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Mechanistic Proposal (mechanistic + proposal)
Selected AbstractsDiisobutylaluminium Hydride (DIBAL-H) Promoted Secondary Rim Regioselective Demethylations of Permethylated ,-Cyclodextrin: A Mechanistic ProposalEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2010Sulong Xiao Abstract Diisobutylaluminium hydride (DIBAL-H) promotes secondary rim regioselective bis-de- O -methylation at the 2A - and 3B -positions of permethylated ,-cyclodextrin. This result contrasts with the selective bis-de- O -benzylation of perbenzylated cyclodextrins in which regioselective deprotection occurs at the primary rim. To gain an insight into the mechanism of this remarkable contrasting behavior, the two corresponding permethylated cyclodextrins with an alcohol function at either the 2- or 3-position were synthesized. The cyclodextrin with the alcohol at the 3-position reacts withDIBAL-H to afford the 2A,3B -diol whereas the cyclodextrin with the alcohol at the 2-position is unreactive. This observation allows us to propose a mechanism for the demethylation reaction that involves for each demethylation step two molecules of aluminium reagent, in accord with the previous hypothesis on the debenzylation reaction. The second demethylation step appears to be faster than the first, the coordination of aluminium now being an intramolecular process. [source] Diastereoselective Synthesis of 2-Aryl-3-vinyl-2,3-dihydrobenzo[b]furans Through a Sakurai Reaction: A Mechanistic Proposal.CHEMINFORM, Issue 16 2007Leticia Jimenez-Gonzalez Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Diastereoselective Synthesis of 2-Aryl-3-vinyl-2,3-dihydrobenzo[b]furans through a Sakurai Reaction: A Mechanistic ProposalCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2007Leticia Jiménez-González Dr. Abstract The condensation of 2,3-dihydrobenzoxasilepins with aromatic aldehydes in the presence of boron trifluoride to form 2,3-dihydrobenzofurans shows a level of diastereoselection which is a function of the electronic nature of the aldehyde and the polarity of the solvent. The study of the mechanism of the reaction demonstrated that it proceeds through a ring-opened allylfluorosilane, which is stable enough to be isolated and characterized. [source] Long-chain silanes as reducing agents part 1: a facile, efficient and selective route to amine and phosphine-stabilized active Pd-nanoparticlesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 3 2010Bhanu P. S. Chauhan Abstract Recently, metal nanoparticles have found applications in various fields, which have necessitated exploration of new avenues to obtain such materials. In this publication, a hydrosilane-based reduction and characterization of resulting palladium nanoparticles is achieved using palladium acetate as nanoparticle precursor and octadecylsilane as a reducing agent. The influence of phosphine and amine ligands in the stabilization of nanoparticles is also investigated. In addition, a brief mechanistic proposal of the reduction process is also discussed. Copyright © 2009 John Wiley & Sons, Ltd. [source] Unprecedented Solvent-Assisted Reactivity of Hydrido W3CuS4 Cubane Clusters: The Non-Innocent Behaviour of the Cluster-Core UnitCHEMISTRY - A EUROPEAN JOURNAL, Issue 18 2009Andrés Abstract Opening the cluster core: Substitution of the chloride ligand in the novel cationic cluster [W3CuS4H3Cl(dmpe)3]+ (see figure; dmpe=1,2-bis(dimethylphosphino)ethane) by acetonitrile is promoted by water addition. Kinetic and density functional theory studies lead to a mechanistic proposal in which acetonitrile or water attack causes the opening of the cluster core with dissociation of one of the CuS bonds to accommodate the entering ligand. Reaction of the incomplete cuboidal cationic cluster [W3S4H3(dmpe)3]+ (dmpe=1,2-bis(dimethylphosphino)ethane) with CuI compounds produces rare examples of cationic heterodimetallic hydrido clusters of formula [W3CuClS4H3(dmpe)3]+ ([1]+) and [W3Cu(CH3CN)S4H3(dmpe)3]2+ ([2]2+). An unexpected conversion of [1]+ into [2]2+, which involves substitution of chloride by CH3CN at the copper centre, has been observed in CH3CN/H2O mixtures. Surprisingly, formation of the acetonitrile complex does not occur in neat acetonitrile and requires the presence of water. The kinetics of this reaction has been studied and the results indicate that the process is accelerated when the water concentration increases and is retarded in the presence of added chloride. Computational studies have also been carried out and a mechanism for the substitution reaction is proposed in which attack at the copper centre by acetonitrile or water causes disruption of the cubane-type core. ESI-MS experiments support the formation of intermediates with an open-core cluster structure. This kind of process is unprecedented in the chemistry of M3M,Q4 (M=Mo, W; Q=S, Se) clusters, and allows for the transient appearance of a new coordination site at the M, site which could explain some aspects of the reactivity and catalytic properties of this kind of clusters. [source] Nd-Doped TiO2 Nanorods: Preparation and Application in Dye-Sensitized Solar CellsCHEMISTRY - AN ASIAN JOURNAL, Issue 5 2006Qiaohong Yao Abstract Monodispersed Nd-doped TiO2 nanorods (20,nm×2,nm) were synthesized by solvothermal methods and characterized by TEM, XRD, and EDS. Application of the nanorods for modifying conventional photoanodes in dye-sensitized solar cells (DSSCs) was investigated. Data show that, after modification, an enhancement of the incident-photon-to-current conversion efficiency (IPCE) in the whole range of visible light was observed and an increase of 33.3,% for overall conversion efficiency was achieved. Our mechanistic proposal is that Nd ions doped on TiO2 nanorods to some extent enhance the injection of excited electrons and decrease the recombination rate of the injected electrons. [source] Ring Expansion versus Cyclization in 4-Oxoazetidine-2- carbaldehydes Catalyzed by Molecular Iodine: Experimental and Theoretical Study in ConcertADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2010Benito Alcaide Abstract Molecular iodine (10,mol%) efficiently catalyzes the ring expansion of 4-oxoazetidine-2-carbaldehydes in the presence of tert -butyldimethylsilyl cyanide, or allylic and propargylic trimethylsilanes to afford protected 5-functionalized-3,4-dihydroxypyrrolidin-2-ones with good yield and high diastereoselectivity, through a C3C4 bond cleavage of the ,-lactam nucleus. Interestingly, in contrast to the iodine-catalyzed reactions of 3-alkoxy-,-lactam aldehydes which lead to the corresponding ,-lactam derivatives (rearrangement adducts), the reactions of 3-aryloxy-,-lactam aldehydes under similar conditions gave ,-lactam-fused chromanes (cyclization adducts) as the sole products, through exclusive electrophilic aromatic substitution involving the C3 aromatic ring and the carbaldehyde. In order to support the mechanistic proposals, theoretical studies have been performed. [source] A Photophysical and Photochemical Study of 6-Methoxy-2-naphthylacetic Acid, the Major Metabolite of the Phototoxic Nonsteroidal Antiinflammatory Drug NabumetonePHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2000F. Boscá ABSTRACT Nabumetone is a phototoxic nonsteroidal antiinflammatory drug used for the treatment of osteoarthritis. However, nabumetone is considered a prodrug with its metabolite 6-methoxy-2-naphthylacetic acid the active form. Photophysical and photochemical studies on this metabolite have been undertaken. It undergoes photodecarboxylation in aerated aqueous and organic solvents. In addition to the accepted photodegradation pathway for related molecules, a new mechanism that implies generation of the naphthalene radical cation from the excited singlet and addition of O2 prior to the decarboxylation process has been demonstrated. Evidence for the involvement of the excited singlet state in this mechanism have been obtained by steady-state and time-resolved fluorescence experiments. The fluorescence quenching by O2 and the shorter singlet lifetime in aerated solvents support this assignment. Laser flash photolysis also supports this mechanism by showing the noninvolvement of the triplet in the formation of the naphthalene radical cation. Finally, the well-known electron acceptor CCl4 acts as an efficient singlet quencher, enhancing the route leading to the radical cation, preventing intersystem crossing to the triplet and thus resulting in a dramatic increase in the yield of 6-methoxy-2-naphthaldehyde, the major oxidative decarboxylation product; this constitutes unambiguous proof in favor of the new mechanistic proposals. [source] | |||||||||||||||||||