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Second-order Rate Constants (second-order + rate_constant)
Selected AbstractsKinetic investigation on the reactions of p -toluenesulfonyl chloride with p -substituted benzoic acid(s) in the presence of triethylamine in aprotic solventsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2009Subbiah Ananthalakshmi Second-order rate constants of the reactions of p -toluenesulfonyl chloride with p -substituted benzoic acids in the presence of triethylamine in acetonitrile/acetone under equimolar and pseudo-first-order conditions have been determined by the conductometric method using the Guggenheim principle at 25, 30, 35, and 40°C. The reactions follow second order with respect to the whole and first order with respect to each of the reactants. The order of reactivity of the substituents in benzoic acid is rationalized. Activation parameters are obtained by applying the usual methods. The Hammett plot has been found nonlinear, whereas the Bronsted plot shows good correlation. This may be explained on the basis of electronic effects of substituents on the reaction center. Kinetic data and the product analyses indicate that the reaction proceeds through direct nucleophilic attack on the sulfur center. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 303,308, 2009 [source] Effect of colloidal self-assemblies on the basic hydrolysis of 2-(4-bromophenoxy)quinoxalineJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6 2003Angela Cuenca Abstract In the presence of cationic surfactants (C16H33NR3Cl; R,=,Me, n-Pr, n-Bu), the shape of rate versus surfactant concentration profiles for the basic hydrolysis of 2-(4-bromophenoxy)quinoxaline depends on substrate concentration. At low substrate concentration there is a single rate maximum and with a 10-fold substrate concentration increase a double rate maximum is observed. The first rate maximum is ascribed to reaction occurring in premicellar aggregates and the second to reaction in micelles. At low substrate concentration the effect of surfactant head group size was examined. Second-order rate constants in the micellar pseudophase increase with increasing head group size. Copyright © 2003 John Wiley & Sons, Ltd. [source] Carboxy Ester Hydrolysis Promoted by a Dicopper(II) Macrocyclic Polyamine Complex with Hydroxypropyl Pendant GroupsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 9 2004Jin Huang Abstract A dinuclear CuII complex containing a hexaaza macrocyclic ligand bearing two 2-hydroxypropyl pendants, 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxypropyl)tricyclo[22.2.2.211,14]triaconta-1,11,13,24,27,29-hexaene (L), was synthesized. The title complex [Cu2(H,2L)Cl2]·6.5H2O was isolated as a blue crystal, orthorhombic, space group Fddd, with a = 16.4581(12), b = 32.248(2), c = 35.830(2) Å, V = 19017(2) Å3, Z = 16, R1 = 0.0690, and wR2 = 0.1546 [I > 2,(I)]. The protonation constants of Cu2L were determined by potentiometric titration, and it was found that the alcoholic hydroxypropyl group of the complex Cu2L exhibits low pKa values of pKa1 = 7.31, pKa2 = 7.83 at 25 °C. The hydrolysis kinetics of 4-nitrophenyl acetate (NA) promoted by the title complex have also been studied. The pH-rate profile for Cu2L gave a sigmoidal curve and showed a second-order rate constant of 0.39 ± 0.02 M,1 s,1 in 10% CH3CN/H2O(v/v), which is greater than that of the dinuclear CuII complex formed by a hexaaza macrocycle without pendants. The reason for the higher catalytic activity of the title complex is discussed. We found that the volume of nucleophile RO, can effect the hydrolysis of the carboxy ester, the nucleophilicity of RO, and the Lewis acidity of the metal macrocycle also affect the carboxy ester hydrolysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Allosteric modulation of anti-HIV drug and ferric heme binding to human serum albuminFEBS JOURNAL, Issue 24 2005Alessio Bocedi Human serum albumin (HSA), the most prominent protein in plasma, is best known for its exceptional capacity to bind ligands (e.g. heme and drugs). Here, binding of the anti-HIV drugs abacavir, atazanavir, didanosine, efavirenz, emtricitabine, lamivudine, nelfinavir, nevirapine, ritonavir, saquinavir, stavudine, and zidovudine to HSA and ferric heme,HSA is reported. Ferric heme binding to HSA in the absence and presence of anti-HIV drugs was also investigated. The association equilibrium constant and second-order rate constant for the binding of anti-HIV drugs to Sudlow's site I of ferric heme,HSA are lower by one order of magnitude than those for the binding of anti-HIV drugs to HSA. Accordingly, the association equilibrium constant and the second-order rate constant for heme binding to HSA are decreased by one order of magnitude in the presence of anti-HIV drugs. In contrast, the first-order rate constant for ligand dissociation from HSA is insensitive to anti-HIV drugs and ferric heme. These findings represent clear-cut evidence for the allosteric inhibition of anti-HIV drug binding to HSA by the heme. In turn, anti-HIV drugs allosterically impair heme binding to HSA. Therefore, Sudlow's site I and the heme cleft must be functionally linked. [source] Mechanism of interaction of DNA bases with dichloro-[1-alkyl-2-(naphthylazo)imidazole]palladium(II) complexes: A cytosine caseINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2009Pradip Kumar Ghosh Interaction of cytosine (C) with dichloro-[1-alkyl-2-(,-naphthylazo)imidazole]-palladium(II) [Pd(,-NaiR)Cl2, 1] and dichloro-[1-alkyl-2-(,-naphthylazo)imidazole]-palladium(II) [Pd(,-NaiR)Cl2, 2] complexes {where alkyl R = Me (a), Et (b) or Bz (c)} in acetonitrile (MeCN)-water (50% v/v) medium to yield [{1-alkyl-2-(,-naphthylazo)-imidazole}bis(cytosine)]palladium(II)dichloride (3a, 3b, 3c) and [{1-alkyl-2-(,-naphthylazo)-imidazole}bis(cytosine)]palladium(II)dichloride (4a, 4b, 4c) was studied. The products were characterized by microanalytical data and spectroscopic techniques (FT-IR, UV,vis, and NMR). The reaction kinetics show first-order dependence of the rate on each of the concentration of Pd(II) complex and C. External addition of Cl, ion (LiCl) did not influence this nucleophilic substitution rate process and has proved the cleavage of first PdCl bond is the rate-determining step. Thermodynamic parameters standard enthalpy of activation (,,Ho) and standard entropy of activation (,,So) were determined from variable temperature kinetic studies. The negative values of ,,So indicate that the reaction proceeds through an associative inner sphere mechanism. The magnitude of the second-order rate constant k2 increases in the following order: Pd(NaiEt)Cl2 (b) < Pd(NaiMe)Cl2 (a) < Pd(NaiBz)Cl2 (c) as well as Pd(,-NaiR)Cl2 (1) < Pd(,-NaiR)Cl2 (2), which corroborates with the experimental ,,Ho values. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 463,472, 2009 [source] Mechanism of oxidation of alanine by chloroaurate(III) complexes in acid medium: Kinetics of the rate processesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2009Pratik K. Sen The kinetics of the oxidation of alanine by chloroaurate(III) complexes in acetate buffer medium has been investigated. The major oxidation product of alanine has been identified as acetaldehyde by 1H NMR spectroscopy. Under the experimental conditions, AuCl and AuCl3(OH), are the effective oxidizing species of gold(III). The reaction is first order with respect to Au(III) as well as alanine. The effects of H+ and Cl, on the second-order rate constant k2, have been analyzed, and accordingly the rate law has been deduced: k2, = (k1[H+][Cl,] + k3K4K5)/(K4K5 + [H+][Cl,]). Increasing dielectric constant of the medium has an accelerating effect on the reaction rate. Activation parameters associated with the overall reaction have been calculated. A mechanism involving the two effective oxidizing species of gold(III) and zwitterionic species of alanine, consistent with the rate law, has been proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 473,482, 2009 [source] Mononuclear Co(III)-complex promoted phosphate diester hydrolysis: dependence of reactivity on the leaving group,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6-7 2004Michela Padovani Abstract The TrpnCo(III)(OH)(OH2)-promoted hydrolysis of a range of methyl aryl phosphate diesters was investigated at 37°C and I,=,0.1,M (NaClO4). The pH,rate profile confirms that the aqua-hydroxy form of the complex is the only kinetically significant ionic form. At pH 6.9, all the reactions are first order in both diester and Co(III) complex. Plotting the second-order rate constant for Co(III) complex-promoted hydrolysis against the pKa of the leaving aryloxy group revealed a bent LFER indicating a change in rate-limiting step. This is discussed in terms of either a change from rate-limiting hydrolysis to rate-limiting binding or the presence of a phosphorane intermediate. Copyright © 2004 John Wiley & Sons, Ltd. [source] Controlled polymerizations of 2-(dialkylamino)ethyl methacrylates and their block copolymers in protic solvents at ambient temperature via ATRPJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 20 2004Baowei Mao Abstract Very well-controlled polymerizations of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and 2-(diethylamino)ethyl methacrylate (DEAEMA) in aqueous and methanolic solutions via atom transfer radical polymerization (ATRP) at ambient temperature were demonstrated. Poly(DMAEMA) and poly(DEAEMA) of low polydispersity index (PDI) of ,1.07 were obtained using the p -toluenesulfonyl chloride/CuCl/1,1,4,7,10,10-hexamethyl-triethylenetetramine (p -TsCl/CuCl/HMTETA) system. Excellent control of polymerization was achieved even in pure methanol. This is in contrast with the very poor control of DMAEMA ATRP in methanol reported previously using a different intiator/catalyst/ligand system. The initiator p -TsCl underwent hydrolysis reaction in aqueous methanolic solutions with a second-order rate constant of 6.1 × 10,4 dm3 mol,1 s,1 at 25 °C. Both poly(DMAEMA) and poly(DEAEMA) retained almost full chlorine-functionization at the chain ends. Well-defined block copolymers of DEAEMA and DMAEMA were successfully obtained by starting with either macroinitiators of DEAEMA or DMAEMA. Other well-defined diblock copolymers could be prepared using these macroinitiators. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5161,5169, 2004 [source] Rhodopsin Regeneration is Accelerated via Noncovalent 11- cis Retinal,Opsin Complex,A Role of Retinal Binding Pocket of Opsin,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2008Hiroyuki Matsumoto The regeneration of bovine rhodopsin from its apoprotein opsin and the prosthetic group 11- cis retinal involves the formation of a retinylidene Schiff base with the , -amino group of the active lysine residue of opsin. The pH dependence of a Schiff base formation in solution follows a typical bell-shaped profile because of the pH dependence of the formation and the following dehydration of a 1-aminoethanol intermediate. Unexpectedly, however, we find that the formation of rhodopsin from 11- cis retinal and opsin does not depend on pH over a wide pH range. These results are interpreted by the Matsumoto and Yoshizawa (Nature258 [1975] 523) model of rhodopsin regeneration in which the 11- cis retinal chromophore binds first to opsin through the , -ionone ring, followed by the slow formation of the retinylidene Schiff base in a restricted space. We find the second-order rate constant of the rhodopsin formation is 6100 ± 300 mol,1 s,1 at 25°C over the pH range 5,10. The second-order rate constant is much greater than that of a model Schiff base in solution by a factor of more than 107. A previous report by Pajares and Rando (J Biol Chem264 [1989] 6804) suggests that the lysyl ,-NH2 group of opsin is protonated when the , -ionone ring binding site is unoccupied. The acceleration of the Schiff base formation in rhodopsin is explained by stabilization of the deprotonated form of the lysyl ,-NH2 group which might be induced when the , -ionone ring binding site is occupied through the noncovalent binding of 11- cis retinal to opsin at the initial stage of rhodopsin regeneration, followed by the proximity and orientation effect rendered by the formation of noncovalent 11- cis retinal,opsin complex. [source] Synthesis and Esterolytic Activity of Catalytic Peptide DendrimersCHEMISTRY - A EUROPEAN JOURNAL, Issue 5 2004David Lagnoux Abstract Peptide dendrimers were prepared by solid-phase peptide synthesis. Monomeric dendrimers were first obtained by assembly of a hexapeptide sequence containing alternate standard , -amino acids with diamino acids as branching units. The monomeric dendrimers were then dimerized by disulfide-bridge formation at the core cysteine. The synthetic strategy is compatible with functional amino acids and different diamino acid branching units. Peptide dendrimers composed of the catalytic triad amino acids histidine, aspartate, and serine catalyzed the hydrolysis of N -methylquinolinium salts when the histidine residues were placed at the outermost position. The dendrimer-catalyzed hydrolysis of 7-isobutyryl- N -methylquinolinium followed saturation kinetics with a rate constant of catalysis/rate constant without catalysis (kcat/kuncat) value of 3350 and a rate constant of catalysis/Michaelis constant (kcat/KM) value 350-fold larger than the second-order rate constant of the 4-methylimidazole-catalyzed reaction; this corresponds to a 40-fold rate enhancement per histidine side chain. Catalysis can be attributed to the presence of histidine residues at the surface of the dendrimers. [source] Nucleophilicities and Nucleofugalities of Organic Carbonates,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2010Nicolas Streidl Abstract The kinetics of the reactions of the methyl carbonate ion with benzhydrylium ions in acetonitrile have been studied by UV/Vis spectrophotometry. Substitution of the resulting second-order rate constants and the electrophilicity parameters E of the benzhydrylium ions into the linear free energy relationship log,k = s(N + E) yielded the nucleophilicity parameters N25 = 16.03 and s25 = 0.64 for methyl carbonate in acetonitrile. The kinetics of the reverse reactions, i.e., of the solvolyses of ring-substituted benzhydryl alkyl carbonates in different aqueous solvents were followed by conductimetry. The obtained first-order rate constants and the known electrofugality parameters Ef of benzhydrylium ions were used to determine the nucleofugality parameters Nf and sf of the ROCO2, groups by using the linear free energy relationship log,k = sf(Nf + Ef). The leaving group abilities of carbonates decrease by a factor of about 300 from PhOCO2, over MeOCO2, and iBuOCO2, to tBuOCO2, in various alcoholic and aqueous solvents. tert -Butyl carbonates (tBocO-R) are, thus, considerably more stable with respect to heterolytic cleavage of the O,R bond than other organic carbonates. [source] Elimination Mechanisms in the Aminolysis of Sulfamate Esters of the Type NH2SO2OC6H4X , Models of Enzyme InhibitorsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 24 2008William J. Spillane Abstract The kinetics of the reaction of 4-nitrophenyl sulfamate NH2SO2OC6H4NO2 -4 (1a) in acetonitrile (ACN) with a series of pyridines (pKa range ca. 8 units) and alicyclic amines (pKa range ca. 3.6 units) has been studied in the presence of excess amine at various temperatures. The compounds 1a,1f are important as model substrates for the medicinally important sulfamate esters 667-coumate and emate and analogues. Pseudo-first-order rate constants (kobsd.) have been obtained mainly by the release of 4-nitrophenol/4-nitrophenoxide. Slopes of plots of kobsd. vs. [amine] gave second-order rate constants (k2), and Brönsted plots were biphasic for the aminolysis (with alicyclic amines) with an initial slope ,1 = 0.53 and a subsequent slope ,2 = 0.19. The change in slope occurs near the first pKa of 1a (17.9) in ACN. Leaving-group effects were probed by using the same series of phenyl sulfamates, i.e. 1a,f and the alicyclic amines N -formylpiperazine and pyrrolidine. The reactions were considered to be dissociative in nature involving E2- and E1cB- type mechanisms with the phenyl sulfamate anion 2 being involved in pyridine and in the weaker alicyclic amines (,1 segment) and a phenyl sulfamate dianion 3 being involved with the stronger alicyclic bases (,2 segment). The calculation of Leffler indices (,) for bond-forming (base···H+) and bond-breaking (S,OAr) steps allows fuller interpretation of the mechanisms occurring, which are seen as having the N -sulfonylamines, HN=SO2 and ,N=SO2 on the reaction pathways leading to products. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Nucleophilic Reactivities of Pyrroles,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2008Tobias A. Nigst Abstract The second-order rate constants of the reactions of alkyl-substituted pyrroles with a series of benzhydrylium ions were determined in acetonitrile, and the reaction products were fully characterized by NMR spectroscopy and mass spectrometry. The formation of the , adducts is the rate-limiting step of these reactions. Because the second-order rate constants correlate linearly with the electrophilicity parameters of the benzhydrylium ions, the determination of the nucleophilicity parameters N and s according to the linear free energy relationship log k2 (20 °C) = s(N + E) was achieved. With these findings, a direct comparison of the nucleophilic reactivities of these ,-excessive heterocycles with other nucleophiles became possible, and the pyrroles were integrated into the comprehensive scale of nucleophilicity, covering a range of 8,9 orders of magnitude from N -(triisopropylsilyl)pyrrole (N = 3.12), the weakest nucleophile of this series, to kryptopyrrole (3-ethyl-2,4-dimethylpyrrole, N = 11.63). Thus, highly reactive pyrroles show similar nucleophilic reactivities as enamines, whereas those of less-reactive pyrroles are comparable to allylsilanes or indoles. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Kinetics and mechanism for oxime formation from benzoylformic acid: Electrostatic interactions in the dehydration of carbinolaminesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 9 2008A. Malpica The evidence establishes that oxime formation from benzoylformic acid in the pH range from 0.25 to 5.5 occurs with acid-catalyzed dehydration of carbinolamines derived from the acid and its anion. The pH-rate profile shows in order of decreasing pH in two regions: (1) In the pH range from 5.5 to ,2.2 the second-order rate constants are linearly dependent on the concentration of hydronium ion and (2) from pH ,2.2 to 0.25 the rate constants deviate slightly from the line of slope = ,1. This slight deviation is a consequence of the very similar values of limiting rate constants of the two forms of the substrate. Electrostatic interactions between charged carbinolamines and hydronium ions are analyzed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 554,558, 2008 [source] Solvent and structural effects on the kinetics of the reactions of 2-substituted cyclohex-1-enylcarboxylic and 2-substituted benzoic acids with diazodiphenylmethaneINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2007J. B. Nikoli The rate constants for the reaction of 2-methyl-cyclohex-1-enylcarboxylic, 2-phenylcyclohex-1-enylcarboxylic, and 2-methylbenzoic and 2-phenylbenzoic acids with diazodiphenyl-methane were determined in 14 various solvents at 30°C. To explain the kinetic results through solvent effects, the second-order rate constants of the examined acids were correlated using the Kamlet,Taft solvatochromic equation. The correlations of the kinetic data were carried out by means of multiple linear regression analysis, and the solvent effects on the reaction rates were analyzed in terms of initial and transition state contributions. The quantitative relationship between the molecular structure and the chemical reactivity has been discussed, as well as the effect of geometry on the reactivity of the examined molecules. The geometric data of all the examined compounds corresponding to the energy minima in solvent, simulated as dielectric continuum, obtained using semiempirical MNDO-PM3 energy calculations. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 664,671, 2007 [source] Effects of alcohols on micellization and on the reaction methyl 4-nitrobenzenesulfonate + Br, in cetyltrimethylammonium bromide aqueous micellar solutionsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 12 2004María Muñoz The effects of n -hexanol, n -pentanol, and n -butanol on the critical micelle concentration (cmc), on the micellar ionization degree (,), and on the rate of the reaction methyl 4-nitrobenzenesulfonate + Br, have been investigated in cetyltrimethylammonium bromide (CTAB) aqueous solutions. An increase in the alcohol concentration present in the solution produces a decrease in the cmc and an increase in the micellar ionization degree. Kinetic data show that the observed rate constant decreases as alcohol concentration increases. This result was rationalized by considering variations in the equilibrium binding constant of the methyl 4-nitrobenzenesulfonate molecules to the micelles, variations in the interfacial bromide ion concentration, and variations in the characteristics of the water,alcohol bulk phase provoked by the presence of alcohols. When these operative factors are considered, kinetic data in this and other works show that the second-order rate constants in the micellar pseudophases of water,alcohol micellar solutions are quite similar to those estimated in the absence of alcohols. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 634,641, 2004 [source] Kinetics of reactions between chlorine or bromine and the herbicides diuron and isoproturonJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2 2007Juan L Acero Abstract The chemical oxidation of two herbicide derivatives of the phenylurea group,diuron and isoproturon,has been carried out by means of chlorine, in the absence and in the presence of bromide ion. Apparent second-order rate constants for the reactions between chlorine and the herbicides were determined to be below 0.45 L mol,1 s,1. Hypobromous acid reacts faster with the investigated herbicides, especially with isoproturon (kapp = 24.8 L mol,1 s,1 at pH 7). While pH exerts a negative effect on the bromination rate, the maximum chlorination rate was found to be at circumneutral pH. In a second stage, the oxidation of each compound was conducted in different natural waters, in order to simulate the processes which take place in water purification plants. Again, chlorine was used as an oxidant, and bromide ion was added in some experiments with the aim of producing the more reactive HOBr oxidant. The herbicide oxidation rate was inversely proportional to the organic matter content of the natural water. However, the formation of trihalomethanes (THMs) was directly proportional to the organic matter content and constitutes a limitation for the application of chlorine during drinking water treatment. Finally, the evolution of herbicide concentration was modeled and predicted by applying a kinetics approach based on the rate constants for the reactions between the herbicides and the active oxidants. Copyright © 2007 Society of Chemical Industry [source] Reactivity parameters for rationalizing iminium-catalyzed reactions,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2010Sami Lakhdar Abstract The correlation equation (1), lg k(20,°C),=,s(E,+,N), where electrophiles are characterized by one (E) and nucleophiles are characterized by two parameters (N, s) was used to rationalize the scope of iminium-catalyzed reactions. Kinetics of the reactions of iminium triflates, pregenerated from cinnamaldehyde and secondary amines, with cyclic ketene acetals were studied by UV,Vis spectroscopy. From the second-order rate constants, electrophilicity parameters ,10,<,E,<,,7 have been derived for these iminium ions. Eqn (1) was found to correctly predict the rate constants for the reactions of the cinnamaldehyde-derived iminium ions with pyrroles, indoles, and sulfur ylides. The zwitterion obtained from cinnamaldehyde and indoline-2-carboxylic acid reacts more than 105 times faster with a sulfur ylide than predicted by Eqn (1), which is explained by MacMillan's ,electrostatic activation'. The failure of imidazolidinones to catalyze cyclopropanations of ,,,-unsaturated carbonyl compounds by sulfur ylides is not due to the low nucleophilic reactivity of sulfur ylides but due to their high Brønsted basicity which inhibits the formation of the iminium ions. Copyright © 2010 John Wiley & Sons, Ltd. [source] SN2 reaction of a sulfonate ester in the presence of alkyltriphenylphosphonium bromides and mixed cationic-cationic systemsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2006Michael M. Mohareb Abstract The effects of alkyltriphenylphosphonium bromides (CnTPB, n,=,10, 12, 14, 16) on the rates of SN2 reactions of methyl 4-nitrobenzenesulfonate and bromide ion have been studied. Observed first-order rate constants are significantly higher than those found for other cationic surfactants for the same reaction. The results have been analyzed by the pseudophase model of micellar kinetics and show true micellar catalysis in the sense that second-order micellar rate constants are higher than the second-order rate constants in water. An attempt has also been made to investigate mixed cationic,cationic surfactant systems with respect to observed rates and pseudophase regression parameters. In addition, modeling of some cationic head groups has illustrated possible differences in head group charges and counterion interactions that may prove kinetically relevant. Copyright © 2006 John Wiley & Sons, Ltd. [source] Substituent effects in reductions of heteroaromatic cationsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2002David Heyes Abstract A set of 11 each of 2,4,6-triphenylpyrylium, -thiopyrylium and - N -methylpyridinium tetrafluoroborates carrying a range of substituents in the phenyl rings were prepared. First and second wave reduction potentials were determined. For the thiopyrylium series there are linear correlations between scaled potentials (E°/0.05915) and summed Hammett constants for substituents in the pendant phenyl groups (,,=,2.29 and 3.38 for first and second waves respectively). For the pyrylium series, a good linear relationship (,,=,2.79) is obtained for all substituent patterns for the first wave reduction potentials, but for the second wave there are separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2- and 6-phenyls. For the pyridinium series, the first wave potentials show separate correlations for salts carrying substituents in the 4-phenyl and for those carrying substituents in 2-and 6-phenyls, but a single linear relationship for the second wave potentials. These are related to particular structural features in the cations, radicals and anions in each series. Rates and products were determined for reductions of the pyrylium and thiopyrylium cations by sodium cyanoborohydride and of all cations by sodium borohydride in acetonitrile solution. Reactions are first order in reducing agent and cation. Primary kinetic isotope effects were determined for borohydride reduction of the least reactive of each of the series of cations. Plots of logarithms of second-order rate constants against summed Hammett constants for substituents in the pendant phenyl groups are linear for all combinations of reagent and cation with 0.91 < , < 1.50 across all substituent patterns. For parent pyrylium and thiopyryliums, kBH4/kCNBH3,=,8.4,×,104 and 1.5,×,104, respectively, and for reductions by borohydride the reactivities of the pyrylium, thiopyrylium and pyridinium, series decrease in the order 1.4,×,105:8.8,×,103:1. Constant selectivities are not observed. Comparison of the correlations for electrochemical reduction and for hydride addition leads to the conclusion that charge neutralization in the hydride addition transition states runs ahead of bonding changes at the originating B,H bond. Copyright © 2002 John Wiley & Sons, Ltd. [source] Synthesis of Peptidyl Ene Diones: Selective Inactivators of the Cysteine ProteinasesCHEMICAL BIOLOGY & DRUG DESIGN, Issue 3 2007Paul Darkins A series of synthetic peptides in which the C-terminal carboxyl grouping (-CO2H) of each has been chemically converted into a variety of ene dione derivatives (-CO-CH=CH-CO-X; X = -H, -Me, -OBut, -OEt, -OMe, -CO-OMe), have been prepared and tested as inactivators against typical members of the serine and cysteine protease families. For example, the sequences Cbz-Pro-Phe-CH=CH-CO-OEt (I) which fulfils the known primary and secondary specificity requirements of the serine protease chymotrypsin, and Cbz-Phe-Ala-CH=CH-CO-OEt (II) which represents a general recognition sequence for cysteine proteases such as cathepsins B, L and S, have been tested as putative irreversible inactivators of their respective target proteases. It was found that, whereas II, for example, functioned as a time-dependent, irreversible inactivator of each of the cysteine proteases, I behaved only as a modest competitive reversible inhibitor of chymotrypsin. Within the simple ester sequences Cbz-Phe-Ala-CH=CH-CO-R, the rank order of inhibitor effectiveness decreases in the order R = -OMe > -OEt >> -OBut. It was also found that the presence of both an unsaturated double bond and an ester (or , -keto ester) moiety were indispensable for obtaining irreversible inactivators. Of the irreversible inactivators synthesized, Cbz-Phe-Ala-CH=CH-CO-CO-OEt (which contains a highly electrophilic , -keto ester grouping) was found to be the most effective exhibiting, for example, second-order rate constants of approximately 1.7 × 106m,1min,1 and approximately 4.9 × 104m,1min,1 against recombinant human cathepsin S and human spleenic cathepsin B, respectively. This initial study thus holds out the promise that this class of inactivator may well be specific for the cysteine protease subclass. [source] | |||||||||||||