic Synthesis [New Synthetic Methods (75)]
Authors: Oppolzer Wolfgang
Published: December 2003

Metallo‐ene reactions, hardly recognized until very recently, have experienced a breathtaking development when applied in an intramolecular sense. Efficient regio‐ and stereoselective magnesium‐ene cyclizations have served as a cornerstone for numerous syntheses of structurally diverse natural products (e.g., sesquiterpenes of marine or plant origin, alkaloids, fragrances, insect defense compounds, and a fungitoxin). A brilliant example is the synthesis of the elusive odorant (+)‐khusimone which outshines 20 years of work in the field of tricyclovetivane synthesis. Palladium‐, platinum‐, and nickel‐catalyzed versions of the metallo‐ene reaction are in a comparatively early stage of exploration, but, nevertheless, reveal intriguing potential. Hence an almost 100% stereospecific CO→C;Pd‐→ CC chirality transfer permits simple and selective, cis‐ or trans‐annelation processes. The mild cyclization conditions are compatible with various functional groups, such as nitrogen moieties, which offer interesting perspectives for the preparation of heterocycles (e.g., alkaloids) difficult to obtain by other methods. Carbon monoxide insertion reactions of the cyclized σ‐metal intermediates were shown to afford annelated cyclopentanones and cyclopentenones with concomitant stereocontrolled formation of four carbon–carbon bonds. These and other observations, highlighted in this article, provide a platform for further extensions and applications of this powerful method in organic synthesis.
Disilene versus Silanediyl (“Silylene”) Additions to CN and NCO Bonds
Authors: Weidenbruch Manfred , Flintjer Bolko , Pohl Siegfried , Saak Wolfgang
Published: December 2003

For corrigendum see DOI:10.1002/anie.198902281
Theoretical Chemistry en route to a Theory of Chemistry
Authors: Amann Anton , Gans Werner
Published: December 2003

Quantum mechanical and classical aspects are of equal importance in chemistry, and theoretical chemistry must be able to deal with this situation. Classical quantities with which a chemist has to deal include, for example, the temperature and the chemical potential of a substance, and chirality, knot‐type and tertiary structures of molecules. Classical concepts of this type are no more consequences of traditional quantum chemistry than is the theory of complex chemical processes (for example adsorption, heterogeneous catalysis) and macroscopic systems. Algebraic quantum mechanics affords a general framework for the discussion of classical quantities and large systems. There are other independent approaches for the theoretical treatment of chemical problems; these can be summarized by using the keywords “fractals”, “chaotic motion”, “quasicrystals” and “knot theory”. The relationships between these four research topics and their connection with algebraic quantum mechanics, as well as their importance for theoretical chemistry, were discussed at a NATO Workshop in 1987 (see Ref. [1]). In an extended theoretical chemistry, the notion of “molecules” will not play a role as central as it does in quantum chemistry. In the theory of large systems in particular, the introduction of complementary approaches appears to be appropriate; the term “molecule” as used by the chemist does not appear either in the description of disordered systems by means of fractals or in the thermodynamic limit of algebraic statistical mechanics.
Polymers as Advanced Materials
Authors: Eisenbach Claus D. , Haarer Dietrich
Published: December 2003

Editorial Essay: The crucial role that polymers play in the electronics industry is just one example of how modern life has been influenced by these multi purpose materials. Development of novel polymers with new and exciting properties and their effective application requires an interdisciplinary effort. This has guided the structuring of the research effort in West Germany. One result of this has been the founding of an Institute for Materials Science in Bayreuth.
Modeling Organic Surfaces with Self‐Assembled Monolayers
Authors: Bain Colin D. , Whitesides George M.
Published: December 2003

The interfacial properties of organic materials are of critical importance in many applications, especially the control of wettability, adhesion, tribology, and corrosion. The relationships between the microscopic structure of an organic surface and its macroscopic physical properties are, however, only poorly understood. This short review presents a model system that has the ease of preparation and the structural definition required to provide a firm understanding of interfacial phenomena. Long‐chain thiols, HS(CH2)nX, adsorb from solution onto gold and form densely packed, oriented monolayers. By varying the terminal functional group, X, of the thiol, organic surfaces can be created having a wide range of structures and properties. More complex systems can be constructed by coadsorbing two or more thiols with different terminal functional groups or with different chain lengths onto a common gold substrate. By these techniques, controlled degrees of disorder can be introduced into model surfaces. We have used these systems to explore the relationships between the microscopic structure of the monolayers on a molecular and supramolecutar scale and their macroscopic properties. Wettability is a macroscopic interfacial property that has proven of particular interest.
Chromatographic Separation of the Excess Enantiomer under Achiral Conditions
Authors: Matusch Rudolf , Coors Carsten
Published: December 2003

The excess enantiomer can be separated chromatographically without difficulty from mixtures of enantiomers in the ratio a: b (a ≠ b), as are usually formed in enantioselective syntheses. This is possible on achiral stationary phases and with achiral mobile phases, but only in utilizable amounts if both homochiral as well as heterochiral associates are formed in solution. These associates are diastereomeric. Hence, the stationary phase is of decisive importance. On aminopropyl silica gel the excess entiomer is eluted first, then the racemate.
Materials Science and Engineering in Britain
Authors: Humphreys Colin
Published: December 2003

Editorial Essay: The funding of the materials science research effort in British universities and polytechnics has recently been reorganized with the founding of the Materials Science and Engineering Commission of the SERC. The chairman of the commission assesses Britain's performance in materials science and outlines the aims and objectives of the funding bodies in the UK. The recent increase in the importance of advanced materials has resulted in a surge of research activity but have the funding levels kept pace?
Tailoring Semiconductor Crystals to Atomic Dimensions
Authors: Joyce Bruce A.
Published: December 2003

This article provides an introduction to the growth by molecular beam epitaxy (MBE) of semiconductor structures which have dimensions of the same order as interatomic distances in solids. The basic process technology is first described, followed by a brief account of surface reaction mechanisms involved in the growth of GaAs from an atomic beam of Ga and molecular beams of As4 and As2.

From the study of growth dynamics using electron diffraction techniques it is shown how reduced dimensionality structures can be grown and some indication is given of the effect of quantum confinement on material properties. Finally, some recent modifications of MBE based on flux interruption are described.
Cyclizations via Palladium‐Catalyzed Allylic Alkylations [New Synthetic Methods (79)]
Authors: Trost Barry M.
Published: December 2003

The history of ring systems in organic chemistry parallels their synthetic accessibility. Transition‐metal‐catalyzed cyclizations offer a new opportunity to create carbo‐ and heterocyclic compounds with great facility. Among these methods, allylic alkylations catalyzed by palladium have proven unusually productive because of the extraordinary chemo‐, regio‐, and diastereoselectivity and the continuing possibility for the development of enantioselectivity. The rules for ring closure differ from those for non‐transition‐metal‐catalyzed reactions. A major benefit is the ability to generate medium (eight‐, nine‐, ten‐, and eleven‐membered) and large rings in preference to normal (five‐, six‐ and seven‐membered) rings. With the appropriate substrate, efficient macrocyclizations are possible under conditions of normal concentrations. A second major benefit derives from the complementary stereochemistry of the metalcatalyzed substitution (net retention of configuration) compared to non‐metal‐catalyzed reactions (inversion of configuration). Further, the requirement for the substrate to conform to the transition‐metal template may impose a stereochemical preference in the intermediate that ultimately translates into the thermodynamically less stable organic product regardless of the stereochemistry of the starting material. While more work has focused on carbocyclic synthesis, the possibilities for heterocyclic synthesis are just beginning to be tapped. In addition to forming heterocycles by CC bond formation, use of a heteroatom as a nucleophile has already proven effective for oxygen and nitrogen, with other nucleophiles awaiting investigation. New dimensions for cyclization via allylic alkylation arise by generating the requisite π‐allylpalladium intermediates by methods other than palladium(0)‐initiated allylic ionizations. In addition, metals other than palladium will clearly expand the possibilities, but as yet remain untapped.
Single‐Source III/V Precursors: A New Approach to Gallium Arsenide and Related Semiconductors
Authors: Cowley Alan H. , Jones Richard A.
Published: December 2003

Until recently, the production of gallium arsenide, indium phosphide, and related compound semiconductors has fallen in the domain of the materials scientist and the electrical engineer. By clever use of classical chemistry, exemplified by the thermal reaction of Me3Ga and AsH3, it is possible to make semiconductors on a commercial scale. However, there are some drawbacks associated with the existing methodology, including the environmental and health hazards of handling pyrophoric and toxic starting materials as well as stoichiometry control problems and undesirable side reactions. Can the synthetic inorganic and organometallic chemist play a useful role in this important area by designing and developing new reagents for the production of semiconductor materials? We believe the answer is yes, and in this article we discuss a new approach to the preparation of GaAs and InP thin films based on single‐source precursors. These compounds feature strong σ‐bonding between the group III and V elements, together with substituents that are capable of facile thermal elimination. The III/V precursors are more stable toward air and moisture and considerably less toxic than either adducts or mixtures of group III and V compounds.
Very Short CC Single Bonds and Their Origin. The Central Bond Length in Bitetrahedryl
Authors: von Ragué Schlever Paul , Bremer Matthias
Published: December 2003

A reduced CC bond length and a lower rotational barrier: these are apparently the consequences of increasing the HCC bond angle in ethane according to ab initio calculation. An explanation is thereby provided for the abnormally short central CC bond in compounds like bicubyl, in which this bond bears “bent‐back” substituents. Since increasing the bond angle causes rehybridization and increased s character in the CC bond, it is appropriate to compare the hybridization of the bond in bitetrahedryl (calculated length 1.438 Å) with that of the CC single bond in butadiyne (1.384 Å).
Sequence Dependence of Template‐Directed Syntheses of Hexadeoxynucleotide Derivatives with 3′–5′ Pyrophosphate Linkage
Authors: von Kiedrowski Günter , Wlotzka Britta , Helbing Jörg
Published: December 2003

Self‐replication in a minimal chemical system represents both a goal and a challenge for bioorganic chemistry, which concerns itself, among other things, with trying to understand the origin of life. A partial success can now be reported in the creation of a model system consisting of nucleic acid building blocks and capable of self replication under replicase‐free conditions. This article presents evidence that a self complementary template can accelerate—in a highly selective way—the further incorporation of subunits into sequentially homologous oligonucleotides.
Hg8 (“Mercubane”) Clusters in Rb15Hg16
Authors: Deiseroth Hans‐Jörg , Strunck Axel
Published: December 2003

Isolated, nearly cubic Hg8 clusters (“mercubanes”) have now been identified for the first time in the rubidium amalgam Rb15Hg16. Metallic, golden single crystals of this previously unknown compound were produced in the course of an unsuccessful attempt to synthesize RbHg. The geometry of the cluster suggests that electron transfer from the electropositive rubidium produces a chemical bond between mercury atoms that exhibits primarily σp (6p) character rather than the σs character familiar from the classical mercury polycations Hg.
Metal Complexes of Small Cycloalkynes and Arynes
Authors: Bennett Martin A. , Schwemlein Heinz P.
Published: December 2003

Cyclooctyne is the smallest unsubstituted cycloalkyne that can be isolated in the free state and it is more reactive than acyclic alkynes towards transition metal complexes. Smaller cycloalkynes such as cycloheptyne, cyclohexyne, benzyne and cyclopentyne, which are transient molecules in the free state, can be stabilized by coordination either to mononuclear, electron‐rich, transition metal‐containing fragments, e.g. [ZrCp2(PMe3)] and M(PR3)2 (M = Ni, Pt), or by formation of dinuclear or polynuclear metal complexes, e.g. [Os3H2(CO)9(C6H4)] and [Pt2(μ‐PPh2)(μ‐C5H6)(PPh3)3]. The alkynes can donate between two and four π‐electrons to the metal centers, the higher number being favored for the early transition metals. The metal‐cycloalkyne and metal‐aryne bonds in the mononuclear complexes readily insert molecules containing CO, CC, CC and CN bonds, a feature that may be useful in organic synthesis. The highly unsaturated species 1, 4‐benzdiyne acts as a bridging ligand between two metal centers in [{Ni(Cy2PCH2CH2PCy2)}2(μ‐C6H2)].
Organic Dications: Gas Phase Experiments and Theory in Concert
Authors: Lammertsma Koop , von Ragué Schleyer Paul , Schwarz Helmut
Published: December 2003

Knowledge of structure and bonding are integral parts in the understanding of the chemistry of neutral, radicaloid and positively or negatively charged species. The interplay of molecular bonding and the inherent electrostatic repulsion in doubly‐charged molecules expands our insight in the versatility of bonding in electron deficient molecules. Small organic dications are now routinely detected in the gas phase by a variety of mass spectrometric techniques. Ab initio molecular orbital theory has advanced to the stage of accurately predicting potential energy surfaces. This review highlights the symbiotic relationship between gas‐phase ion and computational studies and gives a rationale for the salient bonding features in doubly‐charged organic cations.
HoAl3Cl12 in Crystalline Form
Authors: Hake Dietmar , Urland Werner
Published: December 2003

The chemical transport of lanthanoid halides via the gas phase is possible, for example, with A1Cl3. Spectroscopic investigations have revealed the formation of the “gas complexes” LnAl3Cl12 and LnAl4Cl15. The holmium‐containing title compound has now been characterized by X‐ray structure analysis. HoAl3Cl12 contains square HoCl8 antiprisms edge‐linked with AlCl4 tetrahedra, resulting in the overall formation of screw‐shaped chains.
15N CPMAS NMR Study of the Structure of Polyaniline
Authors: Wehrle Bernd , Limbach Hans‐Heinrich , Mortensen John , Heinze Jürgen
Published: December 2003

Communication: The structure and properties of polyaniline are investigated using 15N cross polarization magic angle spinning (CPMAS) solid state NMR spectroscopy. The high resolution spectra obtained demonstrate the potential of the method for studying the structures of other nitrogen containing heterocyclic polymers.
Powder Metallurgy (PM'89)in San Diego
Authors: Laag Rainhard
Published: December 2003

Conference Reports: Rainhard Laag reports on the Powder Metallurgy '89 meeting, Hugh Byrne gives us a taste of macromolecular physics in Poland and Peter Bachmann describes a recent meeting on surface modification technology held in Switzerland.
Surface Modification Technology in Neuchâtel
Authors: Bachmann Peter K.
Published: December 2003

Conference Reports: Rainhard Laag reports on the Powder Metallurgy '89 meeting, Hugh Byrne gives us a taste of macromolecular physics in Poland and Peter Bachmann describes a recent meeting on surface modification technology held in Switzerland.
Anions Isosteric with CO in Cs5SiP3, Cs5SiAs3, Cs5GeP3, and Cs5GeAs3
Authors: Eisenmann Brigitte , Klein Jürgen , Somer Mehmet
Published: December 2003

Planar, isolated anions SiP, SiAs, GeP and GeAs are present in the crystals of the dark, shiny metallic title compounds (Zintl phases). The 24‐valence electron units are the first examples of CO ‐isosteric anions in which central atoms and ligands belong to higher periods. Bond orders (after Pauling) of 1.22, 1.28, 1.12 and 1.26, respectively, can be derived from the measured bond lengths.
About the Li⊕ Gegenion Effect on α‐Sulfonyl Carbanions
Authors: Gais Hans‐Joachim , Hellmann Gunther , Lindner Hans Jörg
Published: December 2003

To what extent do the structure and dynamics of the α‐sulfonyl carbanion l⊖ depend on the gegenion? As shown by the X‐ray structure analyses of the lithium‐ and tetrabutylammonium salts, these are present in the crystal as a dimer (1⊖/Li⊕ · 2THF)2with an eight‐membered (SOLiO)2ring and as a monomer 1⊕/NBu4 with discrete cations and anions, respectively. In contrast, the CαS rotation barrier of 1⊖ is hardly influenced by NBu (in THF or DMSO) and K⊕ (in DMSO).
equation image
Highly Reactive Intermediates from the Cocondensation Reactions of Iron, Cobalt and Nickel Vapor with Arenes
Authors: Zenneck Ulrich
Published: December 2003

In 1969, P. L. Timms reported the first preparative cocondensation reactions of metal vapors with organic and inorganic substrates. The use of this technique in preparative chemistry soon spread rapidly, but in recent years there has been less activity in this sector. If metal atom reactions are not utilized primarily for the formation of new products, but for the synthesis of highly reactive intermediates, a new synthetic strategy may be developed. Our aims are reaction sequences which, based on an effective cocondensation reaction, lead gradually and selectively to new substance classes. This principle can be illustrated by the example of the cocondensation products of arenes and iron, cobalt, or nickel vapor, which decompose between −70 and −50°C. The classes of products accessible by this method extend from clusters, through π‐complexes, organophosphorus and organoboron cage compounds to pure organic cycloaddition products.
Alkyltechnetium Oxides: First Examples and Reactions
Authors: Herrmann Wolfgang A. , Alberto Roger , Kiprof Paul , Baumgärtner Franz
Published: December 2003

A “diagonal relationship” between technetium(VII) and osmium(VIII) can be proposed based on the properties of the newly synthesized title compounds. Like the valence‐isoelectronic OsO4, MeTcO3 adds olefins to form metallacycles (→cis‐diols), whereas the formally analogous MeReO3 epoxidates olefins in the presence of H2O2. This difference is presumably due to the oxidizing strength of TcVII being much greater than that of ReVII.