A new entry to Pd-H chemistry: catalytic asymmetric conjugate reduction of enones with EtOH and a highly enantioselective synthesis of warfarin

[reaction: see text] We report here the catalytic asymmetric conjugate reduction of enones using ethanol as a hydride source. The reaction was carried out in the presence of a chiral Pd complex at ambient temperature in ethanol, and the desired products were obtained in high chemical yield and high enantioselectivity. We applied this novel reaction to the catalytic asymmetric synthesis of warfarin (96% ee), and on the basis of d-labeling experiments, the reaction mechanism is proposed.     

A study of glutamine interference in the acidified ninhydrin reaction for proline

In the application of the acidified ninhydrin reaction to the estimation of free proline in plant extracts, glutamine was a major source of interference, all others being minor, absent or removable with Permutit. Some characteristics of glutamine interference in this reaction are described. A ratio of linear errors method for the correction of proline values to the glutamine free state is introduced. The glycine and proline concentration in the reaction mixtures are prime factors in obtaining a ratio of linear errors by spectrophotometric measurement at 2 wavelengths.     

A study on the induction of stereoselectivity in the Strecker synthesis of basic amino acid-derived α-amino nitriles

Diverse basic amino acid-derived α-amino nitriles have been synthesized via a modified Strecker reaction, using TMSCN as the cyanide source. It has been found that this synthesis is a slow thermodynamically controlled reaction, where it was difficult to induce stereocontrol. Moderate selectivity was induced by thermodynamic control of the reaction in MeOH. The absolute configuration at the stereogenic center generated was assigned by chemical correlation with 2-oxopiperazine derivatives.     

Homeopathic ligand-free palladium as a catalyst in the heck reaction. A comparison with a palladacycle

[reaction: see text] Ligand-free Pd(OAc)(2) can be used as a catalyst in the Heck reaction of aryl bromides as long as the amount of catalyst is kept between 0.01 and 0.1 mol %. At higher concentrations palladium black forms and the reaction stops. The actual catalyst is monomeric. Palladacycles merely serve as a source of ligand-free palladium in Heck reactions of aryl bromides.     

A simple mathematical model to aid quantification of electrophoresis gels by image analysis

In many scientific disciplines, measurements are taken from films that have been exposed to energetic sources. Examples include radiographs where the source is an X-ray tube, autoradiography where the source is a radioactive isotope and electrophoresis gels where the source is an enhanced chemiluminescence reaction. In these situations it is of interest to quantify the darkening of the film and compute the strength of the source which in the cases of autoradiography and electrophoresis can be used to compute unknown concentrations of biochemicals. We developed a simple mathematical model of the darkening of films in radiography, autoradiography and electrophoresis bands disclosed by enhanced chemiluminescence, and present formulae to calculate the strength of the source from measurement of film blackening by image analysis. A simple model is used in two examples to predict blackening of film exposed to electromagnetic radiation. This blackening is measured by image analysis. Results show reasonable agreement between predictions of the model and blackening of film for the examples chosen. This model is proposed as an aid to quantification of electrophoresis gels.     

Protonolysis of the carbon-palladium bond in palladium(II)-catalyzed enyne cyclization in imidazolium-type ionic liquids

The Pd-catalyzed cyclizations of 1,6-enynes are efficient reactions for the synthesis of α-methylene-γ-butyrolactones and lactams. The effects of solvent, proton source, chloride concentration, and temperature on the protonolysis of the carbon-Pd bond were investigated and the optimal reaction conditions were identified. We showed that imidazolium-type ionic liquids played an important role in the reaction both as a ligand for the palladium catalyst and as a solvent. The crystal structure of the Pd complex was obtained and the reaction mechanisms were discussed.     

Titanocene-catalysed, selective reduction of ketones in aqueous media. A safe, mild, inexpensive procedure for the synthesis of secondary alcohols via radical chemistry

We report here a novel procedure for the reduction of ketones to secondary alcohols using catalytic quantities of commercially available Cp₂TiCl₂, inexpensive Zn dust and water as proton source. Mechanistically the reaction presumably proceeds via titanoxy radicals. In practice this reduction process has significant advantages: it shows an interesting selectivity pattern, takes place under mild conditions using safe, cheap reagents and does not require anhydrous solvents. The proton-donor activity of water under these conditions avoids the use of the frequently poisonous hydrogen-atom donors generally required to reduce free radicals. This procedure is also highly convenient for synthesising deuterium-labelled alcohols employing relatively inexpensive D₂O as deuterium source.     

Direct determination of oxygen in organic materials-I A study of the carbon reduction method

The results of an investigation into the nature of the carbon reduction process in the Unterzaucher method for the microdetermination of oxygen are reported and discussed. The reaction between carbon dioxide and carbon is discussed, and its relation to the specific surface area, which contrary to belief has been found not to contribute to the efficiency of the reaction. It is shown that carbon-oxygen complexes give rise to errors in the results, and that an intermediate, SiO(OH), formed on the inner surface of the silica pyrolysis tube might be a precursor to the formation of carbon monoxide as a source of blank error and of the apparent oxygen content of pure hydrocarbons.     

A mechanism for the photochemical transformation of nitrate in snow

Photochemical reactions of trace compounds in snow have important implications for the composition of the atmospheric boundary layer in snow-covered regions and for the interpretation of concentration profiles in snow and ice regarding the composition of the past atmosphere. One of the prominent reactions is the photolysis of nitrate, which leads to the formation of OH radicals in the snow and to the release of reactive nitrogen compounds, like nitrogen oxides (NO and NO₂) and nitrous acid (HONO) to the atmosphere. We performed photolysis experiments using artificial snow, containing variable initial concentrations of nitrate and nitrite, to investigate the reaction mechanism responsible for the formation of the reactive nitrogen compounds. Increasing the initial nitrite concentrations resulted in the formation of significant amounts of nitrate in the snow. A possible precursor of nitrate is NO₂, which can be transformed into nitrate either by the attack of a hydroxy radical or the hydrolysis of the dimer (N₂O₄). A mechanism for the transformation of the nitrogen-containing compounds in snow was developed, assuming that all reactions took place in a quasi-liquid layer (QLL) at the surface of the ice crystals. The unknown photolysis rates of nitrate and nitrite and the rates of NO and NO₂ transfer from the snow to the gas phase, respectively, were adjusted to give an optimum fit of the calculated time series of nitrate, nitrite, and gas phase NO_x with respect to the experimental data. Best agreement was obtained with a ∼25 times faster photolysis rate of nitrite compared to nitrate. The formation of NO₂ is probably the dominant channel for the nitrate photolysis. We used the reaction mechanism further to investigate the release of NO_x and HONO under natural conditions. We found that NO_x emissions are by far dominated by the release of NO₂. The release of HONO to the gas phase depends on the pH of the snow and the HONO transfer rate to the gas phase. However, due to the small amounts of nitrite produced under natural conditions, the formation of HONO in the QLL is probably negligible. We suggest that observed emissions of HONO from the surface snow are dominated by the heterogeneous formation of HONO in the firn air. The reaction of NO₂ on the surfaces of the ice crystals is the most likely HONO source to the gas phase.     

Recent advances in the catalytic conversion of CO_2 to value added compunds

CO2 is a major greenhouse gas,and it can also be used as a chemical feedstock for synthesis of chemicals and fuels by passing the petrochemical source.Herein,we present the recent progress of our research work in the catalytic conversion of CO2 to chemicals,with particular attention paid to catalytic reactivity and reaction mechanism.We also give the recommendations regarding the challenges and potential directions of the future research in this field.     

A general process for the haloamidation of olefins. Scope and mechanism

A methodology is described for the addition of a bromine atom and an amide nitrogen in a trans sense to an olefinic double bond. The process, which is illustrated by numerous examples, involves the use of an N-bromoamide and a Lewis acid as a source of Br+ which reacts with the olefin. The amide group is derived from a nitrile and a water molecule which serve as nucleophiles for the overall three-component reaction. The bromoamidation is general for a broad range of olefins and nitriles. This reaction pathway provides access not only to vicinal bromoamides but also to N-acyl aziridines and oxazolines. From these, many types of amines and amino alcohols can be prepared. Examples are provided which delineate diastereo- and regioselectivity preferences. An analogous chloroamidation reaction is also described.     

A new interpretation of the Baylis-Hillman mechanism

[reaction: see text] On the basis of reaction rate data, we have proposed a new mechanism for the Baylis-Hillman reaction involving the formation of a hemiacetal intermediate. We have determined that the rate-determining step is second order in aldehyde and first order in DABCO and acrylate. We have shown that this mechanism is general to aryl aldehydes under polar, nonpolar, and protic conditions using both rate data and two isotope effect experiments.     

A mechanistic investigation of the three‐component radical photoinitiator system Eosin Y spirit soluble,N‐methyldiethanolamine,and diphenyliodonium chloride

Photo‐DSC and in situ, time‐resolved, laser‐induced, steady‐state fluorescence spectroscopy were used to study the initiation mechanism of the three‐component system: Eosin Y spirit soluble (EYss), N‐methyldiethanolamine, and diphenyliodonium chloride. Kinetic studies based on photo‐DSC revealed that the fastest polymerization occurred when all three components were present (the next fastest was with the dye/amine pair, and the slowest was with the dye/iodonium pair). However, the laser‐induced fluorescence experiments showed that the pairwise reaction between the eosin and iodonium bleaches the dye much more rapidly than does the reaction between the eosin and amine. We concluded that although a direct eosin/amine reaction can produce active radicals in the three‐component system, this reaction is largely overshadowed by the eosin/iodonium reaction, which does not produce active radicals as effectively. We proposed that the amine reduces the oxidized dye radical formed in the eosin/iodonium reaction back to its original state as well as the simultaneous production of an active initiating amine‐based radical. Because of the difference in the pairwise reaction rates for eosin/amine and eosin/iodonium, it is likely that this regeneration reaction was the primary source of active radicals in the three‐component eosin/amine/iodonium system. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 715–723, 2001     

A study of microporous aluminophosphates obtained in the presence of symmetrical di-n-alkylamines

Microporous aluminophosphates and silicoaluminophosphates were obtained by hydrothermal synthesis with a number of symmetrical di-n-alkylamines as templates. The influence of the reagents and chemical composition of the starting reaction gel on the phase composition of the products obtained was studied. Pure ATO-type crystals were formed when the organic component was abundant in the reaction mixture and aluminum isopropoxide served as an aluminum source. Various physicochemical investigations revealed that the properties of the products obtained depend on the amine nature.     

Use of metastable peak shapes in determining ring size in cyclic rearrangements in the mass spectrometer

A general method is described in which the mechanism of a reaction occurring in the ion source is inferred from the kinetic energy release accompanying further fragmentation of metastable product ions. In several cases the probe reaction occurred competitively by two mechanisms, and if high energy resolution is available this allows the detailed metastable peak shapes and not merely the average kinetic energy released, to be used to characterize the product ion formed in the fast (ion source) reaction. It is found that most substituted benzaldoxime O-methyl ethers undergo HCN elimination via a five-centered methoxyl transfer in the ion source, but that the p-methoxy substituted compound reacts through both a four- and a five-membered cyclic intermediate. The slow reactions of the corresponding metastable ions occur predominantly through a four-centered transition state in the p-methoxy compound and probably through both four- and five-membered intermediates for the less strongly electron donating substituents. The fraction of the excess energy of the products is higher than expected from a consideration of energy partitioning data for other systems involving activated complexes of comparable tightness.     

The Influence of Water in the Vapor-Assisted Conversion Synthesis of UiO-67 MOF Thin Films

Water is known to play an important role for the crystallization and stability of Zr-based metal-organic frameworks (MOFs). This work investigates its effect on the vapor-assisted conversion (VAC) synthesis of UiO-67 MOF thin films on Au-coated Si substrates. We demonstrate the equilibration processes taking place during the VAC procedure, confirming the gradual equilibration of all solutions upon heating. The presence of water affects the vapor phase composition but does not significantly impact the acetic acid equilibration rate. However, the preparation of UiO-67 thin films by VAC is highly sensitive to the water content in the reaction. Some water is required for the formation of the zirconium clusters, but excessive water in the reaction vial yields poorly crystalline materials. Atmospheric water that is taken up by the vapor source can be sufficient to reduce crystallinity dramatically. This complication can be partially overcome by increasing the amount of acetic acid in the vapor source.     

A new supported reagent for the photochemical generation of radicals in solution

A new polymer-supported radical source was developed by loading an N-hydroxy thiazole 2(3)-thione on a Wang resin. This new supported reagent can be employed for a solid-phase version of the Hunsdiecker reaction or to liberate free alkoxy radicals, in a variant of the "catch and release" technique, under very mild conditions (irradiation with a discharge lamp) and simplifying the purification procedure.     

A Gerischer phase element in the impedance diagram of the polymer electrolyte membrane fuel cell anode

We study the isothermal hydrogen adsorption and reaction at the E-TEK electrode of a polymer electrolyte fuel cell with a Nafion 117 membrane by impedance spectroscopy at 30 degrees C. We find that the impedance diagram must include a Gerischer phase element. Constant phase elements are not sufficient to describe the experimental data. This means that an adsorption reaction takes place in combination with surface diffusion of hydrogen in the carbon layer located before the platinum surface, separate from the charge transfer step at the platinum particle surface. We are not able to distinguish between molecular or atomic hydrogen diffusion on carbon. We predict and find that the relaxation time of the adsorption step is independent of the applied potential. Water may also enter rate-limiting steps in the electrode reaction, but its role needs further clarification.     

Characterization of a new tailoring domain in polyketide biogenesis: the amine transferase domain of MycA in the mycosubtilin gene cluster

We report the expression and characterization of a truncated form of MycA from the Mycosubtilin gene cluster from Bacillus subtilis. The MycA fragment contains a new amino transferase (AMT) tailoring domain, allowing the first detailed study of a PLP-dependent enzyme operating in cis within the PKS and NRPS biosynthetic paradigm. As the AMT domain acts on covalently bound beta-ketothioesters, and is therefore a single-turnover system, electrospray ionization-Fourier transform mass spectrometry (ESI-FTMS) was used to observe the amine-transfer reaction both for amine donor substrate specificity and to regiospecifically determine enzyme-bound intermediates. We confirm the function of the AMT domain, dissect the mechanistic steps of amine transfer, identify the preferred amine source, and localize the beta-ketothioester substrate during amine transfer.