bluuebunny/crossref_metadata_2025_split huggingface dataset:
SiH and CH Activation by Transition Metal Complexes: A Step Towards Isolable Alkane Complexes?
Authors: Schneider Jörg J. 
Published: January 2004

AbstractThe recognition of the fundamental contributions by G. A. Olah on the elucidation of the structure of nonclassical carbocations, in the form of the award of the Nobel prize for chemistry, has recently emphasized the importance of electron‐deficient bonds in the understanding of chemical bonding in organic chemistry. In the field of coordination chemistry, the formulation of electron‐deficient bonds has been used for some time to describe nonclassical interactions between atoms. Traditional ligands in coordination chemistry such as amines and phosphanes bond to metal centers through their lone pair of electrons. Synergistic bonding effects dominate in the coordination of π‐bonded ligands such as alkenes. In the mid‐1980s the discovery of dihydrogen complexes having side‐on coordination of H2 gave fresh impetus to transition metal chemistry as well as to the understanding of the interaction of σ‐coordinating ligands with transition metals. In the meantime, transiton metal complexes can be obtained with a variety of σ‐coordinated X‐H fragments, and their mode of bonding can be understood by a common and quite general model. The chemistry of σ‐bound silane ligands is particularly varied and well‐investigated. These silane ligands enable the investigation of a large range of σ‐coordinated metal complex fragments up to complete oxidative addition with cleavage of the SiH bond and formation of silyl(hydrido) complexes, which has thus also widened our general understanding of the bonding of other σ‐bound ligands. Whilst there is a large range of isolable and stable H2 and SiR4 complexes available, there are no such alkane analogues known at present. Only when the CH bond is part of a ligand that is already directly bonded to the transition metal center will the resulting chelate effect stabilize this agostic C‐H‐M interaction. The complexation of SiH4, the simplest heavier homologue of CH4, was achieved recently. This is a further step towards the understanding of the factors which govern σ‐complexation of ligands at transition metal centers.
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

AbstractThe 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

AbstractA 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.
Recent Developments of Molecular Chemistry for Sol–Gel Processes
Authors: Corriu Robert J. P. , Leclercq Dominique 
Published: January 2004

AbstractThis 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

AbstractWe 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.
Polymers and the Periodic Table: Recent Developments in Inorganic Polymer Science
Authors: Manners Ian 
Published: January 2004

AbstractPolysiloxanes [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

AbstractNew 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

AbstractFor corrigendum see DOI:10.1002/anie.199619951
Saccharide Sensing with Molecular Receptors Based on Boronic Acid
Authors: James Tony D. , Sandanayake K. R. A. Samankumara , Shinkai Seiji 
Published: January 2004

AbstractThe 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.
Protecting Group Strategies in Organic Synthesis
Authors: Schelhaas Michael , Waldmann Herbert 
Published: January 2004

AbstractThe 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.
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

AbstractFor corrigendum see DOI:10.1002/anie.199721271
Combinatorial Synthesis of Small Organic Molecules
Authors: Balkenhohl Friedhelm , von dem Bussche‐Hünnefeld Christoph , Lansky Annegret , Zechel Christian 
Published: January 2004

AbstractCombinatorial 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.
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

AbstractThis 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

AbstractAmong 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.
Enantioselective Protonation of Enolates and Enols
Authors: Fehr Charles 
Published: January 2004

AbstractEnantioselective 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

AbstractIn 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.
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

AbstractThe 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.