Catalytic Three‐Component Domino Reaction for the Preparation of Trisubstituted Oxazoles

Multicomponent reactions are attractive for assembling functionalized heterocyclic compounds. To this end, an efficient gold‐catalyzed three‐component domino reaction to form oxazoles directly from imines, alkynes, and acid chlorides is presented. The reaction proceeds in a single synthetic step by using a gold(III)–N,N′‐ethylenebis(salicylimine) (salen) catalyst to give trisubstituted oxazoles in up to 96 % yield. The substrate scope, a mechanistic study exploring the role of the gold catalyst, and the synthetic applications of the oxazole products are discussed.     

Determination of the 14N quadrupole coupling constant of nitroxide spin probes by W-band ELDOR-detected NMR

Nitroxide spin probe electron paramagnetic resonance (EPR) has proven to be a very successful method to probe local polarity and solvent hydrogen bonding properties at the molecular level. The g(xx) and the (14)N hyperfine A(zz) principal values are the EPR parameters of the nitroxide spin probe that are sensitive to these properties and are therefore monitored experimentally. Recently, the (14)N quadrupole interaction of nitroxides has been shown to be also highly sensitive to polarity and H-bonding (A. Savitsky et al., J. Phys. Chem. B 112 (2008) 9079). High-field electron spin echo envelope modulation (ESEEM) was used successfully to determine the P(xx) and P(yy) principal components of the (14)N quadrupole tensor. The P(zz) value was calculated from the traceless character of the quadrupole tensor. We introduce here high-field (W-band, 95 GHz, 3.5 T) electron-electron double resonance (ELDOR)-detected NMR as a method to obtain the (14)N P(zz) value directly, together with A(zz). This is complemented by W-band hyperfine sublevel correlation (HYSCORE) measurements carried out along the g(xx) direction to determine the principal P(xx) and P(yy) components. Through measurements of TEMPOL dissolved in solvents of different polarities, we show that A(zz) increases, while |P(zz)| decreases with polarity, as predicted by Savitsky et al.     

Flash photolysis of transient radicals. Benzophenone ketyl radical

A number of processes are found to follow excitation of the benzophenone ketyl radical. The lowest excited state of the radical absorbs, fluoresces and reacts with solvent. Rate constants for reaction of this state with cyclohexane and isopropanol are 4 × 10⁷ and 7 × 10⁸ M^?1 s^?1, respectively. The lifetime of 5.1 ns found in a solution containing 1% cyclohexane in acetonitrile at room temperature is longer than that reported previously.     

Structure-property relationships and moisture sensitivity of PDMS/PTMO mixed soft segment urethane elastomers

The structure-property relationships of polyurethane elastomers based on 4,4′-diphenylmethane diisocyanate (MDI) and 4,4′-dicyclohexylmethane diisocyanate (HMDI) with PTMO, PDMS, and mixed PTMO/PDMS soft segments were investigated using DSC, dynamic mechanical spectroscopy, tensile testing, and SAXS. Changes in water absorption and moisture sensitivity with compositional changes were investigated through water absorption and physical property measurements. Differences in tensile properties and moisture sensitivity with varying hard and soft segment composition are interpreted in terms of the phase mixing characteristics of the various polymers studied.     

Mittlere Schwingungsamplituden eines X2Y2-Molekülmodells mit C2h-Symmetrie: Anwendung auf dastrans-Difluordiazin

Zusammenfassung Es wurde eine Methode zur Berechnung der mittleren Schwingungsamplitudenquadrate für das Modell einesX ₂ Y ₂-Moleküls mit C_2h-Symmetrie unter Verwendung von Symmetriekoordinaten entwickelt und analytische Ausdrücke für die verschiedenen mittleren Amplitudenquadrat-Größen in Termen der symmetrisierten Matrizen der mittleren Amplitudenquadrate abgeleitet. Diese Methode wurde zur Bestimmung der mittleren Schwingungsamplituden sowohl der gebundenen wie auch der nicht gebundenen Atompaare destrans-Difluordiazins bei 298°K aus beobachteten Raman- und Infrarotgrundschwingungsfrequenzen und Angaben zur Molekülstruktur angewendet. Darüber hinaus wurden die molaren Größen der thermodynamischen Funktionen unter Annahme eines starren Rotator—harmonischen Oszillator-Modells für den Temperaturbereich 200 bis 2000°K berechnet. Die Ergebnisse der Berechnungen werden kurz diskutiert.

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Mean amplitudes of vibration of an X₂Y₂ molecular model of C_2h-symmetry: Application to trans-difluorodiazine

A method for the determination of the mean-square amplitudes of vibration for anX ₂ Y ₂ molecular model with C_2h symmetry has been developed using the symmetry coordinates. Analytical expressions for the various mean-square amplitude quantities in terms of the symmetrized mean-square amplitude matrices have been derived. The method has been applied totrans-difluorodiazine for the determination of mean amplitudes of vibration for both bonded and nonbonded atom pairs at 298°K from the observed Raman and infrared fundamental frequencies and molecular structural data. In addition to these, molar thermodynamic functions have also been computed on the assumption of a rigid rotator, harmonic oscillator model for the temperature range 200–2000°K. A brief discussion of the results has been given.

     

Synthesis of poly(ethylene glycol) block copolymers by a redox process and their spectral characterization

Novel block copolymers of poly(ethylene glycol) (PEG) with acrylamide (AAm) and methacrylic acid (MAA) were synthesized using a redox system consisting of ceric ions and PEG in aqueous acidic medium. The molecular weight of PEG in the redox system was varied to obtain a series of block copolymers with differing molecular weights of PEG segment. The polymerization proceeded via macroradical generation, which was substantiated by ESR spectroscopy. This macroradical acted as a redox macroinitiator for the block copolymerization of the vinyl monomers. The formation of the block copolymers was confirmed by fractional precipitation technique.     

Dependence of positron capture rate on dislocation density in solid solutions of KCl—KBr

Generally well-annealed alkali halide crystals exhibit three lifetime components. τ₁ arises due to annihilation with anion and τ₂ due to Bloch Ps. The origin of τ₃ is not well understood. Positron lifetime and etching techniques have been performed on solid solutions of KCl_100−xBr_x with mole % of x = 0.0, 15, 30, 37, 50, 70, and 100. The positron lifetime spectra have been analysed to resolve into three components. The positron capture rate K₃ for the longest lived component and the dislocation density shows a non-linear variation with composition. The positron capture rate varies linearly with the dislocation density of the samples. The origin of τ₃ is identified as due to positronium formation in the dislocations present in the samples. The trapping probability per dislocation has been estimated to be 2.2 × 10⁻¹²/s.