bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of Enzyme Structure and Activity by Protein Engineering
Authors: Fersht Alan R. , Shi Jian‐Ping , Wilkinson Anthony J. , Blow David M. , Carter Paul , Waye Mary M. Y. , Winter Greg P.
Published: December 2003

Structure‐activity relationships of enzymes can now be analyzed for the first time by the systematic alteration of protein structure. Recent developments in the chemical synthesis of DNA fragments and recombinant DNA technology enable the facile modification of proteins by highly specific mutagenesis of their genes. Kinetic analysis of the mutant enzymes combined with high‐resolution structural data from protein X‐ray crystallography allow direct measurements on the relationships between structure and function. In particular, the strength and nature of enzyme‐substrate interactions and their detailed roles in catalysis and specificity can now be studied. We have developed such analysis of enzyme structure‐function by site‐directed mutagenesis of the tyrosyl‐tRNA synthetase from Bacillus stearothermophilus, concentrating so far on the subtle role of hydrogen bonding in both substrate specificity and catalysis. We find that the energetics of tyrosine and ATP binding must be analyzed in terms of an exchange reaction with solvent water. Based on this idea and structural data, we have engineered an enzyme of improved enzyme‐substrate affinity, and there thus appear to be real prospects of engineering proteins of new specificities, activities, and structural properties. We are also using protein engineering to gather direct information on the nature of enzyme catalysis. For example, we find the catalysis of formation of Tyr‐AMP from Tyr and ATP is due largely to electrostatic and hydrogen bonding interactions that are stronger in the transition state than in the ground state—a “strain” mechanism rather than acid‐base or covalent catalysis.
Organolanthanoid Compounds
Authors: Schumann Herbert
Published: December 2003

Up to little more than a decade ago organolanthanoid compounds were still a curiosity. Apart from the description of an isolated number of cyclopentadienyl and indenyl derivatives, very few significant contributions had been made to this interesting sector of organometallic chemistry. However, subsequent systematic studies using modern preparative and analytical techniques, together with X‐ray single crystal structure determinations, enabled the isolation and characterization of a large number of very interesting homoleptic and heteroleptic compounds in which the lanthanoid is bound to hydrogen, to substituted or unsubstituted cyclopentadienyl groups, to allyl or alkynyl groups, or even to phosphorus ylides, trimethylsilyl, and carbonylmetal groups. These compounds, which are all extremely sensitive to oxygen and water, open up new possibilities in the field of catalysis and have great potential in organic synthesis—as recent studies with pentamethylcyclopentadienyl derivatives, organolanthanoid(II) compounds, and hexamethyllanthanoid complexes have already shown.
Topological Charge Stabilization and the Equilibrium between P4S3 and As4S3
Authors: Gimarc Benjamin , Joseph P. J.
Published: December 2003

The cage molecule PS3As3, a beautiful example of the operation of the rule of topological charge stabilization, is formed upon mixing P4S3 and As4S3 in the molar ratio 1:3. The rule states that particularly stable structures result from substitution of electronegative heteroatoms at sites where electron charges are more negative in the homoatomic, isoelectronic reference frame (here P⊖).
Mercury(II)‐ and Palladium(II)‐Catalyzed [3,3]‐Sigmatropic Rearrangements [New Synthetic Methods (46)]
Authors: Overman Larry E.
Published: December 2003

Mercury(II) and palladium(II) salts have found broad applications as catalysts for [3,3]‐sigmatropic rearrangements leading to formation of CO, CN, CS, and CC σ bonds. Increases in reaction rate are often very large (1010 – 1014 at 1 M catalyst concentration) and allow many previously difficult transformations to be conducted at or near room temperature, often with attendant increases in stereoselectivity and decreases in by‐product formation. The mechanism of these catalyzed transformations is briefly discussed, and evidence is summarized to suggest that many follow a cyclization‐induced rearrangement mechanism.
General Entry to Novel Manganese(II) Half‐Sandwich Complexes
Authors: Köhler Frank H. , Hebendanz Nikolaus , Thewalt Ulf , Kanellakopulos Basil , Klenze Reinhardt
Published: December 2003

The chemistry of half‐sandwich complexes is of interest, inter alia, because of the high reactivity of these compounds and because of their potential use for the synthesis of novel complexes. From the manganocene 1, the dimeric PEt3 derivatives 3 were synthesized via the THF complexes 2; 3, X  Cl, is confirmed by X‐ray structure analysis. Surprisingly, 2, X  I, reacts with PMe3 to give the monomeric complex Cp′MnI(PMe3)2. MnII half‐sandwich complexes were hitherto unknown (Cp′  MeC5H4).
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Surprising Stability of Nucleophilic CrIII‐Alkylating and CrIII‐Allylating Reagents towards Ethanol and Water
Authors: Kauffmann Thomas , Abeln Renate , Wingbermühle Dorothea
Published: December 2003

The alkylation and allylation of aldehydes in the presence of ethanol or water are possible with chromium(III) reagents such as 1 or 2. In some cases, the addition of ethanol or water even increases the yields. The maximum yield on addition of 1—3 mol of protic solvent per mol of reagent indicates that 1—3 solvent molecules are incorporated into the ligand spheres of 1 or 2 without cleavage of the CrC bond.
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Gas Chromatographic Separation of Enantiomers on Optically Active Metal‐Complex‐Free Stationary Phases. New Analytical Methods (24)
Authors: Schurig Volker
Published: December 2003

If a stationary phase A employed in gas chromatography possesses a chemical affinity for substance B, which is to be separated, then the retention behavior is not only determined by the normal physical equilibrium between the gas and liquid phases but also by the chemical equilibrium A + B ⇋ AB. If A and B are chiral and A is present in optically active form while B is a racemic mixture, then it is possible to achieve a gas chromatographic enantiomer resolution without the isolation of diastereomers: the energetically different diastereomeric associates AR BR and AR BS are formed rapidly and reversibly. This enantiospecific resolution principle was first demonstrated in 1966 by the quantitative resolution of racemic amino acid derivatives on optically active peptide phases in analogy to the well‐known stereospecificity of enzymes. The anchoring of the chiral resolving agent to thermally stable polysiloxanes together with the employment of high resolution capillary columns and the use of appropriate derivatization strategies has led to the development of enantiomer resolution into a routine modern method for many classes of substances. The demonstration of enantiospecificity in the gas chromatographic separation process is of fundamental interest, and its systematic study can result in a significant contribution to the understanding of the molecular mechanism of “chiral recognition”. The gas chromatographic separation of enantiomers has also proven to be an accurate and sensitive method for the determination of the enantiomeric composition of natural products and products of enantioselective transformations (asymmetric syntheses, “chiral pool” transformations, kinetic resolutions, biomimetic reactions) and for the quantification of racemization, e.g. in the synthesis and hydrolysis of peptides. In any research program devoted to the phenomenon of chirality, the gas chromatographic separation of the enantiomers of volatile compounds constitutes an indispensable modern instrumental technique.
An Azacyclophane Derivative as Catalyst of Substrate‐ and Product‐Selective Reactions with Ambident Anions
Authors: Schneider Hans‐Jörg , Busch Rainer
Published: December 2003

An increase in reaction rate and alteration of the product ratioin nucleophilic exchange reactions is achieved when the azacyclophane1described in the two previous communications is added to the reaction of ambident nucleophiles with 2‐bromomethylnaphthalene. Similarly as in the case of enzymes, saturation kinetics and competitive inhibition are observed as well as substrate and product selectivity.
Di‐(μ‐amido)dirhodium Complexes: Structure of [Rh2(μ‐(NH)2naphth)I2(CO)2(PPh3)2]
Authors: Oro Luis A. , Fernández Maria J. , Modrego Javier , Foces‐Foces Concha , Cano Felix H.
Published: December 2003

A Novel class of binuclear amido complexes containing doubly deprotonated 1,8‐diaminonaphthalene as a bridging bisdihapto ligand has now been synthesized. The title compound is formed by successive reaction of [Rh(acac)(CO)2] with 1,8‐C10H6(NH2)2, PPh3 and I2, whereby all intermediates have been isolated.
The Homoaldol Reaction, or How to Overcome Problems of Regio‐ and Stereo‐selectivity
Authors: Hoppe Dieter
Published: December 2003

Even today, there are conspicuous gaps in the register of organic synthetic methods. Thus, numerous types of homoenolate reagents are listed; yet no generally applicable method for the homologue of the aldol addition, viz. the homoaldol addition, is to be found. The methods documented up to 1980 offer only singular solutions, and, moreover, are not stereoselective. The present article gives an overview of the existing possibilities and then describes how the gap can be closed by using a new class of homoenolate reagents: 2‐alkenyl esters ofN,N‐diisopropylcarbamic acid are lithiated and the resulting allyllithium compounds tailored to high regio‐ and diastereoselectivity in addition reactions to the carbonyl group via metal exchange. Altogether, the homoaldol reactions now offer a general and reliable, highly stereoselective entry to γ‐hydroxycarbonyl compounds, whereby the C(β)C(γ) bond is formed. Furthermore, it is demonstrated how problems associated with the application of allyl anions as synthetic reagents can be solved today.
The in situ Preparation of 2‐Sulfidophenolate Ligands by Copper‐Catalyzed Oxygen Insertion into Thiophenolates; Crystal Structure of [PPh4]2[Mo2O5(SC6H4O)2]
Authors: Garner C. David , Nicholson John R. , Clegg William
Published: December 2003

2‐Mercaptophenol and its mono‐ and dianions are of interest as intermediate members between pyrocatechol and ortho‐benzenedithiol. Complex 3 contains SC6H4O2⊖ ligands. The in‐situ synthesis of 3 can be achieved by coper‐catalyzed atmospheric oxidation of a 1 :3 mixture of 1 and 2.
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Sacrificial Anodes in the Electrocarboxylation of Organic Chlorides
Authors: Silvestri Giuseppe , Gambino Salvatore , Filardo Giuseppe , Gulotta Antonio
Published: December 2003

An increase in yield from below 50 up to 90% is realized on using sacrificial anodes in the electrochemical carboxylation of organic halides. The metal ion liberated from the anodes reacts with the cathodically formed carboxylate to give isolable salts and thus prevents undesired esterification.
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Double Asymmetric Synthesis and a New Strategy for Stereochemical Control in Organic Synthesis
Authors: Masamune Satoru , Choy William , Petersen John S. , Sita Lawrence R.
Published: December 2003

This account examines double asymmetric induction from theoretical and practical viewpoints. In the context of four major organic reactions‐the aldol, Diels‐Alder, catalytic hydrogenation, and epoxidation‐it is shown that a double asymmetric induction can be analyzed in terms of the single asymmetric reactions of each of the two chiral reactants. A rule which qualitatively relates the results of these single asymmetric reactions with the outcome of the double asymmetric reaction is proposed. A powerful new strategy based on this rule for the predictable creation of new chiral centers is discussed and the use of this strategy for the synthesis of sugars and macrolides is presented.
Photooxidation of Chloroform: Isolation and Characterization of Trichloromethyl Hydroperoxide
Authors: Gäb Siegmar , Turner Walter V.
Published: December 2003

Trichloromethyl hydroperoxide 1 is unexpectedly stable; this discovery is of importance for the photooxidative degradation of organochlorine compounds in the atmosphere. 1 is formed upon irradiation of O2‐saturated chloroform with sunlight; it reacts with N2O5 to give 2 and with acetyl chloride to give the perester 3.
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c‐TiS2, a New Modification of Titanium Disulfide with Cubic Structure
Authors: Schöllhorn Robert , Payer Andreas
Published: December 2003

A metastable modification of TiS2 has been prepared by a topotactic solid‐state reaction at room temperature in which CuTi2S4 working electrodes were anodically oxidized in aprotic copper(I) electrolytes. On the basis of X‐ray powder data, it can be determined that the new phase belongs to a very seldom AB2 structural type. This cubic TiS2 is of interest, inter alia, as a material for reversible electrodes in secondary batteries. It has advantages over hexagonal TiS2 (layer lattice).
Cationic 2‐Azaallenylidene Complexes, Novel Compounds with Unusual Photochemical Properties
Authors: Fischer Helmut , Seitz Friedrich , Riede Jürgen , Vogel Jürgen
Published: December 2003

[CNCR2]⊕, a cation unknown in the free state, is present as terminal ligand bound to a metal in the salts 1, MCr, W, RPh. On exposure to low‐energy radiation (λ>300 nm) the complexes 1 fragment with cleavage of CR2; with exclusively short‐wave radiation, however, they are stable. The complexes 1 thus belong to the few compounds in which fluorescence from higher ecited states is observed.
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Oxidation of Organometallic Compounds
Authors: Halpern Jack
Published: December 2003

Stable organometallic compounds, notably of the later transition metals (groups VI–VIII), usually are characterized by closed shell electron configurations (typically 18‐electron valence shells) which are destabilized by electron addition or removal. One‐electron oxidation of such compounds results in the formation of unstable radical ions, whose characteristic reactivity patterns include susceptibility to nucleophilic attack, disproportionation, and metal‐carbon bond dissociation. Two‐electron oxidation may result in dissociation or oxidation of the organic ligand. In this review studies on the chemical and electrochemical oxidations of metal carbonyls, metal‐olefin complexes, and alkyl transition‐metal compounds are described. The studies encompass the following themes: (1) The kinetics and thermodynamics of the initial redox steps; (2) The characterization and reactivity patterns of the resulting oxidation products; (3) The synthetic and catalytic applications of organometallic redox processes.
The 3′‐End of tRNA and Its Role in Protein Biosynthesis
Authors: Chládek Stanislav , Sprinzl Mathias
Published: December 2003

In the course of protein biosynthesis, the 3′‐ends of aminoacyl‐tRNA (aa‐tRNA) and peptidyl‐tRNA specifically interact with macromolecules of the protein biosynthesis machinery. The 3′‐end of tRNA consists of an invariant C‐C‐A single strand. Interaction of the aminoacyl‐tRNA 3′‐end with elongation factor Tu (EF‐Tu) containing bound GTP is necessary for the formation of the aa‐tRNA·EF‐Tu·GTP complex and, after the complex binds to the ribosome, for the GTP hydrolysis. This process is followed by the specific binding of the aminoacyl‐tRNA 3′‐end to the aminoacyl (A) site of the ribosome. In this review, a model is proposed that involves Watson‐Crick base pairing of the CC sequence of the aminoacyl‐tRNA 3′‐end with a specific GG sequence of the ribosomal 23S RNA. Similarly, peptidyl‐tRNA binds with its 3′‐end to the peptidyl (P) site of the ribosome. This binding may also involve Watson‐Crick base pairing of the C‐C‐A sequence with a complementary sequence of 23S RNA. It is proposed that peptide bond formation is catalyzed by a functional site of the 23S RNA located near the 3′‐ends of aminoacyl‐tRNA and peptidyl‐tRNA. A model is suggested in which two loops of the 23S RNA, brought into close proximity via folding, are involved both in binding the 3′‐ends of the tRNAs and in catalyzing peptide bond formation. This model presumes a dynamic structure for ribosomal RNA, which is modulated by interaction with elongation factors and ribosomal proteins.
(Te2)2(I2)—An Unusual Intercalation Compound
Authors: Kniep Rüdiger , Beister Heinz‐Jürgen
Published: December 2003

Tellurium double layers, between which planar layers of iodine molecules are inserted, characterize the new intercalation compound (Te2)2(I2); it contains a tellurium modification that was previously unknown as an independent phase, which is intercalated by elemental iodine. Crystals were obtained by hydrothermal synthesis in concentrated hydrogen iodide.
[{1,3,5‐(CH3)3H3C6}6Tl4][GaBr4]4 — Synthesis and Structure of a Mixed Mono‐ and Bis(arene)thallium Complex
Authors: Schmidbaur Hubert , Bublak Wolfgang , Riede Jürgen , Müller Gerhard
Published: December 2003

A tetrameric Tl4[GaBr4]4 skeletal framework, whose Tl⊕ ions are alternately coordinated by one or two mesitylene molecules, is present in the title compound. It is formed from Tl[GaBr4] and mesitylene and is the first complex containing both mono‐ and bis(arene)thallium units to be characterized by X‐ray analysis.
Combination of Chromatographic Separation Methods with Mass Spectrometry—a Modern Technique for Studying Metabolism
Authors: Spiteller Gerhard
Published: December 2003

Powerful separation methods developed during the past few decades, in particular glass capillary gas chromatography (GC) and high performance liquid chromatography (HPLC), can be combined with sensitive methods of detection such as mass spectrometry (MS) to provide new insights into metabolic processes. Research in a field that was long thought to be exhausted has thus been revitalized. New perspective have been opened up for studying the metabolism of foods, spices and flavorings, drugs, and compounds produced in the body as well as for establishing the way in which the organism rids itself of harmful substances or how and when these compounds can interfere with the metabolic processes occurring in the body.
Host/Guest Chemistry of Organic Onium Compounds—Clathrates, Crystalline Complexes, and Molecular Inclusion Compounds in Aqueous Solution
Authors: Vögtle Fritz , Löhr Hans‐Gerd , Franke Joachim , Worsch Detlev
Published: December 2003

The importance of organic onium compounds for host/guest interactions in biological processes has been long recognized. In contrast, the versatility of synthetic onium compounds as host molecules in abiotic chemistry has only been investigated in detail for the last few years. Due to their inclusion ability and selectivities, organic ammonium, pyridinium, and phosphonium salts and other organic salts with specific structural features are interesting as clathrate‐forming molecules. The crystal structures of the clathrates afford insight into the conformational adaptability of onium clathrands with limited flexibility to the geometric requirements of the guest molecules. Optically active onium hosts can be used for enantiomeric enrichment. In crystalline tetracyanoquinodimethane(TCNQ) complexes and radical anion salts, the structure of the organic heterocation has a marked effect on the electrical conductivity. Macrocyclic onium compounds of the phane type with “exohydrophilic” and “endolipophilic” cavities are suitable as models of enzymes and biological receptors. They can also be used to transfer aromatic and aliphatic guest molecules selectively into aqueous solution. Catalytic activity (including H/D exchange) has already been obtained with synthetic compounds having large cavities. The further development of such host/guest systems may contribute to the understanding of biological systems and the improvement of technical processes (catalysis in molecular cavities).
Synthesis of the Immunologically Essential Saccharide Sequence of the “Enterobacterial Common Antigen”
Authors: Paulsen Hans , Lorentzen Jens Peter
Published: December 2003

A trisaccharide containing an aminouronic acid moiety is the repeat unit of an oligosaccharide that represents the family‐specific immunodeterminant structure of Enterobacteriaceae. The synthesis of the derivative 1 of this trisaccharide has now been accomplished, which should enable the preparation of the corresponding synthetic antigens.
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Cleavage of an Ether Bond via β‐Elimination of Ethanol from [(H2O)5CrCH2CH2OC2H5]2+ in Aqueous Solutions; A Pulse Radiolysis Study
Authors: Cohen Haim , Meyerstein Dan
Published: December 2003

β‐Elimination with cleavage of an ether bond has been observed in studies with the title compound. The rate constant for this reaction (4.6 × 103 M−1 s−1) is similar to that for the elimination of the β‐OH group from the analogous CH2C(CH3)2OH complex and related compounds.
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Dinuclear Complexes with Predictable Magnetic Properties
Authors: Kahn Olivier
Published: December 2003

When two paramagnetic transition metal ions are present in the same molecular entity, the magnetic properties can be totally different from the sum of the magnetic properties of each ion surrounded by its nearest neighbors. These new properties depend on the nature and the magnitude of the interaction between the metal ions through the bridging ligands. If both ions have an unpaired electron (e.g. Cu2+ ions), then the molecular state of lowest energy is either a spin singlet or a spin triplet. In the former case, the interaction is said to be antiferromagnetic, in the latter case ferromagnetic. The nature and the order of magnitude of the interaction can be engineered by judiciously choosing the interacting metal ions and the bridging and terminal ligands, and, thus, by the symmetry and the delocalization of the orbitals centered on the metal ions and occupied by the unpaired electrons (magnetic orbitals). The first success in this “molecular engineering” of bimetallic compounds was in the synthesis of a Cu2+VO2+ heterobimetallic complex in which the interaction is purely ferro‐magnetic. The same strategy could be utilized for designing molecular ferromagnets, one of the major challenges in the area of molecular materials. Another striking result is the possibility of tuning the magnitude of the interaction through a given bridging network by modifying the nature of the terminal ligands, which, in some way, play the role of “adjusting screws”. By careful selection of the bridging and terminal ligands, a very large antiferro‐magnetic interaction can be achieved, even if the metal ions are far away from each other. Some sulfur‐containing bridges are especially suitable in this respect.
A Photochemically “Switched‐on” Crown Ether Containing an Intraannular 4‐Methoxyphenylazo Substituent
Authors: Shinkai Seiji , Miyazaki Kiminori , Manabe Osamu
Published: December 2003

The binding of metal ions can be hindered by the intraannular 4‐methoxyphenylazo substituent in dibenzo[18]crown‐6 analogues in which a CH2OCH2 group is replaced by a (substituted) m‐phenylene ring. When the substituent is photochemically flipped out of the cavity, metal ions may be complexed in the manner characteristic of crown ethers.
2,3‐Dihydro‐1,3‐diborole‐Metal Complexes with Activated CH Bonds: Building Blocks for Multilayered Sandwich Compounds
Authors: Siebert Walter
Published: December 2003

Derivatives of 2,3‐dihydro‐1,3‐diborole acting as four‐electron ligands form various metal complexes in which the ring‐methylene carbon atom is pentacoordinated. The resultant activation of the CH bond can be used to prepare triple‐decker to hexadecker sandwich complexes. Bis(2,3‐dihydro‐1,3‐diborole)nickel complexes undergo a spontaneous stacking and formation of tricarbahexaboranyl oligodecker complexes with stacks of 2,3‐dihydro‐1,3‐diborolylnickel. These findings finally led to the synthesis of the first polydecker sandwich complex which was obtained by vacuum thermolysis of a tris(allyl)(μ‐2,3‐dihydro‐1,3‐diborolyl)dinickel complex. The black polymer possesses remarkable semiconductor properties.
Reaction of Phenylcarbene with Oxygen
Authors: Sander Wolfram
Published: December 2003

The thermal generation of oxygen atoms is very useful in organic chemistry. This reaction has now been observed in the oxidation of phenylcarbene with O2. Carbonyl oxide, frequently postulated as an intermediate, decomposes very rapidly, even at 8 K; this has been demonstrated in an IR spectroscopic investigation of the title compound in an argon matrix at low temperatures.
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Decamethylsilicocene—Synthesis and Structure
Authors: Jutzi Peter , Kanne Dieter , Krüger Carl
Published: December 2003

The first molecular compound of divalent silicon that is stable under normal conditions and the first π‐complex with silicon as the central atom is decamethylsilicocene 2. It was prepared by reduction of 1 with naphthalenelithium, naphthalenesodium, or naphthalenepotassium. Two conformers in the ratio 1:2 are present in the unit cell.
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Reversible Interconversion of CC and CN Multiple Bond Systems on Clusters
Authors: Bernhardt Wolfgang , von Schnering Christine , Vahrenkamp Heinrich
Published: December 2003

Cluster moieties can be so conditioned by metal exchange that they support one and the same reaction in the region of the organic ligands both in the forward and reverse direction. A spontaneous alkylidyne‐vinylidene transformation has been observed for the first time within the sequences of the type shown below.
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Hydroxyacetylene: Generation and Characterization of the Neutral Molecule, Radical Cation and Dication in the Gas Phase
Authors: van Baar Ben , Weiske Thomas , Terlouw Johan K. , Schwarz Helmut
Published: December 2003

The simplest ynol, the title compound 2, like the singly and doubly positively charged species 3 and 4, respectively, could be generated in the mass spectrometer. Cleavage of CO from 1 furnished the radical cation 3, which on neutralization with Xe affords the neutral ynol 2. The radical cation 3 can be converted into the dication 4 by charge stripping.
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Second‐Sphere Coordination–a Novel Rǒle for Molecular Receptors
Authors: Colquhoun Howard M. , Stoddart J. Fraser , Williams David J.
Published: December 2003

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 macrocyclespar excellencefor 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.
High‐Pressure Liquid Chromatography (HPLC) of Proteins [New Analytical Methods (29)]
Authors: Seipke Gerhard , Müllner Hubert , Grau Ulrich
Published: December 2003

The application of high‐pressure liquid chromatography (HPLC) to proteins has undergone a dramatic development in recent years. Nowadays its many variants expand the repertoire of high‐performance analysis methods available to the protein chemist, which, until now, have been dominated by electrophoretic techniques. The advent of gene technology has resulted in a renaissance of protein chemistry. The new analytical and preparative problems that have thereby emerged are often ideally solved by HPLC methods. HPLC has long since ceased to be solely a laboratory technique; HPLC systems are now being developed for the separation of proteins–particularly those of great pharmaceutical interest – on a 100‐g scale. The range of applications of analytical and preparative HPLC will be illustrated by two examples of pharmaceutical importance—insulin and interleukin 2.
Direct Aminolysis of Unactivated Esters at High‐Pressure
Authors: Matsumoto Kiyoshi , Hashimoto Shiro , Otani Shinichi
Published: December 2003

The preparation of amides 3 from nonactivated esters 1 and secondary amines 2 has been achieved at 8 kbar and approximately 45°C. This reaction cannot be carried out at normal pressure without a catalyst; even the reaction with primary amines (uncatalyzed, normal pressure) requires temperatures above 200°C.
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Direct Methods and Anomalous Dispersion (Nobel Lecture)
Authors: Hauptman Herbert
Published: December 2003

Todays triumphant advance of X‐ray structure analysis is due in large part to the development of the “direct methods” by Herbert Hauptman and Jerome Karle. Previously, it was generally assumed to be impossible to determine a structure directly from the diffraction pattern of a crystal, for a normal diffraction experiment afforded only amplitudes, but no phases of the diffraction maxima. This phase problem was overcome in a series of steps that involved recognition that the required phase information was contained in the measured intensities, the derivation of a foundation mathematics that displayed relationships between phases and magnitudes and even among phases alone and, finally, the development of practical methods for structure determination, strategies that brought together in a more or less optimal fashion the mathematical relationships with suitably adjusted and refined experimental data.
185‐nm Photochemistry of Olefins, Strained Hydrocarbons, and Azoalkanes in Solution
Authors: Adam Waldemar , Oppenländer Thomas
Published: December 2003

The possibility to excite directly at 185 nm chromophores that absorb in the vacuum‐UV has stimulated increased activity during the last decade in this field of photochemistry. Whereas photochemical reactions at λ<200 nm have been thoroughly investigated in the gas phase, only recently have intensive studies on the 185‐nm photochemistry of organic compounds in solution provided new insights. Despite the high excitation energies, selective photoreactions are promoted in the short‐lived singlet excited states (Rydberg photochemistry). In contrast to conventional photochemistry (λ > 220 nm), 185‐nm irradiation preferentially results in intramolecular rearrangement, fragmentation, and isomerization reactions. Intermolecular radical couplings and abstractions as well as dimerizations (π, π*‐excitation) compete minimally. Besides the straightforward denitrogenation of photoresistant (“reluctant”) azoalkanes, important applications of the short‐wavelength photolysis are also found in technology (photolithography) and medicine (193‐nm laser). Broadening the scope of the synthetic potential of the 185‐nm photochemistry, which so far has been limited to direct cis/trans isomerizations, presents a challenge for the chemist.
Indirect Electroorganic Syntheses—A Modern Chapter of Organic Electrochemistry [New Synthetic Methods (59)]
Authors: Steckhan Eberhard
Published: December 2003

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.
Total Synthesis of Anthracyclinone [New Synthetic Methods (60)]
Authors: Krohn Karsten
Published: December 2003

The remarkable antitumor properties of anthracycline antibiotics have motivated synthetic chemists, and a large number of groups have been active recently in this field. Initially, there was very little hope for this class of substances due to their extreme toxicity. However, the dividing line between toxicity and desired pharmacological effect has broadened to such an extent, partly through microbiological methods (new strains, mutants) and partly through semisynthetic modifications, that today anthracyclines take number one place (over 20% of the market) in the chemotherapy of (human) cancer. The demands made on modern cytostatica are extremely high, as may be seen from the fact that of the more than 600000 drugs tested in animals, only about 40 are in clinical use. In the meantime, some anthracyclines obtained by total synthesis are also in clinical testing. In this review it will hopefully be demonstrated that all those concerned in the chemotherapeutic use of these substances have also contributed to the general progress made in the solution of regio‐ and stereo‐chemical problems of linearly annelated ring systems.
Photoisomerization of Dihalomethanes
Authors: Maier Günther , Reisenauer Hans Peter
Published: December 2003

Discovering a new type of reaction with simple molecules is not an everyday occurrence: The isomers 2, which are formed upon matrix irradiation of the haloiodomethane 1, can be regarded as unsolvated contact ion pairs
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. In this rearrangement, the iodine atom migrates from the carbon atom to the halogen atom. The isomers 2a (violet), 2b (green) and 2c (yellow) can be reconverted into the educts by changing the wavelength of the radiated light (a, b, c: XI, Br, Cl).
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Oxygen Evolution by Water Oxidation Mediated by Heterogeneous Manganese Complexes
Authors: Ramaraj Ramasamy , Kira Akira , Kaneko Masao
Published: December 2003

The catalytic oxidative decomposition of water has been achieved with the help of the dinuclear manganese complexes 1 and 2, which are models for compounds participating in photosynthesis. Whereas no reaction was observed in homogeneous solution, O2 evolution took place almost immediately in the heterogeneous phase after addition of a CeIV salt as oxidizing agent. This finding supports the assumption that a microheterogeneous environment and a large spatial proximity of the manganese centers are essential for the photosynthetic activity of manganese compunds (bpy2,2′‐bipyridyl; phen1,10‐phenanthroline).
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High‐Resolution Metal NMR Spectroscopy of Organometallic Compounds [New Analytical Methods (30)]
Authors: Benn Reinhard , Rufińska Anna
Published: December 2003

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.
Isothermal Magnetic Phase Transitions Controlled by Reversible Electron/Ion Transfer Reactions
Authors: Schöllhorn Robert , Payer Andreas
Published: December 2003

The ferromagnetic copper chromoselenospinel CuCr2Se4 might serve as the basis for a chemically controlled magentic switch. In copper‐containing electrolytes it can be transformed at 300 K into the cubic, nonstoichiometric phases Cu1 + yCr2Se4 (O ≤ y ≤ 1). At a critical value, yc, the system undergoes a ferromagnetic/paramagnetic phase transition. Since the uptake of copper is a reversible process, this chemical reaction can be used to change the magetic properties. Similar reactions have been observed for the ferromagentic phases CuCr2S4 and CuCr2Te4.
Divinylglyoxal and Methylvinylglyoxal
Authors: Kramme Roland , Martin Hans‐Dieter , Mayer Bernhard , Weimann Ralf
Published: December 2003

The two simplest unsaturated glyoxals 1 and 2 have been prepared by gasphase pyrolysis from 5‐norbornen‐2‐yl‐CO‐CO‐R, R5‐norbornen‐2‐yl and Me, respectively. The UV and PE spectra of the yellow diones 1 and 2 reveal a preferred conformation in which the carbonyl groups—in contrast to those in benzil—are arranged in a nearly antiperiplanar fashion.
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Unsaturated Molecules Containing Main Group Metals
Authors: Veith Michael
Published: December 2003

Molecules in which there are neighboring electrophilic and nucleophilic centers are unusually reactive. Oligomerization can be prevented only by bulky groups attached to the main group metal atom that would act as electron pair acceptor, or to the basic non‐metal atom. The basic and the acid centers behave as a single unit in chemical reactions; the system is similar to a “double bond” whose π‐electron density is largely concentrated at one atom. The unsaturated nature of these molecules can be seen in (for example) their addition reactions with hydrogen compounds of non‐metals, or in reactions that are distantly related to cycloadditions at homopolar double bonds. The selection of suitable reaction partners leads to polycyclic, cage‐like molecules containing metal atoms. If these atoms possess lone pairs (as is usual in the lower oxidation states of the third and fourth main groups), these can be utilized to form bonds to further (Lewis acid) metal centers. In some cases large assemblies can be built up from polycyclic systems in this way; a characteristic of these assemblies is a one‐dimensional array of metal atoms. Commonly occurring structural features of the polycyclic species are tetrahedra, trigonal bipyramids and cubes.
Laser Chemistry–What is Its Current Status?
Authors: Kleinermanns Karl , Wolfrum Jürgen
Published: December 2003

In recent years, various methods have been developed to observe and to influence the course of chemical reactions using laser radiation. By selectively increasing the translational, rotational, and vibrational energies and by controlling the relative orientation of the reaction partners with tunable infrared and UV lasers, direct insight can be gained into the molecular course of the breaking and re‐forming of chemical bonds. Examples for the application of lasers include the synthesis of monomers such as vinyl chloride and polymers such as polyethylene, the synthesis of biologically active substances such as vitamin D3, the separation of isotopes, the removal of impurities, the production of catalysts, glasses, and ceramics, and the deposition and ablation of material on surfaces. Finally, several applications of lasers in medicine are discussed.
(Me3Si)2NPNSiMe3 as Educt for Electron‐Rich Phoshorus‐Nitrogen Ligands
Authors: Scherer Otto J. , Göbel Karin , Kaub Jürgen
Published: December 2003

Three unusual P‐N compounds stabilized in di‐ or trinuclear clusters are formed upon cothermolysis of [Fe3(CO)12] and 1: the 6e‐donor ligand 2, the phosphanediyl 3, and the substituted hydroxy(imiuo)phosphane 4 (R = Si Me3).
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A Biocatalyst for the Preparation of (R) and (S)‐2‐Hydroxycarboxylic Acids
Authors: Skopan Haike , Günther Helmut , Simon Helmut
Published: December 2003

“Biocatalytic enantiomer resolution of 2‐hydroxycarboxylic acids”, rac‐1, has been achieved with the (nonisolated) 2‐oxoacid reductase from Proteus vulgaris. This enzyme is usually employed to reduce 2‐oxomonocarboxylic acids to (R)‐2‐hydroxycarboxylic acids (> 99% ee) with methyl‐ or benzylviologen as electron mediator. The use of carbamoylmethylviologen has allowed, for the first time, the reverse reaction to be carried out. Only the (R) enantiomer of rac‐1 undergoes dehydrogenation; the (S)‐2‐hydroxycarboxylic acid remains unchanged. The mediator can be regenerated.
The Copolymer of 2,6‐Pyridinedimethanethiol and 1,4‐Benzenedimethanethiol
Authors: Modica Giovanni , Maffi Silvia , Mantoneri Enzo , Giuffré Luigi , Grassi Antonio
Published: December 2003

The first polymer containing pyridinc rings as part of the backbone chain, the title compound 3, was prepared from the two bis(chloromethyl) compounds 1 and 2 and Na2S. 3 is insoluble, but melts (m.p. = 177°C) and can undergo extrusion to produce films. With Na2S alone, 1 forms a cyclic tetramer.
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Electrodes for Potential Measurements in Aqueous Systems at High Temperatures and Pressures
Authors: Niedrach Leonard W.
Published: December 2003

The measurement of pH, redox potentials, and corrosion potentials at high temperatures and pressures is often desirable for the control and monitoring of industrial processes; e.g., for controlling water chemistry in conventional and nuclear power generators, for process control in the chemical industry, and for monitoring performance in petroleum extraction and refining. Such measurements are also needed for research on the thermodynamics of high‐temperature aqueous solutions including those associated with geothermal and hydrothermal reactions. Zirconia electrodes for these purposes are described, and their application is illustrated with examples. While not yet routinely available on a commercial basis, useful versions can be readily fabricated in the laboratory. Some of the pitfalls encountered in making such measurements at elevated temperatures are also discussed because great care is often required to assure that valid and useful data are being obtained.
Neutral and Charged Biradicals, Zwitterions, Funnels in S1, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision
Authors: Bonačić‐Koutecký Vlasta , Koutecký Jaroslav , Michl Josef
Published: December 2003

A knowledge of the geometries at which excited molecules return to the electronic ground state (S0) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S1 (lowest excited singlet) and T1(lowest triplet) surfaces, as well as S0–S1 conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the S0 and S1 surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two‐electron two‐orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the S0 and S1 surfaces touch. The results obtained for the simple model are supported by ab initio large‐scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and π‐donor‐to‐π‐acceptor single bonds, and by similar calculations for “push‐pull” perturbed cyclobutadienes, some of which are predicted to have nearly degenerate S0, S1, and T1 states. The likely consequences of these results for the detailed description of the mechanisms of cis‐trans isomerization, the formation of twisted internal charge‐transfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocycloaddition, are summarized.
A Novel Route to Azaalkenylidene‐Metal Complexes
Authors: Werner Helmut , Knaup Wolfgang , Dziallas Michael
Published: December 2003

The oxime complexes 1 have structures analogous to imine complexes. Such oxime complexes have proven suitable as educts for the synthesis of azaalkenylidene complexes 2 of d‐electronrich transition metals. The cleavage of water from 1 with formal hydridic hydrogen and an N‐bonded OH‐group to give 2 proceeds remarkably smoothly [CRR′ = cyclohexylidene or CMe2; {Os} = (C6H6)(PMetBu2)Os].
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The Development of a Carbene‐like Chemistry with Terminal Phosphinidene Complexes
Authors: Mathey François
Published: December 2003

When compared to the chemistry of other well‐known electron‐deficient species such as carbenes, silylenes, and nitrenes, the chemistry of phosphinidenes RP: is extremely underdeveloped. A critical survey of the literature indicates that this state of affairs probably reflects both an inadequate access to these species and an intrinsic lack of reactivity. Theoretical calculations suggest that it is possible to boost the electrophilicity and to stabilize the singlest state of phosphinidenes by complexation with transition‐metal moieties such as M(CO)5 (MCr, Mo, W). The corresponding terminal phosphinidene complexes [RPM(CO)5] show a rich and varied chemistry which often parallels the chemistry of singlet carbenes. Phosphinidene complexes are presently easily accessible through thermal decomposition of 7‐phosphanorbornadiene complexes, but their stabilization remains a challenge to be met.