Reactivity of Metal Carbenes with Olefins: Theoretical Insights on the Carbene Electronic Structure and Cyclopropanation Reaction Mechanism.

Present work addresses the reactivity of several phenyl-substituted metal-carbene complexes with 4-methylstyrene by means of density functional theory OPBE simulations. Different paths that lead to cyclopropanation were explored and compared to the olefin metathesis mechanism. For this purpose, we chose four different catalysts: (i) the Grubbs second-generation olefin metathesis catalyst, (ii) a Grubs second-generation-like complex, in which ruthenium is replaced by iron, and (iii) two iron carbene complexes (a piano stool and a porphyrin iron carbene) that experimentally catalyze alkene cyclopropanation. Results suggest that the nature of the applying mechanism is very sensitive to the coordination around the metal center and the spin state of the metal-carbene complex. Cyclopropanation by open-shell metal-carbene complexes seems to preferentially proceed through a two-step radical mechanism, in which the two C-C bonds are sequentially formed (path C). Singlet-state carbenes proceed either through a direct attack of the olefin to the carbene (path D) when the formation of the metallacycle is not feasible or through a reductive elimination from the metallacyclobutane when this intermediate is accessible both kinetically and thermodynamically (path B).

[Quantitative assessment of medical students' clinical practical learning in department of obstetrics and gynecology of Brest Hospital University]. / Évaluation quantitative de l'enseignement pratique des étudiants hospitaliers en gynécologie-obstétrique au CHU de Brest.

OBJECTIVE: The main objective of this study was to describe how medical students got progressively trained to perform the most frequent medical procedures in obstetrics and gynecology. The secondary objective was to rationalize and plan the training. METHODS: The medical students in our University Hospital were contacted either directly or by e-mail. They were given an anonymous simple choice questionnaire relating to eight most important acts in the specialty. RESULTS: Three hundred and eighty-two medical students have been consulted. We got 173 answers which means a participation rate of 45.3%. The survey showed up that the considered procedures were progressively put into practice with the exception of two: the insertion of an intrauterine contraceptive device (coil) and of an implant. CONCLUSION: The study showed off some insufficiency in the training in two of the considered procedures. An amendment was proposed with simulated performances of the acts and the setting up of a training course booklet.

Dinitrogen dissociation on an isolated surface tantalum atom.

Both industrial and biochemical ammonia syntheses are thought to rely on the cooperation of multiple metals in breaking the strong triple bond of dinitrogen. Such multimetallic cooperation for dinitrogen cleavage is also the general rule for dinitrogen reductive cleavage with molecular systems and surfaces. We have observed cleavage of dinitrogen at 250 degrees C and atmospheric pressure by dihydrogen on isolated silica surface-supported tantalum(III) and tantalum(V) hydride centers [(identical with Si-O)2Ta(III)-H] and [(identical with Si-O)2Ta(V)H3], leading to the Ta(V) amido imido product [(identical with SiO)2Ta(=NH)(NH2)]: We assigned the product structure based on extensive characterization by infrared and solid-state nuclear magnetic resonance spectroscopy, isotopic labeling studies, and supporting data from x-ray absorption and theoretical simulations. Reaction intermediates revealed by in situ monitoring of the reaction with infrared spectroscopy support a mechanism highly distinct from those previously observed in enzymatic, organometallic, and heterogeneous N2 activating systems.