bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic Solvent Effects on Electron‐Transfer Reactions
Authors: Heitele Hans
Published: December 2003

Electron‐transfer processes in solution are among the most important reactions in chemistry and biology. The huge number of redox reactions of transition metal ions and complexes, many preparatively important oxidations and reductions of organic compounds, photosynthesis, and metabolism are only a few examples where electron‐transfer reactions play a pivotal role. This ubiquity, as well as their relative simplicity, makes them excellent models for the study on a molecular level of chemical reactions in solution. A particularly important question in chemical reaction dynamics in solution is the influence of the solvent on the reaction rate. In this context one distinguishes between static and dynamic solvent effects. Static effects refer to the stabilization of reactants, transition state, and products, that is, how the solvent affects the free energies of these species and the energy of activation. This interpretation of solvent effects on all kinds of chemical reactions is well established. A more recent development is the investigation of the influence of solvent dynamics on the rate of a reaction. The transfer of an electron is usually thought to be triggered by a fluctuation of the dielectric polarization in the surrounding solvent. The dynamics of such fluctuations is determined by the finite response time of the orientational polarization of the solvent. Under certain conditions this dielectric response time can become the rate‐determining factor of the reaction. In this article I intend to give a review of these modern developments in the theory and experimental study of electron‐transfer processes. We shall see that solvent dynamics may lead to a whole plethora of phenomena in reaction dynamics. The concepts needed for their description are not limited to electron transfer but bear relevance to many other chemical reactions in solution.
Heterogeneous Catalysis—Still Magic or Already Science?
Authors: Schlögl Robert
Published: December 2003

Heterogeneous catalysis is still regarded as black magic, since the development of practical catalysts is dependent on empirical facts. Research in this area has been restricted by the complexity of solid catalyst systems, the many interrelated factors that determine the reaction mechanism, and the fact that these catalytic processes cannot be analyzed in situ. This highlight describes scientific approaches that could lead catalyst research out of its infancy, but also warns against misinterpretion of new results.
Silylamines with Pyramidal Coordination at Nitrogen
Authors: Ruhlandt‐Senge Karin , Bartlett Ruth A. , Olmstead Marilyn M. , Power Philip P.
Published: December 2003

The π component of the SiN bond in silylamines has little if any influence on their structure—this was established from the X‐ray structures of the silylamines 1–3. The structural analyses show that the coordination at the nitrogen centers is pyramidal rather than planar and that the SiN distances are essentially invariant. Mes = 2,4,6‐Me3C6H2, Triph = 2,4,6‐Ph3C6H2.
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Optical Spectroscopy of Single Impurity Molecules in Solids
Authors: Moerner W. E. , Basché Thomas
Published: December 2003

Optical spectroscopy of a single impurity molecule provides the first truly local probe of host–guest interactions in doped solids. In conventional optical high‐resolution experiments with many molecules only ensemble averages of the microscopic parameters can be obtained. In the single‐molecule regime, the exquisite sensitivity of an individual dopant molecule to both the local environment and to external perturbations has been exploited in recent experiments to reveal a wealth of fascinating novel phenomena such as spectral diffusion in crystals and polymers, optical modification of a single impurity molecule (spectral hole‐burning), local field measurements at the site of a single impurity molecule, and quantum optics in a solid.
Poly(hydroxyalkanoates): A Fifth Class of Physiologically Important Organic Biopolymers?
Authors: Müller Hans‐Martin , Seebach Dieter
Published: December 2003

Along with polyisoprenoids, polypeptides, polysaccharides, and polynucleotides, Nature contains a further group of biopolymers, the poly(hydroxyalkanoates). The commonest member of this group, poly[(R)‐3‐hydroxybutyrate] P(3‐HB), had been identified by Lemoigne as early as the 1920s, as a storage substance in the microorganismBacillus megateriummade up of more than 12000 (3‐HB) units. However, the widespread distribution and significance of these biopolymers has only become clear recently. The work of Reusch, in particular, has shown that low molecular weight P(3‐HB) (100–200 3‐HB units) occurs in the cell membranes of prokaryotic and eukaryotic organisms. The function of P(3‐HB) in the latter sources is largely unknown; it has been proposed that a complex of P(3‐HB) and calcium polyphosphate acts as an ion channel through the membrane. Indeed, it has even been speculated that P(3‐HB) plays a role in transport of DNA through the cell wall. In the present article, the following subjects will be discussed: metabolism of P(3‐HB) and analogous polyesters in the synthesis and degradation of storage materials; P(3‐HB) as a starting material for chiral synthetic building blocks; synthesis of cyclic oligomers (oligolides) of up to ten 3‐HB units, and their crystal structure; high molecular weight bio‐copolymers of hydroxybutyrate and hydroxyvalerate (BIOPOL) as biologically degradable plastics; nonbiological production of polyhydroxyalkanoates from 3‐hydroxy carboxylic acids and the corresponding β‐lactones; specific synthesis of linear oligomers with a narrow molecular weight distribution, consisting of about 100 (R)‐3‐hydroxybutyrate units, by using an exponential coupling procedure; structure of the polyesters, and a comparison with other polymers; the experimental results which led to the postulation of a P(3‐HB) ion channel through the cell wall; modeling of P(3‐HB) helices of various diameters, by using the parameters obtained from the crystal structures of oligolides; formation of a crown ester complex and ion transport experiments with the triolide of 3‐HB. The article describes one example of the contributions that synthetic organic chemists can make to important biological problems in an interdisciplinary framework.
Concerning “An Opinion on the Heterogeneous Photoreduction of N2 with H2O”: First Letter
Authors: Augugliaro Vincenzo , Soria Javier
Published: December 2003

The synthesis of ammonia from nitrogen and water by irradiation in the presence of certain metal oxide powders as catalysts has been published a number of times since 1977 but was called into question last year by Edwards et al. (Angew. Chem. Int. Ed. Engl. 1992, 31, 480). Criticism of those arguments and the authors' reply should allow our readers to draw their own conclusions. This discussion is welcome in view of the importance of the reaction, whose apparently simple stoichiometry is even open to debate.
Concerning “An Opinion on the Heterogeneous Photoreduction of N2 with H2O”: Second Letter
Authors: Palmisano Leonardo , Schiavello Mario , Sclafani Antonino
Published: December 2003

The synthesis of ammonia from nitrogen and water by irradiation in the presence of certain metal oxide powders as catalysts has been published a number of times since 1977 but was called into question last year by Edwards et al. (Angew. Chem. Int. Ed. Engl. 1992, 31, 480). Criticism of those arguments and the authors' reply should allow our readers to draw their own conclusions. This discussion is welcome in view of the importance of the reaction, whose apparently simple stoichiometry is even open to debate.
The Seven‐Component Reaction**
Authors: Dömling Alexander , Ugi Ivar
Published: December 2003

For corrigendum see DOI:10.1002/anie.199309161
A New Type of Hexanuclear Iron(III) Hydroxo(oxo) Cluster
Authors: Harding Charlie J. , Henderson Richard K. , Powell Anne K.
Published: December 2003

Bridging μ3‐oxo and μ2‐hydroxo ligands characterize the structure of the cluster anion 1, which can be prepared as a highly soluble K or Na salt in water. The structure of 1 can be viewed as two parallel, μ3‐oxo‐bridged Fe3 triangles joined by μ‐hydroxo bridges or as two Fe4 butterfly structures with shared wing tips. Cluster 1 has more short FeFe separations than all other related compounds, and its effective magnetic moment μeff increases dramatically at low temperatures. This magnetic behavior is not completely understood.
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Capillary Electrophoresis: Methods and Scope
Authors: Engelhardt Heinz , Beck Wolfgang , Kohr Jörg , Schmitt Thomas
Published: December 2003

Capillary electrophoresis is a new analytical technique that has seen a great upswing in recent years, because this adaptation of electrophoresis may be automated and simple, direct quantification is possible. As in conventional electrophoresis, samples with a large range of molecular weights, from inorganic ions to biopolymers such as DNA and proteins, can be separated. Uncharged molecules can also be separated when micelle‐forming detergents are added to the buffer; the distribution mechanism that then comes into play increases the separation efficiency of this micellar electrokinetic chromatography. Chiral additives such as cyclodextrins make the separation of enantiomers possible. The rapid increase in the popularity of capillary electrophoresis is reflected in instrument sales and the growing number of scientific publications dealing with the method, and it seems that the future of this technique is assured.
The Coordination Chemistry of Multidentate Isocyanide Ligands
Authors: Hahn F. Ekkehardt
Published: December 2003

In spite of the excellent ligation properties of isocyanides, until a few years ago there was only a small number of known multidentate ligands of this type. One of the reasons for this lack of interest, when compared to monodentate isocyanides, was the linear arrangement of the MCNR group, which usually inhibits the formation of mononuclear chelate complexes and leads to the formation of multinuclear or polymeric metal complexes. In these, the multidentate ligand acts in a monodentate fashion towards each metal atom. Only recently has a series of polyisocyanides with large ligand backbones been synthesized successfully. Bidentate isocyanides can bridge two metal atoms or react to give chelates with only one metal center. Tripodal ligands form mono‐ or binuclear complexes, in which the largest organometallic rings observed to date occur (up to 36 atoms). This class of ligands promises to be interesting for the synthesis of stable, diagnostically important technetium complexes of the type [Tc(CNR)6]+. There also appear to be applications for tripodal isocyanides in catalysis. A facial, chiral Cr(CNR*)3 unit might be able to catalyze the hydrogenation or isomerization of prochiral double bonds. It is even possible to bind triisocyanides with suitable backbones to carbonyl trimetal clusters, thereby stabilizing them, or making selective cluster formation possible. Coordinated isocyanides can be transformed readily into carbene ligands, which, in the future, could lead to complexes with polycarbene ligation.
Ordered Molecular Arrays as Templates: A New Approach to the Synthesis of Mesoporous Materials
Authors: Behrens Peter , Stucky Galen D.
Published: December 2003

Mesoporous solids with adjustable pore sizes and narrow pore size distributionshave been lacking in the repertoire of the solid‐state chemist and materials scientist. By extending typical zeolite synthesis procedures to an approach that can be designated as templating by self‐assembled molecular arrays, researchers from the Mobil Oil Company have now been able to crystallize materials with pore sizes adjustable between 16 and over 100 Å; the pore size distributions are as narrow as those of the conventional zeolites.
Hypothetical Carbon Modifications Derived from Zeolite Frameworks
Authors: Nesper Reinhard , Vogel Karlheinz , Blöchl Peter E.
Published: December 2003

sp3‐Hybridized carbon atoms are the building blocks for the hypothetical carbon modifications described here, which were obtained by replacing the MO4/2 units in zeolites (M = Si, Al, etc.) with C. According to calculations the modifications derived from zeolite ZSM‐39 and melanophlogite are the most stable carbon allotropes after graphite and diamond. Their carbon frameworks are constructed of face‐sharing fullerene‐like C20, C24, and C28 cages and can thus be considered polymerization products of fullerenes.
Bromine Azide—Determination of the Molecular Structure by Electron Diffraction in the Gas Phase
Authors: Hargittai Magdolna , Tornieporth‐Oetting Inis C. , Klapötke Thomas M. , Kolonits Maria , Hargittai István
Published: December 2003

From bromine and sodium azide, explosive bromine azide, BrN3, was prepared solvent‐free and structurally characterized for the first time by electron diffraction studies in the gas phase. The BrN3 molecule has a trans configuration with a slightly bent N3 unit, in accord with recent ab initio calculations, and a Br‐N‐N angle that is consistent with sp2 hybridization at the N atom attached to the Br atom.
Phosphane (PH3) in the Biosphere
Authors: Gassmann Günter , Glindemann Dietmar
Published: December 2003

Phosphane is one of the last small molecules whose biogenesis has not yet been elucidated. The first analyses of the PH3, content of the rumen and gut as well as the feces and liquid manure of cattle and pigs, of the feces of man and of the intestinal tract of fish indicate the ubiquitous production of PH3, in the bio‐ and hydrosphere and that its formation is closely correlated with methane biogenesis. In view of the toxicity of phosphane these results are particularly significant; on a humorous note, they may also provide a plausible explanation for “fire‐breathing” dragons and will‐o'‐the wisp.
Superelectrophiles
Authors: Olah George A.
Published: December 2003

Electrophiles that are capable of further interaction (coordination, solvation) with strong Brønsted or Lewis acids can be activated in this way. The resulting enhancement of their reactivity is great compared to that of their parent compounds under conventional conditions and indicates superelectrophile formation (i.e., electrophiles with doubly electrodeficient (dipositive) nature whose reactivity substantially exceeds that of their parent electrophiles). As representative examples the protolytic or electrophilic activation of the following electrophiles in superacidic and related strongly electrophilic media is discussed: oxonium, carboxonium, sulfonium, selenonium, and telluronium ions; acyl cations; protonated CO, CO2, COS, and carbonic and thiocarbonic acids; heteroatom‐substituted carbocations (and some of their boron analogs); halonium ions; azonium (including nitronium) and carbazonium ions. Emphasis is placed on both experimental chemical investigations and theoretical treatment of the involved systems. As prototypes for protolytically activated onium ions, the protohydronium dication (diprotonated water) and its sulfur analogue, the protosulfonium dication, are of particular significance. The protoacetyl and protoformyl dication (diprotonated carbon monoxide), diprotonated carbonic acid and carbon dioxide, diprotonated hydrogen cyanide and nitriles, as well as the protonitronium dication among others—and their emerging chemistry are discussed as examples of superelectrophiles.
Solid‐State Chemistry with Nonmetal Nitrides
Authors: Schnick Wolfgang
Published: December 2003

Among the nonmetal nitrides, the polymeric binary compounds BN and Si3N4are of particular interest for the development of materials for high‐performance applications. The outstanding features of both substances are their thermal, mechanical, and chemical stability, coupled with their low density. Because of their extremely low reactivity, boron and silicon nitride are hardly ever used as starting materials for the preparation of ternary nitrides, but are used primarily in the manufacture of crucibles or other vessels or as insulation materials. The chemistry of ternary and higher nonmetal nitrides that contain electropositive elements and are thus analogous with the oxo compounds such as borates, silicates, phosphates, or sulfates was neglected for many years. Starting from the recent successful preparation of pure P3N5, a further binary nonmetal nitride which shows similarities with Si3N4 with regard to both its structure and properties, this review deals systematically with the solid‐state chemistry of ternary and higher phosphorus(V) nitrides and the relationship between the various types of structure found in this class of substance and the resulting properties and possible applications. From the point of view of preparative solid‐state chemistry the syntheses, structures, and properties of the binary nonmetal nitrides BN, Si3N4, and P3N5 will be compared and contrasted. The chemistry of the phosphorus(V) nitrides leads us to expect that other nonmetals such as boron, silicon, sulfur, and carbon will also participate in a rich nitride chemistry, as initial reports indeed indicate.
Conjugated Enediynes—An Old Topic in a Different Light
Authors: Gleiter Rolf , Kratz Detlef
Published: December 2003

New carbon modifications and extensions to natural products chemistry are the motivation for the current investigations of enediynes. Twenty years ago, when this area was first in the spotlight, the synthesis of aromatic and antiaromatic π‐electron systems was the focus. In this highlight the many different aspects of the research then and now are discussed with particular emphasis on recent work by Nicolaou, Diederich, and Vollhardt.
Gaseous Mercury(IV) Fluoride, HgF4: An Ab Initio Study
Authors: Kaupp Martin , von Schnering Hans Georg
Published: December 2003

HgF4 is more stable than the well‐known KrF2, according to high‐level ab initio calculations. Decomposition of the square‐planar species into HgF2(g) and F2(g) probably is a slightly endothermic process (by about 10–15 kJ mol−1), and oxidation of HgF2(g) by KrF2(g) is substantially exothermic (by about 90 kJ mol−1). Variants of the latter reaction, carried out in solution and at low temperatures, might lead to HgF4; the calculated vibrational frequencies should aid the experimental characterization.
Controlling Valence Tautomerism of Cobalt Complexes Containing the Benzosemiquinone Anion as Ligand
Authors: Adams David M. , Dei Andrea , Rheingold Arnold L. , Hendrickson David N.
Published: December 2003

The transformation from CoII to CoIII (e.g., 1 → 2) is displayed by the compounds of type 1 in solution when the temperature is lowered (L is a bidentate N‐donor ligand with a diimine structure, 3,5‐dtbsq− the 3,5‐di‐tert‐butylbenzosemiquinone anion, and 3,5‐dtbcat− the corresponding catecholate ion). In the process one electron is transferred from the Co center to one of the 3,5‐dtbsq− ligands to form the catecholate ligand. Microcrystalline samples in which L is phenanthroline undergo the transformation abruptly at about 240 K.
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The Role of Chemistry in the Development of Boron Neutron Capture Therapy of Cancer
Authors: Hawthorne M. Frederick
Published: December 2003

A therapeutic method that selectively destroys malignant cells in the presence of normal cells is a highly valued goal of oncologists and the possible salvation of cancer patients afflicted with some incurable forms of the disease. Selective cell destruction is, in principle, possible with a binary therapeutic strategy based upon the neutron capture reaction observed with the 10B nucleus and a neutron of low kinetic energy (thermal neutron). This nuclear fission reaction produces both 4He and 7Li+ nuclei along with about 2.4 MeV of kinetic energy and weak γ‐radiation. Since the energetic and cytotoxic product ions travel only about one cell diameter in tissue one may specify the cell type to be destroyed by placing innocent 10B nuclei on or within only the doomed cells. This article describes the current status of chemical research aimed at the eventual adoption of this therapeutic method (boron neutron capture therapy or BNCT). The multidisciplinary nature of this research effort involves chemistry, biology, nuclear physics, medicine, and related specialties. Methods devised for bringing 10B nuclei to tumor cells in therapeutic amounts are correlated with the structure of a generalized cell and the various cellular compartments available for boron localization. The synthesis methods employed for the creation of boron‐containing biomolecules and drugs are presented along with representative data concerning their efficacy in tumor localization. The outlook for BNCT is especially bright at this time because of rapid developments in the fields of bioorganometallic chemistry, microbiology, immunology, and nuclear science, to name but a few. Very effective boron delivery vehicles have been demonstrated, and through the interaction of chemistry and biology these species are undergoing further improvement and evaluation of their suitability for BNCT.
Classical and Nonclassical Methyleneboranes
Authors: Berndt Armin
Published: December 2003

Methyleneboranes are stable compounds when their reactive CB bonds are shielded by sterically demanding substituents and the electron deficiency at the dicoordinate boron atom is reduced through delocalization of adjacent electron pairs. In the classical amino(methylene)boranes of Nöth and Paetzold, the electron deficiency at the dicoordinate boron atom is relieved through π‐π delocalization of a neighboring nitrogen atom's formally nonbonding electron pair, that is, through formation of a classical two‐center, two‐electron (2c–2e) bond. In nonclassical methyleneboranes, which are the focus of this review, the electrondeficient center at the dicoordinate boron atom forms nonclassical three‐center, two‐electron (3c2e) bonds with neighboring σ bonds. These methyleneboranes exhibit structural and spectroscopic properties similar to carbenium ions having strong σ–π delocalization (hyperconjugation, bridging) and to transition metal complexes having strong agostic interactions.
“Noncoordinating” Anions, II
Authors: Seppelt Konrad
Published: December 2003

An idealized state of a solution without cation–anion interactions can now be quite well approximated with weakly coordinating anions. Consequently, the prerequisites for the stabilization of coordinatively unsaturated cationic compounds that show unusual bonding or that are discussed as catalytically active species have been achieved. Strauss et al. have now established tellurate‐containing anions such as 1–3, which show several advantages over the frequently used tetraarylborates and carborane anions. Cations such as [Ag(CO)2]+ and [Ag(CH2Cl2)3]+ can be isolated with them—in these two examples, for instance, the counterions are 1 and 3, respectively.
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Alternating Ferro‐ and Antiferromagnetic Interactions in a Chainlike CuII Coordination Polymer
Authors: Julve Miguel , Lloret Francesc , Faus Juan , De Munno Giovanni , Verdaguer Michel , Caneschi Andrea
Published: December 2003

2,2′‐Bipyrimidine (bpym) and hydroxo ligands alternately bridge the metal atoms in the complex 1, which exists as a chainlike coordination polymer. In this complex ferro‐ and antiferromagnetic interactions between the CuII centers alternate along the chain. In contrast, in the dimeric parent compound 2, which has a central hydroxo‐bridged Cu2 unit and terminal bpym ligands, a strong ferromagnetic coupling occurs between the CuII centers.
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(C6F5NGaMe)4 and (C6F5NInMe)4: The First Gallium–Nitrogen and Indium–Nitrogen Compounds with Cubane Structures
Authors: Belgardt Thomas , Roesky Herbert W. , Noltemeyer Mathias , Schmidt Hans‐Georg
Published: December 2003

Ga4N4 and In4N4 cubes are the central structural units of the tetrameric iminometalanes 1 and 2, respectively. These are accessible from pentafluoroaniline and trimethylmetalane by methane elimination. Characteristic for these compounds is the increase in the distortion of the cubane framework with the increasing mass of the metal atom.
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Electron Transfer Reactions in Chemistry: Theory and Experiment (Nobel Lecture)
Authors: Marcus Rudolph A.
Published: December 2003

Since the late 1940s, the field of electron transfer processes has grown enormously, both in chemistry and biology. The development of the field, experimentally and theoretically, as well as its relation to the study of other kinds of chemical reactions, presents to us an intriguing history, one in which many threads have been brought together. In this lecture, some history, recent trends, and my own involvement in this research are described.
The Chemistry of the Fullerenes: An Overview
Authors: Hirsch Andreas
Published: December 2003

C60 is a chemical compound just like many others—this is the result of a large number of preparative reactions, which have been reported in the last few months. The selection presented here shows that C60 reacts, for example, as an olefin in [4 + 2], [3 + 2], and [2 + 1 ] cycloadditions. The preparative difficulties encountered at the beginning of fullerene research resulted in part from the hectic state of the initial burst of activity. After the “reign” of the physicists and physical chemists, it now appears to be the time of the preparative chemists.
Prediction of the Secondary Structure of Proteins from the Amino Acid Sequence with Artificial Neural Networks
Authors: Schneider Gisbert , Wrede Paul
Published: December 2003

Secondary structures of amino acid sequences can be predicted with over 70% accuracy in Heidelberg with the aid of artificial neural networks. This improvement over the accuracy of statistical methods is extremely important in view of the rational design of peptides and proteins and the processing of data in sequence data banks. The potential of neural networks is thus demonstrated once again (see also the review “Neural Networks in Chemistry” by J. Gasteiger and J. Zupan in the April issue of Angewandte Chemie).
A Novel Mixed‐Valence Tetranuclear VII/VIII Complex Isolated from 3,5‐Di‐tert‐Butylcatechol/Vanadium(II)/Sodium Methoxide System for Dinitrogen Reduction
Authors: Luneva Nataly P. , Mironova Svetlana A. , Shilov Alexander E. , Antipin Mikhail Yu. , Struchkov Yuri T.
Published: December 2003

Vanadium centers octahedrally surrounded by oxygen atoms are found in the crystals of the title compound 1. This mixed‐valent complex can be isolated from the N2‐reducing system dbcat/VCl2/NaOCH3 in methanol after the catalytic activity has ceased. The structure of 1 supports the conjecture that multinuclear complexes participate in N2‐reducing systems in protic media. dbcat = 3,5‐di‐tert‐butylcatechol.
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A Self‐Replicating System from Three Starting Materials
Authors: Achilles Thomas , von Kiedrowski Günter
Published: December 2003

The reaction scheme may resemble a circuit diagram, but it actually represents a self‐replicating system consisting of a trimeric, a dimeric, and a monomeric building block. The trimeric to hexameric deoxynucleotide derivatives arise in the presence of a carbodiimide and exhibit catalytic, cross‐catalytic, and autocatalytic template effects that feed back into the various synthesis pathways. The major product is a tetramer that is formed preferably in a nonautocatalytic reaction path. In the autocatalytic pathway this product is exclusively “egoistic”, whereas the pentamer and hexamer enter into an “altruistic” relationship and “socialize” (i.e. thus behave as a molecular ensemble).
How Nature Synthesizes Vitamin B12A Survey of the Last Four Billion Years
Authors: Scott A. Ian
Published: December 2003

This review contains an account of recent experimental results in the continuing saga of vitamin B12 biosynthesis (largely from the author's laboratory) as well as a personal view of future directions of research in natural product biosynthesis. The emphasis is on the powerful combination of molecular biology in the search for, and the expression of, the genes encoding the biosynthetic enzymes and state‐of‐the‐art spectroscopic techniques, in order to “view” the biochemical events as they take place in the NMR tube. As a logical development of these approaches, the feasibility of one‐flask, multienzyme synthesis of natural products is addressed.[

Based on a lecture given at the 27th Euchem Conference on Stereochemistry, 26th April to 4th May, 1992. Bürgenstock, Switzerland. A more explicit account of this new field has been given in “Genetically Engineered Synthesis of Complex Natural Products” (A. I. Scott Terruhedron, 1992, 48, 2559).

] Implicit throughout is the profound change in the “tools of the trade” of the bioorganic chemist, thanks to the harnessing and exploitation of cloning techniques and the resultant availability of enzymes which can make carbon‐carbon bonds. It is also our desire to alert organic chemists, who may wish to take advantage of these technical developments, to the fact that they will be rewarded by a new world of natural catalysts capable of high yielding, synthetic chemistry often with a surprising, but welcome, lack of substrate specificity. Finally we hope to convey our enthusiasm for the methods now at the disposal of present and future generations of chemists for studying Nature's synthetic routes to complex natural products.
Peptidomimetics for Receptor Ligands—Discovery, Development, and Medical Perspectives
Authors: Giannis Athanassios , Kolter Thomas
Published: December 2003

Peptides, as neurotransmitters, neuromodulators, and hormones, influence a multitude of physiological processes by signal transduction mediated through receptors. In addition, during the last 20 years their role in the appearance or maintenance of various diseases could be unequivocally proven. Agents that can imitate or block the biological functions of bioactive peptides (agonists or antagonists, respectively) can be considered as aids for the investigation of peptidergic systems and also as therapeutic agents. The suitability of bioactive peptides as therapeutic agents was examined after preliminary pharmacological experiments. It was thereby shown that based on their pharmacological properties, for example degradation by peptidases or poor bioavailability, they could be employed as drugs in only a few cases. To solve this problem peptidomimetics, compounds that act as substitutes for peptides in their interaction with receptors, have been synthesized. In comparison with native peptides they show higher metabolic stability, better bioavailability, and longer duration of action. Peptidomimetics with antagonistic properties were also developed within the range of these investigations. As a result, new types of treatment and therapy for a series of diseases are possible. Although peptidomimetics have been developed largely by empirical methods (e.g. modification of native peptides, optimization of lead structures), methods for rational design based on investigations into the structure of peptidepeptide receptor complexes and studies of conformation energies, among others, are gradually being established.
Ultramicroelectrodes in Electrochemistry
Authors: Heinze Jürgen
Published: December 2003

In the 1950s and 1960s fundamental developments in electrochemical methods included voltammetry and low signal techniques. A generation later, the discovery of the unusual properties of ultramicroelectrodes has opened new possibilities of analyzing electrode processes. The changes in mass transport conditions bring about extremely high current densities at ultramicroelectrodes, whereas the currents themselves become very small. This little‐noticed phenomenon allows for many electroanalytical applications that are not possible with conventional electrodes, especially experiments in solutions with very low electrolyte concentrations, in nonpolar solvents, in solids, and even in gases. In addition, two factors— changes in the experimental time scale at low scan rates because of the size of the electrode, and insignificant iR effects at very high scan rates—make it possible to study very fast homogeneous and heterogeneous electrode processes.
Carboranes, Anti‐Crowns, and Big Wheels
Authors: Grimes Russell N.
Published: December 2003

The supramolecular chemistry of boron–carbon clusters has received significant new impulses from two old hands at boron chemistry: Hawthorne et al. reported in a series of papers on “mercuracarborands”, macrocycles from three or four C2B10 carboranes linked by an equal number of Hg atoms, whose complexation can be considered as that of an anti‐crown (that is, a crown compound with reversed charge distribution); Wade et al. (see this issue p.1328) have achieved the preparation of a macrocycle, in which three meta‐phenylene and three meta‐carboranediyl units alternate. This macrocycle promises interesting chemistry not only as an unusual host, but also as a starting material for novel metal complexes.
[Cp*Ru(acac)] Is Not a Coordinatively Unsaturated, Stable 16‐Valence‐Electron Complex
Authors: Smith Michael E. , Hollander Frederick J. , Andersen Richard A.
Published: December 2003

The bent structure of [C5Me5Ru(acac)] in the solid state (Hacac = acetylacetone) arises from the interaction of the ruthenium center with the γ‐carbon atom of the acac ligand of a second, symmetry‐equivalent molecule of [C5Me5Ru(acac)] and is not an intrinsic characteristic of the complex as claimed by Kölle et al. in 1990 in a communication in this journal. Reinterpretation of the X‐ray data provided these results.
A New Copper Selenide Cluster with PPh3 Ligands: [Cu146Se73(PPh3)30]
Authors: Krautscheid Harald , Fenske Dieter , Baum Gerhard , Semmelmann Marcus
Published: December 2003

The largest cluster characterized to date by a crystal structure analysis, [Cu146Se73(PPh3)30], forms in the reaction of CuCL with Se(SiMe3)2 in the presence of PPh3 in THF. The Cu‐Se core of this huge cluster is shielded perfectly by the PPh3 ligands. In this way, a molecule with a diameter of about 40 Å is formed. As a result of a certain structural similarity with Cu2Se, the cluster could be considered as a Cu2Se particle protected by ligands.
Polynitrile‐ and Polyamine‐Functional Poly(trimethylene imine) Dendrimers
Authors: Wörner Christof , Mülhaupt Rolf
Published: December 2003

By quantitative cyanoethylation of polyamines combined with catalytic hydrogenation (Raney nickel) of the resulting polynitriles, poly(trimethylene imine) cascade molecules are accessible in high yields and high purities. Starting from ammonia, five generations of highly symmetrical dendrimers without structural defects were prepared with this reaction sequence.
Poly(propylene imine) Dendrimers: Large‐Scale Synthesis by Hetereogeneously Catalyzed Hydrogenations
Authors: de Brabander‐van den Berg Ellen M. M. , Meijer E. W.
Published: December 2003

In kilogram quantities; pure poly(propylene imine) dendrimers can be prepared in an extremely simple reaction sequence comprising Michael addition (primary amines to acrylonitrile) and heterogeneous hydrogenation with a Raney cobalt catalyst. Both steps proceed quantitatively and selectively and can be employed with many core and end groups.
Ba9N[N3][TaN4]2: A Nitridotantalate(V) with Nitride and Azide Ions
Authors: Höhn Peter , Kniep Rüdiger , Maier Joachim
Published: December 2003

Within only ten minutes the necessary structural information regarding the sequence of the monosaccharide units in 1 and the position of the glycoside linkages is obtained by selective TOCSY‐ROESY experiments.
Organoelement Compounds with AlAl, GaGa, and InIn Bonds
Authors: Uhl Werner
Published: December 2003

In the attempt to synthesize compounds with aluminum in a low oxidation state and an AlAl bond, by the reduction of alkylaluminum halides with alkali metals, analogous to a Wurtz coupling, in general the deposition of elemental aluminum and the formation of the corresponding trialkylaluminum compounds is observed. Thus, tetrasubstituted dialuminum compounds R2AlAlR2, apart from a few poorly characterized examples, were for a long time considered part of an unverified class of substances. Only with the sterically very highly shielded tetrakis[bis(trimethylsilyl)methyl]dialuminum did we achieve the synthesis of the first completely characterized organoelement compound with unlimited stability, which shows aluminum in the oxidation state + II and has an AlAl bond. In the meantime, corresponding compounds for the elements gallium and indium have been obtained. With the simple access to tetrasubstituted dimetal compounds, a fascinating new research area has opened for preparative chemistry, in which the particular interest will naturally lie in the reactive properties of the metal‐metal bond.
Bacteria and the Biodegradation of Chemicals Achieved Naturally, by Combination, or by Construction
Authors: Gottschalk Gerhard , Knackmuss Hans‐Joachim
Published: December 2003

The natural potential of bacteria for the biological degradation of synthetic compounds is greater than is commonly supposed and extends to many heteroarenes and even some chloroarenes. An increase in the number of substituents on the aromatic ring or a certain substitution pattern is what confers xenobiotic character to a compound. In addition, when enzymes with low substrate specificity encounter foreign compounds with random variations, products with very strong xenobiotic character often result. In this case, changing the conditions or introducing a cooperation between several different types of bacteria can be used to degrade these compounds. Finally, mineralization, the complete breakdown of organic substances into carbon dioxide and inorganic salts, of xenobiotics previously regarded as persistent can be achieved by taking advantage of natural or induced gene transfer to construct hybrid degradative pathways. After an introduction to the world of bacteria and their place in Nature, we will describe their natural potential for biodegradation with reference to aliphatic and aromatic hydrocarbons. The discussion will then turn to the types of the substituents that confer xenobiotic properties to compounds and how these compounds are degraded despite their xenobiotic character.
Triplet States in Organic Chemistry
Authors: Kurreck Harry
Published: December 2003

The investigation of organic multispin systems—with the triplet state as the initial member—has contributed considerably to the understanding of chemical bonding and to the examination of quantum mechanical model concepts. Structural information is accessible from the form and strength of the interaction between unpaired electrons in triplet ground states; in photochemistry triplet excited states play an important role both from a mechanistic and a preparative point of view. Besides triplet states, spin states of higher multiplicity (that is, molecules with more than two unpaired electrons) are attracting increasing interest. One way to superparamagnetic organic molecules has been shown by the synthesis of a branched nonacarbene with 18 unpaired electrons (S = 9 state).
Fast Solid‐State Chemistry: Reactions under the Influence of Microwaves
Authors: Kniep Rüdiger
Published: December 2003

Solid‐state synthesis by microwave irradiation allows the “fascinatingly simple” preparation of crystalline CuInS2 from the elements. The product is not of “semiconductor quality” and can therefore not be used directly for solar cells. The method, however, which has been established for a long time in solvent and molecular chemistry, should rapidly find wider applications in solid‐state chemistry.
The Mechanism of Fullerence Formation
Authors: Schwarz Helmut
Published: December 2003

“Next to nothing is known about how fullerenes actually grow and form.” This statement was published in spring. Soon thereafter Bowers et al. published a series of articles that seem to be the light at the end of the tunnel. According to these studies, in graphite vaporization primarily C, clusters form from single C atoms. These species are linear (for C to C), monocyclic (for C to C), and planar‐bicyclic cations (for C to about C); fullerenes are also present above C. The most important new insight is that the mono‐ and bicyclic C species isomerize directly to fullerenes on addition of energy; „cooling”︁ of the „excited”︁ fullerenes takes place by loss of C1, C2, or C3 units.
Relative Significance of Electrostatic‐ and Hydrophobic‐Induced pKa Shifts in a Model Protein: The Aspartic Acid Residue
Authors: Urry Dan W. , Peng Shao Q. , Parker Timothy M. , Gowda D. Channe , Harris Roland D.
Published: December 2003

Unusual pKa values for amino acids in the active sites of proteins play a large role in their function. A new explanation for these pKa shifts, which considers not electrostatic but mainly hydrophobic interactions as the cause, is presented here. This is substantiated by the correlation between the titrimetrically determined pKa values of a family of polypentapeptides containing aspartic acid and the aspartic acid content in the polypeptide.
Coherence Selection by Gradients without Signal Attenuation: Application to the Three‐Dimensional HNCO Experiment
Authors: Schleucher Jürgen , Sattler Michael , Griesinger Christian
Published: December 2003

Maximum sensitivity and excellent suppression of the water signal is achieved in 3D triple‐resonance NMR experiments by the application of a heteronuclear gradient echo in phase‐modulating pulse sequences. This principle is demonstrated for an HNCO experiment applied to a complex of a uniformly 13C/15Nlabeled protein (calmodulin) and a peptide (C2OW).
Amination of Methane and Ethane by Mercury Photosensitization in the Presence of Ammonia
Authors: Michos Demetrius , Sassano Caroline A. , Krajnik Paul , Crabtree Robert H.
Published: December 2003

When the primary products are condensed in a cold trap to prevent further reaction, CH4/NH3 and C2H6/NH3 mixtures can be converted into imines in the presence of Hg vapor on UV irradiation [Eq. (a)]. Slowing the gas circulation rate during the photolysis leads to CC bond formation; for example, CH3CHNH is formed from CH4/NH3. R  H, CH3.
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Tetranuclear Silver(I) Clusters Linked by Bridging Succinate Anions in a Three‐Dimensional Network: Crystal Structure of Succinatodisilver(I)
Authors: Michaelides Adonis , Kiritsis Vangelis , Skoulika Stavroula , Aubry Andre
Published: December 2003

Silver nitrate and succinic acid react to form the pale yellow prisms of succinatodisilver(I) (1). The structure of 1 (shown on the right) contains Ag4 clusters parallel to the ac plane, each of which are bound to four other clusters through succinato bridges.
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Structures of Organo Alkali Metal Complexes and Related Compounds
Authors: Weiss Erwin
Published: December 2003

The investigation of the reactivity and structure of organometallic compounds of alkali metals has experienced a blustering development in the last decades. This class includes compounds that are especially important for our understanding of chemical bonding and also quite simple, for example methyl alkali metal complexes, whose structures have been unequivocally determined. Organometallic compounds of alkali metals (and also magnesium) generally exist as ion aggregates whose properties can be significantly modified through solvation by, for example, ether or amines. Important advances in the synthesis of new compounds, especially those of the heavier alkali metals, have been based on these results. It was long believed that the alkali metals had little tendency to undergo coordination and that their coordination chemistry would offer few surprises. This picture has now changed completely. Results from crystal structure investigations have revealed a variety of often surprising structure types (rings, heterocubanes, chains, layers, etc.) not only with the organometallic compounds but also with the amides, imides, alkoxides, phenoxides, enolates, and even halides. A comparison reveals interesting similarities between compounds that appear to be so different and leads to a general classification of the structure types possible with C, N, O, and halo ligands.
Hydrophobic Effects. Opinions and Facts
Authors: Blokzijl Wilfried , Engberts Jan B. F. N.
Published: December 2003

The term hydrophobic interactions denotes the tendency of relatively apolar molecules to stick together in aqueous solution. These interactions are of importance in many chemical disciplines, including the chemistry of in vivo processes. Enzyme‐substrate interactions, the assembly of lipids in biomembranes, surfactant aggregation, and kinetic solvent effects in water‐rich solutions are all predominantly governed by hydrophobic interactions. Despite extensive research efforts, the hydration of apolar molecules and the noncovalent interactions between these molecules in water are still poorly understood. In fact, the question as to what the driving force for hydrophobic intractions is shifts the study into a quest for a detailed understanding of the remarkable properties of liquid water. This review highlights some of the novel insights that have been obtained in the past decade. The emphasis is on both hydrophobic hydration and hydrophobic interactions since both phenomena are intimately connected. Several traditional views have been found to be deeply unsatisfactory, and courageous attempts have been made to conceptualize the driving force behind pairwise and bulk hydrophobic interactions. The review presents an admittedly personal selection of the recent experimental and theoretical developments, and when necessary, reference is made to relevant studies of earlier date.
Carbon(IV) Nitride C3N4— A New Material Harder Than Diamond?
Authors: Schnick Wolfgang
Published: December 2003

Numerous complex interacting factors complicate the prediction of the existence, structure, and properties of solid‐state compounds. No success crowned the efforts to synthesize a compound of composition C3N4 having a β‐Si3N4 structure, which should have the hardness and stability of diamond. Recently Lieber et al. managed to prepare a CN solid that, though its defined composition is not C3N4, probably does have the postulated β‐Si3N4 structure and exceptional material properties.
Synthesis, Structure, and Optical Properties of Cd4S6+ Clusters Encapsulated in an Aluminate Cage
Authors: Brenchley Matthew E. , Weller Mark T.
Published: December 2003

Small, well‐ordered CdS semiconductor units are present in the aluminate sodalite 1 as indicated by a significant blue shift of the UV/VIS absorption edge relative to that of bulk (macrocrystalline) CdS. The spectroscopic properties of CdS in 1 are thus similar to those of colloidal cadmium sulfide, although the individual Cd4S6+ clusters in 1 are about 2000 times smaller than the colloid particles. This confirms an interaction between encapsulated clusters in neighboring sodalite cages. Compound 1 was obtained from CdO, CdSO4, and Al2O3 in a two‐stage process via the analogous sulfate sodalite followed by reduction with H2.
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