Application of Pd(II) Complexes with Pyridines as Catalysts for the Reduction of Aromatic Nitro Compounds by CO/H₂O

Many efforts have been undertaken to minimize the cost of large-scale conversion of aromatic nitro compounds to amines. Toward this end, application of CO/H₂O as a reducing agent instead of molecular hydrogen seems to be a promising method, and the process can be catalyzed by Pd­(II) complexes. In this work, the catalytic activity of square planar complexes of general structure PdCl₂(X_<i>n</i>Py)₂ (where X_<i>n</i>Py = pyridine derivative) was studied. Particular attention was paid to the effects of substituents both in the aromatic ring of X_<i>n</i>Py (ligand) and the nitro compound to be reduced (YC₆H₄NO₂). Incorporation of electron-withdrawing Y in the aromatic ring of YC₆H₄NO₂ increases the conversion, indicating that the kinetics of this process is similar to that for the carbonylation of nitrobeznene by CO in the absence of water (described in <i>J. Mol. Catal. A: Chem.</i> 2011, <i>337</i>, 9–16). Surprisingly, the incorporation of electron-withdrawing substituents into the aromatic ring of the X_<i>n</i>Py ligand also increases the conversion of YC₆H₄NO₂ (regardless of the structure of the YC₆H₄NO₂ substrate)

First Experimental Evidence of Dopamine Interactions with Negatively Charged Model Biomembranes

Dopamine is essential for receptor-related signal transduction in mammalian central and peripheral nervous systems. Weak interactions between the neurotransmitter and neuronal membranes have been suggested to modulate synaptic transmission; however, binding forces between dopamine and neuronal membranes have not yet been quantitatively described. Herein, for the first time, we have explained the nature of dopamine interactions with model lipid membranes assembled from neutral 1,2-dimyristoyl-<i>sn</i>-glycero-3-phosphocholine (DMPC), negatively charged 1,2-dimyristoyl-<i>sn</i>-glycero-3-phosphoglycerol (DMPG), and the mixture of these two lipids using isothermal titration calorimetry and differential scanning calorimetry. Dopamine binding to anionic membranes is a thermodynamically favored process with negative enthalpy and positive entropy, quantitatively described by the mole ratio partition coefficient, <i>K. K</i> increases with membrane charge to reach its maximal value, 705.4 ± 60.4 M^–1, for membrane composed from pure DMPG. The contribution of hydrophobic effects to the binding process is expressed by the intrinsic partition coefficient, <i>K</i>⁰. The value of <i>K</i>⁰ = 74.7 ± 6.4 M^–1 for dopamine/DMPG interactions clearly indicates that hydrophobic effects are 10 times weaker than electrostatic forces in this system. The presence of dopamine decreases the main transition temperature of DMPG, but no similar effect has been observed for DMPC. Basing on these results, we propose a simple electrostatic model of dopamine interactions with anionic membranes with the hydrophobic contribution expressed by <i>K</i>⁰. We suggest that dopamine interacts superficially with phospholipid membranes without penetrating into the bilayer hydrocarbon core. The model is physiologically important, since neuronal membranes contain a large (even 20%) fraction of anionic lipids

Media Effects on the Mechanism of Antioxidant Action of Silybin and 2,3-Dehydrosilybin: Role of the Enol Group

Silybin (SIL) and 2,3-dehydrosilybin (DHS) are constituents of milk thistle extract (silymarin) applied in the treatment of cirrhosis, hepatitis, and alcohol-induced liver disease. The molecular mechanism of their action is usually connected with antioxidant action. However, despite experimental and theoretical evidence for the antioxidant activity of SIL and DHS, the mechanism of their antiradical action still remains unclear. We studied the kinetics of SIL/DHS reactions with 2,2-diphenyl-1-picrylhydrazyl radical in organic solutions of different polarity and with peroxyl radicals in a micellar system mimicking the amphiphilic environment of lipid membranes. Kinetic studies together with determination of acidity and electrochemical measurements allowed us to discuss the structure–activity relationship in detail. In nonpolar solvents the reaction with free radicals proceeds via a one-step hydrogen atom transfer (HAT) mechanism, while significant acceleration of the reaction rates in methanol and water/methanol solutions suggests the dominating contribution of a sequential proton-loss electron-transfer (SPLET) mechanism with participation of the most acidic hydroxyl groups: 7-OH in SIL and 7-OH and 3-OH in DHS. In a heterogeneous water/lipid system, both mechanisms operate; however, the reaction kinetics and the antioxidant efficacy depend on the partition between lipid and water phases

Fullerene C₆₀ Derivatives as High-Temperature Inhibitors of Oxidative Degradation of Saturated Hydrocarbons

Fullerene C₆₀ is a free-radical scavenger, but its antioxidant activity is limited to some polymers and hydrocarbons at high temperatures. Here we demonstrate that for high-temperature oxidation of saturated hydrocarbons, the conjugates of C₆₀ with simple phenols are more active antioxidants than the building blocks (pristine C₆₀ and phenols) used alone. The overall kinetic parameters calculated by the Ozawa–Flynn–Wall method for nonisothermal oxidation of model hydrocarbons: saturated stearic acid (STA) and polyunsaturated linolenic acid (LNA) indicate that C₆₀ and its derivatives are effective antioxidants during oxidation of pure STA, but not during oxidation of LNA. These findings indicate that conjugates of C₆₀ and phenols are candidates for potential use in base oils or lubricants as new hybrid antioxidants able to work at temperatures above 100 °C