Enhancing the dienic reactivity of phospholes: an improved access to trivalent 7-phosphanorbornenes.

A structural comparison of 1-cyano- and 1-alkoxy-3,4-dimethylphospholes with 1-benzylphosphole has led to some unexpected conclusions. There is no univocal relationship between phosphole aromaticity and pyramidality at phosphorus. It has been found that both the highly pyramidal 1-cyanophosphole 1 (sigma(CPC angles) = 290 degrees), and the much less pyramidal 1-alkoxyphosphole 6 (sigma(CPC angles) = 310 degrees) have a low Bird aromaticity index (27 for both molecules), when compared to 1-benzylphosphole (sigma(CPC angles) = 303 degrees, BI = 35.5). This low aromaticity is correlated with a high reactivity of the diene in both 1 and 7 (similar to 6) toward acrylonitrile. Good stereochemical control is observed with 7, which gives exclusively the anti,endo [4 + 2] cycloadducts with acrylonitrile and diethyl vinylphosphonate.     

Phosphaalkenes palladium(II) complexes in the suzuki and sonogashira cross‐coupling reactions

The 1‐methoxy‐2‐(supermesitylphosphanylidenemethyl)‐benzene ligand ( 1 ) was prepared by reacting the phospha‐Wittig reagent [Mes*PPMe₃] with o‐methoxybenzaldehyde. Reaction of 1 with one equivalent of the [Pd(allyl)Cl]₂ dimer in the presence of Ag(OTf) affords a neutral complex ( 4 ) in which the triflate ligand is coordinated to the palladium atom. DFT calculations show that the formation of complex 4 is favored by 22.4 kcal/mol with respect to that of a chelate species involving coordination of the ligand through the phosphorus atom of one lone pair at the oxygen of the pendant methoxy group. Reaction of two equivalents of ligand 1 with the [Pd(allyl)Cl]₂ dimer affords complex 5 , in which the two ligands are coordinated through their phosphorus atom. The catalytic activity of complex 5 was compared to that of the 1,3‐bis[2‐(supermesityl)phosphanediylmethyl]benzene palladium chloride complex (6). Performances of the two catalysts were found to be similar in the Suzuki cross‐coupling reaction between phenylboronic acid and some arylbromides (TON between 55.10⁵ and 99.10⁵) as well as in the Sonogashira coupling between phenylacetylene and arylbromides (TON between 400 and 950). © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:363–371, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20307     

An Iron Catalytic Probe for Determination of the O-atom Density in an Ar/O2 Afterglow

An iron fiber optics catalytic probe has been constructed and applied for the real-time measuring of the O-atom density in an Ar/O₂ afterglow. The recombination coefficient for the heterogeneous surface recombination of O atoms on the oxidized iron foil was measured at different temperatures between 400 and 950 K. The coefficient was found to be constant in the entire range of experimental conditions and had a value of 0.41 ± 0.12. The iron fiber optics catalytic probe has an advantage over the classical nickel fiber optics catalytic probe: the probe signal is higher for the iron probe due to a higher recombination coefficient thus causing an easier real-time monitoring of the O-atom density. The O-atom density was measured in an afterglow of microwave plasma created at different discharge powers up to 300 W, at a constant Ar flow rate of 1000 sccm/min and at different oxygen flow rates between 50 and 300 sccm/min. The O-atom density was found to be dependent on oxygen flow. At low oxygen flow rates up to 100 sccm/min, a saturation of the O-atom density was obtained at a certain discharge power, while at high oxygen flow rate the O-atom density kept increasing with the increasing power. The results were explained by gas phase and surface phenomena.     

X-ray diffraction,differential scanning calorimetric and spectroscopic studies of phase transitions in the bidimensional compound (C12H25NH3)2CdCl4

The existence of three main crystalline phases (called III, II and I) in (C₁₂H₂₅NH₃)₂CdCl₄ has been revealed by differential scanning calorimetry. X-ray diffraction and spectroscopic studies. The crystal- lographic evolution with increasing temperature appears to be monoclinic (III) → orthorhombic (II) → tetragonal (I). The low temperature phase III is the only ordered structure. The phase transition (III-II), which is of first order type, corresponds to an order-disorder mechanism involving the organic part of the structure (alkylammonium chains) whereas the phase transition (II-I), which is of second-order type, is related to the arrangement of the mineral matrix (octahedra of perovskite layers). An intermediate disordered form II', stable in a very narrow temperature range and structurally similar to the form II, has also been observed, so that the transformation (III-II) proceeds, in fact, in two steps (III-II'-II). The variation enthalpies observed at the transitions (III-II'-II) and analyzed through an order-disorder mechanism demonstrate the high disorder of the alkylammonium chains in form II, in agreement with spectroscopic results. No thermal anomaly or spectroscopic modification is observed for the high temperature transition (II-I).     

The role of cooperation and parasites in non-linear replicator delayed extinctions

In the present work we study the role of cooperation and parasites on extinction delayed transitions for self-replicating species with catalytic activity. We first use a one-dimensional continuous equation to study the dynamics of both single autocatalytic replicator and symmetric two-member hypercycles, where two well-defined phases involving survival and extinction of replicators are shown to exist. Extinction dynamics is analyzed numerically and analytically and under both deterministic and stochastic scenarios. A ghost is also found for the single autocatalytic replicator and for the asymmetric hypercycle, with an extinction time delay following the square-root scaling law near bifurcation threshold. We find that the extinction delay is longer for the two-member hypercycle than for the single autocatalytic species, indicating that cooperation among replicators might involve to spend a longer time in the bottle-neck region of the ghost. The asymmetry of the network is shown to prolong the extinction time. We also show that an attached parasite decreases the time spent in the bottle-neck region of the ghost, thus accelerating extinction in these systems of replicators. Nevertheless the effect of the parasite is not so important when replicators catalytically cooperate, being the two-member hypercycle less sensitive to the parasite than the autocatalytic species. Here the hypercycle asymmetry can also significantly increase the delaying capacity. These features make the hypercycle to undergo a longer extinction delay, thus increasing the memory effect of the ghost. We finally explore the role of the ghost in fluctuating media, where the extinction delayed transition is shown to increase the survival probability of cooperating catalytic species.     

Electron Energy Distribution Functions in He-CO DC Laser Mixture: The Role of Superelastic Electronic Collisions

Electron energy distribution functions (EDF) in He — CO mixtures have been calculated by solving the Boltzmann equation including superelastic vibrational and electronic collisions. The latter include the metastable states CO(a³II) and He(³S). The results show that the presence of superelastic electronic collisions strongly affect EDF. In particular small concentrations of He(³S) modify the shape of EDF, by creating different structures. The net result is the dramatic dependence of electronic and ionization rate coefficients on the concentrations of He(³S) and CO(a³II), while the vibrational rate coefficients are practically unaffected by the presence of these states.     

Reactions de wittig,wittig-horner et knoevenagel par activation anionique avec l'alumine ou le fluorure de potassium depose sur l'alumine,sans solvant

Wittig, Wittig-Horner and Knoevenagel reactions are achieved in the presence of alumina or potassium fluoride supported on alumina, without organic solvent. The presence of a small amount of water increases the rate of the Wittig and Wittig-Horner reactions. The reaction of the diethyl cyanomethylphosphonate with an aldehyde in the presence of dry alumina is oriented towards the Knoevenagel reaction.