bluuebunny/crossref_metadata_2025 huggingface dataset:
Constraints on the glacial operation of the atlantic ocean's conveyor circulation
Authors: Broecker Wallace S.
Published: April 2004

Circulation in the Atlantic Ocean is currently dominated by a northward flow of upper waters balanced by a return flow of deep water (i.e., the conveyor). Paleoproxies tell us that, unlike today, during the glacial age the deep Atlantic was stratified. Rather than being flooded with one nearly homogeneous water mass, there were two distinctly different ones. In this paper, the paleoproxy results are analyzed in an attempt to constrain the sources and ventilation rate of the deeper of these two glacial Atlantic water masses. Taken together, the cadmium and carbon isotope measurements on benthic foraminifera and the radiocarbon measurements on coexisting benthic and planktonic foraminifera appear to require a conveyor‐like circulation no weaker than half of today's. This conclusion is at odds with geostrophic reconstructions. This seeming disagreement could be eliminated if, as suggested by Keigwin and Schlegel, the radiocarbon measurements by Broecker et al. significantly underestimate the difference between the 14C to C ratio for glacialage surface water and deep water in the equatorial Atlantic.
Dealing with Uncertainty in Diffusion Tensor MR Data
Authors: Basser Peter J. , Pajevic Sinisa
Published: April 2004

This paper explains how radio frequency (RF) background noise produces uncertainty in measured diffusion tensor MRI (DT‐MRI) data, how this noise can be modeled, and how its effects can be mitigated. DT‐MRI data are derived from a series of magnitude diffusion‐weighted images (DWI) in which RF noise is rectified. A new Gaussian distribution is proposed that describes the variability of the estimated diffusion tensor,D, in an ideal experiment in which RF noise is the only artifact present. We show how to improve the design of DT‐MRI experiments by requiring that the statistical distribution ofDbe independent of the laboratory coordinate system. Non‐parametric empirical methods of analyzing uncertainty in DT‐MRI experiments are also described. Monte Carlo simulations are useful in designing and interpreting DT‐MRI experiments. Bootstrap methods help us measure the true variability ofD(and quantities derived from it), and assess the quality of DT‐MRI data. Matrix Perturbation techniques predict how the uncertainty inDpropagates to its eigenvalues and eigenvectors. A method for obtaining a continuous diffusion tensor field from the measured discrete noisy DT‐MRI data also reduces the uncertainty ofDand quantities derived from it. Finally, we describe schemes that use wavelets to remove noise from DWI and DT‐MRI data while preserving boundaries between different tissue regions. Collectively, these parametric and nonparametric methods provide a unified statistical framework to improve the design of DT‐MRI experiments and their subsequent analysis.
Furyl‐Substituted Group‐4 Metallocene Catalysts for Elastomeric Polypropylene Formation—The Role of the Remote Substituents
Authors: Dreier Thorsten , Erker Gerhard
Published: April 2004

Treatment of a series of 2‐alkylfurans (1) and related systems with n‐butyllithium, followed by addition of the resulting lithiofurans to 2‐indanone, H3O+‐catalyzed elimination, deprotonation, and transmetallation to zirconium tetra‐chloride, gave a series of differently substituted bis [(5‐alkyl‐2‐furyl) indenyl] ZrCl2 complexes [alkyl = n‐butyl (5d), n‐hexyl (5e), n‐octyl (5f), tert‐butyl (5g)], as well as the corresponding benzofuryl (5h) and 4,5‐dimethylfuryl (5i) complexes. Their treatment with methylalumoxane (>1000‐fold excess) generated active homogeneous Ziegler‐Natta polymerization catalysts. Propene polymerization at temperatures between −20 °C and +20 °C gave elastomeric polypropylenes. The elastomeric features were characterized by stress/strain curves that revealed a pronounced dependence on the actual alkyl substituent attached at the furyl rings at the periphery of the bent metallocene complexes. Increasing steric bulk of these substituents at the catalyst framework seems to result in increased isotacticities and a corresponding improvement in elastomeric properties of the polymers obtained by these systems.
The Diversity of B0 and B1 Gradient NMR Diffusometry Techniques
Authors: Ardelean Ioan , Kimmich Rainer
Published: April 2004

The conventional pulsed gradient Hahn or stimulated echo methods for diffusion measurements are briefly reviewed. Compared to these techniques, unconventional NMR diffusometry variants based on gradients of the amplitude of the radio frequency field, on nonlinear (or multiple) echoes, and on imaging techniques open promising aspects depending on the application envisaged. A series of typical pulse sequences representing the diversity of NMR diffusometry methods is described and discussed with respect to advantages and disadvantages.
High Molecular Weight Atactic Polypropylene prepared by Zirconium Complexes of an Amine Bis(phenolate) Ligand
Authors: Groysman Stanislav , Tshuva Edit Y. , Reshef Dvora , Gendler Shimrit , Goldberg Israel , Kol Moshe , Goldschmidt Zeev , Shuster Michael , Lidor Gershon
Published: April 2004

The polymerization of propylene employing the recently developed amine bis(phenolate) zirconium catalysts is reported for the first time. The polymerizations were conducted in liquid propylene employing two different experimental setups, and two different precatalysts, based on the same amine bis(phenolate) ligand having a dimethylamino donor on a sidearm. The first precatalyst studied was a dibenzyl zirconium complex, which was found to lead to high molecular weight atactic polypropylene. The effects of the cocatalyst, the ratio between the cocatalyst and the precatalyst, and the polymerization temperature on the nature of the resulting polymer and the activity of the catalyst were studied. Reducing the relative amount of the cocatalyst and lowering the polymerization temperature increased the molecular weight of the polymer, and a temperature of 50 °C led to the most active catalyst. A dichloro zirconium complex of the same ligand, expected to show a higher hydrolytic and thermal stability was developed, its X‐ray structure was solved, and its activity in polymerization of propylene was studied and found to be similar to that of the dibenzyl precatalyst.
“Chain‐End‐Controlled Isotactic” and “Stereoblock‐Isotactic” Polypropylene: Where Is the Difference?
Authors: Busico Vincenzo , Cipullo Roberta , Talarico Giovanni , Castelli Valeria Van Axel
Published: April 2004

This communication describes the microstructure of “stereoblock‐isotactic” polypropylene obtained with sterically hindered “oscillating” metallocene catalysts, points out in which respects it differs from that of “chain‐end‐controlled isotactic” polypropylene, and explains why the two definitions cannot be used as synonymous (as is commonly found in the literature).
Maintaining Current Awareness in Single‐Site Catalysis
Authors: Hlatky Gregory G.
Published: April 2004

The ever‐increasing field of single‐site olefin polymerization catalysis requires the researcher to keep abreast of developments in a number of facets. This paper outlines how one petrochemical company maintains current awareness of publications in both the open literature and the more arcane patent literature.
Application of the triisobutylaluminum‐promoted reductive rearrangement to sucrose‐5‐enes
Authors: Du Roizel Bérengère , Henriques Ana M. P. , Pearce Alan J. , Sinaÿ Pierre
Published: April 2004

Carbohydrate‐based vinyl acetals (5‐hex‐enopyranosides) undergo reductive rearrangement with triisobutylaluminum (TIBAL) to afford highly functionalized cyclohexanes in which both the aglycon and anomeric stereochemistry are retained. Here, we report the first application of this process to the rearrangement of hex‐5‐enopyranosides of sucrose in which the interglycosidic oxygen atom of the vinyl acetal system links the anomeric centers of both monosaccharide units. The sucrose‐derived 5‐hex‐enopyranoside 1 undergoes smooth reductive rearrangement with TIBAL to afford the (1 → 2′) ether‐linked pseudo‐disaccharide 2 in 34% yield. The rearrangement is accompanied by some loss of stereochemical integrity at C‐2′ due to a competitive exo‐cleavage of the interglycosidic (O‐C2′) bond, hence diastereomers at C‐2′ are also obtained in 12% yield. The 4‐O‐allyl‐protected sucrose‐5‐ene 3 is similarly transformed into the corresponding (1 → 2′) ether‐linked pseudo‐disaccharide 4, illustrating the compatibility of the allyl group with the TIBAL reaction conditions.magnified image
Diffusion MR in Biological Systems: Tissue Compartments and Exchange
Authors: Stanisz Greg J.
Published: April 2004

The processes of diffusion in multi‐compartmental tissue models are discussed using an analytical model of diffusion with exchange. Diffusion in the intracellular compartment has been found to be highly restricted, with the apparent diffusion coefficient (ADC1) dependent on the experimental parameters. Extracellular diffusion mediated by tortuosity, depends on tissue geometry, but not on the experimental conditions. Both the intra‐ and extracellular diffusion can be anisotropic and exhibit non‐monoexponential behavior. The cell membrane permeability is shown to significantly influence the diffusion measurement in tissues. In consequence, the phenomenological, bi‐exponential models of diffusion cannot directly estimate the intrinsic characteristics of intra‐ and extracellular water. The mechanisms of the ADC decrease after stroke are also discussed.
Mismatch discrimination in oligonucleotide hybridization reactions using single strand binding protein. A surface plasmon fluorescence study
Authors: Neumann Thomas , Knoll Wolfgang
Published: April 2004

We describe the use of surface plasmon‐ and surface plasmon fieldenhanced fluorescence spectroscopy for the detection of hybridization reactions between surface‐attached probe oligonucleotides and complement strands binding from solution. These targets, exhibiting different base mismatches relative to the probe 15‐mer sequences, carry a fluorophore at their 5'‐end thus allowing for sensitive detection and quantification of association, kon, and dissociation, koff, rate constants, as well as affinity constants, Ka. We demonstrate that by the competitive binding / replacement of single strand binding proteins the mismatch discrimination can be further enhanced.
Optically detected magnetic resonance of thiol‐capped CdTe nanocrystals
Authors: Glozman Alexander , Lifshitz Efrat , Hoppe Kathrin , Rogach Ahdrey L. , Weller Horst , Echymüller Alexander
Published: April 2004

The optical properties of thiol‐stabilized CdTe nanocrystals have been examined. The thiol groups ‐SR generate a CdS shell at the interface, leading to a CdTe/CdS core—shell structure. The present paper describes our efforts to identify the influence of the CdTe–SR interface on the optical properties of the nanocrystals, utilizing photoluminescence and optically detected magnetic resonance (ODMR) spectroscopy. The photoluminescence spectrum consists of an excitonic peak, overlapped by a broad band at lower energies. The ODMR spectrum, in the spectral regime of the broad band, showed two resonance signals. They are associated with a trapped hole at an anisotropic site of a cadmium vacancy at the Cd–SR interface and an electron in the conduction band.
Chemical applications of near‐field scanning optical microscopy: Surface and near‐surface chemical imaging with conventional near‐field optical probes and externally illuminated chemically active ion sensors
Authors: Strinkovski Alina , Shalom Shmuel , Radko Anna , Ignatov Andre , Lewis Aaron , Ottolenghi Michael , Sukenik Chaim
Published: April 2004

Near‐field optics using force sensing cantilevered optical fibers and micropipettes is used to investigate the chemical distribution of protein complexing dyes in chromosomes and the sensing and chemical imaging of surfaces and nearsurface regions. These results are put in the context of what presently can be considered as a broad picture of near‐field optical chemical applications.
Spin Echo Analysis of Restricted Diffusion under Generalized Gradient Waveforms for Spherical Pores with Relaxivity and Interconnections
Authors: Ryland Brett N. , Callaghan Paul T.
Published: April 2004

The problem of restricted diffusion in spherical pores is examined under conditions of finite gradient pulse width in pulsed gradient spin echo (PGSE)‐NMR experiments. Closed form analytic expressions are derived, and the case of interconnected pores is briefly examined. An expression, based on the pore hopping approximation, is presented that predicts the echo attenuation for diffusion between pores under any gradient waveform.
Synthesis, Characterization, and Reactivity of Lanthanide Complexes with Bulky Silylallyl Ligands
Authors: Woodman Timothy J. , Schormann Mark , Bochmann Manfred
Published: April 2004

The synthesis of new lanthanide allyl complexes of enhanced stability and solubility in saturated hydrocarbons based on silyl‐substituted allyl ligands is reported. Thus the potassium salt K(CH2CHCHSiMe3) (1) reacts with YCl3 in tetrahydrofuran to give the tris‐allyl complex Y(CH2CHCHSiMe3)3 (2), while K(CH2CHCHSiMe2tBu) (3) affords Y(CH2CHCHSiMe2tBu)3(THF)1.5 (4). Slow re‐crystallization of 4 from light petroleum in the presence of tert‐butylcyanide led to multiple insertion to give the sec‐amido complex Y{NHC(tBu)(CH)3SiMe2tBu}2{η2‐NHC(tBu)CH=CHCH2SiMe2tBu)CH(CHCHSiMe2tBu)CtBuNH}(THF)·(CH3CH(Me)(CH2)2CH3) (5), which was crystallographically characterized. The reaction of ScCl3(THF)3 with two equivalents of Li{1,3‐C3H3(SiMe3)2} in tetrahydrofuran gives the bis‐allyl complex {1,3‐C3H3(SiMe3)2}2Sc(μ‐Cl)2Li(THF)2 (6), while the analogous reaction of K{1,3‐C3H3(SiMe3)2} (7) with either LaCl3 or YCl3 in tetrahydrofuran affords the bis‐allyl complexes MCl{1,3‐C3H3(SiMe3)2}2(THF)x (8, M = La, x = 1; 9, M = Y, x = 0). An attempt to prepare the similar neodymium complex gave the mono‐allyl complex NdI2{1,3‐C3H3(SiMe3)2}(THF)1.25 (10). The reactions of 8 and 9 with triisobutyl aluminum in benzene‐d6 show allyl exchange between lanthanide and aluminum. Complexes 8, 9, and 10 have been tested with a variety of activator systems as catalysts for the polymerization of 1,3‐butadiene.
Partitioning and pore‐filling: Solubility‐normalized sorption isotherms of nonionic organic contaminants in soils and sediments
Authors: Grathwohl Peter , Rahman Md. Mokhlesur
Published: April 2004

Nonlinear sorption of hydrophobic organic pollutants in soils and sediments can be described by a combination of partitioning and adsorption (pore‐filling) mechanisms. Both the partitioning and the pore‐filling models plot sorptive uptake vs. solubility‐normalized aqueous concentrations, which yields “collapsing” sorption isotherms for similar compounds. With this new concept, sorption of a variety of similar compounds could be predicted based on measured data of one probe chemical. The solubility‐normalized Freundlich model predicts an inverse linear relationship between the sorption coefficient measured at a given relative concentration vs. solubility, S, which is demonstrated for 3 partitioning dominated samples from Bangladesh (soil, aquifer sediment, peat) and 4 compounds (carbofuran, trichloroethene, 1,2‐dichlorobenzene, phenanthrene) as well as for 2 activated carbons (F300, F400), and a wide variety of compounds (literature data). Future research will focus on the improvement of prediction of sorption in soils and sediments by including other normalization parameters, such as molar volume, polarizability, etc., in addition to the water solubility.
Photoconductivity of poly(N‐vinylcarbazole) (PVK) doped with the metallofullerene Dy@C82 and the fullerenes C84 and C60
Authors: Li Xiaogang , Yang Shihe , Xu Yu , Yang Shangfeng , Liu Yunqi , Zhu Daoben , Yang Chunlei , Wang Jiannong , Ge Weikun
Published: April 2004

Photoconductivity has been measured on films of pure poly(Nvinylcarbazole) (PVK) and PVK doped with the metallofullerene Dy@C82 and the fullerenes C84 and C60. The photo‐induced discharge rate of the PVK film increased dramatically when doped with the metallofullerene or the fullerenes. Comparatively, the film doped with Dy@C82 displayed better photoconductivity than that doped with C84, which is attributed to the Dy@C82 being a better electron acceptor than C84. However, the film doped with C60 showed the best photoconductivity. This is attributed to the better electron accepting ability of excited C60* or the better miscibility of the hollow fullerenes with the PVK polymer and organic solvents.
Quality indicators of the state of chemical pollution in israel
Authors: Haran Miriam , Samuels Rana , Gabbay Shoshana , Mingelgrin Uri
Published: April 2004

The state of chemical pollution in Israel is evaluated with the aid of environmental indicators adapted from a list of indicators compiled by the European Union. The precise definition of the individual indicators was modified to suit the specific environmental conditions in Israel. The environmental topics covered are those that have a strong chemical aspect to them, including groundwater quality, pollution of rivers, air pollution by acidifying substances and particulate matter, CO2 emissions, disposal of hazardous waste and handling of hazardous substances, air pollution originating from transportation, wastewater treatment and reuse, and solid waste disposal, as well as the status of environmental research.

The environmental indicators chosen reveal that in recent years there has been a steady improvement in the state of chemical pollution, excepting a deterioration in the quality of groundwater and an increase in vehicular emission of pollutants. The overall situation is, however, still unsatisfactory. A comparison of the state of chemical pollution in Israel to that in the EU and other countries demonstrated that the rate of improvement in environmental quality as determined by the examined indicators is slower in Israel than in most developed countries.
Pressure and gas composition effects on the operation of the pulsed flame photometric detector
Authors: Frishman Gad , Amirav Aviv , Barak Haim
Published: April 2004

The effect of pressure and hydrogen/oxygen ratio of a burning gas mixture on pulsed flame emission time‐dependence was investigated in the range of 0.1–5 atm using a specially designed pulsed flame photometric detector (PFPD). We studied the pressure and gas composition effect on the pulsed flame delayed light emission of sulfur, phosphorus, and nitrogen‐containing organic compounds. The optimal pressure conditions for nitrogen detection, intensity, and emission time delay was found to be 0.4 bar, at which the detection sensitivity could be improved by a factor of 2. For phosphorus, the optimal pressure obtained was 1.3 bar with 40% sensitivity improvement (compared with 1 bar). In the case of sulfur detection, two emission maxima were obtained, at 1.1 and 0.6 bar, at H/O ratio of 5. Increasing the H/O ratio resulted in the appearance of only one peak at 1 bar, and enhancement of the sensitivity by a factor of 2.4 at H/O ratio of 10.3. From the analytical point of view, we found that emission intensity is practically unchanged by the pressure and the H/O ratio for all three elements investigated in the range of 0.8–1.1 bar and H/O of 5–6. Thus, in addition to excellent sensitivity and improved selectivity, the PFPD can be applied under a variety of atmospheric pressure conditions in field environmental applications.
Diffusion MRI and MRS of Skeletal Muscle
Authors: Strijkers Gustav J. , Drost Maarten R. , Heemskerk Anneriet M. , Kruiskamp Marijn J. , Nicolay Klaas
Published: April 2004

MR offers powerful tools to measure molecular diffusion in intact tissues under in vivo conditions. This review focuses on the use of diffusion‐weighted MR imaging (DW‐MRI) and spectroscopy (DW‐MRS) to noninvasively study structural and functional aspects of skeletal muscle. It will be shown that DW‐MRI allows for a high‐resolution reconstruction of the orientation of the muscle fibers, which represents vital input for mathematical models of muscle biomechanics and an improved understanding of contractile dysfunction in muscle disease. DW‐MRS can be used to quantitatively measure the translational displacement of endogenous metabolites in intact muscle. DW‐MRS is of fundamental interest because it enables one to probe the in situ status of the intracellular space from the diffusion characteristics of the metabolites, while at the same time providing information on the intrinsic diffusion properties of the metabolites themselves. This yields information on cellular architecture and on the intrinsic capacity for diffusional mass transport.
The Kinetic Stability of Cationic Benzyl Titanium Complexes that Contain a Linked Amido‐Cyclopentadienyl Ligand: The Influence of the Amido‐Substituent on the Ethylene Polymerization Activity of “Constrained Geometry Catalysts”
Authors: Okuda Jun , Musikabhumma Kittichote , Sinnema Piet‐Jan
Published: April 2004

Cationic benzyl titanium complexes [Ti(η5: η1‐C5Me4SiMe2NR')‐(CH2Ph)]+ were cleanly formed by the reaction of the dibenzyl titanium complexes [Ti(η5: η1‐C5Me4SiMe2NR')(CH2Ph)2] with B(C6F5)3 and [Ph3C][B(C6F5)4] in bromobenzene. NMR spectroscopic studies suggest that the benzyl titanium cations contain a fluxional η2‐coordinated benzyl ligand. Kinetic analysis showed that the benzyl titanium cations decompose according to first‐order kinetics and that the amido substituents R' (R' = Me, iPr, tBu) in the linked amido‐cyclopentadienyl ligand influence the lability of these benzyl titanium cations. The order of the kinetic stability of the benzyl titanium cations was found for both anions to follow the order R' = Me > iPr > tBu. The benzyl titanium cations generated with [Ph3C][B(C6F5)4] were found to undergo faster decomposition than those generated with B(C6F5)3. The ethylene polymerization activity order for both systems was found to be the reverse: R' = tBu > iPr > Me. The decomposition of the benzyl titanium cations was suggested to occur via C—H activation with concomitant toluene elimination.
Centroid dynamics with quantum statistics
Authors: Roy Pierre‐Nicholas , Blinov Nicholas
Published: April 2004

This paper discusses the recent developments of a new molecular dynamics approach for the study of quantum dynamics for systems obeying Bose—Einstein statistics. The formalism is based on the mapping of a quantum mechanical system onto a set of phase space variables. These phase space variables are associated with the centroid (center‐of‐mass) of a Feynman path. We present the essential features of the formalism for the representation of operators and correlation functions and discuss some aspects of its practical implementation. We also present a recently developed simplified model for superfluid environments, based on an effective centroid potential approximation to the partition function. In the context of this model, we introduce a new expression for the calculation of the condensate fraction based on the off‐diagonal one‐particle density matrix. We perform calculations for a many‐particle system where the masses and pair interactions correspond to those of helium. The results show that this simplified model exhibits Bose—Einstein condensation below a certain characteristic temperature.
Synthesis of three tethered trisaccharides to probe entropy contributions in carbohydrate—protein interactions
Authors: Zhang Ping , Bundle David R.
Published: April 2004

Crystal structure data show that the branched trisaccharide 1 constitutes the complete antigenic determinant of the Salmonella serogroup B antigen that is recognized by monoclonal antibody SE155.4. In an effort to characterize the entropic costs associated with immobilization of glycosidic torsional angles in the bound state, three distinct intramolecularly tethered analogues of this trisaccharide 2‐4 have been synthesized. Two trisaccharides are tethered by a methylene acetal via the O‐2 position of the 3,6‐dideoxy‐hexose, abequose to either O‐2 of galactose (2) or O‐4 of mannose (3). The third tether, α,α′‐di‐thio‐p‐xylene, spans the C‐6 atoms of the mannose and galactose residues to create trisaccharide 4. The acetal tethers of 2 and 3 span hydroxyl centers that are known to be involved in intramolecular sugar‐sugar hydrogens bonds, but both trisaccharides are biologically inactive due to distorted conformations that cannot be accommodated in the antibody binding site. Trisaccharide 4 is active since both hydroxymethyl groups of galactose and mannose are solvent exposed in the bound state and the constrained conformation of 4 is virtually superimposable on the bound conformation of 1. Despite the retained complimentary and significant reduction of torsional flexibility, trisaccharide 4 exhibits a ΔG° = ‐7.6 kcal mol−1 compared to ΔG° = ‐7.1 kcal mol−1 for 1. The modest free energy gain for the tethered trisaccharide 4 arises from a small entropy gain (TΔΔS = 0.3 kcal mol−1) and an even smaller enthalpic change (ΔΔH = ‐0.2 kcal mol−1).
TheStille Reaction
Authors: Farina Vittorio , Krishnamurthy Venkat , Scott William J.
Published: April 2004

Examples of the palladium‐catalyzed coupling of organotin compounds with carbon electrophiles were first reported in 1977 by Kosugi, Shimizu, and Migita. The first study by Stille appeared in 1978. The early work of Beletskaya, using “ligandless” catalysts in cross‐coupling reactions, also often employed organostannanes. In recognition of Stille's comprehensive synthetic and mechanistic studies, this coupling is now referred to as the Stille reaction.

R1is typically an unsaturated moiety (e.g., vinyl, aryl, heteroaryl, alkynyl, allyl) or less often an alkyl group, and R2, the nontransferable ligand, is almost always butyl or methyl. Electrophiles participating in the coupling include halides (almost always bromides or iodides) and sulfonates (most often used are the triflates). Other leaving groups have been used in special cases.

The Stille reaction belongs to the larger family of palladium‐ and nickel‐catalyzed cross‐coupling reactions which features, e.g., organomagnesium, organozinc, organoboron, and organosilicon reagents.

Organotin reagents are air‐ and moisture‐stable organometallics, and can be conveniently purified and stored. Since they do not react with most common functional groups, the use of protecting groups is almost always unnecessary in conjunction with the Stille reaction. This is a very unusual and attractive feature for an organometallic process. Also, the reaction is often neither air nor moisture sensitive. In some cases, water and oxygen have actually been shown to promote the coupling. Although the reaction as initially described by Stille is often carried out under rather drastic conditions (temperatures of ≥100° are not uncommon), newly developed ligand11and the addition of copper(I) salts have solved some of the problems associated with low reactivity. The utility and mildness of the Stille reaction are demonstrated by its frequent use in the final stages of complex natural product syntheses.

This chapter attempts a critical and comprehensive coverage of the reaction scope. Mechanistic description of the reaction is rather brief, and the reader is referred to the pertinent literature for a more detailed analysis. All of the relevant literature is covered up to the end of 1994. The reaction was reviewed by Stille in 1986, and by Mitchell in 1992; a rather comprehensive account by Farina and Roth has appeared more recently. Developments that occurred in 1995, as this work was in progress, and that were deemed important were incorporated as much as possible in this review.
The Catalyzed α‐Hydroxyalkylation and α‐Aminoalkylation of Activated Olefins (The Morita—Baylis—Hillman Reaction)
Authors: Ciganek Engelbert
Published: April 2004

In a patent application published in 1972, Baylis and Hillman reported the reaction of acetaldehyde with ethyl acrylate and acrylonitrile in the presence of catalytic amounts of 1,4‐diazabicyclo[2.2.2]octane to give the α‐hydroxyethylated products in good yields. No structure proof was given. The assignment eventually was shown to be correct, but since the initial disclosure was not followed by a journal publication, this remarkably simple, atom‐efficient, and useful reaction was ignored for a number of years, and it has been only fairly recently that its potential has begun to be explored. The transformation is now commonly referred to as the Baylis–Hillman reaction. This is unfortunate because credit for its invention clearly belongs to Morita, who five years earlier, in patents and a brief paper reported the same reaction with the exception that tertiary phosphines were used as the catalysts; he also proposed the currently accepted mechanism. Baylis and Hillman made reference to Morita's work in their patent application. It is true that tertiary amines in general are cheaper, less toxic, and more readily removed than tertiary phosphines. However, the latter sometimes give higher yields in shorter reaction times, and there are a number of examples where they are the only useful catalysts.

The DABCO‐catalyzed reaction is slow, and reaction times at room temperature of days or even weeks are common. Attempts to remedy this situation by use of other amine catalysts, change of reaction temperature, high pressure, or microwave irradiation have been partially successful. Activated olefins other than acrylonitrile and acrylic esters that have now been shown to undergo the reaction include α,β‐unsaturated aldehydes and ketones, vinyl sulfoxides, vinyl sulfones, vinylsulfonates, and vinylphosphonates. β‐Substituted activated olefins do not normally react. Among electrophiles other than aldehydes, unactivated ketones undergo the Morita–Baylis–Hillman reaction only under high pressure, but activation such as in α‐halo ketones, α‐keto esters, α‐keto lactones, and nonenolizable α‐diketones often produces very reactive substrates. Imines also can be employed, provided they carry a sufficiently electronegative group on nitrogen.

This chapter coverscatalyzedα‐hydroxyalkylations and α‐aminoalkylations of activated olefins. Transition metal catalyzed reactions of this type, are also included even though they proceed by a different mechanism. Transformations that require stoichiometric amounts of reagents or more than one step are summarized in the section on Comparison With Other Methods but are not included in the Tabular Survey. The Morita–Baylis–Hillman reaction has been reviewed before.
The Retro–Diels–Alder Reaction PartII. Dienophiles with One or More Heteroatom
Authors: Rickborn Bruce
Published: April 2004

This review (Part II) is topically organized around the dienophile that is generated in a retro [4 + 2] reaction, and to the extent possible follows the principles adopted for Part I, which covers reactions in which both dienophile bonding centers are carbon atoms. The present chapter encompasses retro–Diels–Alder (rDA) reactions in which one or both of the dienophile reaction centers are heteroatoms.

Any of the other atoms in the starting material (cycloadduct) may be carbon or heteroatom. Substituents on all positions are encompassed, and any bond order available to any oxidation state of an element is included, as is bonding to non‐nearest neighbor atoms (e.g., bicyclics).

The reader is directed to Part I (Vol. 52) for general discussion of the rDA reaction, the features that affect rates and outcome, and the use of acids, bases, other catalysts, and scavengers. Certain subclassifications (e.g., processes proposed under MS conditions, polymer applications) of rDA reactions have been omitted from both sections; these topics are listed in the Introduction to Part I. Citations to over 100 reviews on various aspects of the rDA reaction are collected at the beginning of the References to Part I. Those that are especially pertinent to Part II will be repeated here, in the context of the particular dienophile under discussion.

The decision to split the review of the rDA reaction into two parts was based on the volume of literature encountered, as outlined in the Introduction to Part I. Active literature searching was concluded for both Parts in April 1995, although occasional more recent references have been included.
Preparation and Applications of Functionalized Organozinc Compounds
Authors: Knochel Paul , Millot Nicolas , Rodriguez Alain L. , Tucker Charles E.
Published: April 2004

Organozinc compounds have been known for more than 150 years. With the exception of zinc enolates (Reformatsky reagents) and iodomethylzinc derivatives (Simmons‐Smith, Furukawa, and Sawada reagents), their synthetic potential has only been recently recognized. This is certainly due to their low reactivity and to the absence of general methods of preparation. Although the carbon‐zinc bond in diethylzinc has a dissociation energy of 34.5 kcal/mol, it has, because of the similar electronegativities of zinc and carbon, a highly covalent character (ca. 85 %), which is comparable to a carbon‐tin bond. The carbon‐zinc bond is therefore inert to moderately polar electrophiles such as aldehydes, ketones, esters, or nitriles. On the other hand, the presence of empty low‐lyingporbitals at the zinc center allows transmetallations with a number of transition metal complexes. This is favored for both kinetic and thermo‐dynamic reasons. The availability ofdorbitals at the metal center in these compounds allows for new reaction pathways with electrophilic reagents that were not available for the corresponding zinc reagents. This reactivity has been exploited for the formation of new carbon‐carbon bonds and efficient cross‐coupling reactions between organozinc derivatives and unsaturated organic halides, as Negishi has demonstrated using catalytic amounts of palladium(0) salts. Similar catalytic processes have been reported with copper(I) and titanium(IV) complexes, which can mediate numerous reactions of organozinc reagents with organic electrophiles.

The scope and synthetic applications of zinc organometallics were greatly extended when it was found that these species can accommodate a wide range of functional groups. They are ideally suited for the construction of polyfunctional organic molecules without the use of multiple protection and deprotection steps. Although some functionalized organozinc compounds bearing ester groups such had been reported, it was only recently that systematic studies have shown the synthetic potential of these reagents. This chapter describes methods for the preparation of functionalized organozinc halides, diorganozincs, and organozincates and their reactions with electrophilic reagents in the presence of transition metal catalysts, as well as synthetic applications demonstrating their synthetic utility for natural product synthesis. Only the preparation and reactivity of zinc organometallics bearing relatively reactive functional groups are covered. Thus, the chemistry of organozinc compounds bearing an ether, acetal, ketal, trialkylsilyl, or polyfluoroalkyl group is, in general, not covered.
The IntramolecularHeck Reaction
Authors: Link J. T.
Published: April 2004

Since the discovery of the Heck reaction, the process has gained widespread acceptance within the synthetic community both as a practical tool and a research area. The intramolecular Heck reaction has enjoyed a similar renaissance, particularly within the last decade. The reaction has emerged as a reliable method for efficiently constructing small, medium, and large rings. Since the reaction occurs under mild and nearly neutral conditions, the functional group tolerance is high. Tandem reactions, which are initiated by intramolecular Heck reactions, have been developed allowing for multiple ring formations, cyclization and intermolecular coupling, and a multitude of other ever‐expanding possibilities. Furthermore, congested tertiary and quaternary centers can be efficiently constructed diastereoselectively or enantioselectively during the ring‐forming reaction. The numerous advances in reaction technology and capability have led to frequent application of the reaction in complex molecule synthesis and combinatorial library preparation. For the purposes of this chapter, the intramolecular Heck reaction is defined as the palladium[0]‐catalyzed intramolecular coupling of an aryl or alkenyl halide or perfluorosulfonate with an alkene, alkyne, allene, or arene.

The mechanism of the Heck reaction is an active area of research. Precise mechanistic details vary substantially depending upon catalyst, substrate, additives, and reaction conditions. The introduction given here is designed to give the chemist a basic understanding of the currently accepted mechanisms to aid in the rational selection and optimization of reaction conditions. Application to total synthesis of natural products and intramolecular Heck reactions on polymer supports are discussed.
The α‐Hydroxy Ketone (α‐Ketol) and Related Rearrangements†
Authors: Paquette Leo A. , Hofferberth John E.
Published: April 2004

Treatment of suitable alpha‐hydroxy aldehydes and ketones with a base, a Bronsted or Lewis acid, or simply with heat has long been know to induce the 1,2‐shift of an alkyl or aryl substituent to form an isomeric product. The synthetic utility of the process was considerably expanded when applied to steroidal D‐ring homoannulations, and has more recently encompassed novel ring expansion to numerous complex target ring systems. The classical alpha‐ketol rearrangement shares, in common with other base‐promoted ketogenic isomerizations, the property of advancing from an alkoxide to a carbonyl group as the migrating center relocates its bonding electrons to the adjacent trigonal center. A distinctive feature of the title reaction, however, is its reversibility, such that the more stable alpha‐hydroxy carbonyl isomer is favored. The process has sometimes been termed the acyloin rearrangement.

This chapter focuses on the isomerization of alpha‐hydroxy ketones, aldehydes, and imines under various reaction conditions exclusive of photochemical activation. The literature coverage extends to 2000.

Since many of the steroidal transformations were carried out in early days when products were not always adequately characterized; errors in structural assignment were occasionally made. Experimental detail was sometimes lacking. To alleviate this problem, we have included only experiments that lead to reasonably pure products of established structure.
Geometry of adaptive control: optimization and geodesics
Authors: Colon Diego , Pait Felipe M.
Published: April 2004

Two incompatible topologies appear in the study of adaptive systems: the graph topology in control design, and the coefficient topology in system identification. Their incompatibility is manifest in the stabilization problem of adaptive control. We argue that this problem can be approached by changing the geometry of the sets of control systems under consideration: estimating np parameters in an np‐dimensional manifold whose points all correspond to stabilizable systems. One way to construct the manifold is using the properties of the algebraic Riccati equation. Parameter estimation can be approached as an optimal control problem akin to the deterministic Kalman filter, leading to algorithms that can be used in conjunction with standard observers and controllers to construct stable adaptive systems. Copyright © 2004 John Wiley & Sons, Ltd.
Convergence theory for multi‐input discrete‐time iterative learning control with Coulomb friction, continuous outputs, and input bounds
Authors: J. Driessen Brian , Sadegh Nader
Published: April 2004

In this paper we consider the problem of discrete‐time iterative learning control (ILC) for position trajectory tracking of multiple‐input, multiple‐output systems with Coulomb friction, bounds on the inputs, and equal static and sliding coefficients of friction. We present an ILC controller and a proof of convergence to zero tracking error, provided the associated learning gain matrices are scalar‐scaled with a sufficiently small positive scalar. We also show that non‐diagonal learning gain matrices satisfying the same prescribed conditions do not lead to the same convergence property. To the best of our knowledge, for problems with Coulomb friction, this paper represents a first convergence theory for the discrete‐time ILC problem with multiple‐bounded‐inputs and multiple‐outputs; previous work presented theory only for the single‐input, single‐output problem. Copyright © 2004 John Wiley & Sons, Ltd.
Development of an amperometric biosensor based on a redox‐mediator‐doped polypyrrole film
Authors: Rajesh  , Pandey Shyam S. , Takashima W. , Kaneto K.
Published: April 2004

An amperometric biosensor was developed for the quantitative estimation of phenolic compounds in aqueous media. The enzyme tyrosinase [poly(phenol oxidase) (PPO)] was adsorbed onto a hexacyanoferrate(II)‐ion‐doped conducting polypyrrole (PPY) film deposited on an indium tin oxide (ITO) coated glass‐plate support. The PPO activity in the PPO/Fe2+‐PPY/ITO film was assayed as a function of the concentration of phenolic compounds. Cyclic voltammetric studies were carried out on this enzyme electrode, and the surface morphology of the enzyme‐immobilized polymer film was studied with scanning electron microscopy. The results of the amperometric response of the PPO/Fe2+‐PPY/ITO film showed sensitivities of 0.14, 0.21, and 0.36 A M−1cm−2 and linear response ranges of 9.9–84.7, 6.7–72.6, and 3.9–48.8 μM for phenol, catechol, and p‐chlorophenol, respectively. The PPO/Fe2+‐PPY/ITO electrode exhibited a response time of about 50 s and was stable for about 12 weeks at 4°C. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 927–933, 2004
Ethoxylated polyethylenimine gel‐coated on textile‐grade acrylic fiber. A thermally regenerable superfast sorbent for water desalination
Authors: Chanda Manas , Sarkar Amitava , Modak Jayant M.
Published: April 2004

A commercial acrylic fiber containing 92 wt % acrylonitrile was hydrolyzed to convert a part of its nitrile (CN) groups to carboxylic acid (COOH) groups and then was coated chemically with 80% ethoxylated polyethylenimine (EPEI) resin, followed by crosslinking with glutaraldehyde. The resulting sorbent, PAN(CO2H)(EPEI.XG), containing carboxylic acid groups and weakly basic tertiary amine groups in close proximity on the same fiber is found to simulate the well‐known Sirotherm™ resins used for partial desalination of brine solution by adsorbing the salt at ambient temperature and desorbing it at an elevated temperature in the same solution. The sorption behavior of the new sorbent was evaluated for solutes NaCl and MgCl2, showing saturation capacities of 0.797 and 0.877 meq/g (dry) sorbent fiber, respectively, at 30°C. The equilibrium sorption data show good agreement with both Langmuir and Freundlich isotherms for sorption from single‐component solutions and with Butler–Ockrent and LeVan–Vermeulen models for bicomponent sorption. Although the equilibrium uptake of NaCl reaches maximum in neutral solutions (pH ∼ 6.5), falling at both lower and higher pH, that of MgCl2 is augmented in alkaline pH due to additional sorption by cation exchange at the ionic sites formed at higher pH. The initial uptake of the salt, which is nearly instantaneous, exceeds the sorption value attainable at equilibrium. The high initial rate of salt uptake fits a shell‐core kinetic model for sorption on fiber of cylindrical geometry. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 883–893, 2004
Changes in properties of silane–water crosslinked metallocene ethylene‐octene copolymer after prolonged crosslinking time
Authors: Sirisinha Kalyanee , Meksawat Darinya
Published: April 2004

The silane–water crosslinking of metallocene ethylene–octene copolymer is described. Particular consideration is made on the changes in mechanical, thermal, and rheological properties of crosslinked polymers after prolonged crosslinking time. The silane grafting is carried out in a twin‐screw extruder, using a free‐radical reaction of vinyl trimethoxysilane and dicumyl peroxide. The crosslinking reaction is conducted at 70°C in the presence of water. The results show that the rate of gel formation is very high in the early stage of crosslinking process and decreases thereafter. The maximum gel content of 77% is obtained after 60 h of immersing time. Increasing the immersing time beyond 60 h, the gel content does not increase further. However, the FTIR results reveal an increase in siloxane linkages in those samples, indicating an increase in crosslink density without a simultaneous increase in gel content. This finding is supported by a rheological study where a significant increase in storage modulus is clearly observed. As a result of denser network, movement and flexibility of polymer chains are more restricted, leading to some changes in tensile properties. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 901–906, 2004
Polymer reaction in polycarbonate with Na2CO3
Authors: Takeda Kunihiko , Unno Haruo , Zhang Min
Published: April 2004

The recombination of polycarbonate (PC) chains has been researched as one of the studies on self‐repairing materials. The specimens, which included sodium carbonate and Tri(2,4‐di‐t‐buthylphenyl)phosphate, were treated at 120°C in a steam chamber and subsequently at 130°C under nitrogen atmosphere. The molecular weight of the specimen with 0.1 ppm of sodium carbonate increased during the subsequent treatment. The same tendency was observed when sodium carbonate was supplementally added to the specimen. In particular, the molecular weight little changed under severe condition when the concentration of sodium carbonate was controlled to be 100 to 1000 ppm. The result showed PC could be used as a self‐repairing material. The deterioration is expected to be minimized if the recovery rate is the same as the deterioration rate by applying these self‐repairing systems. The tensile strength, the transparency, and the reaction schemes are discussed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 920–926, 2004
Interchain crosslinkable polymer blends of polyurethane and polyacrylic elastomer (sulfur cure)
Authors: Khatua B.B. , Das C.K.
Published: April 2004

Blends of polyurethane and polyacrylic elastomers prepared by three different blending techniques have been studied in different blend ratios. The processability of the polyurethane elastomer was improved as a result of blending with the polyacrylic elastomer. The blending technique has a significant role in determining the physical properties of the blends. Improvement of physical properties was observed in the blends containing the interchain crosslink bonds. IR spectral analysis suggested the formation of interchain crosslink between the two elastomers phases on heat treatment, before the addition of any curatives. Thermal stability of the blends was also improved when preblending and preheating techniques were applied. The extraction of the single phase by solvent was also restricted to a significant extent for the preheated sample probably due to the interchain crosslinking. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 845–853, 2004
Perfusion cultures of human tumor cells: a scalable production platform for oncolytic adenoviral vectors
Authors: Yuk Inn H. Y. , Olsen Matthew M. , Geyer Scott , Forestell Sean P.
Published: April 2004

This study demonstrates the novel use of the HeLaS3 human tumor cell line for propagating ONYX‐411, a recombinant oncolytic adenoviral vector. HeLaS3 cells enabled high levels of vector production without the risk of generating vector recombinants, which is possible with HEK293 cells. The development of a high‐cell‐density perfusion process using ATF technology yielded production levels as high as 6 × 1011 vp/mL, which was approximately sevenfold greater than the titers achieved in fed‐batch bioreactors. Several experiments were performed at the bench (15 L) and pilot (70 L) scales to demonstrate the robust and scalable nature of this industrially relevant technology. © 2004 Wiley Periodicals, Inc.