Recognition and activation by ureas and thioureas: stereoselective reactions using ureas and thioureas as hydrogen-bonding donors

Hydrogen-bonding interaction plays a crucial role in the molecular recognition and activation processes of various biologically important reactions that are mediated by enzymes and antibodies in living organisms. Recently, it has been shown that a hydrogen-bonding donor can be used as a general acid catalyst for various types of reactions in organic chemistry. In this article, we describe enantioselective reactions catalyzed by urea and thiourea derivatives as general acid catalysts as well as diastereoselective reactions. This perspective provides an overview of this rapidly growing field.     

X-ray absorption spectroscopic study on the electronic structure of Li(1)(-)(x)()CoPO(4) electrodes as 4.8 V positive electrodes for rechargeable lithium ion batteries

Changes in the electronic structure of olivine Li(1-x)CoPO(4), 4.8 V positive electrode material for lithium ion batteries, were investigated using the X-ray absorption spectroscopy (XAS) technique. The threshold energy in the Co K-edge increased with electrochemical Li removal, indicating the oxidation of cobalt ions due to charge compensation. Moreover, P and O K-edge XAS showed a slight shift in threshold energy with Li removal. Although it is generally believed that the electrons of PO(4) polyanion do not contribute to the oxidation process, present experimental results indicate changes in the electronic structure around PO(4) units. Such results would be interpreted by the idea of the hybridization effect between the Co 3d and O 2p orbitals and of the polarization effect introduced by Li ions.     

Reactivity of amido ligands on a dinuclear Ru(II) center: formation of imido complexes and C-N coupling reaction with alkyne

Treatment of a dinuclear Ru(II) amido complex [Cp*Ru(mu2-NHPh)]2 (Cp* = eta5-C5Me5) with small organic substrates including CO, tert-butyl isocyanide, a sulfur ylide Ph2S=CH2, and diphenylacetylene resulted in an unexpected disproportionation reaction of the bridging amido ligands to produce a free amine and a series of imido-bridged diruthenium complexes [(Cp*Ru)2(mu2-L)(mu2-NPh)] (L = CO, t-BuNC, CH2). In the case of diphenylacetylene, the bridging imido ligand underwent subsequent coupling reaction with the coordinated alkyne to form an iminoalkenyl complex [(Cp*Ru)2(mu2-PhNCPhCPh)].     

Magnetic circular dichroism study of directly fused porphyrins.

The magnetic circular dichroism (MCD) spectra of doubly and triply linked fused bisporphyrins (2MD and 2MT, M = Ni, Zn, Cu, Pd, and H2) and triply linked higher oligomers (3ZnT and 4ZnT) have been measured, and their Q-bands assigned based on the results of INDO/s calculations. In contrast to the Faraday A term observed for the Q(0,0) band of Ni(II) tetraphenylporphyrin, a single positive Faraday B term was observed for the lowest energy transition of the fused systems. The calculations indicated that the molecular orbitals (MOs) of the directly fused porphyrins consist of linear combinations of the constituent monomeric MOs, and that the effect of lowering the symmetry is always larger on the lowest unoccupied molecular orbital (LUMO) than on the highest occupied molecular orbital (HOMO). On the basis of Michl's perimeter model, these features can be correlated with the observed positive MCD signs in the near infrared region. A weak absorption band at 600-700 nm for the fused dimers can be assigned to a short-axis polarized Q transition.     

Vinyl Polymerization by Metal Complexes. XXIII. UV and Kinetic Studies on the Photopolymerization with Iron(III)-Amine-Carbon Tetrachloride Systems

Abstract

In order to elucidate the initiation reaction of the photopolymerization with iron(III)-amine-carbon tetrachloride systems, the photochemical reaction process among iron(III), amine, and carbon tetrachloride in methanol solution was followed at 0°C by UV spectroscopy as for iron(III) ion. The rate constants of both the reduction of iron (III) under irradiation with light and the oxidation of iron(II) in the dark were measured, and were related to the rates of photopolymerization of methyl methacrylate. Kinetic study on the photopolymerization of methyl methacrylate with iron(III)-triethylenetetramine-carbon tetrachloride system was made in parallel in methanol solution at 0°C. The initiation mechanism of the photopolymerization was postulated.     

Homoleptic Zincate‐Promoted Room‐Temperature Halogen–Metal Exchange of Bromopyridines

Homoleptic lithium tri‐ and tetraalkyl zincates were reacted with a set of bromopyridines. Efficient and chemoselective bromine–metal exchanges were realized at room temperature with a substoichiometric amount of nBu₄ZnLi₂?TMEDA reagent (1/3 equiv; TMEDA=N,N,N′,N′‐tetramethylethylenediamine). This reactivity contrasted with that of tBu₄ZnLi₂?TMEDA, which was inefficient below one equivalent. DFT calculations allowed us to rationalize the formation of N???Li stabilized polypyridyl zincates in the reaction. The one‐pot difunctionalization of dibromopyridines was also realized using the reagent stoichiometrically. The direct creation of C? Zn bonds in bromopyridines enabled us to perform efficient Negishi‐type cross‐couplings.     

Cross‐Coupling of Organolithium with Ethers or Aryl Ammonium Salts by C−O or C−N Bond Cleavage

Various aryl‐, alkenyl‐, and/or alkyllithium species reacted smoothly with aryl and/or benzyl ethers with cleavage of the inert C?O bond to afford cross‐coupled products, catalyzed by commercially available [Ni(cod)₂] (cod=1,5‐cyclooctadiene) catalysts with N‐heterocyclic carbene (NHC) ligands. Furthermore, the coupling reaction between the aryllithium compounds and aryl ammonium salts proceeded under mild conditions with C?N bond cleavage in the presence of a [Pd(PPh₃)₂Cl₂] catalyst. These methods enable selective sequential functionalizations of arenes having both C?N and C?O bonds in one pot.     

Surfactant-mixing effects on the interfacial tension and the microemulsion formation in water/supercritical CO2 system

The effects of surfactant mixing on interfacial tension and on microemulsion formation were examined for systems of air/water and water/supercritical CO2 (scCO2) interfaces and for water/scCO2 microemulsions. A fluorinated surfactant, sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)2), was mixed with the three hydrocarbon surfactants, Pluronic L31, Tergitol TMN-6, and decyltrimethylammonium chloride (DeTAC), at equimolar ratio. For all the cases, the interfacial tension was significantly lowered by the mixing. The positive synergistic effect suggests that the mixed surfactants tend to pack more closely on the interface than the pure constituents. It was found, however, that the microemulsion formation in scCO2 was never facilitated by the mixing, except for the case of TMN-6. This is probably due to the segregation of the surfactants into hydrocarbon-rich and fluorocarbon-rich phases on the microemulsion surface.