bluuebunny/crossref_metadata_2025_split huggingface dataset:
η3‐Allylpalladium Compounds
Authors: Jolly Peter W. 
Published: December 2003

AbstractThe use of transition‐metal complexes as homogeneous catalysts for the production of organic chemicals is of considerable industrial significance. Although palladium complexes have not attained the same importance as, for example, those of rhodium or cobalt, palladium is nonetheless one of the most versatile metals for synthetic organic purposes. An understanding of the role played by the metal in these reactions is essential for their optimal utilization. This necessarily entails a detailed study of the chemistry of the palladium‐carbon bond. In this article we concentrate on η3‐allylpalladium complexes, which are frequently involved as intermediates in the Pd‐catalyzed transformations of dienes. The study of their behavior gives a deeper insight into the individual steps of a catalytic cycle.
Enzymes Hosted in Reverse Micelles in Hydrocarbon Solution
Authors: Luisi Pier Luigi 
Published: December 2003

AbstractReverse micelles are spheroidal aggregates formed by certain surfactants in apolar media. In contrast to normal micelles in water, the polar head groups of the surfactant molecules are directed towards the interior of the aggregate and form a polar core which can solubilize water (the “water pool”); the lipophilic chains are exposed to the solvent. The water of the water pool exhibits properties that (depending on the mole ratio of water to surfactant) differ from those of bulk water. Surprisingly, these reverse micelles are able to solubilize in hydrocarbon solvents hydrophilic molecules, e.g., enzymes and even plasmids, that are much larger than the original water‐pool diameter. These biopolymer‐containing reverse micelles can be viewed as novel microreactors, whose physical properties can be controlled through the water content. Remarkable is the ability of enzyme‐containing micelles to react with water‐insoluble, hydrocarbon‐soluble substrates, as in the example of lipoxygenase with linoleic acid.
Growth and Dissolution of Organic Crystals with “Tailor‐Made” Inhibitors—Implications in Stereochemistry and Materials Science
Authors: Addadi Lia , Berkovitch‐Yellin Ziva , Weissbuch Isabelle , van Mil Jan , Shimon Linda J. W. , Lahav Meir , Leiserowitz Leslie 
Published: December 2003

AbstractWhen crystals of organic compounds are grown in the presence of growth inhibitors, there is a change in crystal morphology. A stereochemical correlation exists between the crystal structure, its modified morphology, and the molecular structure of the inhibitor. This correlation has been successfully exploited for the efficient resolution of conglomerates, the engineering of organic crystals with desired morphologies, the direct and relative assignment of the absolute configurations of chiral molecules and crystals, and for the design of a new model for the spontaneous generation of optical activity. In an analogous way dissolution of organic crystals in the presence of these growth inhibitors induces etch pits at preselected faces. The effect of solvent on crystal growth has been analyzed in some model systems. The experimental results are complemented by atom‐atom potential energy calculations.
Great Fear of Small Amounts of Elements—The Significance of Analytical Chemistry in our Modern Industrialized Community as Exemplified by Trace Element Analysis
Authors: Tölg Günther , Garten Rainer P. H. 
Published: December 2003

AbstractAnalytical chemistry is consolidating an important position within the framework of our modern industrial community; the frontiers of trace (and ultra‐trace) analysis have expanded into new territories, thus demanding a constant change in our mode of thinking in a substance‐related manner in analytical chemistry. An outline of the development of analytical chemistry during this century reveals a period of underdeveloped research and education followed by a current phase of impetuous advancement. However, as a result of increasingly antagonistic sectional convictions in the public mind concerning reservations against, as well as efforts towards, efficient technological progress, this advancement evokes new existential risks for analytical chemistry—viz. either to be used in an uncritical way or to fall into discredit following slogans like ‘high‐performance analytical chemistry is to blame for it all!’ A much more constructive consideration says that risks can be estimated and evaluated solely by means of a highly efficient analytical chemistry, when used with a sense of responsibility. Analysts may help to clarify and to cope with the increasing fear of decreasingly smaller amounts of trace elements—in both adverse groups in our community. Strategies necessary to gain this end are outlined with regard to a methodological as well as a political platform.
Syntheses with Radicals—CC Bond Formation via Organotin and Organomercury Compounds [New Synthetic Methods (52)]
Authors: Giese Bernd 
Published: December 2003

AbstractCC bond formation is one of the most important synthetic steps in the construction of organic molecules. In the last few years it has been increasingly achieved by radical addition to alkenes. In such reactions the adduct radicals have to be trapped by an donor subsequent to the CC bond formation in order to prevent polymerization. This task can be accomplished with organotin and organomercury hydrides, the use of which has led to new synthetic methods. The occurrence of radical chain reactions in which reactions take place between radicals and nonradicals is decisive for the success of the synthesis. In these cases small amounts of radical initiators suffice and numerous functional groups may be used in the CC bond‐forming reactions. The yields and selectivities of these radical reactions are often very high.
Enzymes as Catalysts in Synthetic Organic Chemistry [New Synthetic Methods (53)]
Authors: Whitesides George M. , Wong Chi‐Huey 
Published: December 2003

AbstractEnzymes have great potential as catalysts for use in synthetic organic chemistry. Applications of enzymes in synthesis have so far been limited to a relatively small number of largescale hydrolytic processes used in industry, and to a large number of small‐scale syntheses of materials used in analytical procedures and in research. Changes in the technology for production of enzymes (in part attributable to improved methods from classical microbiology, and in part to the promise of genetic engineering) and for their stabilization and manipulation now make these catalysts practical for wider use in large‐scale synthetic organic chemistry. This paper reviews the status of the rapidly developing field of enzyme‐catalyzed organic synthesis, and outlines both present opportunities and probable future developments in this field.
The Construction of New Proteins and Enzymes‐a Prospect for the Future?
Authors: Mutter Manfred 
Published: December 2003

AbstractOnly a vanishingly small proportion of the almost infinite number of possible proteins occur in nature. Can this remaining potential of structural and functional diversity be used in the construction of new proteins? Is a “second evolution” of proteins and enzymes about to occur? These questions have suddenly become of interest because the recombinant DNA technique allows the synthesis of any given amino acid sequence. Examples of enzyme models demonstrate clearly that the unusual catalytic properties of enzymes are associated with the presence of a specifically folded polypeptide chain which has a complex three‐dimensional form. The critical hurdle in the path of artificial proteins is thus the design of amino acid sequences which are able to fold into tertiary structures. — Recent studies on the topology and the mechanism of folding have provided considerable insight into the occurrence of, and the rules governing the three‐dimensional architecture of proteins. Secondary structures apparently play a key role in the folding process; helices and “β‐structures” act as nucleation centers directing folding and account for the surprisingly small number of different folding topologies. The problem of secondary structure formation can be investigated directly by means of conformational studies on model peptides. Oligopeptides with tailormade physicochemical, structural and conformational properties can already be designed. The theoretical and experimental basis for the construction of polypeptides with stable tertiary structures is therefore established. The path to macromolecules with an immense variety of novel properties lays before us.
Resonance Raman Spectroscopy, and Its Application to Inorganic Chemistry. New Analytical Methods (27)
Authors: Clark Robin J. H. , Dines Trevor J. 
Published: December 2003

AbstractResonance Raman spectra are obtained when the wave number of the exciting radiation is close to, or coincident with, that of an electronic transition of the scattering species. Such spectra are usually characterized by a very large enhancement of the intensities of particular Raman bands, sometimes with the appearance of intense overtone and combination tone progressions. The technique provides detailed information about excited electronic states because it is only the vibrational modes associated with the chromophore that are resonance‐Raman active. Additionally, the high sensitivity is such that compounds at concentrations as low as 10−6 mol/L may be detected, enabling resonance Raman spectroscopy to be used as an analytical tool and for the study of chromophores in molecules of biological interest.
New Methods for the Synthesis of Glycosides and Oligosaccharides—Are There Alternatives to the Koenigs‐Knorr Method? [New Synthetic Methods (56)]
Authors: Schmidt Richard R. 
Published: December 2003

AbstractGlycoproteins, glycolipids, and glycophospholipids (glycoconjugates) are components of membranes. The oligosaccharide residue is responsible for intercellular recognition and interaction; it acts as a receptor for proteins, hormones, and viruses and governs immune reactions. These significant activities have stimulated interest in oligosaccharides and glycoconjugates. With their help it should be possible to clarify the molecular basis of these phenomena and to derive new principles of physiological activity. Major advances in the synthesis of oligosaccharides have been made by the use of the Koenigs‐Knorr method, in which glycosyl halides in the presence of heavy‐metal salts are employed to transfer the glycosyl group to nucleophiles. The disadvantages of this procedure have led to an intensive search for new methods. Such methods will be discussed in this article. Emphasis is placed on glycoside and saccharide formation by 1‐O‐alkylation, on the trichloroacetimidate method, and on activation through the formation of glycosylsulfonium salts and glycosyl fluorides.
The Concept of Strain in Organic Chemistry
Authors: Wiberg Kenneth B. 
Published: December 2003

Abstract“Die Ringschließung ist offenbar diejenige Erscheinung, welche am meisten über die räumliche Anordnung der Atome Auskunft geben kann. Wenn eine Kette von 5 und 6 Gliedern sich leicht, eine von weniger oder mehr Gliedern sich schwierig oder auch gar nicht schließen läßt, so müssen dafür offenbar räumliche Gründe vorhanden sein.… Die vier Valenzen des Kohlenstoffatoms wirken in den Richtungen, welche den Mittelpunkt der Kugel mit den Tetraederecken verbinden, und welche miteinander einen Winkel von 109°28′ machen. Die Richtung der Anziehung kann eine Ablenkung erfahren, die jedoch eine mit der Größe der Letzteren wachsende Spannung zur Folge hat,”[Ring formation is evidently the reaction that can provide the most important information on the spatial arrangement of atoms. Steric reasons should explain the observation that 5‐ or 6‐membered chain undergoes ready closure, whereas a shorter or longer chain undergoes difficult or no closure.… The four valences of the carbon atom act in the directions that connect the center of a sphere with the corners of a tetrahedron and form angles of 109°28′ with one another. The direction of the attraction can deviate, but this in increasing strain as the sizes of the latter increase.
] This is the quintessence of the “ring‐strain theory” formulated by Adolf von Baeyer over one hundred years ago. Although it is today only one facet of the many aspects of strain theory, it has repeatedly stimulated experimental and theoretical chemists. Among the most spectacular of the recent successes in synthetic chemistry are the syntheses of tetra‐tert‐butyltetrahedrane and [1.1.1]propellane. The reasons for the great stability of these two highly strained compounds are completely different. The experimental findings as well as the results of theoretical analysis by means of molecular mechanics and ab initio calculations have contributed decisively to our present state of knowledge of the structure, energy, and reactivity of organic compounds.
Organic Synthesis at High Temperatures. Gas‐Phase Flow Thermolysis [New Synthetic Methods (57)]
Authors: Karpf Martin 
Published: December 2003

AbstractThe preference for carrying out synthetic organic reactions at the lowest possible temperature is due to the expectation that the selectivity often increases with decreasing temperature, as is confirmed by many examples and also theoretically justified. Selectivity, however, is not the only problem at high temperatures; further factors include the frequently limited thermal stability of the functional groups and structural elements not directly involved in the transformation. In spite of these limitations, the advantages of high temperatures and the greatly improved knowledge of the mechanisms of dynamic gas‐phase processes accumulated in recent years can be exploited in directed organic synthesis. In this review the synthetic potential of gas‐phase flow thermolysis will be described from the viewpoint of the synthetic chemist with the aid of typical examples of application.
The Pentagonal Column and the ReO3‐Type Structure
Authors: Marinder Bengt‐Olov 
Published: December 2003

AbstractThe pentagonal column (PC), consisting of MX7 bipyramids sharing their equatorial edges with five MX6 octahedra, is a structural feature frequently occurring in association with frameworks of the ReO3‐type, where it enables accomodation of an anion deficiency. The present article discusses the geometry of the PC and its compatibility with the ReO3‐type structure, which naturally emerges from the finding that a PC may result from twinning of an ReO3‐type structure on the unit cell level. A new symbolism is presented which facilitates visualization of a large variety of crystal and defect structures of MX3–x stoichiometry. This symbolism may also serve as a convenient aid in deriving models of possible new structures that may be of value in the interpretation of HREM images.
The Palladium‐Catalyzed Cross‐Coupling Reactions of Organotin Reagents with Organic Electrophiles [New Synthetic Methods (58)]
Authors: Stille John K. 
Published: December 2003

AbstractThe cross‐coupling of organotin reagents with a variety of organic electrophiles, catalyzed by palladium, provides a novel method for generating a carbon‐carbon bond. Because this mild, versatile reaction is tolerant of a wide variety of functional groups on either coupling partner, is stereospecific and regioselective, and gives high yields of product, it is ideal for use in the synthesis of elaborate organic molecules. When the coupling reaction is carried out in the presence of carbon monoxide, instead of a direct coupling, carbon monoxide insertion takes place, stitching the two coupling partners together and generating a ketone.
Understanding Reaction Dynamics in Coordination Chemistry—Application of High Pressure Techniques
Authors: van Eldik Rudi 
Published: December 2003

AbstractIn order to understand the dynamics of chemical reactions in general, detailed information on electronic, structural and kinetic properties is required. The key questions on how chemical reactions actually occur can in many cases only be answered in terms of information obtained from kinetic studies. In conventional kinetic studies of chemical reactions in solution, the variables usually selected include concentration, acidity, solvent, and temperature. In recent years, pressure has become an additional selected variable in such studies. It enables the measurement of the volume of activation and the construction of reaction volume profiles and thus assists in the elucidation of the underlying mechanism; it also completes the comprehension of reaction kinetics by adding another kinetic parameter that the suggested reaction mechanism must account for. Furthermore, the volume of activation is the only transition state property that can be correlated with the corresponding ground state property in an experimentally simple manner. In this paper, the insights so gained in our understanding of the dynamics of reactions involving coordination complexes will be presented. Such reactions are of fundamental interest to chemists since they often form the basis of catalytic, biological, environmental and energy related processes. Any additional information that will add to the understanding of the reaction dynamics is therefore of exceptional importance.
[m.m]Metallocenophanes: Synthesis, Structure, and Properties
Authors: Mueller‐Westerhoff Ulrich T. 
Published: December 2003

AbstractMetallocenophanes are metallocenes in which the cyclopentadienyl ligands are linked intramolecularly through CmHn or CmHnX (X = heteroatom) chains. Two main classes of metallocenophanes are known: in [m]metallocenophanes the ligands of one metallocene are connected by a bridge (or by several bridges); in [m.n]metallocenophanes two (or more) metallocenes are brought together into one molecule by bridging groups. This article briefly describes the first class (mononuclear complexes) and then concentrates on the second (binuclear complexes). The [1.1]metallocenophanes, which are produced by linking two metallocene moieties through two C1, bridges, are of particular interest and are characterized by a variety of unusual and useful properties: they are very flexible conformationally; hydride abstraction from the CH2 bridges produces carbenium ions that are greatly stabilized by the adjacent metallocene moieties; deprotonation of one of the CH2 bridges is surprisingly easy and leads to the unusual situation that an intramolecular CHċC hydrogen bond stabilizes a carbanion; protonation of both metallocene moieties causes the liberation of one mole of hydrogen per mole of [1.1]metallocenophane, thereby allowing the use of these compounds as catalysts for the formation of hydrogen in acidic aqueous solutions.
Geometric and Topological Thinking in Organic Chemistry
Authors: Turro Nicholas J. 
Published: December 2003

AbstractThe beginning student of organic chemistry is often bewildered by what appears to be an enormous maze of random structural variations and reactions that can be mastered only by tedious memorization. To the organic chemist, however, the same subject is often a beautifully ordered discipline of elegant simplicity. An important value of learning organic chemistry is the mastering of “organic thinking,” an approach to intellectual processing whereby the “sameness” of many families of structures and reactions is revealed. This article offers the author's personal views of organic thinking and explores the intellectual and scientific foundations of organic chemistry and of the powerful methods that provide the field with a platform for making rapid conceptual and experimental advances. It is proposed that these methods involve a geometric and topological approach to scientific reasoning within the framework of scientific paradigms that guide experimental design and execution. The basis of this approach is considered in relation to day‐to‐day thinking, problem solving, and the psychological drive for intellectual closure. The power of the approach is illustrated by the analysis of several photochemical and chemiluminescent reactions.
Totally Synthetic Routes to the Higher Monosaccharides
Authors: Danishefsky Samuel J. , DeNinno Michael P. 
Published: December 2003

AbstractThe evolution of a strategy to synthesize the title compounds is described. Three principal developments allowed realization of this goal: (1) the attainment of high margins of diastereofacial selectivity and regioselectivity in the construction of pyranoid systems via the Lewis acid‐catalyzed cyclocondensation reaction of activated dienes and aldehydes; (2) the exploitation of stereoselective reactions for functionalization of the pyranoid matrix; and (3) the discovery of stereoselective reactions for extending the chiral biases of pyranoid systems to newly emerging stereogenic centers on side chains. The coordination of these components in the synthesis of target systems of high biological interest is described.
Control of the Electronic Structure of Organic Conductors as Exemplified by [NMP][TCNQ] Charge‐Transfer Complexes
Authors: Miller Joel S. , Epstein Arthur J. 
Published: December 2003

AbstractThrough replacement of cations with neutral molecules of similar shape and polarizability in a highly conducting “metal‐like” change‐transfer organic conductor, the deliberate control of the electronic structure from a quarter to a half‐filled band is possible. This goal has been achieved with the N‐methylphenazinium (NMP) salt of the tetracyanoquinodimethane (TCNQ) anion by partial replacement of the cation with phenazine. A detailed study of the optical, electrical, and magnetic properties of these conducting molecular solids has lead to the evolution of a broad understanding of the physics of one‐dimensional organic conductors and a reinterpretation of the mechanism of electron transport in such solids. Phenomena such as switching from a low coulomb repulsion two‐chain conductor to a high coulomb repulsion one‐chain conductor as well as soliton‐assisted electron transport are observed.
Synthesis of Glycopeptides, Partial Structures of Biological Recognition Components [New Synthetic Methods (67)]
Authors: Kunz Horst 
Published: December 2003

AbstractGlycopeptides are partial structures of the connecting regions of glycoproteins and, like these, always contain glycosidic bonds between the carbohydrate and peptide parts. Glycoproteins are not only widely distributed but are also decisive factors in post‐translational biological selectivity, especially in biological recognition. Targeted syntheses of glycopeptides require stereoselective formation of the glycosidic bonds between the carbohydrate and the peptide parts and protective group methods that enable selective deblocking of only one functional group in these polyfunctional molecules. These heavy demands have been met by the well‐established use of benzylic protective groups, which can be removed by hydrogenolysis, combined with the use of base‐labile 2‐phosphonioethoxycarbonyl (Peoc) or 9‐fluorenylmethoxycarbonyl (Fmoc) protective groups or of bromoethyl esters, which can be removed under neutral conditions. The acidolysis of tert‐butyloxycarbonyl (Boc) groups and of tert‐butyl esters has also been successfully used, although, under acidic conditions, anomerization or rupture of the glycosidic bonds may occur, especially when nucleophiles are present. The stable, two‐stage 2‐(pyridyl)ethoxycarbonyl (Pyoc) protective groups allow a more reliable synthesis of complex glycopeptides since they can be removed, after modifications, under mild conditions. Particularly suitable for the synthesis of sensitive glycopeptides are the stable allyl protective groups. They can be removed from the complex glycopeptides in a highly selective and effective manner by means of noble‐metal catalysts under practically neutral conditions. These methods have been employed to synthesize glycopeptides corresponding to partial structures of interesting glycoproteins. Deprotected glyopeptides representing tumor‐associated antigen structures can be coupled to bovine serum albumin, which serves as a biological carrier molecule, without the necessity of using an artificial coupling component (spacer).