51 research outputs found
Dioxomolybdenum(VI) compounds with α-amino acid donor ligands as catalytic precursors for the selective oxyfunctionalization of olefins
A series of cis-dioxomolybdenum(VI) α-amino acid containing compounds I–V has been investigated as potential catalyst precursors for the mild and selective oxyfunctionalization of conjugated or unconjugated olefins like styrene, α-methylstyrene, cis-β-methylstyrene, cyclohexene and cyclooctene, using tert-butyl hydroperoxide (TBHP) as main oxidant. All the I–V complexes behaved as active heterogeneous and recyclable catalysts, showing good to quantitative conversion values of the substrate. In all cases, high selectivity toward the corresponding epoxide formation was detected. No substantial difference in terms of efficiency has been observed among the different catalysts I–V, thus confirming that the different nature of the amino acidic side-chain does not strictly affect the catalytic process. Insights into mechanistic details and reaction free energy profile of catalytic oxidations by means of quantum chemical calculations have been discussed
Double hydrophosphination of alkynes promoted by rhodium: The key role of an N-heterocyclic carbene ligand
The regioselective double hydrophosphination of alkynes mediated by rhodium catalysts is presented. The distinctive stereoelectronic properties of the NHC ligand prevent the catalyst deactivation by diphosphine coordination thereby allowing for the closing of a productive catalytic cycle
Ligand-controlled regioselectivity in the hydrothiolation of alkynes by rhodium N-heterocyclic carbene catalysts
Rh-N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(ν 2- olefin)] 2 and RhCl(IPr)(py)(ν 2-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl(IPr)(py)(ν 2- olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate-hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate-alkyne disposition, favoring formation of 2,2-disubstituted metal-alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl-hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step. © 2012 American Chemical Society.Financial support from the Ministerio de Ciencia e Innovación (MICINN/FEDER) of Spain (Project CTQ2010-15221), the Diputación General de Aragón (E07), the ARAID Foundation under the program “Jóvenes Investigadores”, and CONSOLIDER INGENIO-2010, Projects MULTICAT (CSD2009-00050) and Factoría de Cristalización (CSD2006-0015) are gratefully acknowledged. R.C. thanks the CSIC and the European Social Fund for his Research Contract in the framework of the “Ramón y Cajal” Program.Peer Reviewe
SBA-15 Anchored Metal Containing Catalysts in the Oxidative Desulfurization Process
Recalcitrant sulfur compounds are common impurities in crude oil. During combustion they produce SOx derivatives that are able to affect the atmospheric ozone layer, increasing the formation of acid rains, and reducing the life of the engine due to corrosion. In the last twenty years, many efforts have been devoted to develop conventional hydrodesulfurization (HDS) procedures, as well as alternative methods, such as selective adsorption, bio-desulfurization, oxidative desulfurization (ODS) under extractive conditions (ECODS), and others. Among them, the oxidative procedures have been usually accomplished by the use of toxic stoichiometric oxidants, namely potassium permanganate, sodium bromate and carboxylic and sulfonic peracids. As an alternative, increasing interest is devoted to selective and economical procedures based upon catalytic methods. Heterogeneous catalysis is of relevance in industrial ODS processes, since it reduces the leaching of active species and favors the recovery and reuse of the catalyst for successive transformations. The heterogenization of different types of high-valent metal transition-based organometallic complexes, able to promote the activation of stoichiometric benign oxidants like peroxides, can be achieved using various solid supports. Many successful cases have been frequently associated with the use of mesoporous silicas that have the advantage of easy surface modification by reaction with organosilanes, facilitating the immobilization of homogeneous catalysts. In this manuscript the application of SBA-15 as efficient support for different active metal species, able to promote the catalytic ODS of either model or real fuels is reviewed, highlighting its beneficial properties such as high surface area, narrow pore size distribution and tunable pore diameter dimensions. Related to this topic, the most relevant advances recently published, will be discussed and critically described
Fluorinated bioorganic molecules: stereoselective syntheses of optically active centres
Dottorato di ricerca in chimica industriale. 12. ciclo. A.a. 1996-99. Relatore Pierfrancesco Bravo. Coordinatore Francesco MinisciConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome; Biblioteca Nazionale Centrale - P.za Cavalleggeri, 1, Florence / CNR - Consiglio Nazionale delle RichercheSIGLEITItal
Highly Stereoselective Tandem Pummerer Reaction/α-Hydroxy Imine Rearrangement of E.P. β-Sulfinylenamines
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