bluuebunny/crossref_metadata_2025_split huggingface dataset:
Palladium‐Catalyzed Reactions of Propargylic Compounds in Organic Synthesis
Authors: Tsuji Jiro , Mandai Tadakatsu 
Published: January 2004

AbstractPalladium was used in organic synthesis for long time only as a heterogeneous catalyst for hydrogenating unsaturated compounds. But in recent years palladium compounds have found broad application as homogeneous catalysts. In the last decade, homogeneous catalysis relying on transition metal complexes has led to innovations in organic synthesis. Complexes of various transition metals are now used for organic synthesis, and each metal has its own characteristic features. Palladium complexes are the most versatile and used extensively in organic synthesis, particularly for carbon–carbon bond formation. Palladium‐catalyzed reactions can be classified into several groups based on their substrates: organic halides, allylic compounds, conjugated dienes, alkenes, and alkynes. Propargylic compounds undergo a variety of palladium‐catalyzed transformations and make up one important class of these reactions. It is well known that copper and silver salts promote or catalyze several reactions of propargylic compounds. However, palladium compounds, particularly Pd0 complexes, show catalytic activities entirely different from those of silver and copper salts. Advances in the research on the palladium‐catalyzed reactions of propargylic compounds in the last decade have made the scope of the reactions clear. The ready availability of numerous propargylic alcohols and their esters by the reaction of terminal alkynes with carbonyl compounds clearly enhances the synthetic utility of their reactions. Palladium complexes, in particular palladium phosphane complexes, are soluble in organic solvents and behave as active catalysts under homogeneous conditions.
Computer‐Assisted Planning of Organic Syntheses: The Second Generation of Programs
Authors: Ihlenfeldt Wolf‐Dietrich , Gasteiger Johann 
Published: January 2004

AbstractThe planning of syntheses in organic chemistry has continuously been given more solid foundations during the last decades. Widely applicable rules have been formulated. Nearly parallel with the systematization of this field the potential for the automation of synthesis planning by the use of computers has been promulgated. However, after more than two decades of continuing efforts by a number of groups, computer‐assisted synthesis planning, which relies on large libraries of synthons and transforms, has failed to establish itself firmly. This is in marked contrast to the acceptance of reaction databases. Their use has become routine with a surprisingly short time. Apparently the classical approaches to computer‐assisted synthesis planning do not satisfy the needs of the preparative chemist. However, this lack of success does not yet mean the complete demise of the whole field. The conceptual shortcomings and problems of the first generation of programs are both of technical and psychological nature and need to be critically analyzed. Meanwhile, work has begun on systems of the second generation, which try to support the chemist in the synthesis laboratory with new methods in novel ways. Care is taken to mimic and support the typical planning style of the human chemist, who often on the spur of the moment switches direction in planning as well as his (sub) goals and methods. New tools that correspond better to the thinking patterns and working habits of chemists empower the user to plan syntheses of organic molecules in an interactive and innovative fashion. We feel that this field is far from dead and continue our research activities. In this article we introduce a system with a number of novel concepts and demonstrate its capabilities with some practical examples.
Organic Chemistry on Solid Supports
Authors: Früchtel Jörg S. , Jung Günther 
Published: January 2004

AbstractUntil recently, repetitive solid‐phase synthesis procedures were used predominantly for the preparation of oligomers such as peptides, oligosaccharides, peptoids, oligocarbamates, peptide vinylogues, oligomers of pyrrolin‐4‐one, peptide phosphates, and peptide nucleic acids. However, the oligomers thus produced have a limited range of possible backbone structures due to the restricted number of building blocks and synthetic techniques available. Biologically active compounds of this type are generally not suitable as therapeutic agents but can serve as lead structures for optimization. “Combinatorial organic synthesis” has been developed with the aim of obtaining low molecular weight compounds by pathways other than those of oligomer synthesis. This concept was first described in 1971 by Ugi.[56f,g,59c] Combinatorial synthesis offers new strategies for preparing diverse molecules, which can then be screened to provide lead structures. Combinatorial chemistry is compatible with both solution‐phase and solid‐phase synthesis. Moreover, this approach is conducive to automation, as proven by recent successes in the synthesis of peptide libraries. These developments have led to a renaissance in solid‐phase organic synthesis (SPOS), which has been in use since the 1970s. Fully automated combinatorial chemistry relies not only on the testing and optimization of known chemical reactions on solid supports, but also on the development of highly efficient techniques for simultaneous multiple syntheses. Almost all of the standard reactions in organic chemistry can be carried out using suitable supports, anchors, and protecting groups with all the advantages of solid‐phase synthesis, which until now have been exploited only sporadically by synthetic organic chemists. Among the reported organic reactions developed on solid supports are Diels–Alder reactions, 1,3‐dipolar cycloadditions, Wittig and Wittig–Horner reactions, Michael additions, oxidations, reductions, and Pd‐catalyzed CC bond formation. In this article we present a comprehensive review of the previously published solid‐phase syntheses of nonpeptidic organic compounds.
Copper—A “Modern” Bioelement
Authors: Kaim Wolfgang , Rall Jochen 
Published: January 2004

AbstractCopper is a bioessential element in biology with truly unique chemical characteristics in its two relevant oxidation states +I and +II. Significant progress has been made in recent years in the elucidation of the frequently surprising biochemistry of this trace element. Those advances were especially furthered through mutual stimulation involving results from biochemistry, molecular biology, and medicine on one hand and the synthesis as well as the structural and spectroscopic characterization of low molecular weight model complexes on the other. The most notable features of protein‐bound active copper are its almost exclusive function in the metabolism of O2 or N/O compounds (NO, N2O) and its frequent association with oxidizing organic and inorganic radicals such as tyrosyl, semiquinones, superoxide, or nitrosyl. This unique biological role of copper can be rationalized given its chemical and assumed evolutionary background.
Single‐Chain Polyprenyl Phosphates Form “Primitive” Membranes
Authors: Pozzi Gianluca , Birault Véronique , Werner Birgit , Dannenmuller Olivier , Nakatani Yoichi , Ourisson Guy , Terakawa Susumu 
Published: January 2004

AbstractFor corrigendum see DOI:10.1002/anie.199607811
Metallophosphaalkenes—from Exotics to Versatile Building Blocks in Preparative Chemistry
Authors: Weber Lothar 
Published: January 2004

AbstractThe chemistry of low‐valent organophosphorus compounds such as phosphaalkenes and phosphaalkynes has undergone rapid development in the last two decades. This development also includes the coordination chemistry of these species, which can act as versatile ligands in metal complexes. Metallophosphaalkenes are compounds in which one or more of the organic substituents on the PC unit is replaced by a transition metal complex fragment. Metallophosphaalkenes have emerged from an existence as laboratory curiosities to become a link between main group and organometallic complex chemistry. The great richness of their chemistry not only mirrors the specific properties of the individual building block, but also shows novel and individual traits. Particular examples are cycloadditions of these electron‐rich heteroalkenes with electron‐deficient alkenes, alkynes, azo, and diazo compounds. These often lead to novel types of reaction and compounds. Metallophosphaalkenes are also important as intermediates in all metal‐assisted cyclooligomerizations of phosphaalkynes.
Control of the Structure and Functions of Biomaterials by Light
Authors: Willner Itamar , Rubin Shai 
Published: January 2004

AbstractVision and other light‐triggered biochemical transformations in plants and living organisms represent a sophisticated biological processes in which optical signals are recorded and transduced as (physico)chemical events. Photoswitchable biomaterials are a new class of substances in which optical signals generate discrete “On” and “Off” states of biological functions, resembling logic gates that flip between 0 and 1 states in response to the changes in electric currents in computers. The (photo)chemistry of photochromic materials has been extensively developed in the past four decades. These materials isomerize reversibly upon light absorption, and the discrete photoisomeric states exhibit distinct spectral and chemical features. Integration of photoisomerizable (or photochromic) units into biomaterials allow their secondary functions such as biocatalysis, binding, and electron transfer to be tailored so that they can be switched on or off. This can be accomplished by chemical modification of the biomaterial by photoisomerizable units and by integration of biomaterials in photoisomerizable microenvironments such as monolayers or polymers. The photoswitchable properties of chemically modified biomaterials originate from the light‐induced generation or perturbation of the biologically active site, whereas in photoisomerizable matrices they depend upon the regulation of the physical or chemical features of the photoisomerizable assemblies of polymers, monolayers, or membranes. Light‐triggered activation of catalytic biomaterials provides a means of amplifying the recorded optical signal by biochemical transformations, and photostimulated biochemical redox switches allow its electrochemical transduction and amplification. The field of photoswitches based on biomaterials has developed extensively in the past few years within the general context of molecular switching devices and micromachinery. The extensive knowledge on the manipulation of biomaterials through genetic engineering and the fabrication of surfaces modified by biologically active materials enables us to prepare biomaterials with improved optical‐switching features. Their application in optoelectronic or bioelectronic devices has been transformed from fantasy to reality. The use of photoswitchable biomaterials in information storage and processing devices (biocomputers), sensors, reversible immunosensors, and biological amplifiers of optical signals has already been demonstrated, but still leaves important future challenges.
The Schenck Ene Reaction: Diastereoselective Oxyfunctionalization with Singlet Oxygen in Synthetic Applications
Authors: Prein Michael , Adam Waldemar 
Published: January 2004

AbstractOxyfunctionalized molecules are principal building blocks in organic synthesis. In cellular processes highly efficient enzymes serve as selective catalysts for the formation of such synthetic units, for example the oxygenases oxyfunctionalize substrates by activating molecular oxygen. To date no comparable effective chemical oxidation system has been found. A useful photochemical process is the oxyfunctionalization of allylic substrates by sensitized photooxygenation, for which molecular oxygen and light serve as natural sources. This allylic oxidation of olefins by the ene reaction with singlet oxygen (Schenck reaction) figures as a highly versatile synthetic method. While the regioselectivity of this transformation has been studied for decades, only during the last years has attention focused on stereocontrol. Through these recent efforts it has become possible to control high stereoselectivity in the photooxygenation of organic substrates. This breakthrough has enhanced substantially the utility of singlet oxygen in diastereoselective synthesis.
Friedrich Hund and Chemistry
Authors: Kutzelnigg Werner 
Published: January 2004

AbstractFriedrich Hund, one of the few physicists still alive of the generation that pioneered quantum mechanics, celebrates his 100th birthday this year. This is an occasion to recall how much chemistry owes to him. The important contributions of Hund to theoretical chemistry will not be presented in a primarily historical context—the cooperation of Hund and Mulliken during the development of MO theory will, however, be duly discussed—but the stress will be on the impact of the concepts introduced by Hund on theoretical chemistry today. Nearly all methods of quantum chemical calculations presently in use have a first step based on Hund's MO theory. Hund recognized that use of delocalized molecular orbitals does not necesarily conflict with a description of a molecule in terms of localized bonds. Moreover, he formulated conditions for localizability that are still pertinent today. The classification of the spectra of the spectra diatomic molecules by Hund in 1925 was virtually definitive. Recently Hund's rules, which, among other things, make predictions about the energetic ordering of states with different spin multiplicities within the same electronic configuration, have aroused renewed interest. The concept of atomic states of natural and unnatural parity plays a key role in this context. Hund's first tow rules for atomic states have been found to be special cases of more general rules. Molecules can violate Hund's first rule though spin polarization and though the preference of different equilibrium geometries of the singlet and triplet states.
Understanding and Manipulating Inorganic Materials with Scanning Probe Microscopes
Authors: Lieber Charles M. , Liu Jie , Sheehan Paul E. 
Published: January 2004

AbstractScanning probe microscopies, such as scanning tunneling microscopy and atomic force microscopy, are uniquely powerful tools for probing the microscopic properties of surfaces. If these microscopies are used to study low‐dimensional materials, from two‐dimensional solids such as graphite to zero‐dimensional nanostructures, it is possible to elucidate atomic‐scale structural and electronic properties characteristic of the bulk of a material and not simply the surface. By combining such measurements with chemical synthesis or direct manipulation it is further possible to elucidate relationships between composition, structure, and physical properties, thus promoting an understanding of the chemical basis of material properties. This article illustrates that the combination of scanning probe microscopies and chemical synthesis has advanced our understanding of charge density waves, high‐temperature superconductivity, and nanofabrication in low‐dimensional materials. This new approach to studying materials has directly contributed to our knowledge of how metal dopants interact with charge density waves and elucidated the local crystal chemistry of complex copper oxides, microscopic details of the superconducting states in materials with a high superconducting transition Ic, and new approaches to the fabrication of multi‐component nanostructures. Coupling scanning probe microscopy measurement and manipulation with chemical synthesis should provide an approach to understanding material properties and creating complex nanostructures in general.
Enzyme Mechanisms, Models, and Mimics
Authors: Kirby Anthony J. 
Published: January 2004

Abstract“One of the great intellectual challenges presented to Science by Nature is a proper understanding of how enzymes work. At one level we can ‘explain’ enzyme catalysis—what an enzyme does is bind, and thus stabilize, selectively the transition state for a particular reaction.A. R. Fersht, Enzyme Structure and Mechanism, 2nd ed., Freeman, New York, 1985.
But our current level of understanding fails the more severe, practical test—that of designing and making artificial enzyme systems with catalytic efficiencies which rival those of natural enzymes.” This was the introduction to a recent Highlight in Angewandte Chemie, A. J. Kirby, Angew. Chem. 1994, 106, 573–576; Angew. Chem. Int. Ed. Engl. 1994, 33, 551–553.
prompted by the appearance of a paper that appeared to defy established ideas, by claiming artificial enzyme systems that did indeed attain catalytic efficiency rivaling that of natural enzymes. The “Pepzymes” of Atassi and ManshouriM. Z. Atassi, T. Manshouri, Proc. Natl. Acad. Sci. USA 1993, 90, 8282–8286.
were relatively small (29‐residue) peptides modeled on the active site structures of trypsin and chymotrypsin by “surface simulation.” One was claimed to hydrolyze the simple trypsin “substrate” N‐tosyl‐L‐arginine methyl ester with kcat and Km values comparable to those of the native enzyme, and also to hydrolyze the peptide bonds of test proteins to give comparable peptide profiles. This extraordinary result provoked as much skepticism as excitement, and several groups tried to reproduce the results. They failed, comprehensively., D. R. Corey, M. A. Phillips, Proc. Natl. Acad. Sci. USA 1994, 91, 4106–4109.
Some specific reasons why this failure came as no surprise have been summarized by Matthews et al.J. A. Wells, W. J. Fairbrother, J. Otlewski, M. Lagowski, J. Burnier, Proc. Natl. Acad. Sci. USA 1994, 91, 4110–4114.
This review examines the problems involved in the design of enzyme mimics in more general terms, with the emphasis specifically on the efficiency of catalysis.
The Mechanism of the Claisen Rearrangement: Déjà Vu All Over Again
Authors: Ganem Bruce 
Published: January 2004

AbstractThe great baseball player Yogi Berra, who is quoted in the title, succinctly described what it meant to learn something that others had long ago appreciated. The mechanism by which the enzyme chorismate mutase accelerates the Claisen rearrangement of chorismic acid has for many years perplexed and intrigued chemists and biochemists. Yet classical effects of solvents and catalysts, which were largely ignored by the chemical community, not only provided important clues about the mechanism of the enzyme, but put the Claisen rearrangement in a new perspective. Whereas chorismic acid once seemed exceptional because it rearranged so rapidly under physiological conditions, it is now clear that even allyl vinyl ether rearranges at room temperature in aqueous solution. While the ability of the enzyme to accelerate the rearrangement of chorismate by a factor of 106 was once considered extraordinary, catalysis by trivalent aluminum compounds matches, and in some cases exceeds, those rate enhancements. Therefore, it really should not be surprising to learn that Nature has independently evolved several structurally distinct solutions to the design of enzyme catalysts for the rearrangement of chorismic acid.
Protonation of Unsaturated Hydrocarbon Ligands: Regioselectivity, Stereoselectivity, and Product Specificity
Authors: Henderson Richard A. 
Published: January 2004

AbstractUnderstanding the mechanisms of protonation of hydrocarbon ligands is fundamental to a wide range of chemistry including organic synthesis, organometallic chemistry, and even bioinorganic chemistry. Protonation at carbon or metal sites is often slow, with the result that in species containing both types of sites, initial protonation can be at either the metal or the carbon. This has fundamental consequences on the reactivity of hydrocarbon ligands, which are highlighted in this article. In particular, many reactions are apparently the result of a regioselective protonation on the basis of structural analysis of the isolated products. In fact, these products are often formed by an indirect route involving kinetically controlled protonation at the “wrong” site followed by rearrangement to form the thermodynamically controlled, apparently regioselective, product. Other aspects of the protonation mechanisms of complexes containing hydrocarbon ligands are discussed, with an emphasis on the manner in which competitive protonation at metal or ligand can be exploited to select which hydrocarbon is produced and to control the stereochemistry of the hydrocarbon.
Free‐Radical Carbonylations: Then and Now
Authors: Ryu Ilhyong , Sonoda Noboru 
Published: January 2004

AbstractAlthough known since the 1950s, free‐radical carbonylation has not received much attention until only recently. In the last few years the application of modern free‐radical techniques has revealed the high synthetic potential of this reaction as a tool for introducing CO into organic molecules. Clearly now is the time for a renaissance of this chemistry. Under standard conditions (tributyltin hydride/CO) primary, secondary, as well as tertiary alkyl bromides and iodides can be efficiently converted into the corresponding aldehydes. Aromatic and α,β‐unsaturated aldehydes can also be prepared from the parent aromatic and vinylic iodides. If the reaction is carried out in the presence of alkenes containing an electron‐withdrawing substituent, the initially formed acyl radical subsequently adds to the alkene, leading to a general method for the synthesis of unsymmetrical ketones. This three‐component coupling reaction can be extended successfully to allyltin‐mediated reactions. Thus, β,γ‐enones can be prepared from organic halides, CO, and allyltributylstannanes. In a remarkable one‐pot procedure alkyl halides can be treated with a mixture of alkene, allyltributylstannane, and carbon monoxide in a four‐component coupling reaction that provides β‐functionalized δ,ϵ‐unsaturated ketones by the formation of three new CC bonds. The reaction of 4‐pentenyl radicals with CO leads to acyl radical cyclization, which provides a useful method for the synthesis of cyclopentanones. Certain useful one‐electron oxidations can be combined efficiently with free‐radical carbonylations. These findings and others discussed in this article clearly demonstrate that free‐radical carbonylation can now be considered a practical alternative to transition metal mediated carbonylation.