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Model Reaction (model + reaction)
Selected AbstractsReactions of Cl,/Cl2,, Radicals with the Nanoparticle Silica Surface and with Humic Acids: Model Reactions for the Aqueous Phase Chemistry of the AtmospherePHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2007Paula Caregnato ABSTRACT Reactions of chlorine radicals might play a role in aqueous aerosols where a core of inorganic components containing insulators such as SiO2 and dissolved HUmic-LIke Substances (HULIS) are present. Herein, we report conventional flash photolysis experiments performed to investigate the aqueous phase reactions of silica nanoparticles (NP) and humic acid (HA) with chlorine atoms, Cl,, and dichloride radical anions, Cl2,,. Silica NP and HA may be taken as rough models for the inorganic core and HULIS contained in atmospheric particles, respectively. Both Cl, and Cl2,, were observed to react with the deprotonated silanols on the NP surface with reaction rate constants, k ± ,, of (9 ± 6) × 107 M,1 s,1 and (7 ± 4) × 105 M,1 s,1, respectively. The reaction of Cl, with the surface deprotonated silanols leads to the formation of SiO, defects. HA are also observed to react with Cl, and Cl2,, radicals, with reaction rate constants at pH 4 of (3 ± 2) × 1010 M,1 s,1 and (1.2 ± 0.3) × 109 M,1 s,1, respectively. The high values observed for these constants were discussed in terms of the multifunctional heterogeneous mixture of organic molecules conforming HA. [source] Distributive mixing profiles for co-rotating twin-screw extrudersADVANCES IN POLYMER TECHNOLOGY, Issue 3 2001Gifford Shearer Distributive mixing was experimentally measured during polymer melt blending along the length of a co-rotating twin-screw extruder. A mixing limited interfacial reaction between two reactive polymer tracers was employed to gain direct evidence of the generation of interfacial area. Model reactions were performed to validate this novel experimental method. In particular, the conversion was a direct indicator of the interfacial area available for the reaction. Specially designed sampling ports were used to obtain polymer samples along the length of the extruder during its continuous operation. The mixing capabilities of conveying sections and kneading blocks were compared over a wide range of operating conditions. In conveying sections, distributive mixing was controlled by the fully filled fraction. The mixing in kneading blocks depended on the combination of the operating conditions and the stagger angle. © 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 169,190, 2001 [source] Preparation of single-walled carbon nanotubes-induced poly(p -oxybenzoyl) crystalsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2008Kazufumi Kobashi Abstract Crystallization of oligomers was applied for the preparation of single-walled carbon nanotubes (SWNTs)/poly(p -oxybenzoyl) (POB) crystals using SWNTs as a nucleating agent. Polymerization conditions were investigated to induce the crystallization of POB oligomers through SWNTs. SWNTs/POB plate-like or lozenge-shaped crystals were successfully prepared by direct polymerization of p -hydroxybenzoic acid (HBA) in a mixed solvent of DMF/Py with TsCl in the presence of functionalized SWNTs. The size of the plate-like crystals were ,200 nm to 3 ,m. The crystals consisted of some layers, ,3 nm thick plates. Model reactions showed that esterification reactions proceed between functionalized SWNTs and HBA monomers in the polymerization system. The obtained crystals exhibited unique morphology and high crystallinity, producing a novel SWNT/POB hybrid. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1265,1277, 2008 [source] Condensation routes to polyaniline and its analogsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2007H. K. Hall Abstract To obtain polyanilines which are more structurally perfect than those obtainable by the oxidation of anilines, polycondensation procedures were investigated. Model reactions gave extensive information about yields and about the physical properties of the putative structural polymer units. Condensation of anthraquinones with aromatic diamines using titanium tetrachloride and the unique base, 1,4-diazabicyclo-[2.2.2]-octane Dabco, gave high molecular weight orange polyquinonimines. Alkoxy groups on the anthraquinone ring aided solubility and molecular weight, and appropriately positioned alkoxy groups afforded stereoregular polymers. A bisthiophene benzoquinone also polymerized successfully. Application of the same procedure to 2,5-dimethyl- p -benzoquinone gave stereoregular poly(arylene benzoquinonimines), close analogs of pernigrani line. The factors causing problems in achieving high yields and high molecular weight were identified. Recent synthetic developments in this field are discussed. Reduction of the obtained polyquinonimines proceeded smoothly to the leucoemeraldine analogs. Unlike the results from pernigraniline obtained by oxidative polymerization, no evidence for the formation of the electrically conductive emeraldine form was obtained. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4751,4763, 2007 [source] Theory of chemical bonds in metalloenzymes.INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2008Abstract A first principle investigation has been carried out for intermediate states of the catalytic cycle of a cytochrome P450. To elucidate the whole catalytic cycle of P450, the electronic and geometrical structures are investigated not only at each ground state but also at low-lying energy levels. Using the natural orbital analysis, the nature of chemical bonds and magnetic interactions are investigated. The ground state of the Compound 1 (cpd1) is calculated to be a doublet state, which is generated by the antiferromagnetic coupling between a triplet Fe(IV)O moiety and a doublet ligand radical. We found that an excited doublet state of the cpd1 is composed of a singlet Fe(IV)O and a doublet ligand radical. This excited state lies 20.8 kcal mol,1 above the ground spin state, which is a non-negligible energy level as compared with the activation energy barrier of ,E# = 26.6 kcal mol,1. The reaction path of the ground state of cpd1 is investigated on the basis of the model reaction: 3O(3p) + CH4. The computational results suggest that the reactions of P450 at the ground and excited states proceed through abstraction (3O-model) and insertion (1O-model) mechanisms, respectively. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] The Immobilization of Rhodium-4-(diphenylphosphino)-2- (diphenylphosphinomethyl)-pyrrolidine (Rh-PPM) Complexes: A Systematic StudyADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 12-13 2006Benoît Pugin Abstract A modular toolbox for the immobilization of homogeneous catalysts to various supports is described. It consists of functionalized chiral diphosphines and three different linkers based on isocyanate chemistry and it is used to attach the 4-(diphenylphosphino)-2-(diphenylphosphinomethyl)-pyrrolidine (PPM) ligand to a large variety of soluble, swellable and non-swellable solid organic polymers and to silica gels. As model reaction the hydrogenation of acetamidocinnamic acid derivatives, catalyzed with high enantioselectivity was chosen. Besides information on the usefulness of a particular type of support for synthetic applications, the experiments were also designed to address the question how parameters such as solubility, swellability, cage or pore size and solvent affect the rate and enantioselectivity of an immobilized catalyst. Rhodium complexes of ligands attached to soluble polymers and inorganic supports achieved ees up to 95,% and turnover frequencies between 700 and 1400,h,1, very close to the values of the homogeneous Rh catalyst (ee 95,%, TOF 1320,h,1). Insoluble or strongly cross-linked organic polymers led to catalysts with lower enantioselectivity and activity. PPM ligands attached to water soluble dendrimer fragments allowed hydrogenation in water solution with ees up to 94,%, albeit with much lower activity compared to reactions in methanol with the homogeneous catalyst. [source] Enantioselective Mannich-Type Reaction Catalyzed by a Chiral Brønsted Acid Derived from TADDOLADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-13 2005Takahiko Akiyama Abstract A novel cyclic dialkyl phosphate was synthesized starting from (+)-diethyl tartrate. Its catalytic activity as a chiral Brønsted acid has been examined in the Mannich-type reaction of a ketene silyl acetal with aldimines as a model reaction. The corresponding ,-amino acid esters were obtained with high enantioselectivity. [source] Polymerization of linseed oil with phenolic resinsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2010Gökhan Çayl Abstract In this study, linseed oil was directly polymerized with different oil soluble resoles. p- Ethyl (PEP), p-tertiary butyl (PTB), p-tertiary octyl (PTO), and p- phenyl (PPP) phenols were separately reacted with formaldehyde to give linseed oil soluble resoles. These were then reacted with linseed oil to give transparent rubbery polymers. A model reaction was performed with methyl oleate and PTB phenol resole to clarify the reaction mechanism. Reaction products were characterized with 1H-NMR and IR techniques. Spectral examination of the model reaction showed that polymerization reaction proceeded via ene reaction of the quinone methide formed at the end group of the resole with the allylic positions of the fatty ester. Rubbery polymers were obtained with linseed oil using 10 to 40% of the different resoles. Hard, load bearing and tough materials were obtained at 40% phenolic resin loading. Mechanical properties of the materials were characterized by dynamic mechanical analyzer (DMA) and stress,strain tests. The best mechanical and thermal properties were obtained with PEP resole which showed a storage modulus of 350 MPa and a tan , peak at 65°C. Storage moduli of 275, 250, and 30 were obtained for PPP, PTB, and PTO resoles-linseed oil polymers, respectively. At the same phenolic resin loading, elongation at break and swelling ratios in CH2Cl2 increased with the longer alkyl substitution on the resole resins. The highest thermal stability was observed by PEP resole,linseed oil polymer which has a 5% weight loss temperature of 340°C as determined by TGA. The lowest thermal stability was observed for PTB resole-linseed oil polymer at 220°C. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Synthesis of a new photoreactive gelatin with BTDA and HEMA derivativesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2008Fan-Chun Ding Abstract A novel bio-affinitive, photocuring, and membrane-forming gelatin derivative was synthesized in this study. This process was based on the amide formation between carboxylic acid and the amine in methanol,water media using dicyclohexyl-carbodiimide (DCC) as a condenser. Gelatin and glycine were the sources of amine in the model reaction. Since there were two anhydride groups in each 3,3,,4,4,-benzophenone tetra-carboxylic dianhydride (BTDA) molecule, two 2-hydroxyethyl methacrylate (HEMA) molecules were used to induce the ring-opening reaction of BTDA and release two carboxylic acid groups. The resulting photoreactive gelatin was called GE-BTHE, of which the photoreactive component was the ketone groups of BTDA and HEMA that played the role of double bond supplier. This photoreactive gelatin could be converted from the transparent liquid phase into swollen membrane by a 6-min irradiation of high pressure mercury lamp. The most efficient irradiation was at 267 nm and the highest degree of swelling of the cured GE-BTHE membrane could reach 5.9. The elongation from the dried gel remained 5,10%, i.e., relatively elastic. The properties of this gelatin derivative were investigated using amide formation analysis, calculation of the gel content and the swelling ratio, and monitoring of the photocuring process. The GE-BTHE synthesized in this study should be very potential in applications such as protective wound dressings and hemostatic absorbents for minimally invasive surgery. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Ortho -[18F]Fluoronitrobenzenes by no-carrier-added nucleophilic aromatic substitution with K[18F]F,K222,A comparative studyJOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 13 2002M. Karramkam Abstract The scope of the nucleophilic aromatic ortho -fluorinations from the corresponding ortho -halonitrobenzene precursors (halo-to-fluoro substitutions) with no-carrier-added [18F]fluoride ion as its activated K[18F]F,K222 complex has been evaluated via the radiosynthesis of ortho -[18F]fluoronitrobenzene, chosen as a model reaction. The parameters studied include the influence of the leaving group in the ortho position of the phenyl ring (,Cl, ,Br, ,l), the quantity of precursor used, the type of activation (conventional heating or microwave irradiations), the solvent, the temperature and the reaction time. The iodo-precursor was completely unreactive and the bromo-precursor gave only low incorporation (<10%) in the optimal conditions used (conventional heating at 145°C or microwave activation, 100 W for 120 s). Only the chloro-precursor was found reactive in the conditions described above and up to 70% yield was observed for the formation of ortho -[18F]fluoronitrobenzene ([18F]- 1). In all the experiments, the unwanted ortho -[18F]fluoro-halobenzenes, potentially resulting from the nitro-to-fluoro substitution, could not be detected. These results will be applied for the radiosynthesis of 5-[18F]fluoro-6-nitroquipazine, a potent radioligand for the imaging of the serotonin transporter with PET. Copyright © 2002 John Wiley & Sons, Ltd. [source] Increased gas solubility in nanoliquids: Improved performance in interfacial catalytic membrane contactorsAICHE JOURNAL, Issue 2 2009Marc Pera-Titus Abstract The kinetics of gas-liquid catalytic reactions can be strongly promoted when these are performed in interfacial catalytic membrane reactors instead of other three-phase reactors such as slurry stirrers or trickle beds. The well-defined gas-liquid-catalyst contact in this system avoiding diffusional limitations is usually argued as the main reason for such enhancement. In this work, using nitrobenzene hydrogenation as a model reaction, we propose that this increased catalytic performance might also be attributed, at least partially, to increased gas solubilities in mesoconfined solvents (or simply "nanoliquids") in interfacial contactors overcoming the values predicted by Henry's Law. To support this hypothesis, we provide experimental evidence of a dramatic increase of H2 solubility in confined ethanol using mesoporous ,-Al2O3 as confining solid. Gas-liquid solubilities can be enhanced up to five times over the corresponding bulk values for nanoliquid sizes lower than 15 nm as long as the gas-liquid interface is confined in a mesoporous array. In such a situation, the volume of the gas-liquid interface is no longer negligible compared to the total volume of the confined liquid, and the high surface excess concentrations of the gas adsorbed on the liquid surface make solubility grow up dramatically. According to these measurements, we discuss how nanoliquids might form in catalytic membrane contactors, which gas-liquid configuration in the reactor appears to be more appropriate, and how the structure of the mesoporous catalytic layer contributes to their increased gas solubilization performance. © 2008 American Institute of Chemical Engineers AIChE J, 2009 [source] Comparing flow-reversal and inner recirculation reactors: Experiments and simulationsAICHE JOURNAL, Issue 7 2003Moshe Ben-Tullilah The operation of reactors with flow reversal operate similar to a reactor with internal recirculation, which the feed enters through one (say, inner) reactor and then turns around and flows out through (the outer) another, when the heat-transfer coefficient between the tubes is large. In this study, we compare the behavior of a packed-bed reactor operating in flow-reversal or internal-recirculation modes, using ethylene oxidation on Pt/Al2O3 as a model reaction. The reactor was built from two concentric tubes (with 28.5 and 42.5 mm in diameter), both packed with a 20 cm catalytic bed and 10 cm inert beds (of alumina-pellets) on each side. An adjustable opening between the tubes allowed for an internal recycle mode and the whole system could be operated with periodic flow reversal. The reactor can be employed then either as a simple once-through bed or as a bed with flow reversal in the inner tube or as bed with internal recirculation flowing from the inner to outer tube, or in the opposite direction, as well as an internal-recirculation reactor with flow reversal. Due to heat losses, the latter two modes were inferior to the others. The experiments, backed by simulations using a homogeneous model with independently determined parameters, showed that the technically-simpler inner-outer internal-recycle reactor operated better at low flow rates, than that with flow reversal, but the conclusion is reversed at high flow rates. The domain where the internal-recirculation reactor is superior depends on the heat-transfer coefficient between the streams. By lowering the feed concentration, the extinction point was determined for each mode highlighting again the conclusions drawn above that inner-recirculation operation may be superior to flow reversal at low flow rates. Simulations revealed also the existence of solutions with stationary fronts or oscillatory fronts. [source] Synthesis of 1H -quinazoline-2,4-diones from 2-aminobenzonitriles by fixation of carbon dioxide with amidine moiety supported polymer at atmospheric pressureJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2009Daisuke Nagai Abstract 1H -Quinazoline-2,4-diones, which are key intermediates in the synthesis of medicines, were successfully synthesized from 2-aminobenzonitriles by the fixation of CO2 in the presence of a polystyrene derivative bearing amidine moiety [poly(amidine)]. A model reaction, that is, the reaction of 2-aminobenzonitrile (1a) with CO2 in the presence of N -methyltetrahydropyrimidine (MTHP) revealed that a catalytic amount of MTHP afforded 1H -quinazoline-2,4-dione (2a) quantitatively at atmospheric pressure. Several 1H -quinazoline-2,4-diones (2a - 2c) were successfully synthesized from the corresponding 2-aminobenzonitriles (1a - 1c) in the presence of poly(amidine). The poly(amidine) could easily be separated from the reaction mixture by filtration and reused in subsequent reactions owing to the heterogeneous system. These demonstrated that poly(amidine) is a useful heterogeneous polymer-supported reagent for the synthesis of 1H -quinazoline-2,4-diones from CO2. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 653,657, 2009 [source] Living cationic polymerization of amide-functional vinyl ethers: Specific properties of SnCl4 -based initiating systemJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 18 2008Motomasa Yonezumi Abstract Living cationic copolymerization of amide-functional vinyl ethers with isobutyl vinyl ether (IBVE) was achieved using SnCl4 in the presence of ethyl acetate at 0 °C: the number,average molecular weight of the obtained polymers increased in direct proportion to the monomer conversion with relatively low polydispersity, and the amide-functional monomer units were introduced almost quantitatively. To optimize the reaction conditions, cationic polymerization of IBVE in the presence of amide compounds, as a model reaction, was also examined using various Lewis acids in dichloromethane. The combination of SnCl4 and ethyl acetate induced living cationic polymerization of IBVE at 0 °C when an amide compound, whose nitrogen is adjacent to a phenyl group, was used. The versatile performance of SnCl4 especially for achieving living cationic polymerization of various polar functional monomers was demonstrated in this study as well as in our previous studies. Thus, the specific properties of the SnCl4 initiating system are discussed by comparing with the EtxAlCl3,x systems from viewpoints of hard and soft acids and bases principle and computational chemistry. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6129,6141, 2008 [source] Pd(0)-catalyzed polycondensation of methyl propargyl carbonate and bisphenols under stoichiometrically imbalanced conditionsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 6 2008Tomonori Takemura Abstract We found that a novel Pd(0)-catalyzed polycondensation of methyl propargyl carbonate (1) and bisphenol analogs 4 such as 4,4,-dihydroxydiphenyl ether (4b) proceeded successfully and gave polyethers 5 having exomethylene groups. The polycondensation was affected by the kind of bisphenol analogs used. The molar ratio of 1 and 4 also affected the polymerization behavior. Polyethers 5 with higher molecular weights were obtained by polymerization with 0.5 equiv of 4 toward 1. The use of 1 equiv of 4 gave poor results. When 4,4,-dihydroxybenzophenone (4d) was employed as a nucleophile, benzenesulfonamide (13) was needed as an additive for obtaining the corresponding polyether with higher molecular weight. The Pd(0)-catalyzed condensation of 1 and p -hydroxyacetophenone was conducted as a model reaction. The desired compound and three by-products were produced. Two of the by-products were formed by reaction of 1 and methanol liberated from 1. The other one appeared to be formed via hydrogenolysis. The formation of the by-products suggested that some terminal reactions took place in the polycondensation of 1 and 4. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2250,2261, 2008 [source] Kinetic analysis of the cross reaction between dithioester and alkoxyamine by a Monte Carlo simulation,JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2007Yong Ao Abstract A model reaction of dithioester and alkoxyamine is proposed to probe the reversible addition,fragmentation chain transfer (RAFT) process. The kinetics of the model reaction is analyzed and compared with that of pure alkoxyamine homolysis with a Monte Carlo simulation. Although the pure alkoxyamine obeys the law of persistent radical effect, the model reaction results in higher concentration of the persistent radical during the main period of the reaction. However, for a very fast RAFT process or a very low addition rate constant, the time dependence of the persistent radical concentration is quite close to that of pure alkoxyamine. Furthermore, the cross termination between the intermediate and alkyl radicals causes a retardation effect for the model reaction when the intermediate is relatively long-lived. The Monte Carlo simulation indicates that it is feasible to measure the individual rate constants of the RAFT process, such as the rate constant of addition, with a large excess of alkoxyamine. In addition, the special feature of the system with different leaving groups in the alkoxyamine and dithioester is also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 374,387, 2007 [source] One-pot synthesis of star-shaped aliphatic polyesters with hyperbranched cores and their characterization with size exclusion chromatographyJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2006Tadeusz Biela Star-shaped aliphatic polyesters with hyperbranched cores were prepared by a two-step synthesis with the one-pot, arms-first core method. First, the polymerization of ,-caprolactone (CL) or L,L -lactide (LA) was initiated with aluminum isopropoxide trimer. Then, the resulting poly(CL),OAl< or poly(LA),OAl< living chains were employed as initiators for 5,5,-bis(oxepan-2-one) or 1,6-dioxaspiro[4,4]nonane-2,7-dione) polymerization. A sequence of chain growth and branching reactions led to the formation of starlike macromolecules. The progress of the polymerization was followed with size exclusion chromatography, and the products of the model reaction were also analyzed with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. [source] Synthesis of multicyclic and grafted polystyrenesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 16 2001Bénédicte Lepoittevin Abstract Well-defined multicyclic polystyrenes are prepared in two steps. The first step is the preparation of a cyclic difunctional polystyrene by the reaction of ,,,-dilithiopolystyrene chains with 1,3-bis(phenylethenyl)benzene. Then, this product is covalently grafted to poly(chloromethylstyrene) chains leading to the formation of a high molar mass product containing linear and cyclic parts. As a model reaction and to optimize the previous reaction, a study of coupling of the linear difunctional model polystyrene with poly(chloromethylstyrene) is performed leading to grafted polystyrene. The grafted products are analyzed by size-exclusion chromatography, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and liquid chromatography at the exclusion-adsorption transition point. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2723,2730, 2001 [source] Synthesis of well-defined poly(p -benzamide) from chain-growth polycondensation and its application to block copolymersMACROMOLECULAR SYMPOSIA, Issue 1 2003Tsutomu Yokozawa Abstract Poly(p -benzamide) with a defined molecular weight and a low polydispersity and block copolymers containing this well-defined aramide was synthesized. Phenyl 4-(4-octyloxybenzylamino)benzoate (1b) polymerized at room temperature in the presence of base and phenyl 4-nitrobenzoate (2a) as an initiator in a chain-growth polycondensation manner to give well-defined aromatic polyamides having the 4-octyloxybenzyl groups as a protecting group on nitrogen in an amide. It was confirmed by a model reaction that deprotection of this protecting group proceeded completely with trifluoroacetic acid (TFA) without breaking the amide linkage. The utility of this approach to poly(p -benzamide) with a low polydispersity was demonstrated by the synthesis of block copolymers of poly(p -benzamide) and poly(N -octyl- p -benzamide) or poly(ethylene glycol). The SEM images of the supramolecular assemblies of the former block copolymer showed ,m-sized bundles and aggregates of flake structures. [source] In this issue: Biotechnology Journal 8/2010BIOTECHNOLOGY JOURNAL, Issue 8 2010Article first published online: 12 AUG 2010 Biocatalyst microemulsions Pavlidis et al., Biotechnol. J. 2010, 5, 805,812 Enzymes maintain their catalytic activity when hosted in aqueous nanodroplets like reverse micelles. Researchers from Ioannina, Greece, propose the use of water-in-ionic liquid microemulsionbased organogels (w/IL MBGs) as novel supports for the immobilization of lipase B from Candida antarctica and lipase from Chromobacterium viscosum. These novel lipase-containing w/IL MBGs can be effectively used as solid phase biocatalysts in various polar and non-polar organic solvents or ILs, exhibiting up to 4.4-fold higher esterification activity compared to water-in-oil microemulsion-based organogels. The immobilized lipases retain their activity for several hours at 70°C, while their half life time is up to 25-fold higher compared to that observed in w/IL microemulsions Biocatalyst cryogelation Bieler et al., Biotechnol. J. 2010, 5, 881,885 Entrapment of biocatalysts in hydrogel beads allows stable operation in otherwise deteriorating solvents. Doing this by cryogelation is a gentle method to extend the scope of biocatalysis. To foster the use of this versatile method, researchers from Aachen, Germany, devised an automated injector for the production of PVA/PEG-enzyme immobilisates. The device consists of a thermostated reservoir connected to a programmable injector nozzle and an agitated receiving bath for the droplets. This lab-scale production unit yields up to 1500 beads with immobilized enzyme per minute with a narrow size distribution and good roundness. Biocatalyst membrane reactor Lyagin et al., Biotechnol. J. 2010, 5, 813,821 Screening of biocatalysts, substrates or conditions in the early stages of bioprocess development requires an enormous number of experiments and is a tedious, expensive and time-consuming task. Currently available screening systems can only be operated in batch or fed-batch mode, which can lead to severe misinterpretations of screening results. Researchers from Berlin, Germany, now developed a novel screening system that enables continuous feeding of substrates and continuous removal of products. A prototype based on the membrane reactor concept was designed and operated for a model reaction, the hydrolysis of cellulose. [source] Continuous screening system for inhibited enzyme catalysis: A membrane reactor approachBIOTECHNOLOGY JOURNAL, Issue 8 2010Evgenij Lyagin Abstract The screening of catalysts, substrates or conditions in the early stages of bioprocess development requires an enormous number of experiments and is a tedious, expensive and time-consuming task. Currently available screening systems can only be operated in batch or fed-batch mode, which can lead to severe misinterpretations of screening results. For example, catalysts that are inhibited by substrates or accumulating products will be excluded from further investigations in the early stages of process development despite the fact that they might be superior to other candidates in a different operational mode. Important and advantageous properties such as turnover stability can also be overshadowed by product inhibition. The aim of this study was to develop a novel screening system that enables continuous feeding of substrates and continuous removal of products. A prototype based on the membrane reactor concept was designed and operated for a model reaction, the hydrolysis of cellulose. [source] In situ extraction of polar product of whole cell microbial transformation with polyethylene glycol-induced cloud point systemBIOTECHNOLOGY PROGRESS, Issue 5 2008Zhilong Wang Abstract A novel polyethylene glycol-induced cloud point system (PEG-CPS) was developed for in situ extraction of moderate polar product by setting a microbial transformation of benzaldehyde into L -phenylacetylcarbinol (L -PAC) with Saccharomyces cerevisiae (baker's yeast) as a model reaction. The biocompatibility of the microorganism in PEG-CPS was comparatively studied with a series of water-organic solvent two-phase partitioning systems. The tolerance of microorganism to the toxic substrate benzaldehyde was increased and the moderate polar product L -PAC was extracted into the surfactant-rich phase in the PEG-CPS. The novel PEG-CPS fills the gap of in situ extraction of polar product in microbial transformation left by water-organic solvent two-phase partitioning system. At the same time, the application of PEG-CPS in a microbial transformation also avoids expensive solvent when compared with that of aqueous two-phase system or CPS. [source] Preparation of Imidazolin-2-iminato Molybdenum and Tungsten Benzylidyne Complexes: A New Pathway to Highly Active Alkyne Metathesis CatalystsCHEMISTRY - A EUROPEAN JOURNAL, Issue 29 2010Birte Haberlag Dipl.-Chem. Abstract The reaction of [PhCMBr3(dme)] (dme=1,2-dimethoxyethane) with the hexafluoro- tert- butoxides LiX or KX [X=OC(CF3)2Me] afforded the benzylidyne complexes [PhCMX3(dme)] (2,a: M=W, 2,b: M=Mo), which further reacted with the lithium reagent Li(ImtBuN), generated with MeLi from 1,3-di- tert -butylimidazolin-2-imine (ImtBuNH), to form the imidazolin-2-iminato complexes [PhCMX2(ImtBuN)] (3,a: M=W, 3,b: M=Mo). The propylidyne complex [EtCMoX2(NImtBu)] (4) was obtained by treatment of 3,b with an excess of 3-hexyne. Complexes 3,a and 3,b are able to efficiently catalyse alkyne cross metathesis of various 3-pentynyl benzyl ethers 5 and benzoic esters 7 at room temperature, to afford 2-butyne and the corresponding diethers 6 and diesters 8. The tungsten complex 3,a proved to be a superior catalyst for ring-closing alkyne metathesis, and the [10]cyclophanes 10 and 12 were synthesised in high yield from 1,3-bis(3-pentynyloxymethyl)benzene (9) and bis(3-pentynyl) phthalate (11), respectively. The molecular structures of compounds 2,a, 2,b, 3,a, 3,b, 4, and 12 were determined by single-crystal X-ray diffraction. DFT calculations have been carried out for catalyst systems based on the imidazolin-2-iminato tungsten and molybdenum complexes 3,a and 3,b by choosing the alkyne metathesis of 2-butyne as the model reaction; the studies revealed a lower activation barrier for the tungsten system. [source] Identification of a Highly Efficient Alkylated Pincer Thioimido,Palladium(II) Complex as the Active Catalyst in Negishi CouplingCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2009Jing Liu Dr. Abstract PdIIate complex: A novel alkylated pincer thioimido,Pd complex generated from a catalyst precursor and basic organometallic reagents (RM) was observed by in situ IR, 1H,NMR, and 13C,NMR spectroscopies for the first time and proved to be the active catalyst in stoichiometric and catalytic reactions of aryl iodides with RM (see scheme). The catalyst, as an electron-rich PdII species, promoted the Negishi coupling of aryl iodides and alkylzinc reagents with high efficiency, even at low temperatures (0 or ,20,°C). The induction period of Negishi coupling catalyzed by pincer thioamide,palladium complex 1 was investigated. A heterogeneous mechanism was excluded by kinetic studies and comparison with Negishi coupling reactions promoted by Pd(OAc)2/Bu4NBr (a palladium,nanoparticle system). Tetramer 2 was isolated from the reaction of 1 and organozinc reagents. Dissociation of complex 2 by PPh3 was achieved, and the structure of resultant complex 8 was confirmed by X-ray diffraction analysis. A novel alkylated pincer thioimido,PdII complex, 7, generated from catalyst precursor 1 and basic organometallic reagents (RM), was observed by in situ IR, 1H,NMR, and 13C,NMR spectroscopy for the first time. The reaction of 7 with methyl 2-iodobenzoate afforded 74,% of the cross-coupled product, methyl 2-methylbenzoate, together with 60,% of PdII complex 2. Furthermore, the catalyst, as an electron-rich PdII species, efficiently promoted the Negishi coupling of aryl iodides and alkylzinc reagents without an induction period, even at low temperatures (0,°C or ,20,°C). To evaluate the influence of the catalyst structure upon the induction period, complex 9 was prepared, in which the nBu groups of 1 were displaced by more bulky 1,3,5-trimethylphenyl groups. Trimer 10 was isolated from the reaction of complex 9 and basic organometallic reagents such as CyZnCl or CyMgCl (Cy: cyclohexyl); this is consistent with the result obtained with complex 1. The rate in the induction period of the model reaction catalyzed by 9 was faster than that with 1. Plausible catalytic cycles for the reaction, based upon the experimental results, are discussed. [source] Ring Opening of the Cyclobutane in a Thymine Dimer Radical AnionCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2007Chryssostomos Chatgilialoglu Dr. Abstract The reactions of hydrated electrons (eaq,) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t1/2<35,ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. ,-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T,. is unable to transfer an electron to the thymine dimer 2. [source] Polyamide Synthesis from 6-Aminocapronitrile, Part 1: N -Alkyl Amide Formation by Amine Amidation of a Hydrolyzed NitrileCHEMISTRY - A EUROPEAN JOURNAL, Issue 27 2007Adrianus Abstract The synthesis of N -hexylpentanamide from a stoichiometric amount of pentanenitrile and hexylamine has been studied as a model reaction for the synthesis of nylon-6 from 6-aminocapronitrile. The reaction was carried out under mild hydrothermal conditions and in the presence of a homogeneous ruthenium catalyst. For the mild hydrothermal conditions the presence of hexylamine distinctively increases the nitrile hydrolysis compared to the nitrile hydrolysis in the absence of hexylamine. Amine-catalyzed nitrile hydrolysis mainly produces the N-substituted amide. A clear product development is observed, consisting of first the terminal amide formation and second the accumulation of N -hexylpentanamide. With a maximum conversion of only 80,% after 18,h, the nitrile hydrolysis rate at 230,°C is still much too low for nylon-6 synthesis. Ruthenium dihydride phosphine was therefore used as a homogeneous catalyst, which significantly increases the nitrile hydrolysis rate. At a temperature of 140,°C and with only 0.5,mol,% [RuH2(PPh3)4] a 60,% nitrile conversion is already reached within 2,h. Initially the terminal amide is the sole product, which is gradually converted into N -hexylpentanamide. The reaction has a high initial rate, however, for higher conversions a strong decrease in hydrolysis rate is observed. This is ascribed to product inhibition, which results from the equilibrium nature of the reaction. [source] Polymer Fibers as Carriers for Homogeneous CatalystsCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2007Michael Stasiak Abstract This paper describes a polymer fiber-based approach for the immobilization of homogeneous catalysts. The goal is to generate products that are free of catalysts which would be of great importance for the development of optoelectronic or pharmaceutical compounds. Electrospinning was employed to prepare the non-woven fiber assembly composed of polystyrene. The homogeneous catalyst scandium triflate was immobilized on the polystyrene fibers during electrospinning and on corresponding core shell fibers using a fiber template approach. An imino aldol and an aza-Diels,Alder model reaction were carried out with each fibrous catalytic system. This resulted in the immobilization of homogeneous catalysts in a polymer environment without loss of their catalytic activity and may even be enhanced when compared with reactions carried out in homogeneous solutions. [source] Cyanomethylidyne: A Reactive Carbyne RadicalCHEMPHYSCHEM, Issue 3 2006Jian Wang Abstract The cyanomethylidyne radical (CCN) has been a long-standing subject of extensive structural and spectroscopic studies. However, its chemical reactivity has received rather little attention. Recently, we studied the reaction of CCN with the simplest alkane, CH4, which follows a mechanism of carbyne insertion,dissociation rather than that of direct H abstraction proposed by a recent experimental study. However, we are aware that alkanes like CH4 bear no electron lone pairs and thus are not ideal diagnostic molecules for distinguishing between the carbyne-insertion and H-abstraction mechanisms. Hence, we chose a series of ,-bonded molecules HX (X=OH, NH2, and F) which bear electron lone pairs and are better diagnostics for carbyne-insertion behavior. The new results at the CCSD(T)/6-311+G(2df,p)//B3LYP/6-311G(d,p)+ZPVE, CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p)+ZPVE, G2M(CC1), and MC-QCISD//B3LYP/6-31G(d)+ZPVE levels definitively confirm the carbyne-insertion behavior of the CCN radical towards HX. In addition, we make the first attempt to understand the reactivity of the CCN radical toward ,-bonded molecules, using the CCN+C2H2 model reaction. This reaction involves carbenoid addition to the CC bond without a potential-energy barrier to form a C3 three-membered cyclic intermediate followed by H extrusion. Therefore, the reactions of CCN with both ,- and ,-bonded molecules conclusively show that CCN is a reactive carbyne radical and may be more reactive than the well-known CN radical. Future experimental studies, especially on product characterization, are strongly desired to test our proposed carbyne mechanism. The studied reactions of CCN with CH4,, NH3,, H2O, and C2H2 could be of interest to combustion science and astrophysics, and they could provide efficient routes to form novel cyano-containing molecules in interstellar space. [source] The Origin of the High Enantioselectivity in Asymmetric Cyclopropanation of Unfunctionalized OlefinsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2010Yiying Zheng Abstract The mechanism of the asymmetric Simmons,Smith cyclopropanation for unfunctionalized olefins was investigated using density functional theory (DFT) methods. The calculated results of model reactions showed that the coordinated Lewis acidic zinc halide ZnX2 (X=Cl, I) and/or halomethylzinc halide XZnCH2X in the catalyst play an important role in the enantioselectivity. The catalyst not only forms the ring with the substance in the reaction centre, but also establishes two steric repulsions that can lead to an explanation for the high enantioselectivity. Hence, these results highlight some important insights for the prerequisites of an effective catalyst and a proper substrate towards high enantioselectivity for this kind of reaction. [source] Hard-modelled trilinear decomposition (HTD) for an enhanced kinetic multicomponent analysisJOURNAL OF CHEMOMETRICS, Issue 5 2002Yorck-Michael Neuhold Abstract We present a novel approach for kinetic, spectral and chromatographic resolution of trilinear data sets acquired from slow chemical reaction processes via repeated chromatographic analysis with diode array detection. The method is based on fitting rate constants of distinct chemical model reactions (hard-modelled, integrated rate laws) by a Newton,Gauss,Levenberg/Marquardt (NGL/M) optimization in combination with principal component analysis (PCA) and/or evolving factor analysis (EFA), both known as powerful methods from bilinear data analysis. We call our method hard-modelled trilinear decomposition (HTD). Compared with classical bilinear hard-modelled kinetic data analysis, the additional chromatographic resolution leads to two major advantages: (1) the differentiation of indistinguishable rate laws, as they can occur in consecutive first-order reactions; and (2) the circumvention of many problems due to rank deficiencies in the kinetic concentration profiles. In this paper we present the theoretical background of the algorithm and discuss selected chemical rate laws. Copyright © 2002 John Wiley & Sons, Ltd. [source] | ||||||||||||||||