The aza-Morita-Baylis-Hillman reaction of electronically and sterically deactivated substrates.

The aza-Morita–Baylis–Hillman (azaMBH) reaction has been studied for electronically and sterically deactivated Michael acceptors. It is found that electronically deactivated systems can be converted with electron-rich phosphanes and pyridines as catalysts equally well. For sterically deactivated systems clearly better catalytic turnover can be achieved with pyridine catalysts. This is in accordance with the calculated affinities of the catalysts towards different Michael-acceptors

Mass Spectrometric Back Reaction Screening of Quasi-Enantiomeric Products as a Mechanistic Tool

In this account, we discuss a mass spectrometric method that enables unambiguous identification of intermediates involved in the enantioselective step of a catalytic cycle. This method, which we originally developed for rapid evaluation of chiral catalysts, is based on monitoring the back reaction of mass-labeled quasi-enantiomeric products by ESI-MS. In this way, the intrinsic enantioselectivity of a chiral catalyst can be determined directly by quantification of catalytically relevant intermediates. By comparing the results from the forward and back reaction, direct evidence for the involvement of a catalytic intermediate in the enantioselective step can be obtained. In addition, insights about the energy profile of the catalytic cycle may be gained. The potential of back reaction screening as a mechanistic tool is demonstrated for organocatalytic aldol reactions, 1,4-additions of aldehydes to nitroolefins, Diels-Alder reactions, Michael additions, and Morita-Baylis-Hillman reactions

Mechanism and diastereoselectivity of aziridine formation from sulfur ylides and imines: a computational study.

A computational investigation of the title reaction involving semistabilized (R = Ph) and stabilized (R = CO2Me) sulfur ylides has been performed using DFT methods including a continuum model of solvent. Our results provide support for the generally accepted mechanism and are in very good agreement with observed cis/trans selectivities. This study shows that betaine formation is nonreversible, and that selectivity is thereby determined at the initial addition step, in the case of semistabilized ylides. Our analysis indicates moreover that addition TS structures are governed by the steric strain induced by the N-sulfonyl group, which favors the transoid approach in the case of syn betaine formation and the cisoid mode of addition in anti TSs. The observed low trans selectivity is accounted for by the favorable Coulombic interactions and stabilization by C-H...O hydrogen bonding allowed in the cisoid anti addition TS. In the case of stabilized ylides, the endothermicity of betaine formation combined with the high barrier to ring closure render the elimination step rate- and selectivity-determining. Accordingly, the low cis selectivity observed in stabilized ylide reactions is explained by the lower steric strain in the elimination step generated by the formation of the cis aziridine (as compared to the trans case)

Développement d'une voie générale d'accès aux dérivés β- et γ-aminophosphoniques par réaction Diels-Alder : approches expérimentale et théorique

Malgré leurs activités nombreuses et variées, les dérivés β-, et γ-aminophosphoniques n'ont été préparés jusqu'à present que de façon ponctuelle selon des stratégies de synthèse particulières. Afin de pallier à ce manque nous nous sommes intéressés, au cours de notre doctorat, au développement d'une voie d'accés générale à ces composés. La stratégie que nous avons sélectionée repose sur la réaction Diels-Alder de partenaires rezspectivement aminés et phosphorés. Nous avons dans la première partie de ce travail de doctorat, mis au point la cycloaddition d'aminodiènes et phosphonodiénophiles; l'utilisation d'aminodiènes chiraux nous a permis d'obtenir des β- et γ-amido-phosphonocyclohexènes optiquement actifs avec une excellente sélectivité. Dans la seconde partie, nous nous sommes intéressés à la possibilité de cycloaddition de phosphonodiènes et d'aminodiénophiles. Cette partie de notre travail nous a conduit au développement d'une nouvelle famille de diènes, les dérivés phosphoniques. Parallèlement à ce travail expérimental, nous avons également réalié une étude approfondie de la fonction phosphonate en utilisant les méthodes ab initio de la chimie quantique. L'analyse de l'influence énergétique, structurale et électronique de la substitution d'un système π et de l'état de transition de la réaction D-A par le groupement phosphonate nous apermis d'identifier et de quantifier les effets de ce groupement sur les doubles liaisons et son influence sur leur réactivité en cycloaddition.Doctorat en sciences (chimie) - UCL, 200

The Extraction of Vanadium from Titaniferous Iron Ores

TITANIFEROUS magnetites, often called titano-magnetites, occur in many parts of the world and they mostly contain a variable, but significant content of vanadium. In fact, most ilmenites also contain a small vanadium content