Model studies on the kinetics and mechanism of polyarylate synthesis by acidolysis

Model reactions were carried out to simulate the acidolysis process for polyarylate synthesis by using p-tert-butylphenyl acetate (ptBuPhOAc) and benzoic acid in diphenyl ether. p-tert-Butylphenol was formed in the reaction mixture and its concentration stayed constant throughout the reaction. Acetic benzoic anhydride and benzoic anhydride were detected by NMR. Based on this experimental evidence, a mechanism for the acidolysis was proposed involving the mixed anhydride. The kinetics of the acidolysis reaction was studied for this model reaction. The overall reaction order is two and the reaction order with respect to each reactant is one. Second-order reaction rate constants were measured at different reaction conditions (200–250°C). The activation energy (E_a), activation enthalpy (ΔH^≠), and activation entropy (ΔS^≠) were calculated from these data. The thermodynamic parameters of the acidolysis reaction were also measured for the analogous reaction of p-tert-butylphenyl pivalate (ptBuPhOPiv) and benzoic acid. The kinetics of two other elementary reactions involved in the acidolysis reaction were also studied: p-tert-butylphenol with acetic anhydride or benzoic anhydride, and p-tert-butylphenyl pivalate with benzoic acid.     

Synthesis of 5‐aryl and 5‐amidocamptothecins

A variety of 5‐aryl‐(20S)‐camptothecin derivatives were synthesized by the reaction of 5‐hydroxy‐(20S)‐camptothecin with aromatic hydrocarbons under Friedel‐Craft reaction conditions in moderate to good yield as diastereomeric pairs. The methodology was then extended for the synthesis of 5‐amido‐(20S)‐camptothecin derivatives by reacting 5‐hydroxy‐(20S)‐camptothecin with alkyl and aryl nitriles under Ritter type reaction conditions. The reaction is presumed to proceed through an iminium ion intermediate under Friedel Craft and Ritter type reaction condition, which is further trapped by nucleophile present in the reaction medium. J. Heterocyclic Chem., 00 , 00 (2011).     

The Self Oxidation Reduction of N-Arylhydroxylamines

The reactions of N-phenylhydroxylamine in the presence of dry hydrogen chloride to form azoxybenzene, aniline, 2-chloroaniline and 4-chloroaniline were studied. The molar ratio of the azoxybenzene and aniline obtained was very close to one. A similar reaction was also observed for 2-methyl- and 4-methyl-N-phenylhydroxylamine. A reasonable explanation is that N-phenylhydroxylamine undergoes a self oxidation reduction reaction to give aniline and nitrosobenzene, and the latter subsequently reacts with N-phenylhydroxylamine to give azoxybenzene. The reaction of N-phenylhydroxylamine, catalyzed by trifluoroacetic acid to yield azoxybenzene, was previously studied by Okamoto et al. and was suspected to undergo a similar reaction. We repeated the reaction and evidence for the same self oxidation reduction reaction was found. A mechanism involving the anilenium ion is proposed to account for this reaction.     

Contributions to the spectroscopic–kinetic analysis of linear reaction systems: Systems with three linearly independent reactions using the concept of parallel projection

Linear reaction systems consist by definition of first-order reaction steps. Linearly independent reactions are independent of reaction order. Each reaction mechanism consists of a distinct number (s) of linearly independent reaction steps. Thus, the mechanism ABCD can be described by three linearly independent reactions as it is also true for A→B→C→D (s=3). In the following, a procedure for the analysis of linear reaction systems consisting of three linearly independent reaction steps is described which is based on absorbance (A) as well as absorbance difference quotient (ADQ) diagrams (two graphs of the ‘Mauser diagrams') and the concept of parallel projection. In this way it is possible to determine the ratio of eigenvalues describing the reaction mechanism. Furthermore, the reaction system (s=3) can be reduced to a system which is described only by two linearly independent concentration variables (s=2). The kinetic equations of evaluation are simplified by the concept of parallel projection. This can be helpful, for example, when one independent reaction step shows poor spectroscopic properties. The method can be extended to the analysis of quasilinear photoreactions. The method is demonstrated using a practical example (A′→B′, C′→D′, E′→F′).     

Niederdruck-Oligomerisation von Mono-Olefinen mit löslichen Nickel/Aluminium-Bimetallkatalysatoren Teil III. Reaktionskinetische Untersuchungen über die Dimerisation und Trimerisation des Áthylens

The kinetics of the di- and trimerization of ethylen in organic solvents under the influence of a homogeneous catalyst containing π-tetramethylcyclobutadiene-nickeldichloride and a prereacted mixture of ethylaluminiumdichloride and tri-n-butylphosphine are reported. The primary reaction product is 1-butene, which is isomerized to 2-butene (cis/trans) during the reaction. The C₆-Olefins are formed by the reaction of ethylene with 1-butene and with the 2-butenes. The following primary reaction products are obtained: 3-hexene (cis/trans), 1-hexene, 2-ethyl-1-butene, 3-methyl-1-pentene and 3-methyl-2-pentene (cis/trans). The effect of other phosphines on the reaction was also studied. The relative composition of the reaction product is strongly dependent upon the amount and the LEWIS base strength of the phosphine present. The results are in accordance with a coordinative mechanism on nickel.     

Kinetic analysis of reversible thermal decomposition of solids

The isoconversional method was used to elucidate the kinetics of reversible solid-state reactions occurring under nonisothermal linear heating. The characteristic dependencies of the effective activation energy (E) on the extent of conversion (W) were established for two model processes: a reversible first-order reaction and a reversible reaction followed by an irreversible one. For the first process, E is almost independent of W and varies between the activation energy of the direct and inverse reaction. For the second, process with an endothermic reversible step, the dependence of E on W is of decreasing shape. The effective activation energy is limited by the sum of the activation energy of the irreversible reaction and the enthalpy of the reversible reaction, at low conversions, and by the activation energy of the irreversible reaction at high conversions. Analyses of the kinetic data for the dehydration of crystalhydrates, as well as other processes proceeding through a reversible step, show the dependencies of E on W characteristic of a reversible reaction followed by an irreversible one. © 1995 John Wiley & Sons, Inc.     

Ab initio Mechanism Study on the Reaction of Chlorine Atom with Formic Acid

The potential energy surface(PES) for the reaction of Cl atom with HCOOH is predicted using ab initio molecular orbital calculation methods at UQCIDS(T,full)6-311 G(3df,2p)//UMP2(full)/6-311 G(d,P) level of theory with zero-point vibrational energy (ZPVE) correction.The calculated results show that the reaction mechanism of Cl atom with formic acid is a C-site hydrogen abstraction reaction from cis-HOC(H)O molecule by Cl atom with a 3.73kJ/mol reaction barrier height,leading to the formation of cis-HOCO radical which will reacts with Cl atom or other molecules in such a reaction system.Because the reaction barrier height of O-site hydrogen abstraction reaction from cis-HOC(H)O molecule by Cl atom which leads to the formation of HCO2 radical is 67.95kJ/mol,it is a secondary reaction channel in experiment,This is in good agreement with the prediction based on the previous experiments.     

The Photokinetics of Electron Transfer Reaction of Methylene Blue with Titanium Trichloride in Aqueous–Alcoholic Solvents

The kinetics of photoinduced electron transfer reaction of methylene blue (MB) and titanium trichloride was investigated in water and different aqueous–alcoholic solvents. The reaction is pseudo‐first order, dependent only on the concentration of titanium trichloride at a fixed concentration of MB. The effect of water and aqueous–alcoholic solvents was studied in the acidic pH range (4–7). It was observed that the quantum yield (ϕ ) of the reaction increased with increase in polarity of the reaction medium. The quantum yield was high under acidic conditions and decreased with further increase in acidity. The addition of ions and increase in temperature increased the rate and quantum yield of the reaction. The absence of any reaction intermediate was confirmed by spectroscopic investigations. A mechanism for the reaction has been proposed in accordance with the kinetics of the reaction. The activation energy (E _a) was calculated by the Arrhenius relation. Thermodynamic parameters such as E _a, enthalpy change (ΔH ), free energy change (ΔG ), and entropy change (ΔS ) were also evaluated.     

Ambident anion chemistry: The reaction of sterically-hindered phenolate anions with 1H-pyrrole-2,5-diones

The reaction of N-phenylmaleimide, 4a , with sodium 2,6-di-t-butylphenolate, 5a , in dimethylsulfoxide (DMSO) resulted a complex oligomeric mixture. The dimer 8 was isolated from the reaction of the N-alkyl-maleimide 4b with 5a in DMSO. The reaction of 4a with 5a in tetrahydrofuran (THF), which is an aprotic solvent that is known to promote ion pairing, resulted in the isolation of a low yield of 6a . The reaction of 4a with 5a in the hydrogen-bonding solvent t-butyl alcohol gave 6a in slightly higher yield. The N-alkylpyrrolidine-2,5-diones 6c-f were obtained by the reaction of the maleimides 4c-e with the corresponding sodium phenolate 5a-b in t-butyl alcohol reaction medium. The isolated yield of product increased with the size of the N-alkyl substituent of the maleimide. Surprisingly, the reaction of the 2,6-dimethylphenolate 5c with 4d led to the isolation of the dimer 10 with the formation of a quaternary carbon atom. The yield of 6a was observed to counterion dependent, increasing in the order Na⁺ < Li⁺ < MgBr⁺ in t-butyl alcohol. The bismaleimides 12 and 14 were obtained by the reaction of either 11 or 13 with 5d in THF.     

CHARACTERIZATION OF THE RED LIGHT INDUCED REDUCTION OF A PARTICLE ASSOCIATED b-TYPE CYTOCHROME FROM CORN IN THE PRESENCE OF METHYLENE BLUE*

Methylene blue transfers electrons to a membrane-associated b-type cytochrome in particulate fractions from corn coleoptiles. The K_m for methylene blue is less than 1 µM under optimal conditions. This reaction is destroyed by boiling, but not by 7 M urea. Kinetic analyses of the influence of light intensity on cytochrome reduction suggest that a first order photochemical reaction is limiting. Free EDTA may serve as an electron donor in this system at least at high methylene blue and protein concentrations. The photoactivity does not coincide either with mitochondrial or endoplasmic reticulum markers, and may be localized in plasma membrane. There is an estimated 5 times 10^-10 mol photoreducible cytochrome per g coleoptile tissue. Studies on the effect of pH on the reaction in the presence of methylene blue or thionine indicate that dye photoreduction itself is not rate-limiting. Wavelength dependence studies suggest that it is methylene blue monomer and not dimer which mediates the reaction. Although oxygen is apparently required for the reaction, neither superoxide nor excited singlet oxygen appear to be involved. The reaction mechanism is still unknown.     

Photoinitiation. V. Effect of medium polarity on the acrylonitrile polymerization photoinitiated by naphthalene

The heterogeneous polymerization of acrylonitrile photoinitiated by naphthalene is influenced by the polarity of the reaction medium. The rate of initiation increases with the increasing dielectric strength of the reaction medium. A similar trend is observable for Stern–Volmer constants of naphthalene fluorescence quenching by acrylonitrile. The ratio k_p/k_t^1/2 of the rate constant for propagation and termination reactions is not influenced by a change in the polarity of the reaction medium. The effect of viscosity on the value of k_p/k_t^1/2 known for polymerization in a homogeneous medium was not observed in the reaction systems studied.     

On the photodecomposition of chlorophyll in vitro. I. Reaction rates

Abstract— Chlorophyll solutions are irreversibly bleached by light in the presence of oxygen. The action spectra parallel the absorption spectra for both chlorophyll a and b. The reaction is of second order with a Q₁₀ of 1.26. The reaction rates for chlorophylls a and b are of the same order of magnitude. Depending upon the light source, the initial rate for chlorophyll a is slightly higher, by a factor of 1.15 to 1.30. The rate for pheophytin is lowe 3 by several orders of magnitude. No pheophytin has been detected in the reaction products of the irradiated chlorophyll solutions in the absence of water.     

Diphenylprolinol Silyl Ether Catalyzed Asymmetric Michael Reaction of Nitroalkanes and β,β‐Disubstituted α,β‐Unsaturated Aldehydes for the Construction of All‐Carbon Quaternary Stereogenic Centers

The asymmetric Michael reaction of nitroalkanes and β,β‐disubstituted α,β‐unsaturated aldehydes was catalyzed by diphenylprolinol silyl ether to afford 1,4‐addition products with an all‐carbon quaternary stereogenic center with excellent enantioselectivity. The reaction is general for β‐substituents such as β‐aryl and β‐alkyl groups, and both nitromethane and nitroethane can be employed. The addition of nitroethane is considered a synthetic equivalent of the asymmetric Michael reaction of ethyl and acetyl substituents by means of radical denitration and Nef reaction, respectively. The short asymmetric synthesis of (S)‐ethosuximide with a quaternary carbon center was accomplished by using the present asymmetric Michael reaction as the key step. The reaction mechanism that involves the E/Z isomerization of α,β‐unsaturated aldehydes, the retro‐Michael reaction, and the different reactivity between nitromethane and nitroethane is discussed.     

A model study for coatings containing hexamethoxymethylmelamine

Using a model reaction we have studied the crosslinking chemistry of hydroxy-functional polymers and hexamethoxymethylmelamine. The transetherification of optically active monofunctional alcohols and hexamethoxymethylmelamine was monitored with polarimetry and ¹H-NMR. The reaction rate constants for both the forward (k₁) and the backward (k_?1) reaction of the sulphonic-acid-catalyzed alcoholysis were determined. Primary and secondary alcohols showed the same reaction rate and activation energy (E_a = 96 kJ/mol) for the forward reaction. However, the backward reaction in the equilibrium is considerably slower for primary alcohols than for secondary alcohols, with activation energies of E_a = 96 and 79 kJ/mol, respectively. When amine salts of sulphonic acids are used as catalysts, the E_a is increased from 97 to 116 kJ/mol in the case of primary alcohols. In concentrated aprotic solutions the reaction order in acid is 2.5. The same order in acid is found for the alcoholysis of acetaldehyde diethyl acetal. All the results strongly support the statement that the crosslinking reaction proceeds by an Sn-1 mechanism. The results of this model study are compared with results obtained in network-forming reactions. The important role of the evaporation of the condensation product methanol is discussed.     

Rapid Synthesis of 2‐Alkanol‐substituted Pyrrolo[2,3‐b]quinoxalines from Propargylic Alcohols via Copper‐free Sonogashira Coupling Reaction at Room Temperature

A practical and efficient procedure is established for the synthesis of 2‐alkanol‐substituted pyrrolo[2,3‐b]quinoxalines by the reaction of N‐alkyl‐3‐chloroquinoxaline‐2‐amines with propargylic alcohols. The reaction is carried out in the absence of any copper salt but in the presence of a catalytic amount of Pd(PPh₃)₂Cl₂ at room temperature. The Sonogashira coupling reaction step in this procedure is fast, producing clean products with high yields without contamination by unwanted homocoupling Glaser reaction products. The synthesized pyrroloquinoxaline derivatives are also screened against the three bacterial strains Micrococcus luteus, pseudomonas aeruginosa, and Bacillus subtilis.     

Stereoselectivity of (−)-(1<Emphasis Type="Italic">R</Emphasis>,2<Emphasis Type="Italic">S</Emphasis>)-2-methylamino-1-phenylpropan-1-ol preparation

Stereoselectivity of reductive amination of (R)-1-hydroxy-1-phenylpropan-2-one by methylamine was studied. From the four isomers possible, only two are produced by this reaction. These are marked as (−)-(1R,2S)-ephedrine (desired product) and (+)-(1S,2R)-ephedrine. The reaction stereoselectivity depends both on the type of the catalyst and reaction conditions. The most suitable type is the supported platinum. However, this catalyst rapidly deactivates. With a decreasing activity of Pt catalyst, the stereoselectivity decreases. It is also decreased during the production of the second liquid phase (water) in the reaction mixture.     

Total Synthesis of (+)‐Gracilamine Based on an Oxidative Phenolic Coupling Reaction and Determination of Its Absolute Configuration

The enantioselective total synthesis of (+)‐gracilamine ( 1 ) is described. The strategy features a diastereoselective phenolic coupling reaction followed by a regioselective intramolecular aza‐Michael reaction to construct the ABCE ring system. The configuration at C3a in 1 was controlled by the stereocenter at C9a, which was selectively generated (91 % ee) by an organocatalytic enantioselective aza‐Friedel–Crafts reaction developed by our research group. This synthesis revealed that the absolute configuration of (+)‐gracilamine is 3aR, 4S, 5S, 6R, 7aS, 8R, 9aS.     

Stereoselective Synthesis of (Z)‐ and (E)‐Allyl Aryl Sulfides and Selenides from Baylis?Hillman Acetates under Neutral Conditions Using β‐Cyclodextrin in Water

The first example of the stereoselective synthesis of (Z)‐ and (E)‐allyl aryl sulfides and selenides from Baylis? Hillman acetates under neutral conditions in H₂O by supramolecular catalysis involving β‐cyclodextrin is reported. β‐Cyclodextrin can be recovered and reused. The reaction is very efficient in providing allyl aryl sulfides and selenides in good‐to‐excellent yields with clean reaction profiles under mild reaction conditions.     

Reaction rate analysis of complex kinetic systems

Using the elementary sensitivity densities, a reaction rate sensitivity gradient is obtained which is the derivative of the rate of species concentration change with respect to the rate coefficient. The dimensionless (log-normalized) form of the reaction rate sensitivity gradient is the ratio of the rate of concentration change of species i due to elementary reaction j and the net rate of concentration change of species i. This result provides a mathematical basis for the use of various forms of reaction rate analyses in the study of complex reaction mechanisms. The kinetic information inherent in the relative reaction rate matrix is extracted by principal component analysis. The method is used to analyze the mechanism of high-temperature formaldehyde oxidation and high-temperature propane pyrolysis. Ranking of the elementary reactions allowed us to reduce significantly the original mechanisms and a detailed study of the results revealed the reaction structures and the major reaction paths of the species.     

Kinetics and mechanism of oxidation of dimethyl sulfoxide by chloramine-T in aqueous solution

The kinetics of oxidation of dimethyl sulfoxide (DMSO) by chloramine-T (CAT) is studied in HClO₄ and NaOH media with OsO₄ as a catalyst in the latter medium. In acid medium, the rate law is -d [CAT]/dt = k [CAT][DMSO][H⁺]. Alkali retards the reaction and the rate law takes the form -d [CAT]/dt = k [CAT][DMSO][OsO₄]/[NaOH], but is reduced to -d [CAT]/dt = k [CAT][DMSO] at higher alkali concentrations. The reaction is subjected to changes in (a) ionic strength, (b) concentrations of added neutral salts, (c) concentrations of added reaction product, (d) dielectric constant, and (e) solvent isotope effect, and the subsequent effects on the reaction rate are studied. The reaction mechanism in acid medium assumes an electrophilic attack by the free acid RNHCl (CAT′) at the sulfur site in DMSO, forming a reaction intermediate which subsequently decomposes to dimethyl sulfone on hydrolysis. Formation of a cyclic complex between RNHCl and OsO₄ which interacts with the substrate in a slow step explains the observed results in alkaline medium. The simplification of the rate equation at higher alkali concentrations is attributed to a direct reaction between chloramine-T and the substrate.