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Important Reactions (important + reaction)
Terms modified by Important Reactions Selected AbstractsAsymmetric Ring Opening of Benzo-7-oxabicyclo[2.2.1]heptadienes with Cationic Rhodium ComplexesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2010Angelika Preetz Abstract The efficient design of stereochemically challenging ring systems by ring opening of heterobicyclic alkenes has become a very important reaction in the chemistry of CC and CX bond formation. By using the hitherto applied in situ technique for the generation of the ,2 -bridged, dimeric neutral rhodium complexes, however, the catalytically active species and its concentration remained unidentified. Furthermore, the reaction temperature is at least 80,°C. The application of cationic rhodium(I) solvate complexes (that no longer contain blocking diolefins) shows that a much greater activity and enantioselectivity for the synthesis of 1,2-dihydro-1-naphthols can be reached than was described so far, even at ambient temperature. NMR spectroscopy and X-ray analysis show that a product inhibition during the ring opening reaction takes place that is independent of the nucleophile. [source] A Simple, Effective Boron-Halide Ethoxylation CatalystADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 5 2010Kenneth Abstract Boron esters B(OR)3, readily derived from boric acid and alcohols, combine with iodide or bromide to catalyze the ethoxylation of alcohols and phenols, giving good rates and narrow product distributions. The combined action of a weak electrophile [B(OR)3] and a weak nucleophile (halide) allows for the ethoxylation of base-sensitive alcohols. Experiment suggests a new mechanism for this commercially important reaction proceeding through key ,-haloalkoxy intermediates. [source] Ultrastructural and histochemical study on gills and skin of the Senegal sole, Solea senegalensisJOURNAL OF APPLIED ICHTHYOLOGY, Issue 6 2004J. M. Arellano Summary This study was undertaken to identify the normal ultrastructural features of gills and skin of the Senegal sole, Solea senegalensis, for a comparative measure to morphological alterations caused by environmental stressors such as reduced water quality and diseases. In the Senegal sole skin, four morphologically distinct layers were identified: cuticle, epidermis, dermis and hypodermis. The epidermis was composed of stratified epithelium containing three cellular layers: the outermost or mucosa layer, the middle or fusiform layer and the stratum germinativum or the basal layer. In the mucosa, two mucous cell types were differentiated: type A cells containing several round vesicles of different electron density and type B cells containing mucosomes of uniform electron density. Senegal sole have five pairs of gill arches, each containing two rows of well-developed and compactly organized primary filaments and secondary lamellae. Fingerprint-like microridges were observed on the surface of epithelial cells. The branchial lamellae epithelium consisted of different cell types: pavement, mucous and chloride. Between the chloride cells and the larger pavement cells, accessory cells were observed. Complexes of tight junctions and desmosomes were frequently observed between adjacent chloride and epithelial cells. Neutral mucosubstances and/or glycoconjugates were observed in the epidermis, dermis and hypodermis of S. senegalensis skin. Proteins rich in different amino acids, such as arginine and cysteine, reacted negatively or weakly positive in the epidermis, dermis and hypodermis. In gills, some mucous cells responded weakly positive to periodic acid-Schiff (PAS) reaction but were strongly stained with Alcian Blue at pH 0.5, 1 and 2.5. When Alcian Blue pH 2.5,PAS reaction was performed, most mucous cells were stained blue (carboxylated mucins) and some mucocytes stained purple, indicating a combination of neutral and acid mucins. Proteins rich in cysteine-bound sulphydryl (-SH-) and cystine disulphide (-S-S-) groups were strongly detected in branchial and epidermal mucous cells, whereas lysine, tyrosine and arginine containing proteins showed very weak staining in both epidermal and branchial mucous cells. Protein reactions were strongly positive in the pillar cells, except for those rich in tryptophan, whereas the branchial cartilaginous tissue did not show an important reaction. The performed lipid reactions were negative in goblet and chloride cells. It is concluded from this study that ultrastructural and cytohistochemical features of the Senegal sole skin and gills may serve as control structures in both natural and aquaculture systems to monitor or detect environmental stress responses at the histological level. [source] Use of micro-Raman spectroscopy to study reaction kinetics in blended white cement pastes containing metakaolinJOURNAL OF RAMAN SPECTROSCOPY, Issue 12 2009Moisés Frías Abstract Curing temperature is known to play an important role in the formation, development, and stability of the hydrated phases appearing during pozzolanic reactions (chemical reaction between puzzolanic addition, metakaolin (MK), and calcium hydroxide from cement hydration). A typical example of this important reaction is to be found in metakaolin-bearing cement pastes, characterized by hexagonal phases whose thermodynamic stability declines with rising temperature. These phases cannot be exhaustively researched with traditional techniques (such as X-ray diffraction) due to their poor crystallinity. Consequently, micro-Raman spectroscopy was used in the present study to explore the behavior of white cement paste blends containing 0, 10, and 25% MK at two curing temperatures (20 and 60 °C). This led to the identification, for the first time using Raman spectroscopy, of phases C2ASH81 (stratlingite) and C3ASH6, which appear in the MK,white cement reaction. The CSH gel formed was characterized by Q1 dimers and a C/S ratio of 1.3,1.5. Raising the curing temperature favored the formation of C4AH13. Copyright © 2009 John Wiley & Sons, Ltd. [source] Molecular determinants of ligand specificity in family 11 carbohydrate binding modules , an NMR, X-ray crystallography and computational chemistry approachFEBS JOURNAL, Issue 10 2008Aldino Viegas The direct conversion of plant cell wall polysaccharides into soluble sugars is one of the most important reactions on earth, and is performed by certain microorganisms such as Clostridium thermocellum (Ct). These organisms produce extracellular multi-subunit complexes (i.e. cellulosomes) comprising a consortium of enzymes, which contain noncatalytic carbohydrate-binding modules (CBM) that increase the activity of the catalytic module. In the present study, we describe a combined approach by X-ray crystallography, NMR and computational chemistry that aimed to gain further insight into the binding mode of different carbohydrates (cellobiose, cellotetraose and cellohexaose) to the binding pocket of the family 11 CBM. The crystal structure of C. thermocellum CBM11 has been resolved to 1.98 Å in the apo form. Since the structure with a bound substrate could not be obtained, computational studies with cellobiose, cellotetraose and cellohexaose were carried out to determine the molecular recognition of glucose polymers by CtCBM11. These studies revealed a specificity area at the CtCBM11 binding cleft, which is lined with several aspartate residues. In addition, a cluster of aromatic residues was found to be important for guiding and packing of the polysaccharide. The binding cleft of CtCBM11 interacts more strongly with the central glucose units of cellotetraose and cellohexaose, mainly through interactions with the sugar units at positions 2 and 6. This model of binding is supported by saturation transfer difference NMR experiments and linebroadening NMR studies. [source] Experimental measurements and kinetic modeling of CO/H2/O2/NOx conversion at high pressure,INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2008Christian Lund Rasmussen This paper presents results from lean CO/H2/O2/NOx oxidation experiments conducted at 20,100 bar and 600,900 K. The experiments were carried out in a new high-pressure laminar flow reactor designed to conduct well-defined experimental investigations of homogeneous gas phase chemistry at pressures and temperatures up to 100 bar and 925 K. The results have been interpreted in terms of an updated detailed chemical kinetic model, designed to operate also at high pressures. The model, describing H2/O2, CO/CO2, and NOx chemistry, is developed from a critical review of data for individual elementary reactions, with supplementary rate constants determined from ab initio CBS-QB3 calculations. New or updated rate constants are proposed for important reactions, including OH + HO2 , H2O + O2, CO + OH , [HOCO] , CO2 + H, HOCO + OH , CO + H2O2, NO2 + H2 , HNO2 + H, NO2 + HO2 , HONO/HNO2 + O2, and HNO2(+M) , HONO(+M). Further validation of the model performance is obtained through comparisons with flow reactor experiments from the literature on the chemical systems H2/O2, H2/O2/NO2, and CO/H2O/O2 at 780,1100 K and 1,10 bar. Moreover, introduction of the reaction CO + H2O2 , HOCO + OH into the model yields an improved prediction, but no final resolution, to the recently debated syngas ignition delay problem compared to previous kinetic models. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 454,480, 2008 [source] Ceria in catalysis: From automotive applications to the water,gas shift reactionAICHE JOURNAL, Issue 5 2010Raymond J. Gorte Abstract Ceria is a crucial component of automotive catalysts, where its ability to be reduced and re-oxidized provides oxygen storage capacity. Because of these redox properties, ceria can greatly enhance catalytic activities for a number of important reactions when it is used as a support for transition metals. For reactions that use steam as an oxidant (e.g., the water,gas-shift reaction and steam reforming of hydrocarbons), rates for ceria-supported metals can be several orders of magnitude higher than that for ceria or the transition metal alone. Because the redox properties of ceria are strongly dependent on treatment history and the presence of additives, there are significant opportunities for modifying catalysts based on ceria to further improve their performance. This article will review some of the contributions from my laboratory on understanding and using ceria in these applications. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source] Tetrahedral intermediates in reactions of carboxylic acid derivatives with nucleophiles,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 3 2005Martin Adler Abstract Transacylation reactions of carboxylic acids, carboxylic acid esters, carboxylic acid amides and other carboxylic acid derivatives are among the most widespread and most important reactions in chemistry and biochemistry. Already in 1887, Claisen suggested a tetrahedral intermediate in transformations of carboxylic acid derivatives with nucleophiles. A historical overview gives insight into the studies to detect possible tetrahedral intermediates in such reactions. However, only in recent years has detailed information concerning the structures of such species become available. In this review, neutral, cationic and anionic tetrahedral intermediates are described which serve as models for transacylations under neutral, acid-catalysed or basic conditions. The characteristically different structures correspond nicely with experimental experience with reactions of carboxylic acid derivatives and with quantum chemical model calculations on tetrahedral intermediates. Finally, by means of model calculations, an explanation is given for the fast reactions of Weinreb amides, RC(O)N(CH3)OCH3, with organolithium and even with Grignard reagents: the reactions are determined by comparatively stable chelate transition states. Copyright © 2004 John Wiley & Sons, Ltd. [source] Mathematical Modeling of Atom-Transfer Radical Polymerization Using Bifunctional InitiatorsMACROMOLECULAR THEORY AND SIMULATIONS, Issue 3 2006Mamdouh Al-Harthi Abstract Summary: Bifunctional initiators can produce polymers with higher molecular weight at higher initiator concentrations than monofunctional initiators. In this study, we developed a mathematical model for ATRP with bifunctional initiators. The most important reactions in ATRP were included in the model. The method of moments was used to predict monomer conversion, average molecular weights and polydispersity index as a function of polymerization time in batch reactors. The model was used to understand the mechanism of ATRP and to quantify how polymerization conditions affect monomer conversion and polymer properties by examining the effect of several rate constants (activation, deactivation, propagation and chain termination) and of catalyst and initiator concentration on polymerization kinetics and polymer properties. When compared to monofunctional initiators, bifunctional initiators not only produce polymers with higher molecular weight averages at higher polymerization rates, but also control their molecular weight distributions more effectively. Effect of initial catalyst concentration on polydispersity index as a function of time. [source] | ||||||||||