Pilar Prieto

Exfoliation of graphite with triazine derivatives under ball-milling conditions: preparation of few-layer graphene via selective noncovalent interactions.

A ball-milling treatment can be employed to exfoliate graphite through interactions with commercially available melamine under solid conditions. This procedure allows the fast production of relatively large quantities of material with a low presence of defects. The milling treatment can be modulated in order to achieve graphene flakes with different sizes. Once prepared, the graphene samples can be redispersed in organic solvents, water, or culture media, forming stable dispersions that can be used for multiple purposes. In the present work, we have screened electron-rich benzene derivatives along with triazine derivatives in their respective ability to exfoliate graphite. The results suggest that the formation of a hydrogen-bonding network is important for the formation of multipoint interactions with the surfaces of graphene, which can be used for the exfoliation of graphite and the stabilization of graphene in different solvents. Aminotriazine systems were found to be the best partners in the preparation and stabilization of graphene layers in different solvents, while the equivalent benzene derivatives did not show comparable exfoliation ability. Computational studies have also been performed to rationalize the experimental results. The results provide also the basis for further work in the preparation of noncovalently modified graphene, where derivatives of aminotriazines can be designed to form extensive hydrogen-bond 2D networks on the graphene surface with the aim of manipulating their electronic and chemical properties.

Microwave-Assisted Reactions in Heterocyclic Compounds with Applications in Medicinal and Supramolecular Chemistry

Microwave irradiation has been successfully applied in organic chemistry. Spectacular accelerations, higher yields under milder reaction conditions and higher product purities have all been reported. Indeed, a number of authors have described success in reactions that do not occur under conventional heating and modifications in selectivity (chemo-, regio- and stereoselectivity) have even been reported. Recent advances in microwave-assisted combinatorial chemistry include high-speed solid-phase and polymer-supported organic synthesis, rapid parallel synthesis of compound libraries, and library generation by automated sequential microwave irradiation. In addition, new instrumentation for high-throughput microwave-assisted synthesis continues to be developed at a steady pace. The impressive speed combined with the unmatched control over reaction parameters justifies the growing interest in this application of microwave heating. In this review we highlight our recent advances in this area, with a particular emphasis on cycloaddition reactions of heterocyclic compounds both with and without supports, applications in supramolecular chemistry and the reproducibility and scalability of organic reactions involving the use of microwave irradiation techniques.

Tandem Diels−Alder Aromatization Reactions of Furans under Unconventional Reaction Conditions − Experimental and Theoretical Studies

Silica-supported Lewis acids are good catalysts for Diels−Alder reactions between furan and acrylonitrile and methyl acrylate at room temperature. When 2,5-dimethylfuran is used as the diene, yields of the Diels−Alder adducts with methyl acrylate are lower, due in part to the appearance of aromatization products. The use of microwave activation results in some cases in good yields of aromatic products and, as such, constitutes a good synthetic route to polysubstituted aromatic compounds. Computational studies on the reaction mechanism and the role of the catalyst on the product distribution show that “hard” Lewis acids, such as aluminum derivatives, make ring-opening of the adduct much easier, affording aromatic products. The theoretical results are in excellent agreement with the relative reactivity observed for the different dienes and dienophiles.

A SHORT AND EFFICIENT SYNTHESIS OF KETENE O,O- AND S,S-ACETALS UNDER FOCUSED MICROWAVE IRRADIATION AND SOLVENT-FREE CONDITIONS

Abstract A new methodology leading to ketene O,O- and S,S-acetals is reported. The title compounds were prepared from the corresponding halogenated precursors under microwave irradiation in the absence of solvent within 5–25 minutes with excellent yields. Yields obtained under microwaves are by far the best when compared to those obtained by ultrasound or classical heating in the same conditions of time and temperature.

DFT Studies on Cobalt-Catalyzed Cyclotrimerization Reactions: The Mechanism and Origin of Reaction Improvement under Microwave Irradiation

A DFT computational mechanistic study of the [2+2+2] cyclotrimerization of a diyne with benzonitrile, catalyzed by a cobalt complex, has been carried out. Three alternative catalytic cycles have been examined together with the precatalytic step (responsible for the induction period). The favored mechanism takes place by means of an intramolecular metal-assisted [4+2] cycloaddition. The beneficial role of microwave activation has been studied. It is concluded that microwave irradiation can decrease the catalytic induction period through thermal effects and can also increase the triplet lifetime and promote the reaction, thus improving the final yield.

Use of flow cytometry and confocal microscopy techniques to investigate early CdCl2-induced nephrotoxicity in vitro

CdCl(2) is a well-known toxic compound for the kidney in vivo and in vitro. We report here part of the results of an ECVAM (European Centre for the Validation of Alternative Methods) contract study, aimed at establishing and assessing several flow cytometric and confocal microscopic endpoints for use in an in vitro nephrotoxicity model. Three renal tubule cell lines, OK (opossum, proximal tubule origin), LLC-PK1 (pig, proximal tubule origin) and MDCK (dog, distal tubule origin) were exposed for 1, 5 and 24 h to 25 microM and 100 microM CdCl(2). The results obtained for mitochondrial membrane potential showed a decrease in all the cell lines after 5 h of treatment with both CdCl(2) concentrations. In some cases, this decrease was detected by flow cytometry after a 1-h exposure. On the contrary, intracellular Ca(2+) increased in a time-dependent and concentration-dependent fashion. This increase was especially high in the MDCK cell line after a 24-h exposure to 100 microM CdCl(2). However, cell viability was not affected by 25 microM CdCl(2). Our results demonstrate early changes in mitochondrial membrane potential and cytoplasmic Ca(2+) levels in renal tubular epithelial cell lines treated with CdCl(2).

Influence of Polarity and Activation Energy in Microwave–Assisted Organic Synthesis (MAOS)

The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation.

The effect of focused microwaves on the reaction of ethyl N-trichloroethylidenecarbamate with pyrazole derivatives

Under microwave irradiation vinylpyrazoles react with ethyl N-trichloroethylidenecarbamate1 to give the addition to the imine system through the conjugated vinyl group. Likewise, compound 1 react with the NH group of pyrazolylhydrazones, if present. To the best of our knowledge this reaction type has not been described before and only can be performed under microwave irradiation. By classical heating, in the absence of microwaves, only dimerization or decomposition of the pyrazole derivatives is observed in these reactions.

Cycloadditions of ketene acetals under microwave irradiation in solvent-free conditions

When subjected to microwave irradiation ketene acetals undergo 1,3-dipolar and hetero-Diels–Alder cycloadditions within 5–12 min to give excellent yields of easily purified heterocyclic products. This efficient and rapid synthesis has the advantage of employing milder reaction conditions than those of classical thermal heating.

Preparation of Racemic and Enantiomerically Pure Cyclic Ketene Acetals

Abstract Cyclic ketene acetals have been prepared from a-haloaldehyde dimethylacetals by transacetalization and subsequent elimination in PTC without solvent conditions. No racemization has been observed when an enantiomerically pure diol has been used. Stability and storage conditions have been studied.