The preparation of astatine labelled tyrosine using an electrophilic reaction

Carrier-free astatotyrosine has been synthesized using an electrophilic reaction in acidic media. Temperature of 150–160°C and reaction time of 20–30 min were chosen as optimal conditions for the synthesis of astatotyrosine. Under the selected conditions the yield was about 90%.     

Acceleration of the diels-alder reaction by clays suspended in organic solvents

Rapid and stereoselective Diels-Alder reactions can be run in ethanol or methylene chloride in the presence of Fe^III- doped K10 montmorillonite.     

The effect of the medium polarity on the mechanism of the reaction of hydroxybenzenes with hydrazyl radical in aprotic solvents

Mechanisms of the reaction of di- and trihydroxybenzenes with 2,2′-diphenyl-1-picrylhydrazyl (stable radical) in aprotic media of different polarity have been elucidated by experimental and quantum-chemical methods. Kinetic, stoichiometric, and activation parameters of the reaction have been determined. In benzene (nonpolar solvent), the studied reaction occurs via the hydrogen atom transfer mechanism; in the polar solvent with weak ionizing ability (i.e. DMSO), the reaction predominantly occurs via the faster mechanism of coupled electron and proton transfer.     

The EDTA complexes of astatine

Using the electromigration method the monopositive astatine ion has been shown to form EDTA [H₄L] complexes in 0.1M NaClO₄ solution at pH=3–10. The ion mobility of the AtL^3– complex is 5.60/23/x10^–4 cm².V^–1.s^–1.     

Kinetics and mechanism of formyl- and acetylferrocene oxidation with molecular oxygen in organic solvents

Kinetic laws and the products of autoxidation of formyl- and acetylferrocene in organic solvents in the presence of strong and weak Br?nsted acids were investigated. The special feature of the studied reactions is the extreme dependence of the metal complex oxidation rate on the strong acid concentration. This is explained by protonation of the metal complex at high concentrations of the acids leading to the formation of stable to oxidation ??-ferrocenylcarbenium complexes C₅H₅FeC₅H₄C⁺(OH)R.     

Interaction of tantalum fluoride with organic solvents

A systematic study on the extraction of tantalum fluoride from various acidic solutions and mixed media has been given using HDEHP, TBP and LA-2 as solvents. The results obtained for these three solvents clarify the ionic behaviour of tantalum. The separation of tantalum from other elements is discussed.     

The mechanism of reaction of ebselen with superoxide in aprotic solvents as examined by cyclic voltammetry and ESR

The mechanism of the redox reaction of ebselen with superoxide was investigated using both ESR and electrochemical techniques. The reaction with superoxide in aprotic solvents was followed by means of cyclic voltammetry and ESR spin-trapping. A decrease in the oxidation current due to superoxide as a result of the addition of ebselen was clearly observed in the cyclic voltammograms. Ebselen reduced the ESR signal intensity of 5,5-dimethyl-1-pyrroline N-oxide (DMPO)-superoxide in a dose-dependent manner. The formation of an amidyl radical in this redox reaction was confirmed by rapid mixing continuous-flow ESR. The selenonate form and the seleninate form of ebselen were identified as the final products of the reaction of ebselen with superoxide. The following mechanism for this redox reaction can be proposed: First, ebselen reacts with superoxide and is converted to an ebselen anion radical; second, the ebselen anion radical reacts with superoxide and is converted to the amidyl radical. Hydrogen abstraction by the amidyl radical occurs and gives both a seleninate form and a selenonate form.     

The effect of water-miscible organic solvents on the substitution reaction of Keggin-type heteropolysilicates and -germanates with vanadium(V) ion

The substitution reaction of Keggin-type polyoxometalates, [XM₁₂O₄₀]ⁿ⁻ (X = Si, Ge, P, or As; M = Mo or W), with vanadium(V) ion was investigated with cyclic voltammetry, Raman spectroscopy, and ³¹P NMR. The effect of organic solvents, such as acetonitrile (ACN), tetrahydrofuran (THF), formamide (FA), N-methylformamide (NMF), and N-methylacetoamide (NMA) on their substitution reactions was examined. In aqueous and aqueous-FA, -NMF, and -NMA solutions, the molybdenum or tungsten unit in the Keggin-anions was substituted with vanadium(V) ion to form V(V)-substituted polyoxometalates, while no substitution reaction occurred in aqueous-ACN and -THF solutions. It is suggested that this results from the relationship between the total anion charge and the permittivity of organic solvents.     

The reaction of sulfur with organic compounds

The most effective catalyst for thionation of cumenes to 4-aryl-1, 2-dithiol-3-thiones, have been proven to be sym-diphenylguanidine, mercury acetamide, mercaptobenzthiazole + ZnO. The way in which thionation of cumeme derivatives depends on the nature of the substituent in the aromatic ring is investigated. Eight derivatives (4 previously unknown) of 4-phenyl-1, 2-dithiol-3-thione, with a chlorine atom and an alkyl or alkoxy group in the benzene ring are synthesized and described.For previous papers see [1–6].     

The decamethylferrocene(+/0) electrode reaction in organic solvents at variable pressure and temperature

Half-wave potentials E(1/2) relative to a Ag/Ag(+) electrode, mean diffusion coefficients D, and standard electrode reaction rate constants k(el) are reported for the decamethylferrocene(+/0) couple (DmFc(+/0)) in nine organic solvents at variable pressure and (for five solvents) temperature. Limited data on the ferrocene(+/0) (Fc(+/0)) and Fe(phen)(3)(3+/2+) electrode reactions are included for comparison. Although E(1/2) for DmFc(+/0) correlates only loosely with the reciprocal of the solvent dielectric constant epsilon at ambient pressure, its pressure dependence expressed as the volume of reaction Delta V(cell) is a linear function of Phi = (1/epsilon)( partial differential ln epsilon/ partial differential P)(T) (the Drude-Nernst relation). Interpretation of the temperature dependence data is made difficult by enthalpy-entropy compensation. Measurements of D for solutions containing 0.5 mol L(-1) tetrabutylammonium perchlorate (TBAP) at 25 degrees C and ambient pressure are inversely proportional to the viscosities eta of the pure solvents as expected from the Stokes-Einstein relation, despite the fact that increasing [TBAP] results in increased eta. The activation volume Delta V(diff)(++) for diffusion of DmFc(+/0) ranges from 7 to 17 cm(3) mol(-1) and generally increases with increasing eta and thus with increasing [TBAP]. The activation volumes Delta V(el)(++) for the electrode reactions of DmFc(+/0) and Fc(+/0) are all positive, equaling the corresponding Delta V(diff)(++) values within the experimental uncertainty and contrast sharply with the negative Delta V(ex)(++) values characteristic of the corresponding self-exchange reactions in homogeneous solution. These facts, together with the thermal activation parameters, point to solvent dynamical control of the electrode (but not the homogeneous self-exchange) reactions. The apparent radii of the electroactive species according to the Drude-Nernst and Stokes-Einstein relations cannot be satisfactorily related to their crystallographic radii and are better regarded as adjustable parameters with limited physical significance.     

The utility of cyclodextrins in lipase-catalyzed transesterification in organic solvents: enhanced reaction rate and enantioselectivity

The use of enzymes as valuable catalysts in organic solvents has been well documented. However, some of their features limit their application in organic synthesis, especially the frequently lower enzyme activity under nonaqueous conditions, which constitutes a major drawback in the application of enzymes in organic solvents. In addition, many enzymatic reactions are subject to substrate or product inhibition, leading to a decrease in the reaction rate and enantioselectivity. To overcome these drawbacks and to make enzymes more appealing to organic chemists, we demonstrate the use of cyclodextrins as regulators for the Pseudomonas cepacia lipase (PSL) and macrocyclic additives to enhance the reaction rate and enantioselectivity E in lipase-catalyzed enantioselective transesterification of 1-(2-furyl)ethanol in organic solvents. Both reaction rate and enantioselectivity were significantly enhanced by several orders of magnitude when using co-lyophilized lipase in the presence of cyclodextrins. The effect of cyclodextrin derivatives as well as solvents on the improvement of the reaction parameters has been studied. The observed enhancement was tentatively interpreted in terms of their ability to give a certain flexibility to the enzyme and to form a host-guest complex, thus avoiding product inhibition and leading to enhancement of the reaction rate and enantioselectivity. The effect of cyclodextrin additives on the enzyme morphology has been studied using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) of the co-lyophilized lipase with cyclodextrins. The ability of cyclodextrins to form a host-guest complex to avoid product inhibition, which leads to the observed enhancement, has been proved by NOESY, COSY, 13C and 1H NMR.     

Glycerol and derived solvents: new sustainable reaction media for organic synthesis

The rapid growth of the biodiesel industry has led to a large surplus of its major byproduct, i.e. glycerol, for which new applications need to be found. Research efforts in this area have focused mainly on the development of processes for converting glycerol into value-added chemicals and its reforming for hydrogen production, but recently, in line with the increasing interest in the use of alternative greener solvents, an innovative way to revalorize glycerol and some of its derivatives has seen the light, i.e. their use as environmentally friendly reaction media for synthetic organic chemistry. The aim of the present Feature Article is to provide a comprehensive overview on the developments reached in this field.     

Sonochemical reactions occurring in organic solvents: reaction kinetics and reaction site of radical trapping with 1,1-diphenyl-2-picrylhydrazyl

The reaction of 1,1-diphenyl-2-picrylhydrazyl (DPPH) with a radical intermediate was investigated in the sonolysis of several organic solvents. The rate of the DPPH consumption in the sonolysis of methanol obeyed first-order kinetics at low concentrations of DPPH, while the rate became zero-order as the concentration of DPPH further increased. The radical trapping reactions of DPPH were found to be considerably slow compared with the formation of hydrogen molecules in the sonolysis of alcohols and hydrocarbons. These results indicate that H atoms formed in the solvent sonolysis quickly react to form stable molecules such as hydrogen in the cavitation bubble and/or at the interface region. The rates of the DPPH consumption were strongly dependent on the vapor pressure and relative evaporation rate of the solvents. It was proposed that the optimum vapor pressure exists for an effective formation of radical species.     

Development and initial application of a hybridization-independent, DNA-encoded reaction discovery system compatible with organic solvents

We have developed and applied an approach to reaction discovery that takes advantage of DNA encoding, DNA-programmed assembly of substrate pairs, in vitro selection, and PCR amplification, yet does not require reaction conditions that support DNA hybridization. This system allows the simultaneous evaluation of >200 potential bond-forming combinations of substrates in a single experiment and can be applied in a range of solvent and temperature conditions. In an initial application, we applied this system to explore Au(III)-mediated chemistry and uncovered a simple, mild method for the selective Markovnikov-type hydroarylation of vinyl arenes and trisubstituted olefins with indoles.