bluuebunny/crossref_metadata_2025_split huggingface dataset:
Organic Synthesis Using Metal‐Mediated Metathesis Reactions
Authors: Kissling Rebecca M. , Nolan Steven P. 
Published: March 2006

Abstract
Olefin metathesis is an established reaction, yet it is still growing and diversifying at a remarkable rate. Since 2000, there has been notable progress in a number of areas, especially the development of new catalysts, many of which have N‐heterocyclic carbenes modifying ruthenium. These include phosphine‐free ruthenium catalysts that broaden the scope of efficient ruthenium‐catalyzed olefin metathesis, as well as catalysts with N‐heterocyclic carbenes and electron‐poor phosphines having turnover frequencies >100‐fold faster than the original Grubbs catalyst. In concert with this, some excellent mechanistic experiments and theoretical work have better elucidated the structure‐reactivity relationship and subtle but important points about mechanism relative rates of phosphine. Association, dissociation, and formation of the metallacyclobutane of the ruthenium‐catalyzed reactions are leading the way to further improvements in ruthenium catalysts.

Many methodological applications have also been developed, including improvements in the scope of hetereocycle and macrocycle formation. Cross metathesis (CM) has also undergone a transformation to a high yielding and stereoselective reaction that can be performed with predictable outcomes of yield and alkene stereochemistry. Great strides have been made in asymmetric olefin metathesis reactions, especially desymmetrization and kinetic resolution, with the majority of the results reported using molybdenum‐based catalysts with
C
2
‐symmetric chiral bisaryloxy based ligands. In addition, successful ruthenium‐based catalysts with
C
2
‐ and
C
1
‐symmetric ligands have been reported. Numerous reports of Tandem and sequential olefin metathesis reactions have been published, often in one‐pot, and sometimes utilizing the same metal complex for multiple operations, like ring closure/hydrogenation. A number of examples of tandem reactions utilizing a palladium catalyst and ruthenium‐catalyzed olefin metathesis have also been reported as very efficient routes to desirable organic building blocks.

One of the areas that exploited olefin metathesis is natural product synthesis. There are an ever increasing number of reports that cite olefin metathesis as a key, often step saving, reaction in the successful completion of target molecules by ring closing or cross metathesis. The synthetic advantages the olefin metathesis reaction have also been employed by materials and polymer chemists as part of design strategies for constructing well‐defined polymers, substrates for combinatorial chemistry, and metal‐containing rings.
The olefin metathesis catalysts have been attached to a variety of supports for the purpose of recycling. Many groups have examined methods for tethering both molybdenum and ruthenium‐based catalysts through reversibly coordinating ligands, the alkylidene and the backbone of nonlabile ligands, with reports of high efficiency recycling of up to 10 times.
Low Coordinated Group 13 Chelates
Authors: Dagorne Samuel 
Published: March 2006

Abstract
Recent developments in the area of low‐coordinate group 13 chelates are the focus of this article. Emphasis is placed on the synthesis and reactivity of two new classes of compounds, that is, tri‐coordinate cationic chelates and monovalent two/three‐coordinate chelate species.

Unknown 10 years ago, tri‐coordinate chelate cations {L‐X}MR
+
(M = Al, Ga, In; R = Me, Et, Pr) could be obtained by reaction of the neutral precursors {L‐X}MR
2
with a strong Lewis acid such as [Ph
3
C][B(C
6
F
5
)
4
], provided an extremely crowded L‐X
−
bidentate ligand and an inert counterion are used. Reactivity studies on the Al derivatives show that these highly electrophilic species are potent Lewis acids as expected by the combination of the cationic charge and a low‐coordination number, but are less reactive than neutral AlR
3
compounds with regard to insertion of unsaturated substrates into the AlC bond.

The use of bulky tri‐ and bidentate ligands has also allowed access to unprecedented monovalent and monomeric low‐coordinate group 13 compounds. Tri‐coordinate Ga(I) and In(I) complexes incorporating a sterically encumbered HB(Pz
R, R′
)
−
tridentate ligand were obtained and are quite stable. Similarly, the use of the very crowded HC(CMeNAr)
2
−
and

t

BuNCCN

t

Bu
2−
dinitrogen ligands with an appropriate synthetic methodology has opened the way to stable two‐coordinate and monovalent species: {HC(CMeNAr)
2
}M (M = Al, Ga) and the {

t

BuNCCN

t

Bu}Ga
−
anion, respectively. Such species may behave either as Lewis acids or as Lewis bases due to the intrinsic properties of the metal center (a sextet configuration and the presence of a lone pair of electrons). Reactivity studies on these two‐coordinate complexes have just begun but have already permitted the synthesis of the first monomeric Al and Ga imide complexes {HC(CMeNAr)
2
}M(N‐Trip) (M = Al, Ga).

Finally, representative examples of recent new catalytic applications of tri‐coordinate Al(III) neutral species containing a chiral or achiral X
2
2−
‐type chelating ligand, used as catalysts, are mentioned.
Main Group Carbenes
Authors: Carmalt Claire J. 
Published: March 2006

AbstractThis article describes the chemistry of main group carbene complexes. Although not limited to, the article particularly summarizes the coordination of N‐heterocyclic carbenes to main group species. Usually, carbene ligands cannot be stabilized by electron‐poor coordination fragments because traditional carbenes show a pronounced tendency toward metal‐to‐ligand π back‐donation. However, N‐heterocyclic carbenes do not rely on back‐donation for stabilization because of the delocalization of the nitrogen lone pair in these carbenes. Thus, they are able to form stable complexes with metals that normally show no tendency for π back‐donation. In general, the carbene acts as a two‐electron donor ligand in a similar fashion to phosphines.Carbene complexes of the alkali and alkaline earth metals, which show a preference for σ‐bonding, are described. The high nucleophilicity of the N‐heterocyclic carbenes is shown by alane, gallane, and indane complexes that have much greater stability than normally observed for Lewis base adducts of group 13. Nucleophilic carbenes also react with Lewis acidic group 14 complexes with the formation of neutral or ionic compounds via simple adduct formation or displacement of a halide ion. Furthermore, N‐heterocyclic carbenes are sufficiently nucleophilic to enable depolymerization of cyclopolyphosphines and cyclopolyarsines to occur with the isolation of carbene–pnictinidene adducts. These carbenes can also form stable adducts with strong electron acceptors, such as pnictogen pentafluorides, if the reactivity is moderated. In addition, stable chalcogen–carbene and halide–carbene complexes are also discussed.
Copper Enzymes in Denitrification
Authors: Dooley David M. , Chan Jeannine M. 
Published: March 2006

AbstractDenitrification is an intrinsic part of the global nitrogen cycle, balancing the cycle by returning fixed nitrogen to the atmosphere. During denitrification, a bacterial respiratory process, nitrate or nitrite is reduced to nitrogenous oxides. All of the redox reactions of denitrification are catalyzed by metalloenzymes, two of which contain copper. This chapter reviews the structures, mechanisms, and assembly of these copper‐containing enzymes, copper nitrite reductase and nitrous oxide reductase.
Low Oxidation State Main Group
Authors: Macdonald Charles L. B. , Ellis Bobby D. 
Published: March 2006

AbstractThe synthesis and chemistry of compounds containing groups 13–15 elements in unusually low oxidation states is presented. In general, the oxidation state of an element influences the reactivity of the compounds in which the atom is found. For the main group elements, compounds containing elements in lower than usual oxidation states exhibit significantly different structural features, modes of bonding, and chemistry than is observed for compounds containing the element in a more typical oxidation state. The elements of the s‐block do not form low oxidation state compounds and the elements of groups 16–18 are generally found in the lowest available oxidation state. Thus, this article provides an overview of the following classes of low‐valent main group species: the chemistry of group 13 elements in the +1 and +2 oxidation states, the chemistry of group 14 elements in the 0 oxidation state, and the chemistry of the group 15 elements in oxidation states lower than +3. Numerous exemplary modes of reactivity are presented and the utility of some low oxidation state compounds as catalysts or precursors to materials is acknowledged.
Transition Metal Complexes with Bulky Allyl Ligands
Authors: Hanusa Timothy P. , Carlson Christin N. 
Published: March 2006

Abstract

The allyl anion (C
3
H
5
−
) is often found in combination with other ligands in organometallic complexes. Its compact size, however, means that compounds in which it is the only or principal type of ligand are frequently coordinatively unsaturated and prone to decomposition. Sterically bulky substituents (e.g. – SiMe
3
) placed on the allyl group will increase its effective size, dramatically enhancing the thermal, and in some cases oxidative, stability of the associated metal complexes. Solubility in nonpolar solvents such as aromatic hydrocarbons and alkanes is also improved. Electron‐deficient monomeric first row transition metal allyl complexes (e.g. [π‐(SiMe
3
)
2
C
3
H
3
]
2
M; M = Cr, Fe, Co) have been synthesized that have no (C
3
H
5
)
2
M counterparts. New varieties of early transition metal and f‐element complexes have been made with bulky allyl ligands, some of which are active polymerization catalysts. Research in this area has markedly expanded the range of metals, structural types, and associated reactions that can be incorporated into metal allyl chemistry.
Group 14 Multiple Bonding
Authors: Sasamori Takahiro , Tokitoh Norihiro 
Published: March 2006

Abstract

Multiply bonded compounds between heavier group 14 elements cannot exist as monomers under normal conditions owing to their inherent high reactivity, that is, they readily undergo oligomerization, oxidation, hydrolysis, and so on. The bulkiness of substituents on the reactive sites shows a great effect on the stabilization of such highly reactive compounds. Since the synthesis and isolation of the first stable disilene, Mes
2
SiSiMes
2
(Mes = mesityl), by R. West
et al.
in 1981, the idea of kinetic stabilization using bulky substituents have led to the successful isolation of several types of unsaturated compounds of group 14 elements, such as
heavy alkenes
(double bonds between heavier group 14 elements),
heavy ketones
(heavier group 14–16 double bonds),
heavy imines
(heavier group 14–15 double bonds),
heavy aromatics
(aromatic systems containing heavier group 14 elements),
heavy butadienes
, and triply bonded systems containing heavier group 14 elements, and so on. Now, unsaturated compounds containing heavier group 14 elements are no more imaginary species but are those of real existence even in the case of lead, the heaviest group 14 element, when they are appropriately protected by bulky substituents. The synthesis and properties of the kinetically stabilized unsaturated compounds containing heavier group 14 elements will be described in this section. Although a number of multiple bondings between heavier group 14 elements and transition metal elements have been reported so far, only unsaturated compounds containing main group elements should be considered here from the viewpoint of comparisons with carbon chemistry.
Sol–Gel Encapsulation of Metal and Semiconductor Nanocrystals
Authors: Petruska Melissa A. , Klimov Victor I. 
Published: March 2006

Abstract
The ability to encapsulate nanocrystals in glass matrices affords the opportunity to fabricate a variety of robust materials exhibiting properties tunable with the size, shape, or composition of the dopant. Among the various methods to these composites, sol–gel processing has the distinction of being a ‘wet chemistry’ approach, as it relies on high purity chemical reagents and mild reaction conditions to generate three‐dimensional metal‐oxide polymeric networks. Two general routes to noble metal and semiconductor nanocrystal‐sol–gel composites are explored. In one strategy, the synthesis of the nanocrystal occurs during the fabrication of the host matrix. This direct growth method has been investigated as an express route to nanocrystal‐doped glasses that typically involves the addition of metal salts to sol–gel precursors. Subsequent reduction or precipitation of the embedded metal salts, either with gases or other chemical reagents, leads to the formation of the desired nanocrystals. The second approach exploits the synthetic procedures already available for making high quality, crystalline nanoscale materials and focuses on a decoupled process in which the synthesis of the colloidal nanocrystals is independent of the fabrication of the composite network. In this way, the size, shape, and composition of the nanocrystals can be tailored for specific applications, promising a precise control over the dopant that is unavailable in the direct growth method. Improvements in these procedures have led to the preparation of uniformly dispersed nanocrystal‐sol–gel composites with high nanocrystal volume loadings and narrow nanocrystal size dispersities that have aroused considerable interest for their unique optical, catalytic, and electronic behaviors.
Metal Nanoparticles, Synthesis of
Authors: Schmid Günter 
Published: March 2006

Abstract
The successful synthesis of metal nanoparticles decisively depends on sufficient stabilisation by protecting materials. This can be reached either by electrostatic repulsion of charged species or by covering the surface atoms with polymers, surfactants, or especially ligand molecules binding to the particles via covalent bonds. Noble metals are in focus, since their salts or complexes can easily be reduced to the oxidation state 0 and the sensitivity towards oxygen is reduced or even avoided like in the case of gold. Nevertheless, nanoparticles of less noble metals can be prepared. Above all, there is an increasing need to get monodispersed particles, since their properties depend on size and shape. There exist so‐called physical methods to generate metal atoms from bulk metals that are then allowed to coalesce to nanoparticles. High temperatures or laser ablations are examples. Chemically based procedures use in any case stabilizing agencies. Among numerous methods, salt reduction dominates. Thermal decomposition of appropriate precursor complexes and soft ligand removal from organometallics have become known as alternative routes. Photolytic and radiolytic techniques exist, but do not play important roles. The synthesis of magnetic metal nanoparticles is of increasing interest with respect to applications. The syntheses mainly follow recipes known from noble metals. Fe, Co, Ni as well as magnetic bimetallic alloy‐like nanoparticles are available. The magnetic characteristics of nanoparticles again depend decisively on size, structure, and shape, even more than in case of the nonmagnetic species. Therefore, synthetic procedures resulting in uniform particles are of great importance.
Metal Nanoparticles, Organization & Applications of
Authors: Schmid Günter 
Published: March 2006

AbstractThe need to organize metal nanoparticles in three, two, or one dimension(s) is linked with their properties, the use of which is only possible if they are organized. The most important properties of nonmagnetic and magnetic metal nanoparticles are therefore briefly discussed. Organization in three dimensions (3D) may happen by naturally happening crystallization processes or by linking the particles with the help of spacer molecules of various length and composition. 3D organized metal particles are quite well investigated and electron transport between the building blocks is satisfyingly understood. 2D arrangements result either by spontaneous self‐assembly from solution on appropriate supports, by guided self‐assembly, supporting the organization process, for instance, by means of pressure (Langmuir–Blodgett) or pre‐prepared surfaces, or the most attractive kind of organization, the aimed one, since it results in artificial structures that would be necessary if nanoparticles should find application in future nanodevices. Two routes to aimed structures have become known up to now: the dip‐pen nanolithography and the nanoelectrical modification of surfaces. Dip‐pen nanolithography uses the transport of special molecules from a moving AFM tip to suited surfaces. Chemical modifications of as‐prepared structures end up with formation of aimed patterns of metal nanoparticles. Nanoelectrical modification of CH3‐terminated surfaces, also using an AFM tip, give COOH functions that can further be modified to bind metal nanoparticles. Architectures performed by one of these methods simply depend on the software the AFM is moving on the surfaces.
Carbon: Nanotubes
Authors: Baskaran Durairaj 
Published: March 2006

Abstract
A brief introduction to carbon nanotubes with basic structural classification, synthesis, properties, and surface modification is presented. The growth process of carbon nanotubes and the role of metals catalyst in the arc‐discharge, the laser‐vaporization, and the chemical vapor deposition (CVD) techniques are discussed. The synthesis of aligned carbon nanotubes in a preferred orientation from various substrates and the formation of branched carbon nanotubes are unique features of the CVD technique. The method of purification of carbon nanotubes is chosen on the basis of the amount of catalyst impurities and amorphous carbon present in them that are dependent on the techniques used to produce carbon nanotubes. Oxidative purification methods and the introduction of carboxylic acid on the surface of carbon nanotubes have been described. Optical, electrical, and mechanical properties of carbon nantubes have been highlighted. An overview of the surface modification of carbon nanotubes with covalent and noncovalent chemistry to enhance the dispersion and dissolution properties of carbon nantotubes and the problem associated with size, length, and the functional groups distribution affecting the solution properties of carbon nanotubes have been discussed.
Aktuelle Entwicklungen in der optischen Datenspeicherung mit organischen Farbstoffen
Authors: Mustroph Heinz , Stollenwerk Manfred , Bressau Volker 
Published: March 2006

AbstractDie wesentlichen Motivationen für die Entwicklung digitaler Datenspeicher sind die Verbesserung der Wiedergabequalität und die Erhöhung der Speicherkapazität. Im Jahre 1982 stellten Philips und Sony mit der Compact Disc (CD) und einem entsprechenden Abspielgerät das erste technisch und wirtschaftlich erfolgreiche System dieser Art vor. Heute steht eine Vielfalt an optischen Datenträgerformaten zur Verfügung. Dabei wird zwischen vorbespielten, einmal beschreibbaren und wiederbeschreibbaren Medien unterschieden. Der Aufsatz gibt zunächst eine Übersicht über diese Systeme und die Grundzüge ihrer Herstellung und konzentriert sich dann auf die Eigenschaften der organischen Farbstoffe, die in einmal beschreibbaren Systemen verwendet werden. Die Farbstoffchemie wird im Allgemeinen zu den reifen Arbeitsgebieten gezählt. – Ist dies ein begründeter Abgesang oder nur ein Vorurteil? Anhand von Anwendungen in der optischen Datenspeicherung wird illustriert, dass Fortschritte bei CD‐R und DVD/R auch heute ohne funktionelle Farbstoffe undenkbar wären.
Turn and helical peptide spacers: Combined distance and angular dependencies in the exciton‐coupled circular dichroism of intramolecularly interacting bis‐porphyrins
Authors: Guryanov Ivan , Moretto Alessandro , Campestrini Sandro , Broxterman Quirinus B. , Kaptein Bernard , Peggion Cristina , Formaggio Fernando , Toniolo Claudio 
Published: March 2006

AbstractIn a previous study we examined by the exciton‐coupled circular dichroic method the distance effect generated by three‐rigid‐turn and helical‐peptide spacers. In this connection porphyrins were confirmed to be excellent reporter chromophores. In the present investigation we have completed this research by expanding the original analysis to the assessment of the combined role of the chromophore distance and orientation with use of the same porphyrin derivatives and additional four analogous spacers of different main‐chain lengths. We find that not only the intramolecular separation of the chromophores, but the angular dependence between the directions of their effective transition moments as well, are responsible for the onset and modulation of the intensity of the exciton‐coupling phenomenon of the porphyrin Soret band. © 2006 Wiley Periodicals, Inc. Biopolymers 82: 482–490, 2006This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com
Use of Surface Enhanced Infrared Absorption Spectroscopy (SEIRA) to probe the functionality of a protein monolayer
Authors: Ataka Kenichi , Heberle Joachim 
Published: March 2006

AbstractWe present a novel infrared method to investigate the functionality of a protein monolayer tethered to a metal substrate. The approach employs Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS), which renders high surface sensitivity by enhancing the signal of the adsorbed protein by up to ∼2 orders of magnitude. We demonstrate that the electrochemically induced absorption changes of a cytochrome c monolayer can be observed with excellent signal‐to‐noise ratio when the protein is adhered to a modified gold surface. To probe membrane proteins, a concept is introduced for the oriented incorporation into solid supported lipid bilayers. Recombinant cytochrome c oxidase solubilized in detergent is immobilized on a chemically modified gold surface via the affinity of its histidine (His)‐tag to a nickel‐chelating nitro‐triacetic acid (NTA) surface. The protein monolayer is reconstituted into the lipid environment by detergent removal. Changing the orientation of the protein with respect to the metal surface is achieved by inserting the His‐tag on either side of the membrane protein surface. Orientational control is mandatory for experiments in which electrons are injected from the electrode into the protein. The presented methodology opens new avenues to study the mechanism of the biomedically relevant class of electron and voltage‐gated proteins on the atomic level. © 2006 Wiley Periodicals, Inc. Biopolymers 82: 415–419, 2006This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com