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Mechanistic Investigation (mechanistic + investigation)
Selected AbstractsMechanistic Investigation into the Unique Orientation Textures of Poly(vinylidene fluoride) in Blends with Nylon 11MACROMOLECULAR RAPID COMMUNICATIONS, Issue 10 2003Yongjin Li Abstract Self-seeded crystallization experiments were carried out to detect the mechanism of the unique orientation behavior of poly(vinylidene fluoride) (PVDF) in oriented PVDF/nylon 11 blends. It was found that primary nuclei have no effects on the final orientation textures adopted by PVDF. The results show that the PVDF crystal orientation in the oriented blends is determined in the early stage of crystal growth, thus a trans crystallization mechanism is preferred. Isothermal crystallization kinetics for the self-seeded and non-self-seeded crystallization at 145,°C. [source] Bis-Terminal Hydroxy Polyethers as All-Purpose, Multifunctional Organic Promoters: A Mechanistic Investigation and Applications,ANGEWANDTE CHEMIE, Issue 41 2009Woong Lee Achirale Polyetherderivate beschleunigen SN2-Reaktionen, indem sie sowohl das Nucleophil (KF) als auch das Elektrophil (eine Sulfonatgruppe) aktivieren (siehe Bild). Mit einer chiralen Variante der Katalysatoren gelingt die desilylierende kinetische Racematspaltung der Silylether von sekundären Alkoholen. Die Ergebnisse von Dichtefunktionalrechnungen erklären, wie derartige organische Promotoren funktionieren. [source] ChemInform Abstract: Modeling Dinitrogen Activation by Lithium: A Mechanistic Investigation of the Cleavage of N2 by Stepwise Insertion into Small Lithium Clusters.CHEMINFORM, Issue 48 2009Debjani Roy Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Mechanistic Investigation of a Novel Vitamin B12 -Catalyzed Carbon,Carbon Bond Forming Reaction, the Reductive Dimerization of Arylalkenes.CHEMINFORM, Issue 30 2002Justin Shey Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Mechanistic Investigation of Chiral Phosphoric Acid Catalyzed Asymmetric Baeyer,Villiger Reaction of 3-Substituted Cyclobutanones with H2O2 as the OxidantCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2010Senmiao Xu Dr. Abstract The mechanism of the chiral phosphoric acid catalyzed Baeyer,Villiger (B,V) reaction of cyclobutanones with hydrogen peroxide was investigated by using a combination of experimental and theoretical methods. Of the two pathways that have been proposed for the present reaction, the pathway involving a peroxyphosphate intermediate is not viable. The reaction progress kinetic analysis indicates that the reaction is partially inhibited by the ,-lactone product. Initial rate measurements suggest that the reaction follows Michaelis,Menten-type kinetics consistent with a bifunctional mechanism in which the catalyst is actively involved in both carbonyl addition and the subsequent rearrangement steps through hydrogen-bonding interactions with the reactants or the intermediate. High-level quantum chemical calculations strongly support a two-step concerted mechanism in which the phosphoric acid activates the reactants or the intermediate in a synergistic manner through partial proton transfer. The catalyst simultaneously acts as a general acid, by increasing the electrophilicity of the carbonyl carbon, increases the nucleophilicity of hydrogen peroxide as a Lewis base in the addition step, and facilitates the dissociation of the OH group from the Criegee intermediate in the rearrangement step. The overall reaction is highly exothermic, and the rearrangement of the Criegee intermediate is the rate-determining step. The observed reactivity of this catalytic B,V reaction also results, in part, from the ring strain in cyclobutanones. The sense of chiral induction is rationalized by the analysis of the relative energies of the competing diastereomeric transition states, in which the steric repulsion between the 3-substituent of the cyclobutanone and the 3- and 3,-substituents of the catalyst, as well as the entropy and solvent effects, are found to be critically important. [source] Mechanistic Investigation of the Dipolar [2+2] Cycloaddition,Cycloreversion Reaction between 4-(N,N -Dimethylamino)phenylacetylene and Arylated 1,1-Dicyanovinyl Derivatives To Form Intramolecular Charge-Transfer ChromophoresCHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2010Yi-Lin Wu Abstract The kinetics and mechanism of the formal [2+2] cycloaddition,cycloreversion reaction between 4-(N,N -dimethylamino)phenylacetylene (1) and para -substituted benzylidenemalononitriles 2,b,2,l to form 2-donor-substituted 1,1-dicyanobuta-1,3-dienes 3,b,3,l via the postulated dicyanocyclobutene intermediates 4,b,4,l have been studied experimentally by the method of initial rates and computationally at the unrestricted B3LYP/6-31G(d) level. The transformations were found to follow bimolecular, second-order kinetics, with =13,18,kcal,mol,1, ,,30,cal,K,1,mol,1, and =22,27,kcal,mol,1. These experimental activation parameters for the rate-determining cycloaddition step are close to the computational values. The rate constants show a good linear free energy relationship (,=2.0) with the electronic character of the para -substituents on the benzylidene moiety in dimethylformamide (DMF), which is indicative of a dipolar mechanism. Analysis of the computed structures and their corresponding solvation energies in acetonitrile suggests that the rate-determining attack of the nucleophilic, terminal alkyne carbon onto the dicyanovinyl electrophile generates a transient zwitterion intermediate with the negative charge developing as a stabilized malononitrile carbanion. The computational analysis predicted that the cycloreversion of the postulated dicyanocyclobutene intermediate would become rate-determining for 1,1-dicyanoethene (2,m) as the electrophile. The dicyanocyclobutene 4,m could indeed be isolated as the key intermediate from the reaction between alkyne 1 and 2,m and characterized by X-ray analysis. Facile first-order cycloreversion occurred upon further heating, yielding as the sole product the 1,1-dicyanobuta-1,3-diene 3,m. [source] Mechanistic Investigation of the Oxygen-Atom-Transfer Reactivity of Dioxo-molybdenum(VI) ComplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 28 2006Brian W. Kail Abstract The oxygen-atom-transfer (OAT) reactivity of [LiPrMoO2(OPh)] (1, LiPr=hydrotris(3-isopropylpyrazol-1-yl)borate) with the tertiary phosphines PEt3 and PPh2Me in acetonitrile was investigated. The first step, [LiPrMoO2(OPh)]+PR3,[LiPrMoO(OPh)(OPR3)], follows a second-order rate law with an associative transition state (PEt3, ,H,,=48.4 (±1.9) kJ,mol,1, ,S,,=,149.2 (±6.4) J,mol,1,K,1, ,G,,=92.9 kJ,mol,1; PPh2Me, ,H,,=73.4 (±3.7) kJ,mol,1, ,S,,=,71.9 (±2.3) J,mol,1,K,1, ,G,,=94.8 kJ,mol,1). With PMe3 as a model substrate, the geometry and the free energy of the transition state (TS) for the formation of the phosphine oxide-coordinated intermediate were calculated. The latter, 95 kJ,mol,1, is in good agreement with the experimental values. An unexpectedly large O-P-C angle calculated for the TS suggests that there is significant O-nucleophilic attack on the PC ,* in addition to the expected nucleophilic attack of the P on the MoO ,*. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide with acetonitrile, [LiPrMoO(OPh)(OPR3)] + MeCN , [LiPrMoO(OPh)(MeCN)] + OPR3, follows a first-order rate law in MeCN. A dissociative interchange (Id) mechanism, with activation parameters of ,H,,=93.5 (±0.9) kJ,mol,1, ,S,,=18.2 (±3.3) J,mol,1,K,1, ,G,,=88.1 kJ,mol,1 and ,H,,=97.9 (±3.4) kJ,mol,1, ,S,,=47.3 (±11.8) J,mol,1,K,1, ,G,,=83.8 kJ,mol,1, for [LiPrMoO(OPh)(OPEt3)] (2,a) and [LiPrMoO(OPh)(OPPh2Me)] (2,b), respectively, is consistent with the experimental data. Although gas-phase calculations indicate that the MoOPMe3 bond is stronger than the MoNCMe bond, solvation provides the driving force for the release of the phosphine oxide and formation of [LiPrMoO(OPh)(MeCN)] (3). [source] Facile and Reproducible Synthesis of Nanostructured Colloidal ZnO Nanoparticles from Zinc Acetylacetonate: Effect of Experimental Parameters and Mechanistic InvestigationsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 33 2009Alessia Famengo Abstract A facile and reproducible route to nanostructured colloidal ZnO nanoparticles was developed by controlled hydrolysis and condensation of zinc acetylacetonate in alkaline conditions. By reaction of an ethanolic solution of Zn(acac)2 with NaOH in a 1:2 molar ratio, after reflux, ZnO spherical nanoparticles were obtained that displayed a homogeneous size distribution; particle diameters ranged from 6 to 10 nm, as evidenced by transmission electron microscopy (TEM) analysis. The same reaction was carried out also in water, glycerol and 1,2-propanediol, to investigate the effect of the solvent viscosity and dielectric constant on the final features of the obtained material. Irrespective of the nature of the solvent, X-ray diffraction (XRD) analysis shows the formation ofhexagonal ZnO, whereas the presence of residual unreacted Zn(acac)2 could be ruled out. Indeed, different particle sizes and very different morphologies were obtained. Also the reflux step was shown to be a key factor in avoiding the fast precipitation of a floc and achieving a pure compound, which was isolated and thoroughly characterised. The composition of the obtained ZnO was determined by elemental analysis, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA), showing the formation of pure ZnO. IR spectroscopy evidenced the presence of adsorbed organic ligands on the colloid surfaces. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) revealed the presence of medium- to high-strength acidic sites on the ZnO surface. To gain a deeper insight into the formation mechanisms of these nanostructures, time-resolved UV/Vis and XAS studies were performed on the ethanol solution used for the synthesis of the oxide and also on the solid specimen, obtained after the refluxing step. No remarkable changes could be evidenced in the solution after the addition of an understoichiometric amount of NaOH, but the growth of the ZnO nanoparticles could be followed by UV/Vis spectra. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] 2-Substituted Benzo[b]furans from (E)-1,2-Dichlorovinyl Ethers and Organoboron Reagents: Scope and Mechanistic Investigations into the One-Pot Suzuki Coupling/Direct Arylation,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 29 2010Laina M. Geary Abstract 2-Substituted benzo[b]furans can easily be assembled from simple phenols, boronic acids or other organoboron reagents, and trichloroethylene. The overall process requires only two synthetic steps, with the key step being a one-pot sequential Suzuki cross-coupling/direct arylation reaction. The method tolerates many useful functional groups and does not require the installation of any other activating functionality. The modular nature of the process permits the rapid synthesis of many analogues using essentially the same chemistry, of particular value in drug development. Results of kinetic isotope effect studies and investigations into the regioselectivity of the process indicate that the direct arylation step most likely does not involve an electrophilic palladation. The most likely mechanism lies somewhere on the continuum between a C,H bond metathesis and an assisted palladation or concerted metallation-deprotonation pathway. [source] Zirconia-Based Metastable Solid Solutions through Self-Propagating High-Temperature Synthesis: Synthesis, Characterization, and Mechanistic InvestigationsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2000Filippo Maglia Cubic Zr1,xMexOy(Me = Fe, Co, Ni, Cu) metastable solid solutions with metal content significantly higher than equilibrium levels have been synthesized by the self-propagating high-temperature synthesis method based on a thermite reaction between metallic zirconium and the transition-metal oxides CoO, Fe2O3, CuO, and NiO. Through in situ XRD analysis, it was determined that when heated to 1100°C, the cubic solid solution transformed to the tetragonal phase with the concomitant formation of iron oxide. When cooled to lower temperatures, the tetragonal phase transformed to the monoclinic phase at or below 500°C. Results of auxiliary experiments strongly suggest that the formation of the solid solution takes place behind the combustion front by a reaction between zirconia and the metal. [source] Photoinduced Electron Transfer Reactions by SmI2 in THF: Luminescence Quenching Studies and Mechanistic InvestigationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2005Edamana Prasad Dr. Abstract Photoluminescence quenching studies of SmI2 in dry THF were carried out in the presence of five different classes of compounds: ketone, alkyl chloride, nitrile, alkene and imine. The free energy change (,G,0) of the photoinduced electron transfer (PET) reactions was calculated from the redox potentials of the donor (SmI2) and acceptors. The bimolecular quenching constants (kq) derived from the Stern,Volmer experiments parallel the free energy changes of the PET processes. The observed quenching constants were compared with the theoretically derived electron transfer rate constants (ket) from Marcus theory and found to be in good agreement when a value of ,=167 kJ,mol,1 (40 kcal,mol,1) was used for the reorganization energy of the system. A careful comparison of the excited state dynamics of SmII in the solid state to the results obtained in solution (THF) provides new insight in to the excited states of SmII in THF. The activation parameters determined for the PET reactions in SmI2/1-chlorobutane system are consistent with a less ordered transition state and high degree of bond reorganization in the activated complex compared to similar ground state reactions. Irradiation studies clearly show that SmI2 acts as a better reductant in the excited state and provides an alternative pathway for rate enhancement in known and novel functional group reductions. [source] Pentafluorophenylammonium Trifluoromethanesulfonimide: Mild, Powerful, and Robust Catalyst for Mukaiyama Aldol and Mannich Reactions between Ketene Silyl Acetals and Ketones or Oxime EthersADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7 2010Ryohei Nagase Abstract Pentafluorophenylammonium trifluoromethanesulfonimide (C6F5N+H3,NTf2,) promotes Mukaiyama aldol and Mannich reactions using ketene silyl acetals with ketones and oxime ethers, respectively. The present robust method is mild, but powerful enough to utilize less accessible electrophiles such as enolizable ketones and oxime ethers to produce a variety of ,-hydroxy esters and ,-alkoxyamino esters, respectively. Mechanistic investigation revealed in situ generation of trimethylsilyl bistriflimide [Tf2N(TMS)], the truly active catalyst, which was supported by rational 1H,NMR measurements. [source] Mechanistic investigation of vibrational fine structure in e-H2 scattering using local complex potential-based time dependent wave packet approachINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2008Manabendra Sarma Abstract The structural features of vibrational excitation cross-sections in resonant e-H2 scattering have been investigated using a time dependent wave packet approach and a local complex potential to describe the 2, H anion. An analysis of the partial contributions to the vibrational excitation cross-sections reveals that all features of the excitation profile result from simple interference between bound vibrational levels of H2 and H. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] Mechanistic investigation of the interaction between bisquaternary antimicrobial agents and phospholipids by liquid secondary ion mass spectrometry and differential scanning calorimetryRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 18 2002V. A. Pashynskaya Mechanisms of interaction between the antimicrobial drugs decamethoxinum and aethonium, which are based on bisquaternary ammonium compounds, and a phospholipid component of biological membranes, dipalmitoylphosphatidylcholine, were studied by means of liquid secondary ion mass spectrometry (LSIMS) and differential scanning calorimetry (DSC). Supramolecular complexes of the drugs with this phospholipid were recorded under secondary ion mass spectrometric conditions. The dependence of the structures of these complexes on structural parameters of the dications of the bisquaternary ammonium compounds was demonstrated. Tandem mass spectrometric investigations of the metastable decay of doubly charged ions of decamethoxinum and aethonium complexes with dipalmitoylphosphatidylcholine allowed estimation of structural parameters of these complexes in the gas phase. Interactions of decamethoxinum and aethonium with model membrane assemblies built from hydrated dipalmitoylphosphatidylcholine were studied using DSC. It was shown that while both drugs can interact with model membranes, the mechanisms of such interactions for decamethoxinum and aethonium differ. The correlation between the nature of these interactions and structural and electronic parameters of the dications of the two bisquaternary agents is discussed. Interpretation of combined mass spectrometric and calorimetric experimental data led to proposals that the molecular mechanisms of antimicrobial action of bisquaternary ammonium compounds are related to their effect on the membrane phospholipid components of microbial cells. Copyright © 2002 John Wiley & Sons, Ltd. [source] Improved Synthesis of Pyrroles and Indoles via Lewis Acid-Catalyzed Mukaiyama,Michael-Type Addition/Heterocyclization of Enolsilyl Derivatives on 1,2-Diaza-1,3-Butadienes.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2007Role of the Catalyst in the Reaction Mechanism Abstract The Mukaiyama,Michael-type addition of various silyl ketene acetals or silyl enol ethers on some 1,2-diaza-1,3-butadienes proceeds at room temperature in the presence of catalytic amounts of Lewis acid affording by heterocyclization 1-aminopyrrol-2-ones and 1-aminopyrroles, respectively. 1-Aminoindoles have been also obtained by the same addition of 2-(trimethylsilyloxy)-1,3-cyclohexadiene on some 1,2-diaza-1,3-butadienes and subsequent aromatization. Mechanistic investigations indicate the coordination by Lewis acid of the enolsilyl derivative and its 1,4-addition on the azo-ene system of 1,2-diaza-1,3-butadienes. The migration of the silyl group from a hydrazonic to an amidic nitrogen, its acidic cleavage and the final internal heterocyclization give the final products. Based on NMR studies and ab initio calculations, a plausible explanation for the migration of the silyl protecting group is presented. [source] Mechanistic investigations of antimony-catalyzed polycondensation in the synthesis of poly(ethylene terephthalate)JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2006Faissal-Ali El-Toufaili Abstract The chemical aspects of poly(ethylene terephthalate) synthesis via the antimony-catalyzed polycondensation of hydroxy ethylene terephthalate end groups were studied to elucidate its mechanism. A polycondensation mechanism was proposed in which activation occurs by the formation of a chelate ligand on antimony composed of the hydroxyl end group and alcoholic oxygen of the ester of the same end group. The rate-determining step of the polycondensation reaction was concluded to be the coordination of a second chain end to antimony. The low activity of antimony at a high concentration of hydroxyl end groups was proposed to result from the competition between hydroxyl end groups and the chelate structure leading to the transition state. The high selectivity of antimony is probably due to its relatively low Lewis acidity. Moreover, antimony was found to stabilize hydroxyl end groups against degradation by preventing their complexation to carbonyl functionalities. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1049-1059, 2006 [source] [(NHC)AuI]-Catalyzed Formation of Conjugated Enones and Enals: An Experimental and Computational StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2007Nicolas Marion Abstract The [(NHC)AuI]-catalyzed (NHC = N-heterocyclic carbene) formation of ,,,-unsaturated carbonyl compounds (enones and enals) from propargylic acetates is described. The reactions occur at 60,°C in 8,h in the presence of an equimolar mixture of [(NHC)AuCl] and AgSbF6 and produce conjugated enones and enals in high yields. Optimization studies revealed that the reaction is sensitive to the solvent, the NHC, and, to a lesser extent, to the silver salt employed, leading to the use of [(ItBu)AuCl]/AgSbF6 in THF as an efficient catalytic system. This transformation proved to have a broad scope, enabling the stereoselective formation of (E)-enones and -enals with great structural diversity. The effect of substitution at the propargylic and acetylenic positions has been investigated, as well as the effect of aryl substitution on the formation of cinnamyl ketones. The presence or absence of water in the reaction mixture was found to be crucial. From the same phenylpropargyl acetates, anhydrous conditions led to the formation of indene compounds via a tandem [3,3] sigmatropic rearrangement/intramolecular hydroarylation process, whereas simply adding water to the reaction mixture produced enone derivatives cleanly. Several mechanistic hypotheses, including the hydrolysis of an allenol ester intermediate and SN2, addition of water, were examined to gain an insight into this transformation. Mechanistic investigations and computational studies support [(NHC)AuOH], produced in situ from [(NHC)AuSbF6] and H2O, instead of cationic [(NHC)AuSbF6] as the catalytically active species. Based on DFT calculations performed at the B3LYP level of theory, a full catalytic cycle featuring an unprecedented transfer of the OH moiety bound to the gold center to the CC bond leading to the formation of a gold,allenolate is proposed. Es descriu la reacció de formació de compostos de carbonil ,,, -insaturats (enones i enals) a partir d,acetats propargílics catalitzada per [(NHC)AuI] (NHC=carbè N-heterocíclic). Les reaccions tenen lloc a 60,°C en 8,h en presència d,una mescla equimolar de [(NHC)AuCl] i AgSbF6 i produeix rendiments alts d,enones i enals conjugats. Estudis d,optimització van mostrar que la reacció era sensible al solvent, el NHC, i, en menor grau, a la natura de la sal de plata emprada; això va portar a escollir [(ItBu)AuCl]/AgSbF6 en THF com a sistema catalític eficient. La transformació va mostrar posseir un ampli espectre, i es va aconseguir la formació estereoselectiva d,(E)-enones i -enals amb gran diversitat estructural. La influència de la substitució en les posicions propargílica i acetilènica va ser investigada, així com la substitució d,aril per donar lloc a la formació de cinamilcetones. La presència o absència d,aigua en el medi de reacció es va mostrar com a crucial. A partir dels mateixos fenilpropargilacetats, condicions anhidres van portar a la formació de compostos indènics mitjançant una reacció tàndem de reorganització sigmatròpica [3,3] i hidroarilació intramolecular, mentre que l,addició d,aigua a la reacció portava de manera neta a derivats del tipus enona. Diverses hipòtesis mecanístiques, entre elles la hidròlisi d,un intermedi del tipus al.lenol ester i l,addició SN2, d,aigua, han estat examinades per millorar la comprensió del procés. Les dades mecanístiques i els estudis computacionals suggereixen que l,espècie catalítica activa és [(NHC)AuOH], produït in situ a partir de [(NHC)AuSbF6] i H2O, i no el complex catiònic [(NHC)AuSbF6]. A partir d,estudis DFT realitzats al nivell B3LYP, es proposa un cicle catalític complet, que inclou un nou tipus d,etapa consistent en la transferència d,un grup OH des de l,or cap a l,enllaç CC per produir un complex or-al.lenolat. [source] An Efficient Synthesis of Substituted meta -Halophenols and Their Methyl Ethers: Insight into the Reaction MechanismEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 15 2010Faiz Ahmed Khan Abstract An expeditious synthetic methodology leading to substituted meta -halophenols and their corresponding methyl ether derivatives through acid-mediated fragmentation of suitably substituted dihalonorbornyl ketones has been devised. The reaction sequence consists of TBTH-mediated (TBTH is tri- n -butyltin hydride) selective bridgehead halogen reduction of easily accessible Diels,Alder adducts derived from 1,2,3,4-tetrahalo-5,5-dimethoxycyclopentadiene and ,-substituted vinyl acetates, with subsequent conversion into the requisite bicyclic ketones by a two-step hydrolysis/oxidation approach. An extensive mechanistic investigation based on isotope labeling and cross experiments has been carried out and plausible mechanistic pathways based on these results have been proposed. The absence of halogen atoms at the bridgehead positions steers the reaction through a novel pathway involving the incorporation of proton (or deuterium) followed by elimination of HX (or DX), so the described methodology also provides a reliable route to ortho-para dideuteratedphenolic derivatives. [source] Kinetic and mechanistic investigation into the influence of chelate substituents on the substitution reactions of platinum(II) terpyridine complexesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2008Deogratius Jaganyi The substitution kinetics of the complexes [Pt{4,-(o -CH3 -Ph)-terpy} Cl]SbF6 (CH3PhPtCl(Sb)), [Pt{4,-(o -CH3 -Ph)-terpy}Cl]CF3SO3 (CH3PhPtCl(CF)), [Pt(4,-Ph-terpy)Cl]SbF6 (PhPtCl), [Pt(terpy)Cl]Cl·2H2O (PtCl), [Pt{4,-(o -Cl-Ph)-terpy}Cl]SbF6 (ClPhPtCl), and [Pt{4,-(o -CF3 -Ph)-terpy}Cl]SbF6 (CF3PhPtCl), where terpy is 2,2,:6,,2,-terpyridine, with the nucleophiles thiourea (TU), N,N,-dimethylthiourea (DMTU), and N,N,N,,N,-tetramethylthiourea (TMTU) were investigated in methanol as a solvent. The substitution reactions of the chloride displacement from the metal complexes by the nucleophiles were investigated as a function of nucleophile concentration and temperature under pseudo-first-order conditions using the stopped-flow technique. The reactions followed the simple rate law kobs = k2[Nu]. The results indicate that the introduction of substituents in the ortho position of the phenyl group on the ancillary ring of the terpy unit does influence the extent of ,-backbonding in the terpy ring. This controls the electrophilicity of the platinum center, which in turn controls the lability of the chloro-leaving group. The strength of the electron-donating or -withdrawing ability of the substituents correlates with the reactivity of the complexes. Electron-donating substituents decrease the rate of substitution, whereas electron-withdrawing substituents increase the rate of substitution. This was supported by DFT calculations at the B3LYP/LACVP+** level of theory, which showed that most of the electron density of the HOMO is concentrated on the phenyl ligand rather than on the metal center in the case of the strongest electron-withdrawing substituent in CF3PhPtCl. The opposite was found to be true with the strongest electron-donating substituent in CH3PhPtCl. Thiourea was found to be the best nucleophile with N,N,N,,N,-tetramethylthiourea being the weakest due to steric effects. The temperature dependence studies support an associative mode of activation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 808,818, 2008 [source] A mechanistic investigation into the covalent chemical derivatisation of graphite and glassy carbon surfaces using aryldiazonium saltsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6 2008Poobalasingam Abiman Abstract Modification of carbon materials such as graphite and glassy carbon in bulk quantities using diazonium salts is developed. We used both 4-nitrobenzenediazonium tetrafluoroborate and 1-antharaquinonediazonium chloride to modify graphite and glassy carbon surfaces. Experiments were carried out in the presence and absence of hypophosphorous acid and the mechanism involved in both cases were studied using cyclic voltammetry. The observed peak potentials for both the 4-nitrophenyl and 1-anthraquinonyl modified materials were found to differ depending on whether or not the hypophosphorous acid reducing agent was used. In the absence of hypophosphorous acid the derivatisation reaction was inferred to go through a cationic intermediate, whilst in the presence of the hypophosphorous acid the mechanism likely involves either a purely radical intermediate or a mixture of radical and cationic species. Derivatisation experiments from 5 to 70°C allowed us to determine the optimum derivatisation temperature for both cases, in the presence and absence of hypophosphorous acid. Optimum temperature was 20°C for the former and 35°C for the later. Copyright © 2008 John Wiley & Sons, Ltd. [source] Uniparental disomy and human disease: An overview,AMERICAN JOURNAL OF MEDICAL GENETICS, Issue 3 2010Kazuki Yamazawa Abstract Uniparental disomy (UPD) refers to the situation in which both homologues of a chromosomal region/segment have originated from only one parent. This can involve the entire chromosome or only a small segment. As a consequence of UPD, or uniparental duplication/deficiency of part of a chromosome, there are two types of developmental risk: aberrant dosage of genes regulated by genomic imprinting and homozygosity of a recessive mutation. UPD models generated by reciprocal and Robertsonian translocation heterozygote intercrosses have been a powerful tool to investigate genomic imprinting in mice, whereas novel UPD patients such as those with cystic fibrosis and Prader,Willi syndrome, triggered the clarification of recessive diseases and genomic imprinting disorders in human. Newly developed genomic technologies as well as conventional microsatellite marker methods have been contributing to the functional and mechanistic investigation of UPD, leading to not only the acquisition of clinically valuable information, but also the further clarification of diverse genetic processes and disease pathogenesis. © 2010 Wiley-Liss, Inc. [source] The RNA-Cleaving Bipartite DNAzyme Is a Distinctive MetalloenzymeCHEMBIOCHEM, Issue 1 2006Anat R. Feldman Dr. Abstract Much interest has focused on the mechanisms of the five naturally occurring self-cleaving ribozymes, which, in spite of catalyzing the same reaction, adopt divergent strategies. These ribozymes, with the exception of the recently described glmS ribozyme, do not absolutely require divalent metal ions for their catalytic chemistries in vitro. A mechanistic investigation of an in vitro-selected, RNA-cleaving DNA enzyme, the bipartite, which catalyzes the same chemistry as the five natural self-cleaving ribozymes, found a mechanism of significant complexity. The DNAzyme showed a bell-shaped pH profile. A dissection of metal usage indicated the involvement of two catalytically relevant magnesium ions for optimal activity. The DNAzyme was able to utilize manganese(II) as well as magnesium; however, with manganese it appeared to function complexed to either one or two of those cations. Titration with hexaamminecobalt(III) chloride inhibited the activity of the bipartite; this suggests that it is a metalloenzyme that utilizes metal hydroxide as a general base for activation of its nucleophile. Overall, the bipartite DNAzyme appeared to be kinetically distinct not only from the self-cleaving ribozymes but also from other in vitro-selected, RNA-cleaving deoxyribozymes, such as the 8,17, 10,23, and 614. [source] HPRT mutations, TCR gene rearrangements, and HTLV-1 integration sites define in vivo T-cell clonal lineages,ENVIRONMENTAL AND MOLECULAR MUTAGENESIS, Issue 2-3 2005Mark Allegretta Abstract HPRT mutations in vivo in human T-lymphocytes are useful probes for mechanistic investigations. Molecular analyses of isolated mutants reveal their underlying mutational changes as well as the T-cell receptor (TCR) gene rearrangements present in the cells in question. The latter provide temporal reference points for other perturbations in the in vivo clones as well as evidence of clonal relationships among mutant isolates. Immunological studies and investigations of genomic instability have benefited from such analyses. A method is presented describing a T-cell lineage analysis in a patient with HTLV-1 infection. Lineage reconstruction of an in vivo proliferating HPRT mutant clone allows timing of the integration event to a postthymic differentiated cell prior to the occurrence of HPRT mutations. Environ. Mol. Mutagen., 2005. © 2005 Wiley-Liss, Inc. [source] Genetic and environmental interactions on oral cancer in Southern ThailandENVIRONMENTAL AND MOLECULAR MUTAGENESIS, Issue 2 2001Suparp Kietthubthew Abstract Many countries are interested in understanding the relationship between genetic susceptibility and their prevalent environmental cancers for disease prevention. In Thailand we conducted a population-based case-control study of 53 matched pairs to assess the risk of oral cancer in relation to genetic polymorphism of the glutathione-S-transferase genes (GSTM1 and GSTT1) in cigarette smokers, alcohol drinkers, and betel quid chewers. Interaction of the genes with other potential risk factors such as local bean consumption were also elucidated. Homozygous deletion of GSTM1 has a frequency of 56.6% (n = 30 over 53) among the patients and 30.2% (16/53) among the controls. This gene is associated with a 2.6-fold higher risk for development of oral cancer (95% CI 1.04,6.5). Among the null GSTM1 individuals, those who smoke, consume alcohol, and/or chew betel quid have a significantly increased risk for oral cancer with an odd ratio (OR) = 4.0 (95% CI = 1.2,13.7), OR = 7.2 (95% CI = 1.5,33.8), and OR = 4.4 (95% CI = 1.1,17.8), respectively. Interactions between any two of the lifestyle habits for oral cancer risk, however, are not found. The frequency of the GSTT1 null genotype is 34.0% (18/53) among the patients and 47.2% (25/53) among our controls. There is no association between the GSTT1 null allele and oral cancer risk. In conclusion, our study provides data to indicate that individuals who have homozygous deletion of the GSTM1 gene have increased risk for oral cancer. The risk increases further when these individuals are exposed to environmental toxicants such as chemicals in cigarette smoke, alcohol, and betel quid. These baseline data can be applied to a larger population-based study, both to verify the observation and to conduct mechanistic investigations. Environ. Mol. Mutagen. 37:111,116, 2001 © 2001 Wiley-Liss, Inc. [source] Rational Design of Conjugated Polymer Supramolecules with Tunable Colorimetric ResponsesADVANCED FUNCTIONAL MATERIALS, Issue 10 2009Dong June Ahn Abstract Polydiacetylenes (PDAs), a family of highly ,-conjugated polymers, have unique characteristics associated with their ability to self-assemble. Disruption of the extensively delocalized enyne backbones of molecularly ordered PDA sidechains induces a blue-to-red color change, which has been elegantly applied in the design of chemosensors. Recently, colorimetrically reversible PDAs have received significant attention, not only to gain a better understanding of the fundamentals of PDA chromism, but also to develop methodologies to overcome limitations associated with their colorimetrically irreversible counterparts. In this article, recent progress made in the field of colorimetrically tunable (reversible, stable, or sensitive) PDAs is described. Major emphasis is given to rational design strategies developed in our group. Relevant mechanistic investigations, a diagnostic method to test colorimetric reversibility, as well as future challenges in this area will be also discussed. [source] Olefin Epoxidation with Hydrogen Peroxide Catalyzed by Lacunary Polyoxometalate [,-SiW10O34(H2O)2]4,CHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2007Keigo Kamata Abstract The tetra- n -butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA]4[,-SiW10O34(H2O)2] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcohols, and sulfides with 30,% aqueous hydrogen peroxide. The negative Hammett ,+ (,0.99) for the competitive oxidation of p -substituted styrenes and the low value of (nucleophilic oxidation)/(total oxidation), XSO=0.04, for I -catalyzed oxidation of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans -epoxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I -catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. 29Si and 183W,NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA]4[,-SiW10O32(O2)2] (II), with retention of a ,-Keggin type structure. Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene, epoxidation with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period observed in the catalytic epoxidation. The steric and electronic characters of III are the same as those for the catalytic epoxidation by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidation proceeds via III. [source] | |||||||||||||||||||||||||||||||