Ruthenium(IV)-catalyzed isomerization of the C=C bond of o-allylic substrates: a theoretical and experimental study

A general mechanism to rationalize Ru(IV) -catalyzed isomerization of the C=C bond in O-allylic substrates is proposed. Calculations supporting the proposed mechanism were performed at the MPWB1K/6-311+G(d,p)+SDD level of theory. All experimental observations in different solvents (water and THF) and under different pH conditions (neutral and basic) can be interpreted in terms of the new mechanism. Theoretical analysis of the transformation from precatalyst to catalyst led to structural identification of the active species in different media. The experimentally observed induction period is related to the magnitudes of the energy barriers computed for that process. The theoretical energy profile for the catalytic cycle requires application of relatively high temperatures, as is experimentally observed. Participation of a water molecule in the reaction coordinate is mechanistically essential when the reaction is carried out in aqueous medium. The new mechanistic proposal helped to develop a new experimental procedure for isomerization of allyl ethers to 1-propenyl ethers under neutral aqueous conditions. This process is an unique example of efficient and selective catalytic isomerization of allyl ethers in aqueous medium.     

Homogeneous hydrogels made with acrylic acid,acrylamide and chemically functionalized carbon nanotubes

Carbon nanotubes (CNTs) chemically functionalized were used to synthesize a series of novel nanocomposite hydrogels by in situ polymerization with acrylic acid (AA) and acrylamide (AM). A novel strategy was developed to prepare these hydrogels. CNTs were functionalized following a three-step chemical procedure: (i) purified carbon nanotubes (CNTs_p) were partially surface oxidized to obtain CNTs with hydroxyl, carbonyl and carboxyl groups on their sidewalls (CNTs_oxi), (ii) CNTs_oxi were reacted with oxalyl chloride to obtain CNTs functionalized with acyl chloride groups (CNTs_OCl), and (iii) CNTs_OCl were reacted with acrylic acid (AA). The product, AA modified CNTs_OCl (CNTs_OCl-AA) was used to prepare the (CNTs_OCl-AA-AM) nanocomposite hydrogels, where anhydride groups were tethered to the surface of the CNTs_OCl-AA. The swelling process in water was evaluated as a consequence of the anhydride group hydrolysis, which broke some chemical links between CNTs_OCl-AA and crosslinked AA-AM network. Equilibrium-swelling values of all hydrogels increased as the content of AA increased and were larger for AA-AM hydrogels than for CNTs_OCl-AA-AM nanocomposite hydrogels. Young’s moduli of CNTs_OCl-AA-AM nanocomposite hydrogels prepared with 1 or 2?wt.% AA, reached larger values than those measured for AA-AM hydrogels. This tendency was reversed when the AA content was raised to 3?wt.%.     

Enantioselective conjugate addition of nitro compounds to α,β-unsaturated ketones: an experimental and computational study

A series of chiral thioureas derived from easily available diamines, prepared from α-amino acids, have been tested as catalysts in the enantioselective Michael additions of nitroalkanes to α,β-unsaturated ketones. The best results are obtained with the bifunctional catalyst prepared from L-valine. This thiourea promotes the reaction with high enantioselectivities and chemical yields for aryl/vinyl ketones, but the enantiomeric ratio for alkyl/vinyl derivatives is very modest. The addition of substituted nitromethanes led to the corresponding adducts with excellent enantioselectivity but very poor diastereoselectivity. Evidence for the isomerization of the addition products has been obtained from the reaction of chalcone with [D(3)]nitromethane, which shows that the final addition products epimerize under the reaction conditions. The epimerization explains the low diastereoselectivity observed in the formation of adducts with two adjacent tertiary stereocenters. Density functional studies of the transition structures corresponding to two alternative activation modes of the nitroalkanes and α,β-unsaturated ketones by the bifunctional organocatalyst have been carried out at the B3LYP/3-21G* level. The computations are consistent with a reaction model involving the Michael addition of the thiourea-activated nitronate to the ketone activated by the protonated amine of the organocatalyst. The enantioselectivities predicted by the computations are consistent with the experimental values obtained for aryl- and alkyl-substituted α,β-unsaturated ketones.     

Calorimetric and computational study of the thermochemistry of phenoxyphenols

Thermodynamic properties of 3- and 4-phenoxyphenol have been determined by using a combination of calorimetric and effusion techniques as well as by high-level ab initio molecular orbital calculations. The standard (p° = 0.1 MPa) molar enthalpies of formation in the condensed and gas states, Δ(f)H(m)°(cr or l) and Δ(f)H(m)°(g), at T = 298.15 K, of 3- and 4-phenoxyphenol were derived from their energies of combustion in oxygen, measured by a static bomb calorimeter, and from the enthalpies of vaporization or sublimation derived respectively by Calvet microcalorimetry for the 3-phenoxyphenol and by Knudsen effusion technique for the 4-phenoxyphenol. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies, and, finally, O?H bond dissociation enthalpies. The good agreement between the G3MP2B3-derived values and the experimental gas-phase enthalpies of formation for the 3- and 4-phenoxyphenol gives confidence to the estimate concerning the 2-phenoxyphenol isomer, which was not experimentally studied, and to the estimates concerning the radical and the anion. Additionally, the experimental values of gas-phase enthalpies of formation were also compared with estimates based on the empirical scheme developed by Cox.     

Study of the capillary electrophoresis profile of intact α-1-acid glycoprotein isoforms as a biomarker of atherothrombosis

α-1-Acid glycoprotein (AGP) is a serum glycoprotein that presents several isoforms. Changes in the isoforms of AGP have been related to different pathological states including cardiovascular diseases (CVDs) such as acute myocardial infarction. However, to our knowledge, the role of variations of AGP isoforms as a potential biomarker of atherothrombosis has not been addressed. In this work, a preliminary study about differences in the capillary zone electrophoresis (CZE) profile of intact (non-hydrolyzed) AGP isoforms between healthy individuals and patients with atherothrombosis, specifically abdominal aortic aneurysm (AAA) and carotid atherosclerosis (CTA), has been performed. Biological samples (plasmas and sera) were analyzed by CZE after immunoaffinity chromatography purification. Up to 13 peaks corresponding to groups of isoforms of intact AGP from plasma samples were detected by CZE-UV. Electrophoretic profiles were aligned, peaks assigned, and linear discriminant analysis (LDA) of percentage of the corrected areas of AGP peaks was employed to discriminate and classify the CZE profiles of AGP samples. LDA enabled to accomplish 92.9% of correct classification of the AGP samples when the three groups of samples were considered. Besides, the LDA model showed high predictive power in the groups healthy vs. sick, healthy vs. AAA, and healthy vs. CTA. The described method was a successful approach to study the potential of AGP isoforms profile as a biomarker of atherothrombosis. To the best of our knowledge this has been the first time that a possible role of the CZE profile of intact AGP isoforms as a biomarker of vascular diseases has been demonstrated.     

Determination of Phenolic Compounds and the Antioxidant Capacity of Ximenia parviflora Benth. var. parviflora (Olacaceae) Fruit by High-Performance Liquid Chromatography with Diode Array Detection

The total phenolic and flavonoid content, phenolic composition, and in vitro antioxidant capacity of ethanolic extracts of Ximenia parviflora Benth. var. parviflora fruits collected at Zinaparo, Michoacan (in central Mexico) were determined. Fruit extracts present a high scavenging activity of 2,2-diphenyl-1-picrylhydrazyl and 2,2′-azino-bis[3-ethylbenzothiazoline-6-sulfonic acid] radicals (71.49?±?0.11% and 85.00?±?1.29% inhibition, respectively). The four phenolic compounds identified in fruit extracts by high-performance liquid chromatography with diode array detection were gallic acid, chlorogenic acid, caffeic acid, and quercetin. X. parviflora fruits may be used as a starting material for the extraction of high value antioxidant phenolic compounds with potential applications in the pharmaceutical and dietary supplement industries.     

Radiation polymerization and crosslinking: A viable alternative for synthesis of porous functional polymers

We have earlier shown the possibility of preparation of crosslinked porous polymers by ionizing radiation-initiated reactions, and here we give an overview of the irradiation conditions—porous properties relationship for several methacrylate type copolymers. We illustrate the possible applications by an environment-friendly chromatographic column using water as eluent and a chemically stable scintillating polymer with excellent ion-binding capacity for possible continuous monitoring of radioactivity in natural waters.     

Usefulness of Transgenic Luminescent Bacteria in Direct Bioautographic Investigation of Chamomile Extracts

JPC – Journal of Planar Chromatography – Modern TLC - Direct bioautography performed with luminescence gene-tagged bacteria enables almost real-time detection of antimicrobial compounds...     

Photochemical Sandmeyer-type Halogenation of Arenediazonium Salts

Trihalide salts were found to efficiently promote photochemical dediazotizing halogenations of diazonium salts. In contrast to classical Sandmeyer reactions, no metal catalysts are required to achieve high yields and outstanding selectivities for halogenation over competing hydridodediazotization. Convenient protocols are disclosed for synthetically meaningful brominations, iodinations, and chlorinations of diversely functionalized derivatives.     

Methodology for measuring the thermal conductivity of insulating samples with small dimensions by heat flow meter technique

Journal of Thermal Analysis and Calorimetry - Nowadays, developing advanced, highly insulating materials for minimizing heat losses in buildings is of utmost relevance. Thus, there is a constant...