Evidence for Nonstatistical Dynamics in the Wolff Rearrangement of a Carbene

Two ¹³C-labeled isomers of the formal Diels−Alder adduct of acetylmethyloxirene to tetramethyl 1,2,4,5-benzenetetracarboxylate have been synthesized. Flash vacuum thermolysis of these adducts leads to various isotopic isomers of acetylmethylketene, the ratios of which have been determined by NMR. The surprising finding that the principal product comes from methylpyruvoyl carbene rather than its more stable isomer diacetylcarbene is explained by MPWB1K density functional calculations, which show that the reactant probably undergoes a unimolecular rearrangement to a norcaradiene derivative prior to its fragmentation. Coupled-cluster calculations on the methylpyruvoyl carbene show that it is capable of undergoing three unimolecular isomerizations. The fastest is 1,2-acetyl migration to give acetylmethylketene directly. The next is rearrangement via acetylmethyloxirene to diacetylcarbene and thence by Wolff rearrangement to acetylmethylketene. The least-favorable reaction is degenerate rearrangement via 1,3-dimethyl-2-oxabicyclo[1.1.0]butan-4-one (the epoxide of dimethylcyclopropenone). The combined experimental and computational results indicate that Wolff rearrangement of the diacetylcarbene occurs with a 2.5:1 ratio of the methyl groups despite the fact that they are related by a twofold axis of symmetry in the carbene. Preliminary molecular dynamics simulations are consistent with this conclusion. Taken together, the results suggest that the Wolff rearrangement is subject to the same kind of nonstatistical dynamical effects detected for other kinds of thermally generated reactive intermediates

Situation sanitaire pendant la derniere saison des pluies, des troupeaux ovins-caprins suivis par le CIPEA dans la zone de Djenne

Etude sur la situation sanitaire des troupeaux d'ovins-caprins suivis par le CIPEA dans la zone de Djenne au Mali, pendant la derniere saison des pluies, avec presentation des resultats obtenus montrant d'une part les types de maladies predominants, notamment les ectoparasitoses et le pietin, et d'autre part les taux de mortalite chez les deux especes

Lithium Amino Alkoxide–Evans Enolate Mixed Aggregates: Aldol Addition with Matched and Mismatched Stereocontrol

Building on structural and mechanistic studies of lithiated enolates derived from acylated oxazolidinones (Evans enolates) and chiral lithiated amino alkoxides, we found that amino alkoxides amplify the enantioselectivity of aldol additions. The pairing of enantiomeric series affords matched and mismatched stereoselectivities. The structures of mixed tetramers showing 2:2 and 3:1 (alkoxide-rich) stoichiometries are determined spectroscopically. Rate and computational studies provide a viable mechanistic and stereochemical model based on the direct reaction of the 3:1 mixed tetramers, but they raise unanswered questions for the 2:2 mixed aggregates

Managing Environmental Research Data.

Environmental science researchers are now using and generating ever-increasing volumes of data and information about our natural world. It is estimated that the Environmental Protection Agency’s (EPA's) STRIVE (Science, Technology, Research and Innovation for the Environment) research funding programme will “involve more than 1,000 researchers and company-based scientists over its seven-year lifetime”1. The EPA's Environmental Research Centre (ERC) expects that large volumes of environmental data and information will be generated by projects funded by STRIVE. One of the key objectives of the STRIVE programme is to make the outcomes and data from this research available “in a coherent and timely manner which will ensure synergies across the wider research agenda and early availability of these outputs into the formulation of policy”2. Consequently, the STRIVE programme must adopt best international practice in environmental research data management. Management of these environmental research data is a core activity for the ERC with particular emphasis on the application of appropriate data management techniques to ensure their long-term availability and accessibility. Environmental research data are often irreplaceable; they are always unique particularly in the spatial location and temporal characteristics of their collection. They can also be extremely expensive and difficult to collect or generate. For these reasons the EPA and the ERC attach great importance to the ongoing development of systems that will ensure that maximum benefits are derived from research data once acquired

Metachelins, Mannosylated and N‑Oxidized Coprogen-Type Siderophores from <i>Metarhizium robertsii</i>

Under iron-depleted culture conditions, the entomopathogenic fungus <i>Metarhizium robertsii</i> (Bischoff, Humber, and Rehner) (= <i>M. anisopliae</i>) produces a complex of extracellular siderophores including novel O-glycosylated and N-oxidized coprogen-type compounds as well as the known fungal siderophores <i>N</i>^α-dimethylcoprogen (NADC) and dimerumic acid (DA). Metachelin A (<b>1</b>), the most abundant component in the <i>M. robertsii</i> siderophore mixture, was characterized as a 1094 Da analogue of NADC that is O-glycosylated by β-mannose at both terminal hydroxyl groups and N-oxidized at the dimethylated α-nitrogen. The mixture also contained a 1078 Da analogue, metachelin B (<b>2</b>), which lacks the <i>N</i>-oxide modification. Also characterized were the aglycone of <b>1</b>, i.e., the <i>N</i>-oxide of NADC (<b>3</b>), and the monomannoside of DA (<b>6</b>). <i>N</i>-Oxide and <i>O</i>-glycosyl substituents are unprecedented among microbial siderophores. At high ESIMS source energy and at room temperature in DMSO, <b>1</b> underwent Cope elimination, resulting in loss of the <i>N</i>^α-dimethyl group and dehydration of the α–β bond. High-resolution ESIMS data confirmed that all tri- and dihydroxamate siderophores (<b>1</b>–<b>6</b>) complex with trivalent Fe, Al, and Ga. In a chrome azurol S assay, all of the <i>M. robertsii</i> siderophores showed iron-binding activity roughly equivalent to that of desferrioxamine B

Rapid Determination of Polymer Stereoregularity Using Band-Selective 2D HSQC

We report the use of band-selective 2D HSQC NMR spectroscopy to rapidly determine the stereoregularity of polymers usually analyzed by 1D ¹³C NMR spectroscopy. This approach reduced the time required to characterize the triad stereosequences of polyacrylonitrile from about an hour to a few minutes, and can be performed with sufficient ¹³C resolution to resolve higher-order stereosequences, such as the pentads of polypropylene

Anodically Coupled Electrolysis for the Heterodifunctionalization of Alkenes

The emergence of new catalytic strategies that cleverly adopt concepts and techniques frequently used in areas such as photochemistry and electrochemistry has yielded a myriad of new organic reactions that would be challenging to achieve using orthodox methods. Herein, we discuss the strategic use of anodically coupled electrolysis, an electrochemical process that combines two parallel oxidative events, as a complementary approach to existing methods for redox organic transformations. Specifically, we demonstrate anodically coupled electrolysis in the regio- and chemoselective chlorotrifluoromethylation of alkenes

Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low <i>T</i>_g polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene” comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events

Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low <i>T</i>_g polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene” comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events

Secondary Alkene Insertion and Precision Chain-Walking: A New Route to Semicrystalline “Polyethylene” from α‑Olefins by Combining Two Rare Catalytic Events

While traditional polymerization of linear α-olefins (LAOs) typically provides amorphous, low <i>T</i>_g polymers, chain-straightening polymerization represents a route to semicrystalline materials. A series of α-diimine nickel catalysts were tested for the polymerization of various LAOs. Although known systems yielded amorphous or low-melting polymers, the “sandwich” α-diimines <b>3</b>–<b>6</b> yielded semicrystalline “polyethylene” comprised primarily of unbranched repeat units via a combination of uncommon regioselective 2,1-insertion and precision chain-walking events