bluuebunny/crossref_metadata_2025 huggingface dataset:
Enantioselective CC and CH Bond Formation Mediated or Catalyzed by Chiral ebthi Complexes of Titanium and Zirconium
Authors: Hoveyda Amir H. , Morken James P.
Published: January 2004

The development of catalytic processes that effect enantioselective bond formation under mild conditions is an important and challenging task in modern chemical synthesis. In this connection, chiral C2‐symmetric ansa‐metallocenes (bridged metallocenes) have found notable applications as catalysts. This article discusses the chemistry of this class of chiral metallocene complexes with regard to their utility in catalytic and enantioselective CC and CH bond formation reactions. In addition, where applicable, a brief comparison with other related catalytic enantioselective processes is offered. Many of the reactions effected with high levels of enantioselectivity by catalytic amounts of these complexes are of great significance to the preparation of new materials and in the synthesis of therapeutic agents. For example, zirconocene complexes readily catalyze the enantioselective addition of alkylmagnesium halides to alkenes, and cationic zirconocene complexes may promote the highly stereoregulated copolymerization of terminal alkenes. Furthermore, the related chiral titanocenes are involved in an impressive range of useful asymmetric catalytic reactions, including the enantioselective hydrogenation of olefins and reduction of imines or ketones. This review attempts to bring together the practical aspects of the use of [(ebthi)M] complexes of Group 4 transition metals (catalyst synthesis and resolution), outline the manner in which the C2‐symmetric chiral ligands are believed to initiate stereoselective bond formation, and highlight the aspects of this chemistry that are less well understood and require further research.
First Step Towards Planning of Syntheses in Solid‐State Chemistry: Determination of Promising Structure Candidates by Global Optimization
Authors: Schön J. Christian , Jansen Martin
Published: January 2004

A method is presented here that allows, in principle, the prediction of the existence and structure of (meta)stable solid compounds. It is based on a set of adjustable modules that are applied to the study of the energy function of the chemical system of interest. The main elements are a set of routines for global optimization and local minimization, as well as algorithms for the investigation of the phase space structure near local minima of the potential energy, and the analysis and characterization of the structure candidates. The current implementation focuses on ionic compounds, for which empirical potentials are used for the evaluation of the energy function in the first stage, and a Hartree–Fock algorithm for refinements. The global optimization is performed with a stochastic simulated annealing algorithm, and the local minimization employs stochastic quenches and gradient methods. The neighborhoods of the local minima are studied with the threshold algorithm. The results of this approach are illustrated with a number of examples: compounds of binary noble gases, and binary and ternary ionic compounds. These include several substances that have not been synthesized yet, but should stand a fair chance of being kinetically stable, for example further alkali metal nitrides besides Li3N, as well as Ca3SiBr2 or SrTi2O5.
A Triple Anion Complex Containing Enolate, Amide, and Halide—A New Structural Type in Lithium Chemistry
Authors: Henderson Kenneth W. , Dorigo Andrea E. , Williard Paul G. , Bernstein Peter R.
Published: January 2004

Three different anions are contained in the novel lithium complex 1, which has a “butterfly” structure. The preference of this structure over the common alternatives lies in the relative bridging abilities of the ligands present. The stability of the complex was impressively demonstrated by ab initio calculations.
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Molecular Structure of [Pd6Cl12] in Single Crystals Chemically Grown at Room Temperature
Authors: Dell'Amico Daniela Belli , Calderazzo Fausto , Marchetti Fabio , Ramello Stefano
Published: January 2004

A reduction of the CO partial pressure is sufficient to obtain single crystals of [Pd6Cl12] from the equilibrium (a) in thionyl chloride (solv.) at room temperature. The single‐crystal and powder X‐ray diffraction characterization of this PdII chloride is described.
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Tetrakis(tri‐tert‐butylsilyl)‐tetrahedro‐tetragermane (tBu3Si)4Ge4—The First Molecular Germanium Compound with a Ge4 Tetrahedron
Authors: Wiberg Nils , Hochmuth Walter , Nöth Heinrich , Appel Andrea , Schmidt‐Amelunxen Martin
Published: January 2004

Deep red, oxidation‐ and hydrolysis‐sensitive crystals are formed by the title compound 1, the first derivative of the tetrahedro‐tetragermane Ge4H4. It was prepared according to Equation (a), and isolated and characterized in crystalline form as 2(tBu3Si)4Ge4·(tBu3Si)2. In these crystals three of the four tBu3Si groups of 1—possibly as a result of interactions with the (tBu3Si)2 molecules—are not arranged in radial symmetric manner in the Ge4 tetrahedron, but are slightly bent.
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Recent Developments of Molecular Chemistry for Sol–Gel Processes
Authors: Corriu Robert J. P. , Leclercq Dominique
Published: January 2004

This review focuses on some aspects of the chemistry involved in the preparation of oxides by sol–gel methods. This field is opening interesting possibilities for molecular chemistry and among those presented herein are: (1) the use of precursors containing functional groups for the preparation of solids with specific chemical reactivity; (2) the non‐hydrolytic gelation (NHG) route, a new general method for the synthesis of oxides that allows the use of oxygen donors (ethers, alcohols, alkoxides) other than water; the mechanism is based on oxygen–carbon bond cleavage; (3) organic–inorganic hybrid solids containing covalent bonds for which the chemical reactivity is used to demonstrate the presence of substructure in the globally amorphous solid; the substructure greatly depends on the geometry of the organic moiety.
CC‐1065 and the Duocarmycins: Understanding their Biological Function through Mechanistic Studies
Authors: Boger Dale L. , Johnson Douglas S.
Published: January 2004

We review here studies defining the DNA alkylation properties of a class of potent antitumor antibiotics that includes CC‐1065 and the duocarmycins, as well as investigations that delineate fundamental relationships between their structure, functional reactivity, and biological properties. In conjunction with the study of the natural products themselves, the examination of synthetic agents containing deep‐seated structural changes, the unnatural enantiomers of the natural products, and related analogs has defined the structural basis for the sequence selective alkylation of duplex DNA and proved to be the key to understanding the fundamental relationships between chemical structure, functional reactivity, and biological properties. The characteristic DNA alkylation proceeds by reversible, stereoelectronically controlled adenine‐N3 addition to the least substituted carbon of the activated cyclopropane within AT‐rich minor groove sites. Both the natural and unnatural enantiomers alkylate DNA, and the sequence selectivity of both is controlled and dominated by the preferential noncovalent binding of the agents within the narrower, deeper, AT‐rich minor groove and the steric accessibility to the alkylation site on penetration of this groove. Among the fundamental relationships between structure and properties, the most striking is the direct relationship between chemical or functional stability and biological potency. Within a short time, simplified and readily accessible synthetic agents rationally based on the natural product leads have been developed, which exhibit comparable and exceptional biological potency (IC50 = 50 to 51 pM) and improved efficacy. The fundamental relationships that have been defined to date can be expected to provide the foundation for future developments and advances.
Design of Diradical‐based Hydrogen Abstraction Agents
Authors: Chen Peter
Published: January 2004

Why are diradicals used as hydrogen abstraction agents in the enediyne antitumor antibiotics, and what would happen if a “nonnatural” diradical were to be used? The very slow abstraction rates by arene diradicals suggest an answer to the question of the functional significance of diradicals versus monoradical abstraction agents in the natural products, and furthermore, suggest potential strategies for the rational modification of DNA‐cleaving agents by alteration of the diradical “warhead”.
Template‐Driven Syntheses of Polymeric Metal Cyanides: A Chiral Nanoporous Host for the nBu4N+ Ion
Authors: Schwarz Peter , Eller Stefan , Siebel Eric , Soliman Tarek M. , Fischer R. Dieter , Apperley David C. , Davies Nicola A. , Harris Robin K.
Published: January 2004

A comfortable fit is provided for the large tetrabutylammonium ion within the wide channels of the new host–guest compound [(nBu4N)0.5(Me3Sn)3.5Fe(CN)6]. The lipophilic walls are formed by helical [Fe‐CN‐Sn‐NC]∞ chains involving {Fe(CNSnMe3)6} octahedra that are enantiomorphically derivatized due to cross‐linking.
Intramolecular Movements of Lithium in Crystals
Authors: Veith Michael , Zimmer Michael , Fries Kira , Böhnlein‐Maus Jutta , Huch Volker
Published: January 2004

Rotations of the atoms around the center of gravity of the molecule characterize the cooperative intramolecular movement of the two lithium atoms in the solid amide Li2[(tBuOSiMe2)2N]2 just above room temperature. This movement can be stopped by cooling the sample, whereupon a phase transition occurs. A further phase transition occurs at temperatures above 100°C leading to a “plastic phase” in which whole molecules tumble around their centers of gravity.
The [Mn(μ‐O)(μ‐PhBO2)2]2+ Unit: A New Structural Model for Manganese‐Containing Metalloproteins
Authors: Bossek Ursula , Hummel Helga , Weyhermüller Thomas , Wieghardt Karl , Russell Stephen , van der Wolf Lodewijk , Kolb Uwe
Published: January 2004

By employing boronato instead of carboxylato bridges in the dication [Mn(μ‐O)‐(μ‐PhBO2)2L2]2+ (L = 1,4,7‐trimethyl‐1,4,7‐triazacyclononane), a Mn unit has been stabilized for the first time. Together with the di‐μ‐carboxylato‐μ‐oxodimanganese complexes of analogous structure, there is now an “isostructural series” available containing Mn, MnIIMnIII, Mn, MnIIIMnIV, Mn units. The Mn K‐edge energy of the corresponding complexes increases linearly with increasing oxidation level. This is of importance for EXAFS investigations of metalloproteins containing manganese.
Alkylation of 1‐Naphthol with Alcohols over an Iron Oxide Catalyst
Authors: Grabowska Hanna , Miśta Włodzimierz , Syper Ludwik , Wrzyszcz Józef , Zawadzki Mirosław
Published: January 2004

High yields and selectivities are achieved for the alkylation of 1‐naphthol with alcohols in the presence of an iron catalyst in a continuous gas‐phase process. The resulting 2‐alkyl‐1‐naphthols can be easily separated from the product mixture. This opens up new possibilities for the synthesis of important industrial intermediates.
Polymers and the Periodic Table: Recent Developments in Inorganic Polymer Science
Authors: Manners Ian
Published: January 2004

Polysiloxanes [R2SiO]n, polyphosphazenes [R2PN]n, and polysilanes [R2Si]n illustrate that the incorporation of inorganic elements into a polymer main chain can lead to useful properties. These include low temperature flexibility, high thermal and oxidative stability, flame retardancy, novel forms of chemical reactivity, and intriguing electrical and optical characteristics arising from unusual electronic effects such as the delocalization of σ electrons. However, until recently, the development of inorganic polymer science has been held back by the synthetic problem of finding ways to join atoms of inorganic elements together into long chains. This review surveys many of the exciting advances in the field of inorganic polymers over the past decade and focuses mainly on the new inorganic polymer systems that have been prepared during this period. These include random‐network polysilynes, poly(carbophosphazene)s, sulfur–nitrogen–phosphorus polymers, poly(organooxothiazene)s, and, very recently, the first examples of polystannanes that possess a main chain of tin atoms. Transition metal based polymer science has also experienced a number of synthetic break‐throughs and new materials include high molecular weight metallocene‐based polymers, polymetallaynes incorporating elements such as iron, nickel, and rhodium, liquid crystalline organocobalt and organochromium polymers, and lanthanide‐based polymers.
Chemistry and Biology of the Zaragozic Acids (Squalestatins)
Authors: Nadin Alan , Nicolaou K. C.
Published: January 2004

New natural products seldom arouse much excitement. However, with their unprecedented and heavily oxygenated molecular structure, and potentially lucrative biological activity, the zaragozic acids (squalestatins) provoked a flurry of activity among academic and industrial chemists and biologists in the early 1990s. Since they are powerful inhibitors of squalene synthase, the enzyme catalyzing the first committed step on the biosynthetic pathway to cholesterol, hopes were high that the zaragozic acids would follow the huge commercial success of the mevinic acids by becoming second‐generation cholesterol‐lowering drugs. The unusual and characteristic 2,8‐dioxabicyclo[3.2.1]octane‐3,4,5‐tricarboxylic acid “core” structure of the zaragozic acids stimulated a large number of diverse synthetic studies, ranging from the traditional to the avant garde. In this review we attempt to bring together for the first time the chemistry and biology of this fascinating class of molecules.
Discovery of Chiral Catalysts through Ligand Diversity: Ti‐Catalyzed Enantioselective Addition of TMSCN to meso Epoxides
Authors: Cole Bridget M. , Shimizu Ken D. , Krueger Clinton A. , Harrity Joseph P. A. , Snapper Marc L. , Hoveyda Amir H.
Published: January 2004

For corrigendum see DOI:10.1002/anie.199619951
La9Br5(CBC)3: A New Superconductor
Authors: Mattausch Hansjürgen , Simon Arndt , Felser Claudia , Dronskowski Richard
Published: January 2004

A “fingerprint” that we believe to be an essential condition for superconductivity is shown by the electronic band structure of the novel phase Ce9Br5(CBC)3. Although the magnetic moment of the Ce3+ ion quenches superconductivity, the isostructural lanthanum compound is a superconductor at 6 K.
Diisocyanate
Authors: Maier Günther , Naumann Matthias , Reisenauer Hans Peter , Eckwert Jürgen
Published: January 2004

Sure diisocyanate (1) exists! Those who believed that of the many conceivable C2N2O2 isomers only cyanogendi‐N‐oxide (1) could be synthesized were mistaken. It is possible to generate diisocyanate by means of matrix‐isolation techniques, to characterize it spectroscopically, and to compile data on its stability.
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A Mobility Filter for the Detection and Identification of Charged Species in Complex Liquid Mixtures by ENMR Phase Difference Spectroscopy
Authors: Heil Stefan R. , Holz Manfred
Published: January 2004

In situ electrophoresis is combined with the selectivity of NMR spectroscopy in electrophoretic NMR (ENMR). In a simple new ENMR experiment the signals of ions and electrically charged molecules or aggregates are filtered from the 1D NMR spectrum of a complex liquid mixture. Only the signals of charged species remain in a phase difference spectrum, facilitating the detection and identification of these particles, even when 2D ENMR methods fail.
My Life with O3, NOx, and Other YZOx Compounds (Nobel Lecture)
Authors: Crutzen Paul J.
Published: January 2004

The ozone hole, a nuclear winter, and the greenhouse effect are some of the themes that concerned Paul Crutzen in his work, for which he was awarded the Nobel Prize in 1995. Here he reports on his path to his path to his results and his plans for the future.
Stratospheric Ozone Depletion by Chlorofluorocarbons (Nobel Lecture)
Authors: Rowland F. Sherwood
Published: January 2004

Hope for the atmosphere is the cautiously worded resumé of F. S. Rowland based on his knowledge of the developments in recent years about the influence of the CFCs on the ozone budget. Thanks to the restrictions that came into effect in the 1990s, the concentrations of some CFCs in the atmosphere have already reached their maximum value; for others this point will be reached in the near future. Nevertheless, large ozone losses are expected in the Antarctic spring until the middle of the 21st century.
Alternating Ferro‐ and Antiferromagnetic Interactions in Honeycomb‐Like Layers of an Azidomanganese(II) Compound
Authors: Cortés Roberto , Lezama Luis , Rojo Teófilo , Pizarro J. Luis , Arriortua M. Isabel
Published: January 2004

Manganese(II) nitrate, sodium azide, and 2,2′‐bipyrimidine (bpym) react to give 1 in which the azido groups function as end‐on bridging ligands and the bpym as a bis(bidentate) ligand. The structure of 1 can be described as a honeycomb sheet formed by Mn6 hexagons. Magnetic susceptibility data reveals the existence of ferro‐ and antiferromagnetic interactions that are mediated by the azido ligands and the bpym ligands, respectively.
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Nonlinear Temperature Behavior of Product Ratios in Selection Processes
Authors: Heller Detlef , Buschmann Helmut , Scharf Hans‐Dieter
Published: January 2004

Displaced intermolecular and intramolecular equilibria, which lead to a nonlinear change in the concentration ratios of intermediates, are possibly the cause for the experimentally long‐known nonlinear temperature dependence of logarithmic product ratios in selection processes (isoinversion principle). This interpretation takes into account a change in dominance of differences in the activation enthalpy and entropy for the overall selection process, as well as a change in the rate‐determining step with variation of the temperature.
Saccharide Sensing with Molecular Receptors Based on Boronic Acid
Authors: James Tony D. , Sandanayake K. R. A. Samankumara , Shinkai Seiji
Published: January 2004

The lock‐and‐key principle of natural systems is based on complex interactions like hydrogen bonding. Many synthetic systems that attempt to mimic natural systems have also used hydrogen bonding as the main binding force and have met with great success in non‐hydrogen‐bonding solvents that do not compete with the guest for the binding pocket. In contrast, natural systems function in water, a very competitive solvent. Synthetic hydrogen‐bonding systems may yet evolve to be successful in water. If this transition can not be made, synthetic answers can nevertheless take inspiration from nature without slavishly following the blue print. This is not an attempt to reinvent the “lock”: a new locking mechanism merely replaces the existing one. The inspiration might be the view of the hydrogen bond as an easily reversible “covalent” bond. Screening the literature we rediscovered boronic acids, which have been known for over 100 years. Conveniently, boronic acids rapidly and reversibly form cyclic esters with diols in basic aqueous media. Saccharides and other related “keys” contain a contiguous array of cyclic alcohols. In this work we hope to demonstrate that saccharide “keys” and boronic acid “locks” can open the door to a new and exciting field of research.
Hf7P4: A New Binary Phosphide Synthesized by a Surprising Route
Authors: Kleinke Holger , Franzen Hugo F.
Published: January 2004

A flux is essential for the synthesis of Hf7P4, which has been obtained by the reaction of Hf with Fe and HfP. The compound proved to be thermodynamically stable and iron‐free, but could not be synthesized without the addition of iron as a flux material. Hf7P4 is surprisingly isostructural with Zr7P4, and a possible explanation for this is given based on COOP calculations.
BaYbSi4N7—Unexpected Structural Possibilities in Nitridosilicates
Authors: Huppertz Hubert , Schnick Wolfgang
Published: January 2004

Four silicon atoms have been connected to one central nitrogen atom for the first time in BaYbSi4N7. These N[4] bridges significantly extend the spectrum of possible linkages in Si ‐ N network structures of nitridosilicates. While oxosilicates are built up by SiO4 tetrahedra with either terminal oxygen atoms or simply bridging O[2] atoms, the N[2], N[3], and N[4] linkages in nitridosilicates enable novel highly cross‐linked SiN network structures.
Protecting Group Strategies in Organic Synthesis
Authors: Schelhaas Michael , Waldmann Herbert
Published: January 2004

The choice of protecting groups is one of the decisive factors in the successful realization of a complex, demanding synthetic project. The protecting groups used influence the length and efficiency of the synthesis and are often responsible for its success or failure. A wide range of blocking groups are currently available for the different functional groups; however, an overall strategy combining these different masking techniques in an advantageous and reliable manner has never been proposed or at best only for individual cases. This review attempts to make a contribution to filling this gap. First a very short overview of the most commonly used protecting groups will be given, in which they are classified according to their lability and not according to the functional group they protect. This classification clarifies coherent concepts for the development of blocking strategies. On the basis of this brief summary reliable strategies will then be illustrated and developed with selected examples from the recent literature by which protecting groups may be combined successfully and advantageously in synthetic projects of differing degrees of complexity and difficulty.
The Identification of Genes Controlling Development in Flies and Fishes (Nobel Lecture)
Authors: Nüsslein‐Volhard Christiane
Published: January 2004

Vertebrate genes can often be identified based on analogies to Drosophila genes. This homology has been important in advancing the understanding of vertebrate development on a genetic level, and indicates that a basic body plan is common to the vertebrates and arthropods. Comparisons of patterning in flies and vertebrates have been successful most recently in studies of zebra fish.
From Molecular Patterns to Morphogenesis—The Lessons from Studies on the Fruit Fly Drosophila (Nobel Lecture)
Authors: Wieschaus Eric
Published: January 2004

Establishing inbred lines from single flies carrying mutagenized chromosomes was the strategy behind the mutagenesis experiments conducted by E. Wieschaus and C. Nusslein‐Volhard in Heidelberg. Within two generations, homozygous flies were obtained whose development was compared with that of their heterozygous siblings. The results from 27000 inbred lines led to first indications of the causes of early morphological changes in embryogenesis.
Synthesis and Structure of the Nanoclusters [Hg32Se14(SePh)36], [Cd32Se14(SePh)36‐(PPh3)4],[P(Et)2(Ph)C4H8OSiMe3]5‐ [Cd18I17(PSiMe3)12], and [N(Et)3C4H8OSiMe3]5[Cd18I17(PSiMe3)12]
Authors: Behrens Silke , Bettenhausen Marco , Deveson Anne C. , Eichhöfer Andreas , Fenske Dieter , Lohde Almut , Woggon Ulrike
Published: January 2004

For corrigendum see DOI:10.1002/anie.199721271
Microporous Mixed Oxides—Catalysts with Tunable Surface Polarity
Authors: Klein Stephan , Maier Wilhelm F.
Published: January 2004

For corrigendum see DOI:10.1002/anie.199628811
Isolation and Characterization of the Dimetallofullerene Ce2@C80
Authors: Ding Junqi , Yang Shihe
Published: January 2004

Purer than the driven snow. Ce2@C80 has been isolated with a purity of >99% by an efficient extraction procedure, followed by an HPLC separation. The X‐ray photoelectron spectrum of Ce2@C80 suggests that both Ce atoms inside the cage occur as Ce3+, in agreement with the results of a recent ab initio calculation.
Heavy Element π Donation Is Not Less Effective
Authors: Kapp Jürgen , Schade Christian , El‐Nahasa Ahmed M. , von Ragué Schleyer Paul
Published: January 2004

The inherent stabilizing effects of lone pair substituents from the first five rows of the periodic table on carbocations depend primarily on the group rather than on the period, judging from high‐level ab initio computations (QCISD(T)/DZ + + PP// MP2(fc)/DZ+P). When planarity is imposed on the pnictogen compounds, the stabilization energies from group 15 substituents are almost constant (ca. 95 kcal mol−1 from N to Sb, based on CH) and exceed the stabilization energies of chalcogens (60‐66 kcal mol−1, O <S ≈ Se <Te) and halogens (18‐38 kcal mol−1, F<Cl≈Br<I).
Combinatorial Synthesis of Small Organic Molecules
Authors: Balkenhohl Friedhelm , von dem Bussche‐Hünnefeld Christoph , Lansky Annegret , Zechel Christian
Published: January 2004

Combinatorial synthesis has developed within a few years from a laboratory curiosity to a method that is taken seriously in drug research. Rapid progress in molecular biology and the resulting ability to determine the activity of new substances extremely efficiently have led to a change in paradigm for the synthesis of test compounds: in addition to the conventional procedure of synthesizing one substance after another, new methods allowing simultaneous creation of many structurally defined substances are becoming increasingly important. A characteristic of combinatorial synthesis is that a reaction is performed with many synthetic building blocks at once—in parallel or in a mixture— rather than with just one building block. All possible combinations are formed in each step, so that a large number of products, a so‐called library, is obtained from only a few reactants. Several methods have been developed for combinatorial synthesis of small organic molecules, based on research into peptide library synthesis: single substances are produced by highly automated parallel syntheses, and special techniques enable targeted synthesis of mixtures with defined components. Many structures can be obtained by combinatorial synthesis, and the size of the libraries created ranges from a few individual compounds to many thousand substances in mixtures. This article gives an overview of the combinatorial syntheses of small organic molecules reported to date, performed both in solution and on a solid support. In addition, different techniques for identification of active compounds in mixtures are presented, together with ways to automate syntheses and process the large amounts of data produced. An overview of pionering companies active in this area is also given. The final outlook attempts to predict the future development of this exponentially growing area and the influence of this new thinking in other areas of chemistry.
Solid‐State Chemistry: Restoring the Balance
Authors: Schöllhorn R.
Published: January 2004

A coin with two sides: chemistry has both static and dynamic aspects. Yet in research in solid‐state chemistry attention has focused almost exclusively on the former. Thus, the systematic investigation of the dynamic properties of materials such as the mechanisms of solid‐state processes can be considered long overdue and essential for the development of strategies for rational synthesis and tailor‐made materials.
Low‐Temperature Complete Combustion of Methane over Mn‐, Co‐, and Fe‐Stabilized ZrO2
Authors: Choudhary Vasant R. , Uphade Balu S. , Pataskar Suryakant G. , Keshavaraja Alive
Published: January 2004

Activity comparable to that of noble metal catalysts is shown by the Mn‐, Co‐, or Fe‐stabilized ZrO2 catalysts for the low‐temperature combustion of methane. The incorporation of the transition metal into the ZrO2 bulk structure not only stabilizes the cubic (fluorite) modification, but also increases the reactivity of the lattice oxygen atoms drastically.
Novolak‐Diazoquinone Resists: The Imaging Systems of the Computer Chip
Authors: Reiser Arnost , Shih Hsiao‐Yi , Yeh Tung‐Feng , Huang Jian‐Ping
Published: January 2004

This year some twenty billion integrated circuits will be produced worldwide. Almost all (98%) of them will be made with a photosensitive varnish, the photo resist, that allows the transfer of a fine line‐pattern from an original transparency onto a silicon wafer. This photoresist has two components: one is a novolak resin, a low molecular weight phenol–formaldehyde condensation polymer; the other is a diazonaphthoquinone derivative (DNQ), which is the photoactive part of the system. Novolak–diazoquinone resists have played a crucial role in the microminiaturization of electronics. They are photographic materials of extremely high resolution, able to define features as small as 0.25 μm. Although they are now an indispensible ingredient of modern computer technology, until quite recently their molecular mechanism was not understood. We report here on work at Polytechnic University in New York aimed at solving the mystery of these important and intriguing systems. The solution, in so far as it has revealed itself to us, is rather unique and quite unexpected.
New Developments in the Chemistry of Low‐Valent Titanium
Authors: Fürstner Alois , Bogdanović Borislav
Published: January 2004

Among the applications of low‐valent titanium in organic synthesis, the reductive coupling of carbonyl compounds to produce alkenes (the McMurry reaction) is particularly prominent. Discovered at the beginning of the 1970s, it has been developed and tested repeatedly, for example in numerous syntheses of natural products. This alkene synthesis has become a standard reaction in the repertoire of preparative chemists. However, the possibilities of low‐valent titanium are by no means limited to this process: the last few years have brought some spectacular applications of the conventional McMurry reaction (for example the synthesis of taxol) along with a considerable extension of the scope of reductive carbonyl couplings. Thus, diverse heterocycles are now accessible following novel and efficient pathways based on intramolecular cross‐coupling of functional groups—some of which were hitherto considered to be inert to titanium. The use of this method for the synthesis of indole and pyrrole alkaloids illustrates the new possibilities. At the same time, considerably simplified methods for conducting McMurrytype reactions have been developed. Examples include the particularly convenient “instant” method, the first ketone–amide coupling reactions requiring only catalytic amounts of titanium salts, and the first application of commercially available titanium powder as a coupling agent. Last but not least, the detailed investigation of diverse classical McMurry reagents has afforded a deeper understanding of the nature and mode of action of low‐valent titanium. Revision of some of the current conceptions of the process of reductive carbonyl coupling is thus indispensable.
Electron Transfer and Chemical Reactions—Stepwise or Concerted? On the Competition between Nucleophilic Substitution and Electron Transfer
Authors: Speiser Bernd
Published: January 2004

The subject of an exciting debate—does electron transfer play a role in bimolecular nucleophilic substitution (a)? How would this reaction differ from the classical SN2 reaction (b)? A number of research groups have employed electrochemical and computational methods to determine if alternatives exist for one of the best known mechanisms in organic chemistry.
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Silaethene H2CSiH2: Millimeter Wave Spectrum and Ab Initio Calculations
Authors: Bailleux Stephane , Bogey Marcel , Breidung Jürgen , Bürger Hans , Fajgar Radek , Liu Yuyan , Pola Josef , Senzlober Michael , Thiel Walter
Published: January 2004

139 rotational lines of the 28Si isotopomer of free silaethene H2CSiH2 (1) have been observed in the gas phase by millimeter wave spectroscopy. The search was guided by ab initio calculations at the MP2, CCSD, and CCSD(T) levels with TZ2Pf basis sets. The rotational constants determined experimentally and by ab initio methods for 1 with a planar C2v‐symmetric structure agree within about 0.5%. Compound 1 was best produced by vacuum pyrolysis of 5,6‐bis(trifluoromethyl)‐2‐silabicyclo[2.2.2]octa‐5,7‐diene in an Ar stream; 1 decomposes exponentially with a 1/e lifetime at ambient temperature of 30±2 ms.
Activation of Carbon Dioxide by Coordination with Cations in the Gas Phase: SiF‐Mediated Coupling of CO2 and Aromatic CH Bonds
Authors: Cecchi Patrizio , Crestoni Maria Elisa , Grandinetti Felice , Vinciguerra Vittorio
Published: January 2004

The simultaneous activation of carbon dioxide and benzene by SiF in the gas phase has been investigated by FT‐ICR spectrometry. The key step is the formation of a tightly bound SiF‐(CO2) adduct, which converts the unreactive carbon dioxide into an electrophile able to carbonylate benzene efficiently. At low pressures the benzoyl cation is formed.
Enantioselective Protonation of Enolates and Enols
Authors: Fehr Charles
Published: January 2004

Enantioselective protonation, until recently a largely overlooked reaction, has, in the last five years, developed into a field of intense research activity. The progress achieved parallels or complements that obtained in the understanding of enolate structures and reactivities. The conceptual simplicity and attractiveness of enantioselective protonation results from the fact that after reaction, the chiral proton donor is regenerated in its original protonated form by an extractive workup. Enantioselective protonation has been applied to the synthesis of amino acids, antiinflammatory agents (2‐arylpropanoic acids), and fragrance compounds such as (S)‐α‐damascone, for which its industrial feasibility has been demonstrated.
Erich Hückel, Pioneer of Organic Quantum Chemistry: Reflections on Theory and Experiment
Authors: Berson Jerome A.
Published: January 2004

In the period between 1930 and 1937, Erich Hückel, a theoretical physicist, made profound contributions to organic chemistry in his quantum mechanical descriptions of unsaturated and conjugated compounds. A brief account of his academic career is followed by simplified expositions, from the point of view of an organic chemist, of the highly approximate theoretical methods he used. Of special significance in the case of cyclic molecules of the class CnHnis the concept of filled shells when the number of π electrons is (4N+ 2) (N= 0,1,2,…). Examinations of key applications of Hückel's ideas reveal how they eventually motivated the exploration of new fields in organic synthesis and organic reaction mechanism. Another significant (but until recently, virtually ignored) contribution by Hückel was the recognition that atomic connectivity is a strong determinant of spin multiplicity in non‐Kekulé molecules. This idea provides, in principle, a basis for predicting violations of Hund's rule, as recent computational and experimental developments confirm. The interaction of experiment with different styles of quantum theory, and the impact of this relationship on the development of chemistry, are briefly considered. Speculations are given on why Hückel's work exerted so little influence on organic chemistry for decades before its importance finally began to be recognized.
In Situ Observation of Transient Reaction Phenomena Occurring on Zeolite Catalysts with the Aid of Positron Emission Profiling
Authors: van Santen Rutger A. , Anderson B. G. , Cunningham R. H. , Mangnus A. V. G. , van Ijzendoorn L. J. , de Voigt M. J. A.
Published: January 2004

The total concentrations of reactants and products within a flow‐through reactor can be monitored as a function of both position and time by detecting gamma photons arising from the radioactive positron decay of injected 11C nuclei. The method was demonstrated by directly observing catalyst preconditioning and deactivation of a Pt/H‐mordenite catalyst bed during the industrially important hexane hydroisomerization reaction.
The Ca2+ Ion as a Cofactor for a Novel RNA‐Cleaving Deoxyribozyme
Authors: Faulhammer Dirk , Famulok Michael
Published: January 2004

In vitro selection experiments yield unexpected results. An RNA‐cleaving DNA enzyme that was selected in the presence of Mg2+ ions is much more efficient with Ca2+ as the cofactor. The result is even more surprising because of the lower hydrolytic efficiency of Ca2+ itself.
Transport and Detoxication: Principles, Approaches, and Perspectives for Research on the Blood – Brain Barrier
Authors: Aigner Achim , Wolf Sabine , Gassen Hans Günter
Published: January 2004

The blood–brain barrier is an anatomical constraint that maintains homeostasis within the brain and consists of uniquely structured brain capillaries. In addition to these morphological features, a number of specific proteins and transport systems (which are relevant to pharmaceutical applications) can be described that complement barrier functions and ensure a selective supply of substances from the blood to the brain. The additional role of the blood–brain barrier as a site of active detoxication by metabolizing and thus defending against neurotoxic substances has only become apparent in the past few years. Fundamental to understanding these processes are the principles of detoxication reactions and metabolic pathways of excretory organs, which will be described briefly here. The confirmation of these mechanisms in the brain enhances understanding of the complex protective functions of the blood–brain barrier. Evidently, these detoxication pathways simultaneously produce metabolites with neurotoxic or, in case of leukotriene, blood–brain barrier damaging potential. Finally, a detailed description of the mercapturic acid pathway of detoxication will be used to illustrate how evidence of representative enzymes can be used to distinguish definite functions of particular cell types and marker proteins of the blood–brain barrier.
Electron Crystallography—Now a Handy Method
Authors: Mertin Wilhelm
Published: January 2004

Multiple scattering of electrons in a crystal and the resulting interference problems make electron crystallography of inorganic substances a problematic undertaking. A recent publication by T. E. Weirich et al. now shows that electron crystallography could be the method of choice, for example, to determine the structure of small crystals. Providing that one has access to a high‐resolution electron microscope, the other requirements of this method are relatively small.
Geometry‐Tunable Lewis Acidity of Amidinium Cations and Its Relevance to Redox Reactions of the Thauer Metal‐Free Hydrogenase: A Theoretical Study
Authors: Cioslowski Jerzy , Boche Gernot
Published: January 2004

What are the thermodynamic prerequisites of the Berkessel–Thauer mechanism of action of the first metal‐free hydrogenase? Model reactions of planar and bent amidinium ions with H2 in the absence and presence of H2O and NH3 as bases demonstrate the significance of a proton acceptor in these reactions; the bending of the amidinium ion provides the necessary fine‐tuning.
Detection and Characterization of a Pre‐Reactive Complex in a Mixture of Water and Fluorine: Rotational Spectrum of H2O…︁F2
Authors: Cooke Stephen A. , Cotti Gina , Holloway John H. , Legon Anthony C.
Published: January 2004

A planar (C2v) or effectively planar (Cs) geometry is exhibited by the pre‐reactive intermediate H2O…︁F2, which has been identified in gaseous mixtures of H2O and F2. The rotational spectrum of this intermediate, which was obtained by using a Fourier‐transform microwave spectrometer equipped with a fast‐mixing nozzle, reveals that H2O…︁F2 has an O…︁F distance of 2.719(4) Å and is very weakly bound.
On the Trail of Xanthates: Some New Chemistry from an Old Functional Group
Authors: Zard Samir Z.
Published: January 2004

The use of free radical reactions in organic synthesis has witnessed an extraordinary development in recent years. When properly conceived, radical processes often exhibit many of the properties desired by synthetic organic chemists, such as flexibility, mildness, and selectivity. Unfortunately, the number of synthetically useful radical‐generating systems is still limited, and most applications have relied on tin hydride chemistry. Secondary O‐alkyl‐S‐methyl xanthates, for example, eact with tributyltin hydride to give the corresponding alkane (the Barton‐McCombie reaction). It was, however, found that the isomeric O‐methyl‐S‐alkyl xanthates undergo cleavage of the weaker carbon–sulfur bond and that a chain reaction can be sustained without tin or other heavy metals. A variety of synthetically interesting free radicals can thus be produced and captured, the last propagating step being a reversible transfer of the xanthate group. S‐Propargyl xanthates represent a special class. Their radical chemistry can be easily overshadowed by hitherto unknown but equally interesting nonradical behavior. Upon heating, a thermal [3,3] sigmatropic rearrangement occurs to give the allenyl isomer, which is in equilibrium with a novel betaine structure. This species is at the heart of a number of new transformations, including formal [3+2] cycloadditions, formation of esters with inversion in the case of secondary alcohols, conversion into 1,3‐dithiol‐2‐ones, generation of cisoid dienes, carbon, carbon‐carbon bond formation through reaction with acid chlorides etc. This account provides a brief description of this original radical and nonradical chemistry of xanthates, an old family of compounds that still harbors many mysteries.
Activity of [Sm(C5Me5)3] in Ethylene Polymerization and Synthesis of [U (C5Me5)3], the First Tris(pentamethylcyclopentadienyl) 5f‐Element Complex
Authors: Evans William J. , Forrestal Kevin J. , Ziller Joseph W.
Published: January 2004

For corrigendum see DOI:10.1002/anie.199713711
An FT–Raman Study of the Template –Framework Interaction in AlPO4‐Based Molecular Sieves
Authors: Ashtekar Sunil , Barrie Patrick J. , Hargreaves Mark , Gladden Lynn F.
Published: January 2004

The organic template within molecular sieves may readily be studied by FT–Raman spectroscopy with virtually no interference from bands due to framework vibrations. FT‐Raman spectra show that morpholine and cyclohexylamine are in the protonated form within AlPO4‐based materials with the chabazite structure, while splittings of certain bands indicate specific interactions between the organic template and the framework.
Superstructures of Functional Colloids: Chemistry on the Nanometer Scale
Authors: Antonietti Markus , Göltner Christine
Published: January 2004

Chemistry is classically concerned with the connection of atoms and molecules into new functional units. The rules of connection are yet to be extended to the generation and connection of larger objects, whose dimensions are measured in nanometers. However, linking objects of this size through molecules approaching each other randomly is inefficient, instead the principle of self‐assembly is decisive, in which lyotropic structure formation or amphiphilic interaction play a significant role. As a result of the nature of the energetic driving forces, the objects generated in this way are often well‐defined aggregate structures or highly symmetric volume phases. In contrast to “molecular chemistry”, the linking of larger objects also disregards the inherent borders of classical fields of chemistry: for example, the nanoscale association of inorganic colloids with polymers affords hybrid materials that combine the physical properties of both partners. In such a way, catalytic, optical, and electronic features of inorganic colloids might be combined with the mechanical characteristics of polymers such as film formation, elasticity, and melt processibility.
Amorphous Plastics Pave the Way to Widespread Holographic Applications
Authors: Meerholz Klaus
Published: January 2004

Noncrystalline organic photorefractive materials appear to be on the verge of widespread applications such as holographic storage and real‐time holographic processing. The performance levels of these materials, which are attracting increasing interest, are close to, and in some respects even beyond those of their long‐known inorganic counterparts.
Towards Radiation‐Sensitive Quasi‐Biological Display
Authors: de Groot Franciscus M. H. , Gottarelli Giovanni , Masiero Stefano , Proni Gloria , Spada Gian Piero , Dolci Nicoletta
Published: January 2004

The pitch of the columnar cholesteric mesophase formed by the dinucleoside phosphate d(GpG) and water becomes shorter after exposure to γ radiation. This effect can be measured directly with an optical microscope from the distance between the fingerprint lines. The inverse pitch is linearly correlated to the dose absorbed between 0 and 20 kGy.
Surface X‐ray Scattering Study of Stereospecific Adsorption of Additives onto the Surface of a Molecular Crystal Grown from Solution
Authors: Gidalevitz David , Feidenhans'l Robert , Leiserowitz Leslie
Published: January 2004

An ordered layer of (S)‐methionine is attached to the (010) face of α‐glycine when the crystals are brought into contact with a saturated solution of glycine containing (S)‐methionine as additive. The direct evidence was obtained by grazing incidence X‐ray scattering experiments, which also indicated different behavior for the (010) and (010) faces. The concentration of the surface‐bound methionine molecules was determined to be 4:1.
Design of Antisense (Complementary) Peptides as Selective Inhibitors of Cytokine Interleukin‐1
Authors: Davids Joseph W. , El‐Bakri Ali , Heal Jonathan , Christie Gary , Roberts Gareth W , Raynes John G. , Miller Andrew D.
Published: January 2004

The ability of sense peptides (coded for by sense DNA) and corresponding antisense peptides (coded for by corresponding antisense DNA) to interact specifically was used to design antisense peptide inhibitors of interleukin‐1 isoforms IL‐1β and IL‐1α. These inhibitors appear to act as “mini receptors”, binding to both isoforms and thereby sterically blocking the normal interaction between interleukin‐1 and its receptors.
IR Spectroscopic Evidence for the Absence of Structural Hydroxyl Groups in Cloverite
Authors: Thibault‐Starzyk Frédéric , Janin André , Lavalley Jean‐Claude
Published: January 2004

Little or no Brønsted acidity is exhibited by the large‐pore molecular sieve cloverite according to quantitative IR spectroscopic investigations. Only 3.5% of the expected 192 hydroxyl groups per unit cell suggested by the X‐ray structure are detected.
Cation‐Cyclization Selectivity: Variable Structures of Protonated Cyclopropanes and Selectivity Control by Catalytic Antibodies
Authors: Lee Jeehiun K. , Houk Kendall N.
Published: January 2004

A nonclassical, protonated cyclopropane is the intermediate formed in an antibodycatalyzed cyclization reaction. The selectivity in favor of the cyclopropane product is increased upon binding of the catalytic antibody, which amplifies the inherent preference for one reaction pathway.