Copper-catalyzed dehydrogenative borylation of terminal alkynes with pinacolborane.

LCuOTf complexes [L = cyclic (alkyl)(amino)carbenes (CAACs) or N-heterocyclic carbenes (NHCs)] selectively promote the dehydrogenative borylation of C(sp)-H bonds at room temperature. It is shown that σ,π-bis(copper) acetylide and copper hydride complexes are the key catalytic species

Isolation of bis(copper) key intermediates in Cu-catalyzed azide-alkyne "click reaction".

The copper-catalyzed 1,3-dipolar cycloaddition of an azide to a terminal alkyne (CuAAC) is one of the most popular chemical transformations, with applications ranging from material to life sciences. However, despite many mechanistic studies, direct observation of key components of the catalytic cycle is still missing. Initially, mononuclear species were thought to be the active catalysts, but later on, dinuclear complexes came to the front. We report the isolation of both a previously postulated π,σ-bis(copper) acetylide and a hitherto never-mentioned bis(metallated) triazole complex. We also demonstrate that although mono- and bis-copper complexes promote the CuAAC reaction, the dinuclear species are involved in the kinetically favored pathway

Uses and Misuses of Agricultural Pesticides in Africa: Neglected Public Health Threats for Workers and Population

Pesticides are use in agriculture for their capacity to reduce pest and protect foods. Since their introduction in Africa by colonial masters, the use of these chemicals is constantly growing. Herbicides and insecticides are the two dominant categories. Although they are used in small quantities by farmers who own small exploitation, the frequency of their use, as well as overuses and misuses, constitutes serious factors of exposure and health risks. Farm workers are more vulnerable to occupational effects from pesticide inhalation and skin contacts. Failure to wear protective equipment and observe good agricultural practices explained health symptoms that are frequently experienced: eye and skin irritation, nausea, vomiting, and headache. Population is subject to chronic health effects due to repeated dietary intake of pesticides. Most consumed staple foods on the continent (cereals, vegetables, and fruits) have been found to be contaminated by one or multiple residues of pesticides. The level of residues is often higher than regulatory limits. Organization of surveillance programs to monitor concentration of pesticide residues remains inexistent in most countries, same for toxicovigilance systems to documented poisoning cases. Current data underline the need to carry out pesticide health risk assessment in order to appreciate the threats they pose for public health

Protonolysis of a Ruthenium–Carbene Bond and Applications in Olefin Metathesis

The synthesis of a ruthenium complex containing an N-heterocylic carbene (NHC) and a mesoionic carbene (MIC) is described wherein addition of a Brønsted acid results in protonolysis of the Ru–MIC bond to generate an extremely active metathesis catalyst. Mechanistic studies implicated a rate-determining protonation step in the generation of the metathesis-active species. The activity of the NHC/MIC catalyst was found to exceed those of current commercial ruthenium catalysts

La transformée de Fourier pour les espaces tordus sur un groupe réductif p-adique

le papier a été accepté dans la revue Astérisque— Let G be a connected reductive group defined over a non–Archimedean local field F. Put G = G(F). Let θ be an F –automorphism of G, and let ω be a smooth character of G. This paper is concerned with the smooth complex representations π of G such that π θ = π • θ is isomorphic to ωπ = ω ⊗ π. If π is admissible, in particular irreducible, the choice of an isomorphism A from ωπ to π θ (and of a Haar measure on G) defines a distribution Θ A π = tr(π • A) on G. The twisted Fourier transform associates to a compactly supported locally constant function f on G, the function (π, A) → Θ A π (f) on a suitable Grothendieck group. Here we describe its image (Paley– Wiener theorem) and its kernel (spectral density theorem).Soit G un groupe réductif connexe défini sur un corps local non ar-chimédien F. On pose G = G(F). Soit aussi θ un F –automorphisme de G, et ω un caractère lisse de G. On s'intéresse aux représentations complexes lisses π de G telles que π θ = π • θ est isomorphe à ωπ = ω ⊗ π. Si π est admissible, en particulier irréductible, le choix d'un isomorphisme A de ωπ sur π θ (et d'une mesure de Haar sur G) définit une distribution Θ A π = tr(π • A) sur G. La transformée de Fourier tordue associe à une fonction f sur G localement constante et à support compact, la fonction (π, A) → Θ A π (f) sur un groupe de Grothendieck adéquat. On décrit ici son image (théorème de Paley–Wiener) et son noyau (théorème de densité spectrale)

(Phosphanyl)phosphaketenes as building blocks for novel phosphorus heterocycles.

Although BH3 simply coordinates the endocyclic P of (phospholidino)phosphaketene 1Dipp , the bulkier B(C6F5)3 gives rise to a zwitterionic diphosphirenium, which is a novel type of 2π-electron aromatic system as shown by the calculated NICS values. While the reaction of 1Dipp with Na[PCO(dioxane) x ] is unselective, the same reaction with the sterically bulky (phospholidino)phosphaketene 1Ar** [Ar** = 2,6-bis[di(4-tert-butylphenyl)methyl]-4-methylphenyl selectively affords a sodium bridged dimer containing a hitherto unknown λ3,λ5,λ3-triphosphete core. The latter formally results from "P-" addition to a 1,3-P/C-dipole. Similarly, adamantyl isonitrile adds to 1Dipp giving a 4-membered phosphacycle. In contrast to 1, the phosphaketene derived from the electrophilic diazaphospholidine-4,5-dione is unstable and reacts with a second molecule of Na[PCO(dioxane) x ] to afford a 1,3,4-oxadiphospholonide derivative

Oil Contracts, Progressive Taxation and Government Take in the Context of Uncertainty in Crude Oil Prices: The Case of Chad

Etudes & documentsConcession Contracts (CC) and Production Sharing Contracts (PSC) have quite different implications for Government Take and the properties of the tax system, such as progressivity. In general, taxation via CC introduces significant distortions in activity, particularly due to the balance of royalties which tax production irrespective of the profitability of the project. So CC is normally regressive while PSC is normally progressive, because PSC taxation depends more directly on the profitability of the project. Chad has the distinction of having introduced PSC in the 2007 Chad oil code, while maintaining a royalty on production. Despite this feature, we show with a Cash Flow model and Monte Carlo simulations that the application of the 2007 oil code introduced more progressivity into taxation. This feature is particularly interesting in the current context of falling crude oil prices, because it maintains a favorable tax regime for exploration and exploitation by multinational oil companies. As a result, the Chad government should reactivate a counter-cyclical policy of oil revenue reserves when the crude oil price increases again

Synthesis of Highly Stable 1,3-Diaryl-1H-1,2,3-triazol-5-ylidenes and Their Applications in Ruthenium-Catalyzed Olefin Metathesis

The formal cycloaddition between 1,3-diaza-2-azoniaallene salts and alkynes or alkyne equivalents provides an efficient synthesis of 1,3-diaryl-1H-1,2,3-triazolium salts, the direct precursors of 1,2,3-triazol-5-ylidenes. These N,N-diarylated mesoionic carbenes (MICs) exhibit enhanced stability in comparison to their alkylated counterparts. Experimental and computational results confirm that these MICs act as strongly electron-donating ligands. Their increased stability allows for the preparation of ruthenium olefin metathesis catalysts that are efficient in both ring-opening and ring-closing reactions

Construction et analyse des signaux non-stationnaires adaptés à la caractérisation des phénomènes turbulents

National audienceDans ce papier nous proposons une approche hybride signal-physique pour la détection de la présence de la turbulence dans un fluide. L'idée est de construire une forme d'onde capable de porter, de manière robuste, l'information sur la turbulence et de mettre en place le traitement qui permettra l'extraction des paramètres liés à ce phénomène. Il est donc nécessaire de réaliser une correspondance entre les grandeurs qui caractérisent les écoulements turbulents et les signaux acoustiques. Nous allons montrer que l'utilisation des signaux avec une loi de fréquence instantanée polynomiale est potentiellement porteuse de l'information de turbulence que nous pouvons extraire à partir de l'estimation de la déformation de cette loi