Showing papers in "Angewandte Chemie in 1986"

The Palladium‐Catalyzed Cross‐Coupling Reactions of Organotin Reagents with Organic Electrophiles [New Synthetic Methods (58)]

Abstract: The 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.

New Methods for the Synthesis of Glycosides and Oligosaccharides—Are There Alternatives to the Koenigs‐Knorr Method? [New Synthetic Methods (56)]

Abstract: Glycoproteins, 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.

Biochemistry of Oxidative Stress

Abstract: As a normal attribute of aerobic life, structural damage to organic compounds of a wide variety (DNA, proteins, carbohydrates and lipids) may occur as a consequence of oxidative reactions. Oxidative damage inflicted by reactive oxygen species has been called “oxidative stress”. Biological systems contain powerful enzymatic and nonenzymatic antioxidant systems, and oxidative stress denotes a shift in the prooxidant/antioxidant balance in favor of the former. Diverse biological processes such as inflammation, carcinogenesis, ageing, radiation damage and photobiological effects appear to involve reactive oxygen species. This field of research provides new perspectives in biochemical pharmacology, toxicology, radiation biochemistry as well as pathophysiology.

Charge Separation in Excited States of Decoupled Systems—TICT Compounds and Implications Regarding the Development of New Laser Dyes and the Primary Process of Vision and Photosynthesis

Abstract: The understanding of the dual fluorescence of certain aromatic systems has greatly advanced in recent years. The accompanying large charge separation has been shown to be linked to a twisted (or small overlap) arrangement of the chromophores. Recent theoretical models are able to describe the excited-state twisting of both single bonds (TICT compounds) and double bonds (olefins) in a unified picture. These models can help to elucidate the photophysical behavior of many organic, inorganic, and biologically relevant compounds, and their application to laser dyes and fluorescent probes provides a route to new “tailor-made” fluorescent materials. Applied to the primary processes of vision and photosynthesis, these models can lead to a deeper understanding of basic photobiological processes.

Palladium-katalysierte Kupplungsreaktionen organischer Elektrophile mit Organozinn-Verbindungen

Abstract: Palladium-katalysierte Kupplungsreaktionen von Organozinn-Verbindungen mit unterschiedlichsten organischen Elektrophilen sind eine relativ neue Methode zur Knupfung von Kohlenstoff-Kohlenstoff-Bindungen. Da sie unter milden Bedingungen ablauft und eine Vielfalt funktioneller Gruppen an jedem der beiden Reaktionspartner ermoglicht, daruber hinaus stereospezifisch, regioselektiv und in hohen Ausbeuten verlauft, eignet sie sich ideal fur die Synthese komplizierter organischer Verbindungen. Wird die Kupplungsreaktion in Gegenwart von Kohlenmonoxid ausgefuhrt, so findet eine Verknupfung der beiden Reaktanten unter Kohlenmonoxid-Insertion statt, so das man Ketone erhalt.

The Concept of Strain in Organic Chemistry

Abstract: “Die Ringschliesung ist offenbar diejenige Erscheinung, welche am meisten uber die raumliche 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 schliesen last, so mussen dafur offenbar raumliche Grunde 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 Grose der Letzteren wachsende Spannung zur Folge hat,”[ ] 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.

Preorganization—From Solvents to Spherands

Abstract: This article assesses the importance of molecular preorganization to the rapidly developing field of complexation involving designed synthetic organic compounds. Since its birth as a science, organic chemistry has drawn heavily on biological chemistry as a vast storehouse of evolutionary structures, reactions, and control mechanisms that serve as inspiration for designed organic-compounds mimics. Biological systems, through highly structured complexation, accomplish complicated tasks. The receptor sites of enzymes, the genes, the antibodies, and ionophores possess high degree of preorganization. In other words, their functional groups act cooperatively as binding or catalytic sites which are largely collected and oriented prior to complexation.—The strength of the organic chemist derives from his ability to design organic compounds, organic reactions, synthetic sequences, and test systems to evaluate hypotheses. The design of highly structured complexs and the discovery of the rules that govern their behavior are described here. Research in this field is particularly rewarding because scientific and aesthetic content merge and become visible in the structures of many of the complexes.

Neue Methoden zur Glycosid- und Oligosaccharidsynthese – gibt es Alternativen zur Koenigs-Knorr-Methode?

Abstract: Glycoproteine, Glycolipide und Glycophospholipide (=Glycokonjugate) sind Bestandteile von Membranen. Dabei ist der oligosaccharidische Teil fur Zell-Zell-Erkennung und -Wechselwirkung zustandig; er fungiert als Rezeptor fur Proteine, Hormone und Viren und determiniert Immunreaktionen. Diese Bedeutung hat das Interesse an Oligosacchariden und Glycokonjugaten stimuliert. Mit ihnen sollte es gelingen, die molekulare Basis dieser Phanomene aufzuklaren und neue physiologische Wirkprinzipien abzuleiten. Grose Fortschritte der Oligosaccharidsynthese sind mit der Koenigs-Knorr-Methode erzielt worden, bei der Halogenosen in Gegenwart von Schwermetallsalzen zur Glycosylubertragung auf Nucleophile dienen. Die Nachteile dieses Verfahrens haben zu einer intensiven Suche nach neuen Methoden gefuhrt. In diesem Beitrag werden solche Methoden diskutiert. Dabei ist insbesondere auf die Glycosid- und Saccharidbildung durch 1-O-Alkylierung, auf die Trichloracetimidat-Methode und auf die Aktivierung uber Glycosylsulfoniumsalze und Glycosylfluoride hinzuweisen.

[3+2] Cycloaddition Approaches to Five-Membered Rings via Trimethylenemethane and Its Equivalents [New Synthetic Methods (55)]

Abstract: The power of cycloadditions for ring formation derives from the potential for chemo-, regio-, diastereo-, and enantioselectivity. Trimethylenemethane (TMM) and its equivalents offer the possibility of extending such benefits to the synthesis of five-membered rings. Three types of conjunctive reagents appear to be promising candidates: (1) selected 4-alky-lidene-4,5-dihydro-3H-pyrazoles, (2) 2-[(trimethylsilyl)methyl]allyl esters and halides, and (3) alkylidenecyclopropanes. Thermal reactions, especially intramolecular ones, effect the cycloaddition of the pyrazoles to olefins–especially electron-poor olefins. The latter two precursors require a catalyst, notably a palladium complex, to unlock the synthetic potential. With the 2-[(trimethylsilyl)methyl]allyl esters, a general chemo-, regio-, and diastereoselective cycloaddition with typical Diels-Alder dienophiles forms methylenecyclopentanes in both inter- and intramolecular fashion. A tin analogue allows extension of this cycloaddition to aldehydes, and, in a related sequence, to ketones and imines to form 3-methylenete-trahydrofurans and 3-methylenepyrrolidines with excellent diastereoselectivity. The 2-[(tri-methylsilyl)methyl]allyl esters also serve to effect net methylenecyclopentane annulation to cyclic enols. Such adducts can further undergo three-carbon intercalation or combined ring contraction-spiroannulation simply by the use of either a nucleophilic or electrophilic trigger. Alkylidenecyclopropanes undergo cycloadditions to both electron-rich and electron-poor olefins when stimulated by metal catalysts, especially palladium. A complementary regiochemistry to the approach based upon silicon conjunctive reagents is observed. The availability of these cycloadditions provides new strategies for the synthesis of natural products.

Resonance Raman Spectroscopy, and Its Application to Inorganic Chemistry. New Analytical Methods (27)

Abstract: Resonance 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.

Selective Synthesis by Use of Lewis Acids in the Presence of Organocopper and Related Reagents. New Synthetic Methods (61)

Abstract: Organometallic compounds with pronounced nucleophilicity such as organocopper and organolithium compounds, and powerful electrophiles, such as BF3 and AlCl3, are generally thought to be incompatible under normal, homogeneous reaction conditions. As a matter of course, it is anticipated that the two species cannot coexist and undergo transmetalation reactions. To our surprise, however, RCu and BF3 are compatible at low temperature. RCu·BF3 and related organocopper-Lewis acid reagents exhibit new and unique reactivities and selectivities: in conjugate additions, e.g., not only activation but also high regio-, stereo-, and chemo-selectivity as well as very high asymmetric induction is observed; allylic derivatives are alkylated regioselectively, and reaction towards carbonyl groups, imines, epoxides, aziridines, ethers, acetals, orthoformates, and pyridinium salts is in each case found to be selective. RLi·BF3 and related organolithium-Lewis acid reagents also exhibit noteworthy selectivities and reactivities. These complex reagents are utilized in the key steps of the total synthesis of many important natural products, such as certain terpenes, steroids, and alkaloids.

Indirect Electroorganic Syntheses—A Modern Chapter of Organic Electrochemistry [New Synthetic Methods (59)]

Abstract: The 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.

Ladungstrennung in angeregten Zuständen entkoppelter Systeme – TICT-Verbindungen und Implikationen für die Entwicklung neuer Laserfarbstoffe sowie für den Primärprozeß von Sehvorgang und Photosynthese

Abstract: Das Verstandnis der dualen Fluoreszenz bestimmter aromatischer Systeme hat in den letzten Jahren grose Fortschritte gemacht. Es wurde gezeigt, das die damit verbundene starke Ladungstrennung mit einer verdrillten Anordnung (oder geringen Uberlappung) der Chromophore verknupft ist. Neuere theoretische Modelle konnen die Verdrillung von Systemen in einem angeregten Zustand sowohl um eine Einfachbindung (TICT-Verbindungen) als auch um eine Doppelbindung (Olefine) in einem einheitlichen Bild beschreiben. Dadurch kann das photophysikalische Verhalten vieler organischer, anorganischer und biologisch relevanter Verbindungen besser verstanden werden, und es wird durch Anwendung dieser Modelle auf Laserfarbstoffe und Fluoreszenzsonden ein Weg zu neuen „masgeschneiderten” Fluoreszenzmaterialien gewiesen. Auf die Primarprozesse des Sehvorgangs und der Photosynthese angewandt, konnen diese Modelle zu einem tieferen Verstandnis grundlegender photobiologischer Prozesse fuhren.

Second‐Sphere Coordination–a Novel Rǒle for Molecular Receptors

Abstract: Although 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 macrocycles par excellence for 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.

Multiple Bonds between Transition Metals and “Bare” Main Group Elements: Links between Inorganic Solid State Chemistry and Organometallic Chemistry

Abstract: The last two decades have seen a dramatic development in the study of metal-metal multiple bonds, particular successes being recorded in the field of organometallic chemistry. Syntheses designed to produce novel transition metal complexes with single, double, triple and quadruple metal-metal bonds occupy a most important place in such research, as also do reactivity studies. A striving to establish general principles has provided much of the motivation for such work, but one less obvious goal—the commercial application of the catalytic properties of metal-metal multiple bonding systems, in the medium and long term—should not be overlooked. All aspects of the investigations of metal-metal multiple bonds also apply to a particular class of compound that has, however, enjoyed little lime-light and thus deserves the present review: complexes with multiple bonds between transition metals and substituent-free (“bare”) main group elements. Although based mostly on accidental discoveries, the few noteworthy examples are now beginning to unfold general concepts of synthesis that are capable of being extended and thus are deserving of exploitation in preparative chemistry. The availability of further structural patterns exhibiting multiple bonds between transition metals and ligand-free main group elements might enable preparative organometallic chemistry to expand in a completely new direction (for instance by the stabilizing or activation of small molecules at the metal complex). This essay discusses the chemistry of complexes of bare carbon, nitrogen, and oxygen ligands (carbido-, nitrido-, and oxo-complexes) and their relationships to higher homologues from both a synthetic and a structural point of view.

High‐Resolution Metal NMR Spectroscopy of Organometallic Compounds [New Analytical Methods (30)]

Abstract: Great 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.

3‐Chloropropyltrialkoxysilanes—Key Intermediates for the Commercial Production of Organofunctionalized Silanes and Polysiloxanes

Abstract: Silicon and carbon may be looked upon as the two fundamental antipodes with respect to the naturally occurring forms of matter and life on earth: Silicon plays the key role in the “petrification” of inanimate matter, while carbon is the representative element for the “incarnation” of living organisms. If a scientist were to search for a highly efficient method of uniting inorganic and organic matter, he would, at a very early stage of attempting to solve this problem, combine these two natural principles of matter and think of “bireactive” molecules containing a silicon functionality for bonding to inorganic material on the one hand and a carbon functionality for anchoring to an organic counterpart on the other. The title compounds and their derivatives epitomize such molecules: they are wanderers between both (chemical) worlds. Owing to their bifunctionality they are capable of binding to inorganic (especially siliceous) systems as well as to organic polymers. Whether their commercial application concerns the strengthening of polyester resins with glass fibers for use in boatbuilding or the incorporation of silica as a filler in rubber mixtures for the manufacture of wear-resistant tires or even the immobilization of enzymes on glass spheres for carrying out reactions in enzyme reactors—in all cases organofunctionalized silanes guarantee a reliable and permanent union between two otherwise “incompatible” material systems.

[m.m]Metallocenophanes: Synthesis, Structure, and Properties

Abstract: Metallocenophanes 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 CHċC 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.

Das Konzept der Spannung in der Organischen Chemie

Abstract: „Die Ringschliesung ist offenbar diejenige Erscheinung, welche am meisten ube dir raumliche 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 schliesen last, so mussen dafur offenbar raumliche Grunde 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 Grose der Letzteren wachsende Spannung zur Folge hat.” Das ist die Quintessenz der vor einem Jahrhundert von Adolf von Baeyer formulierten „Ringspannungstheorie”. Zwar ist sie heute nur noch eine Facette unter den vielen Aspekten der Spannungstheorie, doch hat sie praparative und theoretisch arbeitende Chemiker immer wieder stimuliert. Zu den spektakularsten neueren Erfolgen der Synthetiker gehoren zweifellos die Darstellung von Tetra-tert-butyl-tetrahedran und [1.1.1]Propellan. Die Grunde fur die grose Stabilitat dieser hochgespannten Verbindungen sind vollig verschieden. Die auf dem Weg dorthin und bei der theoretischen Durchdringung der Probleme mit Molekul-Mechanik- und ab-initio-Rechnungen gewonnenen Erkenntnisse haben entscheidend zu unserem heutigen Verstandnis von Struktur, Energie und Reaktivitat organischer Verbindungen beigetragen.