bluuebunny/crossref_metadata_2025 huggingface dataset:
Supramolecular Synthons in Crystal Engineering—A New Organic Synthesis
Authors: Desiraju Gautam R.
Published: January 2004

A 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

A 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.
Alkali Metallocene Anions: Syntheses and Structures
Authors: Wessel Jens , Lork Enno , Mews Rüdiger
Published: January 2004

The breakdown of the polymeric structure of alkali metal cyclopentadienides can be achieved by (Me2N)3S+ Cp− [(TAS)Cp]. This leads to the salts TAS+[Cp2M]− (MLi, Na, K). On the other hand, [TAS2Cp]+[Cp2M]− salts are obtained if this reaction is carried out in the molar ratio 2:1, the structure analysis of the Na compound shows that sandwich and inverse sandwich units are present side by side.
Cyclopentadienyl‐Free Organolanthanide Chemistry
Authors: Edelmann Frank T.
Published: January 2004

During 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.
Reactions of Neopentyl 2,2,2,‐Trifluoroethanesulfonate (Tresylate) with Nucleophiles: A Model Study for the Coupling of Nucleophiles with Tresyl Agarose
Authors: King James F. , Gill Manjinder Singh
Published: January 2004

Is it prudent to compare homogeneous and heterogeneous phase reactions? One side of this discussion on the reactivity of certain tresylates to nucleophiles such as butylamine and butanethiol says “Yes”, but the other says “Not always”.
A Novel Method for the Synthesis of Isocyanates Under Mild Conditions
Authors: Knölker Hans‐Joachim , Braxmeier Tobias , Schlechtingen Georg
Published: January 2004

Not the amine‐N‐Boc derivatives, but the isocyanates form in the reaction of alkyl‐ or arylamines with di‐tert‐butyl dicarbonate ((Boc)2O) in the presence of a catalytic amount of 4‐dimethylaminopyridine (DMAP) according to Equation (a). The procedure provides a simple laboratory method of generating sterically hindered alkyl and aryl isocyanates without the use of phosgene.
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Coordination of Dioxygen in Three Representative Transition Metal Cations: FeO, CrO, and [CH3Re(O2)2O]+
Authors: Schröder Detlef , Fiedler Andreas , Herrmann Wolfgang A. , Schwarz Helmut
Published: January 2004

How is dioxygen activated by “naked” or ligated metals? Details of the potential energy surfaces of the three representative cations [Fe,O2]+, [Cr,O2]+, and [CH3Re(O2)2O]+ were correlated with mass spectrometric data. The results may lead to a better understanding of O2 activation in synthetic, industrial, and biological processes.
Commensurate Freezing of n‐Alkanes in Silicalite
Authors: van Well Willy J. M. , Wolthuizen Jillus P. , Smit Berend , van Hooff Jan H. C. , van Santen Rutger A.
Published: January 2004

Very surprising desorption behavior from silicalite is shown by n‐hexane and n‐heptane among the n‐alkanes ranging from n‐butane to n‐decane. While n‐hexane and n‐heptane show a two‐step desorption, longer and shorter n‐alkanes desorb in a single step. These results can be explained by commensurate freezing, a phenomenon which was predicted very recently by computer simulations.
Palladium‐Catalyzed Reactions of Propargylic Compounds in Organic Synthesis
Authors: Tsuji Jiro , Mandai Tadakatsu
Published: January 2004

Palladium 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

The 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.
X‐ray Crystal Structures of Cytochrome c Oxidases from Paracoccus denitrificans and Bovine Heart and Their Implications for the Molecular Mechanism of Cell Respiration
Authors: Kadenbach Bernhard
Published: January 2004

Pores with rotational symmetry, each formed from five transmembrane helices in cytochrome c oxidase, form the postulated pathways for the protons required for the biochemical reduction of O2 as well as for the protons transported across the cell (or inner mitochondrial) membrane resulting in a H+ gradient. The structures of cytochrome c oxidase from a bacterium recently described by Michel et al. and from bovine heart by Yoshikawa et al. corroborate many previous results on the structure and function of this enzyme and disprove others.
Synthesis and Structures of Triamino‐ and Tetraaminosilanes
Authors: Wraage Karsten , Künzel Antje , Noltemeyer Mathias , Schmidt Hans‐Georg , Roesky Herbert W.
Published: January 2004

The first compounds of the type RSi(NH2)3 containing substituents R bound through oxygen or nitrogen to silicon are presented herein. These compounds are expected, like the isoelectronic silanetrioles RSi(OH)3, to exhibit a wide range of reactivity—selective substitutions have already been carried out—and should be ideal precursors for the preparation of dendrimers and ceramics.
Two Diorganopotassates: Crystal Structure of [K(C6H6)][K{C(SiMe3)2(SiMe2Ph)}2]
Authors: Eaborn Colin , Hitchcock Peter B. , Izod Keith , Smith J. David
Published: January 2004

The different environments for the two potassium ions in 1 justify its formulation as a potassium salt of a potassate. In an attempt to synthesize the analogous compound containing [C(SiMe3)2{SiMe2(HCCH2)}]− as anion, crystals of 2 were obtained after four weeks, by the adventitious reaction with silicone grease.
The Formal Potentials of Solid Metal Hexacyanometalates
Authors: Scholz Fritz , Dostal Aleš
Published: January 2004

The first report of the formal potentials of a series of solid metal hexacyanometalates (M+)nM′[M″ (CN)6] is given (M+ = Li+, K+, Na+, Rb+, Cs+; M′ = Ag, Al, Cd, Co, Cr, Cu, Fe, Ga, In, Mn, Ni, Pb; M″ = Cr, Fe, Mn). A linear correlation exists between the ionic potential Φ = z/reff of the nitrogen‐bonded M′ ions and the formal potentials of the hexacyanometalate ions. This results from increased π back bonding when the acid–base interaction between the nitrogen atom of the cyanide ions and the M′ ions is stronger.
A Strategy of “Random Glycosylation” for the Production of Oligosaccharide Libraries
Authors: Kanie Osamu , Barresi Frank , Ding Yili , Labbe Jill , Otter Albin , Forsberg L. Scott , Ernst Beat , Hindsgaul Ole
Published: January 2004

Six at one blow! The creation of small oligosaccharide libraries was achieved by a new strategy of random glycosylation of nonprotected sugars. With tri‐O‐benzyl‐L‐fucopyranosyl trichloroacetimidate as donor and βGal(1 → n)βGlcNAcOR (n = 3, 4, 6; R = (CH2)8OC6H4‐p‐OMe) as acceptors, all six trisaccharides form in DMF in roughly equal quantities. It is astounding that all free OH groups are fucosylated at a similar rate.
Organic Chemistry on Solid Supports
Authors: Früchtel Jörg S. , Jung Günther
Published: January 2004

Until 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

Copper 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.
Selective Catalytic Reactions at the External Surface of Zeolite Crystals
Authors: Ernst Stefan
Published: January 2004

The arrangement of the pore openings serves as a template for the positions of the branchings in the isomerization of n‐alkanes—this is the explanation proposed by Martens et al. (Angew. Chem. Int. Ed. Engl. 1995, 34, 2528) to rationalize their results in the isomerization of n‐heptadecane on the zeolite ZSM‐22. This reaction would thereby be the first example of key–lock catalysis on the external surface of a zeolite.
New, Efficient Routes to Cyclic Enediynes
Authors: König Burkhard
Published: January 2004

The search for simple and pharmacologically active enediynes should be considerably facilitated by a new sequence leading to acyclic and also cyclic enediynes, which proceeds via a carbenoid intermediate. The recently reported intramolecular variant yields functionalized enediynes on a multigram scale from readily accessible starting materials in yields of up to 95%.
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

For corrigendum see DOI:10.1002/anie.199607811
Shape‐Selective Catalysis with Microporous Amorphous Mixed Oxides
Authors: Maier Wilhelm F. , Martens Johan A. , Klein Stephan , Heilmann Jens , Parton Rudy , Vercruysse Karen , Jacobs Pierre A.
Published: January 2004

Ti‐ and V‐containing amorphous glasses with inner surfaces of several hundred m2g−1 and pore diameters of 0.7 nm can be prepared by a sol‐gel process. Like zeolites, these amorphous microporous mixed oxides AMM‐MxSi show catalytic shape‐selectivites. The epoxidation of alkenes with tert‐butylhydroperoxide on Ti‐containing glasses is catalyzed size‐selectively. In the hydrocracking test of decane, these materials show a product distribution that indicates the presence of Brønstedt centers in a shape‐selective environment, as well as tubular pores, the lack of additional cavities, and an effective pore size comparable to those of 10‐ and 12‐ring zeolites.
Asymmetric Dihydroxylation Enables Rapid Construction of Chiral Dendrimers Based on 1,2‐Diols
Authors: Chang Han‐Ting , Chen Chien‐Tien , Kondo Teruyuki , Siuzdak Gary , Sharpless K. Barry
Published: January 2004

Ninety chiral centers and a molecular mass of 11354 Da are the outstanding features of the largest C3‐symmetric, chiral dendrimer to date. As general approach to these chiral polyether compounds (1) styrenes or stilbenes are converted into 1,2‐diols by asymmetric dihydroxylation; (2) the resulting building blocks are then linked together to form dendrons by a uniquely effective ether‐forming reaction, in which the vincinal hydroxyls are alkylated with benzylic bromides by using KOH pellets in refluxing toluene; (3) the dendrons are coupled to a core unit such as 1,3,5‐benzene‐tricarbonyl trichloride.
A Sustainable Catalyst for the Partial Oxidation of Methane to Syngas: Ni/Ca1‐xSrxTiO3, Prepared In Situ from Perovskite Precursors
Authors: Hayakawa Takashi , Harihara Hideaki , Andersen Arnfinn G. , York Andrew P. E. , Suzuki Kunio , Yasuda Hiroyuki , Takehira Katsuomi
Published: January 2004

High activity and negligible coke formation in the course of the catalytic partial oxidation of CH4 to syngas are characteristics of the Ni/Ca4‐xSrxTiO3 catalyst, which is readily accessible by the citrate method from nickel nitrate, alkaline earth metal carbonates, and tetraisopropoxidotitanium. The properties and efficiency of the catalyst are normally only associated with catalysts that contain considerably more expensive metals such as Rh, Ir, and Ru.
Poly(A·A·U) Triple Helix Formation Promoted by Porphyrin Assembly
Authors: Lauceri Rosaria , Campagna Tiziana , Contino Annalinda , Purrello Roberto
Published: January 2004

A multicomponent molecular recognition process leads to a self‐assembled “quaternary” supramolecular complex formed from porphyrins and an “unusual” poly(A·A·U) triple helix. The formation of this superstructure is promoted by porphyrin aggregation and a number of recognition processes (Watson‐Crick and Hoogsteen pairing, coulombic interactions, and stacking interactions).
Metallophosphaalkenes—from Exotics to Versatile Building Blocks in Preparative Chemistry
Authors: Weber Lothar
Published: January 2004

The 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.
Pericyclic Reactions in Biological Systems—Does Nature Know About the Diels—Alder Reaction?
Authors: Laschat Sabine
Published: January 2004

Not only synthetic chemists, but also nature makes use of the spontaneous Diels‐Alder reaction. Now there is also increasing evidence for a Diels–Alder‐ase, for example in the biosynthesis of solanapyrones. The search for an enzyme‐catalyzed [4 + 2]cycloaddition in nature will therefore certainly continue.
Tris(chalcogenato)carbenium Ions [C(XR)3]+(X  O, S, Se, Te): An Experimental and Quantum‐Chemical Comparison
Authors: Ohlmann Dietmar , Marchand Christina M. , Grützmacher Hansjörg , Chen Grace Shiahuy , Farmer David , Glaser Rainer , Currao Antonio , Nesper Reinhard , Pritzkow Hans
Published: January 2004

[C(OR)3]+ions are stabilized differently than [C(XR)3]+ions (X = S, Se, Te).While the stability of the RO‐substituted carbenium ions can be attributed to the high polarity of the Cδ+‐Oδ−bond, the polarity of the C‐X bond in the homologous S‐, Se‐, and Te‐containing ions is reversed, and the stabilization is achieved by the σ‐and π‐dative effect of the chalcogen centers. These results were obtained by ab initio calculations on [C(XH)3]+ions and by experimental studies on the new compounds [C(XR)3][PF6] (X = S, Se, Te; R = 2,4,6‐iPr3C6H2).
Control of the Structure and Functions of Biomaterials by Light
Authors: Willner Itamar , Rubin Shai
Published: January 2004

Vision 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.
Transition Metal Stabilized AlI and GaI Complexes; Structure of [(CO)5CrGa{(CH3)(tmeda)}]
Authors: Schulte Markus M. , Herdtweck Eberhardt , Raudaschl‐Sieber Gabriele , Fischer Roland A.
Published: January 2004

Redox reactions in the initial products formed from K2[Cr(CO)5] and Cl2ER (E = Al, Ga; R = Cl, alkyl) lead to the formation of aluminum(I) and gallium(I) transition metal complexes of type 1 (shown below) that are surprisingly stable. Stoichiometric CrGa films were prepared from 1 (E = Ga, R = CH2CH3) by MOCVD. tmeda = N,N,N′,N′,‐tetramethylethylenediamine.
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Synthesis of a Stable Hexagonally Packed Mesoporous Niobium Oxide Molecular Sieve Through a Novel Ligand‐Assisted Templating Mechanism
Authors: Antonelli David M. , Ying Jackie Y.
Published: January 2004

Since the porous structure is completely retained upon surfactant removal, Nb‐TMS1, a hexagonally packed mesoporous transition metal oxide, is an interesting molecular sieve. The synthesis of this material was accomplished by a novel route in which the inorganic precursor 1 and the surfactant 2 were attached by a covalent NbN bond; this bond is probably retained throughout the course of the synthesis.
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The Schenck Ene Reaction: Diastereoselective Oxyfunctionalization with Singlet Oxygen in Synthetic Applications
Authors: Prein Michael , Adam Waldemar
Published: January 2004

Oxyfunctionalized 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.
Voids in Variable Chemical Surroundings: Mesoporous Metal Oxides
Authors: Behrens Peter
Published: January 2004

Structural design in the synthesis of solids is now a step closer, at least for mesoporous compounds. Surfactant assemblies are used as templates to structure metal oxides, which are formed from metal alkoxides. By adapting the headgroup of the surfactant to the inorganic component and exploiting the coordination chemistry of the metal atoms in the alkoxides, a wide range of mesoporous metal oxides, including alumina, titania, niobia, and zirconia, becomes available.
Friedrich Hund and Chemistry
Authors: Kutzelnigg Werner
Published: January 2004

Friedrich 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

Scanning 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

“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 Manshouri

M. 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.
Wittig Olefination of Lactones
Authors: Lakhrissi Mohammed , Chapleur Yves
Published: January 2004

For corrigendum see DOI:10.1002/anie.199628811
Superconductivity in Intercalated and Substituted Y2Br2C2
Authors: Bäcker Michael , Simon Arndt , Kremer Reinhard K. , Mattausch Hans‐Jürgen , Dronskowski Richard , Rouxel Jean
Published: January 2004

Layer compounds of the type Y2X2C2 (X = Cl, Br, I) show superconductivity at temperatures between 2.3 and 11.2 K. The transition temperature is related to the tendency of conduction electrons toward pairwise localization in C2* states at the Fermi level, and changes with the concentration of valence electrons, which is varied by intercalation of Na.
Schellman Motif in Dehydrooligopeptides: Crystal and Molecular Structure of Boc‐Val‐ΔPhe‐Leu‐Phe‐Ala‐ΔPhe‐Leu‐OMe
Authors: Mal Tapas Kumar , Jain Ratan Mani , Chauhan Virander Singh , Rajashankar Kanagalaghatta Ramabhatta , Ramakumar Suryanarayanarao
Published: January 2004

Secondary structural motifs found in peptides are also generated in model compounds containing α,β‐didehydrophenylalanine residues (ΔPhe). A short peptide containing two ΔPhe units, Boc‐Val‐ΔPhe‐Leu‐Phe‐Ala‐ΔPhe‐Leu‐OMe, was found to assume a 310/α‐helical conformation terminated by a Schellman motif.
The “Bare” Uranyl(2+) Ion, UO
Authors: Cornehl Hans H. , Heinemann Christoph , Marçalo Joaquim , de Matos António Pires , Schwarz Helmut
Published: January 2004

Ion–molecule reactions between U2+ and oxygen donors or “charge‐stripping” collisions between singly charged UO ions and O2 collision partners generate uranyl(2 +) ions in the gas phase. These do not readily dissociate into singly charged fragments. The standard enthalpy of formation for UO is estimated to be 371 ± 60 kcalmol−1, in accord with the results of ab initio calculations.
The Mechanism of the Claisen Rearrangement: Déjà Vu All Over Again
Authors: Ganem Bruce
Published: January 2004

The 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

Understanding 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.
Hückel Arenes with Ten π Electrons: Cyclic Zintl Anions Si and Ge, Isosteric to P and As
Authors: von Schnering Hans Georg , Bolle Urs , Curda Jan , Peters Karl , Carrillo‐Cabrera Wilder , Somer Mehmet , Schultheiss Martin , Wedig Ulrich
Published: January 2004

A new Zintl anion X (X = Si, Ge) appears in the compounds Ba4Li2Si6, B4Li2Ge6, and Ba10Ge7O3. Bonding within the anion and its interaction with the crystal field is described in terms of the electron localization function (ELF), which allows the visualization of covalent bonds, n electron pairs, and core shells. The existence of such cyclic anions indicates that in the solid‐state structures of heavier Group 14 elements the formation of ten π electron Hückel arenes is preferred over the formation of discrete double bonds.
Charge Transfer Interactions in Dyed Crystals of Aromatic Carboxylic Acids and Their Relevance to MALDI Mass Spectrometry
Authors: Mitchell Christine A. , Lovell Scott , Thomas Keith , Savickas Phil , Kahr Bart
Published: January 2004

A new facet of an old theme! Dyed crystals of aromatic carboxylic acids, a common topic in the scientific literature 100 years ago, are rarely discussed today. These mixed crystals are now reintroduced as superior model systems for studying the host–guest interactions that are so important in MALDI mass spectrometry. Orientations for included dyes are determined through polarization spectroscopy. The role of charge transfer is indicated by the X‐ray structure of Malachite Green · hydrogen phthalate.
Free‐Radical Carbonylations: Then and Now
Authors: Ryu Ilhyong , Sonoda Noboru
Published: January 2004

Although 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.
SiH and CH Activation by Transition Metal Complexes: A Step Towards Isolable Alkane Complexes?
Authors: Schneider Jörg J.
Published: January 2004

The recognition of the fundamental contributions by G. A. Olah on the elucidation of the structure of nonclassical carbocations, in the form of the award of the Nobel prize for chemistry, has recently emphasized the importance of electron‐deficient bonds in the understanding of chemical bonding in organic chemistry. In the field of coordination chemistry, the formulation of electron‐deficient bonds has been used for some time to describe nonclassical interactions between atoms. Traditional ligands in coordination chemistry such as amines and phosphanes bond to metal centers through their lone pair of electrons. Synergistic bonding effects dominate in the coordination of π‐bonded ligands such as alkenes. In the mid‐1980s the discovery of dihydrogen complexes having side‐on coordination of H2 gave fresh impetus to transition metal chemistry as well as to the understanding of the interaction of σ‐coordinating ligands with transition metals. In the meantime, transiton metal complexes can be obtained with a variety of σ‐coordinated X‐H fragments, and their mode of bonding can be understood by a common and quite general model. The chemistry of σ‐bound silane ligands is particularly varied and well‐investigated. These silane ligands enable the investigation of a large range of σ‐coordinated metal complex fragments up to complete oxidative addition with cleavage of the SiH bond and formation of silyl(hydrido) complexes, which has thus also widened our general understanding of the bonding of other σ‐bound ligands. Whilst there is a large range of isolable and stable H2 and SiR4 complexes available, there are no such alkane analogues known at present. Only when the CH bond is part of a ligand that is already directly bonded to the transition metal center will the resulting chelate effect stabilize this agostic C‐H‐M interaction. The complexation of SiH4, the simplest heavier homologue of CH4, was achieved recently. This is a further step towards the understanding of the factors which govern σ‐complexation of ligands at transition metal centers.
Formation of FeIV and NiIV by Electrochemical and Chemical Oxidation of an Iron‐Substituted Nickel(II) Hydroxide: The Direct Two‐Electron Step NiII → NiIV + 2e−
Authors: Axmann Peter , Erdbrügger Christina Freire , Buss Dieter H. , Glemser Oskar
Published: January 2004

Six NiIV ions make up the coordination environment of each FeIV ion in the γ‐NiOOH‐like layer of the (electrochemically) oxidized form of the iron‐substituted nickel(II) hydroxide 1, as can be deduced from Mössbauer spectroscopic and electrochemical investigations. No NiIII ions are present in the redox‐active material. The direct two‐electron step NiII → NiIV + 2e− must be assumed.
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Application of π‐Arene–Ruthenium Complexes in Peptide Labeling and Peptide Synthesis
Authors: Krämer Roland
Published: January 2004

The potential of bioorganometallic chemistry as a new interdisciplinary research area is illustrated impressively by organoruthenium complexes with peptide ligands. Ruthenium‐π‐arene fragments are of interest both as markers for peptides that can be readily introduced and as promoters of complex synthetic transformations such as the preparation of cyclic peptide antibiotics and of linear oligopeptides in a one‐pot procedure.
Molybdenum Blue: A 200 Year Old Mystery Unveiled
Authors: Müller Achim , Meyer Jochen , Krickemeyer Erich , Diemann Ekkehard
Published: January 2004

“This is a substance or group of substances about which there has been much discussion” is a statement we read in an older standard textbook of inorganic chemistry with reference to a (usually) amorphous blue material that was first mentioned by C. W. Scheele in 1778 and discussed by J. J. Berzelius in 1826, and that today is known as “molybdenum blue”. Now based on the knowledge of compound 1 with an archetypal structure, the more than 200 year old mystery regarding the nature of this multifunctional material with its interesting properties has been partly unveiled.
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