bluuebunny/crossref_metadata_2025_split huggingface dataset:
Patterns in Hydrogen Bonding: Functionality and Graph Set Analysis in Crystals
Authors: Bernstein Joel , Davis Raymond E. , Shimoni Liat , Chang Ning‐Leh 
Published: January 2004

AbstractWhereas much of organic chemistry has classically dealt with the preparation and study of the properties of individual molecules, an increasingly significant portion of the activity in chemical research involves understanding and utilizing the nature of the interactions between molecules. Two representative areas of this evolution are supramolecular chemistry and molecular recognition. The interactions between molecules are governed by intermolecular forces whose energetic and geometric properties are much less well understood than those of classical chemical bonds between atoms. Among the strongest of these interactions, however, are hydrogen bonds, whose directional properties are better understood on the local level (that is, for a single hydrogen bond) than many other types of non‐bonded interactions. Nevertheless, the means by which to characterize, understand, and predict the consequences of many hydrogen bonds among molecules, and the resulting formation of molecular aggregates (on the microscopic scale) or crystals (on the macroscopic scale) has remained largely enigmatic. One of the most promising systematic approaches to resolving this enigma was initially developed by the late M. C. Etter, who applied graph theory to recognize, and then utilize, patterns of hydrogen bonding for the understanding and design of molecular crystals. In working with Etter's original ideas the power and potential utility of this approach on one hand, and on the other, the need to develop and extend the initial Etter formalism was generally recognized. It with that latter purpose that we originally undertook the present review.
Scalar Coupling Constants—Their Analysis and Their Application for the Elucidation of Structures
Authors: Eberstadt Matthias , Gemmecker Gerd , Mierke Dale F. , Kessler Horst 
Published: January 2004

AbstractSince their discovery in the early fifties, scalar/coupling constants have been of great interest to the NMR spectroscopist. Their impact on structure determination by NMR spectroscopy is founded on the fact that the size of the coupling constant is directly related to molecular conformation. Today, for most chemical substances the parameters for the Karplus relationship, which relates the vicinial (3‐bond) coupling constant to the dihedral angle, have been determined. In addition to proton–proton distances, the application of coupling constants in modern conformational analysis is indispensable. In the study of larger molecules which are of current interest, more and more involved experiments are necessary in order to overcome signal overlap and increasing line widths. A large number of experimental techniques for the determination of coupling constants has been developed; however, for this reason the choice of the most appropriate experiment to use has become more difficult. This decision must be made carefully to maximize instrument usage and obtain the largest number of couplings with the greatest accuracy possible. Many of the computer programs used in structure calculations can directly apply coupling constant restraints, similar to proton–proton distances developed from NOEs. Therefore, not only is the quality of the structure improved, but the molecular motions (internal dynamics) are better described. In this article, we review the techniques that exist today with particular attention paid to helping the non‐expert to choose the appropriate experiment for the problem at hand. In addition, the use of coupling constants in computer simulations are discussed.
Thermotropic Liquid Crystals Formed by Intermolecular Hydrogen Bonding Interactions
Authors: Paleos Constantinos M. , Tsiourvas Dimitris 
Published: January 2004

AbstractThe role of hydrogen bonding in the formation or stabilization of liquid crystalline phases has only recently been appreciated. Following the first, wellestablished examples of liquid crystal formation from the dimerization of aromatic carboxylic acids, through hydrogen bonding, several classes of compounds have recently been synthesized, the liquid crystalline behavior of which is also dependent on intermolecular hydrogen bonds between similar or dissimilar molecules. In this review the main classes of compounds exhibiting liquid crystallinity due to hydrogen bonding are presented to show the diversity of organic compounds that can be used as building elements in liquid crystals. The molecules are either of the rigid‐rod anisotropic or amphiphilic types such as molecules appropriately functionalized with pyridyl and carboxyl groups, whose interaction leads to the formation of liquid crystals; amphiphilic carbohydrates and amphiphilic and bolaamphiphilic compounds with multiple hydroxyl groups whose dimerization or association is indispensable for the formation of liquid crystals; and certain amphiphilic carboxylic acids with monomeric or polymeric mesogens and amphiphilic‐type compounds bearing different moieties, whose interaction may lead to the formation of mesomorphic compounds. Associated with the macroscopic display of liquid crystalline phases is the supramolecular structure, and therefore rather extended discussion of these structures are included in this review.
Novel Porphyrinoids for Chemistry and Medicine by Biomimetic Syntheses
Authors: Franck Burchard , Nonn Ansgar 
Published: January 2004

AbstractIn 1926 Hans Fischer and Bruno Walach synthesized the first porphyrins.H. Fischer, B. Walach, Justus Liebigs Ann. Chem. 1926, 450, 164–181.
Currently more than 1400 new articles concerning the synthesis and uses of porphyrins are published every year.CAS Online search for 1994.
However, the strong interest in these compounds indicated by this is in sharp contrast to their restricted availability. This is reflected in the current price of up to 500 DM for 5 mg of the most important porphyrins used in research and other applications (see Scheme 3).Biomimetic syntheses offer possibilities for an improved approach to porphyrins. By following the example set by nature it is also possible to obtain novel porphyrinoids which are different from naturally occurring porphyrins. This is exemplified by N,N′‐bridged porphyrinogens, which have cage structures, inverted porphyrinoids (N atoms in the outer periphery) and, in particular, porphyrins with expanded systems. Among the family of expanded porphyrins are superarenes with up to 34 π electrons. Their pronounced aromaticity is indicated by 1H NMR spectra, bond length equivalence, planar structures, and electrophilic substitution. With their strong absorption bands, the strongest of which have ϵ values of > 1000000, a value which exceeds the absorption intensity of all other organic pigments observed until now, and their ability to act as efficient photosensitizers, the expanded porphyrins open interesting perspectives in the fields of photochemistry and photomedicine.
Chemistry of Inositol Lipid Mediated Cellular Signaling
Authors: Potter Barry V. L. , Lampe Dethard 
Published: January 2004

AbstractIt is now slightly more than a decade since Michael Berridge and his collaborators reported in Nature “…micromolar concentrations of Ins(1,4,5)P3 (1D‐myo‐inositol 1,4,5‐trisphosphate) release Ca2+ from a non‐mitochondrial intracellular Ca2+ store in pancreatic acinar cells. Our results strongly suggest that this is the same Ca2+ store that is released by acetylcholine”. This observation ushered in an a new era in the field of signal transduction with the discovery of a small‐molecule second messenger liniking the spatially separated events of cell surface receptor activation and intracellular Ca2+ mobilization. This event, which has spawned what is now one of the most active fields of current biology, also stimulated a renaissance in inositol and inositol phosphate chemistry. The synthesis of inositol polyphosphates presents a number of problems: the regiospecific protection of inositol and the optical resolution of the resulting precursors, the phosphorylation of the polyol, removal of all phosphate protecting groups without phosphate migration, and finally the purification of the water‐soluble target polyanion. With the solution of these problems over the last few years it is now possible to look beyond the synthesis of naturally occurring inositol polyphosphates, whose number has been steadily increasing, to the design of chemically modified inositol phosphate analogues with the prospect of developing enzyme inhibitors, rationally modified receptor ligands and antagonists, and perhaps, through pharmacological intervention in signal transduction pathways, even the therapeutical agents of the future.
CH and CC Bond Activation by Bare Transition‐Metal Oxide Cations in the Gas Phase
Authors: Schröder Detlef , Schwarz Helmut 
Published: January 2004

AbstractOver the last decade the gas‐phase chemistry of bare transition‐metal oxide cations MO+ has received considerable attention. This interest is primarily due to the particular role of metal oxides in the oxidation of organic compounds in a variety of chemical and biochemical transformations. At a molecular level the simplest model system for these processes deals with reactions of bare metal‐oxide ions in the gas phase. Due to the high oxophilicities of the early transition metals, their monoxide cations MO+ do not mediate O‐atom transfer to any organic compounds at all. In contrast, monoxide cations of late transition metal can oxygenate a variety of hydro‐carbons, and the most reactive ions, MnO+, FeO+, NiO+, OsO+, and PtO+, even activate methane. Insight into the reaction mechanisms of these oxidation processes can be obtained by analysis of reaction kinetics, isotope effects, product distributions etc., and for the reactions of MO+ with alkanes the initial CH bond activation by MO+ is often rate‐determining. Interestingly, the high reactivity of some MO+ ions is not always associated with a decrease in regioselectivity; for example, FeO+ ions induce regiospecific γ‐CH bond activation of dialkylketones in the gas phase. The situation for the epoxidation of olefins in the gas phase turns out to be even more complex than for condensedphase analogues. This is primarily because the metal ion that mediates O‐atom transfer to the olefin also catalyzes the isomerization of the epoxides formed, to afford the energetically more stable aldehydes or ketones. Aromatic compounds can also be hydroxylated by MO+ ions, and particularly the oxidation of benzene by bare FeO+ ions in the gas phase reveals striking parallels to the metabolism of arenes. Furthermore, the storing capabilities of ion cyclotron resonance mass spectrometers even permit the design of catalytic processes in which a single metal ion converts more than one substrate molecule into an oxygenated product in a sequence of strictly bimolecular reactions. The most outstanding examples are the Pt+‐mediated oxidation of methane by molecular oxygen and the Co+‐mediated hydroxylation of benzene by N2O as oxidant. Finally, the key features of the gas‐phase reactions are compared with observations in condensed‐phase systems in which metal oxides are anticipated as central intermediates. The result of this comparison is promising in the sense that, in general, the understanding of transition‐metal‐mediated oxidations in the gas phase may lead to a more uniform description of these processes at a molecular level. Ultimately, it is hoped that gas‐phase studies will serve as one of the building blocks in the evolution of tailor‐made catalysts.
The Conquest of Taxol
Authors: Nicolaou Kyriacos C. , Guy Rodney K. 
Published: January 2004

AbstractMuch has been written about taxol, one of the newest weapons against cancer, and its producer, the Pacific Yew tree (Taxus brevifolia).K. C. Nicolaou, W.‐M. Dai, R. K. Guy, Angew. Chem. 1994, 106, 38–69; Angew. Chem. Int. Ed. Engl. 1994, 33, 15–44.
In this article, the authors give a frank and behind‐the‐scenes account of their encounter with this well‐known molecule, which they and their collaborators faced as a synthetic target.K. C. Nicolaou, Z. Yang, J.‐J. Liu, H. Ueno, P. G. Nantermet, R. K. Guy, C. F. Claiborne, J. Renaud, E. A. Couladouros, K. Paulvannan, E. J. Sorensen, Nature (London) 1994, 367, 630–634.
K. C. Nicolaou, J.‐J. Liu, Z. Yang, H. Ueno, R. K. Guy, E. A. Couladouros, E. J. Sorensen, J. Am. Chem. Soc. 1995, 117, 624–633.
K. C. Nicolaou, J.‐J. Liu, Z. Yang, H. Ueno, E. J. Sorensen, C. F. Claiborne, R. K. Guy, C.‐K. Hwang, M. Nakada, P. G. Nantermet, J. Am. Chem. Soc. 1995, 117, 634–644.
K. C. Nicolaou, Z. Yang, J.‐J. Liu, P. G. Nantermet, C. F. Claiborne, J. Renaud, R. K. Guy, K. Shibayama, J. Am. Chem. Soc. 1995, 117, 645–652.
K. C. Nicolaou, H. Ueno, J.‐J. Liu, P. G. Nantermet, Z. Yang, J. Renaud, K. Paulvannan, R. Chadha, J. Am. Chem. Soc. 1995, 117, 653–659.
Once again total synthesis is found to offer excellent opportunities for developing new synthetic strategies and novel reactions. The team of chemists who took up this challenge emerged with valuable experience and confidence in their skills.
Cytomimetic Organic Chemistry: Early Developments
Authors: Menger Fredric M. , Gabrielson Kurt D. 
Published: January 2004

AbstractThis article describes how chemical and physical stimuli cause a simple system, the giant vesicle, to undergo a variety of “cytomimetic” transformations such as fusion, fission, endocytosis, budding, aggregation, birthing, and foraging. For example, when a giant vesicle, which happens to have a smaller vesicle inside it, is exposed to octyl glucoside, the smaller vesicle can pass through the outer membrane into the external medium (“birthing”). The resulting injury to the membrane of the host vesicle heals immediately. Addition of cholic acid, on the other hand, induces a feeding frenzy in which a vesicle grows rapidly as it consumes its smaller neighbors. After the food is gone, the giant vesicle then self‐destructs (a case of “birth, growth, and death”). Such lifelike morphological changes were obtained by using commercially available chemicals; thus these processes should be assigned to organic chemistry, and not to biology or even biochemistry, and not to biology or even biochemistry. Experimental details (e.g. the preparation and observation of the vesicles) are included in this review in hopes of helping others enter this undeveloped field.
Volatile Metal Alkoxides according to the Concept of Donor Functionalization
Authors: Herrmann Wolfgang A. , Huber Norbert W. , Runte Oliver 
Published: January 2004

AbstractSince the fundamental works of D. C. Bradley and R. C. Mehrotra, metal alkoxides have attracted attention because of the diversity of their low‐ and high‐molecular‐weight structures; they are also generating increasing interest as precursor compounds for solving technical materials problems. The understanding of the hydrolytic nucleation behavior is a prerequisite for the optimization of materials from sol–gel processes. For metal alkoxides to be precursors in chemical vapor deposition (CVD) processes in the preparation of inorganic oxidic materials, they should be sufficiently volatile, and sublimation should occur without decomposition at as low a temperature as possible (< 150 °C). Only recently, using the “donor functionalization” concept, was a ligand type systematically developed that unifies the advantages of both steric demands and σ‐donor stabilization and so stabilizes low‐molecularweight metal alkoxides. Even large metal ions of low charge (for example Ba2+) can thus form volatile alkoxides. O‐ and N‐donor functions in bidentate and multidentate alkoxo ligands are particularly advantageous; hence, for example, the vanadium derivative [V(OCMe2CH2OMe)3] is one of the most volatile metal alkoxides known to date. The first alkoxides of the alkaline earth metals calcium, strontium, and barium, which sublime without decomposition, have the formula [M2{OC(CH2OiPr)2tBu}4]. This article presents a critical inventory of the metal alkoxides with particular regard to the aspect of volatility. It also describes successes of the donor functionalization concept and shows—in prespective—how alkoxo ligands can be “tailor made” for metals according to their charge‐to‐radius ratio by further development of the concept.
Carbon Dioxide as a Raw Material: The Synthesis of Formic Acid and Its Derivatives from CO2
Authors: Leitner Walter 
Published: January 2004

AbstractThe use of carbon dioxide as a raw material for chemical syntheses is an ecologically and economically valuable extension to the carbon sources used at the present time. In order to convert the thermodynamically stable and comparatively unreactive CO2 molecule into the desired product in an efficient manner, suitable reaction conditions and activation mechanisms must be found. The catalytic reduction of CO2 to formic acid and its derivatives has been intensively studied in recent years. A number of new approaches to the synthesis of formic acid from CO2 have reached such a state of knowledge that continuing development may well lead to industrial‐scale operation in the near future. This can to a large extent be attributed to the fruitful interaction between investigative work into reaction mechanisms and the development of new catalytic systems.
Supramolecular Synthons in Crystal Engineering—A New Organic Synthesis
Authors: Desiraju Gautam R. 
Published: January 2004

AbstractA crystal of an organic compound is the ultimate supermolecule, and its assembly, governed by chemical and geometrical factors, from individual molecules is the perfect example of solid‐state molecular recognition. Implicit in the supramolecular description of a crystal structure is the fact that molecules in a crystal are held together by noncovalent interactions. The need for rational approaches towards solid‐state structures of fundamental and practical importance has led to the emergence of crystal engineering, which seeks to understand intermolecular interactions and recognition phenomena in the context of crystal packing. The aim of crystal engineering is to establish reliable connections between molecular and supramolecular structure on the basis of intermolecular interactions. Ideally one would like to identify substructural units in a target supermolecule that can be assembled from logically chosen precursor molecules. Indeed, crystal engineering is a new organic synthesis, and the aim of this article is to show that rather than being only nominally relevant to organic chemistry, this subject is well within the mainstream, being surprisingly similar to traditional organic synthesis in concept. The details vary because one is dealing here with intermolecular interactions rather than with covalent bonds; so this article is divided into two parts. The first is concerned with strategy, highlighting the conceptual relationship between crystal engineering and organic synthesis and introduces the term supramolecular synthon. The second part emphasizes methodology, that is, the chemical and geometrical properties of specific intermolecular interactions.
Supramolecular Inorganic Chemistry: Small Guests in Small and Large Hosts
Authors: Müller Achim , Reuter Hans , Dillinger Stephan 
Published: January 2004

AbstractA key reaction in the biological and material world is the controlled linking of simple (molecular) building blocks, a reaction with which one can create mesoscopic structures, which, for example, contain cavities and display specifically desired properties, but also compounds that exhibit typical solid‐state structures. The best example in this context is the chemistry of host–guest interactions, which spans the entire range from three‐ and two‐dimensional to one‐ and “zero‐dimensional”, discrete host structures. Members of the class of multidimensional compounds have been classified as such for a long time, for example, clathrates and intercalation compounds. Thus far, however, there are no classifications for discrete inorganic host–guest compounds. The first systematic approach can be applied to novel polyoxometalates, a class of compounds which has only recently become known. Molecular recognition; tailor‐made, molecular engineering; control of fragment linkage of spin organization and crystallization; cryptands and coronands as “cages” for cations, anions or anion–cation aggregates as sections of ionic lattices; anions within anions, receptors; host–guest interactions; complementarity, as well as the dialectic terms reduction and emergence are important terms and concepts of supramolecular inorganic chemistry. Of particular importance for future research is the comprehension of the mesoscopic area (molècular assemblies)—that between individual molecules and solids (“substances”)—which acts in the biological world as carrier of function and information and for which interesting material properties are expected. This area is accessible through certain variations of “controlled” self‐organization processes, which can be demonstrated by using examples from the chemistry of polyoxometalates. The comprehension of the laws that rule the linking of simple polyhedra to give complex systems enables one to deal with numerous interdisciplinary areas of research: crystal physics and chemistry, heterogeneous catalysis, bioinorganic chemistry (biomineralization), and materials science. In addition, conservative self‐organization processes, for example template‐directed syntheses, are of importance for natural philosophy in the context of the question about the inherent properties of material systems.
Cyclopentadienyl‐Free Organolanthanide Chemistry
Authors: Edelmann Frank T. 
Published: January 2004

AbstractDuring the past ten years the organometallic chemistry of the lanthanides has witnessed enormous growth. One of the main reasons for this development was the finding that lanthanide metallocenes can exhibit an unusually high catalytic activity. Considerable advances have also been achieved in the chemistry of organolanthanide complexes that are not stabilized by cyclopentadienyl ligands. For example, the successful synthesis of the first unsolvated homoleptic trialkyllanthanides, [Ln{CH(SiMe3)2}3], has recently been reported. Anionic allyl complexes of the lanthanides show promising catalytic properties and cyclooctatetraene complexes of these elements are also under active investigation. This review is intended to provide an overview of this rapidly developing area of research.