bluuebunny/crossref_metadata_2025 huggingface dataset:
Solid acids and their use as environmentally friendly catalysts in organic synthesis
Authors: Wilson Karen , Clark James H.
Published: October 2007

Tightening environmental legislation is driving the fine and speciality chemicals industries to consider alternative processes that avoid the use of conventional mineral acids. The use of heterogeneous catalysts in these processes would vastly simplify catalyst removal, minimizing the amount of waste formed. However, diffusion limitation of liquids within porous solids dictates that effective solid acids for liquid-phase reactions require the use of mesoporous materials <20_100Å. Recent developments in materials chemistry has led to the discovery of a family of ordered mesoporous silicas which opens up new possibilities for preparing solid-acid catalysts for liquid-phase reactions. This review concentrates on recent developments in the synthesis of new mesoporous solid acids for liquid-phase organic synthesis.
Environment-friendly organic synthesis. The photochemical approach
Authors: Albini Angelo , Fagnoni Maurizio , Mella Mariella
Published: October 2007

Photochemical reactions are expected to have an increasing role in organic synthesis with the drive towards environment-friendly reactions. Some examples illustrating the scope and versatility of radical alkylation of electrophilic alkenes via photoinduced electron transfer are presented. A few applications in special fields are mentioned.
Frontiers in green chemistry utilizing carbon dioxide for polymer synthesis and applications
Authors: Young Jennifer L. , DeSimone Joseph M.
Published: October 2007

In the last ten years, there has been incredible growth in research involving the use of carbon dioxide as an environmentally benign solvent. This article will highlight polymer synthesis, characterization, and applications in CO2 and place this research in the context of both green chemistry and the latest developments in CO2 technology.
The environmental issue. A challenge for new generation polyolefins
Authors: Romano Ugo , Garbassi Fabio
Published: October 2007

The recent developments in the field of polyolefin technologies is reviewed in the light of the positive impact that they will have on several aspects of environmental issues.
Phase-transfer catalysis. A general green methodology in organic synthesis
Authors: Makosza Mieczyslaw
Published: October 2007

Basic concept of phase-transfer catalysis (PTC), its field of applications and specific features as the most general, efficient, and environment-friendly green methodology of organic synthesis, particularly for industrial processes, is discussed.
Quantum chemistry in the 21st century (Special topic article)
Authors: Barden Christopher J. , Schaefer Henry F.
Published: October 2007

Quantum chemistry is the field in which solutions to the Schrödinger equation are used to predict the properties of molecules and solve chemical problems. This paper considers possible future research directions in light of the discipline's past successes. After decades of incremental development—accompanied by a healthy dose of skepticism from the experimental community—the ready availability of fast computers has ushered in a "golden age" of quantum chemistry. In this new era of acceptance, theoretical predictions often precede experiment in small molecule chemistry, and quantum chemical methods play an ever greater role in biochemical and other larger systems. Quantum chemists increasingly divide their efforts along three fronts: high-level (spectroscopic) accuracy for small molecules, characterized by such techniques as Brueckner methods, r12 formalisms, and multireference calculations; parameterization- or extrapolation-based intermediate-level schemes (such as Gaussian-N theory) for medium molecules; and lower-level (chemical) accuracy for large molecules, characterized by density functional theory and linear scaling techniques. These tools, and quantum chemistry as a whole, are examined here from a historical perspective and with a view toward their future applications.
Guidelines for terms related to chemical speciation and fractionation of elements. Definitions, structural aspects, and methodological approaches (IUPAC Recommendations 2000)
Authors: Templeton Douglas M. , Ariese Freek , Cornelis Rita , Danielsson Lars-Göran , Muntau Herbert , van Leeuwen Herman P. , Lobinski Ryszard
Published: October 2007

This paper presents definitions of concepts related to speciation of elements, more particularly speciation analysis and chemical species. Fractionation is distinguished from speciation analysis, and a general outline of fractionation procedures is given. We propose a categorization of species according to isotopic composition of the element, its oxidation and electronic states, and its complex and molecular structure. Examples are given of methodological approaches used for speciation analysis. A synopsis of the methodology of dynamic speciation analysis is also presented.
Internet and other sources of methods for the assessment of workplace air quality (Technical report)
Authors: Brown R. H.
Published: October 2007

Methods for assessing occupational exposure to toxic chemicals have been developed semi-independently by a number of standards organizations and regulatory authorities in Europe and beyond. Few of these have been systematically reviewed against Comité Europeán de Normalisation (CEN) criteria for accuracy and reliability. Some of these methods have been complied into indexes, either in hard copy or on the Internet, but existing indices are incomplete. There is a clear need for the harmonization and validation of these methods and for a full compendium of available methods and their validation status, while avoiding duplication of essentially equivalent procedures. Many of the methods are accessible on the Internet, so that in such instances the compendium need not contain full details of the methods themselves but can provide links to the pertinent web sites.
Designing efficient synthetic routes to polyfunctionality
Authors: Katritzky Alan R. , Denisko Olga V.
Published: October 2007

Benzotriazole-mediated methodology was successfully applied for the preparation of many classes of polyfunctional organic compounds.
New developments in radical chemistry. Applications to total synthesis and asymmetric processes
Authors: Aïssa Christophe , Delouvrié Bénédicte , Dhimane Anne-Lise , Fensterbank Louis , Malacria Max
Published: October 2007

This article reviews our contribution in the field of radical synthetic chemistry, focusing on cascades relying on the use of propargylic BMDMS ethers and asymmetric synthesis using enantiopure vinylsulfoxides.
Stereoselective C,C-bond formation. Cyclizations of biradicals
Authors: Giese Bernd , Barbosa Frédérique , Stähelin Christian , Sauer Stefan , Wettstein Philipp , Wyss Caroline
Published: October 2007

The formation of C,C-bonds via photolytically generated biradicals can occur with high stereoselectivity. If triplet biradicals are involved, chiral induction is highly likely. Syntheses that occur only via singlet biradicals have a good chance to show a memory effect of chirality.
Total synthesis of herboxidiene, a complex polyketide from Streptomyces species A7847
Authors: Banwell Martin , McLeod Malcolm , Premraj Rajaratnam , Simpson Gregory
Published: October 2007

A formal total synthesis of the polyketide herboxidiene (1) has been achieved by Horner-Wittig coupling of the side-chain fragment 12-epi-(2) with the tetrahydropyran-2-acetic acid derivative (3) followed by desilylation of the resulting triene (19) and hydroxyl-directed mono-epoxidation of the ensuing bis-homoallylic alcohol (20).
Synthetic studies toward pyranonaphthoquinone antibiotics
Authors: Brimble Margaret A.
Published: October 2007

A furofuran annulation/oxidative rearrangement strategy was used to construct the basic skeleton of the pyranonaphthoquinone family of antibiotics. This synthetic methodology has been applied to the synthesis of the spiroacetal-containing pyranonaphthoquinone antibiotic griseusin A, to an analog of the C-glycoside medermycin, and to a dimeric pyranonaphthoquinone.
Fluorous methods for synthesis and separation of organic molecules
Authors: Curran Dennis P.
Published: October 2007

Fluorous molecules partition out of an organic phase and into a fluorous (highly fluorinated) phase in a liquid-liquid extraction. New fluorous techniques allow simple yet substantive separations of organic reaction mixtures based on the presence or absence of a fluorous tag. Fluorous-tagged molecules can also be separated from nontagged molecules by solid phase extraction over fluorous reverse-phase silica gel. This technique is ideal for solution-phase parallel synthesis because it allows simple yet substantive separations of organic reaction mixtures.
Hypervalent siloxane derivatives. An alternative to Stille and Suzuki couplings
Authors: DeShong Philip , Handy Christopher J. , Mowery Molly E.
Published: October 2007

Siloxane methodology is extended to couplings with iodo- and bromoanilines, hindered arylbromides, and substituted siloxanes as substrates. Additionally, initial studies with 5-bromoindole and other heteroaromatics suggest that the siloxane-mediated cross coupling methodology will also be applicable in these systems.
Synthetic studies on the A83586C and bryostatin antitumor macrolides and the monamycin antibiotics
Authors: Hale K. J. , Hummersone M. G. , Cai J. , Manaviazar S. , Bhatia G. S. , Lennon J. A. , Frigerio M. , Delisser V. M. , Chumnongsaksarp A. , Jogiya N. , Lemaitre A.
Published: October 2007

After a brief summary of our asymmetric total syntheses of A83586C and 4-epi-A83586C, we will go on to describe some of our synthetic work on the monamycins, and our most recent total synthesis studies on the bryostatin antitumor macrolides.
Stereoselective and enantioselective synthesis of five-membered rings via conjugate additions of allylsulfone carbanions
Authors: Hassner Alfred , Ghera Eugene , Yechezkel Tamar , Kleiman Victoria , Balasubramanian Thiagarajan , Ostercamp Daryl
Published: October 2007

This lecture describes some of our studies of lithio derivatives of allyl sulfone carbanions which add α-regioselectively as well as anti diastereoselectively to Michael acceptor olefins. This can be ascribed to chelation in the Michael addition step. When the reaction leads to subsequent ring closure by using a bromoallyl sulfone, the latter acts as a methylenemethane synthon in a (3+2) Michael-initiated ring closure, affording highly functionalized cyclopentane derivatives. Such additions proceed with high stereoselectivity and with asymmetric induction leading to nonracemic substituted cyclopentanones. Additions of allyl sulfone carbanions also proceed stereoselectively to C=N systems containing a chiral auxiliary on N. These can be used in the synthesis of optically active five- and six-membered ring N-heterocycles. Furthermore, chiral groups on the allyl sulfone moiety can induce significant remote asymmetric induction, made possible by the presence of an aromatic π-system which promotes intramolecular chelation to the Li cation.
Ring constructions by the use of fluorine substituent as activator and controller
Authors: Ichikawa Junji
Published: October 2007

By using the properties of fluorine such as electronic effects and leaving-group ability, two types of ring-forming reactions have been achieved starting from fluoroolefins: (i) fluorinated vinyl ketones with a vinyl and/or an aryl group, which undergo fluorine-directed and/or -activated Nazarov, Friedel-Crafts, and tandem cyclizations in their combination to construct highly functionalized and fused ring systems and (ii) gem-difluoroolefins bearing a nucleophilic center on the carbon δ to the flourines undergo intramolecular substitution for the fluorine via "anti-Baldwin" 5-endo-trig closures leading to ring-fluorinated heterocycles. Throughout these reactions, fluorines function as an activator of the substrates and a controller over the reaction pathways.
Stereocontrolled asymmetric synthesis
Authors: Kim Yong Hae , Kim Sam Min , Park Doo Han , Youn So Won
Published: October 2007

Stereo differentiated asymmetric syntheses have been achieved by S-indoline derivations. Diels-Alder cycloadditions of S-indoline chiral acrylamides with cyclopentadiene proceed with high diastereofacial selectivity, giving either endo-R or endo-S products depending on Lewis acid and the structures of chiral dienophiles. Diastereo- and enantio-selective pinacol coupling reactions of chiral α-ketoamides mediated by samarium diiodide afforded extremely high diastereoselectivities. Enantiopure (S,S)- or (R,R)-2,3-dialkyltar-taric acid and derivatives can be synthesized for the first time depending on the structure of α-ketoamides.
Selective transformations mediated by main-group organometallics
Authors: Knochel Paul , Boudier Andreas , Bromm Lars O. , Hupe Eike , Varela Jesús A. , Rodriguez Alain , Koradin Christopher , Bunlaksananusorn Tanasri , Laaziri Hamid , Lhermitte Frédéric
Published: October 2007

Main-group organometallics are useful intermediates for the chemoselective and diastereoselective C-C bond formation. The boron-zinc exchange is a unique way for preparing chiral secondary alkylzinc reagents which are configurationally stable over a wide temperature scale. Coupled with the thermal rearrangement of tertiary organoboranes, a broad range of open-chain and cyclic polyfunctional molecules have been prepared. In addition, several examples of a diastereoselective remote C-H activation have been studied. The second part shows that the main-group metallic alkoxides such as t-BuOK can be used for the performance of catalytic reactions such as a new indole synthesis.
New nucleoside analogs, synthesis, and biological properties
Authors: Wanner M. J. , Deghati P. Y. F. , Rodenko B. , Koomen G. J.
Published: October 2007

In view of the importance of nucleoside analogs as enzyme inhibitors and adenosine receptor (ant)agonists, new adenosine analogs were prepared and their activities studied on adenosine deaminase in vitro and on A1, A2A, and A3 receptors. Particularly useful for the synthesis of new analogs were the applications of the Pd-catalyzed Buchwald reaction and a radical nitration reaction of purine- and 1-deazapurine nucleosides.
Reactions involving inorganic compounds
Authors: Krief Alain , Delmotte Cathy , Colaux-Castillo Catherine
Published: October 2007

Selenium chemistry became, over the last 30 years, particularly useful for synthetic organic chemistry [1]. Inorganic as well as organic selenium compounds allow transformation which otherwise cannot be done or require much more drastic conditions to proceed. We have over the last 25 years explored the reactivity of elemental selenium as well as its inorganic and organic derivatives. We report here our recent finding concerning (i) organic diselenols and -diselenolates and (ii) the role of selenoxides in the enantioselective dihydroxylation of C,C double bonds using catalytic amounts of osmium tetroxide.
Titanacyclopropanes as versatile intermediates for carbon-carbon bond formation in reactions with unsaturated compounds
Authors: Kulinkovich O. G.
Published: October 2007

Dialkoxytitanacyclopropane intermediates [or titanium (II)-olefin complexes] generated in situ from ethylmagnesium bromide and titanium (IV) isopropoxide react with allylic alcohols and allylic ethers to afford SN2' allylic ethylation products. The reaction proceeds with high regioselectivity and with low to high trans-/cis-stereoselectivity. This observation and others suggest a reaction mechanism involving an EtMgBr-initiated formation of titanacyclopentane ate complex 10 from titanacyclopropane-olefin complex 7 as a key step. Based on this assumption, a modified mechanism of titanium-mediated cyclopropanation of esters with Grignard reagents is proposed.
Oxazoline-N-oxide mediated asymmetric cycloadditions. Recent progress in the stereo-selective syntheses of β-lactones and β-lactams
Authors: Dirat O. , Kouklovsky C. , Mauduit M. , Langlois Y.
Published: October 2007

Camphor-derived oxazoline-N-oxides are versatile dipoles in a new kind of asymmetric [2+3] cycloadditions. Recent applications of this methodology allowed the stereoselective syntheses of several b-lactones natural products such as 1233A and tetrahydrolipstatine. Two formal syntheses of b-lactams antibiotics, b-methyl thienamycin and carpetimycin A, have also been achieved using this type of cycloaddition.
Gary A. Molander
Authors: Molander Gary A.
Published: October 2007

Metallocene complexes of the Group 3 metals and lanthanides have been utilized for cyclization/silylation reactions of polyunsaturated organic substrates. Acyclic dienes, trienes, enynes, and dienynes can be selectively cyclized to provide five- and six-membered rings in high yields.
Anionic cyclization approach toward perhydroindoles. Total synthesis of montanine-type Amaryllidaceae alkaloids
Authors: Sha C.-K. , Huang S.-J. , Huang C.-M. , Hong A.-W. , Jeng T.-H.
Published: October 2007

Hexahydro-1H-indol-3-one can be used as a building block for alkaloid synthesis. Radical and anionic cyclization approaches toward this useful structure were developed. Approaches toward total synthesis of montanine-type Amaryllidaceae alkaloids using hexahydro-1H-indol-3-one as a key intermediate were studied.
Stereochemical support for Serratia endonuclease active-site geometry
Authors: Stec W. J. , Koziolkiewicz M. , Taira K.
Published: October 2007

Earlier observation that stereoregular All-Rp-PS-oligonucleotides in the presence of nonsequence-specific Serratia endonuclease undergo nucleolytic degradation, confronted with recently published results on the active-site architecture of this enzyme, strongly supports an involvement of 3'-bridging and pro-Sp-nonbridging oxygen atoms of scissile internucleotide bond in the interactions with hydrated magnesium ion anchored by Asn-119 residue of this endonuclease.
Synthetic study of ravidomycin, a hybrid natural product
Authors: Suzuki Keisuke
Published: October 2007

Strategies and tactics associated with the total synthesis of hybrid natural products are discussed. The target is ravidomycin (2), one of the gilvocarcin-class antitumor antibiotics with an aryl C-glycoside structure. The first total synthesis of 2, which was achieved along similar lines of that of gilvocarcin V (1), served for the determination of the relative as well as the absolute stereochemistry of 2. Also revealed was a limitation of the synthetic scheme so long as the amino sugar congener was concerned. A preliminary result is discussed on the [2+2+2] approach that relies on the ready availability of various benzocyclobutene derivatives via regioselective [2+2] cycloaddition of α-alkoxybenzynes and ketene silyl acetals.
Challenges and opportunities in solid-state chemistry
Authors: DiSalvo Francis J.
Published: October 2007

We are largely unable to predict the composition and crystal structure of new extended structure compounds. Yet we find that as materials become more complex in stoichiometry and structure, new, interesting, and perhaps useful, phenomena are encountered. The need for improved experimental and theoretical approaches to seize more rapidly the opportunities provided in this complexity is discussed.
Thermodynamics of structured fluids. Hard science for soft materials
Authors: Prausnitz John M.
Published: October 2007

At liquid-like densities, molecules of complex fluids can assume a variety of structures (or positions) in space; when the molecules contain many atoms as, for example, in polymers, that variety becomes very large. Further, when confined to a narrow space, it is possible to achieve structures that are not normally observed. Thanks to recent advances in statistical mechanics and molecular physics, and thanks to increasingly fast computers, it is now possible to calculate a fluid's structure, that is, the positions of molecules at equilibrium under given conditions. Calculation of fluid structure is useful because thermodynamic properties depend strongly on that structure, leading to possible applications for new materials. Three examples illustrate some recent developments; each example is presented only schematically (with a minimum of equations) to indicate the physical basis of the mathematical description. The first example considers the effect of branching on self-assembly (micellization) of copolymers (with possible long-range applications in medicine). The second and third examples consider the effect of confinement on fluid structure: first, crystallization in a narrow, confined space to produce a desired crystal structure (with possible applications for light-emitting diodes) and second, suppression of micellization of a diblock copolymer in a thin film (with possible application in lithography). Whenever possible, theoretical calculations are compared with experimental results.
Thermochemical studies of advanced nuclear fuels and materials
Authors: Yamawaki Michio , Inoue Tadashi , Ogawa Toru
Published: October 2007

Thermochemical analyses related with development of advanced nuclear fuels (i.e., metalic, nitride, and hydride fuels) with respect to the transmutation of transuranium elements and relevant pyrochemical reprocessing and fabrication processes, which have been carried out mainly in Japan, are summarized. In addition, application of atmosphere-controlled high-temperature mass spectrometry on solid-gas reactions, in particular on the prediction of gas-interacting behaviors of cesium and lithium on respective oxide ceramics, are delineated in relation to nuclear material-relevant researches.
Hydrocarbon oxidation high-temperature chemistry
Authors: Warnatz Jürgen
Published: October 2007

The exact knowledge of hydrocarbon oxidation kinetics is very important due to the fact that this process is involved in many technological processes: combustion in engines and furnaces, flame synthesis of materials, partial oxidation processes in chemical technology, catalytic combustion, and exhaust gas treatment, etc. An overview is given on the present state of the art with respect to kinetic data on gas-phase and (shortly) surface oxidation of hydrocarbons. Furthermore, some applications are described in the areas mentioned above. Examples for the importance of the gas-phase oxidation of hydrocarbons are ignition and combustion in engines and furnaces and partial oxidation processes in industrial chemical reactors. In many applications, both gas-phase and surface chemistry are taking place. Examples here are flame generation of diamonds and syngas generation.
Properties of high-temperature melts using levitation
Authors: Nordine Paul C. , Weber J. K. Richard , Abadie Johan G.
Published: October 2007

Containerless conditions allow well-controlled investigation of liquids at high temperatures. Levitation methods used for this purpose are reviewed, and their application is illustrated by discussion of the properties and behavior of deeply undercooled yttrium-aluminum-oxide melts.
Chemical kinetics at solid-solid interfaces
Authors: Schmalzried Hermann
Published: October 2007

The kinetics of solid-solid interfaces controls in part the course of heterogeneous reactions in the solid state, in particular in miniaturized systems. In this paper, the essential situations of interface kinetics in solids are defined, and the basic formal considerations are summarized. In addition to the role interfaces play as resistances for transport across them, they offer high diffusivity paths laterally and thus represent two-dimensional reaction media. Experimental examples will illustrate the kinetic phenomena at static and moving boundaries, including problems such as exchange fluxes, boundary-controlled solid-state reactions, interface morphology, nonlinear phenomena connected with interfaces, and reactions in and at boundaries, among others.
Localization and bandwidth of the 3d-orbitals in magnetic Ni and Co clusters
Authors: Morenzin J. , Kietzmann H. , Bechthold P. S. , Ganteför G. , Eberhardt W.
Published: October 2007

Photoelectron spectra of negatively charged Nin
- , Cun
-, and Con
- clusters are presented. The spectra of the Nin
- clusters contain information about the degree of localization of the 3d valence orbitals, which is strongly related to the magnetic properties. In the clusters with less than 7 atoms, the 3d orbitals seem to be almost totally localized. For Co13
- we find indications of a narrowing of the 3d-derived valence "bandwidth" corresponding to a presumed high symmetry and a high degree of degeneracy of the 3d electronic states.
Transport processes in metal halide gas discharge lamps
Authors: van Erk W.
Published: October 2007

An overview is given of transport reactions and corrosion phenomena in metal halide gas discharge lamps filled with a mixture of alkali halides with scandium or rare-earth iodides. The phenomena that are discussed are: (a) pressures of light-emitting species above the molten salt mixture, (b) interaction of the metal halides with the vessel wall, quartz glass as well as polycrystalline alumina, (c) transport processes along the vessel wall giving rise to wall corrosion, and (d) transport to and from the electrodes (i.e., electrode corrosion and wall blackening).
High-temperature chemical vapor deposition. An effective tool for the production of coatings
Authors: Wahl G. , Stadel O. , Gorbenko O. , Kaul A.
Published: October 2007

Chemical vapor deposition (CVD) processes have a large throwing power and can operate at atmospheric pressure. Therefore, they are economical for many applications. High-temperature CVD processes give a good control of the crystal structure (e.g., epitaxial layers or other special layer structures) and support diffusion processes. The are used, for example, for the following deposition processes: 1) perovskites, 2) yttrium-stabilized ZrO2 layers as ion conductors or heat barrier coatings, 3) aluminide diffusion coatings in long tubes for corrosion protection, and 4) BN on fibers for fiber-reinforced materials. The special properties of the perovskite CVD are discussed in more detail. These processes are described for small- and large-scale applications. The CVD process is simulated by the computer code Fluent.
Modeling and experimental studies on the effect of thermophysical properties on field-activated combustion synthesis reactions
Authors: Munir Z. A.
Published: October 2007

The influence of thermal and electrical conductivities and relative density on the dynamics of self-sustaining reactions activated by an electric field was investigated by modeling. Under a given field, the variation of the wave velocity and maximum temperature with normalized electrical and thermal conductivities and density exhibited a maximum at an optimum value of each of the three parameters. The results are discussed in terms of a Fourier relationship modified by the Joule heat contribution of the imposed field. A simulation was also made to investigate the configuration of the reaction front in systems that are ignited and sustained by a current. A transition from a volume (uniform) combustion reaction to a wavelike form was found to occur as the sample size is increased. These changes are also attributed to the interrelation between the thermophysical parameters.
Mechanism of Fe(OH)2+(aq) photolysis in aqueous solution(Technical Report)
Authors: Pozdnyakov Ivan P. , Glebov Evgeny M. , Plyusnin Victor F. , Grivin Vyacheslav P. , Ivanov Yuri V. , Vorobyev Dmitry Yu. , Bazhin Nikolai M.
Published: October 2007

Experiments on laser flash photolysis (308 nm) of Fe(OH)2+(aq) complex in aqueous solution with addition of nitrobenzene demonstrate the formation of hydroxyl radical in the primary photochemical process.
Foliar Interception and Retention Values after Pesticide Application. A Proposal for Standardized Values for Environmental Risk Assessment (Technical Report)
Authors: Linders J. , Mensink H. , Stephenson G. , Wauchope D. , Racke K.
Published: October 2007

In performing risk assessments for plant protection products by applicants or regulators in relation to the registration of the products, an important aspect to take into account is the foliar interception and retention of the active substance of the product on the plant. An overview is given of the approaches to this item in several parts of the world. The relevant circumstances and influencing variables, such as growth phase, planting density, and some physicochemical characteristics (e.g., vapor pressure and Henry's coefficient) are dealt with. Finally, a proposal is presented for how to take into account the phenomenon of foliar interception and retention in the initial phase, first tier, of the risk assessment process.
From rotaxanes to knots. Templating, hydrogen bond patterns, and cyclochirality
Authors: Reuter Carin , Schmieder Roland , Vögtle Fritz
Published: October 2007

Rotaxanes of the amide type have been accessible in preparative yields by a variety of reactions. Beneath SN2- and SN2t-mechanisms we developed a synthesis of [2]rotaxanes that comes off a Michael addition. The motif of the attractive interactions between an axle-shaped and a macrocyclic wheel part to form rotaxanes consists of multiple hydrogen bonds in the nonionic strategy (threading), as well as in a new high yield anionic template synthesis (trapping). We introduce new synthetic routes for the preparation of [n]rotaxanes using nonionic as well as anionic templates. Furthermore, we report on the latest results of the statistical synthesis (slipping) by melting together axle and wheel to form rotaxanes. The chiroptical properties of a homologous series of cycloenantiomeric [1]rotaxanes as well as a cyclodiastereomeric [3]rotaxane have been described. The differences in the Cotton effects obtained show that small structural changes have an impact on the chiroptical properties of rotaxanes. The first X-ray structures obtained of cycloenantiomerically chiral amide-based [2]- and [1]rotaxanes as well as of the first topologically chiral amide-based knot compound were solved which show networks of H-bonds between the entities of the rotaxanes and the segments of the knot-shaped molecule. Our investigations in template effects based on hydrogen bonding for the synthesis of supramolecular structures open up a variety of strategies for the preparation of catenanes, rotaxanes andrecentlyeven molecular knots.
Mechanisms and consequences of oxygen transfer reactions
Authors: Beak Peter , Anderson David R. , Jarboe Stephen G. , Kurtzweil Mitchell L. , Woods Keith W.
Published: October 2007

The geometry about oxygen in the transition-state structures for oxygen transfers from a nitrone to phosphorous, from a percarboxylic acid to a carbon­carbon double bond, and from an N-sulfonyl oxaziridine to a carbon­carbon double bond have been evaluated by the endocyclic restriction test. The former can proceed at an oblique angle, while the latter two require a large angle between the entering and leaving groups on oxygen. This information is used to determine the mechanism of the aldehyde-dependent oxygen transfer from molecular oxygen to a carbon­carbon double bond.
Adventures in molecular recognition. The ins and outs of templating
Authors: Sanders Jeremy K. M.
Published: October 2007

Two different approaches are described for the creation of supramolecular systems potentially capable of recognition and catalysis. Using the design approach, we have been able to accelerate and influence two different Diels­Alder reactions within the cavities of porphyrin dimers and trimers; this is templating from the outside inwards. The selection approach is a synthetic chemical attempt to capture some of the key evolutionary features of biological systems: dynamic combinatorial chemistry is used to create equilibrating mixtures of potential receptors, and then a template is used to select and amplify the desired system. Five potential reactions for such dynamic chemistry are discussed: base-catalyzed transesterification, hydrazone exchange, disulfide exchange, alkene metathesis, and Pd-catalyzed allyl exchange, and preliminary templating results (inside outwards) are presented.
Microreactor-controlled selectivity in organic photochemical reactions
Authors: Tung Chen-Ho , Wu Li-Zhu , Zhang Li-Ping , Li Hong-Ru , Yi Xiu-Yu , Song Kai , Xu Ming , Yuan Zhen-Yu , Guan Jing-Qu , Wang Hong-Wei , Ying Yun-Ming , Xu Xiao-He
Published: October 2007

Molecular-sieve zeolites, Nafion membranes, low-density polyethylene films, and mixed surfactant vesicles have been used as microreactors to carry out organic photochemical reactions. The photo-cycloadditions of diaryl compounds with long flexible chains included in NaY zeolite or low-density polyethylene films yield intramolecular photocyclomers to the exclusion of intermolecular products. The photosensitized oxidation of alkenes included in pentasil zeolites or Nafion membranes or vesicles can be directed selectively toward either the singlet oxygen-mediated or the superoxide radical anion-mediated products by controlling the status and location of the substrate and sensitizer molecules in the reaction media. The photo-Fries rearrangement of phenyl phenylacetates included within NaY and pentasil zeolites or Nafion membranes gives either ortho-hydroxyphenones or decarbonylation products depending on the size/shape of the microreactors and the substrate molecules. All these results demonstrate the utility of microreactors to control the product selectivity in organic photochemical reactions.
Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes
Authors: Sommer J. , Jost R.
Published: October 2007

Acid-catalyzed hydrocarbon reactions involve by far the largest amount of catalyst and the largest volume of transformation in oil refinery and chemical industry. However, despite the general agreement on the carbocationic nature of the reaction intermediates, the initial steps and the true nature of the cations on the surface are still open to debate. On these points our basic knowledge has gained enormously from landmark experiments in physical organic chemistry with spectroscopic observations using liquid superacids as solvents. As the range of superacidity covers over 11 logarithmic units of the Hammett acidity scale, the mechanistic behavior of small alkanes vary accordingly. This allows for comparison with strong solid acids such as zeolite or others used in industry. Isotopic labeling of the alkane and the acids is a powerful tool used to understand the nature of the intermediates, as well as the reaction mechanism in which carbenium and carbonium ions intervene.
Concepts of metal-mediated methane functionalization. An intersection of experiment and theory
Authors: Schwarz Helmut , Schröder Detlef
Published: October 2007

Concepts for the activation of methane are derived from ion-molecule reactions of mass-selected, ground-state transition-metal cations M+. Elementary steps of industrially important processes are uncovered (e.g., oxygenation of methane or its coupling with carbon dioxide). In addition, the implications of the electronic structures of M(CH2)+ complexes for their reactions with nucleophiles are discussed, and the crucial role of contemporary quantum mechanical calculations in the elucidation of mechanistic details is emphasized.
Analytical advantages of using electrochemistry for atomic spectrometry
Authors: Bulska Ewa
Published: October 2007

Atomic spectrometry and electrochemistry are usually recognized as independent analytical tools used for different purposes. Here, a brief review is given of the advantages of using electrochemistry in the various fields of atomic spectrometry techniques. In the first part, the application of electrochemical preconcentration before the atomic spectrometry will be addressed and exemplified. Electrochemical preconcentration could be used with flame atomic absorption spectrometry (AAS) or graphite furnace AAS as well as with atomic emission plasma sources. The second area of the applications of electrochemistry will be directly focused on the graphite furnace AAS where the electrodeposition onto the graphite surface of the atomizer could be used for both in situ analyte preconcentration or modification of the surface by noble metals.
A glance into the bulk of solvent polymeric pH membranes
Authors: Lindner Ernö , Gyurcsányi Robert E. , Pendley Bradford D.
Published: October 2007

The pH-sensitive chromoionophores brought the dream of the ion-selective membrane scientist close to realization. With the help of these molecules, one can build pH-sensitive, ion-selective electrodes and look into the bulk of solvent polymeric membranes during potentiometric measurements (spectropotentiometry) and image concentration profiles in situ with high spatial and temporal resolution. The combination of electrochemical and optical information helped to interpret non-idealities in the potentiometric responses, suppress or tailor the undesirable transport across sensor membranes, and estimate the residual lifetime of chronically implanted sensors. These novel opportunities provide feedback in membrane optimizations and are expected to lead to sensor systems with picomolar detection limits and superb selectivities.
New developments in anti-HIV chemotherapy
Authors: De Clercq Erik
Published: October 2007

Virtually all the compounds that are currently used, or under advanced clinical trial, for the treatment of HIV infections, belong to one of the following classes: (i) nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs): i.e., zidovudine, didanosine, zalcitabine, stavudine, lamivudine, abacavir, emtricitabine, tenofovir (PMPA), and disoproxil fumarate; (ii) non-nucleoside reverse transcriptase inhibitors (NNRTIs): i.e., nevirapine, delavirdine, efavirenz, and emivirine; and (iii) protease inhibitors (PIs): i.e., saquinavir, ritonavir, indinavir, nelfinavir, and amprenavir. In addition, various other events in the HIV replicative cycle are potential targets for chemotherapeutic intervention: (i) viral adsorption, through binding to the viral envelope glycoprotein gp120; (ii) viral entry, through blockade of the viral coreceptors CXCR4 and CCR5; (iii) virus-cell fusion; (iv) viral assembly and disassembly; (v) proviral DNA integration; and (vi) viral mRNA transcription. Also, new NRTIs, NNRTIs, and PIs have been developed that possess respectively improved metabolic characteristics, or increased activity against NNRTI-resistant HIV strains or, as in the case of PIs, a different, nonpeptidic scaffold. Given the multitude of molecular targets with which anti-HIV agents can interact, one should be cautious in extrapolating from cell-free enzymatic assays to the mode of action of these agents in intact cells.
Continuing evolution of the drug discovery process in the pharmaceutical industry
Authors: Ratti Emiliangelo , Trist David
Published: October 2007

Many early discoveries in the pharmaceutical industry were through serendipity. Later, targets were mainly identified in animals and systematically exploited through the identification of potent and selective molecules. A disease association was normally obtained through the clinical testing of candidate molecules in patients. The technological advances in the last few years offer the possibility of knowing more about the disease, and this is driving the industry toward a disease-based approach where understanding the disease becomes central to the process. This is now possible thanks to the recent explosion in molecular and cellular biology, together with the application of genetics and genomics. New screening technologies have also revolutionized the identification of chemical leads. Now, high-throughput screening allows a wide chemical diversity to be applied in order to obtain tractable leads, which can then be optimized by the medicinal chemist. It is envisaged that these trends of continuously searching for process improvement will continue, being driven by the need to find medicines that add value in treating unmet medical need.
Development of bioreactors for application of biocatalysts in biotransformations and bioremediation
Authors: Burton Stephanie G.
Published: October 2007

Biotransformation systems, whether used for environmentally benign biocatalysis of synthetic reactions, or bioremediation of pollutants, require suitable biocatalysts and suitable bioreactor systems with particular characteristics. Our research focuses on the bioconversion of organic compounds, many of which are industrial residues, such as phenols, poly-aromatic hydrocarbons, heterocyclic compounds, and polychlorinated biphenyls. The purpose of such biotransformations can be twofold: firstly, to remove them from effluents and convert them to less toxic forms, and secondly, to convert them into products with economic value. We conduct research in utilizing various isolated-enzyme and whole-cell biological agents; bioreactors, including novel membrane bioreactors, are used as a means of supporting/immobilizing, and hence applying, these biocatalysts in continuous systems. In addition, the enzyme systems are characterized biochemically, to provide information which is required in modification, adaptation, and scale-up of the bioreactors. The paper summarizes research on application of biofilms of fungal and bacterial cells and their enzymes, including hydrolases, polyphenol oxidase, peroxidase and laccase, in bioreactor systems including continuously operating membrane bioreactors.
Nanostructured amorphous metals, alloys, and metal oxides as new catalysts for oxidation
Authors: Kesavan V. , Dhar D. , Koltypin Y. , Perkas N. , Palchik O. , Gedanken A. , Chandrasekaran S.
Published: October 2007

The oxidation of cyclohexane with molecular oxygen in the presence of isobutyraldehyde catalyzed by nanostructured iron and cobalt oxides and iron oxide supported on titania has been studied. Nanostructured cobalt oxide on MCM-41 is found to be efficient for catalytic aerobic epoxidation of olefins.
Peroxometal-mediated environmentally favorable route to brominating agents and protocols for bromination of organics
Authors: Bora Upasana , Chaudhuri Mihir K. , Dey Deepa , Dhar Siddhartha S.
Published: October 2007

Higher-valent transition metals react with H2O2 to form peroxometallates, thereby activating the coordinated peroxide. Based on the reaction profiles of peroxometal species, environmentally acceptable newer syntheses of tetrabutylammonium tribromide (TBATB), Bu4NBr3, cetyltrimethylammonium tribromide (CTMATB), cetyl(Me)3NBr3, and tetraethyl-ammonium tribromide (TEATB), Et4NBr3, have been developed from the reactions of the corresponding quaternary ammonium bromides with H2O2 and a catalytic amount of vanadium(V) or molybdenum(VI). Other transition metals capable of activating peroxide give similar results. The quaternary ammonium tribromides (QATBs) thus produced, especially TBATB and CTMATB, very efficiently act as clean and selective brominating agents for a variety of organic substrates. Very facile bromination of organic substrates, including aromatics, is also possible by tetrabutylammonium bromide (TBAB) Bu4NBr, either promoted by V2O5­H2O 2 or catalyzed by MoO4
2­­H2 O2. The scope of the protocols has been underscored, and the relevance to green chemistry has been highlighted.
Catalysis for green chemistry
Authors: Clark James H.
Published: October 2007

The use of heterogenization as a method for achieving clean synthesis is discussed. The chemical modification of mesoporous solids can be used to make a range of catalysts, including solid acids and bases, and stable metal complexes for selective oxidations and other reactions. By avoiding an aqueous quench stage in the separation, the heterogenization of catalysts and reagents can lead to substantial reductions in waste produced in organic chemical manufacturing processes.
Rapid screening and combinatorial methods in homogeneous organometallic catalysis
Authors: Loch Jennifer A. , Crabtree Robert H.
Published: October 2007

Methods are discussed for rapid screening of soluble and polymer-bound homo-geneous catalysts for activity. A polymer-bound phosphine library is synthesized, and a modular tridentate pincer CNC bis-carbene Pd complex is described. The possibility of C-bound His in metalloenzymes is raised.
Polymer-metal complexes for environmental protection. Chemoremediation in the aqueous homogeneous phase
Authors: Geckeler Kurt E.
Published: October 2007

The concept of chemoremediation in the liquid homogeneous phase using polymer reagents as a green technology is presented and outlined. It is based on the retention of polymer­metal complexes in aqueous solution according to their molecular size by using membrane filtration. The fundamentals of the metal ion interaction with respect to enrichment and separation procedures are discussed, and the role of polymer­metal complexes is assessed. The formation conditions of these polymer­metal complexes as well as major factors influencing the process are investigated. In addition, recent developments using this method are evaluated, and several application examples are highlighted.
Human exposure to traffic pollution. Experience from Danish studies
Authors: Hertel Ole , Jensen Steen Solvang , Andersen Helle Vibeke , Palmgren Finn , Wåhlin Peter , Skov Henrik , Nielsen Ivan Vejsgaard , Sørensen Mette , Loft Steffen , Raaschou-Nielsen Ole
Published: October 2007

Air pollution may have severe long-term as well as short-term health effects. The determination of possible links between pollution levels and impact on human health is, however, not a straightforward task. A key problem is the assessment of human exposure to ambient pollution levels. In later years, the possible role of particulate pollution as a health hazard has drawn major attention and is, therefore, the subject of research projects in many countries including Denmark. The present paper gives a review of recent and ongoing/planned Danish air pollution exposure studies. Furthermore, key results from Danish studies of ultrafine particles from urban traffic are outlined. The exposure studies show that air pollution models may be strong tools in impact assessment studies, especially when used in combination with personal exposure monitoring and application of biomarkers. Personal exposure measurements in Copenhagen indicate that indoor pollution levels may be very important for the personal exposure to fine fraction particles (PM2.5). Measurements with a differential mobility analyzer (DMA) in Danish urban areas show that number concentrations of ultrafine particles (<100 nm) in busy streets are strongly correlated with classic traffic pollutants such as nitrogen oxides and carbon monoxide. The number concentrations in urban Danish streets have decreased considerably between two campaigns in 1999 and 2000, apparently as a result of reductions in sulfur contents in Danish diesel fuels that took place in July 1999.
Reactivity and selectivity under microwaves in organic chemistry. Relation with medium effects and reaction mechanisms
Authors: Loupy André , Perreux Laurence , Liagre Marion , Burle Karine , Moneuse Michel
Published: October 2007

Typical applications of solvent-free conditions and microwave activation are described. Non-purely thermal specific effects are evidenced and discussed in terms of reaction medium and mechanisms, taking into account the variations in polarity of the systems.