Lewis acid mediated SN2-type nucleophilic ring opening followed by [4+2] cycloaddition of N-tosylazetidines with aldehydes and ketones: synthesis of chiral 1,3-oxazinanes and 1,3-amino alcohols

A highly efficient strategy for Cu(OTf)₂ mediated S_N2-type nucleophilic ring opening followed by [4+2] cycloaddition reactions of enantiopure 2-phenyl-N-tosylazetidines with various aldehydes and ketones afforded a variety of substituted 1,3-oxazinanes and 1,3-amino alcohols in excellent yields, excellent de and good to excellent ee. The proposed S_N2-type mechanism of the cycloaddition reaction is supported by experimental evidence.     

A DFT study of the Huisgen 1,3-dipolar cycloaddition between hindered thiocarbonyl ylides and tetracyanoethylene

The mechanism for the 1,3-dipolar cycloaddition between the hindered thiocarbonyl ylide 1 and tetracyanoethylene 2 has been studied at the B3LYP/6-31G^∗ level. Formation of the [3+2] cycloadduct 4 takes place through a stepwise mechanism that is initiated by the nucleophilic attack of the thiocarbonyl ylide 1 to the ethylene derivative 2 to give a zwitterionic intermediate IN. The subsequent cyclization of IN yields a seven-membered cyclic ketene imine 6, which equilibrates with the thermodynamically more stable [3+2] cycloadduct 4. The computed free energies are in agreement with the experimental outcomes.     

Particle formation mechanism and kinetic model of ultrasonically initiated emulsion polymerization

A general kinetic model of particle formation in an ultrasonically initiated emulsion polymerization system is presented. This model takes into account homogeneous, micelle entry, and monomer droplet nucleation mechanism. The effects of the ultrasound in producing free radical, degrading free radical and influencing the fashion of the nucleation are also considered. Moreover, chain transfer to monomer and termination in the aqueous phase, capture of oligomer radicals by particles, and coagulation of particles are also considered. An analytical solution is obtained for the initial particle stage consideration. This model predicts that, if the desorption of radical from particles can be neglected, the concentration of the total radical in the aqueous phase is directly proportional to the cavitation concentration. Model predictions are in good agreement with experimental data obtained from the literature.     

Kinetics and mechanism of oxidation of allyl alcohol byN-bromosuccinimide

The kinetics of oxidation of allyl alcohol byN-bromosuccinimide (NBS) has been studied at 35 °C in aqueous medium. The reaction shows first order dependence on bothNBS and allyl alcohol. In fairly high acid concentration, there is no change in the rate of the reaction but at low acid concentration, the rate is considerably enhanced. There is no primary salt effect. At varying mercuric acetate concentrations, the rate constant remains the same. But in the absence of mercuric acetate, the rate is enhanced. The kinetic parameters,E _a,Arrhenius factorA, H, G and S have been calculated. A rate law in agreement with experimental results has been derived. A mechanism is proposed.

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Kinetik und Mechanismus der Oxidation von Allylalkohol mixN-Bromsuccinimid

Zusammenfassung Die Kinetik der Oxidation von Allylalkohol mitN-Bromsuccinimid (NBS) wurde bei 35 °C in wäßrigem Medium untersucht. Die Reaktion zeigt erste Ordnung gegenüberNBS und Allylalkohol. Bei relativ hoher Säurekonzentration zeigt sich keine Änderung der Reaktionsgeschwindigkeit, bei niedriger Säurekonzentration wird die Reaktionsgeschwindigkeit beträchtlich erhöht. Es wurde kein primärer Salzeffekt festgestellt. Bei varriierender Quecksilberacetatkonzentration bleibt die Reaktionsgeschwindigkeit gleich, bei Abwesenheit von Quecksilberacetat wird jedoch die Geschwindigkeitskonstante erhöht. Die kinetischen Parameter,E _a, derArrheniusfaktorA, H , G und S wurden bestimmt. Ein Geschwindigkeitsgesetz in Übereinstimmung mit den experimentellen Befunden wurde abgeleitet und ein Mechanismus vorgeschlagen.

     

Synthetic Crystalline Calcium Silicate Hydrate (I): Cation Exchange and Caesium Selectivity

Summary.  Solid crystalline calcium silicate hydrate (I) synthesized from equimolar amounts of Ca and Si under hydrothermal conditions at 120°C shows cation exchange properties towards divalent metal cations such as Ni, Cu, Cd, or Hg. It also exhibits caesium selectivity in the presence of Na⁺. The exchange capacity and selectivity of the solid can be increased by 10 and 28%, respectively, upon substitution of 0.01 mol of the Ca²⁺ in its structure by Na⁺. The ability of metal cation uptake by the solid was found to obey the order Ni²⁺ > Hg²⁺ > Cu²⁺ > Cd²⁺. The different affinities of calcium silicate hydrate (I) towards these ions can be used for their separation from solutions and also in nuclear waste treatment. The mechanism of the exchange reaction is discussed. Received June 11, 2001. Accepted September 10, 2001     

Kinetics of the Ru(III) catalyzed oxidation of formaldehyde and acetaldehyde by alkaline hexacyanoferrate(III)

The kinetics of ruthenium(III) catalyzed oxidation of formaldehyde and acetaldehyde by alkaline hexacyanoferrate(III) has been studied spectrophotometrically. The rate of oxidation of formaldehyde is directly proportional to [Fe(CN) _3– ⁶ ] while that of acetaldehyde is proportional tok[Fe(CN) _3– ⁶ ]/{k +k[Fe(CN) _3– ⁶ ]}, wherek, k andk are rate constants. The order of reaction in acetylaldehyde is unity while that in formaldehyde falls from 1 to 0. The rate of reaction is proportional to [Ru(III)] _T in each case. A suitable mechanism is proposed and discussed.

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Die Kinetik der Ru(III)-katalysierten Oxidation von Formaldehyd und Acetaldehyd mittels alkalischem Hexacyanoferrat(III)

Zusammenfassung Die Untersuchung der Kinetik erfolgte spektrophotometrisch. Die Geschwindigkeitskonstante der Oxidation von Formaldehyd ist direkt proportional zu [Fe(CN) _3– ⁶ ], währenddessen die entsprechende Konstante für Acetaldehyd proportional zuk[Fe(CN) _3– ⁶ ]/{k +k[Fe(CN) _3– ⁶ ]} ist, wobeik,k undk Geschwindigkeitskonstanten sind. Die Reaktionsordnung für Acetaldehyd ist eine erste, die für Formaldehyd fällt von erster bis zu nullter Ordnung. Die Geschwindigkeitskonstante ist in jedem Fall proportional zu [Ru(III)] _T . Es wird ein passender Mechanismus vorgeschlagen.

     

Trifluoromethylation of Alkyl Radicals: Breakthrough and Challenges†

Trifluoromethylation of alkyl radicals is emerging as a powerful tool for C(sp³)–CF₃ bond formations. Based on the hypothesis of CF₃ group transfer from Cu(II)–CF₃ to alkyl radicals, a number of trifluoromethylation reactions have been developed, including trifluoromethylation of alkyl halides, decarboxylative trifluoromethylation of aliphatic carboxylic acids, C(sp³)–H trifluoromethylation, amino‐ and carbo‐trifluoromethylation of alkenes, etc. Challenges in this intriguing field are also discussed.     

The thermal degradation of poly(diethyl fumarate)

The kinetics and mechanism of the thermal degradation of poly(diethyl fumarate) (PDEF) were studied by thermogravimetry, as well as by analysis of the thermolysis volatiles and polymer residue. The characteristic mass loss temperatures were determined, as were the overall thermal degradation activation energies of three PDEF samples of varying molar mass. Ethylene and ethanol were present in the thermolysis volatiles at degradation temperatures below 300 °C, while diethyl fumarate was also evidenced at higher degradation temperatures. The amount of monomer increased with increasing degradation temperature. The dependence of the molar mass of the residual polymer on the degradation time and temperature was established and the number of main-chain scissions per monomer unit, s/P₀, calculated. A thermal degradation mechanism including de-esterification and random main-chain scission is proposed. The thermal degradation of PDEF was compared to the thermolysis of poly(ethyl methacrylate) (PEMA), poly(diethyl itaconate) (PDEI) and poly(ethyl acrylate) (PEA).     

Kinetic simulation of living carbocationic polymerizations. II. Simulation of living isobutylene polymerization using a mechanistic model

This paper discusses the kinetic simulation of TiCl₄--coinitiated living carbocationic isobutylene (IB) polymerizations governed by dormant-active equilibria, using a mechanistic model. Two kinetic models were constructed from the same underlying mechanism: one using a commercial simulation software package (Predici^®), and the other using the method of moments. Parameter estimation from experimental batch reactor data with Predici yielded a rate constant of propagation k_p = 4.64 × 10⁸ ± 2.75 × 10⁸ L/mol s, with no constraints imposed. This agrees with k_p data measured with diffusion clock and competition methods, but disagrees with kinetically obtained k_p values. Estimation of rate constants with Predici^® and the GREG parameter estimation software packages revealed that it was difficult to estimate the complete set of kinetic parameters, due to correlated effects of the parameters on model predictions. Estimability analysis confirmed that some of the strongly correlating parameters could not be estimated simultaneously using the available experimental data. Using k_p = 6 × 10⁸ ± 2.75  × 10⁸ L/mol s measured by Mayr, and using starting estimates of other rate constants defined by experimentally observed correlations, yielded the set of rate constants required for the simulations. Both kinetic models yielded good agreement with experimental data, with the exception of M_w values that slightly diverged from the theoretically predicted ‘M_w−M_n = constant’ relationship. This may indicate the occurrence of a minor side reaction. However, the k_p/k₋₁ = 17.5 L/mol average run length calculated from measured and simulated MWD data agrees well with earlier literature values.     

Synthesis of novel copolymer：Poly(p-dioxanone-co-L-phenylalanine)

In order to expand the application of poly(p-dioxanone) or PPDO in biomedical area,a series of novel copolymers were synthesized successfully by one-step,melted copolymerization of p-dioxanone(PDO) and L-phenylalanine N-carboxyanhydride(L-Phe-NCA) monomers.With the in-feed molar ratio of L-PheNCA /PDO equal to 1/20,the conversions of the two kinds of monomers were calculated from ~1H NMR. The average molecular weight and polydispersity of the copolymer increase with the increasing reaction time and catalyst concentration.However,the conversions of the two kinds of monomers did not change with the reaction conditions.A three-step mechanism is presented and proved by high resolution ~1H NMR and IR spectrums.