bluuebunny/crossref_metadata_2025_split huggingface dataset:
Microscopical Structuring of Solids by Molecular Beam Epitaxy—Spatially Resolved Materials Synthesis
Authors: Ploog Klaus 
Published: December 2003

AbstractInterfaces 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

AbstractParallel 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

AbstractFor corrigendum see DOI:10.1002/anie.198812001For corrigendum see DOI:10.1002/anie.198812001For corrigendum see DOI:10.1002/anie.198812001For corrigendum see DOI:10.1002/anie.198812001For 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

AbstractThe 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.
Say It in English, Please!
Authors: Schoenfeld† Robert 
Published: December 2003

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

AbstractThe 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

AbstractBiological 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

AbstractThe 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.
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

AbstractThe 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

AbstractManaging 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

AbstractMetallo‐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.
Theoretical Chemistry en route to a Theory of Chemistry
Authors: Amann Anton , Gans Werner 
Published: December 2003

AbstractQuantum 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.
Modeling Organic Surfaces with Self‐Assembled Monolayers
Authors: Bain Colin D. , Whitesides George M. 
Published: December 2003

AbstractThe 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.
Tailoring Semiconductor Crystals to Atomic Dimensions
Authors: Joyce Bruce A. 
Published: December 2003

AbstractThis 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

AbstractThe 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

AbstractUntil 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.