Decarboxylation catalytique et quantitative d'acides arylacetiques en presence'de cuivre I

Copper(I) salts of arylacetic acids $\text{1}$ RCO₂H readily undergo a catalytic and quantitative decarboxylation in acetonitrile leading to hydrocarbons $\text{2}$ RH when the corresponding carbanions R are stabilized by resonance and/or electronwithdrawing substituents.     

All particle primary nucleon spectrum derived from the latest jacee data

The all particle primary nucleon spectrum has been estimated on the basis of the latest Japanese American Emulsion Experiments on the balloon borne emulsion chamber measurements of the fluxes of p, He, C, N, O, Ne, Mg, Si, Fe and low energy fit extrapolated results have been found to follow the power law fit of the formN(E) dE=2·03E ^–2·7 dE (cm² s sr GeV/nucleon)^–1. The concept of nuclear fragmentation has been used to estimate the nucleon flux from nuclei spectra. The derived result agrees with the predicted primary spectrum after Yodh, Ellsworth, Stanev and Gaisser.Equipe de Recherche associée au C.N.R.S.Av. H. Fellow, on leave fromIndian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India.     

High-energy interactions

Summary This paper, based on the rapporteur talk given at the XXIV International Cosmic-Ray Conference (Roma, 28 August–8 September 1995), concerns the sessions HE1, HE2, HE6. Multihadron production in cosmic rays, models, emulsion chamber measurements, simulation of electromagnetic and hadronic cascades, new particles and processes are summarized in the frame of the special phenomenology adapted to the collisions of ultrahigh-energy particles in the atmosphere. Rapporteur talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

Dimerisation des polyphenyl-allenes-II : Formation et interconversion des dimeres cyclobutaniques du triphenyl-allene. stereoselectivite de la dimerisation des allenes

Progressive heating of triphenyl-allene 1 leads first to the kinetic dimers 3b and 3c, then the less stable dimer 3c undergoes thermal opening, and so isomerizes to 3a and 3b, which can equilibrate further. Finally, the diradicals 2 produced by thermal reopening of these dimers 3 can give the dihydronaphthacene 4 through recombination involving the ortho position of a phenyl group and dehydrogenation. The kinetically formed diradical, which gives 3b and 3c by conrotatory closures, is shown to possess structure 2a, and this fact suggests that the meeting of two allene molecules 1 must lead to a transition state of T_1,2 type, corresponding to optimum electronic and steric stabilization. The analysis of thermochemical data for the dimerization of aliene to 1,2-dimethylene cyclobutane demonstrates their compatibility with such a structure for the transition state. Stereochemistry of various polysubstituted allenes dimerizations appears to conform with the same factors of stability for the corresponding transition states.     

The search for cosmic strangelets with the supersonic concorde and with JACEE's circumpolar balloon flight in Antarctica

The search for cosmic strangelet nuclei was carried out by two experiments with emulsion chambers. A balloon-borne JACEE emulsion chamber was flown at 3.5 g/cm² for 200 h in Antarctica (JACEE-10 experiment) and the Concorde flights were made by ECHOS at an atmospheric depth of 110 g/cm² between Paris and New York. No nuclei withZ⩾30 survived after traversing 60–120 g/cm² of the detector materials in the JACEE instruments. No evidence for a long mean free path were found in the zenith angle distribution forZ/β⩾26 nuclei. The exposure factor used by the JACEE was 72 m²hsr. The intensity upperbounds,I⩽(2.2–9.7)×10⁻²/m²h sr, were obtained for strangelets having an atmospheric attenuation length of 220−50 g/cm², which corresponds to the case for mass numberA=100–10000 andZ/β > 13. Concorde experiments (ECHOS) used both a thin and a thick emulsion chamber. The total exposure was 209 m² h sr and no candidates with chargeZ⩾30 were found. The largest track hadZ/β=28.6±1.29 withβ ∼ 1. Nuclei observed with charge 13⩽Z⩽30 were consistent with the survival intensity of ordinary nuclei. The flux bounds from the ECHOS experiments were I⩽(2.1–5.0) x 10⁻²/m²h for strangelets with mass number 100⩽A⩽1000.     

Ortho-hydroxylation selective des phenols : I - vers un modele chimique simple des tyrosinases

The oxydation of cuprous phenoxides by molecular oxygen gives rise to c and ortho-benzoquinones. Such a new reaction provides a mechanistic approach to tyrosinase catalyzed oxidation of phenols.     

Decarboxylation oxydante des acides carboxyliques aliphatiques par un systeme cuivre(i)/oxygene.

A new oxidative decarboxylation reaction converts carboxylic acids 1 or 2 (Z = OH) to the corresponding carbonyl compounds 3, in the presence of oxygen and catalytic amounts of copper(I)probably via transient copper(III) carboxylates.     

The titration of carboxyl-terminated monolayers revisited: in situ calibrated fourier transform infrared study of well-defined monolayers on silicon

The acid-base equilibrium at the surface of well-defined mixed carboxyl-terminated/methyl-terminated monolayers grafted on silicon (111) has been investigated using in situ calibrated infrared spectroscopy (attenuated total reflectance (ATR)) in the range of 900-4000 cm (-1). Spectra of surfaces in contact with electrolytes of various pH provide a direct observation of the COOH <--> COO (-) conversion process. Quantitative analysis of the spectra shows that ionization of the carboxyl groups starts around pH 6 and extends over more than 6 pH units: approximately 85% ionization is measured at pH 11 (at higher pH, the layers become damaged). Observations are consistently accounted for by a single acid-base equilibrium and discussed in terms of change in ion solvation at the surface and electrostatic interactions between surface charges. The latter effect, which appears to be the main limitation, is qualitatively accounted for by a simple model taking into account the change in the Helmholtz potential associated with the surface charge. Furthermore, comparison of calculated curves with experimental titration curves of mixed monolayers suggests that acid and alkyl chains are segregated in the monolayer.     

Formal chemoselective synthesis of leucascandrolide A

A chemoselective synthesis of the macrocyclic core of leucascandrolide A has been achieved, utilizing highly enantioselective allylmetalations, an enantioselective Noyori reduction of a propargylic ketone and olefin metatheses as the key steps.