bluuebunny/crossref_metadata_2025_split huggingface dataset:
2,3‐Dihydro‐1,3‐diborole‐Metal Complexes with Activated CH Bonds: Building Blocks for Multilayered Sandwich Compounds
Authors: Siebert Walter 
Published: December 2003

AbstractDerivatives of 2,3‐dihydro‐1,3‐diborole acting as four‐electron ligands form various metal complexes in which the ring‐methylene carbon atom is pentacoordinated. The resultant activation of the CH bond can be used to prepare triple‐decker to hexadecker sandwich complexes. Bis(2,3‐dihydro‐1,3‐diborole)nickel complexes undergo a spontaneous stacking and formation of tricarbahexaboranyl oligodecker complexes with stacks of 2,3‐dihydro‐1,3‐diborolylnickel. These findings finally led to the synthesis of the first polydecker sandwich complex which was obtained by vacuum thermolysis of a tris(allyl)(μ‐2,3‐dihydro‐1,3‐diborolyl)dinickel complex. The black polymer possesses remarkable semiconductor properties.
Second‐Sphere Coordination–a Novel Rǒle for Molecular Receptors
Authors: Colquhoun Howard M. , Stoddart J. Fraser , Williams David J. 
Published: December 2003

AbstractAlthough it has been appreciated for most of this century that the coordinating influence of many transition metal complexes extends beyond their covalently‐bonded first‐sphere ligands to non‐covalently bound chemical species in the so‐called second‐sphere, it is only recently that the fundamental importance of the phenomenon has been recognized and researched in its most general context. Rapid progress has been possible in this relatively new area of supramolecular chemistry by appealing to the symbiotic relationship that exists between X‐ray crystallographic investigations and synthetic work. The gamut of non‐covalent bonds, including electrostatic forces, hydrogen bonding, charge transfer, and van der Waals interactions are available for exploitation in choosing or designing suitable molecular receptors for particular transition metal complexes. Crown ethers are the synthetic macrocyclespar excellencefor forming adducts with neutral and cationic complexes carrying protic ligands (NH3, H2O, CH3CN, etc.) in their first coordination spheres. Anionic complexes with electron‐rich first‐sphere ligands (e.g. CN⊖) form adducts with polyammonium macrocyclic receptors. In both situations, hydrogen bonds and electrostatic interactions provide the dominant sources of supramolecular stabilization. Spectroscopic studies (UV, IR, and NMR) reveal that the structural integrity of the adducts is usually maintained in solution. The transition metal complexes can be organometallic as well as inorganic. In complexes supporting organic ligands, such as 1,5‐cyclooctadiene, norbornadiene, cyclopentadiene, 2,2′‐bipyridine, and trimethylphosphane, the opportunity exists to match their steric and electronic characteristics with appropriate binding sites in the molecular receptor. With these ligands, the weaker categories of non‐covalent bonding, e.g. charge transfer and van der Waals interactions, assume considerable importance. The phenomenon is also observable with naturally‐occurring receptors such as the cyclodextrins and polyether antibiotics. Moreover, it extends beyond the territory of transition metal complexes to main group complexes such as ammonia‐borane. As far as applications are concerned, it finds expression in areas as diverse as separation science and drug delivery systems.
High‐Pressure Liquid Chromatography (HPLC) of Proteins [New Analytical Methods (29)]
Authors: Seipke Gerhard , Müllner Hubert , Grau Ulrich 
Published: December 2003

AbstractThe application of high‐pressure liquid chromatography (HPLC) to proteins has undergone a dramatic development in recent years. Nowadays its many variants expand the repertoire of high‐performance analysis methods available to the protein chemist, which, until now, have been dominated by electrophoretic techniques. The advent of gene technology has resulted in a renaissance of protein chemistry. The new analytical and preparative problems that have thereby emerged are often ideally solved by HPLC methods. HPLC has long since ceased to be solely a laboratory technique; HPLC systems are now being developed for the separation of proteins–particularly those of great pharmaceutical interest – on a 100‐g scale. The range of applications of analytical and preparative HPLC will be illustrated by two examples of pharmaceutical importance—insulin and interleukin 2.
185‐nm Photochemistry of Olefins, Strained Hydrocarbons, and Azoalkanes in Solution
Authors: Adam Waldemar , Oppenländer Thomas 
Published: December 2003

AbstractThe possibility to excite directly at 185 nm chromophores that absorb in the vacuum‐UV has stimulated increased activity during the last decade in this field of photochemistry. Whereas photochemical reactions at λ<200 nm have been thoroughly investigated in the gas phase, only recently have intensive studies on the 185‐nm photochemistry of organic compounds in solution provided new insights. Despite the high excitation energies, selective photoreactions are promoted in the short‐lived singlet excited states (Rydberg photochemistry). In contrast to conventional photochemistry (λ > 220 nm), 185‐nm irradiation preferentially results in intramolecular rearrangement, fragmentation, and isomerization reactions. Intermolecular radical couplings and abstractions as well as dimerizations (π, π*‐excitation) compete minimally. Besides the straightforward denitrogenation of photoresistant (“reluctant”) azoalkanes, important applications of the short‐wavelength photolysis are also found in technology (photolithography) and medicine (193‐nm laser). Broadening the scope of the synthetic potential of the 185‐nm photochemistry, which so far has been limited to direct cis/trans isomerizations, presents a challenge for the chemist.
Indirect Electroorganic Syntheses—A Modern Chapter of Organic Electrochemistry [New Synthetic Methods (59)]
Authors: Steckhan Eberhard 
Published: December 2003

AbstractThe electrochemical formation and regeneration of redox agents for organic syntheses (indirect electrolysis) widens the potential of electrochemistry, as higher or totally different selectivities can often be obtained while at the same time the energy input can be lowered significantly. Higher current densities can also be obtained by preventing otherwise often encountered electrode inhibition. New types of redox catalysts can be formed in‐situ and can be regenerated after reaction with the substrates. This principle is of increasing importance also for the application of already known redox agents with regard to environmental protection, since large amounts of a product can be generated in a closed circuit using only relatively small amounts of the redox reagent. Consequently the operation of such a process can be greatly simplified, and the release of ecologically objectionable spent reagents into the environment can be prevented. The broad spectrum of redox catalysts currently in use includes, inter alia, metal salts in very low or high oxidation states, halogens in various oxidation states, and, in particular, a wide variety of transition‐metal complexes. A great deal of progress has recently been made in the application of organic electron transfer agents, since compounds have been found that are sufficiently stable in both the reduced as well as the oxidized state.
Total Synthesis of Anthracyclinone [New Synthetic Methods (60)]
Authors: Krohn Karsten 
Published: December 2003

AbstractThe remarkable antitumor properties of anthracycline antibiotics have motivated synthetic chemists, and a large number of groups have been active recently in this field. Initially, there was very little hope for this class of substances due to their extreme toxicity. However, the dividing line between toxicity and desired pharmacological effect has broadened to such an extent, partly through microbiological methods (new strains, mutants) and partly through semisynthetic modifications, that today anthracyclines take number one place (over 20% of the market) in the chemotherapy of (human) cancer. The demands made on modern cytostatica are extremely high, as may be seen from the fact that of the more than 600000 drugs tested in animals, only about 40 are in clinical use. In the meantime, some anthracyclines obtained by total synthesis are also in clinical testing. In this review it will hopefully be demonstrated that all those concerned in the chemotherapeutic use of these substances have also contributed to the general progress made in the solution of regio‐ and stereo‐chemical problems of linearly annelated ring systems.
High‐Resolution Metal NMR Spectroscopy of Organometallic Compounds [New Analytical Methods (30)]
Authors: Benn Reinhard , Rufińska Anna 
Published: December 2003

AbstractGreat advances have been made in the past decade in the field of NMR spectroscopy. Apart from the development of completely new areas of application, such as in solid‐state chemistry, in materials science, in physiological chemistry, and in medicine, with the introduction of new pulse spectroscopic methods and the application of high magnetic field strengths important progress has also been made in the traditional field of high‐resolution NMR spectroscopy. Thus, among other things, the observation of metal resonances has been facilitated and new areas of application have been opened up in inorganic and organometallic chemistry. In this review, recent detection methods for spin‐1/2 and quadrupolar metal nuclei are presented and discussed. The use of metal‐NMR spectroscopy with respect to problems of a typical chemical nature, mainly from the field of organometallics, is demonstrated for a number of selected metal nuclei (25Mg, 27Al, 49Ti, 57Fe, 59Co, 61Ni, 91Zr, 103Rh, and 195Pt). Relations found empirically between chemical shifts and coordination number, oxidation number, and electronic configuration of a metal bound in a complex are emphasized. Furthermore, cases in which the chemical shifts of metal nuclei can be interpreted in terms of the energy difference of frontier orbitals are presented. This aspect leads to the establishment of a relationship between chemical reactivity and NMR parameters for a series of related compounds.
Unsaturated Molecules Containing Main Group Metals
Authors: Veith Michael 
Published: December 2003

AbstractMolecules in which there are neighboring electrophilic and nucleophilic centers are unusually reactive. Oligomerization can be prevented only by bulky groups attached to the main group metal atom that would act as electron pair acceptor, or to the basic non‐metal atom. The basic and the acid centers behave as a single unit in chemical reactions; the system is similar to a “double bond” whose π‐electron density is largely concentrated at one atom. The unsaturated nature of these molecules can be seen in (for example) their addition reactions with hydrogen compounds of non‐metals, or in reactions that are distantly related to cycloadditions at homopolar double bonds. The selection of suitable reaction partners leads to polycyclic, cage‐like molecules containing metal atoms. If these atoms possess lone pairs (as is usual in the lower oxidation states of the third and fourth main groups), these can be utilized to form bonds to further (Lewis acid) metal centers. In some cases large assemblies can be built up from polycyclic systems in this way; a characteristic of these assemblies is a one‐dimensional array of metal atoms. Commonly occurring structural features of the polycyclic species are tetrahedra, trigonal bipyramids and cubes.
Laser Chemistry–What is Its Current Status?
Authors: Kleinermanns Karl , Wolfrum Jürgen 
Published: December 2003

AbstractIn recent years, various methods have been developed to observe and to influence the course of chemical reactions using laser radiation. By selectively increasing the translational, rotational, and vibrational energies and by controlling the relative orientation of the reaction partners with tunable infrared and UV lasers, direct insight can be gained into the molecular course of the breaking and re‐forming of chemical bonds. Examples for the application of lasers include the synthesis of monomers such as vinyl chloride and polymers such as polyethylene, the synthesis of biologically active substances such as vitamin D3, the separation of isotopes, the removal of impurities, the production of catalysts, glasses, and ceramics, and the deposition and ablation of material on surfaces. Finally, several applications of lasers in medicine are discussed.
Electrodes for Potential Measurements in Aqueous Systems at High Temperatures and Pressures
Authors: Niedrach Leonard W. 
Published: December 2003

AbstractThe measurement of pH, redox potentials, and corrosion potentials at high temperatures and pressures is often desirable for the control and monitoring of industrial processes; e.g., for controlling water chemistry in conventional and nuclear power generators, for process control in the chemical industry, and for monitoring performance in petroleum extraction and refining. Such measurements are also needed for research on the thermodynamics of high‐temperature aqueous solutions including those associated with geothermal and hydrothermal reactions. Zirconia electrodes for these purposes are described, and their application is illustrated with examples. While not yet routinely available on a commercial basis, useful versions can be readily fabricated in the laboratory. Some of the pitfalls encountered in making such measurements at elevated temperatures are also discussed because great care is often required to assure that valid and useful data are being obtained.
Neutral and Charged Biradicals, Zwitterions, Funnels in S1, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision
Authors: Bonačić‐Koutecký Vlasta , Koutecký Jaroslav , Michl Josef 
Published: December 2003

AbstractA knowledge of the geometries at which excited molecules return to the electronic ground state (S0) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S1 (lowest excited singlet) and T1(lowest triplet) surfaces, as well as S0–S1 conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the S0 and S1 surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two‐electron two‐orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the S0 and S1 surfaces touch. The results obtained for the simple model are supported by ab initio large‐scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and π‐donor‐to‐π‐acceptor single bonds, and by similar calculations for “push‐pull” perturbed cyclobutadienes, some of which are predicted to have nearly degenerate S0, S1, and T1 states. The likely consequences of these results for the detailed description of the mechanisms of cis‐trans isomerization, the formation of twisted internal charge‐transfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocycloaddition, are summarized.
The Development of a Carbene‐like Chemistry with Terminal Phosphinidene Complexes
Authors: Mathey François 
Published: December 2003

AbstractWhen compared to the chemistry of other well‐known electron‐deficient species such as carbenes, silylenes, and nitrenes, the chemistry of phosphinidenes RP: is extremely underdeveloped. A critical survey of the literature indicates that this state of affairs probably reflects both an inadequate access to these species and an intrinsic lack of reactivity. Theoretical calculations suggest that it is possible to boost the electrophilicity and to stabilize the singlest state of phosphinidenes by complexation with transition‐metal moieties such as M(CO)5 (MCr, Mo, W). The corresponding terminal phosphinidene complexes [RPM(CO)5] show a rich and varied chemistry which often parallels the chemistry of singlet carbenes. Phosphinidene complexes are presently easily accessible through thermal decomposition of 7‐phosphanorbornadiene complexes, but their stabilization remains a challenge to be met.
Glycolate and Thioglycolate Complexes of Rhenium and Their Oxidative Elimination of Ethylene and of Glycol
Authors: Herrmann Wolfgang A. , Marz Dieter , Herdtweck Eberhardt , Schäfer Adolf , Wagner Werner , Kneuper Heinz‐Josef 
Published: December 2003

AbstractSelenium dioxide and osmium tetroxide are effective reagents and catalysts for olefin oxidation, although, owing to their toxicity, reservations remain as to their applicability.[1] We are therefore seeking more easily handled metal oxides that are soluble in organic solvents and that are as effective as osmium tetroxide in carrying out stereospecific cis hydroxylation of olefins. The rhenium(VII) oxide 1, which has meanwhile become readily accessible, is a favorable candidate.[2]
New Cisplatin Analogues—On the Way to Better Antitumor Agents
Authors: Pasini Alessandro , Zunino Franco 
Published: December 2003

AbstractThe clinical success of cisplatin (cis‐diamminedichloroplatinum(II)) in antitumor chemotherapy has encouraged an all‐out search for analogues with lower toxicity, improved therapeutic index and increased activity. Literally thousands of analogues, obtained by replacement of the ammine‐ and chloro‐ligands by other amines and anionic ligands, respectively, have been systematically screened for activity in experimental tumor models. Some of these analogues have been selected for clinical evaluation, but only very few of them appear to be promising antitumor agents. More recently, cisplatin analogues have been designed and synthesized on the basis of, inter alia, the following considerations: 1) platinum complexes with carrier molecules as ligands should prove useful for achieving increasing drug concentration in tumor tissues; 2) platinum complexes with chemotherapeutic agents as ligands could afford polyfunctional drugs with synergistic action; 3) complexes containing more than one platinum atom might be more effective than complexes containing only one platinum atom; 4) platinum complexes could be used as sensitizers in radiation therapy. In this paper, we shall give a brief account of the “traditional” analogues, and then critically discuss what we believe could be the new trends in the design of cisplatin analogues.
Biological Exploitation of Solar Energy by Photosynthetic Water Splitting
Authors: Renger Gernot 
Published: December 2003

AbstractThe cleavage of water by solar radiation into dioxygen and metabolically bound hydrogen during photosynthesis is of central importance for the existence of higher forms of life on earth. The realization of this process in biological organisms made possible the use of the earth's huge water reservoir for the exploitation of solar energy and, at the same time, led to the creation of an aerobic atmosphere. The dioxygen thereby formed is a powerful oxidant which permits an energetically highly efficient nutrient turnover. In recent years considerable progress has been made in understanding the functional and structural organization of photosynthetic water splitting. This article attempts to give a review of our current state of knowledge with special emphasis on the oxidation of water to O2in biological systems.
The Generation and Characterization of Molecules by Neutralization‐Reionization Mass Spectrometry (NRMS). New Analytical Methods (33)
Authors: Terlouw Johan K. , Schwarz Helmut 
Published: December 2003

AbstractThe diluted gas phase within a mass spectrometer is ideally suited for performing collision experiments of fast moving ions with thermalized target atoms. One important reaction which can be brought about with a given cation is its selective reduction, i. e. its neutralization. This allows the synthesis in tailored experiments of a variety of unusual chemical species, which because of intermolecular processes cannot be prepared or studied in solution or in a matrix. Among the molecules which have been generated in this way are hypervalent species (Rydberg radicals) like H, van der Waals complexes like He2, mono‐ or disubstituted acetylene derivatives XCCY (X  H, Y  OH, NH2; X  Y  OH, NH2), and elusive compounds like carbonic acid (H2CO3) or carbamic acid (NH2CO2H). Collision experiments can also be used advantageously for the structural analysis of the neutral molecule N generated in unimolecular dissociation reactions of the type m → m+N. The dissociation of ionized aniline for example yields, besides C5H cations, neutral isocyanic acid, HNC, and not cyanic acid, HCN. The technique of Neutralization Reionization Mass Spectrometry (NRMS) is performed in the diluted gas phase, a medium where intermolecular interactions are entirely absent. The method may therefore increasingly be applied to the study of molecular species whose reactivity precludes their characterization in matrices. Molecules in specific excited states and fragments of organometallic compounds with unsaturated valencies are two examples of systems which could fruitfully be studied.
Di‐Grignard Compounds and Metallacycles
Authors: Bickelhaupt Friedrich 
Published: December 2003

AbstractJust as the by now famous Grignard reaction can be used to prepare organomagnesium halides from organic monohalides, organic dihalides can be used under certain conditions to prepare di‐Grignard compounds. Difficulties are encountered in particular when the two halide functionalities are separated by only a short carbon chain (i. e. by 1‐3 carbon atoms): in such cases side reactions predominate. It has however recently become possible to obtain such “short” di‐Grignard compounds on a preparatively useful scale. This opens up new perspectives for the synthesis of metallacyclic compounds of other main group and transition metal elements and in particular for metallacyclobutanes. The preparation, structure and application of a selected number of di‐Grignard reagents will be treated in the present review.
How Nature Adapts Chemical Structures to Curved Surfaces
Authors: von Schnering Hans Georg , Nesper Reinhard 
Published: December 2003

AbstractGuided by the symmetry in a natural way, periodic potential surfaces partition the space in solid crystalline compounds. The arrangement of atoms, clusters and molecules obviously follows the (in general) curved shape of these ‘space partitioners’. In structures, the atoms therefore choose only a very limited subset of the infinite set of possible positions. In collective structures the periodic surfaces separate areas of different interactions between atoms, clusters, and molecules. In a certain sense, they can be considered as inner surfaces, a knowledge of which reveals insights into the organization of crystalline matter. There are many good indications that the weakly bonded electrons in the highest occupied orbitals are preferably localized in the region of the space partitioners. Dynamic processes as well can be correlated to the shape of the periodic surfaces. Moreover, the surfaces are didactically very helpful in making accessible the complicated three‐dimensional relations in collective structures because the main features are projected onto (although curved) two‐dimensional creations. The application of periodic potential surfaces to such a variety of compounds as quartz, brass and alpha‐amylose underscores their general significance. Simple qualitative considerations already reveal the manifold relations to animate and inanimate nature through to mathematics, art and architecture. It appears that, in a very universal sense, the adaption of structures to a collective order finds a natural solution through curvature.