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Mechanistic Considerations (mechanistic + consideration)
Selected AbstractsMethoxycarbonylation of Aliphatic Diamines with Dimethyl Carbonate Promoted by in situ Generated Hydroxide Ion: A Mechanistic ConsiderationADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010Dae Won Kim Abstract The methoxycarbonylation reactions of aliphatic diamines with dimethyl carbonate are accelerated greatly in the presence of water. Theoretical investigations on the mechanistic aspects of the methoxycarbonylation of 1,6-hexanediamine strongly suggest that the hydroxide ion, generated in situ from the interaction of 1,6-hexanediamine with water, is an active catalytic species and plays a pivotal role in the rate-determining hydrogen abstraction step from the amino group. [source] The Reaction of Ozone with the Hydroxide Ion: Mechanistic Considerations Based on Thermokinetic and Quantum Chemical Calculations and the Role of HO4, in Superoxide DismutationCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2010Gábor Merényi Prof. Abstract The reaction of OH, with O3 eventually leads to the formation of . OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol.1982, 16, 676,681), it was suggested that the first step occurred by O transfer: OH,+O3,HO2,+O2 and that . OH was generated in the subsequent reaction(s) of HO2, with O3 (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one-step O transfer such as that mentioned above would require the release of O2 in its excited singlet state (1O2, O2(1,g)); this state lies 95.5,kJ,mol,1 above the triplet ground state (3O2, O2(3,g,)). The low experimental rate constant of 70,M,1,s,1 is not incompatible with such a reaction. However, according to our calculations, the reaction of OH, with O3 to form an adduct (OH,+O3,HO4,; ,G=3.5,kJ,mol,1) is a much better candidate for the rate-determining step as compared with the significantly more endergonic O transfer (,G=26.7,kJ,mol,1). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO4, have been probed: HO4,,HO2,+1O2 is spin allowed, but markedly endergonic (,G=23.2,kJ,mol,1). HO4,,HO2,+3O2 is spin forbidden (,G=,73.3,kJ,mol,1). The decay into radicals, HO4,,HO2.+O2.,, is spin allowed and less endergonic (,G=14.8,kJ,mol,1) than HO4,,HO2,+1O2. It is thus HO4,,HO2.+O2., by which HO4, decays. It is noted that a large contribution of the reverse of this reaction, HO2.+O2.,,HO4,, followed by HO4,,HO2,+3O2, now explains why the measured rate of the bimolecular decay of HO2. and O2., into HO2,+O2 (k=1×108,M,1,s,1) is below diffusion controlled. Because k for the process HO4,,HO2.+O2., is much larger than k for the reverse of OH,+O3,HO4,, the forward reaction OH,+O3,HO4, is practically irreversible. [source] Stereoelectronic Effects in the Iodine-Promoted Oxidation of Pentacyclic TetrahydroisoquinolinesHELVETICA CHIMICA ACTA, Issue 8 2004Oliver Koepler Iodine-promoted oxidations of the pentacyclic tetrahydroisoquinolines 4a, 4b, and 10 were investigated. Whereas the all- cis diastereoisomer 4a containing an arylamino moiety gave the iminium ion 5 as the primary product, which subsequently underwent intramolecular aminal formation to 6, the corresponding all- trans diastereoisomer 4b epimerized to the all- cis diastereoisomer 4avia iminium ion 7. In contrast, tetrahydroisoquinoline 10 could be cleanly oxidized to the corresponding isoquinolizidinium ion 11. Mechanistic considerations were supported by molecular-modeling calculations. [source] Microstructure analysis and thermal property of copolymers made of glycolide and ,-caprolactone by stannous octoateJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2002Ji Won Pack Abstract Glycolide (GL) and ,-caprolactone (CL) were copolymerized in bulk at relatively high temperatures using stannous octoate as a catalyst. To investigate the relationship among microstructure, thermal properties, and crystallinity, three series of copolymers prepared at various reaction temperatures, times, and comonomer feed ratios were prepared and characterized by 1H and 13C NMR, DSC, and wide-angle X-ray diffraction (WAXD). The 600-MHz 1H NMR spectra provided information about not only the copolymer compositions but also about the chain microstructure. The reactivity ratios (rG and rC) were calculated from the monomer sequences and were 6.84 and 0.13, respectively. In terms of overall feed compositions, the sequence lengths of the glycolyl units calculated from the reactivity ratios exceeded those measured from the polymeric products. Mechanistic considerations based on reactivity ratios, monomer consumption data, and average sequence lengths are discussed. The unusual phase diagram of GL/CL copolymers implies that the copolymer melting temperature does not depend on its composition alone but rather on the nature of the sequence distribution. The DSC and WAXD measurements show a close relationship between polymer crystallinity and the nature of the polymer sequence. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 544,554, 2002; DOI 10.1002/pola.10123 [source] Catalysis of intumescent flame retardancy of polypropylene by metallic compounds,POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 1 2003Menachem Lewin Abstract Divalent and multivalent metallic compounds catalyze the flame retardancy performance of intumescent systems based on ammonium polyphosphate (APP) and pentaerythritol (petol) in poly(propylene) (PP). The catalytic effect is shown by increases in the oxygen index (OI) and UL-94 ratings. The effect is exerted by small concentrations of the metallic compounds in the range of 0.1,2.5 wt% of the compositions. The effect increases with the concentration of the catalyst until a maximum is reached. At higher concentrations of the catalyst a decrease in the flame retardancy parameters is observed, accompanied in several cases by a degradation and discoloration of the composition. The catalyst replaces melamine in intumescent systems. Catalytic effectiveness is defined and calculated for a number of compounds. Thermogravimetric parameters, such as initial decomposition temperature, temperature of the transition point and residue-after-transitions (RAT) change in parallel with the catalytic effect of the metal compound concentration. Metal compounds investigated include oxides, acetates, acetyl acetonates, borates and sulfates of Mn, Zn, Mg, Al, Ca, Ba,,V, Co, Ni, Cu, Mo, Zr, and Cr. Mechanistic considerations on the activity of the catalysts are presented. Copyright © 2003 John Wiley & Sons, Ltd. [source] Mechanistic study of membrane concentration and recovery of Listeria monocytogenesBIOTECHNOLOGY & BIOENGINEERING, Issue 3 2005Wan-Tzu Chen Abstract Detection of the foodborne pathogen Listeria monocytogenes requires that food samples be processed to remove proteins and lipids, concentrate microorganisms to a detectable concentration, and recover the concentrated cells in a small volume compatible with micron-scale biochips. Mechanistic considerations addressed in this research include the roles of membrane structure, pore size, and detergents in maximizing recovery of cells from a complex biological fluid. The fluid in this case was a food sample (hotdog extract) inoculated with L. monocytogenes. This study showed how membrane filtration using a syringe filter is able to concentrate L. monocytogenes by 95× with up to 95% recovery of living microorganisms by concentrating 50 mL of food sample into a volume of 500 ,L. Tween 20 was added to the sample to prevent irreversible adsorption of the microorganism to the membrane and thereby help to ensure high recovery. Comparison of polycarbonate, mixed cellulose, nylon, and PVDF membranes with 0.2 to 0.45 ,m pores showed the 0.2 ,m polycarbonate membrane with straight through, mono-radial pores gives the highest recovery of living microorganisms. The mixed cellulose, nylon, and PVDF membranes have a fibrous structure whose characteristic openings are much larger than their effective pore size cut-offs of 0.22 or 0.45 ,m. We define conditions for rapid membrane-based cell concentration and recovery that has the potential to supplant enrichment steps that require a day or more. This approach has the added benefit of facilitating examination of a large amount of fluid volume by reducing its volume to a range that is compatible with the microliter scales of biochip or other biosensor detection systems. © 2004 Wiley Periodicals, Inc. [source] Catalytic cleavage of methyl oleate or oleic acidEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 1 2010Angela Köckritz Abstract Different reaction pathways are discussed for the Os-catalyzed oxidation of methyl oleate and oleic acid using O2/aldehyde as oxidation system. Monomethyl azelate and pelargonic acid were the main products obtained in yields of approximately 50,70% starting from methyl oleate. Besides, varying amounts of methyl 9,10-epoxystearate and methyl 9,10-dihydroxystearate were found as by-products. Azelaic acid and pelargonic acid were obtained exclusively from oleic acid used as reactant. Some mechanistic considerations led to the conclusion that the observed products are formed in parallel reaction paths. The effective oxidant for the scission of the C=C double bond seems to be very likely an in situ formed peracid generated by Os-catalysis from O2/aldehyde. Additional investigations concerning the cleavage of oleic acid and methyl oleate with in situ formed performic acid from H2O2/formic acid corroborate this assumption. [source] Interactions between N,acetylcysteine and sodium selenite in modulating the clastogenicity of urethane and 2,acetylaminofluorene in miceINTERNATIONAL JOURNAL OF CANCER, Issue 1 2004Roumen M. Balansky Abstract Combined treatment with different agents represents a promising approach in cancer chemoprevention. Therefore, it is useful to assess in preclinical models the efficacy of combinations that are selected by taking into account mechanistic considerations. We designed 2 studies evaluating the interaction between N,acetylcysteine (NAC) and sodium selenite (Se), both given with the drinking water to Balb/c mice, in modulating clastogenic effects in bone marrow polychromatic erythrocytes. In a first study, a single i.p. injection of urethane considerably enhanced the frequency of micronucleated cells. While NAC produced a significant inhibition, Se further enhanced urethane clastogenicity. When given in combination at the same doses, NAC prevented the adverse effect of Se. In a second study, a single i.p. injection of 2,acetylaminofluorene enhanced the frequency of micronucleated cells. Se did not reduce this effect to a significant extent, while NAC produced a dose,dependent inhibition. When tested at the lower dose in combination with Se, the protective effect of NAC was unchanged. Especially in association with Se, NAC also prevented the toxicity of 2,acetylaminofluorene by normalizing the ratio of polychromatic to normochromatic erythrocytes. In conclusion, NAC attenuated the clastogenicity of both urethane and 2,acetylaminofluorene and the toxicity of this aromatic amine. In addition, NAC prevented the clastogenic and toxic effects resulting from the interaction of Se with urethane. Together with the findings of previous studies, it appears that, besides its intrinsic protective properties in carcinogenesis, NAC is capable of attenuating the adverse effects of several cytotoxic drugs and chemopreventive agents. © 2003 Wiley-Liss, Inc. [source] Copper-Catalyzed Asymmetric Allylic Alkylation of Racemic Cyclic Substrates: Application of Dynamic Kinetic Asymmetric Transformation (DYKAT)ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010Jean-Baptiste Langlois Abstract The copper-catalyzed asymmetric allylic alkylation (AAA) is of great interest in organic synthesis. This reaction was extensively studied using a broad range of substrates, ligands and organometallic reagents. However, the use of racemic substrates was still limited. Although some processes of kinetic resolution are reported in the literature, no examples of quantitative deracemization are described as is the case for the Pd-catalyzed allylic alkylation. We present here a full account of our investigations through the development of the first example of such a process in copper-catalyzed AAA. High enantioselectivities (up to 99% ee), scope of the reaction and mechanistic considerations are reported herein. [source] A novel approach for the kinetic and mechanistic modeling of acid-catalyzed degradation of polymersASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 2 2009Y.-H. Lin Abstract A new approach is presented that combines kinetic and mechanistic considerations which take into account chemical reactions and catalyst deactivation in the modeling of the catalytic degradation of polymers. Though acid-catalyzed hydrocarbon cracking reactions involve a large number of compounds, reactions and catalyst deactivation and are very complex, the model gives a good representation of experimental results from the degradation of polypropylene over fluidized acidic catalysts. This model provides the benefits of product selectivity for the chemical composition, such as alkanes, alkenes, aromatics and coke, in relation to the effect of structurally different polymer feeds, and the performance of the catalyst used. It is an improvement of the currently available empirical ,lumping' techniques which usually are severely restricted in terms of product definitions. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] | ||||||||||||||||