bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the Trail of Xanthates: Some New Chemistry from an Old Functional Group
Authors: Zard Samir Z. 
Published: January 2004

AbstractThe 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

AbstractFor corrigendum see DOI:10.1002/anie.199713711
Superstructures of Functional Colloids: Chemistry on the Nanometer Scale
Authors: Antonietti Markus , Göltner Christine 
Published: January 2004

AbstractChemistry 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.
Oxidative Transformations of Organic Compounds Mediated by Redox Molecular Sieves
Authors: Arends Isabel W. C. E. , Sheldon Roger A. , Wallau Martin , Schuchardt Ulf 
Published: January 2004

AbstractZeolites are viewed by some as the “philosopher's stone” of modern chemistry.[1] They are more or less indispensable in oil refining and petrochemicals manufacture where they are widely applied as solid acid catalysts. More recently attention has been focused on their use in the manufacture of fine chemicals. The synthetic utility of zeolites and related molecular sieves (zeotypes) has been considerably extended by the incorporation of redox metals into their frameworks. The resulting redox molecular sieves catalyze a variety of selective oxidations under mild conditions in the liquid phase. Their structural diversity–including variation of the redox metal, incorporation of metal complexes, and the size and polarity of the micropores–provides the possibility of designing tailor‐made solid catalysts (“mineral enzymes”) for liquid‐phase oxidations with clean oxidants such as O2, H2O2, and RO2H. Hence, they have enormous potential in industrial organic synthesis as environmentally friendly alternatives to traditional oxidations employing inorganic oxidants in stoichiometric amounts. A primary aim of this review is to familiarize organic chemists with the synthetic potential of redox molecular sieves. An outline of their synthesis, structures, and chemical properties, highlighting their unique advantages, is followed by a discussion of general (mechanistic) features that influence the choice of a suitable catalyst for a particular type of oxidation. The main part of the review deals with the oxidation of various substrates of synthetic interest–such as alkanes, alkenes, (alkyl)arenes, alcohols, and amines–and emphasizes the advantages of redox molecular sieves (including selectivity and stability) over their homogeneous counterparts. New directions towards truly biomimetic solid catalysts, for example zeolite‐encapsulated chiral metal complexes as heterogeneous catalysts for asymmetric oxidations, are high‐lighted.
Alkenylidenes in Organic Synthesis
Authors: Kirmse Wolfgang 
Published: January 2004

AbstractAlkenylidenes R2CC: (= alkylidene carbenes) undergo regio‐ and stereoselective intramolecular CH insertion reactions that are excellently suited for the synthesis of cyclopentenes. The 1, 2‐shifts occurring with RH and RAr are useful for the preparation of alkynes. Alkenylidenes are efficiently generated from carbonyl compounds by diazomethylation, from vinyl halides by α‐elimination, from alkynyliodonium salts by addition of nucleophiles, and from alkynes by retro‐1, 2‐shifts. Specific applications of the various methods, particularly in the synthesis of natural products, are discussed.
Novel Reactions of Organomolybdenum and Organotungsten Compounds: Additive–Reductive Carbonyl Dimerization, Spontaneous Transformation of Methyl Ligands into μ‐Methylene Ligands, and Selective Carbonylmethylenation
Authors: Kauffmann Thomas 
Published: January 2004

AbstractHitherto there was no reaction known that permits transformations of R1R2‐CO → 0.5 R1R2R3C–CR1R2R3 in one step. This type of additive–reductive carbonyl dimerization is now possible using alkoxy(alkyl)tungsten(v) complexes with aromatic, heteroaromatic or α,β‐unsaturated aldehydes and ketones. When a corresponding phenyl complex was employed in a test experiment, it was revealed that an aliphatic ketone could be used as the substrate in this reaction. A second interesting type of reaction is the transformation of CH3 ligands into μ‐CH2 ligands, which occurs during the treatment of MeLi or Me3Al with molybdenum or tungsten chlorides (oxidation states VI and V, for Mo additionally IV) at low temperatures with liberation of CH4. Here, the question arises as to whether the intermediate involved has a terminal CH2 ligand (Schrock carbene complex) or a μ‐CH3 ligand (CH3 bound by a two‐electron three‐center bond to two metal atoms). Of all the μ‐CH2 complexes obtained, those which were synthesized by the action of MeLi on molybdenum chlorides can be recommended as reagents for carbonylmethylenation of aldehydes and ketones. They display high selectivity, very low basicity, a surprising resistance to protons, they are readily available, can be easily modified and, as regards their selective behavior, they have been investigated more thoroughly than other readily accessible carbonylmethylenation reagents of comparable selectivity. The results of NMR spectroscopic investigations on the structure of the μ‐CH2 complexes, and associated reaction mechanisms are discussed. A survey of carbonylmethylenation reagents, which have been reported in the literature, permits comparisons to be made with carbonylmethylenating molybdenum and tungsten complexes.
Stereoisomeric Peptide Libraries and Peptidomimetics for Designing Selective Inhibitors of the αvβ3 Integrin for a New Cancer Therapy
Authors: Haubner Roland , Finsinger Dirk , Kessler Horst 
Published: January 2004

AbstractCell–cell and cell–matrix interactions play a major role in biology. An important class of surface receptors are the integrins, which exist in a number of subtypes and are involved in many biological processes, for example embryonic development, blood coagulation, osteoporosis, cancer, and regulation of the balance between proliferation and death (apoptosis) of a cell. Therefore, selective inhibition of specific integrin subtypes is of great pharmaceutical interest. It is possible to develop highly selective and active inhibitors of integrins, especially the αvβ3 subtype, by the novel procedure of spatial screening. The resulting αvβ3 antagonists exhibit activity in the nanomolar range and suppress tumor‐induced angiogenesis (formation of new blood vessels). Tissues with intact blood vessels are not influenced. This opens a promising new way for cancer therapy.
Molecular Beam Studies on Semiconductor Clusters: Polarizabilities and Chemical Bonding
Authors: Becker Jörg August 
Published: January 2004

AbstractThis article discusses experiments that measure polarizabilities, permanent dipole moments, and absorption spectra of semiconductor clusters such as SiN, GaNAsM, and GeNTeM isolated in a molecular beam. The experiments show that the transition from the typical optoelectronic properties of macroscopic semiconductor crystals to the molecular properties of small polyhedral semiconductor molecules occurs in several steps. The bonding in nanometer‐sized semiconductor colloids with approximately 103–105 atoms is similar to that in the solid state. The bonding orbitals of the semiconductor crystals, which extend over a nanometer range, are, however, somewhat more strongly localized in these clusters and are therefore already slightly modified. Over the size range from 103 to 10 atoms, a gradual transition occurs from localized solid‐state‐like bonds to molecule‐like bonds. Interestingly, quantum size effects are found even in relatively small particles with about 102 atoms. These effects can be observed in the form of unusually small static polarizabilities of SiN clusters or in the absorption bands of GaNAsM clusters, which are strongly blue‐shifted relative to those of the solid state. For smaller clusters. These effects disappear over the size range 102−10 atoms, which is manifested by an increasing red‐shift of the absorption bands with decreasing cluster size.
Uncatalyzed Transfer Hydrogenation and Transfer Hydrogenolysis: Two Novel Types of Hydrogen‐Transfer Reactions
Authors: Rüchardt Christoph , Gerst Matthias , Ebenhoch Jochen 
Published: January 2004

AbstractUncatalyzed transfer hydrogenations are H2‐transfer reactions in which donors that contain weakly bound hydrogen atoms undergo an H‐atom transfer (retrodisproportionation) onto an acceptor containing unsaturated bonds such as CC, CO, CN, NN, NO. Transfer hydrogenolyses are reactions in which σ bonds are cleaved upon additon of H2. These hydrogenations are terminated by H, transfer; they do not follow a radical‐chain mechanism. The initial steps of both types of reactions, H‐atom transfer, complement the bimolecular formation of 1,4‐diradicals from alkenes or heteroalkenes within the scope of bimolecular radical formation (Molecule‐Induced Radical Formation, MIRF). The title reactions play an important role in coal liquefaction, aromatization reactions with nitroarenes or quinones, and possibly biochemical dehydrogenations. This review focuses on mechanistic studies, structure–reactivity relationships, and current applications of these reactions.
Chemistry and Molecular Biology of Transmissible Spongiform Encephalopathies
Authors: Edenhofer Frank , Weiss Stefan , Winnacker Ernst‐Ludwig , Famulok Michael 
Published: January 2004

AbstractPrion diseases are currently in the spotlight. Among them, the Creutzfeldt–Jakob disease in humans, scrapie in sheep, and bovine spongiform encephalopathy, or mad cow disease, are most commonly known. The term “spongiform” refers to the characteristic appearance of the lesions found in affected brains. It is likely that prion diseases originate from a causative agent that replicates independently of nucleic acids. Current research assumes that a structural isoform of prion protein, the scrapie form PrPSc, is the responsible pathogen. The three‐dimensional structure, but not the amino acid sequence of the isoform differs from that of the normal cellular isoform, PrPc. According to a widely accepted hypothesis, the normal isoform of the protein is converted by an autocatalytic process into the scrapie form upon contact with the latter. This hypothesis has not yet been proven. However, considerable progress has been made in the last few years, which might provide answers to many open questions about prion diseases, the subject of this review.
ELF: The Electron Localization Function
Authors: Savin Andreas , Nesper Reinhard , Wengert Steffen , Fässler Thomas F. 
Published: January 2004

AbstractThe chemical bond is always considered from different points of view, depending or on the chemical and physical aspects to be examined. In both cases, description of the chemical bond are chosen that are appropriate for the particular research or application. Therefore, there are significant differences in the understanding as what constitutes chemical bonding. This is acceptable in practice but proves to be a hindrance for true interdisciplinarity. The concept of the chemical bond offer a firm basis upon which to forge links not only within chemistry but also to all related sciences. The general desire and the growing necessity for interdisciplinary collaboration requires a careful treatment of these concepts and, if possible, a tightening and standardization to a level that is widely acceptable and beneficial. In the age of tremendously fast development of computers and computer science, we believe that the electron localization function (ELF) provides a new description of the chemical bond for almost all classes of compounds. Its graphical language earns it the ultimate qualification for enhanced interdisciplinarity.
Olefin Metathesis in Organic Chemistry
Authors: Schuster Matthias , Blechert Siegfried 
Published: January 2004

AbstractTransition metal catalyzed CC bond formations belong to the most important reactions in organic synthesis. One particularly interesting reaction is olefin metathesis, a metal‐catalyzed exchange of alkylidene moieties between alkenes. Olefin metathesis can induce both cleavage and formation of CC double bonds. Special functional groups are not necessary. Although this reaction—which can be catalyzed by numerous transition metals—is used in industry, its potential in organic synthesis was not recognized for many years. The recent abrupt end to this Sleeping‐Beauty slumber has several reasons. Novel catalysts can effect the conversion of highly fictionalized and sterically demanding olefins under mild reaction conditions and in high yields. Improved understanding of substrate–catalyst interaction has greatly contributed to the recent establishment of olefin metathesis as a synthetic method. In addition to the preparation of polymers with fine‐tuned characteristics, the metathesis today also provides new routes to compounds of low molecular weight. The highly developed ring‐closing metathesis has been proven to be key step in the synthesis of a growing number of natural products. At the same time interesting applications can be envisioned for newly developed variants of bimolecular metathesis. Improvements in the selective cross‐metathesis of acyclic olefins as well as promising attempts to include alkynes as viable substrates provide for a vivid development of the metathesis chemistry.
N‐Heterocyclic Carbenes
Authors: Herrmann Wolfgang A. , Köcher Christian 
Published: January 2004

AbstractThe chemistry of N‐heterocyclic carbenes has long been limited to metal coordination compounds derived from azolium precursors, a development that was started by Öfele and Wanzlick in 1968. Since free carbenes are now available through the work of Arduengo (1991), a renaissance in this little‐recognized area of chemistry has occurred. A leading motive is the advantages of N‐heterocyclic carbenes as ligands in organometallic catalysts, where they extend the scope of application reached by phosphanes (functionalized, chiral, water‐soluble, and immobilized derivatives). The present review summarizes the state of the art with regard to synthesis, structure, bonding theory, metal coordination chemistry, and catalysis. Chelating, functionalized, chiral, and immobilized ligands can be generated and attached to metal centers in straightforward procedures under mild conditions. A wealth of new chemistry is thus opened. It is also shown how carbenes derived from imidazoles and triazoles behave as ligands in catalysis. It is reasonable to assume that N‐heterocyclic carbenes surpass the ubiquitous phosphanes as ligands in a number of organometallic catalytic reactions.
Chemistry in Microstructured Reactors
Authors: Jähnisch Klaus , Hessel Volker , Löwe Holger , Baerns Manfred 
Published: January 2004

AbstractThe application of microstructured reactors in the chemical process industry has gained significant importance in recent years. Companies that offer not only microstructured reactors, but also entire chemical process plants and services relating to them, are already in existence. In addition, many institutes and universities are active within this field, and process‐engineering‐oriented reviews and a specialized book are available. Microstructured systems can be applied with particular success in the investigation of highly exothermic and fast reactions. Often the presence of temperature‐induced side reactions can be significantly reduced through isothermal operations. Although microstructured reaction techniques have been shown to optimize many synthetic procedures, they have not yet received the attention they deserve in organic chemistry. For this reason, this Review aims to address this by providing an overview of the chemistry in microstructured reactors, grouped into liquid‐phase, gas‐phase, and gas–liquid reactions.
Importance of addressing National Electrical Code® violations that result in unusual exposure to 60 Hz magnetic fields
Authors: Adams Jack , Samuel Bitler J. , Riley Karl 
Published: January 2004

AbstractWe evaluated wiring in multifamily developments containing National Electrical Code® (NEC®) violations as a source of unusual exposure to 60 Hz magnetic fields. Two methods were used in this evaluation: measurement and modeling. We measured the building wiring as a source of magnetic fields in six multifamily developments in Michigan. In this small sample, building wiring proved to be an important source of exposure in four of the six cases. In all four cases with exposure from building wiring, one or more NEC violations were involved. To supplement our measurement efforts, we used computer modeling to compare magnetic field exposure due to building wiring with magnetic field exposure from external power lines. Our calculations showed that where the building wiring has a NEC violation leading to net current loops, the exposure due to wiring is likely to be more important than that from external power lines. Our results support the results obtained in a recent study of the exposure of Californian K‐12 students to magnetic fields, where building wiring with one or more NEC violation was found to be the single most important exposure source. If 60 Hz magnetic fields are important to avoid, then improved enforcement of the NEC, as required by law, is perhaps the single most important mitigation policy to adopt. Bioelectromagnetics 25:102–106, 2004. © 2004 Wiley‐Liss, Inc.
Teratological studies of prenatal exposure of mice to a 20 kHz sawtooth magnetic field
Authors: Kim Sung‐Ho , Song Ji‐Eun , Kim Se‐ra , Oh Heon , Gimm Youn‐Myoung , Yoo Done‐Sik , Pack Jeong‐Ki , Lee Yun‐Sil 
Published: January 2004

AbstractIn order to evaluate the importance of gestational age in possible effects due to exposure to a 20 kHz sawtooth magnetic field, pregnant ICR mice at gestational 2.5–15.5 days post‐coitus, which is the most sensitive stage for the induction of major congenital malformations, were exposed in a carrousel irradiator. The mice were exposed to a 20 kHz intermediate frequency (IF) sawtooth magnetic field had a 6.5 μT peak intensity for 8 h/day. The animals were sacrificed on the 18th day of gestation; and the fetuses were examined for mortality, growth retardation, changes in head size, and other morphological abnormalities. From the above conditions, it is concluded that the exposure to a 20 kHz sawtooth magnetic field with 6.5 μT peak intensity does not inflict any adverse effect on fetuses of pregnant mice. Bioelectromagnetics 25:114–117, 2004. © 2004 Wiley‐Liss, Inc.
Cellular phone use does not acutely affect blood pressure or heart rate of humans
Authors: Tahvanainen Kari , Niño Juanita , Halonen Pirjo , Kuusela Tom , Laitinen Tomi , Länsimies Esko , Hartikainen Juha , Hietanen Maila , Lindholm Harri 
Published: January 2004

AbstractA recent study raised concern about increase of resting blood pressure after a 35 min exposure to the radiofrequency (RF) field emitted by a 900 MHz cellular phone. In this randomized, double blind, placebo controlled crossover trial, 32 healthy subjects were submitted to 900 MHz (2 W), 1800 MHz (1 W) cellular phone exposure, and to sham exposure in separate sessions. Arterial blood pressure (arm cuff method) and heart rate were measured during and after the 35 min RF and sham exposure sessions. We evaluated cardiovascular responses in terms of blood pressure and heart rate during controlled breathing, spontaneous breathing, head‐up tilt table test, Valsalva manoeuvre and deep breathing test. Arterial blood pressure and heart rate did not change significantly during or after the 35 min RF exposures at 900 MHz or 1800 MHz, compared to sham exposure. The results of this study indicate that exposure to a cellular phone, using 900 MHz or 1800 MHz with maximal allowed antenna powers, does not acutely change arterial blood pressure and heart rate. Bioelectromagnetics 25:73–83, 2004. © 2004 Wiley‐Liss, Inc.