Ruthenium catalysts bearing N-heterocyclic carbene ligands in atom transfer radical reactions

The catalytic activity of ruthenium-p-cymene complexes bearing N-heterocyclic carbene ligands in atom transfer radical addition (ATRA) or polymerisation (ATRP) strongly depends on the substituents of the carbene ligand, thereby providing a nice illustration of the importance of organometallic engineering and ligand fine tuning in homogeneous catalysis.     

Cross-coupling of aryl iodides with paramagnetic terminal acetylenes derived from 4,4,5,5-tetramethyl-2-imidazoline-1-oxyl 3-oxide

2-(Arylylethynylphenyl)-4,4,5,5-tetramethyl-2-imidazoline-1-oxyl 3-oxides 12 and 13 were synthesized by cross-coupling of aryl iodides with 1-alkynes containing the 4,4,5,5-tetramethyl-2-imidazoline-1-oxyl 3-oxide fragment. A procedure was developed for the preparation of 3- and 4-ethynylbenzaldehydes with the use of 2-methylbut-3-yn-2-ol.     

Regularities of reversible radical dissociation in liquid crystals

The effect of liquid-crystalline ordering of alkylcyanobiphenyls on the thermodynamic parameters of reversible dissociation of the dimeric 2,2"-bis[2-(p-dimethylaminophenyl)indane-1,3-dione] to the 2-(p-dimethylaminophenyl)indane-1,3-diyl radicals was found. The angular dependence of the ESR spectra of the radicals in the smectic mesophase is related to their orientation along the liquid crystal director.     

Radical reactions of N-heterocyclic compounds, XV. Radial intermediates and end products of the oxidation of 3-anilino-1,5-diphenylpyrazoles with Pb(OAc)4

Summary Substituted 3-anilino-1,5-diphenylpyrazoles1a–i were oxidized with Pb(OAc)₄ in benzene or CH₂Cl₂ solution. The ESR measurements confirmed the formation of aminyl radicals3f–i frompara-CH₃ substituted pyrazoles1f–i. The radical intermediates from unsubstituted pyrazoles1a–e were assigned to triarylaminium cation radicals These were generated by consecutive oxidation of the dimeric products4a–e whose structure was proven by NMR spectroscopy and₁₅N labeling.

---

Radikalreaktionen von N-Heterocyclen, 15. Mitt. Intermediäre Radikale und Endprodukte der Oxidation von 3-Anilino-1,5-diphenylpyrazolen mit Pb(OAc)₄

Zusammenfassung Die substituierten 3-Anilino-1,5-diphenylpyrazole1a–i wurden mit Pb(OAc)₄ in Benzol oder CH₂Cl₂ oxidiert. Die intermediären Aminylradikale3f–i wurden ESR-spektroskopisch bei der Oxidation der im Anilinrest methylsubstituierten Pyrazole1f–i nachgewiesen. Die ESR-spektroskopisch nach Oxidation der Pyrazole1a–e beobachteten Radikalintermediate interpretieren wir als Kationradikale6a–e. Diese werden durch Weiteroxidation der Dimeren4a–e gebildet, deren Struktur durch NMR-Untersuchungen und¹⁵N-Markierung bewiesen wurde.

     

Samarium(II) iodide-induced tandem reductive coupling-Dieckmann condensation reaction: one-step synthesis of bicyclic oxacyclopentanecarboxylate from bis-α,β-unsaturated esters

The tandem cyclization of bis-α,β-unsaturated esters with SmI₂-Sm-THF in the presence of catalytic amount of methanol was found to stereoselectively provide bicyclo[4.3.0]nonan-8-ones and bicyclo[3.3.0]octan-3-ones.     

Significant reduction of on-site Coulomb energy U due to short-range correlation in an organic Mott insulator.

By carrying out a first-principles T-matrix calculation on multiple scatterings between electrons, we show that the intramolecular electron-electron interaction energy U, of a Mott insulator of the organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is significantly reduced from the naive expectation value of the Coulomb interaction (7.3 eV and 5.3 eV, respectively, for the bare and screened Coulomb interactions) to 2.9 eV due to the short-range correlation. This result together with the intermolecular interaction energy D=1.3 eV explains the experimental optical gap (1.5 eV). The associated two-particle wavefunction clearly shows the Coulomb hole indicating that two electrons with antiparallel spins cannot approach because of the Coulomb repulsion. We also discuss the energetics and magnetics of this system.     

Free Radical Studies at Low Temperatures

Free radicals can be produced in the gas phase and then condensed together with the molecules of an inert gas; they can also be generated and trapped in isolated sites in rigid solids. IR and ESR spectra of trapped radicals provide information concerning their structure and chemical properties. The techniques employed for producing and studying radicals are reviewed and a number of recent IR and ESR investigations of trapped radicals are discussed. The last part of the article deals exclusively with the applications of the rotating cryostat to the study of the ESR spectra and reactions of trapped radicals at low temperatures.     

Studies on Oxidants' Generation in a Foaming Column with a Needle to Dielectric Covered Plate Electrode

The generation of oxidative products in the newly investigated foaming system was improved by a discharge between a high voltage multineedle electrode and a dielectric covered plate electrode. A uniformity of the discharge was observed. The presence of alumina dielectric seems to be responsible of the homogenous spatial distribution of the discharge. The absolute power and energy of the discharge was determined. Besides, in larger gap space the foam quality was improved.This way of oxidant's generation, which might be prospective for the treatment of exhaust gas and wastewater, has not been investigated by the other researchers' groups as yet. During the laboratory test 875 ppm of gaseous ozone, 0.5 mg/l of dissolved ozone and 62 mg/l of hydrogen peroxide was obtained at 14 kV of applied voltage and 5 l/min of oxygen gas flow in the present configuration used for the discharge generation. The average yield of gaseous ozone production ranged 55 gO₃/kWh.     

Radicals in the synthesis of sulphur dioxide-based copolymers

Free radicals generated from a number of olefinic, acetylenic, and vinyl monomers in the presence of sulphur dioxide and tert-butyl hydroperoxide were investigated. In all reaction systems a sulphonyl propagating radical first appears which depropagates into the corresponding carbon radical at higher temperatures. The sulphonyl radicals were observed between ?119 and ?44°C, whereas the carbon radicals were observed from ?63 to ?1°C. The order of decreasing reactivity of the reaction system was: 3-hexyne > 1-heptene > methyl acrylate > acrylonitrile > 1-heptyne. © 1993 John Wiley & Sons, Inc.     

Behandlung der Strukturen 3 atomiger Dublett- und Triplett-Radikale auf der Grundlage von Slater-Exponenten

The sequence of the atoms M, Q, and R in neutral triatomic radicals of formula MQR is discussed in terms of the Slater-exponent (Z-s)/n ^* of the atoms involved. It is shown that the atom with the relatively smallest numerical value of (Z-s)/n ^* occupies the central position. This result is supplemented by Walsh's rule (linearity or nonlinearity of the radicals under consideration) and vice versa.