Cyclopalladation and Reactivity of Amino Esters through C?H Bond Activation: Experimental,Kinetic, and Density Functional Theory Mechanistic Studies

The orthopalladation, through C? H bond activation, of a large number of amino esters and amino phosphonates derived from phenylglycine, and having different substituents at the aryl ring and the C‐α atom, as well as on the N‐amine atom, has been studied. The experimental observations indicated an improvement in the yields of the orthopalladated compounds when the N‐amine and/or the C‐α atom are substituted, when compared with the unsubstituted methyl phenylglycinate derivatives. In contrast, substitutions at the aryl ring do not promote significant changes in the orthometalation results. Furthermore, the use of hydrochloride salts of the amino esters has also been shown to have a remarkably favorable effect on the process. All these observations have been fully quantified at different temperatures and pressures by a detailed kinetic study in solution in different solvents and in the presence and absence of added Brønsted acids and chloride anions. The data collected indicate relevant changes in the process depending on these conditions, as expected from the general background known for cyclopalladation reactions. An electronic mechanism of the orthopalladation has been proposed based on DFT calculations at the B3LYP level, and a very good agreement between the trends kinetically measured and the theoretically calculated activation barriers has been obtained. The reactivity of the new orthopalladated amino phosphonate derivatives has been tested and it was found that their halogenation, alkoxylation and carbonylation resulted in formation of the corresponding functionalized ortho‐haloaminophosphonates, ortho‐alkoxyaminophosphonates and oxoisoindolinylphosphonates.     

Enzyme‐Responsive Silica Mesoporous Supports Capped with Azopyridinium Salts for Controlled Delivery Applications

The preparation of a new capped silica mesoporous material, Rh‐Azo‐S , for on‐command delivery applications in the presence of target enzymes is described. The material consists of nanometric mesoporous MCM‐41‐like supports loaded with Rhodamine B and capped with an azopyridine derivative. The material was designed to show “zero delivery” and to display a cargo release in the presence of reductases and esterases, which are usually present in the colon, mainly due to intestinal microflora. The opening and cargo release of Rh‐Azo‐S in vitro studies were assessed and seen to occur in the presence of these enzymes, whereas no delivery was noted in the presence of pepsine. Moreover, Rh‐Azo‐S nanoparticles were used to study controlled Rhodamine B dye delivery in intracellular media. HeLa cells were employed for testing the “non”‐toxicity of nanoparticles. Moreover, delivery of the dye in these cells, through internalization and enzyme‐mediated gate opening, was confirmed by confocal microscopy. Furthermore, the nanoparticles capped with the Azo group and loaded with a cytotoxic camptothecin ( CPT ) were also prepared (solid CPT‐Azo‐S ) and used as delivery nanodevices in HeLa cells. When this solid was employed, the cell viability decreased significantly due to internalization of the nanoparticles and delivery of the cytotoxic agent.     

Unzipping Nucleoside Channels by Means of Alcohol Disassembly

Gold nanoparticles capped with simple adenosine derivatives can form colloidal aggregates in nonpolar solvents. Theoretical calculations indicate the formation of organic channels by the supramolecular assembly of the nanoparticles by means of hydrogen bonds between the adenine moieties. The aggregates were only negligibly sensitive to nPrOH, iPrOH, and tBuOH, whereas some showed a similar response to MeOH and EtOH, and others showed high selectivity toward MeOH. DNA nucleoside derivatives (1‐(2‐deoxy‐β‐D ‐ribofuranosyl)‐5‐methyluracil and 2′,3′‐O‐isopropylideneadenosine) as well as thymine and other aromatic compounds such as pyrene derivatives (pyrene, 1‐chloropyrene, 1‐hydroxypyrene, (1‐pyrenyl)methanol, and 2‐hydroxynapthalene) did not induce disassembly of the nanoparticle aggregates. Data suggest that the nucleoside channels allow access to alcohols according to their size, and an efficient interaction between the alcohol and the adenine units destabilizes the hydrogen bonds, which eventually leads to nanoparticle disassembly.     

Use of green alternative solvents in dispersive liquid-liquid microextraction: A review

Dispersive liquid-liquid microextraction is one of the most widely used microextraction techniques currently in the analytical chemistry field, mainly due to its simplicity and rapidity. The operational mode of this approach has been constantly changing since its introduction, adapting to new trends and applications. Most of these changes are related to the nature of the solvent employed for the microextraction. From the classical halogenated solvents (e.g., chloroform or dichloromethane), different alternatives have been proposed in order to obtain safer and non-pollutants microextraction applications. In this sense, low-density solvents, such as alkanols, switchable hydrophobicity solvents, and ionic liquids were the first and most popular replacements for halogenated solvents, which provided similar or better results than these classical dispersive liquid-liquid microextraction solvents. However, despite the good performances obtained with low-density solvents and ionic liquids, researchers have continued investigating in order to obtain even greener solvents for dispersive liquid-liquid microextraction. For that reason, in this review, the evolution over the last five years of the three types of solvents already mentioned and two of the most promising solvent alternatives (i.e., deep eutectic solvents and supramolecular solvents), have been studied in detail with the purpose of discussing which one provides the greenest alternative.     

Synthesis of 1D Bragg gratings by a layer-aggregation method

We present what we believe to be a novel method for the synthesis of complex 1D (fiber and waveguide) Bragg gratings, which is based on an impedance reconstruction layer aggregation technique. The main advantage brought by the method is the possibility of synthesizing structures containing defects or discontinuities of the size of the local period, a feature that is not possible with prior reported methods. In addition, this enhanced spatial resolution allows the synthesis of very strong fiber Bragg grating devices providing convergent solutions. The method directly renders the refractive index profile n(z) as it does not rely on the coupled-mode theory.     

Molecular Recognition of Nucleotides in Water by Scorpiand‐Type Receptors Based on Nucleobase Discrimination

The detection of nucleotides is of crucial importance because they are the basic building blocks of nucleic acids. Scorpiand‐based polyamine receptors functionalized with pyridine or anthracene units are able to form stable complexes with nucleotides in water, based on coulombic, π–π stacking, and hydrogen‐bonding interactions. This behavior has been rationalized by means of an exploration with NMR spectroscopy and DFT calculations. Binding constants were determined by potentiometry. Fluorescence spectroscopy studies have revealed the potential of these receptors as sensors to effectively and selectively distinguish guanosine‐5′‐triphosphate (GTP) from adenosine‐5′‐triphosphate (ATP).     

Orbital‐Like Motion of Hydride Ligands around Low‐Coordinate Metal Centers

Hydrogen atoms in the coordination sphere of a transition metal are highly mobile ligands. Here, a new type of dynamic process involving hydrides has been characterized by computational means. This dynamic event consists of an orbital‐like motion of hydride ligands around low‐coordinate metal centers containing N‐heterocyclic carbenes. The hydride movement around the carbene–metal–carbene axis is the lowest energy mode connecting energy equivalent isomers. This understanding provides crucial information for the interpretation of NMR spectra.     

Effect of Ionic Liquid Structure on the Electrochemical Response of Dopamine at Room Temperature Ionic Liquid‐modified Carbon Paste Electrodes (IL–CPE)

In this work 12 different ionic liquids (ILs) have been used added as co‐binders in the preparation of modified carbon paste electrodes (IL–CPEs) used for the voltammetric analysis of dopamine in Britton‐Robinson buffer. The ionic liquids studied were selected based on three main criteria: (1) increasing chain length of alkyl substituents (studying 1‐ethylimidazolium and ethyl, propyl, butyl, hexyl and decylmethylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids); (2) nature of the counter ion (dicyanamide, bis(trifluoromethylsulfonyl)imide and hexafluorophosphate) in 1‐butyl‐3‐methylimidazolium ionic liquids; and (3) cation ring structures (1‐butyl‐3‐methylimidazolium, 1‐butyl‐1‐methylpiperidinium, 1‐butyl‐1‐methylpyrrolidinium and 1‐butyl‐3‐methylpyridinium) in bis(trifluoromethylsulfonyl)imide or hexafluorophosphate (1‐butyl‐3‐methylimidazolium or 1‐butyl‐3‐methylpyridinium as cations) ionic liquids. The use of IL as co‐binders in IL–CPE results in a general enhancement of both the sensitivity and the reversibility of dopamine oxidation. In square wave voltammetry experiments, the peak current increased up to a 400 % when 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide was used as co‐binder, as compared to the response found with the unmodified CPE. Experimental data provide evidence that electrostatic and steric effects are the most important ones vis‐à‐vis these electrocatalytic effects on the anodic oxidation of dopamine on IL–CPE. The relative hydrophilicity of dicyanamide anions reduced the electrocatalytic effects of the corresponding ionic liquids, while the use of 1‐ethyl‐3‐methylimidazolium hexafluorophosphate or 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (two relatively small and highly hydrophobic ionic liquids) as co‐binders in IL–CPE resulted in the highest electrocatalytic activity among all of the IL–CPE studied.     

Chemical triple-mutant boxes for quantifying cooperativity in intermolecular interactions

Chemical double-mutant cycles have been used to quantify intermolecular functional-group interactions in H-bonded zipper complexes in chloroform. If the same interaction is measured in zippers of different overall stability, the double-mutant cycles can be combined to produce a triple-mutant box. This construct quantifies cooperativity between the functional group interaction of interest and the other interactions that are used to change the overall stability of the complexes. The sum of two edge-to-face aromatic interactions (-2.9 +/- 0.5 kJ mol-1) is shown to be insensitive to changes of up to 13.7 +/- 0.2 kJ mol-1 in the overall stability of the complex. In principle, enthalpic cooperative effects caused by entropy-enthalpy compensation could perturb the measurement of intermolecular interactions when using the double-mutant cycle approach, but these experiments show that, for this system, the magnitude of the effect lies within the error of the measurements.