Browsing by Subject "Palladium catalysts"
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Item Direct comparison of homogeneous and heterogeneous palladium(II) catalysts for Suzuki-Miyaura cross-coupling reactions(2014-05) Crawford, Katherine Alexis; Cowley, Alan H.; Humphrey, Simon M.The syntheses and catalytic properties of four new 1,2-acenaphthenyl N-heterocyclic carbene-supported palladium(II) catalysts are presented. The acenaphthenyl carbene can be prepared using either mesityl or 2,6-diisopropyl N-aryl substituents. In addition, two new heterogeneous analogs were synthesized with 2,6-diisopropyl N-aryl substituents that were anchored through the backbone to an insoluble silica-support. Comprehensive catalytic studies of the Suzuki coupling of aryl halides with aryl boronic acids were carried out. In general, the homogeneous diisopropyl-functionalized catalyst was found to exhibit superior selectivity and reactivity. A comparison of the performances of the aforementioned catalysts in toluene, dichloromethane and aqueous solutions are also presented. In organic solvents, the catalysts were found to be proficient for the homogeneous Suzuki coupling of aryl iodides, bromides and chlorides with boronic acids at low temperatures (35‒40 °C). Similar reactions that were carried out in aqueous media resulted in the formation of insoluble colloidal catalytic species. Nevertheless, these species still retained high activities in terms of in the Suzuki reaction with aryl chlorides. Moreover, the heterogeneous Pd precipitates can be easily recovered for subsequent use by means of filtration. The activation energies that were determined for the aryl bromide-based Suzuki reactions were found to fall in the range, 159.2‒171.2 kJ mol⁻¹ in organic solvents and 111.3‒115.9 kJ mol⁻¹ in water. The corresponding activation energy for the aryl chloride was found to be 321.8 kJ mol⁻¹ in aqueous media using the homogeneous diisopropyl-functionalized carbene catalyst. Conversely, the heterogeneous catalyst exhibited reactivity toward aryl iodides and bromides exclusively, and required significantly higher temperatures and catalyst loadings in both toluene and water. Additional experimental trials that were performed in tetrahydrofuran solution at lower temperatures resulted in substantially larger catalytic conversions. The heterogeneous catalyst allowed for easy separation and recovery. However, the catalyst exhibited a significant decrease in reactivity toward the aryl halides after two consecutive trials.Item Palladium-mediated cyclizations of gamma-oxoallenes: synthesis of structurally modified nucleic acids(Texas Tech University, 1993-08) Mosher, Michael D.The goals of the research described in this dissertation were to study the palladium-mediated intramolecular nucleometallatlon/methoxycarbonylation of yoxoallenes (see Scheme 1) and to use the methyl furanoside products obtained from such reactions in the syntheses of deoxy- and dideoxyribonucleoside analogues. Nucleoside analogues are currently of particular interest due to their ability to act as anti-tumor and anti-AIDS (Acquired Immunodeficiency Syndrome) medicines. A section of this introduction has been dedicated to information about AIDS and antiretrovlral nucleoside analogues. A convenient route to enantiomerically pure C-nucleoslde derivatives will be mentioned as a side project. (C-Nucleosides have also been found to possess potent biological activity). In addition, the cyclizatlon of 4,5-hexadienoic acid to give butyrolactone products will be described (see Scheme 1; R'" = OH).Item Stereoselective syntheses of functionalized tetrahydrofurans via intramolecular oxymetallations of allenes and syntheses of methyl nonactate homologues(Texas Tech University, 1988-12) Park, GyoosoonMy research objective was to investigate two-step intramolecular nucleometallationcoupling reactions of allenes to form polyfiinctional heterocyclic molecules. This reaction sequence takes place by either: a) an intramolecular oxy- or aminomercuration of an allene followed by a transmetallation of the mercury by palladium(II), then a coupling reaction (e.g., with carbon monoxide and an alcohol to form estCTs); or b) an intramolecular oxy- or aminopalladation of an allene followed, in situ, by the coupling. The results of this investigation have led to: 1) the development of a stereospecific approach to cis-2-15-alkyltetrahydrofiiran-2-yl)-2-propenoates; 2) the discovery of a stereospecific reduction of cis-2-(5-alkyltetrahydrofuran-2-yl)-2-propenoates; and 3) the development of a general synthetic approach to natural and unnatural nonactic acid esters. Scheme 1 represents our basic idea for the intramolecular nucleometallation of allenes (1) using mercury or palladium salts. A metal-complexed allene (2) should cyclize to form an intermediate a-vinyl complex of the metal (3). The complex 3 (where M = palladium) can undergo carbonylation and alkoxide coupling in the presence of carbon monoxide and an alcohol to form substituted acrylate esters (4) which should be valuable intermediates for organic synthesis.