Kinetics and mechanism of back-extraction: selected palladium extraction systems

The equilibrium and kinetics of back-extraction (stripping) of palladium originally extracted as PdCl^2?₄ from the chloroform extracts obtained with 1-(2-pyridylazo)-2-naphthol (PAN), 7-(4-ethyl-1-methyloctyl)quinolin-8-ol (Kelex 100) or dioctyl sulfide (R₂S) were investigated. Replacement of chloride in extracted species by thiocyanate occurs prior to back-extraction. The back-extraction equilibria have been described by Pd(SCN)₂(R₂S)₂(o) + 2SCN^?K_BX1 Pd(SCN)^2?₄ + 2R₂S(o), with K_BX1=10^{?(1.16±0.05)}, and Pd(SCN)PAN(o) + 3SCN^? + H⁺K_BX2 Pd(SCN)^2?₄ + PAN(o), with K_BX2=10^4.89±0.06. The rate of stripping from PdPAN and PdKelex 100 displayed an inverse first-order dependence on the solution pH, a second-order dependence on the thiocyanate concentration and was zero order in both the chloride and the organic phase chelate concentration. More complicated kinetics were observed for palladium stripping in the dioctyl sulfide system. In all systems, the enhancement in stripping rate parallels the size of the “trans effect”.     

The mass spectra of small-ring heterocycles. III. Aroylaziridines

The mass spectra of cis-trans isomeric aroylaziridines are presented. Attempts to extend the method for distinguishing between cis and trans isomers previously established for aroylazetidines are described and the results rationalized. A simple fission of the 1-alkylnitrogen bond is described and detailed fragmentation mechanisms are presented and discussed.     

Chemically Modified Carbon Nanotubes for Use in Electroanalysis

The discovery of carbon nanotubes has had a profound impact on many areas of science and technology, not least that of electroanalysis. The properties and applications of carbon nanotubes themselves have been well reviewed in the literature and a number of reviews with an electrochemical emphasis have been published. However, the modification of carbon nanotubes has recently been the focus of much research, primarily to improve their solubility in various solvents. Yet modified carbon nanotube electrodes also allow the electrochemist to tailor the properties of the carbon nanotubes, or the electrode surface to impart desired properties such as enhanced sensing capabilities. In this review we attempt to comprehensively cover the different chemical and electrochemical modification strategies and research carried out using modified carbon nanotubes for electroanalytical and bioanalytical applications. Furthermore we also discuss the use of modified carbon nanotubes in electrocatalysis and biocatalysis from an analytical aspect, as well as seeking to dispel some of the myths surrounding the “electrocatalytic” properties of carbon nanotubes.     

Structural organization of G-protein-coupled receptors

Atomic-resolution structures of the transmembrane 7--helical domains of 26 G-protein-coupled receptors (GPCRs) (including opsins, cationic amine, melatonin, purine, chemokine, opioid, and glycoprotein hormone receptors and two related proteins, retinochrome and Duffy erythrocyte antigen) were calculated by distance geometry using interhelical hydrogen bonds formed by various proteins from the family and collectively applied as distance constraints, as described previously [Pogozheva et al., Biophys. J., 70 (1997) 1963]. The main structural features of the calculated GPCR models are described and illustrated by examples. Some of the features reflect physical interactions that are responsible for the structural stability of the transmembrane -bundle: the formation of extensive networks of interhelical H-bonds and sulfur–aromatic clusters that are spatially organized as 'polarity gradients' the close packing of side-chains throughout the transmembrane domain; and the formation of interhelical disulfide bonds in some receptors and a plausible Zn2+ binding center in retinochrome. Other features of the models are related to biological function and evolution of GPCRs: the formation of a common 'minicore' of 43 evolutionarily conserved residues; a multitude of correlated replacements throughout the transmembrane domain; an Na+-binding site in some receptors, and excellent complementarity of receptor binding pockets to many structurally dissimilar, conformationally constrained ligands, such as retinal, cyclic opioid peptides, and cationic amine ligands. The calculated models are in good agreement with numerous experimental data.     

Synthesis of Aminated Naphthacenetriones: Precursors to Aminated Anthracyclines

An application of aminoacetaldehyde dialkoxy acetal to introduce an amino fuctionality in the 6 and/or 11 position of the naphthacenetrione ring system, resulting in key intermediates to aminated anthracyclines, is described.     

Butyllithium Polymerization of Butadiene. II. Effects of Solvent and Temperature on Association of Polybutatadienyl Lithium

Abstract

In the butyllithium polymerization of butadiene in aliphatic solvents at 25°C, the first few monomer units are incorporated largely in a 1, 2 manner. With increasing degree of polymerization the extent of 1, 2 addition decreases to a limiting value of about 10% at a degree of polymerization of about 50. However, within this region of high 1, 2-addition increasing solvent basicity, i.e., changing from aliphatic to aromatic solvents, markedly reduces extent of 1, 2 addition and also narrows the molecular weight distribution. Further, in aliphatic solvents, increasing polymerization temperature from 25 to 60°C also results in a marked reduction in 1, 2 addition. These results are consistent with the concept that an ion pair of the associated organolithium complex, [R (_n-1 Li_n] ⊕R?, is the active polymerization species and that changes in mode of monomer incorporation are due to changes in the degree of association of the entire organolithium system.     

Cover Feature: Contrasting the Mechanism of H2 Activation by Monomeric and Potassium-Stabilized Dimeric AlI Complexes: Do Potassium Atoms Exert any Cooperative Effect? (Chem. Eur. J. 69/2021)

Aluminyl anions are low-valent, anionic, and carbenoid aluminum species commonly found stabilized with potassium cations from the reaction of Al-halogen precursors and alkali compounds. These systems are very reactive toward the activation of σ-bonds and in reactions with electrophiles. Various research groups have detected that the potassium atoms play a stabilization role via electrostatic and cation interactions with nearby (aromatic)-carbocyclic rings from both the ligand and from the reaction with unsaturated substrates. Since stabilizing K⋯H bonds are witnessed in the activation of this class of molecules, we aim to unveil the role of these metals in the activation of the smaller and less polarizable H₂ molecule, together with a comprehensive characterization of the reaction mechanism. In this work, the activation of H₂ utilizing a NON-xanthene-Al dimer, [K{Al(NON)}]₂ ( D ) and monomeric, [Al(NON)]⁻ ( M ) complexes are studied using density functional theory and high-level coupled-cluster theory to reveal the potential role of K⁺ atoms during the activation of this gas. Furthermore, we aim to reveal whether D is more reactive than M (or vice versa), or if complicity between the two monomer units exits within the D complex toward the activation of H₂. The results suggest that activation energies using the dimeric and monomeric complexes were found to be very close (around 33 kcal mol⁻¹). However, a partition of activation energies unveiled that the nature of the energy barriers for the monomeric and dimeric complexes are inherently different. The former is dominated by a more substantial distortion of the reactants (and increased interaction energies between them). Interestingly, during the oxidative addition, the distortion of the Al complex is minimal, while H₂ distorts the most, usually over 0.77 . Overall, it is found here that electrostatic and induction energies between the complexes and H₂ are the main stabilizing components up to the respective transition states. The results suggest that the K⁺ atoms act as stabilizers of the dimeric structure, and their cooperative role on the reaction mechanism may be negligible, acting as mere spectators in the activation of H₂. Cooperation between the two monomers in D is lacking, and therefore the subsequent activation of H₂ is wholly disengaged.