Photochemical acetalization of carbonyl compounds in protic media using an in situ generated photocatalyst

Carbonyl compounds are conveniently converted into their corresponding dimethyl acetals in good yields and short reaction times by means of a photochemical reaction in methanol with a catalytic amount of chloranil (2,3,5,6-tetrachloro-1,4-benzoquinone, CA) as the sensitizer. Using aldehydes gives better results than using ketones, which also tend to form enol ethers as side products. These results are similar to those of simple acid-catalyzed acetalization reactions, suggesting the involvement of a photochemically generated acid. On the basis of steady state and laser flash photolysis data the reaction is proposed to involve the in situ generation of a photocatalyst (2,3,5,6-tetrachloro-1,4-hydroquinone, TCHQ) via reaction of CA with the solvent. The acetalization process is initiated by ionization of TCHQ, followed by loss of a proton to the solvent or the carbonyl, which starts a catalytic reaction. The photocatalyst is regenerated via a disproportionation reaction.     

Origin of the synchronicity on the transition structures of polar Diels-Alder reactions. Are these reactions [4 + 2] processes?

The transition structures (TSs) for a series of related Diels-Alder reactions between cyclopentadiene and mono-, di-, tri-, and tetracyanoethylene derivatives have been studied with use of DFT methods at the B3LYP/6-31G computational level. The increase of the electron-withdrawing substitution on ethylene increases the rate of these polar cycloadditions. However, the symmetric arrangement of cis and trans 1,2-di- and tetracyanoethylenes decreases the effectiveness of the substitution, which can be related to the symmetry found at the corresponding TSs. A DFT analysis of the global and local electrophilicity power of these series of cyano ethylenes provides a sound explanation about the nature of these synchronous processes. The present theoretical study is in agreement with the experimental outcomes.     

An unusually stable trinuclear manganese(II) complex bearing bulk carboxylic radical ligands

A trinuclear MnII cluster based on the versatile polychlorinated triphenylmethyl carboxylic radical ligand has been characterized by X-ray crystallography and magnetic measurements, representing a rare example of a MnII trinuclear linear structure with six open-shell ligands.     

Photoreaction between 2-benzoylthiophene and phenol or indole

Laser flash photolysis, density functional theory (DFT) calculations, and product studies have been performed to understand the mechanism of photoreduction of 2-benzoylthiophene (BT) in the presence of phenol or indole. Time-resolved experiments showed that BT ketyl (BTH) and phenoxy (PhO) or indolyl (In) radicals are generated with high rate constants and quantum yields close to 1. However, low conversions (specially in the case of indole) of the starting reagents are obtained upon prolonged lamp irradiation, indicating that recombination within the radical pairs must occur to a large extent, regenerating the starting materials. The solvent-dependence of the quenching rate constants, together with DFT theoretical studies, indicate fundamental differences between the mechanisms of the reaction of BT triplet with phenol and indole. Thus, data for phenol agree with the involvement of a hydrogen-bonded exciplex BT(.)HOPh, where concerted electron and proton transfer leads to the BTH(.)OPh radical pair. However, in the case of indole, electron transfer at the BT(.)HIn stage precedes proton transfer. Finally, C-C cross-coupling products have been isolated and characterized in the preparative irradiation of BT in the presence of phenol and indole. The structures of the products have been confirmed by alternative synthesis.     

Gas diffusion sequential injection system for the spectrophotometric determination of free chlorine with o-dianisidine

A gas diffusion sequential injection system for spectrophotometric determination of free chlorine is described. The detection is based in the colorimetric reaction between free chlorine and a low toxicity reagent o-dianisidine. A gas diffusion unit is used to isolate free chlorine from the sample in order to avoid possible interferences. This feature results from the conversion of free chlorine to molecular chlorine (gaseous) with sample acidification. With minor changes in the operating conditions, two different dynamic ranges were obtained enhancing the application both to water samples and bleaches. The results obtained with the developed system were compared to the reference method, iodometric titration and proved not to be statistically different. A detection limit of 0.6 mg ClO⁻/L was achieved. Repeatability was evaluated from 10 consecutive determinations being the results better than 2%. The two dynamic ranges presented different determination rates: 15 h⁻¹ for 0.6-4.8 mg ClO⁻/L (water samples) and 30 h⁻¹ for 0.047-0.188 g ClO⁻/L (bleaches).     

The effect of the starting material on the thermal decomposition of iron oxyhydroxides

The effect of the iron precursor on the thermal decomposition of iron oxyhydroxides was studied by DSC, DTA and TG in this work. Samples were prepared from iron nitrate, iron sulfate and iron chloride and the thermal curves obtained were analyzed by specific area measurements, X-ray diffraction and Mössbauer spectroscopy. It was found that the iron oxyhydroxide precursors affect the temperatures of the hematite formation as well as the textural properties of the final hematite producing particles with different diameters as following: iron sulfate (3.3 nm)相似文献   

A double-line sequential injection system for the spectrophotometric determination of copper, iron, manganese, and zinc in waters

A double-line sequential injection system was developed for the spectrophotometric determination of several metal ions in waters. The proposed double-line configuration was used to enable adding sample and chromogenic reagents as merging zones. The methodology was applied to the spectrophotometric determination of copper, iron, manganese, and zinc in samples of diverse origins at the range of 0.15-5.00, 0.10-10.0, 0.48-4.00, and 0.11-5.00 mg/L, respectively. Different chromogenic reagents and detection wavelengths were used. The chromogenic reagents for iron and manganese were 1,10-phenanthroline and formaldoxime, respectively. Copper and zinc were both determined using the analytical reagent zincon. Analytical characteristics of the methodology, such as manifold parameters, buffer pH, and reagent concentrations were optimized, and interference of some of the metal ions commonly present in water sample was assessed. Results of the analysis were in agreement with those obtained by atomic absorption spectrometry. Repeatability, expressed as the relative standard deviation for 10 consecutive injections of water samples, was lower than 6%. The determination rate was approximately 36/h.     

The closure of a similarity orbit is always arcwise connected

If T, A are Hilbert space operators such that A is the norm-limit of operators similar to T, then there exists a continuous function from [0,1] into the algebra of operators such that (t) is a limit of similarities of T for all t in the interval [0,1], (0)=T and (1)=A.This research was partially supported by a Grant of the National Science Foundation.     

Polar [3 + 2] cycloaddition of ketones with electrophilically activated carbonyl ylides. Synthesis of spirocyclic dioxolane indolinones

The [3 + 2] cycloaddition reaction between carbonyl ylides generated from epoxides and ketones (ethyl pyruvate, ethyl phenylglyoxylate, isatin, N-methylisatin and 5-chloroisatin) to give substituted dioxolanes and spirocyclic dioxolane indolinones was investigated. The effect of microwave irradiation on the outcome of the reaction was studied. The thermal reaction between 2,2-dicyano-3-phenyloxirane and N-methylisatin was theoretically studied using DFT methods. This reaction is a domino process that comprises two steps. The first is the thermal ring opening of the epoxide to yield a carbonyl ylide intermediate, whereas the second step is a polar [3 + 2] cycloaddition to yield the final spiro cycloadducts. The cycloaddition presents a low stereoselectivity and a large regio- and chemoselectivity. Analysis of the electrophilicity values and the Fukui functions of the reagents involved in the cycloaddition step allowed the chemical outcome to be explained.