Effective recognition of H2PO4- by a new series of dendrimers functionalized with ferrocenyl-urea termini

A new series of dendrimers with poly(propylene imine) backbones and 4, 8, 16, or 32 peripheral ferrocenyl-urea groups were prepared and characterized; their voltammetric behavior in DMSO solution was very sensitive to the presence of hydrogenphosphate anions at submillimolar concentration levels.     

Synthesis of fused oxabicyclic systems by metal-catalyzed intramolecular addition of 1,3-cycloalkyldiones to alkynes

[reaction: see text] Readily available 4-propargyl-1,3-cyclohexanediones and cyclopentanediones can be chemo- and regioselectively cycloisomerized to synthetically appealing fused oxabicyclic systems by simply stirring at room temperature with catalytic amounts of an appropriate metal complex.     

Longitudinal ultrasonic attenuation in the spin spiral state of single crystal holmium

Ultrasonic attenuation experiments have been performed using 15–255 MHz longitudinal sound waves along the c-axis of single crystal holmium. Without an external magnetic field, the attenuation increases in the spin-spiral state. This anomalous increase originates, at least in part, from the spin-phonon relaxation mechanism we have proposed, which states that the attenuation coefficient Δα₁ ∞ ω²/(1+ω²τ²) where τ is the longitudinal spin phonon relaxation time. The frequency dependence of the attenuation varies from ω^1·0 to ω^1·5 which suggests a frequency-dependent character for the spin-phonon relaxation time τ. A broad longitudinal attenuation maximum, which presumably is due to the competing processes between $\overline{S}$, J and τ, is observed in the spin-spiral state, where $\overline{S}$ is the thermal average of the spin angular momentum per trivalent ion and J the Fourier transform of the exchange integral. An anomalous suppression of the longitudinal attenuation spike at T_N for frequencies higher than 165 MHz is unexplainable at present. The longitudinal attenuation spike at T_N for frequencies higher than 165 MHz is unexplainable at present. The longitudinal spin-phonon relaxation time for Ho has been determined using experimental data. It has a T^?3 temperature dependence. In the presence of an external magnetic filed in the basal plane the attenuation is decreased and a new peak and a plateau appear at the intermediate phase transitions.     

Does aromaticity in a reaction product increase or decrease the intrinsic barrier? Kinetics of the reversible deprotonation of benzofuran-3(2H)-one and benzothiophene-3(2H)-one

A kinetic study of the reversible deprotonation of benzofuran-3(2H)-one (3H-O) and benzothiophene-3(2H)-one (3H-S) by amines and hydroxide ion in water at 25 degrees C is reported. The respective conjugate bases, 3--O and 3--S, represent benzofuran and benzothiophene derivatives, respectively, and thus are aromatic. The main question addressed in this paper is whether this aromaticity has the effect of enhancing or lowering intrinsic barriers to proton transfer. These intrinsic barriers were either determined from Br?nsted plots for the reactions with amines or calculated on the basis of the Marcus equation for the reaction with OH-; they were found to be lower for the more highly aromatic benzothiophene derivative, indicating that aromaticity lowers the intrinsic barrier. It is shown that the reduction in the intrinsic barrier is not an artifact of other factors such as inductive, steric, resonance, polarizability, and pi-donor effects, although these factors affect the intrinsic barriers in a major way. Our results imply that aromatic stabilization of the transition state is ahead of proton transfer; this contrasts with simple resonance effects, which typically lag behind proton transfer at the transition state, thereby increasing intrinsic barriers.     

First kinetic discrimination between carbon and oxygen reactivity of enols

Nitrosation of enols shows a well-differentiated behavior depending on whether the reaction proceeds through the carbon (nucleophilic catalysis is observed) or the oxygen atom (general acid-base catalysis is observed). This is due to the different operating mechanisms for C- and O-nitrosation. Nitrosation of acetylacetone (AcAc) shows a simultaneous nucleophilic and acid-base catalysis. This simultaneous catalysis constitutes the first kinetic evidence of two independent reactions on the carbon and oxygen atom of an enol. The following kinetic study allows us to determine the rate constants for both reaction pathways. A similar reactivity of the nucleophilic centers with the nitrosonium ion is observed.     

Catalysis by metallic nanoparticles in aqueous solution: model reactions

Catalysis by metallic nanoparticles is certainly among the most intensely studied problems in modern nanoscience. However, reliable tests for catalytic performance of such nanoparticles are often poorly defined, which makes comparison and benchmarking rather difficult. We tackle in this tutorial review a subset of well-studied reactions that take place in aqueous phase and for which a comprehensive kinetic analysis is available. Two of these catalytic model reactions are under consideration here, namely the reduction of (i) p-nitrophenol and (ii) hexacyanoferrate (iii), both by borohydride ions. Both reactions take place at the surface of noble metal nanoparticles at room temperature and can be accurately monitored by UV-vis spectroscopy. Moreover, the total surface area of the nanoparticles in solution can be known with high precision and thus can be directly used for the kinetic analysis. Hence, these model reactions represent cases of heterogeneous catalysis that can be modelled with the accuracy typically available for homogeneous catalysis. Both model reactions allow us to discuss a number of important concepts and questions, namely the dependence of catalytic activity on the size of the nanoparticles, electrochemistry of nanoparticles, surface restructuring, the use of carrier systems and the role of diffusion control.     

First evidence of simultaneous different kinetic behaviors at the interface and the continuous medium of w/o microemulsions

A study was carried out on the butylaminolysis reaction of 4-nitrophenyl caprate in AOT/chlorobenzene/water microemulsions, with the observed rate constant, kobs, showing both first- and second-order dependence on butylamine concentration. The first-order term in [BuNH2] is due to the reaction occurring at the interface of the microemulsion while the second-order term is due to the reaction in the continuous medium. The different kinetic behavior is accounted for by the mechanism by which the reaction proceeds: at the interface of the microemulsion, the rate-determining step is the formation of the addition intermediate, T+/-, whereas in the continuous medium the slow step is the base-catalyzed decomposition of this intermediate. The application of the pseudophase formalism allows the observed kinetic behavior to be explained and to obtain the rate constants at the interface, ki2=0.13 M-1 s-1, and in the continuous medium, ko2KT=2.46x10(-2) M-2 s-1. These values indicate that the reaction rate decreases approximately 23 times upon going from the aqueous medium to the interface of the microemulsion, whereas the rate constant in the continuous medium is consistent with that obtained in pure chlorobenzene, ko2KT=2.09x10(-2) M-2 s-1.     

Palladium-catalyzed [3 + 2] intramolecular cycloaddition of alk-5-enylidenecyclopropanes

Readily accessible alk-5-enylidenecyclopropanes undergo [3 + 2] intramolecular cycloaddition reactions upon treatment with appropriate palladium complexes. The method allows the rapid and efficient assembly of a variety of bicyclo[3.3.0]octane systems with up to three stereocenters. Preliminary theoretical calculations uncovered previously unsuspected mechanistic possibilities based on either a concerted pallada-ene-like rearrangement or a stepwise process involving zwitterionic intermediates.     

On the low-lying excited states of sym-triazine-based herbicides.

We report a joint computational and luminescence study on the low-lying excited states of sym-triazines, namely, 1,3,5-triazine (1) and the ubiquitous herbicides atrazine [6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (2)] and ametryn [6-methylthio-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine (3)]. Geometrical structures, energetics, and transition and state properties of I and 2 were computed at the TD-DFT, CASSCF, and CASPT2 levels of theory. The fluorescence and phosphorescence emission spectra, lifetimes, and fluorescence quantum yields were measured for the three compounds, and from these, the energies of the lowest excited states and their corresponding radiative rates were determined. The predictions from CASPT2 calculations are in good agreement with the experimental results obtained from the luminescence studies and allow the interpretation of different absorption and emission features.     

Rhodium‐Catalyzed Intramolecular [3+2+2] Cycloadditions between Alkylidenecyclopropanes,Alkynes, and Alkenes

A Rh‐catalyzed intramolecular [3+2+2] cycloaddition is reported. The cycloaddition affords synthetically relevant 5,7,5‐fused tricyclic systems of type 2 from readily available dienyne precursors. The transformation takes place with moderate or good yields, high diastereoselectivity, and total chemoselectivity.