Kinetics and mechanism of the reaction between dibenzyl triselenocarbonate and oxygen-bases

The base-induced solvolysis of dibenzyl triselenocarbonate in DMSO/water (4:1) has been studied by UV spectrophotometric measurements under pseudo-first order conditions, using KOH, Me₄NOH, Bu₄ⁿNOH and Bu₄ⁿNHCO₃ as bases. The reaction is first order with respect to the substrate as well as to the base. Rate constants and activation parameters for the four bases indicate that the rate determining step is the nucleophilic attack by the base, as an ionic couple, on the substrate. Fast reactions, due to strong nucleophilic selenolate species, may follow this slow step; a tentative mechanism is proposed on the basis of experimental evidence.     

Magnetic Hyperfine Mössbauer Spectra of Ferrocenyaldimines-FeCl3 Adducts

Ferrocenylaldimines (Fc-CH=N-R) were prepared by condensation of Schiff base of formylferrocene with polyacenylamines RNH₂; R = naphthyl, anthracenyl and pyrenyl groups. This step was followed by oxidation with FeCl₃ to give [Fc-CH=NR]-FeCl₃ adducts. The electronic state of iron in these adducts was investigated by means of ⁵⁷Fe Mössbauer spectra except for [Fc-CH=N-Pyr]-FeCl₃ showed that only a doublet was observed at ambient temperature. As the temperature was decreased below 200 K, additional magnetic hyperfine splitting was observed characteristic of ferric oxide.     

Copper-catalyzed domino reaction between 1-(2-halophenyl)methanamines and amidines or imidates for the synthesis of 2-substituted quinazolines

The CuI-catalyzed domino reaction between 1-(2-bromophenyl)methanamines and amidines using K₃PO₄ as the base, pivalic acid as the additive, and aerial oxygen as the oxidant gives access to substituted quinazolines in a single step with yields in the range between 43 and 90%. It is assumed that the reaction proceeds as a Cu(I)-catalyzed intermolecular N-arylation followed by an intramolecular nucleophilic substitution and a Cu(II)-catalyzed oxidation. The amidines can be replaced with imidates and the reaction can also be run with 1-(2-iodophenyl)methanamine.     

An FTIR study of the mechanism for the reaction HO + CH3SCH3

Product studies were made using the Fourier transform infrared method in the uv (300–400-nm) photolysis of mixtures containing CH₃SCH₃, C₂H₅ONO, and NO in ppm concentrations in 700 torr of O₂–N₂ diluent. Methyl thionitrite, CH₃SNO, arising from the reaction CH₃S + NO, was detected as an intermediate product. In addition, the yields of the major sulfur-containing products SO₂ and CH₃SO₃H coincided with those of the oxidation of the CH₃S radicals generated directly by the photodissociation of CH₃SNO. The formation of CH₃S in the HO-initiated oxidation of CH₃SCH₃ in the presence of NO suggests a reaction scheme involving the H-abstraction reaction HO + CH₃SCH₃ → CH₃SCH₂ + H₂O as the primary step.     

Chromotropic acid-formaldehyde reaction in strongly acidic media. The role of dissolved oxygen and replacement of concentrated sulphuric acid

The procedure for formaldehyde analysis recommended by the National Institute for Occupational Safety and Health (NIOSH) is the Chromotropic acid spectrophotometric method, which is the one that uses concentrated sulphuric acid. In the present study the oxidation step associated with the aforementioned method for formaldehyde determination was investigated. Experimental evidence has been obtained indicating that when concentrated H₂SO₄ (18 mol l⁻¹) is used (as in the NIOSH procedure) that acid is the oxidizing agent. On the other hand, oxidation through dissolved oxygen takes place when concentrated H₂SO₄ is replaced by concentrated hydrochloric (12 mol l⁻¹) and phosphoric (14.7 mol l⁻¹) acids as well as by diluted H₂SO₄ (9.4 mol l⁻¹). Based on investigations concerning the oxidation step, a modified procedure was devised, in which the use of the potentially hazardous and corrosive concentrated H₂SO₄ was eliminated and advantageously replaced by a less harmful mixture of HCl and H₂O₂.     

Study of the reaction between styrene and electrogenerated superoxide ion in dimethylformamide

The oxidation of some hydroxamic acids (RCONHOH, R=CH₃, C₆H₅, C₆H₅CH₂) has been investigated on platinum electrodes in aqueous solution buffered at different pH values. The voltammetric data showed that in acid media the dissociation of the acids in the chemical reaction preceding the quasi-reversible charge transfer step, while in alkaline media a weak adsorption of the reagent is operative. The reaction mechanism proposed in alkaline medium involves the formation of transient radical-anions, evidentiated by cyclic voltammetry, which dimerize and, by intramolecular rearrangement, quantitatively give (N,O-diaceyl)hydroxylamines. In acid media a two-electron oxidation process leads to carboxylic acids as final products.     

Functionalization of superparamagnetic Fe3O4@SiO2 nanoparticles with a Cu(II) binuclear Schiff base complex as an efficient and reusable nanomagnetic catalyst for N‐arylation of α‐amino acids and nitrogen‐containing heterocycles with aryl halides

Fe₃O₄@SiO₂ nanoparticles was functionalized with a binuclear Schiff base Cu(II)‐complex (Fe₃O₄@SiO₂/Schiff base‐Cu(II) NPs) and used as an effective magnetic hetereogeneous nanocatalyst for the N‐arylation of α‐amino acids and nitrogen‐containig heterocycles. The catalyst, Fe₃O₄@SiO₂/Schiff base‐Cu(II) NPs, was characterized by Fourier transform infrared (FTIR) and ultraviolet‐visible (UV‐vis) analyses step by step. Size, morphology, and size distribution of the nanocatalyst were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and dynamic light scatterings (DLS) analyses, respectively. The structure of Fe₃O₄ nanoparticles was checked by X‐ray diffraction (XRD) technique. Furthermore, the magnetic properties of the nanocatalyst were investigated by vibrating sample magnetometer (VSM) analysis. Loading content as well as leaching amounts of copper supported by the catalyst was measured by inductive coupled plasma (ICP) analysis. Also, thermal studies of the nanocatalyst was studied by thermal gravimetric analysis (TGA) instrument. X‐ray photoelectron spectroscopy (XPS) analysis of the catalyst revealed that the copper sites are in +2 oxidation state. The Fe₃O₄@SiO₂/Schiff base‐Cu(II) complex was found to be an effective catalyst for C–N cross‐coupling reactions, which high to excellent yields were achieved for α‐amino acids as well as N‐hetereocyclic compounds. Easy recoverability of the catalyst by an external magnet, reusability up to eight runs without significant loss of activity, and its well stability during the reaction are among the other highlights of this catalyst.     

Photoelectrochemical oxidation behavior of organic substances on TiO2 thin-film electrodes

The photoelectrocatalytic oxidation characteristics of salicylic acid, formic acid and methanol on anodized nanoporous titanium dioxide (TiO₂) thin-films were investigated by using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. From dark to ultraviolet illumination, the open circuit potential (OCP) and film resistance of TiO₂ films decreased markedly. A general equivalent circuit model was proposed for the photoelectrochemical system anodic TiO₂ thin-film electrode/test solution. The photoelectrochemical oxidation process of the organic compounds showed similar impedance features at OCP and was controlled by the charge transfer step. According to the polarization curves of the base solution and organic solutions, the kinetic rate curves for the photoelectrocatalytic oxidation of pure organic species were obtained as a function of the potential bias. One photooxidation peak was first observed at a bias potential of ca. 0.26 V for these species with low concentrations.     

Single‐Site Copper(II) Water Oxidation Electrocatalysis: Rate Enhancements with HPO42− as a Proton Acceptor at pH 8

The complex Cu^II(Py₃P) ( 1 ) is an electrocatalyst for water oxidation to dioxygen in H₂PO₄^?/HPO₄^2? buffered aqueous solutions. Controlled potential electrolysis experiments with 1 at pH 8.0 at an applied potential of 1.40 V versus the normal hydrogen electrode resulted in the formation of dioxygen (84 % Faradaic yield) through multiple catalyst turnovers with minimal catalyst deactivation. The results of an electrochemical kinetics study point to a single‐site mechanism for water oxidation catalysis with involvement of phosphate buffer anions either through atom–proton transfer in a rate‐limiting O? O bond‐forming step with HPO₄^2? as the acceptor base or by concerted electron–proton transfer with electron transfer to the electrode and proton transfer to the HPO₄^2? base.     

The effects of isotopic substitution and competitive adsorption on the oxidation of aldehydes and alcohols at gold electrodes

The nature of the mechanism of the anodic oxidation of aldehydes in aqueous base on gold electrodes has been probed using isotopic substitution and competitive adsorption studies. A primary kinetic isotope effect of k_H/k_D=3?4 was observed upon substitution of deuterium for protium on the formyl group of benzaldehyde and cyclohexanecarboxaldehyde on gold in aqueous base using the techniques of cyclic voltammetry, rotating disc electrode voltammetry and chronoamperometry. Similar results are reported for the same aldehydes on a silver electrode, and also for the anodic oxidation of 2-propanol and 2-propanol-2-d on gold under similar conditions. Inhibition of the anodic cyclic voltammetric peak for benzaldehyde on gold by a variety of adsorbed species including CN^?, I^?, Br^?, (C₂H₅)₄N⁺ and diethylenetriamine is also described. These observations are used to propose a mechanism for the low potential oxidations of aldehydes and alcohols on gold involving a rate limiting dissociative adsorption step with cleavage of the α-carbon-hydrogen bond.     

Copper(II) Schiff Base Complex Immobilized on Superparamagnetic Fe3O4@SiO2 as a Magnetically Separable Nanocatalyst for Oxidation of Alkenes and Alcohols

A new heterogeneous catalyst containing a copper(II) Schiff base complex covalently immobilized on the surface of silica‐coated Fe₃O₄ nanoparticles (Fe₃O₄@SiO₂‐Schiff base‐Cu(II)) was synthesized. Characterization of this catalyst was performed using various techniques. The catalytic potential of the catalyst was investigated for the oxidation of various alkenes (styrene, α‐methylstyrene, cyclooctene, cyclohexene and norbornene) and alcohols (benzyl alcohol, 3‐methoxybenzyl alcohol, 3‐chlorobenzyl alcohol, benzhydrol and n ‐butanol) using tert ‐butyl hydroperoxide as oxidant. The catalytic investigations revealed that Fe₃O₄@SiO₂‐Schiff base‐Cu(II) was especially efficient for the oxidation of norbornene and benzyl alcohol. The results showed that norbornene epoxide and benzoic acid were obtained with 100 and 87% selectivity, respectively. Moreover, simple magnetic recovery from the reaction mixture and reuse for several times with no significant loss in catalytic activity were other advantages of this catalyst     

The oxidation kinetics study of ultrafine iron powders by thermogravimetric analysis

The oxidation kinetics of ultrafine metallic iron powder to hematite (α-Fe₂O₃) up to temperatures 800 °C were studied in air using non-isothermal and isothermal thermogravimetric (TG) analysis. The powders with average particles size of 90, 200, and 350 nm were made by the electric explosion of wire. It was observed that the reactivity of the iron powder is increased with the decreasing particle size of powder. The experimental TG curves clearly suggest a multi-step process for the oxidation, and therefore a model-fitting kinetic analysis based on multivariate non-linear regressions was conducted. The complex reaction can be best described with a three-step reaction scheme consisting of two concurrent and one parallel reaction step. In one reaction pathway Fe is oxidized to α-Fe₂O₃. The other pathway is described by the oxidation of Fe to magnetite (Fe₃O₄). At higher temperatures the formed Fe₃O₄ is further oxidized in a α-Fe₂O₃. It is established that the best fitting three-step mechanism employed a branching set of n-order equations for each step.     

Kinetics and mechanism of the reaction of manganese(III) octaethylporphine with hydrogen peroxide

A spectroscopic and kinetic study of the oxidation of (chloro)(octaethylporphinato)manganese(III) (Cl)MnOEP with hydrogen peroxide in an aqueous-organic medium at 288–308 K was made. The nature and composition of the reaction products differ depending on the reaction conditions (H₂O₂ concentration). Based on the data on reaction rates, thermodynamic parameters of activation, and form of the rate equations of the (C1)MnOEP oxidation, a multistep reaction mechanism is suggested and substantiated, in which the decisive role is played by the limiting step, two-electron oxidation of the metal porphyrin with the coordinated peroxide or partial reduction of the oxidized form of the manganese porphyrin with the second peroxide molecule (in the form of HO ₂ ^? ), and by acid-base equilibria of the peroxide.     

A novel approach for the one-pot synthesis of linear and angular fused quinazolinones

A convenient and inexpensive one step methodology has been developed for the synthesis of linear and angular fused quinazolinones. The protocol, which uses amino heterocycles and o-bromo benzyl/naphthyl bromides as reactants, CuI as catalyst, Cs₂CO₃ as base, l-proline as ligand, and DMF as solvent, proceeds via nucleophilic aromatic substitution of the N-heteroaromatic cationic intermediate followed by in situ aerial oxidation at the benzylic position to the quinazolinone scaffold.     

The comparative study of CO2 + Hads reaction on platinum electrode in H2O and D2O

The observed isotopic effect in CO₂ + H_ads reaction showed that at 0.05 V the rate determining step is

formation whereas at 0.2 V the reorientation of adsorbed intermediate:

is probably the slowest step of reaction. The oxidation of adsorbed product is slower in D₂SO₄ than H₂SO₄ solution like the surface oxidation of platinum. The rate determining step of COOH_ads oxidation is a reaction with OH_ads.     

An improved sequential extraction method to determine element mobility in pyrite-bearing siliciclastic rocks

A sequential extraction method was developed for pyrite-bearing (FeS₂) siliciclastic rocks. The focus of this study was to enhance the procedure by an improved oxidation step to completely dissolve not only organic matter but also microcrystalline pyrite. In the first experiment, four oxidation procedures were compared for pure pyrite at extraction temperatures of 25°C and 85°C with hydrogen peroxide (H₂O₂) as the main oxidant. It was found that pyrite dissolution was most effective by using a mixture of H₂O₂, ammonium acetate (NH₄OAc) and nitric acid (HNO₃) at 25°C. This procedure dissolved >90% pyrite, and detected >75% using solute iron measurements. The difference between these two results was explained by reprecipitation of secondary iron minerals. The procedure worked best at 25°C, since solvent evaporation at 85°C amplified iron oversaturation and precipitation. For the pyrite-bearing siliciclastic rocks, two sequential extraction schemes were compared to optimise solid–solvent ratio, extraction step order and type of solvent. Eventually, the most effective step order identified for siliciclastic rocks containing pyrite and little organic matter was to first (1) remove the exchangeable fraction, followed by (2) dissolution with acid and afterwards (3) with a reducing agent. The (4) oxidation step was performed last.     

Enhanced ammonia oxidation on BDD induced by inhibition of oxygen evolution reaction

Electrochemical oxidation of ammonia (NH₃ and NH₄ ⁺ ) on boron-doped diamond (BDD) electrode was studied using differential electrochemical mass-spectrometry (DEMS) and chronoamperometry. Electro-oxidation of ammonia induces inhibition of the oxygen evolution reaction (OER) due to adsorption of the ammonia oxidation products on the BDD surface. The inhibition of the OER enhances ammonia electro-oxidation, which becomes the main reaction. The amino radicals, formed during ammonia oxidation, trigger a reaction chain in which molecular oxygen dissolved in solution is involved in the ammonia electro-oxidation. Nitrogen, nitrous oxide, and nitrogen dioxide were detected as the ammonia oxidation products, with nitrogen being the main gaseous product of the oxidation.     

Structure of zinc-5,15-di(o-methoxyphenyl)octaalkylporphyrines and their reaction with organic peroxides in the presence of pyridine

By combined spectral and calculation methods the structure of zinc 5,15-di(o-methoxyphenyl)-2,8,12,18 3,7,13,17-octaalkylporphyrinates (I, II) and their properties in the reaction with organic peroxides with addition of different amounts of pyridine were studied. The reaction of zinc porphyrinates with peroxides in the presence of pyridine leads to destruction of the complex chromophore. Kinetic parameters of the investigated reaction (effective k _ef and true k _V rate constants) are obtained. The presence of base in the reaction medium is found to lead to a change in the structure of the zinc porphyrinates and affects the rate of oxidation. By quantum-chemical method PM3 the geometry of the reagents was calculated and the deformation distortions of the reactants molecules and intermediates in the course of the oxidation reaction was demonstrated. The influence of electronic effects of substituents and the degree of deformation of the zinc porphyrinate macrocycle on their redox properties is revealed.     

The reaction of C3F6 with dioxygen under IR laser initiation

The mechanism of oxidation of C₃F₆ by molecular O₂ under IR multiphoton excitation was studied. The activation energy of C₃F₆ oxidation was estimated from the dependence of the reagent conversion on the O₂ pressure. The data obtained for the final reaction products, CF₃CFO and CF₂O, suggest the dioxetane mechanism of C₃F₆ oxidation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 45–49, January, 1999.     

Oxidation of cycloalkanones with hydrogen peroxide: an alternative route to the Baeyer-Villiger reaction. Synthesis of dicarboxylic acid esters

The acid-catalyzed oxidation of cycloalkanones C₅-C₈ and C₁₂ with hydrogen peroxide in alcohols was performed, and dicarboxylic acid esters were obtained as the major products in 53-70% yields. In the first step, geminal bishydroperoxides are generated from five-to-seven-membered cyclic ketones. The Baeyer-Villiger reaction is a side process accompanied by the formation of ω-hydroxycarboxylic acid esters.