A new interpretation of the Baylis-Hillman mechanism

[reaction: see text] On the basis of reaction rate data, we have proposed a new mechanism for the Baylis-Hillman reaction involving the formation of a hemiacetal intermediate. We have determined that the rate-determining step is second order in aldehyde and first order in DABCO and acrylate. We have shown that this mechanism is general to aryl aldehydes under polar, nonpolar, and protic conditions using both rate data and two isotope effect experiments.     

Kinetics and mechanism of the oxidation of glycine by N-bromosuccinimide

A systematic kinetic study on the oxidation of glycine by N-bromosuccinimide (NBS) in presence of mercuric acetate in acetic acid—water media has been made. Near first order dependence inNBS and glycine and near inverse first order dependence in hydrogen ion concentrations have been observed. A negligible ionic strength effect and a positive dielectric effect have been observed. Various rate parameters have been computed and hydrocyanic acid identified as the end product. On the basis of the kinetic data, a mechanism of the reaction has been proposed.With 3 Figures     

The Catalytic Mechanism of Human Transketolase

We have computationally determined the catalytic mechanism of human transketolase (hTK) using a cluster model approach and density functional theory calculations. We were able to determine all the relevant structures, bringing solid evidences to the proposed experimental mechanism, and to add important detail to the structure of the transition states and the energy profile associated with catalysis. Furthermore, we have established the existence of a crucial intermediate of the catalytic cycle, in agreement with experiments. The calculated data brought new insights to hTK′s catalytic mechanism, providing free-energy values for the chemical reaction, as well as adding atomistic detail to the experimental mechanism.     

Polymerisation anionique de l'acroleine—I. Polymerisations amorcees par des organo-sodiques dans le tetrahydrofuranne

In tetrahydrofuran, with Na⁺ as counter-ion, the anionic polymerization of acrolein involves numerous transfer reactions to monomer and to polymer; on the other hand, termination of growing chains does not occur. The use of initiators, like carbanions or oxanions, does not affect the polymerization rate. The kinetic order of the reaction is unity for monomer and unity for initiator; these results indicate that the living ends are not associated at the studied concentrations of initiator. Without stating precisely the mechanism of the transfer reactions, we have proposed a kinetic scheme.In tetrahydrofuran, with Na⁺ as counter-ion, the anionic polymerization of acrolein involves numerous transfer reactions to monomer and to polymer; on the other hand, termination of growing chains does not occur. The use of initiators, like carbanions or oxanions, does not affect the polymerization rate. The kinetic order of the reaction is unity for monomer and unity for initiator; these results indicate that the living ends are not associated at the studied concentrations of initiator. Without stating precisely the mechanism of the transfer reactions, we have proposed a kinetic scheme.     

Oxidation of aniline and some para-substituted anilines by benzimidazolium fluorochromate in aqueous acetic acid medium – A kinetic and mechanistic study

The oxidation kinetics of some para-substituted anilines by benzimidazolium fluorochromate (BIFC) have been studied in aqueous acetic acid media in the presence of perchloric acid. The reaction is first order with respect to both aniline and BIFC and is catalysed by a hydrogen ion. The rate data obey Hammett relationship. The products of oxidation are the corresponding azo benzenes. Based on the kinetic results and product analysis, a suitable mechanism has been proposed for the reaction of BIFC with anilines.     

Conversion of C5 into C6 cyclic species through the formation of C7 intermediates

We have recently proposed that the addition of C2H2 to the cyclopentadienyl radical can lead to the rapid formation of the cycloheptatrienyl radical and, in succession, of the indenyl radical. These reactions represent an interesting and unexplored route for the enlargement of gas-phase cyclic species. In this work we report ab initio calculations we performed with the aim of investigating in detail the gas-phase reactivity of cycloheptatrienyl and indenyl radicals. We found that the reaction of the cycloheptatrienyl radical with atomic hydrogen can lead to its fast conversion into the more stable benzyl radical. This reaction pathway involves the intermediate formation of heptatriene, norcaradiene, and toluene. Successively we investigated whether this reaction mechanism can be extended to polycyclic aromatic hydrocarbons (PAHs). For this purpose we studied the reaction of C2H2 with the indenyl radical, which can be considered as a superior homologue of the cyclopentadienyl radical. This reaction proceeds through a pathway similar to that proposed for C5H5 but with a reaction rate about an order of magnitude smaller. The present calculations extend thus the previously proposed C5-C7-C9 mechanism to bicyclic PAH and suggest a fast route for the conversion of C5 into C6 cyclic radicals, mediated by the formation of C7 cyclic species.     

The mechanism of the rearrangement of 1-benzyl-1,2-dihydroisoquinolines: some criticisms answered

The rearrangement of 2-methyl-1,2-dihydropapaverine (1a) to the corresponding 2-methyl-3-benzyl-3,4-dihydroisoquinolinium ion (3a) has been shown to be a second order rate process with an unusually high entropy of activation. These data, together with an analysis of the orbital symmetry requirements, have been shown to be consistent with the previously proposed double exchange mechanism for this reaction.     

Enzyme-catalyzed rearrangement of a diepoxy-germacrane compound into new 7-epi-eudesmane derivatives

Two new 7-epi-eudesmane derivatives, together with two new germacrane compounds, have been isolated from the microbial-transformation of a (1alpha,10beta),(4beta,5alpha)-diepoxygermacrane using the hydroxylating fungi Rhizopus nigricans. The rearranged skeleton and the stereochemistry of the chiral centers have been determined by means of their spectral data, and the absolute configuration has been confirmed by single-crystal X-ray analyses. A possible mechanism based on an enzyme-catalyzed isomerization to a 1alpha-hydroxy-(4beta,5alpha)-epoxygermacr-9(E)-ene intermediate and a subsequent cyclization process is proposed in order to explain the formation of the 7-epi-eudesmane compounds.     

The reversible consecutive mechanism for the reaction of trinitroanisole with methoxide ion

Although the competitive mechanism for Meisenheimer complex formation during the reaction of 2,4,6-trinitroanisole with methoxide ion in methanol is generally accepted, no kinetic evidence has been presented to rule out a reversible consecutive mechanism. Simulation of the competitive mechanism revealed that a fractional order in [MeO(-)] is predicted by the latter. Conventional pseudo-first-order analysis of the kinetics resulted in cleanly first-order in [MeO(-)], which rules out the competitive mechanism. The kinetic data are consistent with the reversible consecutive mechanism, which is proposed for this important reaction. An intermediate is required for this mechanism, and we propose that a dianion complex (III) is formed reversibly from the initial 1,3-σ complex (I). The trimethoxy complex (III), the (1)H NMR spectrum of which was observed earlier by Servis (Servis, K. L. J. Am. Chem. Soc. 1965, 87, 5495; 1967, 89, 1508), then eliminates methoxide ion reversibly to form the 1,1-σ complex product (II).     

Kinetic simulation of oxidative coupling of methane over perovskite catalyst by genetic algorithm： Mechanistic aspects

The reaction kinetics of oxidative coupling of methane catalyzed by perovskite was studied in a fixed bed flow reactor. At atmospheric pressure, the reactions were carried out at 725, 750 and 775 ℃, inlet methane to oxygen ratios of 2 to 4.5 and gas hourly space velocity (GHSV) of 100 min-1. Correlation of the kinetic data has been performed with the proposed mechanisms. The selected equations have been regressed with experimental data accompanied by genetic algorithm (GA) in order to obtain optimized parameters. After investigation the Langmuir-Hinshelwood mechanism was selected as the best mechanism, and Arrhenius and adsorption parameters of this model were obtained by linear regression. In this research the Marquardt algorithm was also used and its results were compared with those of genetic algorithm. It should be noted that the Marquardt algorithm is sensitive to the selection of initial values and there is possibility to trap in a local minimum.     

双桥反应机理的从头计算研究

有大量文献[1-3]对有机化学反应中的反式加成、邻基参与下的反式消去反应所涉及的三元环过程（我们称之为单桥反应机理）进行了研究·负离子以相等的几率进攻成桥的二个碳原子中的任意一个，因而对大量反应的立体选择性给出了很好的解释．但是有些反应却不能用这一机理解释．FinarN］指出其原因还不清楚，并推测这是由于桥键可以断裂而形成开环经典离子，使原来的桥原子所跨碳碳单键旋转．这儿相当于给出了两种并存的机理．HoPkhson等人＊以乙烯和氟或氯加成为模型，对单桥机理进行了量子化学从头算研究·得到这两个反应的活化能分别大于5…     

Manganese(III) oxidation of L‐serine in aqueous sulfuric acid medium: Kinetics and mechanism

Kinetics and mechanism of oxidation of L‐serine by manganese(III) ions have been studied in aqueous sulfuric acid medium at 323 K. Manganese(III) sulfate was prepared by an electrolytic oxidation of manganous sulfate in aqueous sulfuric acid. The dependencies of the reaction rate are: an unusual one and a half‐order on [Mn(III)], first‐order on [ser], an inverse first‐order on [H⁺], and an inverse fractional‐order on [Mn(II)]. Effects of complexing agents and varying solvent composition were studied. Solvent isotope studies in D₂O medium were made. The dependence of the reaction rate on temperature was studied and activation parameters were computed from Arrhenius‐Eyring plots. A mechanism consistent with the observed kinetic data has been proposed and discussed. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 525–530, 1999     

Hydrotropic Enhancement of Rate of Ninhydrin‐α‐Amino Acid Reaction: A Kinetic Study

The kinetics of ninhydrin‐α‐amino acid (alanine, phenylalanine, and valine) reaction have been studied in the absence and in the presence of a common hydrotrope, sodiumbenzoate. A strong enhancement in the rate of reaction has been observed in the presence of the hydrotrope above its minimal hydrotrope concentration (MHC). The order of reaction in ninhydrin was always found to be unity while that in amino acid decreases from unity to zero at higher [Aminoacid]. A mechanism consistent with kinetic data and involving the formation of a mixed aggregation assemblies between amino acid and hydrotrope has been proposed.     

Bray–Liebhafsky and non-catalysed Briggs–Rauscher oscillating reactions

In order to propose mechanisms of complicated chemical systems, it is necessary to study simpler subsystems. The mechanism we have proposed for the Bray–Liebhafsky (BL) oscillating reaction is based on kinetic studies of several reactions of iodine compounds between them and with hydrogen peroxide. Because the reactants of the non-catalysed Briggs–Rauscher (BR) oscillating reaction are the same as those of the BL reaction plus malonic acid, we propose now to extend the mechanism of the BL reaction to the BR reaction. With this aim, we add radical reactions of iodine compounds and of malonic acid. The choice of these reactions is based on our recent study of the unusual kinetics of the iodate reduction by high concentrations of hydrogen peroxide.     

A Simple Equation for Predicting the Volumetric Properties of Mixtures Involving Ionic Liquids

A simple equation has been employed to predict the volumetric properties of binary mixtures involving an ionic liquid. Moreover, we have calculated excess volumetric properties of the studied mixtures using the proposed equation. The outcomes of our computations have been compared with experimental data. The reliability of the proposed model was checked by taking 854 experimental data points over a temperature range from 278 to 358 K; the average absolute deviation of the calculated density and the excess molar volume were found to be 0.50 and 0.80 %, respectively, while the accuracies of our calculations were of the order of ±3 %.     

Neural network-based correlations for the thermal conductivity of propane

An alternative approach, exploiting neural networks, is proposed to develop thermal conductivity correlation of propane for the first time. In order to test the accuracy of the proposed technique and demonstrate its utility in fitting the thermal conductivity surface of propane, we have established a thermal conductivity correlation in terms of temperature and density, and then compared its predictions with those obtained by the conventional method. The results obtained are so impressive that the neural network correlation has lower overall average absolute deviations (AADs) in each data set.     

Enhanced cooperativity through design: pendant Co(III)--salen polymer brush catalysts for the hydrolytic kinetic resolution of epichlorohydrin (salen=N,N'-bis(salicylidene)ethylenediamine dianion)

The Co(III)--salen-catalyzed (salen=N,N'-bis(salicylidene)ethylenediamine dianion) hydrolytic kinetic resolution (HKR) of racemic epoxides has emerged as a highly attractive and efficient method of synthesizing chiral C(3) building blocks for intermediates in larger, more complex molecules. HKR reaction rates have displayed a second order dependency on the concentration of active sites, and thus researchers have proposed a bimetallic transition state for the HKR mechanism. Here we report the utilization of pendant Co(III)--salen catalysts on silica supported polymer brushes as a catalyst for the HKR of epichlorohydrin. The novel polymer brush architecture provided a unique framework for promoting site-site interactions as required in the proposed bimetallic transition state of the HKR mechanism. Furthermore, the polymer brushes mimic the environment of soluble polymer-based catalysts, whereas the silica support permitted facile recovery and reuse of the catalyst. The polymer brush catalyst displayed increased activities over the soluble Jacobsen Co--salen catalyst and was observed to retain its high enantioselectivities (>99 %) after each of five reactions despite decreasing activities. Analysis indicated decomposition of the salen ligand as an underlying cause of catalyst deactivation.     

Use of block copolymer surfactants in the inverse-suspension polymerization of acrylamide

A comprehensive experimental investigation of the inverse microsuspension polymerization of acrylamide using an oil-soluble initiator and a block copolymeric surfactant whose hydrophobic miety is poly(12-hydroxystearic acid) and whose hydrophilic moeity is polyethylene oxide was carried out. It was found that the initial polymerization rate was first order with respect to molar monomer concentration, first order with respect to molar initiator concentration and zeroth order with respect to molar emulsifier concentration. Based on these experimental findings, a mechanism was proposed which includes initiation, propagation transfer to monomer and termination. It also includes transfer to impurities which are believed to be found in the surfactant. The kinetic model developed from the proposed mechanism is found to be in good agreement with the experimental conversion and weight-average molecular weight data. Comparing with sorbitan esters of fatty acids, the copolymeric surfactant provides higher polymerization rate and very high and linear molecular weight comparable to those obtained by solution polymerization.     

Hydrolysis of 3,4-diphenyl-1,2,5-thiadiazole-1,1-dioxide. Acid and base catalysis

We have performed a product and kinetic study of the hydrolysis of 3,4-diphenyl-1,2,5-thiadiazole-1,1-dioxide in aqueous solution. Benzil and sulfamide are the only products of hydrolysis and are formed in equimolar yields. The kinetic results indicate that a first order law is followed up to 90% conversion. The observed rate constant is independent of substrate concentration. Ionic strength and buffer concentration do not affect the rate constant, but the reaction is acid-base catalyzed. The rate-pH profile has been determined and a mechanism that fits the experimental data satisfactorily is proposed. Corresponding rate constants and equilibrium constants for the protonation of the substrate are reported.